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

Abstract

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 μM). 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 μM) plus Cd²⁺ (200 μM). Thus, nicotine at very low concentrations induced LHRH release via a Na⁺-dependent, Ca²⁺-independent mechanism.