Nigrothalamic projections in the rat as demonstrated by orthograde and retrograde tracing echniques

The nigrothalamic projection of the rat was investigated by means of horseradish peroxidase (HRP) histochemistry and by the autoradiographic tracing technique. The use of these techniques has permitted an extension of previous descriptions by identifying a neuron population in the substantia nigra, pars reticulata (SNR) as the nigrothalamic cells of origin. These calls (20-25 mu) were multipolar and were distributed throughout the SNR. There was a tendency however for more labelled neurons to be observed in the rostrolateral aspects of the nucleus. HRP-labelled neurons comprised roughly one-sixth of SNR neurons of comparable size, ventromedial (VM) thalamic nucleus. In addition, a smaller projection to the ipsilateral parafascicular nucleus was observed. The possible functional sifificance of the nigrothalamic projections is discussed in the light of recent anatomical, physiological and behavioral findings.