Adjacent laminar terminations of two centrifugal afferent pathways to the accessory olfactory bulb in the mouse

Anterograde and retrograde axonal tracing methods have been combined with transection of the stria terminalis to investigate the centrifugal afferent connections of the accessory olfactory bulb in the mouse. Injection of tritiated proline into the postero-medial cortical amygdaloid nucleus (C3) gives rise to anterograde autoradiographic labelling of a pathway terminating in the internal granular layer of the accessory olfactory bulb (AOB). Transection of the ipsilateral stria terminalis completely abolishes labelling of this pathway. Injections further rostral, in the bed nucleus of the accessory olfactory tract (bnAOT) and medial amygdaloid nucleus (M), give rise to labelling of a second ipsilateral afferent pathway to the AOB which terminates in the internal plexiform layer (IPL) and is unaffected by strial transection. Injections of wheat germ lectin-HRP conjugate into the AOB confirm that it receives afferents from the ipsilateral bnAOT, M and C3, and from a few cells in the contralateral C3. Transection of the ipsilateral stria terminalis prevents retrograde labelling of any cells in the ipsilateral C3, but does not affect labelling of cells in M or bnAOT (or contralateral C3). The conjugate is also transported anterogradely in this system, labelling the efferent projections of the AOB to bnAOT, M and C3. It is concluded that the AOB receives at least two sets of ipsilateral afferents: one set from C3, via the stria terminalis, terminating in the internal granular layer, and a second set from M and/or bnAOT terminating in the IPL and probably running in the accessory olfactory tract.