CIRCADIAN-RHYTHM IN 5-AMINOLEVULINATE SYNTHASE MESSENGER-RNA LEVELS IN THE HARDERIAN-GLAND OF THE SYRIAN-HAMSTER - INVOLVEMENT OF LIGHT-DARK CYCLE AND PINEAL FUNCTION

Aminolevulinate synthase (ALV-S) is the first and rate-controlling ste p in heme biosynthesis. In the current paper we describe a circadian r hythm for the ALV-S mRNA in the Harderian gland (HG) of female Syrian hamsters. ALV-S mRNA levels are low during the day and increase at the end of the light phase to reach maximum values after lights off. Ther eafter, ALV-S mRNA levels decrease progressively to again reach low le vels at the end of the dark phase. The circadian rhythm in ALV-S gene expression is independent of the estrous cycle. The administration of Actinomycin-D prevents the increase in ALV-S mRNA observed in the late light phase, suggesting that the regulation of this circadian rhythm is at the transcriptional level. The exposure of hamsters for 3 days t o continuous light does not abolish the circadian rhythm of ALV-S mRNA in HC but both the evening rise and the amplitude of the nocturnal pe ak are modified under these conditions. The administration of an NMDA receptor antagonist, MK-801, prevents the evening rise of ALV-S mRNA, suggesting that the involvement of light in this rhythm involves the s uprachiasmatic nucleus. Surgical ablation of pineal gland modify the n ormal rhythmicity of ALV-S mRNA levels, advancing the cycle to an earl ier hour. Melatonin administration (25 mu g) in the morning partially prevents the evening rise in ALV-S mRNA, indicating its participation in the regulation of this enzyme.