Cells in the glomerular layer express NAPE-PLD, an enzyme implicated in the synthesis of anandamide (Egertovaá et al. 2007) targeting external tufted cells, “short axon: cells, and periglomerular cells. Several types of neurons collectively referred to as juxtaglomerular cells send dendrites into the glomerular neuropil (reviewed in Ennis et al. Sensory transmission from olfactory nerve terminals to principal neurons of the MOB, mitral and tufted cells, is regulated by juxtaglomerular cells in glomeruli. The MOB is the first central relay station for olfactory information conveyed from the nasal epithelium by olfactory receptor neurons. However, little is known about the relevance of CB 1 for mitral cell activity in MOB glomeruli. Intriguingly, cannabinoid signaling in the MOB is implicated in regulating appetite and olfactory threshold through centrifugal fiber input to inhibitory granule cells as a means of cortical feedback to the MOB ( Pouille and Schoppa 2018 Soria-Gómez et al. These lipid messengers are produced and broken down enzymatically. 1995) and arachidonoyl ethanolamine (AEA anandamide Devane et al. Two eCBs are strongly implicated in cannabinoid signaling, 2-arachidonoyl glycerol (2-AG Mechoulam et al. 2001 Wilson and Nicoll 2001) in the hippocampus and cerebellum. 2002 Hoffman and Lupica 2000 Katona et al. 1998 Takahashi and Linden 2000) and GABA release ( Diana et al. Functionally, eCBs can act on CB 1 at presynaptic terminals to reduce transmitter release, diminishing glutamate ( Kreitzer and Regehr 2001a Lévénés et al. Many CB 1-expressing neurons in the central nervous system are GABAergic ( Tsou et al. In the granule cell layer, CB 1 is abundantly expressed on axon terminals of centrifugal cortical glutamatergic neurons that project to inhibitory granule cells ( Soria-Gómez et al. Moldrich and Wenger (2000) observed a moderate density of CB 1 immunoreactive cell bodies and fibers in several layers of the MOB: glomerular layer, mitral cell layer, internal plexiform layer, and granule cell layer. 1991 Moldrich and Wenger 2000 Pettit et al. Immunohistochemical and autoradiographic studies indicate that CB 1 is present in the main olfactory bulb (MOB) with moderate to intense levels of staining ( Herkenham et al. 2016), which involves cannabinoid receptors, CB 1 and CB 2, and their endogenous activators, the endocannabinoids (eCBs). The endocannabinoid system has emerged as an important neuromodulatory system ( Iannotti et al. CB 1 directly regulates GABAergic processes to control GABA release and, in turn, regulates mitral cell activity with potential effects on olfactory threshold and behavior. CB 1 was present in periglomerular processes of a GAD65-positive subpopulation of interneurons. We detected endocannabinoids in the mouse MOB. NEW & NOTEWORTHY Cannabinoid signaling with cannabinoid receptor type 1 (CB 1) is involved in the regulation of glomerular activity in the main olfactory bulb (MOB). We conclude that CB 1 directly regulates GABAergic processes in the glomerular layer to control GABA release and, in turn, regulates mitral cell activity with potential effects on olfactory threshold and behavior. This hypothesis was supported by the finding that cannabinoids modulated synaptic transmission to mitral cells. We previously observed that GABAergic periglomerular cells show the inverse response pattern to CB 1 activation compared with mitral cells, suggesting that CB 1 indirectly regulates mitral cell activity as a result of cellular activation of glomerular GABAergic processes. CB 1 effects on mitral cells were absent in subglomerular slices in which the olfactory nerve layer and glomerular layer were removed, suggesting the glomerular layer as the site of CB 1 action. Patch-clamp electrophysiology demonstrated that CB 1 agonists activated mitral cells and evoked an inward current, while CB 1 antagonists reduced firing and evoked an outward current. We detected eCBs in the mouse MOB as well as the expression of CB 1 and other genes associated with cannabinoid signaling in the MOB. CB 1 was present in periglomerular processes of a GAD65-positive subpopulation of interneurons but not in mitral cells. Using anatomical and functional approaches we have explored this question. However, the cannabinoid-related circuitry of inputs to mitral cells in the MOB has not been fully determined. Cannabinoid signaling is involved in regulating glomerular activity in the main olfactory bulb (MOB). Our understanding of the role of cannabinoid receptor type 1 (CB 1) in olfactory processing remains limited. The endocannabinoid (eCB) signaling system has been functionally implicated in many brain regions.
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