A study of the relationship between photoperiod and pinealocyte granulated vesicles in the golden Syrian hamster.

Previous studies in rabbits and mice have revealed distinct circadian rhythms in the number of pinealocyte granulated vesicles (PGVs) and control of their synthesis and/or secretion by sympathetic nerves. The present study demonstrates the absence of a circadian rhythm in PGV content in hamsters "functionally pinealectomized" by maintenance under long photoperiod (L/D = 14/10 h). On the other hand, a highly significant rhythm of low amplitude was noted in PGVs of hamsters placed in photoperiods (less than 12.5 h) which are known to initiate pineal antigonadotropic activity. Bilateral optic enucleation, which also leads to pineal-mediated gonadal atrophy in the hamster, produced a significant decrease in the number of perivascular PGVs when compared to intact control animals. Daily late afternoon injections of melatonin produced no significant difference in the number of PGVs between treated and control animals at any sample time examined.