Neurotransmitters are chemicals that are used to relay, amplify and modulate 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.
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:
Around 10 "small-molecule neurotransmitters" are known:
monoamines (norepinephrine NE, dopamine DA & serotonin 5-HT)
3 or 4 amino acids, depending on exact definition used: (primarily glutamic acid, GABA, aspartic acid & glycine)
Purines, (Adenosine, ATP, GTP and their derivatives)
Fatty acids are also receiving attention as the potential endogenous cannabinoid.
Over 50 neuroactive peptides (vasopressin, somatostatin, neurotensin, etc.) have been found, among them hormones such as LH or insulin that have specific local actions in addition to their long-range signalling properties.
Single ions, such as synaptically-released zinc, are also considered neurotransmitters by some.
The major "workhorse" neurotransmitters of the brain are glutamic acid (=glutamate) and GABA.
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 neurotransmitters are usually packaged in vesicles. When an action potential reaches the cell body, the rapid depolarization causes calcium ion (Ca2) channels to open. Calcium then stimulates the transport of vesicles to the synaptic membrane and their release at synaptic boutons - a form of exocytosis. These neurotransmitters are released in quanta, whereby a single quantum consists of a vesicle containing possibly thousands of neurotransmitters.
The neurotransmitters then diffuse across the synaptic cleft to bind to densely and geometrically arranged 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.
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.
Cocaine, for example, blocks the reuptake of dopamine, leaving these neurotransmitters in the synaptic gap longer.
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.
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.
A neurohormone is any hormone produced by neurosecretory cells, usually in the brain. Neurohormonal activity is distinguished from that of classical neurotransmitters as it can have effects on cells distant from the source of the hormone.
A hormone (from Greek όρμή - "to set in motion") is a chemical messenger that carries a signal from one cell (or group of cells) to another. All multicellular organisms produce hormones (including plants - see phytohormone).
The function of hormones is to carry information to the target cells; the action of hormones is determined by the pattern of secretion and response of the receiving tissue - the signal transduction response.
The best-known animal hormones are those produced by endocrine glands of vertebrate animals, but hormones are produced by nearly every organ system and tissue type in a multicellular organism.
Endocrine hormone molecules are secreted (released) directly into the bloodstream, while exocrine hormones (or ectohormones) are secreted directly into a duct, and from the duct they either flow into the bloodstream or they flow from cell to cell by diffusion in a process known as paracrine signalling.