Preoperative housing in an enriched environment significantly reduces the duration of post-operative pain in a rat model of knee inflammation.

The influence of the environment on clinical post-operative pain received recently more attention in human. A very common paradigm in experimental pain research to model the effect of housing conditions is the enriched environment (EE). During EE-housing, rats are housed in a large cage (i.e. social stimulation), usually containing additional tools like running wheels (i.e. physical stimulation). Interestingly, only postsurgical housing effect on post-operative pain was developed during clinical and experimental studies while little is known on the influence of preoperative housing. In this study, our aim was to investigate the influence of housing conditions prior to an operation on the development of post-operative pain, using a rat model of carrageenan-induced inflammatory pain. Four housing conditions were used: a 3-week pre-housing in standard conditions (S-) followed by a post-housing in an EE; a 3-week pre-housing in EE followed by a post-operation S-housing; a pre- and post-housing in EE; a pre- and post-S-housing. The development of mechanical allodynia was assessed by the means of the von Frey test, preoperatively and at day post-operative (DPO) 1, 3, 7, 10, 14, 17, 21, 24 and 28. Our results show that a 3-week preoperative exposure to EE leads to a significant reduction in the duration of the carrageenan-induced mechanical allodynia, comparable with a post-operative exposure to EE. Strikingly, when rats were housed in EE prior to as well as after the carrageenan injection into the knee, mechanical allodynia lasted only 2 weeks, as compared to 4 weeks in S-housed rats.