Reciprocal connections of the motor neocortical area with the contralateral thalamus in the hedgehog (Erinaceus europaeus) brain.

Horseradish peroxidase unilateral injections in various neocortical areas (prefrontal, somatosensory, auditory, visual) of the hedgehog (Erinaceus europaeus) brain resulted in labelling of nuclei in the ipsilateral thalamus known from studies in other species and in the hedgehog to project to these areas. However, injections in the motor area resulted in retrograde and anterograde labelling of nuclei in both the ipsilateral and contralateral thalamus. These nuclei included the ventral lateral nucleus (VL), the intralaminar nuclei (ILN), the mediodorsal nucleus (MD) and midline nuclei. Large unilateral injections located mainly laterally in the thalamus labelled cells, contralaterally, in the ventral lateral geniculate nucleus, the intergeniculate leaflet and the reticular nucleus of the thalamus, but never in VL, ILN and MD. The present results confirm previously described bilateral thalamocortical projections from the VL to the somatosensorimotor area in this species (Regidor and Divac, Brain Behav. Evol., 39, 265-269, 1992) and in addition demonstrate that (i) bilateral thalamocortical projections are established preferentially with the motor area, (ii) several nuclei are involved in such connections, (iii) these connections are reciprocal and topographically organized, and (iv) labelling in the contralateral thalamus observed in the present study is not a result of transneuronal transport of the tracer through thalamothalamic connections. This organization is unique among mammals and supports previous anatomical and electrophysiological findings, on the basis of which it has been suggested that the hedgehog retains a primitive character in neocortical and thalamic evolution.