THE AXONAL ARBORIZATION OF SINGLE THALAMIC RETICULAR NEURONS IN THE SOMATOSENSORY THALAMUS OF THE RAT

This study describes the axonal projections of single neurons of the t halamic reticular complex within the somatosensory thalamic nuclei in rats, Experiments were performed under urethane anaesthesia and reticu lar cells were labelled by extracellular microiontophoretic applicatio ns of biocytin. The axonal arborization of 25 thalamic reticular cells projecting to the ventrobasal (VB) nucleus and/or to the posterior th alamic (Po) complex were reconstructed from serial horizontal sections . Reticular cells labelled with biocytin display somatodendritic featu res similar to those reported previously. Their cell body is fusiform and their dendrites bear few spines and show a high degree of streamin g along the horizontal curved axis of the nucleus. In most cells, axon -like beaded processes stem out from dendrites but, contrary to previo us descriptions, no intrareticular axonal collateral was observed. The axonal arborization of most thalamic reticular cells is confined with in the limits of a single thalamic nucleus; only two neurons were seen projecting to both the VB and the Po nuclei. In VB, termination field s form short rods (diameter approximate to 150 mu m, length approximat e to 200-300 mu m) densely packed with grape-like boutons and varicosi ties; termination fields in Po are larger, much less dense, and they a re contained within a horizontal slab of tissue (thickness approximate to 200 mu m, mediolateral width approximate to 400 mu m, rostrocaudal length approximate to 1 mm. By charting the position of all labelled cells within the thickness of the thalamic reticular complex, a strip- like arrangement was revealed, Cells projecting to Po occupy the inner most portion of the nucleus whereas those projecting to the ventral-po steromedial and ventral-posterolateral nuclei are located respectively in the middle and in the outer tiers of the nucleus. This strip-like reciprocity was confirmed by separate biocytin injections performed in VB and in Po. These results show that inhibition of reticular origin is distributed within the rat dorsal thalamus in a highly specific man ner, most likely according to a principle of reciprocity within the so matotopic representation of the body.