RECIPROCAL CONNECTIONS OF THE MOTOR NEOCORTICAL AREA WITH THE CONTRALATERAL THALAMUS IN THE HEDGEHOG (ERINACEUS-EUROPAEUS) BRAIN

Horseradish peroxidase unilateral injections in various neocortical ar eas (prefrontal, somatosensory, auditory, visual) of the hedgehog (Eri naceus europaeus) brain resulted in labelling of nuclei in the ipsilat eral thalamus known from studies in other species and in the hedgehog to project to these areas. However, injections in the motor area resul ted in retrograde and anterograde labelling of nuclei in both the ipsi lateral and contralateral thalamus. These nuclei included the ventral lateral nucleus (VL), the intralaminar nuclei (ILN), the mediodorsal n ucleus (MD) and midline nuclei. Large unilateral injections located ma inly laterally in the thalamus labelled cells, contralaterally, in the ventral lateral geniculate nucleus, the intergeniculate leaflet and t he 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 specie s (Regidor and Divac, Brain Behav. Evol., 39, 265 - 269, 1992) and in addition demonstrate that (i) bilateral thalamocortical projections ar e established preferentially with the motor area, (ii) several nuclei are involved in such connections, (iii) these connections are reciproc al and topographically organized, and (iv) labelling in the contralate ral thalamus observed in the present study is not a result of transneu ronal transport of the tracer through thalamothalamic connections. Thi s organization is unique among mammals and supports previous anatomica l and electrophysiological findings, on the basis of which it has been suggested that the hedgehog retains a primitive character in neocorti cal and thalamic evolution.