ALPHA-4-1 SUBUNIT MESSENGER-RNA OF THE NICOTINIC ACETYLCHOLINE-RECEPTOR IN THE RAT OLFACTORY-BULB - CELLULAR EXPRESSION IN ADULT, PRENATAL AND POSTNATAL STAGES

In addition to their role in signal transduction, nicotinic acetylchol ine receptors have been shown in vitro to be involved in neuronal grow th cone regulation during development. This idea is supported by recen t histochemical findings showing that iso- and archicortical nicotinic alpha 4-1 receptor mRNA expression precedes cholinergic fiber ingrowt h. To test whether this also holds true for rhinencephalic parts of th e telencephalon, we have studied the olfactory bulb by digoxigenin-med iated in situ hybridization, using an alpha 4-1 isoform-specific ribop robe and an alkaline-phosphatase-based detection system. Development i s characterized by early intense alpha 4-1 mRNA expression (embryonic day 14), reaching a peak around postnatal day 2 when all olfactory bul b layers are invested with numerous alpha 4-1 transcript-bearing neuro ns. Subsequently, the density of labeled neurons decreases to reach ad ult levels (postnatal day 120), where strongly labeled neurons remain in the mitral cell layer, outer external plexiform layer, and glomerul ar layer. The unifying pattern of iso-, archicortical, and rhinencepha lic alpha 4-1 mRNA expression is its early onset, i.e. preceding choli nergic innervation. This points to a possible role of nicotinic recept ors regarding neuronal migration in all three regions.