Enriched environment and the recovery from inflammatory pain: Social versus physical aspects and their interaction

In this study, we aimed at comparing the effect of the social versus the physical enrichment of the environment on inflammatory pain. Hence, a rat model of carrageenan-induced knee inflammation was used. Four housing conditions were investigated: a physically enriched environment (PE), a socially enriched environment (SE), an enriched environment (EE) (i.e. physically and socially enriched) and a restricted environment (RE) (i.e. non-physically or socially enriched housing). Mechanical allodynia was assessed using the von Frey test preoperatively and at day post-operative (DPO) 1, 3, 7, 10, 14, 17, 21, 24 and 28. Besides, anxiety was evaluated at DPO29, using the Elevated Plus-Maze test. Results show that RE housing resulted in a duration of mechanical allodynia of 4 weeks and of only 3 weeks in EE housing. Housing in a physically enriched environment also resulted in a reduction of the duration of mechanical allodynia of 1 week. Finally, if housed in a SE, the mechanical allodynia lasted for 3 weeks and an half. From these data, we conclude that both physical and social aspects of the environment are involved in the reduction of inflammatory pain duration, although the PE has a larger effect than the SE in this experimental setting. Interestingly, an inter-dependent relationship was noted between the PE and SE. Moreover, no significant difference in the rat anxiety was measured between groups, suggesting that the pain outcomes are likely not biased by the mean of potential housing condition-induced anxiety.