Effects of environmental cross-over on inflammation-induced nociception

Abstract

There are many studies in which environmental enrichment (EE) has focused on modifying animal behavior leading to improved cognitive functions, such as learning and memory. There are also increasing numbers on how EE positively affects a subject's pain tolerance by increasing thresholds. In order to better understand how an environment can affect nociceptive behaviors, this study looked at the effects of cross-over from one environment to another after induced inflammation. Male Sprague-Dawley rats were initially randomly assigned three to a cage; four of the cages were standard, only containing bedding and food/water, while the other four cages were supplemented with toys (enriched). Baseline behavioral measurements, paw thickness, paw withdrawal latency (PWL) and paw withdrawal threshold (PWT) were collected. After five weeks, CFA was injected into the left hind paw of all 24 rats and the cross-over was conducted. Two of the initial standard cages stayed standard, while the remaining two standard cages changed to enriched. Two of the initial enriched cages changed to standard, while the remaining two enriched cages stayed enriched. The behavioral measurements were then conducted post CFA at 2 hours, 7 days, 14 days, 21 days, 28 days, and 35 days. A repeated measures ANOVA was conducted on paw thickness for all four treatment groups (standard-standard, standard-enriched, enriched-standard, enriched-enriched) at baseline, 2 hours, 7 days, 14 days, 21 days, 28 days, and 35 days post-inflammation. F₁₀.₁₂₅,₄₄=1.403 with no significant difference among home cage treatment groups (p-value = 0.180). There was, however, a significant difference across testing times (2h, 7d, 14d, etc.; p-value < 0.001) and F₃.₄₈₅,₄₄=1.403. The repeated measures ANOVA conducted on PWT revealed F₃.₈₀₁,₄₄= 2.520 and no significant difference among the four housing groups (p-value = 0.311). There was a slight significant differe