Effects of continuous light, continuous darkness and pinealectomy on pineal-thyroid-gonadal axis of the female Indian palm squirrel, Funambulus pennanti.

Effects of continuous light (CL; 24L:0D), continuous darkness (CD; 24D:0L) and pinealectomy (Px) were tested on pineal-thyroid-gonadal axis of a tropical seasonally breeding rodent, Funambulus pennanti during its sexually active reproductive phase (February-March). CL had no effect on pineal, thyroid and ovarian functions as judged by weight, plasma level of thyroxine (T4) and estradiol (E2). However, CD reduced significantly thyroid and ovarian weight and plasma T4, and E2 levels. Px was stimulatory for thyroid and ovarian functions under normal photoperiod, and under CL, Px significantly increased thyroid and ovarian weights with no observable changes in their hormonal levels. Px under CD condition prevented the reduction in thyroid and ovarian weights but the hormonal levels were increased. These results suggest that CD could be a effective stimulator for the pineal function and increased pineal activity in this mammal. The inhibitory effect of CD on thyroid and ovarian functions could be due to the increased pineal activity in term of induced melatonin by CD itself.