C-fos Expression in Vomeronasal Pathways During Mating Behavior in Male Hamsters

Abstract

Male hamsters are very dependent on chemosensory input for their mating behavior, especially input from the vomeronasal system. This accessory olfactory system projects to the accessory olfactory bulb (AOB), to the medial (Me) and posterior medial cortical nuclei (PMCN) of the amygdala, the bed nucleus of the stria terminalis (BNST), and the medial preoptic area (MPOA). These are relatively direct projections to central structures important in reproductive behavior, compared to the indirect access of the main olfactory system to these areas. The expression of immediate early gene, c-fos, was used to study the role of chemosensory input during mating behavior. Intact hamsters and those with vomeronasal organs removed (VNX) were stimulated with a receptive female, or female hamster vaginal fluid (HVF), or left unstimulated for 45 min Their olfactory bulbs and brains were processed 45 min later for immunocytochemical detection of FOS, the protein product of c-fos and an indicator of neuronal activation. Since mated animals receive a variety of inputs, Fos activation could be attributed to chemosensory inputs, to other sensory inputs, to “mating-related” inputs, or to the motor and integrative aspects of copulatory performance. Contributions from these different sources were studied by examining c-fos expression patterns in animals with different sensory inputs and/or different behavior. In animals exposed to HVF, the stimulus is restricted to the vomeronasal and main olfactory systems. Activity related to mating performance and female cues other than HVF are eliminated here. Activation due to non-vomeronasal input was studied in VNX animals stimulated with a female or HVF. Most VNX animals do not mate, but do perform intense chemoinvestigatory behavior. Any FOS activation here would be non-vomeronasal. In this study, two VNX animals did mate and their c-fos expression pattern is attributed to mating-related activation and to non-VN inputs. Unstimulated animals placed in clean cages without a female or HVF had little activation and none attributable to mating-related stimuli. FOS activation was studied quantitatively in vomeronasal and main olfactory targets by counting FOS-positive nuclei in vomeronasal and main olfactory targets.