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Day-night variation of the preprosomatostatin mRNA level in the suprachiasmatic nucleus (SCN), the site of the circadian pacemaker, was determined in rats maintained under light-dark cycles by semiquantitative Northern blot hybridization of micropunched tissues. Preprosomatostatin mRNA abundance in the SCN showed significant variation during a day with a peak level at around light on and a trough at around 4 h after light off. This time course was essentially identical with that previously found in blinded rats, indicating that environmental light did not produce an immediate effect on the preprosomatostatin mRNA level in the SCN. Moreover, this rhythm in the preprosomatostatin mRNA appeared specific to the SCN, since levels in the cortex and anterior hypothalamus did not oscillate even under light-dark conditions.

Entrainment of the circadian pacemaker in the suprachiasmatic nucleus is accomplished by two neural pathways, the retinohypothalamic and geniculohypothalamic tracts. The geniculohypothalamic tract, which originates from the intergeniculate leaflet and a portion of the ventral lateral geniculate nucleus, is composed of fibers immunoreactive to neuropeptide Y. To assess the processing of photic information by the geniculohypothalamic tract, neuropeptide Y immunoreactivity in the suprachiasmatic nucleus of rats kept under various external lighting conditions was determined by enzyme immunoassay of micropunched tissues. Neuropeptide Y levels in the suprachiasmatic nucleus steadily increased when rats were exposed to continuous light and reached a peak in 2 h before returning to basal level. The amount of increase did not depend on intensity and duration of light exposure. A light pulse as short as 5 min elicited a similar rise in neuropeptide Y, indicating that the response is due to the sudden transition from dark to light. This response, however, was only observed when the dark to light transition occurred at circadian time 0 (subjective dawn) of the pacemaker. A light pulse at circadian time 0, which effectively induces the increase in neuropeptide Y level, does not significantly shift the phase of the circadian rhythm. This observation indicates that the photic pathway utilizing neuropeptide Y may be functional only when the endogenous circadian rhythm is synchronized to external light and dark cycles. Administration of an excitatory amino acid antagonist (MK-801) blocked the increase of neuropeptide Y by light, while an agonist (N-methyl-D-aspartate) induced similar facilitatory effects to that of light on the neuropeptide Y level in the rat suprachiasmatic nucleus. These results suggest that the geniculohypothalamic tract processes photic information so as to facilitate distinction of the transition between light and darkness that occurs either at subjective dawn or dusk.

The content of substance P (SP)-like immunoreactivity (LI) within the suprachiasmatic nucleus (SCN) of rats was determined by enzyme immunoassay to evaluate the effect of light on SP-LI in the rat SCN. Male rats were kept under various lighting conditions: light-dark cycles, constant darkness, continuous light exposure for 24 h or light pulse interrupting constant darkness. Animals were also subjected to ocular enucleation. The present study showed that SP-LI in the SCN was unaffected by environmental lighting conditions or by bilateral ocular enucleation. Immunohistochemical studies also confirmed that SP immunoreactivity, which was found in the ventrolateral (VL) subdivision of the SCN, was not reduced significantly even after ocular enucleation. These results suggest that, in contrast to other neurotransmitters in the VL portion of the SCN such as vasoactive intestinal polypeptide (VIP), gastrin releasing peptide (GRP) and neuropeptide Y (NPY), SP level in the SCN is quite stable to light and arises from an area other than the retina.

The suprachiasmatic nucleus (SCN) of the hypothalamus has been established as the locus of the circadian pacemaker in mammals. The SCN is histochemically divided into two subdivisions: dorsomedial and ventrolateral subfields. The dorsomedial SCN is characterized, in part, by dense somatostatin-like immunoreactivity (SS-LI), but its functional significance in circadian pacemaking remains unclear. Our previous study revealed that 24 h SS-LI contents of the SCN in rats kept under light-dark (LD) conditions or blinded by orbital enucleation showed a distinct circadian rhythm. In the present study, 24 h SS-LI contents of the SCN in sighted rats kept under constant darkness (DD) conditions for prolonged periods were measured by enzyme immunoassay. Cellular contents of SS-LI exhibited a clear circadian rhythm on the third day of constant darkness (DD) with a peak at circadian time (CT) 5, corresponding to the time of peak levels found in LD conditions and in enucleated animals. This endogenous free-running rhythm continued to oscillate without attenuation of the amplitude even at 14 days in constant darkness. Moreover, SS-LI rhythm was found to be similar to the vasopressin rhythm in the SCN. In summary, these findings further strengthen the idea that the cellular content of SS-LI in the SCN is under the control of the endogenous circadian pacemaker.

The suprachiasmatic nucleus (SCN) of the anterior hypothalamus contains a circadian pacemaker in mammals. We determined the circadian profiles of arginine-vasopressin (AVP), a major peptide in the dorsomedial SCN, in rats under light-dark (LD), constant dark (DD) and constant light (LL) conditions. Under LD conditions, AVP levels in the SCN showed circadian rhythmicity with a peak at early light phase and a broad trough during the dark phase. This rhythm in the AVP contents was maintained even after 14 days of free-running under DD conditions and 3 days under LL conditions. These circadian patterns of AVP are similar to those of somatostatin, another peptide in the dorsomedial SCN. This indicates a common mode of regulation for peptides in this subfield of the SCN.