PRESYNAPTIC NICOTINIC RECEPTORS STIMULATE INCREASES IN INTRATERMINAL CALCIUM OF CHICK SYMPATHETIC NEURONS IN CULTURE

Stimulation of chick sympathetic neurons in culture by the cholinergic agonists acetylcholine, nicotine, and 1,1-dimethyl-4-phenylpiperazini um (all at 10-1,000 mu mol/L) induced concentration-dependent increase s of free calcium levels measured by fura 2 fluorescence in neuronal p rocesses. The response evoked by acetylcholine had both nicotinic and muscarinic components, whereas that induced by 1,1-dimethyl-4-phenylpi perazinium was purely nicotinic. Tetrodotoxin (0.3 mu mol/L) blocked c ompletely the increase of intraterminal free calcium level evoked by e lectrical stimulation. On the other hand, stimulation with 1,1-dimethy l-4-phenylpiperazinium still evoked 20-25% of the control response in the presence of tetrodotoxin. The concentration-response relationship of 1,1-dimethyl-4-phenylpiperazinium stimulation did not differ in the absence and in the presence of tetrodotoxin. The nicotinic antagonist s d-tubocurarine (10 mu mol/L) and mecamylamine (10 mu mol/L), but not alpha-bungarotoxin (125 nmol/L), prevented the increase of intratermi nal free calcium level evoked by 1,1-dimethyl-4-phenylpiperazinium (10 0 mu mol/L) in the presence of tetrodotoxin. These observations indica te the presence of nicotinic receptors on neuronal processes that incr ease the intraterminal concentration of free calcium and probably modu late transmitter release. Their pharmacological properties are similar to those of nicotinic receptors located on neuronal cell bodies.