title: Serotonergic systems in the balance: CRHR1 and CRHR2 differentially control stress-induced serotonin synthesis.
abstract: Anxiety and affective disorders are often associated with hypercortisolism and dysfunctional serotonergic systems, including increased expression of TPH2, the gene encoding the rate-limiting enzyme of neuronal serotonin synthesis. We previously reported that chronic glucocorticoid exposure is anxiogenic and increases rat Tph2 mRNA expression, but it was still unclear if this also translates to increased TPH2 protein levels and in vivo activity of the enzyme. Here, we found that adult male rats treated with corticosterone (CORT, 100 μg/ml) via the drinking water for 21 days indeed show increased TPH2 protein expression in the dorsal and ventral part of the dorsal raphe nucleus (DRD, DRV) during the light phase, abolishing the enzyme's diurnal rhythm. In a second study, we systemically blocked the conversion of 5-hydroxytryptophan (5-HTP) to serotonin immediately before rats treated with CORT or vehicle were either exposed to 30 min acoustic startle stress or home cage control conditions. This allowed us to measure 5-HTP accumulation as a direct readout of basal versus stress-induced in vivo TPH2 activity. As expected, basal TPH2 activity was elevated in the DRD, DRV and MnR of CORT-treated rats. In response to stress, a multitude of serotonergic systems reacted with increased TPH2 activity, but the stress-, anxiety-, and learned helplessness-related dorsal and caudal DR (DRD/DRC) displayed stress-induced increases in TPH2 activity only after chronic CORT-treatment. To address the mechanisms underlying this region-specific CORT-dependent sensitization, we stereotaxically implanted CORT-treated rats with cannulae targeting the DR, and pharmacologically blocked either corticotropin-releasing hormone receptor type 1 (CRHR1) or type 2 (CRHR2) 10 min prior to acoustic startle stress. CRHR2 blockade prevented stress-induced increases of TPH2 activity within the DRD/DRC, while blockade of CRHR1 potentiated stress-induced TPH2 activity in the entire DR. Stress-induced TPH2 activity in the DRD/DRC furthermore predicted TPH2 activity in the amygdala and in the caudal pontine reticular nucleus (PnC), while serotonin synthesis in the PnC was strongly correlated with the maximum startle response. Our data demonstrate that chronically elevated glucocorticoids sensitize stress- and anxiety-related serotonergic systems, and for the first time reveal competing roles of CRHR1 and CRHR2 on stress-induced in vivo serotonin synthesis.
authors.affiliation: Department of Integrative Physiology and Center for Neuroscience, University of Colorado Boulder, 1725 Pleasant Street, 114 Clare Small, UCB 354, 80309 Boulder, CO, USA; Department of Stress Neurobiology and Neurogenetics, Max Planck Institute of Psychiatry, Kraepelinstr. 2-10, 80804 Munich, Germany. Electronic address: nina_donner@psych.mpg.de.
authors.forename: Nina C
authors.initials: NC
authors.lastname: Donner
authors.affiliation: Department of Integrative Physiology and Center for Neuroscience, University of Colorado Boulder, 1725 Pleasant Street, 114 Clare Small, UCB 354, 80309 Boulder, CO, USA.
authors.forename: Philip H
authors.initials: PH
authors.lastname: Siebler
authors.affiliation: Department of Integrative Physiology and Center for Neuroscience, University of Colorado Boulder, 1725 Pleasant Street, 114 Clare Small, UCB 354, 80309 Boulder, CO, USA.
authors.forename: Danté T
authors.initials: DT
authors.lastname: Johnson
authors.affiliation: Department of Integrative Physiology and Center for Neuroscience, University of Colorado Boulder, 1725 Pleasant Street, 114 Clare Small, UCB 354, 80309 Boulder, CO, USA.
authors.forename: Marcos D
authors.initials: MD
authors.lastname: Villarreal
authors.affiliation: Department of Integrative Physiology and Center for Neuroscience, University of Colorado Boulder, 1725 Pleasant Street, 114 Clare Small, UCB 354, 80309 Boulder, CO, USA.
authors.forename: Sofia
authors.initials: S
authors.lastname: Mani
authors.affiliation: Department of Integrative Physiology and Center for Neuroscience, University of Colorado Boulder, 1725 Pleasant Street, 114 Clare Small, UCB 354, 80309 Boulder, CO, USA.
authors.forename: Allison J
authors.initials: AJ
authors.lastname: Matti
authors.affiliation: Department of Integrative Physiology and Center for Neuroscience, University of Colorado Boulder, 1725 Pleasant Street, 114 Clare Small, UCB 354, 80309 Boulder, CO, USA.
authors.forename: Christopher A
authors.initials: CA
authors.lastname: Lowry
mesh_terms: D000679:Amygdala / Q000187:drug effects / Q000378:metabolism; D000818:Animals; D001007:Anxiety / Q000235:genetics / Q000378:metabolism; D003345:Corticosterone / Q000494:pharmacology; D065847:Dorsal Raphe Nucleus / Q000187:drug effects / Q000378:metabolism; D006706:Homeostasis / Q000187:drug effects / Q000235:genetics; D008297:Male; D053858:Metabolic Networks and Pathways / Q000187:drug effects / Q000235:genetics; D051381:Rats; D017207:Rats, Sprague-Dawley; D018019:Receptors, Corticotropin-Releasing Hormone / Q000502:physiology*; D012701:Serotonin / Q000378:metabolism*; D013315:Stress, Psychological / Q000235:genetics / Q000378:metabolism*; D014365:Tryptophan Hydroxylase / Q000235:genetics / Q000378:metabolism; D000099052:CRF Receptor, Type 1
chemical_list: C101531:CRF receptor type 2; D018019:Receptors, Corticotropin-Releasing Hormone; D012701:Serotonin; D000099052:CRF Receptor, Type 1; D014365:Tryptophan Hydroxylase; C504569:tph2 protein, rat; D003345:Corticosterone
keywords: Corticotropin receptor; Dorsal raphe nucleus; Glucocorticoids; Serotonin; Stress; Tryptophan hydroxylase