Convergence of basolateral amygdaloid and mediodorsal thalamic projections in different areas of the frontal cortex in the rat.

The extent of convergence of mediodorsal thalamic and amygdalar afferents on the rat's frontal cortex was studied by tracing retrogradely labeled cells following injections of horseradish peroxidase (HRP). HRP was applied iontophoretically in extremely small injections throughout all areas of the frontal cortex. The following organization was revealed: Converging inputs from the mediodorsal nucleus and the amygdala are observed in the posterior parts of the pre- and infralimbic areas, in the posterior half of the dorsal and ventral agranular insular areas and in the lateral and dorsal precentral areas. Both mediodorsal and amygdaloid afferents reach the dorsal tip of the frontal cortex. Only the mediodorsal afferents were found to terminate in the anterior parts of the pre- and infralimbic areas and in the anterior part of the dorsal division of the anterior cingulate area and in the medial precentral area. On the lateral side of the hemisphere the anterior halves of the dorsal and ventral agranular insular areas receive mediodorsal afferents. Amygdaloid, but not mediodorsal afferents, were found following injections into the more posterior parts of the lateral precentral area. These results are discussed with respect to the extent of the prefrontal cortex in the rat and its definability as a target area of subcortical nuclei. Functional aspects of the anatomical convergence of connections within the so-called basolateral limbic circuit are outlined.