The role of alpha- and beta-presynaptic receptors in the regulation of noradrenaline release elicited by nerve stimulation.

1. The presynaptic mechanisms appear to be involved in the regulation of noradrenaline release during nerve stimulation. The first one. mediated by beta-adrenoceptors, operates at low frequencies of nerve stimulation, leading to an increase in transmitter release. The second one, mediated through alpha-adrenoceptors, is triggered when higher concentrations of the transmitter are reached in the synaptic cleft, leading to inhibition of transmitter release, probably through a restriction in the availability of calcium for the secretory process. 2. It is postulated that part of the anti-hypertensive effects of drugs like clonidine, alpha-methyldopa and beta-receptor-blocking agents may be related to their long-term effects on presynaptic adrenoceptors.