Analysis of projections from the medial prefrontal cortex to the thalamus in the rat, with emphasis on nucleus reuniens.

The medial prefrontal cortex (mPFC) is involved in high-order cognitive processes, including, but not limited to, decision making, goal directed behavior, and working memory. Although previous reports have included descriptions of mPFC projections to the thalamus in overall examinations of mPFC projections throughout the brain, no previous study has comprehensively examined mPFC projections to the thalamus. The present report compares and contrasts projections from the four divisions of the mPFC, i.e., the infralimbic, prelimbic, anterior cingulate and medial agranular cortices, to the thalamus in the rat by using the anterograde anatomic tracer Phaseolus vulgaris-leucoagglutinin. We showed that (1) the infralimbic, prelimbic, anterior cingulate cortices distribute heavily and selectively to midline/medial structures of the thalamus, including the paratenial, paraventricular, interanteromedial, anteromedial, intermediodorsal, mediodorsal, reuniens, and the central medial nuclei; (2) the medial agranular cortex distributes strongly to the rostral intralaminar nuclei (central lateral, paracentral, central medial nuclei) as well as to the ventromedial and ventrolateral nuclei of thalamus; and (3) all four divisions of the mPFC project densely to the nucleus reuniens (RE) of the thalamus. The nucleus reuniens is the major source of thalamic afferents to the hippocampal formation. There are essentially no direct projections from the mPFC to the hippocampus. The present demonstration of pronounced mPFC projections to RE suggests that the nucleus reuniens is a critical relay in the transfer of information from the medial prefrontal cortex to the hippocampus. Our further demonstration of strong mPFC projections to several additional thalamic nuclei, particularly to the mediodorsal nucleus, suggests that these thalamic nuclei, like RE, represent important output stations (or gateways) for the actions of mPFC on diverse subcortical and cortical structures of the brain.