Efferent connections of the medial prefrontal cortex in the rabbit

The different cytoarchitectonic regions of the medial prefrontal cortex (mPFC) have recently been shown to play divergent roles in associative learning in rabbits. To determine if these subareas of the mPFC, including 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 retrograde tracing experiments were performed using the Phaseolus vulgaris leukoagglutinin (PHA-L), and horseradish peroxidase (HRP) techniques. All three areas showed local dorsal-ventral projections into each of the other areas, and a contralateral projection to the homologous area on the other side of the brain. All three also revealed a trajectory through the striatum, resulting in heavy innervation of the caudate nucleus, the claustrum, and a lighter projection to the agranular insular cortex. The thalamic projections of areas 24 and 32 were similar, but not identical, with projections to the mediodorsal nucleus (MD) and all of the midline nuclei. However, the primary thalamic projections 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 limb of the fornix. Amygdala projections from areas 32 and 24 were primarily to the lateral, basolateral and basomedial nuclei, but area 25 also projected to the central nucleus. All three areas also showed projections to the midbrain periaqueductal central gray, median raphe nucleus, ventral tegmental area, substantia nigra, locus coeruleus and pontine nuclei. However, only areas 24 and the more dorsal portions of area 32 projected to the superior colliculus. Area 25 and the ventral portions of area 32 also showed a bilateral projection to the parabrachial nuclei and dorsal and ventral medulla. The dorsal portions of area 32, and all of area 24 were, however, devoid of these projections. It is suggested that these differential projections are responsible for the diverse roles that the cytoarchitectonic subfields of the mPFC have been demonstrated to play in associative learning.Abbreviations ACC anterior cingullate cortex - ACN amygdaloid central nucleus - AD anterodorsal nucleus of thalamus - AIC, Iag agranular insular cortex - AM anteromedial nucleus of thalamus - AMG amygdala - AV anteroventral nucleus of thalamus - BL basolateral nucleus of amygdala - BM basomedial nucleus of amygdala - CdN, CD caudate nucleus - CL claustrum - CN centromedian nucleus of thalamus - D MV, DVM dorsal motor nucleus of vagus - IC internal capsule - L lateral nucleus of amygdala - LC locus coeruleus - LH lateral hypothalamus - MB mammillary bodies - MDN mediodorsal nucleus of thalamus - mPFC medial prefrontal cortex - MRN, R median raphe nucleus - MV medioventral nucleus of thalamus - NA nucleus ambiguus - NTS nucleus of solitary tract - PAG periaqueductal central gray - PAV, PV para ventricular nucleus of thalamus - PC paracentral nucleus of thalamus - PF parafascicular nucleus of thalamus - PN,LP pontine nuclei - PS posterior subiculum - PS CG posterior cingulate cortex - PT paratenial nucleus of thalamus - Put putamen - ReN nucleus reuniens of thalamus - RF reticular formation - RN reticular nucleus of thalamus - RhN rhomboid nucleus of thalamus - RS CX retrosplenial cortex - S septum - SC superior colliculus - SN substantia nigra - tt tenia tecta - VL ventrolateral nucleus of thalamus - VM ventromedial nucleus of thalamus - VP ventroposterior nucleus of thalamus - VTA ventral tegmental area