Manipulation of dorsal raphe serotonergic neurons modulates active coping to inescapable stress and anxiety-related behaviors in mice and rats

Abstract

Major depression and anxiety disorders are a social and economic burden worldwide. Serotonergic signaling has been implicated in the pathophysiology of these disorders and thus has been a crucial target for pharmacotherapy. However, the precise mechanisms underlying these disorders are still unclear. Here, we used species-optimized lentiviral vectors that were capable of efficient and specific transduction of serotonergic neurons in mice and rats for elucidation of serotonergic roles in anxiety-like behaviors and active coping behavior in both species. Immunohistochemical analyses revealed that lentiviral vectors with an upstream sequence of tryptophan hydroxylase 2 gene efficiently transduced serotonergic neurons with a specificity of approximately 95% in both mice and rats. Electrophysiological recordings showed that these lentiviral vectors induced sufficient expression of optogenetic tools for precise control of serotonergic neurons. Using these vectors, we demonstrate that acute activation of serotonergic neurons in the dorsal raphe nucleus increases active coping with inescapable stress in rats and mice in a time-locked manner, and that acute inhibition of these neurons increases anxiety-like behaviors specifically in rats. These findings further our understanding of the pathophysiological role of dorsal raphe serotonergic neurons in different species and the role of these neurons as therapeutic targets in major depression and anxiety disorders.

The Role of Dorsal Raphe Serotonin Neurons in the Balance between Reward and Aversion

“…We and others previously reported that lentiviral vectors bearing the mouse and rat TPH2 promoter can selectively transduce 5-HT neurons in mice and rats, respectively [31,37]. To manipulate 5-HT neuronal activity not only at the cell soma region but also at the axon terminal region, we made an adeno-associated virus (AAV) bearing CheRiff, an optogenetic actuator that shows good expression and membrane trafficking [38], under the control of the mouse TPH2 promoter.…”

“…Previous reports have indicated the importance of 5-HT neuronal signaling in the pathophysiology and therapeutics of major depressive disorders [26][27][28][29]. Indeed, previous reports have demonstrated that the activation of DRN 5-HT neurons induces an antidepressant-like effect in mice and rats [30,31]. Similar to an antidepressive effect, rewarding potency of DRN 5-HT neurons has been studied by using several lines of genetically-modified mice [9][10][11][32][33][34][35].…”

“…To address these issues, in the present study, we examined the link between the excitation of DRN 5-HT neurons and reward potency, and that between inhibition of DRN 5-HT neurons and aversive potency using two different methods selectively transducing 5-HT neurons. We examined the rewarding potency of optogenetic manipulation of DRN 5-HT neurons using an adeno-associated virus (AAV) expressing optogenetic actuators under the control of the mouse tryptophan hydroxylase 2 (TPH2) promoter [31]. We also employed the Tph2-tTA transgenic mouse line [36] in combination with tTA-dependent AAVs, to further investigate the reward/aversive potency of these neurons.…”

Genetic approaches to the investigation of serotonergic neuron functions in animals

“…Based on these constructs, lentiviral and adeno-associated viral vectors were created for serotonin neuron targeting (Benzekhroufa et al, 2009). Moreover, these vectors were improved by optimizing the IRES site and adding the WPRE sequence to increase the expression level of the genes delivered by the retroviral vectors (Nishitani et al, 2019).…”

“…To deliver optogenetic proteins to serotonergic neurons of animals that had not been subjected to genetic modifications, lentiviral vectors based on the rat and mouse tph2 promoters were designed. They made it possible to study the differences in the depressive-like and anxious behavior of these species, stimulating the neurons of the dorsal raphe nucleus (Nishitani et al, 2019).…”