Olfactory relationships of the telencephalon and diencephalon in the rabbit. III. The ipsilateral centrifugal fibers to the olfactory bulbar and retrobulbar formations

Abstract

The axoplasmic retrograde transport of horseradish peroxidase (HRP) from axon terminals to their parent cell bodies and histochemical fluorescence microscopy have been used to study the ipsilateral centrifugal fibers to the olfactory bulbs and anterior olfactory nucleus in the rabbit. Focal injections of peroxidase were placed unilaterally into the main or accessory olfactory bulb or into the anterior olfactory nucleus. In animals with injected HRP confined within the main bulb, perikarya retrogradely labeled with the protein in the ipsilateral forebrain were observed in the anterior prepyriform cortex horizontal limb of the nucleus of the diagonal band, and far lateral preoptic and rostral lateral hypothalamic areas. Brain stem cell groups that contained HRP-positive somata include the locus coeruleus and midbrain dorsal raphe nucleus. Except for the prepyriform cortex, the basal forebrain structures with labeled perikarya correlate well with locations of cell bodies containing acetylcholinesterase and choline acetyltransferase. These somata may represent a cholinergic afferent system to the main olfactory bulb. Peroxidase-labeled cell bodies in the locus coeruleus and midbrain raphe are indicative of noradrenergic and serotonergic innervations respectively of the olfactory bulb. In rabbits in which peroxidase was injected or diffused into the accessory olfactory bulb and anterior alfactory nucleus, HRP-positive somata were identified in the prepyriform cortex bilaterally, the horizontal limb of the diagonal band nucleus, lateral hypothalamic region, nucleus of the lateral olfactory tract, corticomedial complex of the amygdala, mitral and tufted cell layers of the ipsilateral main olfactory bulb, locus coeruleus, and the midbrain raphe. Evidence for centrifugal fibers to the accessory olfactory bulb from the corticomedial complex of the amygdala, locus coeruleus, and possibly the nucleus of the lateral olfactory tract and midbrain raphe is discussed. A similar distribution of labeled perikarya in the forebrain and brain stem was seen in rats in which peroxidase injected into the main olfactory bulb had spread into the accessory bulb and anterior olfactory nucleus. Histochemical fluorescence microscopy of the main and accessory olfactory bulbs in the rabbit and rat revealed fine caliber, green fluorescent fibers and varicosities predominantly in the granule cell layer and less so among cells in the glomerular layer. In sections through the root of the main olfactory bulb, a similar fluorescence was seen in the deep half of the plexiform layer of the pars externa of the anterior alfactory nucleus. These fluorescent fibers likely represent the noradrenergic innervation of the olfactory bulbar and retrobulbar formations. A fluorescent yellow hue was observed in the glomerular layer of the main bulb and may signify a serotonergic innervation of this lamina...