Intermale social aggression: Reinstatement in castrated rats by implants of testosterone propionate in the medial hypothalamus

Abstract

Male hooded rats were castrated, subcutaneously implanted with testosterone-filled silastic tubes, and individually housed with an intact adult female rat. An unfamiliar male intruder was introduced into each colony on a weekly basis and the aggressive behavior of the resident male was recorded. When the intermale social aggressive behavior of the resident male toward the intruder reached a high level in terms of a composite aggression score, the subcutaneous testosterone tubes were removed. Weekly tests of aggression toward unfamiliar intruders continued until the aggression of the resident male dropped to a low level for two successive weeks in terms of our composite aggression score. Bilateral implants of pellets of testosterone propionate were then made into the medial hypothalamus or adjacent tissue. A control group was implanted with cholesterol pellets into the medial hypothalamus. During four weekly tests following the implant, rats with testosterone propionate implants in the medial hypothalamus showed increases in lateral attacks, lateral attack duration, bites, and piloerection. The increase in aggression was not consistently displayed by animals with testosterone propionate implants dorsal or anterior to the medial hypothalamus or by animals with cholesterol implants in the medial hypothalamus. These results suggest that the medial hypothalamus or closely adjacent tissue contains testosterone-sensitive neural circuitry modulating intermale social aggression.