HETEROGENEOUS AXONAL ARBORIZATIONS OF RAT THALAMIC RETICULAR NEURONS IN THE VENTROBASAL NUCLEUS

The gamma-aminobutyric acid (GABA)-containing neurons of the thalamic reticular nucleus (nRt) are a major source of inhibitory innervation i n dorsal thalamic nuclei. Individual nRt neurons were intracellularly recorded and labelled in an in vitro rat thalamic slice preparation to investigate their projection into ventrobasal thalamic nuclei (VB). C amera lucida reconstructions of 37 neurons indicated that nRt innervat ion ranges from a compact, focal projection to a widespread, diffuse p rojection encompassing large areas of VB. The main axons of 65% of the cells gave rise to intra-nRt collaterals prior to leaving the nucleus and, once within VB, ramified into one of three branching patterns: c luster, intermediate, and diffuse. The cluster arborization encompasse d a focal region averaging approximately 25,000 mu m(2) and contained a high density of axonal swellings, indicative of a topographic projec tion. The intermediate structure extended across an area approximately fourfold greater and also contained numerous axonal swellings. The di ffuse arborization of nRt neurons covered a large region of VB and con tained a relatively low density of axonal swellings. Analysis of somat ic size and shape revealed that diffuse arborizations arose from signi ficantly smaller, fusiform-shaped somata. Cytochrome oxidase reactivit y or parvalbumin immunoreactivity was used to delineate a discontinuou s staining pattern representing thalamic barreloids. The size of a clu ster arborization closely approximated that of an individual barreloid . The heterogeneous arborizations from nRt neurons may reflect a dynam ic range of inhibitory influences of nRt on dorsal thalamic activity. (C) 1996 Wiley-Liss, Inc.