Effects of social housing on conditioned place aversion

Abstract

Although pain represents a source of considerable loss to both economic productivity and quality of life for millions of people worldwide, its underlying causes and sources of modulation are not always well understood. The present study sought to identify differential changes in behavior in response to long-lasting inflammation among rats housed in groups consisting of diverse ratios of inflamed to uninflamed individuals. Behavioral changes were quantified using a model of conditioned place aversion (CPA), with an intraplantar injection of complete Freund's adjuvant (CFA) producing inflammation in the left hindpaw for use as a noxious, conditioning stimulus. It was hypothesized that housing with uninjured cagemates might reduce the effect of conditioning in injured individuals, or vice versa. Persistent inflammation was induced in the CFA-injected individuals, and significant CPA was achieved between baseline and day 15 among subjects on average. However, significant differences were not found in developed place aversion between CFA- and saline-injected (control) individuals, nor was there evidence of a significant difference in conditioning between different housing groups. The conditioning power of inflammation is believed to have been masked by the initial stress of anesthesia and injection. Further analysis of the data revealed the existence of a floor effect , whereby subjects only developed significant aversion if the noxious stimulus was paired with the specific floor of the conditioning apparatus that was initially preferred at baseline. No additional aversion was measured in response to conditioning on the initially aversive, non-preferred floor. This effect can be linked to the biased design of the CPA apparatus, in which subjects exhibited a significant mean preference for the circle floor at baseline compared to the square floor.