Effects of estrous stage and time of day on prepulse inhibition in female rats.

Prepulse inhibition (PPI) of the acoustic startle response is a measure of sensory motor gating, and is affected in various neuropsychiatric disorders. Although PPI has been used extensively to study both the neural effects of such conditions, as well as in the search for animal models, a number of critical issues have been encountered. Published methods for testing PPI vary widely across many parameters, two of the most common being the phase of the light/dark cycle during which the subjects are tested and the inclusion or exclusion of females. While previous research has attempted to clarify the effect of these factors, results for both human and animal studies have often been contradictory. This study investigated the relevance of the estrous cycle and time of day as variables that may influence PPI in adult female Sprague-Dawley rats. Results indicate that PPI is not affected by estrous phase, but may be affected by the time of day of testing, particularly at higher prepulse levels. At the 86 dBs prepulse level, rats tested during the light phase of the light/dark cycle displayed significantly lowered PPI as compared to the animals tested during the dark phase. Additionally, other measures such as baseline startle, habituation and activity during testing did not vary across the estrous or light cycles. These findings indicate that while estrous phase does not have any effect on PPI in female Sprague-Dawley rats when tested under these parameters, the time of day during which testing occurs does have the potential to alter PPI.