title: Differential Muscarinic Modulation in the Olfactory Bulb.
abstract: Neuromodulation of olfactory circuits by acetylcholine (ACh) plays an important role in odor discrimination and learning. Early processing of chemosensory signals occurs in two functionally and anatomically distinct regions, the main and accessory olfactory bulbs (MOB and AOB), which receive extensive cholinergic input from the basal forebrain. Here, we explore the regulation of AOB and MOB circuits by ACh, and how cholinergic modulation influences olfactory-mediated behaviors in mice. Surprisingly, despite the presence of a conserved circuit, activation of muscarinic ACh receptors revealed marked differences in cholinergic modulation of output neurons: excitation in the AOB and inhibition in the MOB. Granule cells (GCs), the most abundant intrinsic neuron in the OB, also exhibited a complex muscarinic response. While GCs in the AOB were excited, MOB GCs exhibited a dual muscarinic action in the form of a hyperpolarization and an increase in excitability uncovered by cell depolarization. Furthermore, ACh influenced the input-output relationship of mitral cells in the AOB and MOB differently showing a net effect on gain in mitral cells of the MOB, but not in the AOB. Interestingly, despite the striking differences in neuromodulatory actions on output neurons, chemogenetic inhibition of cholinergic neurons produced similar perturbations in olfactory behaviors mediated by these two regions. Decreasing ACh in the OB disrupted the natural discrimination of molecularly related odors and the natural investigation of odors associated with social behaviors. Thus, the distinct neuromodulation by ACh in these circuits could underlie different solutions to the processing of general odors and semiochemicals, and the diverse olfactory behaviors they trigger.
  
  
  CONCLUSIONS
  State-dependent cholinergic modulation of brain circuits is critical for several high-level cognitive functions, including attention and memory. Here, we provide new evidence that cholinergic modulation differentially regulates two parallel circuits that process chemosensory information, the accessory and main olfactory bulb (AOB and MOB, respectively). These circuits consist of remarkably similar synaptic arrangement and neuronal types, yet cholinergic regulation produced strikingly opposing effects in output and intrinsic neurons. Despite these differences, the chemogenetic reduction of cholinergic activity in freely behaving animals disrupted odor discrimination of simple odors, and the investigation of social odors associated with behaviors signaled by the Vomeronasal system.
authors.affiliation: Department of Biology, University of Maryland, College Park, Maryland 20742.
authors.forename: Richard S
authors.initials: RS
authors.lastname: Smith
authors.affiliation: Department of Biology, University of Maryland, College Park, Maryland 20742.
authors.forename: Ruilong
authors.initials: R
authors.lastname: Hu
authors.affiliation: Department of Biology, University of Maryland, College Park, Maryland 20742.
authors.forename: Andre
authors.initials: A
authors.lastname: DeSouza
authors.affiliation: Department of Biology, University of Maryland, College Park, Maryland 20742.
authors.forename: Christian L
authors.identifier: http://orcid.org/0000-0003-4352-8625
authors.initials: CL
authors.lastname: Eberly
authors.affiliation: Department of Biology, University of Maryland, College Park, Maryland 20742.
authors.forename: Krista
authors.identifier: http://orcid.org/0000-0001-8036-5082
authors.initials: K
authors.lastname: Krahe
authors.affiliation: Department of Biology, University of Maryland, College Park, Maryland 20742.
authors.forename: Wilson
authors.initials: W
authors.lastname: Chan
authors.affiliation: Department of Biology, University of Maryland, College Park, Maryland 20742 raraneda@umd.edu.
authors.forename: Ricardo C
authors.identifier: http://orcid.org/0000-0001-8728-3063
authors.initials: RC
authors.lastname: Araneda
mesh_terms: D000109:Acetylcholine / Q000378:metabolism*; D000818:Animals; D018678:Cholinergic Agents / Q000494:pharmacology; D005260:Female; D007150:Immunohistochemistry; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D018613:Microscopy, Confocal; D009830:Olfactory Bulb / Q000187:drug effects / Q000378:metabolism*; D009833:Olfactory Pathways / Q000187:drug effects / Q000378:metabolism*; D055696:Olfactory Perception / Q000187:drug effects / Q000502:physiology*; D018408:Patch-Clamp Techniques; D011976:Receptors, Muscarinic / Q000502:physiology
chemical_list: D018678:Cholinergic Agents; D011976:Receptors, Muscarinic; D000109:Acetylcholine
keywords: accessory olfactory bulb; aggression; cholinergic; muscarinic; olfactory; social behavior