Presynaptic autoinhibition of central noradrenaline release in vitro: operational characteristics and effects of drugs acting at alpha-2 adrenoceptors in the presence of uptake inhibition

Abstract

Functional characteristics of autoinhibition of central noradrenaline release were studied in the presence of uptake inhibition. Slices of rat cerebral cortex were incubated with [3H]noradrenaline, superfused and field-stimulated with 1 to 16 monophasic rectangular pulses at frequencies of 0.02 to 40 Hz. 1) Substances acting at presynaptic alpha-2 adrenoceptors were identified as antagonists, agonists or partial agonists by comparing their effects on 3H-overflow evoked by a single pulse or by two consecutive pulses at 1 Hz. 2) When 1 to 16 pulses were delivered at 0.02, 0.08, 0.3 and 1 Hz to stimulate outflow of tritium, a frequency-dependent suppression of responses to the second and the following pulses was observed. In the presence of the alpha-2 adrenoceptor antagonist idazoxan (10(-6) M), comparable amounts of tritium were released by the first stimulus and each of the following stimuli at 0.02 Hz. In contrast, at 0.08, 0.3 and 1 Hz the amount of 3H-overflow evoked by the first pulse was not reached in response to the following pulses. Clonidine (10(-6) M) diminished markedly the response to the first as well as to the following stimuli, irrespective of the frequency of stimulation. 3) Using two consecutive pulses delivered with decreasing pulse intervals, an apparent reduction or complete abolition of autoinhibition was observed at intervals of less than 100 msec, indicated by reduction or loss of the facilitatory effects of alpha-2 adrenoceptor antagonists. The present results provide detailed insights in operational characteristics of alpha-2 adrenoceptor-mediated autoinhibition and the effects of drugs on this regulatory mechism.