Circadian variations in intracranial pressure and translaminar pressure difference in Sprague-Dawley rats.

PURPOSE To study the circadian (24-hour) change in intracranial pressure (ICP) in conscious, freely moving rats and to project the circadian change in translaminar pressure difference. METHODS Telemetric pressure transmitters were implanted to monitor ICP in the lateral ventricle in nine light-dark-entrained Sprague-Dawley rats. ICP and locomotor activity data were collected. The mean results for the 12-hour light period and the 12-hour dark period were compared. The light-dark change in ICP was also determined in six rats under an acute 24-hour constant dark condition. The circadian translaminar pressure difference was projected based on the ICP data and the previously established circadian pattern of intraocular pressure (IOP). RESULTS Under the standard light-dark condition, the hourly average ICP was relatively constant (7.47-10.90 mm Hg). The light-dark ICP difference was -0.11 ± 1.45 mm Hg (mean ± SD, P = 0.823), whereas the locomotor activity was significantly higher during the dark period (P < 0.01). Under the acute constant dark condition, the subjective light-dark ICP difference remained small. Compared with a significant light-dark IOP elevation of 5.15 ± 4.47 mm Hg (P = 0.037) in rats housed under the same laboratory conditions, the light-dark ICP variation was considered minimal. The translaminar pressure difference was projected to be 5.26 mm Hg higher in the dark period (mean, 17.10 mm Hg) than in the light period (mean 11.84 mm Hg). CONCLUSIONS There is no significant circadian ICP variation in Sprague-Dawley rats. The translaminar pressure difference is projected to be higher during the dark period because of the change in IOP.