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

ABSTRACT

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]