SUPRACHIASMATIC NUCLEUS NEUROCHEMISTRY IN THE CONSCIOUS BRAIN - CORRELATION WITH CIRCADIAN ACTIVITY RHYTHMS

The aim of the research reported here was to provide information on th e neurochemical processes that underlie the generation and entrainment of mammalian circadian rhythms. The studies were centered principally around the in vivo brain microdialysis technique for assessing the da ily pattern of neurotransmitter activity in the suprachiasmatic hypoth alamus of freely behaving Syrian and Siberian hamsters. This approach yielded several findings related to the activities of serotonergic and excitatory amino acid systems in the region of the suprachiasmatic nu clei (SNC). Specifically, we found that (1) there were daily variation s in the extracellular concentrations of 5-hydroxyindoleacetic acid (5 -HIAA) and glutamate (GLU) in the SCN region (highest levels of 5-HIAA occurred soon after lights-off , whereas GLU peaked later in the dark phase); (2) the daily rhythm of GLU, but not serotonin, in the SCN re gion appeared to be circadian in nature and was not driven by an exter nal influence; (3) the rhythm in GLU measured in SCN microdialysate in volved a tetrodotoxin-insensitive mechanism and did not appear to be d irectly linked to the expression of locomotor behavior; and (4) direct application of serotonin receptor agonists via the microdialysis prob e suppressed the concentration of extracellular GLU in the SCN region, suggesting that serotonin may modulate GLU release in the SCN.