PHOTIC CONTROL OF NITRIC-OXIDE SYNTHASE ACTIVITY IN THE HAMSTER SUPRACHIASMATIC NUCLEI

Circadian rhythms are controlled by an endogenous clock, which in mamm als is located in the hypothalamic suprachiasmatic nuclei (SCN). A rol e for nitric oxide in circadian responses to light has been Indicated. To test the role of nitric oxide synthase (NOS) in the SCN and in cir cadian responses to light, we examined NOS specific activity at differ ent time points and photic conditions. NOS activity was determined by the conversion of H-3-arginine to H-3-citrulline. NOS enzymatic activi ty in the SCN was significantly higher during the dark phase than duri ng the day, without any changes in the levels of the NOS protein. Howe ver, this difference disappeared when animals were placed under consta nt darkness, and NOS activity was similar at CT 8 and CT 18 (with CT 1 2 defined as the onset of the subjective night). When 5-min light puls es were administered at these time points (when light would induce no phase shift or a phase advance, respectively), NOS activity was signif icantly increased almost equally. A spectrophotometric assay was used to determine NO content in the SCN, showing relatively high constituti ve levels enhanced by 100 mu M glutamate. These results suggest that N OS activity is not controlled by the circadian clock, although it migh t mediate some of the effects of Light on biological rhythms. (C) 1998 Elsevier Science B.V, All rights reserved.