Electron microscopic demonstration of terminations of posterior thalamic axons on identified rubrospinal neurons in the rat.

In the rat, the major output of the posterior thalamic nucleus (PT) ends in the ventrolateral sector of the rostral two-thirds of the red nucleus. The aim of this study is to identify the rubral cells contacted by these thalamic efferents. In a first set of experiments, an anterograde neurotracer (PHA-L) was injected into the rostral part of the red nucleus. The only structure consistently and densely labeled was the contralateral spinal cord. Therefore, in a second set of experiments, massive HRP injections were made at different cervical levels in the spinal cord in combination with either electrolytic lesion or PHA-L injection in the contralateral PT. Both anterograde and retrograde labelings obtained in the RN were examined by correlated light and electron microscopy. Our findings indicate that anterogradely degenerated or labeled axons arising from the PT form synaptic contacts on HRP-filled dendritic processes of rubrospinal neurons. The thalamo-rubral articulation is direct and seems to be mainly axo-dendritic. These results support the possible participation of the PT in the modulatory control of spinal interneurons through the rubrospinal tract.