Diurnal behavioral outcomes and BBB integrity in adult male SHR and WKY rats.

Day and night neurobehavioral tests were performed for Spontaneously Hypertensive rats (SHR) and Wistar Kyoto rats (WKY) to establish an optimal testing window. Also, blood-brain barrier (BBB) integrity in brain areas related to cognition was measured to determine strain-specific differences. Six months old male SHR and WKY were randomly divided into day (i.e., 9-11 a.m.) and nighttime (i.e., 7-9 p.m.) testing regimen. Neurobehavioral tests included: Y-maze spontaneous alternation, open field, novel object recognition (NOR), and tail suspension test (TST). BBB integrity was measured in the cortex, hippocampus and striatum. WKY showed higher recognition memory at night compared to SHR and daytime test groups. Working memory and exploratory behavior were higher for SHR at both testing times when compared to WKY. During the day and when compared to WKY, SHR showed significant depression-like behavior which was absent during the night. SHR exhibited a higher anxiety-like behavior at both testing times when compared to WKY. Immunofluorescent signal for FITC-Dextran significantly increased in brain regions of SHR compared to WKY, but BBB tight junction proteins decreased suggesting impaired BBB integrity in SHR. In sum, NOR and TST are best performed during the dark phase to observe actual baseline outcomes for SHR and WKY. Neurobehavioral outcome differences between strains are independent of BBB integrity.