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.

Short-wavelength light reduces circadian elevation of intraocular pressure in rabbits.

In light-dark entrained rabbits, the intraocular pressure (IOP) increases around the onset of dark due to the increased activities of ocular sympathetic nerves. This circadian elevation of IOP can be eliminated by exposing the rabbits to constant white light in the early subjective dark phase. The spectral effectiveness of light causing the reduction in IOP was studied using lights selectively filtered through various optic filters. Light which passed through a long-pass filter, with a cut-on wavelength at 570 nm or 530 nm, caused no change of the circadian IOP elevation. A significant reduction of the IOP elevation was observed with filtered light via the 495 nm long-pass filter and, to a larger extent, with light via the 475 nm long-pass filter. Short-wavelength light (380-480 nm, peak at 432 nm) passed through a band-pass filter completely eliminated the circadian IOP elevation. The aqueous humor concentration of norepinephrine (NE) under the short-wavelength light was significantly lower than the NE concentration under the filtered light via the 530 nm long-pass filter. Sensation of the short-wavelength light plays an important role in synchronizing the circadian elevation of IOP in rabbits.