EFFERENT CONNECTIONS OF THE MEDIAL PREFRONTAL CORTEX IN THE RABBIT

The different cytoarchitectonic regions of the medial prefrontal corte x (mPFC) have recently been shown to play divergent roles in associati ve learning in rabbits. To determine if these subareas of the mPFC, in cluding areas 24 (anterior cingulate cortex), 25 (infralimbic cortex), and 32 (prelimbic cortex) have differential efferent connections with other cortical and subcortical areas in the rabbit, anterograde and r etrograde tracing experiments were performed using the Phaseolus vulga ris leukoagglutinin (PHA-L), and horseradish peroxidase (HRP) techniqu es. All three areas showed local dorsal-ventral projections into each of the other areas, and a contralateral projection to the homologous a rea on the other side of the brain. All three also revealed a trajecto ry through the striatum, resulting in heavy innervation of the caudate nucleus, the claustrum, and a lighter projection to the agranular ins ular cortex. The thalamic projections of areas 24 and 32 were similar, but not identical, with projections to the mediodorsal nucleus (MD) a nd all of the midline nuclei. However, the primary thalamic projection s from area 25 were to the intralaminar and midline nuclei. All three areas also projected to the ventromedial and to a lesser extent to the ventral posterior thalamic nuclei. Projections were also observed in the lateral hypothalamus, in an area just lateral to the descending li mb of the fornix. Amygdala projections from areas 32 and 24 were prima rily to the lateral, basolateral and basomedial nuclei, but area 25 al so projected to the central nucleus. All three areas also showed proje ctions to the midbrain periaqueductal central gray, median raphe nucle us, ventral tegmental area, substantia nigra, locus coeruleus and pont ine nuclei. However, only areas 24 and the more dorsal portions of are a 32 projected to the superior colliculus. Area 25 and the ventral por tions of area 32 also showed a bilateral projection to the parabrachia l nuclei and dorsal and ventral medulla. The dorsal portions of area 3 2, and all of area 24 were, however, devoid of these projections. It i s suggested that these differential projections are responsible for th e diverse roles that the cytoarchitectonic subfields of the mPFC have been demonstrated to play in associative learning.