Organization of visceral and limbic connections in the insular cortex of the rat

The anterograde and retrograde transport of horseradish peroxidase was used to study the anatomical organization of visceral and limbic terminal fields in the insular cortex. Following injections into the ventroposterolateral parvicellular (VPLpc) and ventroposteromedial parvicellular (VPMpc) visceral relay nuclei of the thalamus, dense anterograde and retrograde labeling was present in the posterior granular and dysgranular insular cortices, respectively. The parabrachial nucleus had extensive connections with the posterior dysgranular cortex and to a lesser degree with the anterior dysgranular and granular cortices. In contrast, injections into the medial prefrontal cortex and mediodorsal nucleus of the thalamus resulted in dense anterograde and retrograde labeling primarily in the anterior agranular cortex, whereas injections in the amygdala resulted in axonal labeling in the agranular and dysgranular insular cortices. Injections into the lateral hypothalamic area resulted in dense anterograde and retrograde labeling mainly in the agranular and dysgranular cortices and moderate to light labeling in the granular cortex. Our results indicate that ascending visceral afferents, VPLpc, VPMpc, and parabrachial nuclei, are topographically organized in the granular and dysgranular fields of the insular cortex, whereas the agranular cortex appears to receive highly integrated limbic afferents from the infralimbic cortex and the mediodorsal nucleus of the thalamus. Although these visceral and limbic inputs to the insular cortex are segregated for the most part into different longitudinally oriented strips of cortex, limbic input from the lateral hypothalamic area and the amygdala, which have extensive autonomic as well as limbic connections, are more diffusely distributed over the different regions of the insular cortex. This organization may subserve a role for the insular cortex in integration of autonomic response with ongoing behaviour and emotion.