title: Serotonin increases synaptic activity in olfactory bulb glomeruli.
abstract: Serotoninergic fibers densely innervate olfactory bulb glomeruli, the first sites of synaptic integration in the olfactory system. Acting through 5HT2A receptors, serotonin (5HT) directly excites external tufted cells (ETCs), key excitatory glomerular neurons, and depolarizes some mitral cells (MCs), the olfactory bulb's main output neurons. We further investigated 5HT action on MCs and determined its effects on the two major classes of glomerular interneurons: GABAergic/dopaminergic short axon cells (SACs) and GABAergic periglomerular cells (PGCs). In SACs, 5HT evoked a depolarizing current mediated by 5HT2C receptors but did not significantly impact spike rate. 5HT had no measurable direct effect in PGCs. Serotonin increased spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) in PGCs and SACs. Increased sEPSCs were mediated by 5HT2A receptors, suggesting that they are primarily due to enhanced excitatory drive from ETCs. Increased sIPSCs resulted from elevated excitatory drive onto GABAergic interneurons and augmented GABA release from SACs. Serotonin-mediated GABA release from SACs was action potential independent and significantly increased miniature IPSC frequency in glomerular neurons. When focally applied to a glomerulus, 5HT increased MC spontaneous firing greater than twofold but did not increase olfactory nerve-evoked responses. Taken together, 5HT modulates glomerular network activity in several ways: 1) it increases ETC-mediated feed-forward excitation onto MCs, SACs, and PGCs; 2) it increases inhibition of glomerular interneurons; 3) it directly triggers action potential-independent GABA release from SACs; and 4) these network actions increase spontaneous MC firing without enhancing responses to suprathreshold sensory input. This may enhance MC sensitivity while maintaining dynamic range.
authors.affiliation: Department of Anatomy and Neurobiology, University of Maryland School of Medicine, Baltimore, Maryland; and.
authors.forename: Julia
authors.initials: J
authors.lastname: Brill
authors.affiliation: Department of Anatomy and Neurobiology, University of Maryland School of Medicine, Baltimore, Maryland; and.
authors.forename: Zuoyi
authors.initials: Z
authors.lastname: Shao
authors.affiliation: Department of Anatomy and Neurobiology, University of Maryland School of Medicine, Baltimore, Maryland; and.
authors.forename: Adam C
authors.initials: AC
authors.lastname: Puche
authors.affiliation: Department of Neurobiology and Anatomy, and Brain Institute, University of Utah, Salt Lake City, Utah.
authors.forename: Matt
authors.initials: M
authors.lastname: Wachowiak
authors.affiliation: Department of Anatomy and Neurobiology, University of Maryland School of Medicine, Baltimore, Maryland; and mshipley@som.umaryland.edu.
authors.forename: Michael T
authors.initials: MT
authors.lastname: Shipley
mesh_terms: D000818:Animals; D059290:Dopaminergic Neurons / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D059330:GABAergic Neurons / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D007395:Interneurons / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D051379:Mice; D008810:Mice, Inbred C57BL; D009830:Olfactory Bulb / Q000166:cytology / Q000378:metabolism / Q000502:physiology*; D044402:Receptor, Serotonin, 5-HT2A / Q000378:metabolism; D012701:Serotonin / Q000494:pharmacology*; D013569:Synapses / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D055366:Synaptic Potentials; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
chemical_list: D044402:Receptor, Serotonin, 5-HT2A; D012701:Serotonin; D005680:gamma-Aminobutyric Acid
keywords: glomerulus; neuromodulation; olfactory bulb; serotonin; spike-independent neurotransmission