Pineal serotonin N-acetyltransferase and hydroxyindole-O-methyltransferase activities in vizcacha (Lagostomus maximus maximus) maintained under various photoperiods.

The experiments described in the present paper were aimed to study the vizcacha pineal N-acetyltransferase (NAT) and hydroxyindole-o-methyl-transferase (HIOMT) activities under different lighting regimens. Pineal NAT activity (LD 12:12) exhibits a marked diurnal variation, which decreased during the illumination hours and increased 3-fold after 4 h in darkness. NAT activity of the vizcacha exposed to permanent darkness exhibited an abrupt increase, values being lower during permanent illumination. Vizcacha pineal HIOMT activity (LD 12:12) does not exhibit significant change during the light and darkness phases. In animals exposed continuously to darkness HIOMT activity increased, while under constant illumination a decrease of HIOMT activity to about 1/7 of those under permanent darkness was found. When vizcachas were exposed to natural illumination (day-night), pineal HIOMT activity did exhibit a diurnal rhythm. Either continuous darkness or continuous light abolished the daily variations of vizcacha pineal HIOMT activity. Our results also show that permanent darkness abolished the daily variations of NAT activity suggesting an exogenous rhythm.