N-METHYL-D-ASPARTATE INDUCES PHASE-SHIFTS IN CIRCADIAN-RHYTHM OF NEURONAL-ACTIVITY OF RAT SCN IN-VITRO

The suprachiasmatic nucleus (SCN) acts as a pacemaker for mammalian circadian rhythms. Receptors for excitatory amino acids like N-methyl-D-aspartate (NMDA) and non-NMDA receptors have both been found to play an important role in the transmission of photic information from the retina to the SCN. Therefore, we investigated whether the application of glutamate receptor agonists could reset the phase of the circadian rhythm of SCN firing activity in vitro. Treatment with NMDA (0.1-10 mu M) for 15 min or 1 h during the early part of the subjective night produced phase delay, whereas treatment during the late subjective night caused an advance in phase. The phase-response curve for NMDA was similar to that previously obtained in response to light pulses in vivo. Application of DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid hydrobromide (AMPA) (1 or 10 mu M), a non-NMDA-receptor agonist, also produced a dose-dependent phase delay of SCN activity. The NMDA-induced phase delay was antagonized by an NMDA-receptor antagonist MK-801. These findings suggest that both NMDA and non-NMDA receptors may be involved in the transmission of information to the SCN via the retinohypothalamic tract. In addition, both the advances and delays in phase caused by NMDA were potentiated by cotreatment with neuropeptide Y, whereas AMPA-induced phase delay was not potentiated by neuropeptide Y. This points to a functional link between NMDA and neuropeptide Y receptor-mediated mechanisms in the SCN.