NEUROCHEMICAL ORGANIZATION OF CIRCADIAN-RHYTHM IN THE SUPRACHIASMATICNUCLEUS

The circadian rhythm in mammals is under control of the pacemaker loca ted in the suprachiasmatic nucleus (SCN) of the hypothalamus. This tin y nucleus contains a number of neurochemicals, including peptides, ami nes and amino acids. Heterogeneous distribution of these neurochemical s defines the substructures of the SCN. In the present review, functio nal significance of such neurochemical heterogeneity in the SCN is dis cussed in the light of circadian patterns of the concentrations of the se neurochemicals in the SCN and their effects on SCN neurons in in vi tro slice preparation. In particular, the hypothesis that the dorsomed ial SCN is involved in maintaining the circadian rhythm, while the ven trolateral SCN is involved in adjusting the phase of the rhythm, is cr itically discussed. These considerations suggest that distinct sub-com ponents of the SCN as marked by neurochemicals, interact with each oth er and this organizational architecture could be the basis of the prop er operation of the circadian time keeping system in this nucleus.