Effect of the estrous cycle on olfactory bulb response to vaginocervical stimulation in the rat: results from electrophysiology and Fos immunocytochemistry experiments.

To determine whether the stage of the estrous cycle modified the response of olfactory bulb neurons to vaginocervical stimulation, (1) vaginocervical stimulation was applied to animals in proestrus-estrus and metestrus-diestrus and the extracellular electrophysiological response of units in the mitral cell layer of the main olfactory bulb was compared, and (2) the effect of vaginocervical or sham stimulation and the effect of the estrous cycle on the number of neurons stained immunocytochemically for Fos in the main and accessory olfactory bulb was examined. Animals in proestrus-estrus had basal firing rates of 21.8 +/- 1.8 spikes per 5 s and vaginocervical stimulation produced an increase in firing rate. In contrast, animals in metestrus-diestrus had a slower basal firing rate (14.3 +/- 2.3 spikes per 5 s) and vaginocervical stimulation produced a decrease in the firing rate. For animals in proestrus-estrus, vaginocervical stimulation increased the number of Fos-stained cells in the granular cell layer of the accessory olfactory bulb, and in the glomerular and in external plexiform layers of the main olfactory bulb. In contrast, the number of Fos-stained cells decreased in the granular cell layer of the main olfactory bulb after stimulation was applied to animals in proestrus-estrus. The number of Fos-stained cells in the granular layer of the accessory olfactory bulb and the granular and glomerular cell layers of the main olfactory bulb was modulated by the estrous cycle. Therefore, olfactory bulb activity, measured both electrophysiologically and by Fos staining, was affected by the estrous cycle and vaginocervical stimulation, and the two variables interacted. It is likely that integration of interoceptive and environmental stimulation is important for the normal expression of sexual behavior in the female rat.