Chemical structure of D-aspartic acid, a common amino acid neurotransmitter.

Neurotransmitters are chemicals that are used to relay, amplify and modulate signals between a neuron and another cell.^[1] 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 enzymes located in the presynaptic side of the synapse;
• 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.

Types of neurotransmitters

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

Some more precise divisions are as follows:

• Around ten "small-molecule neurotransmitters" are known:
  • Acetylcholine (Ach)
  • Monoamines (epinephrine (E), norepinephrine (NE), dopamine (DA), serotonin (5-HT) and melatonin)
  • Three or four amino acids, depending on exact definition used: (primarily glutamic acid, gamma aminobutyric acid (GABA), aspartic acid & glycine)
  • Purines, (Adenosine, ATP, GTP and their derivatives)
  • Fatty acids are also receiving attention as the potential endogenous cannabinoid.^{citation needed}
• Over 50 neuroactive peptides (vasopressin, somatostatin, neurotensin, etc.) have been found, among them hormones such as Luteinizing hormone (LH) or insulin that have specific local actions in addition to their long-range signalling properties.
• Histamine
• Single ions, such as synaptically released zinc, are also considered neurotransmitters by some.^{citation needed}
• Gaseous, including nitric oxide (NO) and carbon monoxide (CO)

The major "workhorse" neurotransmitters of the brain are glutamic acid and GABA.

Excitatory and inhibitory

Release of excitatory neurotransmitters from the presynaptic membrane opens channels in the postsynaptic membrane and leads to an increase in the concentration of sodium ions within the postsynaptic cell and a decrease in that of potassium ions. This leads to a 'depolarisation' of the postsynaptic cell, which is propagated further along the cell membrane by an action potential.

Inhibitory neurotransmitters encourage the hyperpolarization of the postsynaptic cell, making it less likely to generate an action potential.

Whether a neurotransmitter acts in an excitatory or inhibitory manner is determined by the reaction of the receptor to its binding. Thus a given chemical can be excitatory at some receptors and inhibitory at others.

Actions

Some examples of neurotransmitter action:

• Acetylcholine - voluntary movement of the skeletal muscles (via the sympathetic pathways) and movement of the viscera (via the parasympathetic pathways)
• Norepinephrine - wakefulness or arousal - via the sympathetic pathways
• Epinephrine - similar to nor-epinephrine. Large amounts of it are produced and are released by the adrenal glands. Also called adrenaline.
• Dopamine - voluntary movement and motivation, "wanting", pleasure, associated with addiction and love
• Serotonin - memory, emotion, wakefulness, sleep and temperature regulation
• Glutamate - the most abundant excitatory neurotransmitter in the central nervous system.
• GABA - the most abundant inhibitory neurotransmitter in the central nervous system.
• Glycine - spinal reflexes and motor behaviour
• Neuromodulators - sensory transmission, especially pain
• Histamine - involved in the sleep/wake cycle and inflammatory response. Also has a modulating action on norepinephrine, serotonin, and acetylcholine.
• Taurine - inhibitory neurotransmitter. Has similar functions to GABA and can even mimic it by binding to its receptors. Some well-known energy drinks combine taurine with stimulants to produce their effect.

NB: Neurotransmitters may be specific to particular target organs and have multiple roles around the body. For instance, Acetylcholine is released from both sympathetic and parasympathetic neurons. ACH can be either excitatory to skeletal muscle cells or inhibitory to both smooth muscle and cardiac muscle.

Neurotransmitter systems

Main article: Neuromodulation

Neurons expressing certain types of neurotransmitters sometimes form distinct systems, where activation of the system affects large volumes of the brain, called volume transmission. The major neurotransmitter systems are the noradrenaline (norepinephrine) system, the dopamine system, the serotonin system and the cholinergic system.

Drugs targeting the neurotransmitter of such systems affects the whole system; this fact explains the mode of action of many drugs. Cocaine, for example, blocks the reentering of dopamine back into the presynaptic neuron, leaving these neurotransmitters in the synaptic gap longer. Since the dopamine is in the synapse longer, the neurotransmitter rapidly hit the receptors on the postsynaptic neuron cell, and therefore causing happiness. Excess intake of cocaine can lead to physical addiction. The physical addiction of cocaine is when the neurotransmitters stay in the synapse so long , the body removes some receptors from the postsynaptic neuron. After the effects of the drug wear off, the person usually feels unhappy, because now the neurotransmitters are less likely to hit the receptor since the body removed many of them during the drug intake. Prozac is a selective serotonin reuptake inhibitor (SSRI), hence potentiating the effect of naturally released serotonin. AMPT prevents the conversion of tyrosine to L-DOPA, the precursor to dopamine; reserpine prevents dopamine storage within vesicles; and deprenyl inhibits monoamine oxidase (MAO)-B and thus increases dopamine levels.

Diseases may affect specific neurotransmitter systems. For example, Parkinson's disease is at least in part related to failure of dopaminergic cells in deep-brain nuclei, for example the substantia nigra. Treatments potentiating the effect of dopamine precursors have been proposed and effected, with moderate success.

A brief comparison of the major neurotransmitter systems follows:

Common neurotransmitters

Degradation and elimination

Neurotransmitter must be broken down once it reaches the post-synaptic cell to prevent further excitatory or inhibitory signal transduction. For example, acetylcholine, (ACH) (an excitatory neurotransmitter), is broken down by acetylcholinesterase (AchE). Choline is taken up and recycled by the pre-synaptic neuron to synthesize more ACH. Other neurotransmitters such as dopamine are able to diffuse 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.

Sub-Neurotransmitters

In the process of neurotransmission, certain unknown components are existent and active, though their activities are subtle.

These components assist and desist procession of neurotransmission by aiding communication or stimulating the building blocks of the neurotransmitter itself. Examples are:

Mesoendodextrine Mesotetradextrine

See also

• Neuropsychopharmacology
• Nervous system
• Gasotransmitters

References

External links

Wikimedia Commons has media related to:

Neurotransmitter

• Molecular Expressions Photo Gallery: The Neurotransmitter Collection
• Brain Neurotransmitters
• Endogenous Neuroactive Extracellular Signal Transducers
• MeSH Neurotransmitter