ORGANIZATION OF THE RETICULAR THALAMIC PROJECTION TO THE INTRALAMINARAND MIDLINE NUCLEI IN RATS

This study examines the projection of the reticular thalamic nucleus t o the classic ''nonspecific'' dorsal thalamic nuclei of rats. Individu al nuclei of the intralaminar (central-lateral, paracentral, central-m edial, parafascicular) and the midline (reuniens/rhomboid, parataenial ) nuclear groups, together with the reticular nucleus itself, were inj ected with the neuronal tracers biotinylated dextran or fluorescent la tex microspheres (red or green). Reticular cells projecting to the int ralaminar and midline nuclei are limited largely to the rostral pole o f the nucleus. Within the rostral pole, most reticular cells projectin g to the intralaminar and midline nuclear groups are found in largely distinct sectors; cells that project to the intralaminar nuclei tend t o lie more laterally, whereas those projecting to the midline nuclei l ie more medially within the pole. Among the individual nuclei of both the intralaminar and midline nuclear groups, however, the segregation is far less distinct. For instance, the reticular cells that project t o the intralaminar central-lateral, central-medial, paracentral, and p arafascicular nuclei are intermixed completely on the lateral edge of the rostral pole. After separate injections of different colored latex microspheres into individual intralaminar nuclei, the incidence of do uble-labelled reticular cells is about 37%, st percentage much higher than among the ''specific'' dorsal thalamic nuclei (<1%). All the abov e-mentioned results refer to the reticular labelling seen on the side ipsilateral to the injection. After separate injections into the intra laminar central-medial nucleus, the midline nuclei, and the reticular nucleus itself, we also see a very small group of reticular cells labe lled on the contralateral side. In general, our results indicate that the reticular projection to the intralaminar and midline nuclei is far more diffuse than the reticular projection to the specific dorsal tha lamic nuclei. (C) 1997 Wiley-Liss, Inc.