3,4-DIAMINOPYRIDINE MASKS THE INHIBITION OF NORADRENALINE RELEASE FROM CHICK SYMPATHETIC NEURONS VIA PRESYNAPTIC ALPHA(2)-ADRENOCEPTORS - INSIGHTS INTO THE ROLE OF N-TYPE AND L-TYPE CALCIUM CHANNELS

We have investigated the participation of the N-type (omega-conotoxin GVIA-sensitive) and L-type (nifedipine-sensitive) calcium channels in the alpha(2)-adrenoceptor mediated autoinhibition of the release of [H -3]noradrenaline from chick sympathetic neurons in culture. Blockade o f 3,4-diaminopyridine-sensitive potassium channels resulted in tetrodo toxin-sensitive and calcium-dependent increase of the release of [H-3] noradrenaline evoked by electrical stimulation. Nifedipine attenuated the evoked release under control conditions by 20%, but in the presenc e of 3,4-diaminopyridine by 51%, while omega-conotoxin decreased the r elease under control conditions by 87% and in the presence of 3,4-diam inopyridine by only 43%. The L-type calcium channel activator Bay k 86 44 increased the evoked release of the transmitter both in the absence and in the presence of 3,4-diaminopyridine. Under control conditions, the alpha(2)-adrenoceptor agonist UK 14 304 decreased the evoked rele ase by 57% and the alpha(2)-adrenoceptor antagonist rauwolscine increa sed it by 14%. Nifedipine did not prevent this modulation. In the pres ence of 3,4-diaminopyridine, UK 14304 lost its effect on the release o f noradrenaline, but its inhibitory action was restored when nifedipin e, but not omega-conotoxin, was added. Changes in the increase of intr acellular calcium concentration ([Ca2+](i)) evoked by electrical stimu lation, measured in the cell processes by microfluorimetry, paralleled the changes in the release of [H-3]noradrenaline. Under control condi tions, nifedipine attenuated the rise of intracellular calcium by only 16%, while omega-conotoxin did so by 66%. 3,4-Diaminopyridine enhance d the evoked rise of [Ca2+](i); in its presence the rise of intracellu lar calcium was about equally reduced by nifedipine and omega-conotoxi n (by 46 and 36%, respectively). These effects were additive. UK 14304 diminished the peak concentration of [Ca2+](i) elicited by the standa rd electrical stimulation by 31% and rauwolscine antagonised this effe ct. UK 14304 did not measurably inhibit the stimulation-evoked rise of intraterminal [Ca2+](i) in the presence of 3,4-diaminopyridine but it produced an inhibition by 26% if nifedipine had been applied together with 3,4-diaminopyridine. Our observations show that, under control c onditions, the stimulated release of [H-3]noradrenaline is mainly asso ciated with the opening of N-type channels, while in the presence of 3 ,4-diaminopyridine the contribution of L-type channels becomes more im portant. The alpha(2)-adrenoceptor stimulation by UK 14304 inhibits th e release of [H-3]noradrenaline but, in the presence of 3,4-diaminopyr idine, the inhibition of release can only be observed if the massive i nflux through L-type calcium channels is prevented. These data suggest that presynaptic alpha(2)-adrenoceptors of chick sympathetic neurons preferentially influence the N-type calcium channels.