Long-lasting depolarization of leech neurons mediated by receptors with a nicotinic binding site

The serotonergic Retzius neurons of the leech midbody ganglia respond in a complex manner to pressure pulses of acetylcholine (ACh) applied onto their soma with a fast depolarization followed by a slower hyperpolarization and an additional delayed long-lasting depolarization. The delayed depolarization is the subject of the present study. The delayed depolarization could be elicited by long (>1 s) ACh pressure pulses or by short pulses (10 ms) of carbachol, nicotine and DMPP, but not by muscarinic agonists. It was inhibited by bath application of nicotine (10-100 micromol l-1), strychnine (100 micromol l-1) and atropine (10-100 micromol l-1). Nicotinic antagonists that blocked the fast depolarization and the slow hyperpolarization (100 micromol l-1 mecamylamine and d-tubocurarine) did not affect the delayed depolarization induced by carbachol. Partial replacement of the extracellular Na+ by glucamine caused a decrease in the amplitude of the response and a shift of its reversal potential to more negative values. Carbachol pulses applied to Retzius neurons of the ganglia innervating the reproductive segments elicited delayed depolarizations of much smaller amplitude than the ones recorded in Retzius neurons from standard segments. The delayed depolarization could be elicited by the application of short agonist pulses onto different loci over the surface of the ganglion, at a distance from the soma. Isolated cultured Retzius neurons did not exhibit the delayed depolarization although they readily expressed the earlier phases of the complex cholinergic response. Carbachol pulses applied to the soma of other neurons in the leech ganglion produced a variety of specific responses.The results suggest that the delayed depolarization was produced by the activation of a cationic conductance mediated by receptors with a pharmacological profile similar to that of the 9 nicotinic receptors and was not a byproduct of the early phases of the cholinergic response. The response seemed to be initiated in the extensive neuropilar processes of the Retzius cell, enabling a persistent excitatory signal.