Relationship between Pineal N-Acetyltransferase Activity, Pineal Melatonin and Serum Melatonin in Rats under Different Lighting Conditions

Pineal N-acetyltransferase (NATase) activity has often been used as an indicator of pineal melatonin (MT) production, and the rhythms of pineal and serum MT levels are assumed to be parallel. However, these issues require further examination in view of some recent findings. In the present study, the 24-hour rhythms of pineal NATase, pineal MT content and serum MT concentrations were examined in groups of rats exposed to four artificial lighting regimens: (a) 16 h of light, 8 h of darkness (LD 16:8) for 2 weeks; (b) LD 12:12 for 2 weeks; (c) LD 4:20 for 2 weeks, and (d) LD 4:20 for 4 weeks. Under the above lighting conditions, the rhythms of pineal NATase, serum and pineal MT contents when tested demonstrated a closely parallel relationship. These results supported the idea that pineal NATase accurately reflected MT synthesis and that the rhythms of pineal and serum MT content were in phase under long and short photoperiods, in spite of extrapineal sites of MT synthesis. Further, the results also demonstrated that the timing of the MT peaks in the serum and the pineal may depend on the length of the adaptation period to the environmental lighting. When the rats were housed under a short photoperiod (LD 4:20), the peak MT levels appeared in the middle of the dark period after 2 weeks but shifted towards the end of the dark period after 4 weeks.