Thalamocortical and thalamo-amygdaloid projections from the parvicellular division of the posteromedial ventral nucleus in the cat.

Projections from the parvicellular division of the posteromedial ventral thalamic nucleus (VPMpc) of the cat were examined. After injection of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) into the VPMpc, both anterogradely labeled axon terminals and retrogradely labeled neuronal cell bodies were found ipsilaterally in three discrete regions of the cerebral cortex, i.e., in the orbital cortex, caudoventral part of the infralimbic cortex, and medial part of the fundus of the posterior rhinal sulcus (perirhinal area); in the subcortical regions, anterogradely labeled axon terminals were seen ipsilaterally in the rostrodorsal part of the lateral amygdaloid nucleus. Neuronal connections between these VPMpc-recipient regions were further verified by injecting WGA-HRP into each of the three cortical and the lateral amygdaloid regions. After injection of WGA-HRP into each of the three cortical regions, labeled neuronal cell bodies and axon terminals were seen ipsilaterally in the VPMpc, especially in its medial part, and in the other two of the three VPMpc-recipient cortical regions. In the rostrodorsal part of the lateral amygdaloid nucleus, both axon terminals and neuronal cell bodies were labeled after WGA-HRP injection into the perirhinal area, and only axon terminals were labeled after WGA-HRP injection into the orbital cortex, but no labeling was observed after WGA-HRP injection into the infralimbic cortex. After injection of WGA-HRP into the rostrodorsal portion of the lateral amygdaloid nucleus, both axon terminals and neuronal cell bodies were labeled ipsilaterally in the perirhinal area and the ectorhinal area, and only neuronal cell bodies were labeled ipsilaterally in the VPMpc (especially in its medial part) and orbital cortical region; no labeling was observed in the infralimbic cortex. The present results indicate that the VPMpc of the cat is connected reciprocally with the orbital, infralimbic, and perirhinal cortical regions on the ipsilateral side, that the three VPMpc-recipient cortical regions are reciprocally connected with each other, that the VPMpc sends fibers ipsilaterally to the rostrodorsal part of the lateral amygdaloid nucleus, which may relay information from the VPMpc to the perirhinal cortical area, and that the VPMpc-recipient area in the lateral amygdaloid nucleus receives cortical fibers from the orbital and perirhinal cortical regions.