The rat olfactory bulb is innervated by basal forebrain cholinergic neurons
and is endowed with both nicotinic and muscarinic receptors. The developme
nt of this centrifugal cholinergic innervation occurs mainly in early postn
atal stages. This developmental time-course and the demonstration that acet
ylcholine can modulate some aspects of neuronal proliferation, differentiat
ion or death, suggests the possible involvement of cholinergic afferents in
the morphogenesis and/or plasticity of the olfactory bulb. The purpose of
the present work was to assess whether acetylcholine could modulate neurona
l morphogenesis in the olfactory bulb. Toward this aim, we developed a prim
ary culture model of rat olfactory bulbs. Three major cell types were ident
ified on the basis of their morphological and immunocytochemical phenotype:
neuronal-shaped cells expressing the neuronal markers neuron specific enol
ase, microtubule associated protein 2, neural cell adhesion molecule and be
ta-tubulin III; glial-like cells immunoreactive for glial fibrillary acidic
protein and flattened cells immunolabelled with antibodies against beta-tu
bulin III and nestin, most likely neuronal precursors. After three to six d
ays of treatment with 100-mu M carbachol, a cholinergic agonist, significan
t increase in neuritic length was observed in cultured olfactory bulb neuro
ns. The neurite outgrowth effect of carbachol was abolished by cotreatment
with 1 mu M alpha-bungarotoxin, an alpha 7 subunit nicotinic receptor antag
onist, but was not affected by the addition of 10 mu M atropine, a general
muscarinic antagonist. The effect of carbachol was also mimicked by the nic
otinic agonists, nicotine (100 mu M) and epibatidine (10 mu M) This pharmac
ological profile suggested the involvement of nicotinic receptors of the al
pha 7-like subtype as confirmed using I-125-alpha-bungarotoxin receptor aut
oradiography.
Taken together, these data argue for a role for nicotinic receptors in neur
itic outgrowth in the rat olfactory bulb and provide a cellular support to
the previously described effects of acetylcholine on olfactory bulb plastic
ity in vivo. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights r
eserved.