α4-1 Subunit mRNA of the nicotinic acetylcholine receptor in the rat olfactory bulb: Cellular expression in adult, pre- and postnatal stages

In addition to their role in signal transduction, nicotinic acetylcholine receptors have been shown in vitro to be involved in neuronal growth cone regulation during development. This idea is supported by recent histochemical findings showing that iso- and archicortical nicotinic α4-1 receptor mRNA expression precedes cholinergic fiber ingrowth. To test whether this also holds true for rhinencephalic parts of the telencephalon, we have studied the olfactory bulb by digoxigenin-mediated in situ hybridization, using an α4-1 isoform-specific riboprobe and an alkaline-phosphatase-based detection system. Development is characterized by early intense α4-1 mRNA expression (embryonic day 14), reaching a peak around postnatal day 2 when all olfactory bulb layers are invested with numerous α4-1 transcript-bearing neurons. Subsequently, the density of labeled neurons decreases to reach adult levels (postnatal day 120), where strongly labeled neurons remain in the mitral cell layer, outer external plexiform layer, and glomerular layer. The unifying pattern of iso-, archicortical, and rhinencephalic α4-1 mRNA expression is its early onset, i.e. preceding cholinergic innervation. This points to a possible role of nicotinic receptors regarding neuronal migration in all three regions.