Does presynaptic regulation of sympathetic transmission occur within a limited range of neuronal activity?

Recent reports indicate that the presynaptic negative feedback mechanism involved in regulating sympathetic neurotransmission is only functional within a narrow range of neuronal activity. The specific aim of this report is to examine the release of sympathetic transmitter at varying frequencies of stimulation, and the effects of alpha-adrenoceptor antagonists on the overflow. The experiments were carried out in the 3H-noradrenalin-labeled heart of the guinea pig. The overflow of tritium (per pulse) increased from a stimulation frequency of 0.125 to 10 Hz and declined at 30 Hz. Phentolamine or yohimbine produced facilitation of the overflow from 0.125 to 10 Hz; the effect was more pronounced at lower frequencies of stimulation. We conclude that the presynaptic alpha-adrenoceptor-mediated negative feedback mechanism operates over a wide range of cardiac sympathetic nerve activity.