PROJECTION AND INNERVATION PATTERNS OF INDIVIDUAL THALAMIC RETICULAR AXONS IN THE THALAMUS OF THE ADULT-RAT - A 3-DIMENSIONAL, GRAPHIC, ANDMORPHOMETRIC ANALYSIS

The gamma-aminobutyric acid-ergic thalamic reticular nucleus (Rt), whi ch carries matching topographical maps of both the thalamus and cortex and in which constituent cells can synaptically communicate between e ach other, is the major extrinsic source of thalamic inhibitions and d isinhibitions. Whether all the Rt axonal projections into the thalamus are similarly organized and have common projection and innervation pa tterns are questions of great interest to further our knowledge of the functioning of the Rt. The present study provides architectural and m orphometric data of individual, anterogradely labeled axonal arbors th at arose from distinct parts of the Rt. One hundred twenty-seven Rt ne urons from all regions of Rt were marked juxtacellularly with biocytin or Neurobiotin in urethane-anesthetized adult rats. Eighteen two-dime nsional and 14 three-dimensional reconstructions of single tracer-fill ed Rt neurons were made from serial, frontal, horizontal, or sagittal sections. Both the somatodendritic and axonal fields of tracer-filled Rt cells were mapped in three dimensions and illustrated to provide a complementary stereotaxic reference for future studies. Most marked un its projected to a single nucleus of the anterior, dorsal, intralamina r, posterior, or ventral thalamus. Axons emerging from cells in distin ct sectors of the Rt projected to distinct nuclei. Within a sector, ne urons with separate dendritic fields innervated separate regions eithe r in a single nucleus or into different but functionally related thala mic nuclei. Neurons with an overlap of their dendritic fields gave ris e either to overlapping axonal arborizations or, more rarely, to disti nct axonal arbors within two different thalamic nuclei implicated in t he same function. In rare instances, an Rt axon could project within t hese two nuclei. Thalamic reticular axons commonly displayed a single well-circumscribed arbor containing a total of about 4,000 +/- 1,000 b outons. Every arbor was composed of a dense central core, which encomp assed a thalamic volume of 5-63 x 10(6) mu m(3) and was made up of pat ches of maximal innervation density (10 +/- 4 boutons/tissue cube of 2 5 mu m each side), surrounded by a sparse component. The metric relati onships between the Rt axonal arbors and the dendrites of their target thalamocortical neurons were determined. Both the size and maximmal i nnervation density of the axonal patches were found to fit in with the somatodendritic architecture of the target cells. The Rt axonal proje ctions of adult rats are thus characterized by their (1) well-focused terminal field with a patchy distribution of boutons and (2) parallel organization with a certain degree of divergence. The role of the Rt m ediated thalamic inhibition and disinhibition may be to contrast signi ficant with nonrelevant ongoing thalamocortical information. (C) 1998 Wiley-Liss, Inc.