Distribution of alpha2, alpha3, alpha4, and beta2 neuronal nicotinic receptor subunit mRNAs in the central nervous system: A hybridization histochemical study in the rat

Abstract

Previous studies have revealed the existence of a gene family that encodes a group of neuronal nicotinic acetylcholine receptor (nAChR) subunits. Four members of this family have been characterized thus far; three of these subunits (alpha2, alpha3, and alpha4) are structurally related to the ligand binding subunit expressed in muscle and form functional nAChRs when combined with the beta2 gene product in Xenopus oocytes. In addition, the alpha4 gene appears to encode two different products (alpha4‐1 and alpha4‐2) that have been proposed to arise by alternative mRNA splicing. Nine different [35 S]‐complementary ribonucleic acid (cRNA) probes were used in the present study to map the distribution of these nAChR subunit mRNAs throughout the central nervous system (CNS) of the rat. It was found that the beta2 gene is expressed in most regions of the CNS, as are the alpha subunit genes as a group. However, each alpha gene is expressed in a unique, although partly overlapping, set of neuronal structures. Alpha4 is the most widely expressed alpha gene, and the evidence suggests that mRNAs for the alpha4‐1 and alpha4‐2 products are virtually always found in the same regions, in approximately the same ratios (alpha4‐2 greater than alpha4‐1). In addition, there are several examples of cell groups that express beta2 but none of the alpha subunit mRNAs examined here (particularly in the hypothalamus), as well as all groups that express the converse, thus suggesting that additional neuronal nAChR subunits remain to be characterized. Finally, the extensive expression of multiple alpha subunits in certain regions, particularly for alpha3 and alpha4 in the thalamus, suggests that there is microheterogeneity in a small population of cells or that some neurons may express more than one alpha subunit. This problem needs to be examined directly with double labeling methods but raises the possibility that some neuronal nAChRs may be composed of more than one kind of alpha subunit. The wide expression of these receptor genes suggests that nAChRs constitute major excitatory systems in the CNS.