Environmental housing affects the duration of mechanical allodynia and the spinal astroglial activation in a rat model of chronic inflammatory pain.

In this study, we aimed at investigating the effect of an enriched environment (EE) on the recovery from chronic inflammatory pain. Inflammatory pain was induced by the injection of 2 mg of carrageenan (CAR) into the right knee of male Sprague-Dawley rats (n=34). Rats were housed either singly (S-housed) or in an EE (EE-housed). The EE consisted of a large cage (L x W x H=2.0 x 1.0 x 0.8 m) containing various attributes (e.g. running wheels, shelter house, climbing frame). Withdrawal response to von Frey filament was used to assess mechanical allodynia at days post-operative (DPO) -1, 1, 7, 14, 21 and 28. S-housed animals showed a marked tactile sensitivity in the ipsilateral paw from DPO1 to DPO21. Four weeks after the CAR injection, S-housed rats were no longer allodynic. In contrast, EE-housed rats showed a significantly faster recovery: already at DPO21, they were no longer allodynic. In a first attempt to analyse the possible role of astroglial cells in the EE-induced effect, histological analysis at DPO21 was performed. Immunohistochemical staining of the spinal dorsal horn at L3-L5 indeed showed that spinal levels of astroglial activation are different between the two housing groups and therefore may play a role in the EE-induced effect on the duration of mechanical allodynia. In conclusion, our results showed that EE-housing results in a reduced duration of mechanical allodynia in chronic inflammatory pain in rats. Astroglial activation is suggested to be involved in this housing effect.