NICOTINE INCREASES [CA2-ACTIVITY IN INSECT NEUROSECRETORY-CELLS (DUM NEURONS) VIA AN ACETYLCHOLINE-RECEPTOR WITH MIXED NICOTINIC-MUSCARINICPHARMACOLOGY(](I) AND REGULATES ELECTRICAL)

An increase in intracellular free calcium concentration ([Ca2+](i)) wa s observed following the application of nicotine to isolated adult dor sal unpaired median (DUM) neurons of the cockroach (Periplaneta americ ana) terminal abdominal ganglion (TAG) using Fura-2 fluorescence measu rements. Bath-applied nicotine (1 mM) induced a transient increase in [Ca2+](i). Calcium responses to bath-applied nicotine were blocked com pletely by alpha-bungarotoxin (100 nM) and were reduced by 50% in the presence of pirenzepine (1 mu M). The sensitivity of the response to b oth nicotinic and muscarinic antagonists suggested that it was mediate d by an acetylcholine receptor with 'mixed' pharmacology. In whole cel l current-clamp experiments, nicotine reduced the frequency of evoked action potentials by decreasing the slope of the predepolarization in the last two-thirds of the pacemaker potential. Voltage-clamp studies revealed that nicotine modified the inactivation properties of the mai ntained low-voltage-activated (LVA) calcium current increasing the rat e of relaxation of this current and transforming a U-shaped voltage de pendence of inactivation into a monotonic relationship to voltage, The se effects were blocked when isolated DUM neurons were pretreated with 0.5 mu M alpha-bungarotoxin. Our findings suggested a novel calcium-d ependent regulation of firing behavior in TAG DUM neurons following ac tivation of an acetylcholine receptor with 'mixed' pharmacology, resul ting in a rise in [Ca2+](i) which reduces firing frequency by modulati ng a maintained LVA calcium current responsible for the action potenti al predepolarization. Copyright (C) 1996 Elsevier Science Ireland Ltd.