ABSTRACT

Inactivation is necessary to ensure that the synapse can respond to rapid changes in the firing frequency of the presynaptic neuron. There are three ways in which transmitter can be inactivated and they are not mutually exclusive: enzymic degradation, transport out of the synaptic cleft back into neurons or glia, or by passive diffusion away from the synapse. Many enzymes are involved in the catabolism of both classical and peptide transmitters and pharmacological manipulation of many of these can have consequences for synaptic transmission. Acetylcholine is hydrolyzed by acetylcholinesterase, which cleaves the transmitter molecule into choline and acetate. The amino acid transmitters may be transported into both neurons or glia whereas amines are transported only into neurons. Despite the existence of transporters, diffusion out of the synapse may be important for the inactivation of glutamate and glutamate and γ-aminobutyrate at synapses in the cerebral cortex.