Supraoptic nucleus afferents from the main olfactory bulb—I. Anatomical evidence from anterograde and retrograde tracers in rat

Abstract

The morphological features of a putative connection between the main olfactory bulb and the supraoptic nucleus of the rat was studied using a combination of anatomical techniques. Immunocytochemistry of neuropffysin-containing processes were employed to delineate morphological features of supraoptic dendrites. Main olfactory bulb efferents to the supraoptic nucleus were studied by injection of the anterogradely transported substances, wheatgerm agglutinin conjugated horseradish peroxidase or Phaseolus vulgaris leucoagglutinin, into the main olfactory bulb. To confirm the results of these studies, the distribution of retrogradely labeled cells within the main olfactory bulb was determined after injection of rhodamine-labeled latex microspheres or Fluoro-Gold R into the supraoptic nucleus. Neurophysin immunocytochemistry revealed the supraoptic nucleus dendritic plexus which coursed anteroposteriorly beneath supraoptic somata. Additionally, a portion of this plexus also projected ventrolaterally into periamygdaloid areas, a feature of supraoptic architecture which is not generally appreciated. The anterograde tracers labeled main olfactory bulb efferents including a dense plexus of terminals and fibers ventrolateral to the ipsilateral supraoptic nucleus. The pattern of anterogradely labeled fibers and terminals appeared to overlap with the distribution of ventrolaterally projecting neurophysin-containing processes. Since the latter consists of dendritic processes of supraoptic origin, this suggests that the main olfactory bulb projects to the supraoptic nucleus. Injections of rhodamine-labeled latex microspheres or Fluoro-Gold R resulted in retrogradely labeled mitral cells throughout the ipsilateral main olfactory bulb. Taken together, these anatomical studies demonstrate a direct projection from the main olfactory bulb to the supraoptic nucleus of the rat. A companion electrophysiological study confirmed these results.