DEVELOPMENT OF VASOACTIVE-INTESTINAL-PEPTIDE MESSENGER-RNA RHYTHM IN THE RAT SUPRACHIASMATIC NUCLEUS

Development of the daily rhythm of vasoactive intestinal peptide (VIP) mRNA in the rat suprachiasmatic nucleus (SCN), a main locus of circad ian oscillation, was investigated by in situ hybridization. The phenot ypic expression of VIP neurons occurred in two developmental stages in the ventrolateral portion of the SCN (VLSCN): the first was found bef ore birth in the rostral part, and the second occurred in the main par t between postnatal day (P) 10 and P20. The latter period coincided wi th the time that the massive VIP-efferent fibers project to the subpar aventricular zone, In the adult and P20, the VIP mRNA signals of the S CN showed a clear diurnal rhythm with a trough in the light phase and a peak in the dark phase under light/dark (LD) conditions, but under c onstant dark (DD) conditions, no VIP mRNA fluctuations were observed. At P10, however, it was found that SCN VIP mRNA showed a peak at the t ransition from night to day and a trough at early dark period in LD co nditions, in sharp contrast to the night peak in the adult rhythm. In DD conditions, a light-phase peak and a dark-phase trough were also ob served at P10, contrasting the arrhythmic feature at adult stage. The present findings suggest that daily VIP rhythm was first generated in the early developed clock-controlled rostral SCN neurons, and later re gulated by light-dependent main VLSCN neurons.