THE INFLUX OF CA2-TYPE, N-TYPE, OR P-TYPE CALCIUM CHANNELS( AND THE RELEASE OF NORADRENALINE EVOKED BY THE STIMULATION OF PRESYNAPTIC NICOTINIC RECEPTORS OF CHICK SYMPATHETIC NEURONS IN CULTURE ARE NOT MEDIATED VIA L)

We have shown earlier that nicotinic agonists induce the release of no radrenaline from chick sympathetic neurons in culture in two ways: (a) by activating the postsynaptic nicotinic receptors on nerve cell bodi es, giving rise to spreading electrical activity and opening of voltag e operated calcium channels in neuronal processes; (b) by activating t he presynaptic nicotinic receptors on neuronal processes. In the prese nt work, we investigated the contribution of various pathways to the o bserved Ca2+ influx and subsequent noradrenaline release. Sympathetic neurons in culture were stimulated either by the nicotinic agonist dim ethylphenylpiperazinium or electrically, in the presence or absence of tetrodotoxin and of specific blockers of calcium or nicotinic channel s, and the effects on [Ca2+](i) in the area of neuronal processes and on noradrenaline release were measured, Under control conditions, the N-type channel blocker omega-conotoxin (0.1 mu mol/l) diminished the r elease of noradrenaline and the increase of intraterminal Ca2+ by 48% and 55%, respectively, whereas the L-type channel blocker (+)Bay k 864 4 (1 mu mol/l) diminished the release of noradrenaline by 25% and the increase of [Ca2+](i) by 39%. The P-type channel blocker omega-agatoxi n (0.3 mu mol/l) had no effect. The effects of the L-type channel liga nds were complex and could only be explained on the assumption that, a t high concentrations, these drugs also act as nicotinic antagonists. Tetrodotoxin blocked the Ca2+ response evoked by electrical stimulatio n whereas DMPP applied in the presence of tetrodotoxin still evoked an increase of [Ca2+](i) and the release of noradrenaline (27% and 30% o f control without tetrodotoxin, respectively). These residual response s were not blocked by any of the calcium channel blockers used or by t heir combination. Apparently, a substantial part of the influx of Ca2 induced by the activation of presynaptic nicotinic receptors is not c arried by the N-, L- or P-type channels and probably occurs directly v ia the open channels of nicotinic receptors.