PRESYNAPTIC ALPHA(2)-ADRENOCEPTORS INHIBIT CALCIUM INFLUX IN TERMINALS OF CHICKEN SYMPATHETIC NEURONS AND NORADRENALINE RELEASE EVOKED BY NICOTINIC STIMULATION

The changes of intracellular calcium concentration evoked by stimulati on of nicotinic receptors and its modulation by alpha(2)-adrenoceptors were investigated in chicken sympathetic neurons grown in culture. St imulation of nicotinic receptors increased the intracellular calcium b oth in the area of cell bodies and processes. In normal extracellular calcium (1.3 mmol/l), the specific alpha(2)-adrenoreceptor agonist UK 14 304 (10 mu mol/l) diminished the response in cell bodies but not in processes. The same treatment in medium with reduced calcium (0.13 mm ol/l) resulted in the decrease of the response evoked by nicotinic sti mulation in the area of processes but not in the cell bodies. The effe ct of UK 14 304 on evoked noradrenaline release paralleled its influen ce on intracellular calcium in processes. These data indicate that nic otinic stimulation-induced influx of calcium is inhibited by alpha(2)- adrenoceptor stimulation both in cell bodies and in processes. It is c oncluded that the nicotinic stimulation-evoked release of noradrenalin e is triggered by influx of calcium through alpha(2)-adrenoreceptor se nsitive as well as insensitive pathways.