A POPULATION OF NICOTINIC RECEPTORS IS ASSOCIATED WITH THALAMOCORTICAL AFFERENTS IN THE ADULT-RAT - LAMINAL AND AREAL ANALYSIS

In the adult rat brain, a prominent population of nicotinic cholinocep tors binds H-3-nicotine with nanomolar affinity. These receptors are a bundant in most thalamic nuclei and in neocortical layers 3/4, which r eceive a major thalamic input. To test whether cortical nicotinic rece ptors are associated with thalamocortical afferents, unilateral excito toxic (N-methyl-D-aspartate) lesions were made in one of four thalamic nuclear groups (anterior, ventral, medial geniculate, or dorsal later al geniculate) or in temporal cortex. After 1 or 4 weeks of survival, cortical H-3-nicotine binding was quantified via autoradiography. Thal amic lesions resulted in a partial loss of H-3-nicotine binding in ips ilateral cerebral cortex. In each thalamic lesion group, the greatest decrease (35-45%) occurred within the cortical layers and area (i.e., cingulate, parietal, temporal, or occipital cortex) receiving the dens est thalamocortical innervation. Binding of H-3-nicotine was also redu ced within the thalamus local to the lesion, particularly at the longe r survival time. Saturation analysis, performed in frontoparietal cort ical tissue homogenates following ventral thalamic lesions, revealed a significant (34%) reduction in receptor density but not affinity. Dir ect excitotoxic lesions of the neocortex (temporal cortex) tended to p reserve H-3-nicotine binding in layers 3/4, despite local neuronal los s. These results, taken with other published findings, suggest that so me nicotinic cholinoceptors in adult rat cerebral cortex are located o n thalamocortical terminals. This organizing principle appears to appl y not only to sensory and motor relay projections but also to associat ion nuclei that project to allocortical areas. These receptors may pro vide a local mechanism for nicotinic cholinergic modulation of thalamo cortical input. (C) 1997 Wiley-Liss, Inc.