Photoperiodic response of pineal-thyroid axis of the female Indian palm squirrel, Funambulus pennanti.

Effect of short (SP: 10 L: 14 D: Light: Dark) and long (LP: 14 L: 10 D) photoperiod was tested on pineal-thyroid axis in a tropical seasonally breeding rodent F. pennanti during sexually regressive phase of reproduction. SP reduced thyroid weight and plasma thyroxine (T4) as well as increased melatonin (aMT) while pinealectomy (Px) prevented such an effect of SP. LP and Px plus LP had no significant effect on thyroid activity when compared with their respective controls. These results suggest that SP stimulated pineal activity which increased the concentration of aMT, hence inhibited thyroid gland activity. However, the plasma aMT concentration under LP regime had no significant difference from controls, as the ambient photoperiodic length (13.6 hr) was almost similar to the experimental one (14.0 hr). It is suggested that being innervated from the same ganglion (SCG) which conveys photic stimuli to the pineal gland, the thyroid gland like the pineal is also under the influence of photoperiodic responses. Further pineal/melatonin also influences thyroid function indirectly.