SUPRACHIASMATIC NUCLEUS IN THE NEURAL CIRCUITRY FOR THE CIRCADIAN ELEVATION OF INTRAOCULAR-PRESSURE IN RABBITS

Intraocular pressure (IOP) in light-dark entrained rabbits changes in a circadian pattern. Previous studies indicate that signals for the ci rcadian IOP elevation in the early dark phase come from the central ne rvous system (CNS) via the ocular sympathetic nerves. Its neural circu itry in the CNS was totally unclear. In the present study, the role of the suprachiasmatic nucleus in the circuitry was investigated. Adult New Zealand albino rabbits were entrained in a daily 12 hr/12 hr light -dark cycle, and their circadian IOP elevations around the onset of da rk were determined. Bilateral lesions of the suprachiasmatic nuclei we re made by heat probe and verified by histology. Change in the circadi an IOP elevation was monitored for up to 3 months. After the lesion, t here was a significant reduction of the circadian IOP elevation for 2 weeks. Elevations in the plasma concentration of melatonin and the aqu eous humor concentration of norepinephrine (NE) in the early dark phas e were significantly reduced. When exposed to constant light, the resi dual circadian IOP elevation in the postoperative rabbits was further reduced to the level seen in intact rabbits exposed to constant light. In postoperative weeks 6-10, the circadian IOP elevation returned to the initial value. A full, inhibitory effect of constant light on the circadian IOP elevation was observed. In postoperative months 2-3, con centrations of plasma melatonin and aqueous humor NE regained the circ adian pattern. These results indicate that the suprachiasmatic nucleus is in the neural circuitry which propagates the circadian elevation o f IOP in rabbits.