Testosterone augments neuronal Fos responses to estrous odors throughout the vomeronasal projection pathway of gonadectomized male and female rats.

Pheromonal signals emanating from female rats' soiled bedding have previously been shown to attract male conspecifics and to augment the number of Fos-immunoreactive neurons present in portions of the vomeronasal projection pathway, ranging from the accessory olfactory bulb (AOB) to the medial preoptic area (mPOA) of gonadectomized, testosterone-treated male as well as female subjects. In the present study we extended these findings by showing that these neuronal Fos responses to estrous odors occurred only in gonadectomized subjects which received testosterone propionate (TP), as opposed to oil vehicle, at the time of testing. Previously, when treated with TP, all subjects had displayed mounting with an estrous female. In subsequent tests, gonadectomized male and female subjects spent significantly more time investigating soiled estrous bedding as opposed to clean or anestrous bedding, again, provided they were receiving TP. We propose that testosterone facilitates odor-induced neuronal Fos expression either via its conversion to estradiol, and the subsequent action of this steroid at estrogen response elements on the c-fos gene, or via some indirect mechanism involving centrifugal control of AOB neurotransmission. The ability of female odors to stimulate equivalent numbers of Fos-IR cells in gonadectomized, TP-treated male and female rats indicates that the functional responsiveness of neurons throughout the vomeronasal projection pathway is made more male-like by the fetal actions of testosterone in female as well as in male rats. Previous studies established that males have more neurons than females in several segments of the vomeronasal pathway; however, this morphological sexual dimorphism does not account for the observed isomorphic neuronal Fos responses to female odors.