CELLULAR-LOCALIZATION OF THROMBIN RECEPTOR MESSENGER-RNA IN RAT-BRAIN- EXPRESSION BY MESENCEPHALIC DOPAMINERGIC-NEURONS AND CODISTRIBUTIONWITH PROTHROMBIN MESSENGER-RNA

Cell culture studies demonstrating that the serine protease thrombin c an induce neuronal and glial process retraction, glial proliferation, and changes in gene expression suggest a role for thrombin in CNS deve lopment, plasticity, and response to injury. Most cellular responses t o thrombin are mediated by proteolytic activation of the cloned thromb in receptor (TR), a member of the seven transmembrane domain, G-protei n-coupled receptor superfamily. As a step toward understanding the rol e of thrombin and its receptor in the CNS, Northern blot, in situ hybr idization, and immunohistochemical techniques were used to analyze the cellular localization of TR mRNA in weanling-age rat brain. TR mRNA w as broadly distributed across the neuraxis, although expression was ve ry focal and often anatomically limited within specific neural structu res. The greatest hybridization was associated with individual neurons in neocortex, cingulate/retrosplenial cortex, and subiculum, subsets of nuclei in hypothalamus, thalamus, pretectum, and ventral mesencepha lon, and discrete cell layers in the hippocampus, cerebellum, and olfa ctory bulb. Patterns of hybridization included neuronal, glial, and ep endymal cells, although white matter was uniformly negative, as were m ost cerebrovascular endothelial cells. Expression of TR mRNA by astrog lia and dopaminergic neurons was confirmed by colocalization with immu noreactivity for glial fibrillary acidic protein (GFAP) in hippocampus and tyrosine hydroxylase in the substantia nigra. Comparison between TR and prothrombin (thrombin's precursor) cRNA hybridization demonstra ted distinct but overlapping brain distributions of these transcripts, most clearly evident in postnatally developing, laminated structures. These results suggest widespread utilization of, and multiple physiol ogic, and possibly pathophysiologic, functions for, the thrombin/TR ce ll signaling system in the CNS.