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Dopamine plays key roles in the processing of the olfactory information that takes place in the olfactory glomeruli. Previous studies using autoradiography demonstrate that, at the glomerular level, these actions are mainly mediated via activation of D2 dopamine receptors. Moreover, it has been suggested that D2 receptors could be present in the olfactory nerve, where they might modulate the entrance of olfactory input into the brain. Nevertheless, the precise subcellular localization of D2 receptors in the glomerular neuropil has not been investigated. In this report, we show the subcellular distribution of D2 receptors in the glomerular circuits of Wistar rats, using pre-embedding immunogold-silver labelling and electron microscopy. Present results demonstrate for the first time the presence of D2 dopamine receptors into the terminals of the olfactory axons. In addition, we demonstrate that D2 receptors are located into presynaptic elements of the glomerular neuropil other than the olfactory axons. These elements include the dendrites of the mitral/tufted cells and the dendrites of a subset of periglomerular cells that are GABAergic and dopaminergic. This distribution pattern provides anatomical support for a wide range of actions of dopamine in the glomerular circuits through presynaptic mechanisms mediated by D2 receptors. These actions would include: (i) modulation of the glutamate release from the olfactory axons to the dendrites of mitral/tufted cells and periglomerular cells; (ii) modulation of glutamatergic synapses from the dendrites of mitral/tufted cells to the dendrites of periglomerular cells and (iii) modulation of the neurotransmission from a subset of GABAergic/dopaminergic periglomerular cells to mitral/tufted cells. [ABSTRACT FROM AUTHOR]

In the brain, nitric oxide acts as an atypical messenger in cellular nonsynaptic transmission. In the olfactory bulb, this gas is produced at the level of the olfactory glomeruli by a subpopulation of periglomerular cells that participates in the first synaptic relay of the olfactory information between the olfactory nerve and the dendritic tufts of principal cells. It has been proposed that nitric oxide modulates intraglomerular synaptic integration of sensory inputs, but its specific role in the glomerular circuitry remains to be understood. In this article, we demonstrate that, in the glomerular circuits, a specific subset of periglomerular cells, most of them expressing the calcium binding protein calbindin D-28 k, expresses theβ1 subunit of the soluble guanylyl cyclase. These cells could be the targets for the action of nitric oxide at the glomerular level via activation of soluble guanylyl cyclase and production of cGMP. [ABSTRACT FROM AUTHOR]

This study investigates the targets of the population of vasoactive intestinal polypeptide (VIP)-containing deep short-axon cells of the rat olfactory bulb (OB), combining single- and double-immunocytochemical approaches under light and electron microscopy. It has been assumed that deep short-axon cells innervate granule cells in the mammalian OB, but their synaptic connectivity has not been demonstrated to date. Our results indicate that, instead of the accepted scheme of the bulbar circuitry, VIP-containing deep short-axon cells are γ-aminobutyric acid (GABA)ergic interneurons specialized in the selective innervation of other GABAergic deep short-axon cells. Their axons contact with the perisomatic region and the dendritic portions of subsets of deep short-axon cells that contain VIP, calbindin D-28k and neuropeptide Y. Electron microscopy reveals axo-somatic and axo-dendritic symmetrical synapses from VIP-containing boutons. Taken altogether, our data show that the VIP-containing deep short-axon cells of the rat OB form an interneuronal network that modulates the function of other interneurons different from granule cells. They might be involved indirectly in the inhibition or disinhibition of principal cells or might participate in the generation of oscillatory activity and in the synchronization of populations of interneurons and, then, of principal cells. Present data demonstrate that modulation of the OB by local circuits is more complex than the simple inhibition from periglomerular cells and granule cells, and remark the importance of considering the contribution of other classes of GABAergic interneurons different from periglomerular cells and granule cells to the bulbar circuitry. [ABSTRACT FROM AUTHOR]

AbstractWe 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 γ-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. [ABSTRACT FROM AUTHOR]

In the olfactory glomeruli of the rat olfactory bulb, there is a population of periglomerular cells (PG) that contains the neuronal isoform of the nitric oxide synthase (nNOS). To date, these PG have not been characterized neurochemically and it has not been determined whether they are type 1 (GABAergic PG that receive synaptic contacts from the olfactory axons) or type 2 PG (non-GABAergic PG that do not receive synapses from the olfactory axons). Combining pre-embedding NADPH-diaphorase histochemistry and post-embedding immunoperoxidase detection of GABA, we demonstrate that nNOS-containing PG are GABAergic and therefore, belong to the type 1 PG. The possible actions of nitric oxide in the olfactory glomeruli are discussed. [Copyright &y& Elsevier]