Developmental profiles of various cholinergic markers in the rat main olfactory bulb using quantitative autoradiography

The existence of possible relationships among the developmental profile of various cholinergic markers in the main olfactory bulb (OB) was assessed by using in vitro quantitative autoradiography. Muscarinic receptors were visualized with ³H]pirenzepine (muscarinic M1-like sites) and ³H]AF-DX 384 (muscarinic M2-like sites); nicotinic receptors by using ³H]cytisine (nicotinic 42-like subtype) and ¹²⁵I]α-bungarotoxin (nicotinic 7-like subtype); cholinergic nerve terminals by using ³H]vesamicol (vesicular acetylcholine transport sites) and ³H]hemicholinium-3 (high-affinity choline uptake sites). These various cholinergic markers exhibited their lowest levels at birth and reached adult values by the end of the 4–5 postnatal weeks. However, the density of presynaptic cholinergic markers and nicotinic receptors at postnatal day 2 represented a large proportion of the levels observed in adulthood, and displays a transient overexpression around postnatal day 20. In contrast, the postnatal development of cholinergic muscarinic M1-like and M2-like receptors is apparently regulated independently of the presynaptic cholinergic markers and nicotinic receptors. Two neurochemically and anatomically separate olfactory glomeruli subsets were observed in the posterior OB of the developing rat. These atypical glomeruli expressed large amounts of ³H]vesamicol- and ³H]hemicholinium binding sites without significant amounts of muscarinic M1, M2, or nicotinic α4β2 receptor binding sites. A significant density of ¹²⁵I]α-bungarotoxin binding sites could be detected only at early postnatal ages. A few olfactory glomeruli specifically restricted to the dorsal posterior OB expressed a high density of ³H]cytisine binding sites but lacked significant binding of the two presynaptic cholinergic markers used here, suggesting their noncholinergic but cholinoceptive nature. © 1996 Wiley-Liss, Inc.