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) en ds in the ventrolateral sector of the rostral two-thirds of the red nu cleus. The aim of this study is to identify the rubral cells contacted by these thalamic efferents. In a first set of experiments, an antero grade neurotracer (PHA-L) was injected into the rostral part of the re d 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 in jection in the contralateral PT. Both anterograde and retrograde label ings obtained in the RN were examined by correlated light and electron microscopy. Our findings indicate that anterogradely degenerated or l abeled axons arising from the PT form synaptic contacts on HRP-filled dendritic processes of rubrospinal neurons. The thalamo-rubral articul ation is direct and seems to be mainly axo-dendritic. These results su pport the possible participation of the PT in the modulatory control o f spinal interneurons through the rubrospinal tract.