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Abstract

Conditioned place preference (CPP) is a classical conditioning paradigm used to evaluate the rewarding or aversive properties of a stimulus. A stimulus can be an audio, visual, or sensory prompt but can also be stimuli associated with behaviors. Environments associated with sexual and aggressive encounters can become rewarding to both male and female Syrian hamsters regardless of social status. However, we have observed that individually-housed, non-aggressive hamsters find social interaction without aggression or sexual behaviors rewarding. Therefore, we expanded upon our previous experiments using CPP to test the hypothesis that group-housed, male hamsters (n=12) can develop a preference for a negative social experience such as social defeat. The CPP paradigm consisted of an initial preference test (15 min X 2), a conditioning phase (10 min X 5 days), and a final preference test (15 min). The hamster was placed into a CPP apparatus containing two main testing chambers, black and white, connected by a neutral, clear chamber. The pretest was conducted to establish a baseline for comparison to the posttest to determine changes in the amount of time spent in either (black/white) chamber. An individually-housed male (n=11) was paired with a group-housed male in their non-preferred chamber for conditioning. The order of placement in the chambers was alternated daily. Control animals (n=5) were used to evaluate effects of habituation across conditioning trials. The result of our experiment showed that group- housed animals developed a CPP for social defeat. We observed that the preference scores increased from 0.34 (± 0.01) to 0.41 (± 0.04), p = .08 and the difference scores decreased from 243.67 (± 21.72) to 171.25 (± 60.34), p < .05. There were no significant differences between pretest and posttest scores for controls for both preference scores and for difference scores from pretest to posttest. The results of this experiment suggest that that our subjects developed a CPP for social defeat and that their response may have been influenced by the type of social defeat. We believe that an escapable defeat with different aggressors’ parings, as was conducted in our experiment, produced social interaction with novelty but with lower stress levels, leading the animals to develop a CPP for social defeat. The hamster’s reactions to the experiment conducted suggest that their response may have been influenced by the type of social defeat that was experienced. There are two types of social defeat models: inescapable and escapable. An inescapable defeat implies that an animal cannot avoid its aggressor whereas an escapable defeat implies that the animal can. Animals exposed to an inescapable defeat have been found to produce generalized aversion and avoidance towards not only a familiar aggressor but a novel, non-aggressive aggressor with the avoidance greater with the familiar aggressor than the unfamiliar; whereas, animals exposed to an escapable defeat only produce avoidance towards its previous aggressor. However, our previous and current research indicates that all Syrian hamsters regardless of social (aggressive/submissive) and housing (group/individual) status all formed a CPP for social behavior, albeit social aggression or social defeat. Our CPP experiment paradigm, similar to that of an inescapable defeat model in that the animal can avoid its aggressors, perhaps produced a positive CPP for social defeat because the hamsters found the controllability of the defeat with novel aggressors across the conditioning trials to be stimulating. While, the hamsters displayed a CPP for social defeat they were not completely resilient and demonstrated higher vigilance to their surroundings as evidenced by the increased duration in the neutral chamber following social defeat. The hamsters’ behaviors suggest that the conducted experiment produced a combination of mild social defeat with novel interactions that produced less aversion and perhaps lower generalized anxiety which was sufficient to develop a CPP for social defeat.

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