An anterograde and retrograde tract-tracing study on the projections from the thalamic gustatory area in the rat: distribution of neurons projecting to the insular cortex and amygdaloid complex.

Projections from the thalamic gustatory nucleus, i.e. the parvicellular part of the posteromedial ventral thalamic nucleus (VPMpc) to the forebrain regions were studied in the rat by the tract-tracing methods with anterograde tracer (biotinylated dextran amine, BDA) and anterograde/retrograde tracer (wheat-germ agglutinin-horseradish peroxidase, WGA-HRP). After BDA injection into the VPMpc, terminal labeling was observed in the insular cortex, amygdaloid complex, and fundus striati. The terminal labeling in the amygdaloid complex was distributed in dorsolateral area of the rostral part of the lateral amygdaloid nucleus and the rostral part of the lateral subdivision of the central amygdaloid nucleus. The terminal labeling in the central amygdaloid nucleus extended to the fundus striati. The retrograde tracing study with WGA-HRP revealed that the projection fibers from the VPMpc to the amygdaloid complex originated from the medial part of the VPMpc and also from the thalamic area medial to the VPMpc. In the rats injected with Fluoro-Gold and WGA-HRP, respectively into the insular cortex and amygdaloid complex, no double-labeled neuronal cell bodies were found in the VPMpc, although neurons labeled singly with Fluoro-Gold were intermingled with those singly labeled with WGA-HRP in the medial part of the VPMpc. The results indicated that VPMpc neurons projecting to the amygdaloid complex constituted a population different from VPMpc neurons projecting to the insular cortex.