Effects of photoperiod alterations on day-night variations in hypothalamic serotonin content and turnover, and monoamine oxidase activity in the female catfish, Heteropneustes fossilis (Bloch).

The effects of different photoperiod regimes, and total darkness on day-night variations in hypothalamic serotonin (5-HT) content and turnover index (TI), and monoamine oxidase (MAO) activity that occur exclusively during the gonadal preparatory phase (February-March) were investigated in female Heteropneustes fossilis. Exposure of the fish to long photoperiods (16L:8D; light between 06.00-22.00h, for 40 days elevated both the midphotophase and midscotophase values of 5-HT content and MAO, and abolished their day-night differences. The daily variations of 5-HT-TI was further intensified by the treatment. Under short photoperiods (4L:20D; light between 18.00-22.00h for 40 days), there was a significant decrease in both 5-HT content and TI, a reversal of the day-night variations of 5-HT content and MAO activity, and loss of the 5-HT-TI pattern. In fish maintained in total darkness the day-night variations were not found and there was a significant inhibition of 5-HT. Exposure of the fish to continuous light abolished the day-night variations of these correlates and elevated their values at both intervals. When the LD cycle was reversed, there was a corresponding shift in the day-night patterns. The gonadosomatic index (GSI) was significantly elevated in the long photoperiod and continuous illumination groups, significantly decreased in the short photoperiod and total darkness groups, and unchanged in the reversed LD cycle regime. The results indicate that the day-night variations of 5-HT and MAO are photoperiod-dependent and are controlled by the prevailing LD cycle during the gonadal preparatory phase of the annual reproductive cycle. The photoperiod effects on the gonadal activity may be mediated through the hypothalamic serotonergic system.