Annual variation of plasma melatonin following pinealectomy and melatonin administration in Indian tropical rodent, Funambulus pennanti

Abstract

Pinealectomy (Px) and melatonin (Mel) administration is known for its effect on reproductive and behavioural rhythms. To date no report is available to suggest the effect of Px and Mel administration on the annual variations of plasma Mel itself in any tropical seasonally breeding rodent. We report here that a statistically significant annual oscillation of Mel existed in sham operated control squirrel, F. pennanti. The exact peak value of Mel was noted some time in November (~13th ± 8 days), which was certainly due to short photoperiod and low ambient temperature. Px decreased the melatonin level most significantly. Mel injection to Px squirrel entrained the phase of Mel, though the amplitude was not completely achieved but had a peak melatonin concentration in January (~2 months phase delay). Exogenous Mel administration to sham operated squirrel decreased amplitude of melatonin during the months of March-July leading to early gonadal regression but could not alter the amplitude of Mel in November and December, while the peak value was phase delayed by ~2 months (January). We may suggest, therefore, that the pineal gland of this squirrel is essential for maintaining its own rhythm, which is an adaptive significance of this tropical rodent for the perpetuation of the species.