EFFECT OF NMDA RECEPTOR ANTAGONIST MK-801 ON LIGHT-INDUCED FOS EXPRESSION IN THE SUPRACHIASMATIC NUCLEI AND ON MELATONIN PRODUCTION IN THE SYRIAN-HAMSTER

In mammals, circadian rhythms generated by the suprachiasmatic nuclei (SCN) are daily synchronized by a light-dark cycle. Photic information is transmitted to the SCN mainly through the direct retinohypothalami c tract, the neurotransmitters involved being excitatory amino acids. It is also commonly accepted that photoperiodic information coming fro m the retina via the SCN is transduced by the pineal into a nocturnal signal, i.e. melatonin production. Light exposure at night induces (1) an inhibition of melatonin synthesis and (2) an expression of c-fos i n numerous cells of SCN. To determine the role of the NMDA receptor in these effects, we treated Syrian hamsters with ip injections of MK-80 1, a noncompetitive NMDA receptor antagonist. Several subpopulations o f light-sensitive cells in the SCN are affected by MK-801:. According to previous studies, MK-801 inhibits light-induced Fos immunoreactivit y mainly in the most ventral part of the SCN. However, we observed tha t numerous other cells are still activated by light. When light is app lied in the middle of the night, MK-801 pretreatment does not reduce F os-ir in the dorsal SCN. At the beginning of the night, labeled cells in this part of the nucleus appear even more numerous after MK-801. We also found that MK-801 fails to reduce the light-induced inhibition o f melatonin synthesis. Moreover, in control animals, which received no light stimulation, ip injection of MK-801 induces by itself a dose-de pendent inhibition of melatonin production.