Why does castrated male odor attract sexually active male rats?–Attractivity induced by hypothalamus-pituitary-gonad axis block.

Neural and Hormonal Basis of Opposite-Sex Preference by Chemosensory Signals

“…We considered it unlikely that the lack of testosterone (T) due to castration may newly produce an attractant for males, but that the lack of T disinhibits the negative feedback of the hypothalamus-pituitary-gonadal (HPG) axis, resulting in increased circulating levels of gonadotropin-releasing hormone (GnRH) and gonadotropins, and consequently, that those became male incentive odors. Therefore, we examined the effect of GnRH antagonist administration to castrated male rats, and found that the castrated male attractiveness decreased to levels comparable to gonadally intact males [ 56 ]. Furthermore, we compared the responses of sexually active male rats to the odors of castrated males with and without surgical removal of the pituitary gland (hypophysectomy, HPx) to remove the source of gonadotropin, demonstrating longer exploration of the odor of pituitary-intact mice than that of HPx males [ 56 ].…”

“…Therefore, we examined the effect of GnRH antagonist administration to castrated male rats, and found that the castrated male attractiveness decreased to levels comparable to gonadally intact males [ 56 ]. Furthermore, we compared the responses of sexually active male rats to the odors of castrated males with and without surgical removal of the pituitary gland (hypophysectomy, HPx) to remove the source of gonadotropin, demonstrating longer exploration of the odor of pituitary-intact mice than that of HPx males [ 56 ]. However, the odor of HPx males was still more attractive than that of gonadally intact males, indicating that the attractiveness of castrated males is not due only to elevated gonadotropin levels but also to increased GnRH.…”

“…However, the odor of HPx males was still more attractive than that of gonadally intact males, indicating that the attractiveness of castrated males is not due only to elevated gonadotropin levels but also to increased GnRH. Furthermore, we examined the effect of administration of human and equine chorionic gonadotropins (hCG and eCG) on luteinizing hormone (LH) and follicle stimulating hormone (FSH) action in the rat, respectively; in HPx males, the recovery of attractiveness followed eCG, but not hCH, injection [ 56 ]. We thus concluded that the attractiveness of castrated males to sexually active male rats is due to elevated levels of both GnRH and FSH due to the lack of circulating T.…”