Cascade projections from somatosensory cortex to the rat basolateral amygdala via the parietal insular cortex

The pathways by which somatosensory information could be relayed from the cortex to the amygdaloid complex were investigated by using the anterograde axonal transport of biocytin following cortical microinjections. Injections of biocytin into head and limb areas of secondary somatosensory cortex (S2) produced heavy labeling of fibers and terminals in granular and dysgranular parietal insular cortex from bregma to 3.8 mm behind bregma but only extremely sparse labeling in the lateral and basolateral amygdaloid nuclei. Biocytin injections into granular parietal insular cortex produced a heavy labeling of the subjacent dysgranular parietal insular cortex, but only sparse labeling in the basolateral amygdala. Biocytin injections into dysgranular parietal insular cortex resulted in heavy labeling of the subjacent agranular parietal insular cortex and strong labeling of fibers and terminals in the dorsal part of lateral nucleus, with moderate labeling of fibers in the anterior and posterior basolateral nuclei, and the central nucleus. Injections into S2 labeled the ventroposterior medial, ventroposterior lateral and posterior thalamic nuclei; injections in rostral granular and dysgranular parietal insular cortex labeled the ventral posterior and parvicellular part of ventroposterior lateral thalamic nuclei; and injections in middle to caudal dysgranular parietal insular cortex labeled only the posterior nucleus. These results suggest that whereas somatosensory cortex projects only very sparsely to the amygdala, somatosensory-related inputs to the amygdala arise in the dysgranular parietal insular cortex. The association of dysgranular parietal insular cortex with the posterior thalamus suggests it may relay nociceptive information to the amygdala.