Olfactory relationships of the telencephalon and diencephalon in the rabbit. II. An autoradiographic and horseradish peroxidase study of the efferent connections of the anterior olfactory nucleus.

The efferent connections of the anterior olfactory nucleus in the female albino rabbit have been studied using the autoradiographic and horseradish peroxidase methods for tracing axonal pathways. Following a unilateral injection of 3H-leucine into the olfactory peduncle, radioactively labeled efferent projections from the anterior olfactory nucleus were traced into all layers of the ipsilateral main olfactory bulb beneath the olfactory nerve layer and through the ipsilateral anterior limb of the anterior commissure and plexiform layer of the medial side of the cerebral hemisphere to the deep half of the plexiform (IB) and pyramidal cell (II) layers of the prepyriform cortex, the tenia tecta, and the entire surface of the olfactory tubercle. Labeled projections crossing the midline within the anterior commissure were followed to the layers IB and II of the contralateral anterior prepyriform cortex and pars externa, pars lateralis, and pars dorsalis of the anterior olfactory nucleus, and through the periventricular layer of the olfactory peduncle to all layers of the main olfactory bulb beneath the olfactory nerve layer. No well-defined labeled projection was traced to the contralateral accessory olfactory bulb. Evidence for possible anterior olfactory nucleus and/or prepyriform cortical projections to the ipsilateral paleocortical half of the claustrum, horizontal limb of the nucleus of the diagonal band, the posterior lateral hypothalamus at the level of the mammillary complex, and to the bed nucleus of the stria terminalis is discussed. Intra-axonal retrograde transport of horseradish peroxidase from axon terminals to parent cell bodies after unilateral injection of the protein into the main olfactory bulb or anterior olfactory nucleus revealed that anterior olfactory nucleus projections to the olfactory bulbs and the contralateral anterior olfactory nucleus arise predominately from the pars externa. The autoradiographic data indicate that the anterior olfactory nucleus projects to olfactory cortical structures which also receive afferent input from the olfactory bulb and that the termination of these projections is complementary to those from the olfactory bulb.