CALCIUM AND MELATONIN PRODUCTION IN DISSOCIATED TROUT PINEAL PHOTORECEPTOR CELLS IN CULTURE

Trout pineal cells maintained in primary culture produce melatonin in high amounts during night time and low amounts during daytime. The dar k-induced increase in melatonin production was enhanced, in a dose-dep endent manner, by elevating extracellular calcium concentration. Low e xternal calcium concentration reduced nocturnal and diurnal melatonin production. Bay K 8644 increased, in a dose-dependent manner, the dark -induced rise in melatonin output, and this effect was antagonized by nifedipine and verapamil. This suggests a role for the dihydropyridine calcium channels in the regulation of the melatonin output. To confir m this, patch-clamp recordings (whole-cell perforated) were run in a 2 0 mmol/l barium medium at different holding potentials from -80 mV. A voltage-dependent inward current was activated from -30 mV to +40 mV w ith a maximal amplitude being observed at 0 mV. This current was drast ically increased in the presence of Bay K 8644. Nifedipine inhibited t he current both in the absence or in the presence of Bay K 8644. Our r esults are consistent with the idea that extracellular calcium partici pates in the control of melatonin secretion by photoreceptor cells. It is suggested that activation of the voltage-dependent L-type channel may modulate this secretion.