Neuronal Fos activation in olfactory bulb and forebrain of male rats having erections in the presence of inaccessible estrous females

Volatile odors from estrous female rats are necessary and sufficient to ind uce non-contact penile erections in male rats. It is not known whether thes e pheromones are detected by the accessory as opposed to the main olfactory system or whether they are processed by forebrain regions that receive olf actory inputs. Using nuclear Fos immunoreactivity as a marker of neuronal a ctivation, we asked how the detection and processing of distal cues from in accessible estrous females, which elicited non-contact penile erections, co mpared with the processing of sensory cues from soiled estrous bedding whic h did not elicit non-contact penile erections. In Experiment I, groups of s exually experienced males were given one of five treatments. A control grou p was placed on clean bedding. A second group displayed noncontact penile e rections when exposed to the smell, sight and sound of an estrous female re strained behind a permeable barrier. A third group was exposed to the same stimuli as the second (an estrous female) but failed to exhibit non-contact penile erections during the first hour of testing. A fourth group was plac ed on soiled estrous bedding, and a fifth group was allowed two ejaculation s with an estrous female. All males were perfused with 4% paraformaldehyde 2 h after the onset of these respective treatments, and their brains were l ater processed for Fos immunoreactivity. Non-contact penile erections were observed in males that were exposed to distal cues from an estrous female b ut not in males exposed to soiled estrous bedding. Males that displayed non -contact penile erections or that were exposed to estrous bedding showed si gnificantly more neuronal Fos immunoreactivity than clean-bedding controls in the nucleus accumbens core and shell, anterior and posterior medial amyg dala, bed nucleus of the stria terminalis and the medial preoptic nucleus. Even greater neuronal Fos responses occurred in these regions in mated male s. In Experiment 2 these same treatments were given to another cohort of se xually experienced males. Increased neuronal Fos immunoreactivity was obser ved in the granule and mitral cell layers of the accessory olfactory bulb o f males that were either mated or exposed to estrous bedding, but not in ma les that displayed non-contact penile erections in response to distal cues from an estrous female. The volatile odors which presumably caused non-contact penile erections fai led to stimulate significant neuronal Fos immunoreactivity in five main olf actory bulb sites examined. Even so, it seems likely that these pheromones are detected via the main olfactory system and are subsequently processed b y the same projection circuit that responds to other pheromones present in estrous bedding that are incapable of eliciting non-contact penile erection s. (C) 1999 IBRO. Published by Elsevier Science Ltd.