Subcellular localization of m2 muscarinic receptors in GABAergic interneurons of the olfactory bulb

Abstract

We analysed the ultrastructural distribution of the m2 muscarinic receptor (m2R) in the rat olfactory bulb (OB) using immunohistochemical techniques and light and electron microscopy. m2R was differentially distributed within the cellular compartments of gamma-aminobutyric acid (GABA)ergic bulbar interneurons. It is located in the gemmules of granule cells and in the synaptic loci of the interneurons of the external plexiform layer, suggesting that m2R activation could modulate the release of GABA from these interneurons onto principal cells by a presynaptic mechanism. By contrast, the receptor appears in the somata and dendritic trunks of second-order short-axon interneurons located in the inframitral layers, suggesting that postsynaptic muscarinic activation in these cells could elicit the inhibition of granule cells, leading to a disinhibition of principal cells. We also detail the anatomical substrate for a new putative muscarinic modulation that has not been previously described, and that could influence the reception of sensory information within the olfactory glomeruli. m2R appears in a subset of GABAergic/dopaminergic juxtaglomerular cells innervated by olfactory axons but is absent in juxtaglomerular cells that do not receive sensory inputs. This finding suggests that m2R activation could modify, through dopaminergic local circuits, the strength of olfactory nerve inputs onto principal cells. Activation of the muscarinic receptor may modulate the olfactory information encoding within olfactory glomeruli and may facilitate the bulbar transmission to superior centres influencing the GABA release by presynaptic and postsynaptic mechanisms. Taken together, our data provide the neuroanatomical basis for a complex action of m2R at different levels in the mammalian OB.