Expression of alpha5 nicotinic acetylcholine receptor subunit mRNA during hippocampal and cortical development.

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated pentameric cation channels of alpha and beta subunits. The alpha5 subunit, when added to heteromeric complexes, alters pharmacological and physiological properties of nAChRs, which may be important during development. Here we have evaluated the pre- and postnatal expression of alpha5 subunit mRNA in rat cortex and hippocampus using highly sensitive in situ hybridization. In the cortex, alpha5 mRNA was detected in the subplate, claustrum, and endopiriform nucleus at embryonic day 18 (E18), areas with sustained expression into adulthood. Transient cortical expression was detected in numerous cells, scattered mainly in layers V and II/III, during the first 2 postnatal weeks. In the hippocampus, alpha5 transcripts first appeared in the CA1 region at E18. During postnatal development, increased hybridization signal was detected in CA1 and CA3 pyramidal neurons and granule cells of the dentate gyrus, which strongly contrasted with the low levels in the adult. Interneurons in CA1/CA3 stratum radiatum and moleculare, and in the hilus region of the dentate gyrus, strongly expressed alpha5 mRNA in postnatal and adult animals. With double in situ hybridization, co-expression of alpha5 and alpha7 mRNAs was detected in a subpopulation of hippocampal interneurons. In contrast, in the subiculum, numerous cells exhibited strong hybridization signal for alpha5 and alpha7 mRNAs, but co-expression was rarely detected. In conclusion, similar to other nAChR subunits, there is transient expression of alpha5 mRNA during cortical and hippocampal development. This expression pattern places nicotinic receptors containing the alpha5 subunit in a position to regulate glutamatergic and GABAergic transmission during the postnatal period.