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] alpha-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 alpha 4 beta 2 receptor binding sites. A significant density of [125I] alpha-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.