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 circadian IOP elevation in the early dark phase come from the central nervous system (CNS) via the ocular sympathetic nerves. Its neural circuitry 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 dark were determined. Bilateral lesions of the suprachiasmatic nuclei were made by heat probe and verified by histology. Change in the circadian IOP elevation was monitored for up to 3 months. After the lesion, there was a significant reduction of the circadian IOP elevation for 2 weeks. Elevations in the plasma concentration of melatonin and the aqueous humor concentration of norepinephrine (NE) in the early dark phase were significantly reduced. When exposed to constant light, the residual 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, concentrations of plasma melatonin and aqueous humor NE regained the circadian pattern. These results indicate that the suprachiasmatic nucleus is in the neural circuitry which propagates the circadian elevation of IOP in rabbits.