Enduring effects of environmental enrichment from weaning to adulthood on pituitary-adrenal function, pre-pulse inhibition and learning in male and female rats

Abstract

Environmental enrichment (EE) increases stimulation and provides richer sensory, cognitive and motor opportunities through the interaction with the social and physical environment. EE produces a wide range of neuroanatomical, neurochemical and behavioural effects in several animal species. However, the effects of EE have mainly been studied shortly after the treatment, so its long-lasting effects remain to be elucidated. Thus, we studied in male and female Sprague-Dawley rats the enduring effects of EE on tasks that measured emotional reactivity, social exploration and memory, sensorimotor gating and learning. After weaning, rats reared in EE were housed in single-sex groups of 12-14 in enriched cages during 12 weeks, whereas control rats were housed in single-sex groups of 2-3 animals in standard cages. Then, all rats were housed in pairs and successively exposed to different tests between 4 and 60 weeks post-EE. The results indicated that animals of both sexes reared in EE gained less weight during the enrichment period; differences disappeared in females during the post-EE period, but were maintained intact in males. Rats reared in EE showed an altered daily pattern of corticosterone and a lower hormone response to a novel environment (hole board, HB), although no differences in ACTH were found. EE resulted in more exploratory behaviour in the HB and higher number of entries in the open arms of the elevated plus maze (with no changes in the time spent in the open arms), suggesting a greater motivation to explore. Unexpectedly, rats reared in EE showed reduced pre-pulse inhibition (PPI), a measure of sensorimotor gating, suggesting lower capability to filter non-relevant information compared with control rats. EE increased social exploratory behaviour towards juvenile rats and social discrimination in males, but decreased social discrimination in females. Finally, in the Hebb-Williams maze, rats reared in EE showed better performance in terms of reduced number of errors and shorter distances travelled in the mazes. It is concluded that EE exposure from weaning to adulthood has important and long-lasting consequences on physiological and behavioural variables, most of them similar in both sexes, although sex differences in response to the EE are also reported.