Abstract
Expression of TPH2, the rate-limiting enzyme for brain serotonin synthesis, is elevated in the dorsal raphe nucleus (DR) of depressed suicide victims. One hypothesis is that this increase in TPH2 expression is stress-induced. Here, we used an established animal model to address whether exposure to an acute stressor, inescapable tail shock (IS), increases tph2 mRNA and Tph2 protein expression, and if IS sensitizes the DR to a subsequent, heterotypic stressor. In Experiment 1, we measured tph2 mRNA expression 4 h after IS or home cage (HC) control conditions in male rats, using in situ hybridization histochemistry. In Experiment 2, we measured Tph2 protein expression 12 h or 24 h after IS using western blot. In Experiment 3, we measured tph2 mRNA expression following IS on Day 1, and cold swim stress (10 min, 15 °C) on Day 2. Inescapable tail shock was sufficient to increase tph2 mRNA expression 4 h and 28 h later, selectively in the dorsomedial DR (caudal aspect of the dorsal DR, cDRD; an area just rostral to the caudal DR, DRC) and increased Tph2 protein expression in the DRD (rostral and caudal aspects of the dorsal DR combined) 24 h later. Cold swim increased tph2 mRNA expression in the dorsomedial DR (cDRD) 4 h later. These effects were associated with increased immobility during cold swim, elevated plasma corticosterone, and a proinflammatory plasma cytokine milieu (increased interleukin (IL)-6, decreased IL-10). Our data demonstrate that two models of inescapable stress, IS and cold swim, increase tph2 mRNA expression selectively in the anxiety-related dorsomedial DR (cDRD).
Highlights
Anxiety disorders, stress- or trauma-related disorders, and affective disorders are common, with a life-time prevalence of up to 20% of the population, and represent a significant social and economic burden (Kessler et al, 2005; Wittchen et al, 2011)
Inescapable tail shock (IS) increases tph2 mRNA expression in the dorsomedial dorsal raphe nucleus (DR) 4 h later In Experiment 1, analysis of inescapable tail shock (IS)-induced increases in tph2 mRNA expression throughout the rostrocaudal extent of each subdivision of the entire DR using linear mixed models (LMMs) revealed that IS-induced increases in tph2 mRNA expression 4 h after the onset of IS were dependent on rostrocaudal level (Fig. 2b; IS x rostrocaudal level interaction, F(12, 25.3) = 2.90, p < .05)
Exposure to one 100 min-long session of IS was sufficient to increase tph2 mRNA expression in the dorsomedial DR, as measured 4 h after the onset of IS; rats that were exposed to IS on Day 1 responded with increased tph2 mRNA in the dorsomedial DR 4 h after exposure to the heterotypic cold swim stressor on Day 2, relative to rats maintained in the home cage on Day 1 and exposed to cold swim on Day 2
Summary
Stress- or trauma-related disorders, and affective disorders are common, with a life-time prevalence of up to 20% of the population, and represent a significant social and economic burden (Kessler et al, 2005; Wittchen et al, 2011). One well-studied model of how uncontrollable stress can lead to dysregulation of neuronal circuits and behavioral responses is inescapable tail shock (IS) in adult male rats (Maier and Watkins, 2005). Inescapable tail shock leads to large increases in extracellular 5-hydroxytryptamine (5-HT, serotonin) in the DR (Maswood et al, 1998), and increases in anxiety-related behaviors measured 24 h later, such as exaggerated fear conditioning and escape deficits in a shuttle box escape task, behavioral consequences that are absent if the DR is lesioned or inhibited during IS (Maier et al, 1993, 1995). Hyperexcitability of DR serotonergic neurons, measured 24 h following IS, is thought to be dependent on a functional desensitization of inhibitory 5-HT1A autoreceptors in the dorsal DR (DRD) (Rozeske et al, 2011), leading to exaggerated release of serotonin in forebrain circuits mediating anxiety
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