Neurotransmitter

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Neurotransmitters are chemicals that are used to relay, amplify and modulate electrical signals between a neuron and another cell. According to the prevailing beliefs of the 1960s, a chemical can be classified as a neurotransmitter if it meets the following conditions:

  • It is synthesized endogenously, that is, within the presynaptic neuron;
  • It is available in sufficient quantity in the presynaptic neuron to exert an effect on the postsynaptic neuron;
  • Externally administered, it must mimic the endogenously-released substance; and
  • A biochemical mechanism for inactivation must be present.

However, there are other materials, such as the zinc ion, that are neither synthesized nor catabolized (i.e., degraded; see Anabolism) and are considered neurotransmitters by some. Thus, the old definitions are being revised.

Contents

Types of neurotransmitters

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

Some more precise divisions are as follows:

The major "workhorse" neurotransmitters of the brain are glutamic acid (=glutamate) and GABA.

Effects

Some examples of neurotransmitter action:

  • Acetylcholine - voluntary movement of the muscles
  • Norepinephrine - wakefulness or arousal
  • Dopamine - voluntary movement and motivation, "wanting"
  • Serotonin - memory, emotions, wakefulness, sleep and temperature regulation
  • GABA (gamma aminobutyric acid) - inhibition of motor neurons
  • Glycine - spinal reflexes and motor behaviour
  • Neuromodulators - sensory transmission-especially pain

It is important to appreciate that it is the receptor that dictates the neurotransmitter's effect.

Mechanism of action

Within the cells, small-molecule neurotransmitter molecules are usually packaged in vesicles. When an action potential travels to the synapse, the rapid depolarization causes calcium ion channels to open. Calcium then stimulates the transport of vesicles to the synaptic membrane; the vesicle and cell membrane fuse, leading to the release of the packaged neurotransmitter, a mechanism called exocytosis.

The neurotransmitters then diffuse across the synaptic cleft to bind to receptors. The receptors are broadly classified into ionotropic and metabotropic receptors. Ionotropic receptors are ligand-gated ion channels that open or close through neurotransmitter binding. Metabotropic receptors, which can have a diverse range of effects on a cell, transduct the signal by secondary messenger systems, or G-proteins.

Neuroactive peptides are made in the neuron's soma and are transported through the axon to the synapse. They are usually packaged into dense-core vesicles and are released through a similar, but metabolically distinct, form of exocytosis used for small-molecule synaptic vesicles.

Post-synaptic effect

A neurotransmitter's effect is determined by its receptor. For example, GABA can act on both rapid or slow inhibitory receptors (the GABA-A and GABA-B receptor respectively). Many other neurotransmitters, however, may have excitatory or inhibitory actions depending on which receptor they bind to.

Neurotransmitters may cause either excitatory or inhibitory post-synaptic potentials. That is, they may help the initiation of a nerve impulse in the receiving neuron, or they may discourage such an impulse by modifying the local membrane voltage potential. In the central nervous system, combined input from several synapses is usually required to trigger an action potential. Glutamate is the most prominent of excitatory transmitters; GABA and glycine are well-known inhibitory neurotransmitters.

Many neurotransmitters are removed from the synaptic cleft by neurotransmitter transporters in a process called reuptake (or often simply 'uptake'). Without reuptake, the molecules might continue to stimulate or inhibit the firing of the postsynaptic neuron. Another mechanism for removal of a neurotransmitter is digestion by an enzyme. For example, at cholinergic synapses (where acetylcholine is the neurotransmitter), the enzyme acetylcholinesterase breaks down the acetylcholine. Neuroactive peptides are often removed from the cleft by diffusion, and eventually broken down by proteases.

Specifications

While some neurotransmitters (glutamate, GABA, glycine) are used very generally throughout the central nervous system, others can have more specific effects, such as on the autonomic nervous system, by both pathways in the sympathetic nervous system and the parasympathetic nervous system, and the action of others are regulated by distinct classes of nerve clusters which can be arranged in familiar pathways around the brain. For example, Serotonin is released specifically by cells in the brainstem, in an area called the raphe nuclei, but travels around the brain along the medial forebrain bundle activating the cortex, hippocampus, thalamus, hypothalamus and cerebellum. Also, it is released in the Caudal serotonin nuclei, so as to have effect on the spinal cord. In the peripherial nervous system (such as in the gut wall) serotonin regulates vascular tone. Dopamine classically modulates two systems: the brain's reward mechanism, and movement control.

Neurotransmitters that have these types of specific actions are often targeted by drugs.

Some neurotransmitter/neuromodulators like zinc not only can modulate the sensitivity of a receptor to other neurotransmitters (allosteric modulation) but can even penetrate specific, gated channels in post-synaptic neurons, thus entering the post-synaptic cells. This "translocation" is another mechanism by which synaptic transmitters can affect postsynaptic cells.

Diseases may affect specific neurotransmitter pathways. 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.

Common neurotransmitters

Category Name Abbreviation Metabotropic Ionotropic
Small: Amino acids Aspartate - -
Small: Amino acids Glutamate (glutamic acid) Glu Metabotropic glutamate receptor NMDA receptor, Kainate receptor, AMPA receptor
Small: Amino acids Gamma-aminobutyric acid GABA GABAB receptor GABAA receptor, GABAC receptor
Small: Amino acids Glycine Gly - Glycine receptor
Small: Acetylcholine Acetylcholine Ach Muscarinic acetylcholine receptor Nicotinic acetylcholine receptor
Small: Monoamine (Phe/Tyr) Dopamine DA Dopamine receptor -
Small: Monoamine (Phe/Tyr) Norepinephrine (noradrenaline) NE - -
Small: Monoamine (Phe/Tyr) Epinephrine (adrenaline) Epi - -
Small: Monoamine (Phe/Tyr) Octopamine - -
Small: Monoamine (Phe/Tyr) Tyramine -
Small: Monoamine (Trp) Serotonin (5-hydroxytryptamine) 5-HT Serotonin receptor, all but 5-HT3 5-HT3
Small: Monoamine (Trp) Melatonin Mel Melatonin receptor -
Small: Monoamine (His) Histamine H Histamine receptor -
PP: Gastrins Gastrin - -
PP: Gastrins Cholecystokinin CCK Cholecystokinin receptor -
PP: Neurohypophyseals Vasopressin Vasopressin receptor -
PP: Neurohypophyseals Oxytocin Oxytocin receptor -
PP: Neurohypophyseals Neurophysin I - -
PP: Neurohypophyseals Neurophysin II - -
PP: Neuropeptide Y Neuropeptide Y NY Neuropeptide Y receptor -
PP: Neuropeptide Y Pancreatic polypeptide PP - -
PP: Neuropeptide Y Peptide YY PYY - -
PP: Opioids Corticotropin (adrenocorticotropic hormone) ACTH Corticotropin receptor -
PP: Opioids Dynorphin - -
PP: Opioids Endorphin - -
PP: Opioids Enkephaline - -
PP: Secretins Secretin Secretin receptor -
PP: Secretins Motilin Motilin receptor -
PP: Secretins Glucagon Glucagon receptor -
PP: Secretins Vasoactive intestinal peptide VIP Vasoactive intestinal peptide receptor -
PP: Secretins Growth hormone-releasing factor GRF - -
PP: Somtostatins Somatostatin Somatostatin receptor -
SS: Tachykinins Neurokinin A - -
SS: Tachykinins Neurokinin B - -
SS: Tachykinins Substance P - -
PP: Other Bombesin - -
PP: Other Gastrin releasing peptide GRP - -
Gas Nitric oxide NO - -
Gas Carbon monoxide CO - -
Other Anandamide AEA Cannabinoid receptor -
Other Adenosine triphosphate ATP P2Y12 P2X receptor


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