Neurotransmitters

Neurotransmitters are chemicals in the central nervous system that relay signals between neurons by crossing the small gap (synapse) between neurons. The neurotransmitters are stored in vesicles close to the synaptic membrane, and when they are released into the synapse they bind to receptors in the synaptic membrane of the opposite neuron. The effect of this depends upon the properties of the receptor. In most cases receptor binding causes depolarisation of the receptor site. In general this results in the cell firing an action potential, and therefore has an excitatory effect. Some neurotransmitters cause hyperpolarisation of the receptor site, and this results in inhibition of the target neuron.

For a chemical to be regarded as a neurotransmitter it must fulfil a number of criteria (Fig. 1). There must be evidence that it is synthesised in the presynaptic neuron. The precursors and enzymes associated with synthesis must be found in the presynaptic neuron. It must be released when the presynaptic receptor is stimulated, and bind to the postsynaptic receptor, causing a biological effect. There must also be evidence of a mechanism for deactivating the chemical in the synapse, or for its reuptake.

Fig. 1 Properties of neurotransmitters.

The first neurotransmitter to be described was acetylcholine, in 1914. Since then a wide variety of neurotransmitters have been identified. The most common neurotransmitter in the CNS is glutamate, present in more than 80% of synapses in the brain. Gamma-aminobutyric acid (GABA) is present in the majority of other synapses. Other neurotransmitters are present in fewer synapses, but are of greater significance in the aetiology and treatment of mental illness – in particular dopamine, serotonin, noradrenaline and acetylcholine.