CIRCADIAN-RHYTHM OF INTRAOCULAR-PRESSURE IN THE RAT

To define the characteristics of the diurnal variation of intraocular pressure (IOP) in eyes of awake rats, ten male brown Norway rats were entrained to a 12-hour light:12-hour dark (12L:12D) lighting schedule and were conditioned to IOP measurement with the TonoPen XL tonometer while awake, using only 0.5% proparacaine HCl anesthesia. The IOP meas urements were performed in 4 experiments: Preliminary-IOP was measured at 6-hour intervals in both eyes of each animal, to determine correla tion between right and left eyes; Light:Dark-lighting remained the sam e as in the preliminary experiment, but the measurement schedule was a ltered so that measurements were obtained at 4-hour intervals in alter nating eyes, over two 24-hour light cycles; Dark:Dark-animals were pla ced in constant dark (OL:24D) and, after 72 h, measurements were obtai ned at 4-hour intervals in alternating eyes. Animals were then re-entr ained to the previous 12L:12D schedule for 7 days, after which they we re returned to constant dark and the experiment was repeated; and Dark :Light-animals were entrained to a reversed light:dark cycle (12D:12L) for 28 days, after which measurements were obtained in the same fashi on as in the Light:Dark experiment. Close agreement was found between right- and left-eye IOPs. Animals on a 12L:12D schedule exhibited lowe st IOP while the lights were on (19.3 +/- 1.9 mm Hg), and highest (31. 3 +/- 1.3 mm Hg) while the lights were off. Pressure changes anticipat ed the change from light to dark and dark to light. This pattern persi sted in constant dark, and was reversed when the cycle was changed to 12D:12L. Brown Norway rats possess a regular rhythm of IOP that is ent rained by the cycle of light and dark, and persistence of this rhythm in constant dark establishes it as a circadian rhythm. Furthermore, ou r results indicate that reliable and physiologically meaningful IOP me asurements can be obtained in awake rats using the TonoPen XL tonomete r.