Diurnal variations in hypothalamic monoamine levels in the teleostChanna punctatus (bloch) in response to melatonin under two photothermal conditions.

Hypothalamic dopamine (DA), noradrenaline (NA) and 5-hydroxytryptamine (5-HT) levels exhibited marked day-night variations under ambient photoperiod and temperature (12L∶12D; 17±1°C) with peak values at mid-light phase. The 16L∶8D; 22±1°C treatment reversed the diurnal rhythm of 5-HT, but not that of DA and NA. However, there was an overall increase in the levels of the catecholamines on exposure to the long photoperiod and high temperature. The administration of melatonin in the fish held on 16L∶8D; 22±1°C regime restored the 5-HT rhythm to that of the 12L∶12D; 17±1°C control group, but with elevated mid-photophase value. However, there was no effect of the indole treatment on the 5-HT rhythm under the 12L∶12D; 17±1°C regime. Melatonin causes a significant reduction of NA level in both the groups, while DA level did not change in either group.