Response of reptilian gonad to melatonin.

The effect of melatonin treatment on the weight and volume of gonads and accessory sex organs of sham-operated and pinealectomized lizards (Calotes versicolor) was examined during the different phases of the annual gonadal cycle of the animal. Melatonin injections to sham-operated lizards inhibited gonadal activity during the progressive (April-May), reproductive (June-July) and early regression (September-October) phases of the cycle, but had no effect on the atrophied gonad present during the inactive and early progressive phases. Pinealectomy increased the weight and volume of the gonads and accessory sex organs at all phases of the gonadal cycle. Melatonin treatment in pinealectomized lizards counteracted the effect of pinealectomy. It is concluded that melatonin is antigonadotropic in this species of tropical lizard.