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 thalamic reticular complex within the somatosensory thalamic nuclei in rats. Experiments were performed under urethane anaesthesia and reticular cells were labelled by extracellular microiontophoretic applications of biocytin. The axonal arborization of 25 thalamic reticular cells projecting to the ventrobasal (VB) nucleus and/or to the posterior thalamic (Po) complex were reconstructed from serial horizontal sections. Reticular cells labelled with biocytin display somatodendritic features similar to those reported previously. Their cell body is fusiform and their dendrites bear few spines and show a high degree of streaming along the horizontal curved axis of the nucleus. In most cells, axon-like beaded processes stem out from dendrites but, contrary to previous descriptions, no intrareticular axonal collateral was observed. The axonal arborization of most thalamic reticular cells is confined within the limits of a single thalamic nucleus; only two neurons were seen projecting to both the VB and the Po nuclei. In VB, termination fields form short rods (diameter approximately 150 microns, length approximately 200-300 microns) densely packed with grape-like boutons and varicosities; termination fields in Pro are larger, much less dense, and they are contained within a horizontal slab of tissue (thickness approximately 200 microns, mediolateral width approximately 400 microns, rostrocaudal length approximately 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 innermost portion of the nucleus whereas those projecting to the ventral-posteromedial 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 manner, most likely according to a principle of reciprocity within the somatotopic representation of the body.