Parabrachio-cortical connections with the lateral hemisphere in the madagascan hedgehog tenrec: prominent projections to layer 1, weak projections from layer 6

Abstract

The present study was undertaken to further characterize and subdivide the rhinal cortex (insular and perirhinal areas) in the hedgehog tenrec (Echinops telfairi), a placental mammal with a rather low encephalisation index. Injections of wheat germ agglutinin-horseradish peroxidase into the dorsolateral pontine tegmentum revealed a prominent layer 1 projection to several rhinal target areas, while the rhinal cortex only stained weakly for the calcitonin gene-related peptide. Among the regions retrogradely labeled following tracer injections into the rhinal cortex, the parabrachial nucleus was considered the main origin of the tegmento-cortical projection. This conclusion was based on the circumscribed pattern of termination, as well as the differences noted between the pattern of anterograde labeling and the pattern obtained by thyrosine hydroxylase immunohistochemistry. The tracer injections into the dorsolateral tegmentum also revealed numerous retrogradely labeled cells in the layer 5 of the dorsomedial frontal cortex. In contrast, the rhinal cortex only showed few labeled cells and most of these cells were located in the layer 6/7. A comparison with other species indicates that the tenrec’s parabrachial nucleus gives rise to the most extensive cortical projections but receives the least prominent input from the lateral cerebral hemisphere. The layer 6/7 projection may be a common mammalian feature but it is overshadowed by the layer 5 projection in higher mammals.