Caffeine injection in the dark phase prolongs the nocturnal rise in serotonin N-acetyltransferase activity and melatonin content in the pineal gland of male rats

Caffeine, an important member of methylxanthines, induced a prolonged nocturnal rise in pineal melatonin content and an increase in its rate-limiting enzyme serotonin N-acetyltransferase [NAT] activity. The highest levels were reached five hours after subcutaneous caffeine injection to male rats in the dark phase, where the NAT activity increased from 920 +/- 70 pM pineal-1h-1 in the control group to 1190 +/- 120 pM pineal-1h-1 [p<0.001] in the treated group. The pineal melatonin content, as well, was elevated from 520 +/- 40 pg pineal-1 in the control group to 1120 +/- 80 pg pineal-1 [p<0.001] in caffeine treated group. These changes could be attributed to the depressive effect of caffeine on the activity of phosphodiesterase [PDE], the enzyme responsible for the hydrolysis of the intracellular second messenger cyclic adenosine monophosphate [cAMP]

Pituitary gondatrophin levels in female Syrian hamsters exposed to short photoperiod

Pituitary gonadotrophin levels in female golden Syrian hamsters [Mesocricetus auratus] were elevated by shortphotoperiod treatment than their long photoperiod treated controls. It was found that after short period of treatment, gonadotrophins resemble the same pattern as in pineal-induced gonadally regressed female hamsters. The date of the present study show that although short photoperiod elevated the pituitary levels of LH and FSH than those of the long photoperiod treated controls, still there is an afternoon surge at proestrous for LH and at diestrous for FSH