Activation of nicotinic receptor-induced postsynaptic responses to luteinizing hormone-releasing hormone in bullfrog sympathetic ganglia via a Na(+)-dependent mechanism

Nicotine at very low doses (5–30 nM) induced large amounts of luteinizing hormone-releasing hormone (LHRH) release, which was monitored as slow membrane depolarizations in the ganglionic neurons of bullfrog sympathetic ganglia. A nicotinic antagonist, d-tubocurarine chloride, completely and reversibly blocked the nicotine-induced LHRH release, but it did not block the nerve-firing-evoked LHRH release. Thus, nicotine activated nicotinic acetylcholine receptors and produced LHRH release via a mechanism that is different from the mechanism for evoked release. Moreover, this release was not caused by Ca(2+) influx through either the nicotinic receptors or the voltage-gated Ca(2+) channels because the release was increased moderately when the extracellular solution was changed into a Ca(2+)-free solution that also contained Mg(2+) (4 mM) and Cd(2+) (200 μM). The release did not depend on Ca(2+) release from the intraterminal Ca(2+) stores either because fura-2 fluorimetry showed extremely low Ca(2+) elevation (≈30 nM) in response to nicotine (30 nM). Moreover, nicotine evoked LHRH release when [Ca(2+)] elevation in the terminals was prevented by loading the terminals with 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid and fura-2. Instead, the nicotine-induced release required extracellular Na(+) because substitution of extracellular NaCl with N-methyl-d-glucamine chloride completely blocked the release. The Na(+)-dependent mechanism was not via Na(+) influx through the voltage-gated Na(+) channels because the release was not affected by tetrodotoxin (1–50 μM) plus Cd(2+) (200 μM). Thus, nicotine at very low concentrations induced LHRH release via a Na(+)-dependent, Ca(2+)-independent mechanism.