INHIBITION OF N-TYPE CALCIUM CHANNELS - THE ONLY MECHANISM BY WHICH PRESYNAPTIC ALPHA(2)-AUTORECEPTORS CONTROL SYMPATHETIC TRANSMITTER RELEASE

alpha(2)-Adrenoceptors are known to inhibit voltage-dependent Ca2+ cha nnels located at neuronal cell bodies; the present study investigated whether this or alternative mechanisms, possibly downstream of Ca2+ en try, underlie the presynaptic alpha(2)-adrenergic modulation of transm itter release from chick sympathetic neurons. Using chick sympathetic neurons, overflow of previously incorporated [H-3]noradrenaline was el icited in the presence of extracellular Ca2+ by electrical pulses, 25 mM K+ or 10 mu M nicotine, or by adding Ca2+ to otherwise Ca2+-free me dium when cells had been made permeable by the calcium ionophore A2318 7 or by alpha-latrotoxin. Pretreatment of neurons with the N-type Ca2 channel blocker omega-conotoxin GVIA and application of the alpha(2)- adrenergic agonist UK 14304 reduced the overflow elicited by electrica l pulses, K+ or nicotine, but not the overflow caused by Ca2+ after pe rmeabilization with alpha-latrotoxin or A23187. In contrast, the L-typ e Ca2+ channel blocker nitrendipine reduced the overflow due to K+ and nicotine, but not the overflow following electrical stimulation or al pha-latrotoxin- and A23187-permeabilization. The inhibition of electri cally evoked overflow by UK 14304 persisted in the presence of nitrend ipine and the L-type Ca2+ channel agonist BayK 8644, which per se enha nced overflow. In omega-conotoxin GVIA-treated cultures, electrically evoked overflow was also enhanced by BayK 8644 and almost reached the value obtained in untreated neurons. However, UK 14304 lost its effect under these conditions. Whole-cell recordings of voltage-activated Ca 2+ currents corroborated these results: UK 14304 inhibited Ca2+ curren ts by 33%, nitrendipine caused a 7% reduction, and BayK 8644 increased the currents by 30%. Moreover, the dihydropyridines failed to abolish the inhibition by UK 14304, but pretreatment with omega-conotoxin GVI A, which reduced mean amplitude from 0.95 to 0.23 nA, entirely prevent ed alpha(2)-adrenergic effects. Our results indicate that the alpha(2) -autoreceptor-mediated modulation of noradrenaline release from chick sympathetic neurons relies exclusively on the inhibition of omega-cono toxin GVIA-sensitive N-type Ca2+ channels. Mechanisms downstream of th ese channels and voltage-sensitive Ca2+ channels other than N-type app ear not to be important.