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 Ca2+influx through either the nicotinic receptors or the voltage-gated Ca2+channels because the release was increased moderately when the extracellular solution was changed into a Ca2+-free solution that also contained Mg2+(4 mM) and Cd2+(200 μ M). The release did not depend on Ca2+release from the intraterminal Ca2+stores either because fura-2 fluorimetry showed extremely low Ca2+elevation (≈ 30 nM) in response to nicotine (30 nM). Moreover, nicotine evoked LHRH release when [Ca2+] 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 μ M) plus Cd2+(200 μ M). Thus, nicotine at very low concentrations induced LHRH release via a Na+-dependent, Ca2+-independent mechanism.