Neurotransmitter

Neurotransmitter

Overview
Neurotransmitters are endogenous
Endogenous
Endogenous substances are those that originate from within an organism, tissue, or cell. Endogenous retroviruses are caused by ancient infections of germ cells in humans, mammals and other vertebrates...

 chemicals
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

 that transmit signals from a neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

 to a target cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 across a synapse
Synapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...

. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to receptors in the membrane on the postsynaptic side of the synapse. Release of neurotransmitters usually follows arrival of an action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

 at the synapse, but may also follow graded electrical potentials.
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Encyclopedia
Neurotransmitters are endogenous
Endogenous
Endogenous substances are those that originate from within an organism, tissue, or cell. Endogenous retroviruses are caused by ancient infections of germ cells in humans, mammals and other vertebrates...

 chemicals
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

 that transmit signals from a neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

 to a target cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 across a synapse
Synapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...

. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to receptors in the membrane on the postsynaptic side of the synapse. Release of neurotransmitters usually follows arrival of an action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

 at the synapse, but may also follow graded electrical potentials. Low level "baseline" release also occurs without electrical stimulation. Neurotransmitters are synthesized from plentiful and simple precursors, such as amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s, which are readily available from the diet and which require only a small number of biosynthetic steps to convert.

Discovery


Until the early 20th century, scientists assumed that the majority of synaptic communication in the brain was electrical. However, through the careful histological
Histology
Histology is the study of the microscopic anatomy of cells and tissues of plants and animals. It is performed by examining cells and tissues commonly by sectioning and staining; followed by examination under a light microscope or electron microscope...

 examinations of Ramón y Cajal (1852–1934), a 20 to 40 nm gap between neurons, known today as the synaptic cleft, was discovered. The presence of such a gap suggested communication via chemical messengers traversing the synaptic cleft, and in 1921 German pharmacologist Otto Loewi
Otto Loewi
Otto Loewi was a German born pharmacologist whose discovery of acetylcholine helped enhance medical therapy. The discovery earned for him the Nobel Prize in Physiology or Medicine in 1936 which he shared with Sir Henry Dale, whom he met in 1902 when spending some months in Ernest Starling's...

 (1873–1961) confirmed that neurons can communicate by releasing chemicals. Through a series of experiments involving the vagus nerves of frogs, Loewi was able to manually control the heart rate of frogs by controlling the amount of saline solution present around the vagus nerve. Upon completion of this experiment, Loewi asserted that sympathetic regulation of cardiac function can be mediated through changes in chemical concentrations. Furthermore, Otto Loewi is accredited with discovering acetylcholine
Acetylcholine
The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...

 (ACh)—the first known neurotransmitter. Some neurons do, however, communicate via electrical synapse
Electrical synapse
An electrical synapse is a mechanical and electrically conductive link between two abutting neurons that is formed at a narrow gap between the pre- and postsynaptic neurons known as a gap junction. At gap junctions, such cells approach within about 3.5 nm of each other, a much shorter...

s through the use of gap junctions, which allow specific ions to pass directly from one cell to another.

Identifying neurotransmitters


The chemical identity of neurotransmitters is often difficult to determine experimentally. For example, it is easy using an electron microscope to recognize vesicles on the presynaptic side of a synapse, but it may not be easy to determine directly what chemical is packed into them. The difficulties led to many historical controversies over whether a given chemical was or was not clearly established as a transmitter. In an effort to give some structure to the arguments, neurochemists worked out a set of experimentally tractable rules. According to the prevailing beliefs of the 1960s, a chemical can be classified as a neurotransmitter if it meets the following conditions:
  • There are precursors and/or synthesis enzyme
    Enzyme
    Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

    s located in the presynaptic side of the synapse
    Synapse
    In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...

    .
  • The chemical is present in the presynaptic element.
  • It is available in sufficient quantity in the presynaptic neuron to affect the postsynaptic neuron.
  • There are postsynaptic receptors and the chemical is able to bind to them.
  • A biochemical mechanism for inactivation is present.


Modern advances in pharmacology, genetics, and chemical neuroanatomy have greatly reduced the importance of these rules. A series of experiments that may have taken several years in the 1960s can now be done, with much better precision, in a few months. Thus, it is unusual nowadays for the identification of a chemical as a neurotransmitter to remain controversial for very long periods of time.

Types of neurotransmitters


There are many different ways to classify neurotransmitters. Dividing them into amino acids, peptides, and monoamines is sufficient for some classification purposes.

Major neurotransmitters:
  • Amino acid
    Amino acid
    Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

    s: glutamate, aspartate, D-serine, γ-aminobutyric acid (GABA), glycine
    Glycine
    Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...

  • Monoamines and other biogenic amines: dopamine
    Dopamine
    Dopamine is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this substituted phenethylamine functions as a neurotransmitter, activating the five known types of dopamine receptors—D1, D2, D3, D4, and D5—and their...

     (DA), norepinephrine
    Norepinephrine
    Norepinephrine is the US name for noradrenaline , a catecholamine with multiple roles including as a hormone and a neurotransmitter...

     (noradrenaline; NE, NA), epinephrine
    Epinephrine
    Epinephrine is a hormone and a neurotransmitter. It increases heart rate, constricts blood vessels, dilates air passages and participates in the fight-or-flight response of the sympathetic nervous system. In chemical terms, adrenaline is one of a group of monoamines called the catecholamines...

     (adrenaline), histamine
    Histamine
    Histamine is an organic nitrogen compound involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter. Histamine triggers the inflammatory response. As part of an immune response to foreign pathogens, histamine is produced by...

    , serotonin
    Serotonin
    Serotonin or 5-hydroxytryptamine is a monoamine neurotransmitter. Biochemically derived from tryptophan, serotonin is primarily found in the gastrointestinal tract, platelets, and in the central nervous system of animals including humans...

     (SE, 5-HT)
  • Others: acetylcholine
    Acetylcholine
    The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...

     (ACh), adenosine, anandamide
    Anandamide
    Anandamide, also known as N-arachidonoylethanolamide or AEA, is an endogenous cannabinoid neurotransmitter. The name is taken from the Sanskrit word ananda, which means "bliss, delight", and amide. It is synthesized from N-arachidonoyl phosphatidylethanolamine by multiple pathways...

    , nitric oxide
    Nitric oxide
    Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...

    , etc.


In addition, over 50 neuroactive peptide
Peptide
Peptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...

s have been found, and new ones are discovered regularly. Many of these are "co-released" along with a small-molecule transmitter, but in some cases a peptide is the primary transmitter at a synapse. β-endorphin is a relatively well known example of a peptide neurotransmitter; it engages in highly specific interactions with opioid receptors in the central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

.

Single ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s, such as synaptically released zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...

, are also considered neurotransmitters by some, as are some gaseous molecules such as nitric oxide (NO) and carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

 (CO). These are not classical neurotransmitters by the strictest definition, however, because although they have all been shown experimentally to be released by presynaptic terminals in an activity-dependent way, they are not packaged into vesicles.

By far the most prevalent transmitter is glutamate, which is excitatory at well over 90% of the synapses in the human brain. The next most prevalent is GABA, which is inhibitory at more than 90% of the synapses that do not use glutamate. Even though other transmitters are used in far fewer synapses, they may be very important functionally—the great majority of psychoactive drugs exert their effects by altering the actions of some neurotransmitter systems, often acting through transmitters other than glutamate or GABA. Addictive drugs such as cocaine and amphetamine exert their effects primarily on the dopamine system. The addictive opiate
Opiate
In medicine, the term opiate describes any of the narcotic opioid alkaloids found as natural products in the opium poppy plant.-Overview:Opiates are so named because they are constituents or derivatives of constituents found in opium, which is processed from the latex sap of the opium poppy,...

 drugs exert their effects primarily as functional analogs of opioid peptides, which, in turn, regulate dopamine levels.

Excitatory and inhibitory


Some neurotransmitters are commonly described as "excitatory" or "inhibitory". The only direct effect of a neurotransmitter is to activate one or more types of receptors. The effect on the postsynaptic cell depends, therefore, entirely on the properties of those receptors. It happens that for some neurotransmitters (for example, glutamate), the most important receptors all have excitatory effects: that is, they increase the probability that the target cell will fire an action potential. For other neurotransmitters, such as GABA, the most important receptors all have inhibitory effects (although there is evidence that GABA is excitatory during early brain development). There are, however, other neurotransmitters, such as acetylcholine, for which both excitatory and inhibitory receptors exist; and there are some types of receptors that activate complex metabolic pathways in the postsynaptic cell to produce effects that cannot appropriately be called either excitatory or inhibitory. Thus, it is an oversimplification to call a neurotransmitter excitatory or inhibitory—nevertheless it is so convenient to call glutamate excitatory and GABA inhibitory that this usage is seen very frequently.

Actions



As explained above, the only direct action of a neurotransmitter is to activate a receptor. Therefore, the effects of a neurotransmitter system depend on the connections of the neurons that use the transmitter, and the chemical properties of the receptors that the transmitter binds to.

Here are a few examples of important neurotransmitter actions:
  • Glutamate is used at the great majority of fast excitatory synapses in the brain and spinal cord. It is also used at most synapses that are "modifiable", i.e. capable of increasing or decreasing in strength. Modifiable synapses
    Synaptic plasticity
    In neuroscience, synaptic plasticity is the ability of the connection, or synapse, between two neurons to change in strength in response to either use or disuse of transmission over synaptic pathways. Plastic change also results from the alteration of the number of receptors located on a synapse...

     are thought to be the main memory-storage elements in the brain. Excessive glutamate release can lead to excitotoxicity
    Excitotoxicity
    Excitotoxicity is the pathological process by which nerve cells are damaged and killed by excessive stimulation by neurotransmitters such as glutamate and similar substances. This occurs when receptors for the excitatory neurotransmitter glutamate such as the NMDA receptor and AMPA receptor are...

     causing cell death.
  • GABA
    Gabâ
    Gabâ or gabaa, for the people in many parts of the Philippines), is the concept of a non-human and non-divine, imminent retribution. A sort of negative karma, it is generally seen as an evil effect on a person because of their wrongdoings or transgressions...

     is used at the great majority of fast inhibitory synapses in virtually every part of the brain. Many sedative/tranquilizing drugs act by enhancing the effects of GABA. Correspondingly glycine
    Glycine
    Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...

     is the inhibitory transmitter in the spinal cord.
  • Acetylcholine
    Acetylcholine
    The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...

     is distinguished as the transmitter at the neuromuscular junction
    Neuromuscular junction
    A neuromuscular junction is the synapse or junction of the axon terminal of a motor neuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract...

     connecting motor nerves to muscles. The paralytic arrow-poison curare
    Curare
    Curare is a common name for various arrow poisons originating from South America. The three main types of curare are:* tubocurare...

     acts by blocking transmission at these synapses. Acetylcholine also operates in many regions of the brain, but using different types of receptors.
  • Dopamine
    Dopamine
    Dopamine is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this substituted phenethylamine functions as a neurotransmitter, activating the five known types of dopamine receptors—D1, D2, D3, D4, and D5—and their...

     has a number of important functions in the brain. It plays a critical role in the reward system
    Reward system
    In neuroscience, the reward system is a collection of brain structures which attempts to regulate and control behavior by inducing pleasurable effects...

    , but dysfunction of the dopamine system is also implicated in Parkinson's disease
    Parkinson's disease
    Parkinson's disease is a degenerative disorder of the central nervous system...

     and schizophrenia
    Schizophrenia
    Schizophrenia is a mental disorder characterized by a disintegration of thought processes and of emotional responsiveness. It most commonly manifests itself as auditory hallucinations, paranoid or bizarre delusions, or disorganized speech and thinking, and it is accompanied by significant social...

    .
  • Serotonin
    Serotonin
    Serotonin or 5-hydroxytryptamine is a monoamine neurotransmitter. Biochemically derived from tryptophan, serotonin is primarily found in the gastrointestinal tract, platelets, and in the central nervous system of animals including humans...

     is a monoamine neurotransmitter
    Monoamine neurotransmitter
    thumb|right|350px| A phylogenetic tree showing how a number of monoamine receptors are related to each other.Monoamine neurotransmitters are neurotransmitters and neuromodulators that contain one amino group that is connected to an aromatic ring by a two-carbon chain...

    . Most is produced by and found in the intestine (approximately 90%), and the remainder in central nervous system
    Central nervous system
    The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

     neurons. It functions to regulate appetite, sleep, memory and learning, temperature, mood, behaviour, muscle contraction, and function of the cardiovascular system and endocrine system
    Endocrine system
    In physiology, the endocrine system is a system of glands, each of which secretes a type of hormone directly into the bloodstream to regulate the body. The endocrine system is in contrast to the exocrine system, which secretes its chemicals using ducts. It derives from the Greek words "endo"...

    . It is speculated to have a role in depression, as some depressed patients are seen to have lower concentrations of metabolites of serotonin in their cerebrospinal fluid
    Cerebrospinal fluid
    Cerebrospinal fluid , Liquor cerebrospinalis, is a clear, colorless, bodily fluid, that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord...

     and brain tissue.
  • Substance P
    Substance P
    In the field of neuroscience, substance P is a neuropeptide: an undecapeptide that functions as a neurotransmitter and as a neuromodulator. It belongs to the tachykinin neuropeptide family. Substance P and its closely related neuropeptide neurokinin A are produced from a polyprotein precursor...

     is an undecapeptide responsible for transmission of pain from certain sensory neurons to the central nervous system.


Neurons expressing certain types of neurotransmitters sometimes form distinct systems, where activation of the system affects large volumes of the brain, called volume transmission. Major neurotransmitter systems include the noradrenaline (norepinephrine) system, the dopamine
Dopamine
Dopamine is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this substituted phenethylamine functions as a neurotransmitter, activating the five known types of dopamine receptors—D1, D2, D3, D4, and D5—and their...

 system, the serotonin
Serotonin
Serotonin or 5-hydroxytryptamine is a monoamine neurotransmitter. Biochemically derived from tryptophan, serotonin is primarily found in the gastrointestinal tract, platelets, and in the central nervous system of animals including humans...

 system and the cholinergic
Cholinergic
The word choline generally refers to the various quaternary ammonium salts containing the N,N,N-trimethylethanolammonium cation. Found in most animal tissues, choline is a primary component of the neurotransmitter acetylcholine and functions with inositol as a basic constituent of lecithin...

 system.

Drugs targeting the neurotransmitter of such systems affect the whole system; this fact explains the complexity of action of some drugs. Cocaine
Cocaine
Cocaine is a crystalline tropane alkaloid that is obtained from the leaves of the coca plant. The name comes from "coca" in addition to the alkaloid suffix -ine, forming cocaine. It is a stimulant of the central nervous system, an appetite suppressant, and a topical anesthetic...

, for example, blocks the reuptake of dopamine
Dopamine
Dopamine is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this substituted phenethylamine functions as a neurotransmitter, activating the five known types of dopamine receptors—D1, D2, D3, D4, and D5—and their...

 back into the presynaptic neuron, leaving the neurotransmitter molecules in the synaptic gap
Synapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...

 longer. Since the dopamine remains in the synapse longer, the neurotransmitter continues to bind to the receptors on the postsynaptic neuron, eliciting a pleasurable emotional response. Physical addiction to cocaine may result from prolonged exposure to excess dopamine in the synapses, which leads to the downregulation of some postsynaptic receptors. After the effects of the drug wear off, one might feel depressed because of the decreased probability of the neurotransmitter binding to a receptor. Prozac is a selective serotonin reuptake inhibitor (SSRI), which blocks re-uptake of serotonin by the presynaptic cell. This increases the amount of serotonin present at the synapse and allows it to remain there longer, hence potentiating the effect of naturally released serotonin. AMPT
AMPT
Alpha-methyl-p-tyrosine is a tyrosine hydroxylase enzyme inhibitor. It has been used in the treatment of pheochromocytoma. It has been demonstrated to inhibit the production of melanin.-Side-effects:...

 prevents the conversion of tyrosine to L-DOPA, the precursor to dopamine; reserpine
Reserpine
Reserpine is an indole alkaloid antipsychotic and antihypertensive drug that has been used for the control of high blood pressure and for the relief of psychotic symptoms, although because of the development of better drugs for these purposes and because of its numerous side-effects, it is rarely...

 prevents dopamine storage within vesicles
Synaptic vesicle
In a neuron, synaptic vesicles store various neurotransmitters that are released at the synapse. The release is regulated by a voltage-dependent calcium channel. Vesicles are essential for propagating nerve impulses between neurons and are constantly recreated by the cell...

; and deprenyl inhibits monoamine oxidase
Monoamine oxidase
L-Monoamine oxidases are a family of enzymes that catalyze the oxidation of monoamines. They are found bound to the outer membrane of mitochondria in most cell types in the body. The enzyme was originally discovered by Mary Bernheim in the liver and was named tyramine oxidase...

 (MAO)-B and thus increases dopamine levels.

Diseases may affect specific neurotransmitter systems. For example, Parkinson's disease
Parkinson's disease
Parkinson's disease is a degenerative disorder of the central nervous system...

 is at least in part related to failure of dopaminergic cells in deep-brain nuclei, for example the substantia nigra
Substantia nigra
The substantia nigra is a brain structure located in the mesencephalon that plays an important role in reward, addiction, and movement. Substantia nigra is Latin for "black substance", as parts of the substantia nigra appear darker than neighboring areas due to high levels of melanin in...

. Treatments potentiating the effect of dopamine precursors have been proposed and effected, with moderate success.

A brief comparison of the major neurotransmitter systems follows:
Neurotransmitter systems
System Origin Effects
Noradrenaline system locus coeruleus
  • arousal
  • reward
Lateral tegmental field
Dopamine system dopamine
Dopamine
Dopamine is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this substituted phenethylamine functions as a neurotransmitter, activating the five known types of dopamine receptors—D1, D2, D3, D4, and D5—and their...

 pathways
Neural pathway
A neural pathway, neural tract, or neural face, connects one part of the nervous system with another and usually consists of bundles of elongated, myelin-insulated neurons, known collectively as white matter...

:
  • mesocortical pathway
    Mesocortical pathway
    The mesocortical pathway is a neural pathway that connects the ventral tegmentum to the cerebral cortex, particularly the frontal lobes. It is one of the four major dopamine pathways in the brain...

  • mesolimbic pathway
    Mesolimbic pathway
    The mesolimbic pathway is one of the dopaminergic pathways in the brain. The pathway begins in the ventral tegmental area of the midbrain and connects to the limbic system via the nucleus accumbens, the amygdala, and the hippocampus as well as to the medial prefrontal cortex...

  • nigrostriatal pathway
    Nigrostriatal pathway
    The nigrostriatal pathway is a neural pathway that connects the substantia nigra with the striatum. It is one of the four major dopamine pathways in the brain, and is particularly involved in the production of movement, as part of a system called the basal ganglia motor loop.Loss of dopamine...

  • tuberoinfundibular pathway
    Tuberoinfundibular pathway
    The tuberoinfundibular pathway refers to a population of dopamine neurons in the arcuate nucleus of the mediobasal hypothalamus that project to the median eminence . It is one of the four major dopamine pathways in the brain...

  • motor system
    Motor system
    The motor system is the part of the central nervous system that is involved with movement. It consists of the pyramidal and extrapyramidal system....

    , reward, cognition
    Cognition
    In science, cognition refers to mental processes. These processes include attention, remembering, producing and understanding language, solving problems, and making decisions. Cognition is studied in various disciplines such as psychology, philosophy, linguistics, and computer science...

    , endocrine, nausea
    Nausea
    Nausea , is a sensation of unease and discomfort in the upper stomach with an involuntary urge to vomit. It often, but not always, precedes vomiting...

    Serotonin system caudal dorsal raphe nucleus Increase (introversion
    Extraversion and introversion
    The trait of extraversion-introversion is a central dimension of human personality theories.Extraverts tend to be gregarious, assertive, and interested in seeking out external stimulus. Introverts, in contrast, tend to be introspective, quiet and less sociable. They are not necessarily loners but...

    ), mood
    Mood (psychology)
    A mood is a relatively long lasting emotional state. Moods differ from emotions in that they are less specific, less intense, and less likely to be triggered by a particular stimulus or event....

    , satiety, body temperature and sleep
    Sleep
    Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than...

    , while decreasing nociception
    Nociception
    Nociception is defined as "the neural processes of encoding and processing noxious stimuli." It is the afferent activity produced in the peripheral and central nervous system by stimuli that have the potential to damage tissue...

    .
    rostral dorsal raphe nucleus
    Cholinergic system pontomesencephalotegmental complex
  • learning
    Learning
    Learning is acquiring new or modifying existing knowledge, behaviors, skills, values, or preferences and may involve synthesizing different types of information. The ability to learn is possessed by humans, animals and some machines. Progress over time tends to follow learning curves.Human learning...

  • short-term memory
  • arousal
  • reward
  • basal optic nucleus of Meynert
    medial septal nucleus

    Common neurotransmitters

    Category Name Abbreviation Metabotropic Ionotropic
    Small: Amino acids Aspartate  - >- Neuropeptide
    Neuropeptide
    Neuropeptides are small protein-like molecules used by neurons to communicate with each other. They are neuronal signaling molecules, influence the activity of the brain in specific ways and are thus involved in particular brain functions, like analgesia, reward, food intake, learning and...

    s
    N-Acetylaspartylglutamate
    N-Acetylaspartylglutamate
    N-Acetylaspartylglutamic acid is a neuropeptide that is the third-most-prevalent neurotransmitter in the mammalian nervous system. NAAG consists of N-acetylaspartic acid and glutamic acid coupled via a peptide bond...

     
    NAAG Metabotropic glutamate receptor
    Metabotropic glutamate receptor
    The metabotropic glutamate receptors, or mGluRs, are a type of glutamate receptor that are active through an indirect metabotropic process. They are members of the group C family of G-protein-coupled receptors, or GPCRs...

    s; selective agonist of mGluR3 
    >- Small: Amino acids Glutamate (glutamic acid) Glu Metabotropic glutamate receptor
    Metabotropic glutamate receptor
    The metabotropic glutamate receptors, or mGluRs, are a type of glutamate receptor that are active through an indirect metabotropic process. They are members of the group C family of G-protein-coupled receptors, or GPCRs...

     
    NMDA receptor
    NMDA receptor
    The NMDA receptor , a glutamate receptor, is the predominant molecular device for controlling synaptic plasticity and memory function....

    , Kainate receptor
    Kainate receptor
    Kainate receptors, or KARs, are non-NMDA ionotropic receptors which respond to the neurotransmitter glutamate. They were first identified as a distinct receptor type through their selective activation by the agonist kainate, a drug first isolated from red algae Digenea simplex. KARs are less well...

    , AMPA receptor
    AMPA receptor
    The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor is a non-NMDA-type ionotropic transmembrane receptor for glutamate that mediates fast synaptic transmission in the central nervous system . Its name is derived from its ability to be activated by the artificial glutamate analog AMPA...


    >-
    Small: Amino acids Gamma-aminobutyric acid
    Gamma-aminobutyric acid
    γ-Aminobutyric acid is the chief inhibitory neurotransmitter in the mammalian central nervous system. It plays a role in regulating neuronal excitability throughout the nervous system...

     
    GABA GABAB receptor  A, GABAA-ρ receptor
    >-
    Small: Amino acids Glycine
    Glycine
    Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...

     
    Gly - Glycine receptor
    Glycine receptor
    The glycine receptor, or GlyR, is the receptor for the amino acid neurotransmitter glycine. GlyR is an ionotropic receptor that produces its effects through chloride current...


    >-
    Small: Acetylcholine Acetylcholine
    Acetylcholine
    The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...

     
    Ach Muscarinic acetylcholine receptor
    Muscarinic acetylcholine receptor
    Muscarinic receptors, or mAChRs, are acetylcholine receptors that form G protein-coupled in the plasma membranes of certain neurons and other cells...

     
    Nicotinic acetylcholine receptor
    Nicotinic acetylcholine receptor
    Nicotinic acetylcholine receptors, or nAChRs, are cholinergic receptors that form ligand-gated ion channels in the plasma membranes of certain neurons and on the postsynaptic side of the neuromuscular junction...


    >-
    Small: Monoamine (Phe
    Phenylalanine
    Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form...

    /Tyr
    Tyrosine
    Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...

    )
    Dopamine
    Dopamine
    Dopamine is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this substituted phenethylamine functions as a neurotransmitter, activating the five known types of dopamine receptors—D1, D2, D3, D4, and D5—and their...

     
    DA Dopamine receptor
    Dopamine receptor
    Dopamine receptors are a class of metabotropic G protein-coupled receptors that are prominent in the vertebrate central nervous system . The neurotransmitter dopamine is the primary endogenous ligand for dopamine receptors....

     
    >- Small: Monoamine (Phe
    Phenylalanine
    Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form...

    /Tyr
    Tyrosine
    Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...

    )
    Norepinephrine
    Norepinephrine
    Norepinephrine is the US name for noradrenaline , a catecholamine with multiple roles including as a hormone and a neurotransmitter...

     (noradrenaline)
    NE Adrenergic receptor
    Adrenergic receptor
    The adrenergic receptors are a class of metabotropic G protein-coupled receptors that are targets of the catecholamines, especially noradrenaline and adrenaline ....

     
    >- Small: Monoamine (Phe
    Phenylalanine
    Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form...

    /Tyr
    Tyrosine
    Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...

    )
    Epinephrine
    Epinephrine
    Epinephrine is a hormone and a neurotransmitter. It increases heart rate, constricts blood vessels, dilates air passages and participates in the fight-or-flight response of the sympathetic nervous system. In chemical terms, adrenaline is one of a group of monoamines called the catecholamines...

     (adrenaline)
    Epi Adrenergic receptor
    Adrenergic receptor
    The adrenergic receptors are a class of metabotropic G protein-coupled receptors that are targets of the catecholamines, especially noradrenaline and adrenaline ....

     
    >- Small: Monoamine (Phe
    Phenylalanine
    Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form...

    /Tyr
    Tyrosine
    Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...

    )
    Octopamine
    Octopamine
    Octopamine is an endogenous biogenic amine that is closely related to norepinephrine, and has effects on the adrenergic and dopaminergic systems. It is also found naturally in numerous plants, including bitter orange. Biosynthesis of the D--enantiomer of octopamine is by β-hydroxylation of...

     
    - >- Small: Monoamine (Phe
    Phenylalanine
    Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form...

    /Tyr
    Tyrosine
    Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...

    )
    Tyramine
    Tyramine
    Tyramine is a naturally occurring monoamine compound and trace amine derived from the amino acid tyrosine. Tyramine acts as a catecholamine releasing agent...

     
    >- Small: Monoamine (Trp
    Tryptophan
    Tryptophan is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG...

    )
    Serotonin
    Serotonin
    Serotonin or 5-hydroxytryptamine is a monoamine neurotransmitter. Biochemically derived from tryptophan, serotonin is primarily found in the gastrointestinal tract, platelets, and in the central nervous system of animals including humans...

     (5-hydroxytryptamine)
    5-HT Serotonin receptor, all but 5-HT3 >- Small: Monoamine (Trp
    Tryptophan
    Tryptophan is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG...

    )
    Melatonin
    Melatonin
    Melatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring compound found in animals, plants, and microbes...

     
    Mel Melatonin receptor
    Melatonin receptor
    A melatonin receptor is a G protein-coupled receptor which binds melatonin.Three types of melatonin receptor have been cloned. The MT1 and MT2 receptor subtypes are present in humans and other mammals, while an additional melatonin receptor subtype MT3 has been identified in amphibia and...

     
    >- Small: Monoamine (His
    Histamine
    Histamine is an organic nitrogen compound involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter. Histamine triggers the inflammatory response. As part of an immune response to foreign pathogens, histamine is produced by...

    )
    Histamine
    Histamine
    Histamine is an organic nitrogen compound involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter. Histamine triggers the inflammatory response. As part of an immune response to foreign pathogens, histamine is produced by...

     
    H Histamine receptor
    Histamine receptor
    The histamine receptors are a class of G protein-coupled receptors with histamine as their endogenous ligand.There are four known histamine receptors:*H1 receptor*H2 receptor*H3 receptor*H4 receptor-Comparison:...

     
    >- PP: Gastrins Gastrin
    Gastrin
    In humans, gastrin is a peptide hormone that stimulates secretion of gastric acid by the parietal cells of the stomach and aids in gastric motility. It is released by G cells in the antrum of the stomach, duodenum, and the pancreas...

     
    - >- PP: Gastrins Cholecystokinin
    Cholecystokinin
    Cholecystokinin is a peptide hormone of the gastrointestinal system responsible for stimulating the digestion of fat and protein...

     
    CCK Cholecystokinin receptor
    Cholecystokinin receptor
    Cholecystokinin receptors or CCK receptors are a group of G-protein coupled receptors which bind the peptide hormones cholecystokinin or gastrin...

     
    >- PP: Neurohypophyseals Vasopressin
    Vasopressin
    Arginine vasopressin , also known as vasopressin, argipressin or antidiuretic hormone , is a neurohypophysial hormone found in most mammals, including humans. Vasopressin is a peptide hormone that controls the reabsorption of molecules in the tubules of the kidneys by affecting the tissue's...

     
    AVP Vasopressin receptor
    Vasopressin receptor
    A vasopressin receptor is a cell surface receptor which binds vasopressin. The three types of vasopressin receptor are members of the A6 subfamily of G-protein coupled receptors.-Subtypes:Humans express three subtypes: 1A, 1B and 2-Function:...

     
    >- PP: Neurohypophyseals Oxytocin
    Oxytocin
    Oxytocin is a mammalian hormone that acts primarily as a neuromodulator in the brain.Oxytocin is best known for its roles in sexual reproduction, in particular during and after childbirth...

     
    OT Oxytocin receptor
    Oxytocin receptor
    The oxytocin receptor, also known as OXTR, is a protein which functions as receptor for the hormone and neurotransmitter oxytocin. In humans, the oxytocin receptor is encoded by the OXTR gene., which has been localized to human chromosome 3p25....

     
    >- PP: Neurohypophyseals Neurophysin I
    Neurophysin I
    Neurophysin I is a carrier protein with a size of 10 KDa and containing 90 to 97 aminoacids that is a cleavage product of preprooxyphysin...

     
    - >- PP: Neurohypophyseals Neurophysin II
    Neurophysin II
    Neurophysin II is a carrier protein which binds vasopressin. It is generated from the same precursor as vasopressin.It can be associated with neurohypophyseal diabetes insipidus.Neurophysin II is also known as a stimulator of Prolactin secretion....

     
    - >- PP: Neuropeptide Y Neuropeptide Y
    Neuropeptide Y
    Neuropeptide Y is a 36-amino acid peptide neurotransmitter found in the brain and autonomic nervous system."NPY has been associated with a number of physiologic processes in the brain, including the regulation of energy balance, memory and learning, and epilepsy." The main effect is increased food...

     
    NY Neuropeptide Y receptor
    Neuropeptide Y receptor
    Neuropeptide Y receptors are a class of G-protein coupled receptors which are activated by the closely related peptide hormones neuropeptide Y, peptide YY and pancreatic polypeptide...

     
    >- PP: Neuropeptide Y Pancreatic polypeptide
    Pancreatic polypeptide
    Pancreatic polypeptide is a polypeptide secreted by PP cells in the endocrine pancreas predominantly in the head of the pancreas. It consists of 36 amino acids and has molecular weight about 4200 Da....

     
    PP - >- PP: Neuropeptide Y Peptide YY
    Peptide YY
    Peptide YY is a short protein released by cells in the ileum and colon in response to feeding. In humans it appears to reduce appetite.It is also known as PYY, Peptide Tyrosine Tyrosine, or Pancreatic Peptide YY3-36....

     
    PYY - >- PP: Opioid
    Opioid
    An opioid is a psychoactive chemical that works by binding to opioid receptors, which are found principally in the central and peripheral nervous system and the gastrointestinal tract...

    s
    Corticotropin (adrenocorticotropic hormone) ACTH Corticotropin receptor  >- PP: Opioids Dynorphin
    Dynorphin
    Dynorphins are a class of opioid peptides that arise from the precursor protein prodynorphin. When prodynorphin is cleaved during processing by proprotein convertase 2 , multiple active peptides are released: dynorphin A, dynorphin B, and α/β-neo-endorphin...

     
    - >- PP: Opioids Endorphin
    Endorphin
    Endorphins are endogenous opioid peptides that function as neurotransmitters. They are produced by the pituitary gland and the hypothalamus in vertebrates during exercise, excitement, pain, consumption of spicy food, love and orgasm, and they resemble the opiates in their abilities to produce...

     
    - >- PP: Opioids Enkephaline  - >- PP: Secretins Secretin
    Secretin
    Secretin is a hormone that controls the secretions into the duodenum, and also separately, water homeostasis throughout the body. It is produced in the S cells of the duodenum in the crypts of Lieberkühn...

     
    Secretin receptor
    Secretin receptor
    Human secretin receptor is a G protein-coupled receptor which binds secretin and is the leading member of the class B GPCR subfamily.-Interactions:...

     
    >- PP: Secretins Motilin
    Motilin
    Motilin is a 22-amino acid polypeptide hormone in the motilin family that, in humans, is encoded by the MLN gene.Motilin is secreted by endocrine M cells that are numerous in crypts of the small intestine, especially in the duodenum and jejunum. Based on amino acid sequence, motilin is unrelated...

     
    Motilin receptor
    Motilin receptor
    Motilin receptor is a G protein-coupled receptor that binds motilin. Motilin in turn is an intestinal peptide that stimulates contraction of gut smooth muscle.-External links:...

     
    >- PP: Secretins Glucagon
    Glucagon
    Glucagon, a hormone secreted by the pancreas, raises blood glucose levels. Its effect is opposite that of insulin, which lowers blood glucose levels. The pancreas releases glucagon when blood sugar levels fall too low. Glucagon causes the liver to convert stored glycogen into glucose, which is...

     
    Glucagon receptor
    Glucagon receptor
    The glucagon receptor is a 62 kDa protein that is activated by glucagon and is a member of the class B G-protein coupled family of receptors, coupled to G alpha i, Gs and to a lesser extent G alpha q. Stimulation of the receptor results in activation of adenylate cyclase and increased levels of...

     
    >- PP: Secretins Vasoactive intestinal peptide
    Vasoactive intestinal peptide
    Vasoactive intestinal peptide also known as the vasoactive intestinal polypeptide or VIP is a peptide hormone containing 29 amino acid residues that is produced in many tissues of vertebrates including the gut, pancreas and suprachiasmatic nuclei of the hypothalamus in the brain...

     
    VIP Vasoactive intestinal peptide receptor
    Vasoactive intestinal peptide receptor
    There are two known receptors for the vasoactive intestinal peptide termed VPAC1 and VPAC2. These receptors bind both VIP and pituitary adenylate cyclase-activating polypeptide to some degree...

     
    >- PP: Secretins Growth hormone-releasing factor  GRF - >- PP: Somatostatins Somatostatin
    Somatostatin
    Somatostatin is a peptide hormone that regulates the endocrine system and affects neurotransmission and cell proliferation via interaction with G-protein-coupled somatostatin receptors and inhibition of the release of numerous secondary hormones.Somatostatin...

     
    Somatostatin receptor
    Somatostatin receptor
    There are five known somatostatin receptors:* SST1 * SST2 * SST3 * SST4 * SST5 All are G protein-coupled seven transmembrane receptors.-External links:...

     
    >- SS: Tachykinins Neurokinin A
    Neurokinin A
    Neurokinin A is a member of the tachykinin family of neuropeptide neurotransmitters. It is produced from the same preprotachykinin A gene as the neuropeptide substance P. It has various roles in the body of humans and other animals...

     
    - >- SS: Tachykinins Neurokinin B
    Neurokinin B
    Neurokinin B is a tachykinin peptide.It is found in higher concentration in pregnant women suffering pre-eclampsia and can bind the immune-cloaking molecule phosphocholine....

     
    - >- SS: Tachykinins Substance P
    Substance P
    In the field of neuroscience, substance P is a neuropeptide: an undecapeptide that functions as a neurotransmitter and as a neuromodulator. It belongs to the tachykinin neuropeptide family. Substance P and its closely related neuropeptide neurokinin A are produced from a polyprotein precursor...

     
    - >- PP: Other Bombesin
    Bombesin
    Bombesin is a 14-amino acid peptide originally isolated from the skin of a frog. It has two known homologs in mammals called neuromedin B and gastrin-releasing peptide. It stimulates gastrin release from G cells. It activates three different G-protein-coupled receptors known as BBR1, -2, and -3. It...

     
    - >- PP: Other Gastrin releasing peptide
    Gastrin releasing peptide
    Gastrin-releasing peptide, also known as GRP, is an important regulatory molecule that has been implicated in a number of physiological and pathophysiological processes in humans....

     
    GRP - >- Gas Nitric oxide
    Nitric oxide
    Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...

     
    NO Soluble guanylyl cyclase  >- Gas Carbon monoxide
    Carbon monoxide
    Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

     
    CO - Heme
    Heme
    A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...

     bound to potassium channel
    Potassium channel
    In the field of cell biology, potassium channels are the most widely distributed type of ion channel and are found in virtually all living organisms. They form potassium-selective pores that span cell membranes...

    s
    >-
    Other Anandamide
    Anandamide
    Anandamide, also known as N-arachidonoylethanolamide or AEA, is an endogenous cannabinoid neurotransmitter. The name is taken from the Sanskrit word ananda, which means "bliss, delight", and amide. It is synthesized from N-arachidonoyl phosphatidylethanolamine by multiple pathways...

     
    AEA Cannabinoid receptor
    Cannabinoid receptor
    The cannabinoid receptors are a class of cell membrane receptors under the G protein-coupled receptor superfamily. As is typical of G protein-coupled receptors, the cannabinoid receptors contain seven transmembrane spanning domains...

     
    >- Other Adenosine triphosphate
    Adenosine triphosphate
    Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

     
    ATP P2Y12
    P2Y12
    In the field of molecular biology, the P2Y12 protein is found mainly but not only on the surface of blood platelet cells and is an important regulator in blood clotting....

     
    P2X receptor
    P2X Receptor
    P2X receptors are a family of cation-permeable ligand gated ion channels that open in response to the binding of extracellular adenosine 5'-triphosphate . They belong to a larger family of receptors known as the purinergic receptors...


    Precursors of neurotransmitters


    While intake of neurotransmitter precursors
    Precursor (chemistry)
    In chemistry, a precursor is a compound that participates in the chemical reaction that produces another compound. In biochemistry, the term "precursor" is used more specifically to refer to a chemical compound preceding another in a metabolic pathway....

     does increase neurotransmitter synthesis, evidence is mixed as to whether neurotransmitter release (firing) is increased. Even with increased neurotransmitter release, it is unclear whether this will result in a long-term increase in neurotransmitter signal strength, since the nervous system can adapt to changes such as increased neurotransmitter synthesis and may therefore maintain constant firing. Some neurotransmitters may have a role in depression, and there is some evidence to suggest that intake of precursors of these neurotransmitters may be useful in the treatment of mild and moderate depression.

    Dopamine precursors


    L-DOPA, a precursor of dopamine
    Dopamine
    Dopamine is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this substituted phenethylamine functions as a neurotransmitter, activating the five known types of dopamine receptors—D1, D2, D3, D4, and D5—and their...

     that crosses the blood-brain barrier
    Blood-brain barrier
    The blood–brain barrier is a separation of circulating blood and the brain extracellular fluid in the central nervous system . It occurs along all capillaries and consists of tight junctions around the capillaries that do not exist in normal circulation. Endothelial cells restrict the diffusion...

    , is used in the treatment of Parkinson's disease
    Parkinson's disease
    Parkinson's disease is a degenerative disorder of the central nervous system...

    .

    Norepinephrine precursors


    For depressed patients where low activity of the neurotransmitter norepinephrine
    Norepinephrine
    Norepinephrine is the US name for noradrenaline , a catecholamine with multiple roles including as a hormone and a neurotransmitter...

     is implicated, there is only little evidence for benefit of neurotransmitter precursor administration. L-phenylalanine and L-tyrosine are both precursors for dopamine
    Dopamine
    Dopamine is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this substituted phenethylamine functions as a neurotransmitter, activating the five known types of dopamine receptors—D1, D2, D3, D4, and D5—and their...

    , norepinephrine
    Norepinephrine
    Norepinephrine is the US name for noradrenaline , a catecholamine with multiple roles including as a hormone and a neurotransmitter...

    , and epinephrine
    Epinephrine
    Epinephrine is a hormone and a neurotransmitter. It increases heart rate, constricts blood vessels, dilates air passages and participates in the fight-or-flight response of the sympathetic nervous system. In chemical terms, adrenaline is one of a group of monoamines called the catecholamines...

    . These conversions require vitamin B6
    Vitamin B6
    Vitamin B6 is a water-soluble vitamin and is part of the vitamin B complex group. Several forms of the vitamin are known, but pyridoxal phosphate is the active form and is a cofactor in many reactions of amino acid metabolism, including transamination, deamination, and decarboxylation...

    , vitamin C
    Vitamin C
    Vitamin C or L-ascorbic acid or L-ascorbate is an essential nutrient for humans and certain other animal species. In living organisms ascorbate acts as an antioxidant by protecting the body against oxidative stress...

    , and S-adenosylmethionine. A few studies suggest potential antidepressant effects of L-phenylalanine and L-tyrosine, but there is much room for further research in this area.

    Serotonin precursors


    Administration of L-tryptophan, a precursor for serotonin
    Serotonin
    Serotonin or 5-hydroxytryptamine is a monoamine neurotransmitter. Biochemically derived from tryptophan, serotonin is primarily found in the gastrointestinal tract, platelets, and in the central nervous system of animals including humans...

    , is seen to double the production of serotonin in the brain. It is significantly more effective than a placebo in the treatment of mild and moderate depression. This conversion requires vitamin C
    Vitamin C
    Vitamin C or L-ascorbic acid or L-ascorbate is an essential nutrient for humans and certain other animal species. In living organisms ascorbate acts as an antioxidant by protecting the body against oxidative stress...

    . 5-hydroxytryptophan
    5-Hydroxytryptophan
    5-Hydroxytryptophan , also known as oxitriptan , is a naturally occurring amino acid and chemical precursor as well as metabolic intermediate in the biosynthesis of the neurotransmitters serotonin and melatonin from tryptophan....

     (5-HTP), also a precursor for serotonin
    Serotonin
    Serotonin or 5-hydroxytryptamine is a monoamine neurotransmitter. Biochemically derived from tryptophan, serotonin is primarily found in the gastrointestinal tract, platelets, and in the central nervous system of animals including humans...

    , is also more effective than a placebo.

    Degradation and elimination


    A neurotransmitter must be broken down once it reaches the post-synaptic cell to prevent further excitatory or inhibitory signal transduction. For example, acetylcholine (ACh)
    Acetylcholine
    The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...

    , an excitatory neurotransmitter, is broken down by acetylcholinesterase
    Acetylcholinesterase
    "Acetylcholinesterase, also known as AChE or acetylcholine acetylhydrolase, is an enzyme that degrades the neurotransmitter acetylcholine, producing choline and an acetate group. It is mainly found at neuromuscular junctions and cholinergic nervous system, where its activity serves to terminate...

     (AChE). Choline
    Choline
    Choline is a water-soluble essential nutrient. It is usually grouped within the B-complex vitamins. Choline generally refers to the various quaternary ammonium salts containing the N,N,N-trimethylethanolammonium cation....

     is taken up and recycled by the pre-synaptic neuron to synthesize more ACh.
    Other neurotransmitters such as dopamine
    Dopamine
    Dopamine is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this substituted phenethylamine functions as a neurotransmitter, activating the five known types of dopamine receptors—D1, D2, D3, D4, and D5—and their...

     are able to diffuse
    Diffusion
    Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

     away from their targeted synaptic junctions and are eliminated from the body via the kidneys, or destroyed in the liver. Each neurotransmitter has very specific degradation pathways at regulatory points, which may be the target of the body's own regulatory system or recreational drugs.

    See also


    • Neuropsychopharmacology
      Neuropsychopharmacology
      Neuropsychopharmacology is an interdisciplinary science related to psychopharmacology and fundamental neuroscience...

    • Neuropeptide
      Neuropeptide
      Neuropeptides are small protein-like molecules used by neurons to communicate with each other. They are neuronal signaling molecules, influence the activity of the brain in specific ways and are thus involved in particular brain functions, like analgesia, reward, food intake, learning and...

    • Nervous system
      Nervous system
      The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...

    • Gasotransmitters
      Gasotransmitters
      Gasotransmitters are gaseous molecules synthesized in the body. They include nitric oxide, hydrogen sulfide, carbon monoxide, and possibly nitrous oxide.-Overview:...

    • Neuromuscular transmission

    External links