Electrophysiological characterization of nicotinic acetylcholine receptors in cat petrosal ganglion neurons in culture: effects of cytisine and its bromo derivatives.

Petrosal ganglion neurons are depolarized and fire action potentials in response to acetylcholine and nicotine. However, little is known about the subtype(s) of nicotinic acetylcholine receptors involved, although alpha4 and alpha7 subunits have been identified in petrosal ganglion neurons. Cytisine, an alkaloid unrelated to nicotine, and its bromo derivatives are agonists exhibiting different affinities, potencies and efficacies at nicotinic acetylcholine receptors containing alpha4 or alpha7 subunits. To characterize the receptors involved, we studied the effects of these agonists and the nicotinic acetylcholine receptor antagonists hexamethonium and alpha-bungarotoxin in isolated petrosal ganglion neurons. Petrosal ganglia were excised from anesthetized cats and cultured for up to 16 days. Using patch-clamp technique, we recorded whole-cell currents evoked by 5-10 s applications of acetylcholine, cytisine or its bromo derivatives. Agonists and antagonists were applied by gravity from a pipette near the neuron surface. Neurons responded to acetylcholine, cytisine, 3-bromocytisine and 5-bromocytisine with fast inward currents that desensitized during application of the stimuli and were reversibly blocked by 1 microM hexamethonium or 10 nM alpha-bungarotoxin. The order of potency of the agonists was 3-bromocytisine >> acetylcholine approximately = cytisine >> 5-bromocytisine, suggesting that homomeric alpha7 neuronal nicotinic receptors predominate in cat petrosal ganglion neurons in culture.