Using A Novel Optogenetic Approach To Directly Assess 5-Ht1a Somatodendritic Autoreceptor Function In Response To Chronic Selective Serotonin Reuptake Inhibitor Treatment

USING A NOVEL OPTOGENETIC APPROACH TO DIRECTLY ASSESS 5-HT1ASOMATODENDRITIC AUTORECEPTOR FUNCTION IN RESPONSE TO CHRONICSELECTIVE SEROTONIN REUPTAKE INHIBITOR TREATMENTbyKELLY MARIE MCGREGORAugust 2015Advisor: Dr. Rodrigo Andrade Major: Pharmacology (Molecular Neuropharmacology)Degree: Doctor of Philosophy Antidepressant drugs are widely used but their mechanism of action remains only partiallyunderstood. One leading hypothesis holds that a key effect of chronic treatment with a SelectiveSerotonin Reuptake Inhibitor (SSRI) is loss of somatodendritic 5-HT1A receptor-mediatedautoinhibition in serotonergic neurons of the dorsal raphe nucleus (DRN). However, technicallimitations have prevented direct testing of this hypothesis. In the current study we took advantage of optogenetic strategies to assess the effects of the classic SSRI fluoxetine on 5-HT1A receptor-mediated autoinhibition. We conducted these experiments in mice expressing the light-sensitiveion channel Channelrhodopsin (ChR) in 5-HT neurons to facilitate their unambiguous identificationand achieve precise temporal control over endogenous 5-HT release and 5-HT1A autoreceptoractivation. Whole-cell intracellular recordings of DRN 5-HT neurons in in vitro brainstem slicesrevealed that light-induced 5-HT1A autoreceptor-mediated currents in chronically treated mice (14days) were smaller in amplitude but longer in duration, thereby resulting in an overall greater charge transfer compared to controls. Consistent with this, 5-HT dose response curvesconstructed in the presence of bath fluoxetine also provided no evidence for a reduction inautoreceptor sensitivity. To test whether an attenuation of 5-HT clearance could potentiallyaccount for the alterations observed from chronic treatment, responses were compared to thoseobtained following acute or subchronic fluoxetine treatment (bath application or 3 days). In both