PRESYNAPTIC RECEPTORS

Activation of different types of G-protein-linked and ionotropic presy naptic receptors has been shown to regulate neurotransmitter release t hroughout the central and peripheral nervous systems. In the case of G -protein-linked receptors, three major mechanisms have been suggested: (a) inhibition of Ca channels in the nerve terminal; (b) the activati on of presynaptic K channels, resulting in a reduction in the effectiv eness of the action potential; and (c) direct modulation of one or mor e components of the neurotransmitter vesicle release apparatus. In the case of ionotropic presynaptic receptors, inhibition of release may b e achieved through depolarization of the terminal and inactivation of Na and Ca channels. Activation of presynaptic ionotropic receptors tha t are appreciably Ca permeable can also enhance the release of transmi tters as a result of their ability to raise [Ca](i) in the terminal di rectly. Many transmitters employ several of these mechanisms, thus all owing considerable flexibility in the presynaptic regulation of transm itter release.