Mode of action of (−)nicotine on GtH2 secretion from pituitary cells of common carp (Cyprinus carpio L.)

Abstract

Abstract Enzymatically dispersed carp pituitary cells in a continuous perifusion system were used to investigate the mode of the action of nicotine on GtH2 secretion. Nicotine given in a 12-min pulse stimulated GtH2 release in a dose-dependent manner. Doses of 10−7 M and 10−6 M regularly evoked significant increase in GtH2 secretion up to 140% and to 250% of the baseline, respectively. Prolonged (60 min) administration of nicotine showed that the secretory response of the cells consists of two distinct phases: first-acute phase (4 to 14 min) and second-sustained release phase (20 to 40 min). The magnitude of the response during the first phase was dose-dependent and significantly different between doses. In contrast, the magnitude of the response during second phase showed no significant differences between doses and was stabilized at 140% of the baseline. Repeated administration of several 12-min pulses of nicotine showed distinct reaction of the cells according to the dose. Using higher doses (10−6 M) a progressive desensitization of the cells for consecutive stimuli was observed, including absence of response for the last (fourth) pulse. In the case of 10−7 M nicotine, the stimulatory action was observed only during the first pulse. All subsequent next pulses had no effect on the GtH2 secretion rate and this reaction cannot be explained by a desensitization phenomenon. Even though specific nicotinic receptors in fish pituitary secretory cells have been elusive to date, our results bring indirect proof that stimulatory action of nicotine on GtH2 secretion from carp pituitary cells is exhibited by a receptor mediated mechanism.