Substantia nigra pars reticulata projects to the reticular thalamic nucleus of the cat: a morphological and electrophysiological study.

The projections of substantia nigra pars reticulata (SNr) toward the reticular (RE) thalamic complex of cat were studied morphologically and electrophysiologically. Numerous SNr cells were retrogradely labeled following injections of horseradish peroxidase conjugated to wheat-germ agglutinin (WGA-HRP) in the rostral and rostrolateral part of the RE thalamic nucleus. Iontophoretic injections of Phaseolus vulgaris leucoagglutinin (PHA-L) in the SNr confirmed that this retrograde labeling was not a consequence of tracer diffusion in neighboring structures, but reflected a genuine SNr projection to the RE thalamic nucleus. Indeed, widely branched and varicose PHA-L-positive fibers were found in the rostral and rostrolateral pole of the RE thalamic nucleus following PHA-L injections in the SNr. Furthermore, in agreement with previous data indicating that nigrothalamic cells are GABAergic, electrical stimulation of the SNr evoked a short-latency inhibitory effect acting on both spontaneous and cortically-evoked discharges of most RE thalamic neurons. These results are discussed in light of the possible role of the SNr and RE thalamic complex in attentional processes.