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

BACKGROUND: Reward processing is fundamental for animals to survive and reproduce. Many studies have shown the importance of dorsal raphe nucleus (DRN) serotonin (5-HT) neurons in this process, but the strongly correlative link between the activity of DRN 5-HT neurons and rewarding/aversive potency is under debate. Our primary objective was to reveal this link using two different strategies to transduce DRN 5-HT neurons. METHODS: For transduction of 5-HT neurons in wildtype mice, adeno-associated virus (AAV) bearing the mouse tryptophan hydroxylase 2 (TPH2) gene promoter was used. For transduction in Tph2-tTA transgenic mice, AAVs bearing the tTA-dependent TetO enhancer were used. To manipulate the activity of 5-HT neurons, optogenetic actuators (CheRiff, eArchT) were expressed by AAVs. For measurement of rewarding/aversive potency, we performed a nose-poke self-stimulation test and conditioned place preference (CPP) test. RESULTS: We found that stimulation of DRN 5-HT neurons and their projections to the ventral tegmental area (VTA) increased the number of nose-pokes in self-stimulation test and CPP scores in both targeting methods. Concomitantly, CPP scores were decreased by inhibition of DRN 5-HT neurons and their projections to VTA. CONCLUSION: Our findings indicate that the activity of DRN 5-HT neurons projecting to the VTA is a key modulator of balance between reward and aversion.

Identification of neuron-type specific promoters in monkey genome and their functional validation in mice.

Viral gene delivery is one of the most versatile techniques for elucidating the mechanisms underlying brain dysfunction, such as neuropsychiatric disorders. Due to the complexity of the brain, expression of genetic tools, such as channelrhodopsin and calcium sensors, often has to be restricted to a specified cell type within a circuit implicated in these disorders. Only a handful of promoters targeting neuronal subtypes are currently used for viral gene delivery. Here, we isolated conserved promoter regions of several subtype-specific genes from the macaque genome and investigated their functionality in the mouse brain when used within lentiviral vectors (LVVs). Immunohistochemical analysis revealed that transgene expression induced by the promoter sequences for somatostatin (SST), cholecystokinin (CCK), parvalbumin (PV), serotonin transporter (SERT), vesicular acetylcholine transporter (vAChT), substance P (SP) and proenkephalin (PENK) was largely colocalized with specific markers for the targeted neuronal populations. Moreover, by combining these results with in silico predictions of transcription factor binding to the isolated sequences, we identified transcription factors possibly underlying cell-type specificity. These findings lay a foundation for the expansion of the current toolbox of promoters suitable for elucidating these neuronal phenotypes.

Ketamine-Induced Prefrontal Serotonin Release Is Mediated by Cholinergic Neurons in the Pedunculopontine Tegmental Nucleus.

Background: Ketamine rapidly elicits antidepressive effects in humans and mice in which serotonergic activity is involved. Although α4ß2 nicotinic acetylcholine receptor (α4ß2 nAChR) in the dorsal raphe nucleus plays a key role in the ketamine-induced prefrontal serotonin release, the source of cholinergic afferents, and its role is unclear. Methods: Prefrontal serotonin levels after ketamine injection were measured by microdialysis in rats. Electrolytic lesion of pedunculopontine tegmental nucleus and laterodorsal tegmental nucleus was made with constant direct current. Results: Bilateral lesion of the pedunculopontine tegmental nucleus, but not laterodorsal tegmental nucleus, attenuated prefrontal serotonin release induced by systemic ketamine. Intra-pedunculopontine tegmental nucleus, but not intra-laterodorsal tegmental nucleus ketamine perfusion, increased prefrontal serotonin release. This increase was attenuated by intra-dorsal raphe nucleus injection of dihydro-ß-erythroidine, an α4ß2 nAChR antagonist, or NBQX, an AMPA receptor antagonist. Conclusions: These results suggest the ketamine-induced serotonin release in medial prefrontal cortex is mediated by cholinergic neurons projecting from pedunculopontine tegmental nucleus to dorsal raphe nucleus via α4ß2 nAChRs.

Dorsal raphe serotonergic neurons preferentially reactivate dorsal dentate gyrus cell ensembles associated with positive experience.

Major depressive disorder (MDD) is among the most common mental illnesses. Serotonergic (5-HT) neurons are central to the pathophysiology and treatment of MDD. Repeatedly recalling positive episodes is effective for MDD. Stimulating 5-HT neurons of the dorsal raphe nucleus (DRN) or neuronal ensembles in the dorsal dentate gyrus (dDG) associated with positive memories reverses the stress-induced behavioral abnormalities. Despite this phenotypic similarity, their causal relationship is unclear. This study revealed that the DRN 5-HT neurons activate dDG neurons; surprisingly, this activation was specifically observed in positive memory ensembles rather than neutral or negative ensembles. Furthermore, we revealed that dopaminergic signaling induced by activation of DRN 5-HT neurons projecting to the ventral tegmental area mediates an increase in active coping behavior and positive dDG ensemble reactivation. Our study identifies a role of DRN 5-HT neurons as specific reactivators of positive memories and provides insights into how serotonin elicits antidepressive effects.

Median raphe serotonergic neurons projecting to the interpeduncular nucleus control preference and aversion.

Appropriate processing of reward and aversive information is essential for survival. Although a critical role of serotonergic neurons in the dorsal raphe nucleus (DRN) in reward processing has been shown, the lack of rewarding effects with selective serotonin reuptake inhibitors (SSRIs) implies the presence of a discrete serotonergic system playing an opposite role to the DRN in the processing of reward and aversive stimuli. Here, we demonstrated that serotonergic neurons in the median raphe nucleus (MRN) of mice process reward and aversive information in opposite directions to DRN serotonergic neurons. We further identified MRN serotonergic neurons, including those projecting to the interpeduncular nucleus (5-HTMRN→IPN), as a key mediator of reward and aversive stimuli. Moreover, 5-HT receptors, including 5-HT2A receptors in the interpeduncular nucleus, are involved in the aversive properties of MRN serotonergic neural activity. Our findings revealed an essential function of MRN serotonergic neurons, including 5-HTMRN→IPN, in the processing of reward and aversive stimuli.

An Adenosine A2A Receptor Antagonist Improves Multiple Symptoms of Repeated Quinpirole-Induced Psychosis.

Obsessive-compulsive disorder (OCD) is a neuropsychiatric disorder characterized by the repeated rise of concerns (obsessions) and repetitive unwanted behavior (compulsions). Although selective serotonin reuptake inhibitors (SSRIs) is the first-choice drug, response rates to SSRI treatment vary between symptom dimensions. In this study, to find a therapeutic target for SSRI-resilient OCD symptoms, we evaluated treatment responses of quinpirole (QNP) sensitization-induced OCD-related behaviors in mice. SSRI administration rescued the cognitive inflexibility, as well as hyperactivity in the lateral orbitofrontal cortex (lOFC), while no improvement was observed for the repetitive behavior. D2 receptor signaling in the central striatum (CS) was involved in SSRI-resistant repetitive behavior. An adenosine A2A antagonist, istradefylline, which rescued abnormal excitatory synaptic function in the CS indirect pathway medium spiny neurons (MSNs) of sensitized mice, alleviated both of the QNP-induced abnormal behaviors with only short-term administration. These results provide a new insight into therapeutic strategies for SSRI-resistant OCD symptoms and indicate the potential of A2A antagonists as a rapid-acting anti-OCD drug.

TRPM2 confers susceptibility to social stress but is essential for behavioral flexibility.

Transient receptor potential melastatin 2 (TRPM2) is a Ca2+-permeable, nonselective cation channel and a member of the TRP channel superfamily that acts as a sensor of intracellular redox states. TRPM2 is widely distributed in many tissues and highly expressed in the brain, but the physiological roles of TRPM2 in the central nervous system remain unclear. In this study, TRPM2-deficient mice were examined in a series of behavioral tests. TRPM2-deficient mice did not significantly differ from wild-type littermates in muscle strength, light/dark transition test, rotarod, elevated plus maze, social interaction, prepulse inhibition, Y-maze, forced swim test, cued and contextual fear conditioning, and tail suspension test. In the Barnes circular maze, TRPM2-deficient mice learned the fixed escape box position at similar extent to wild-type littermates, suggesting normal reference memory. However, performance of the first reversal trial and probe test were significantly impaired in TRPM2-deficient mice. In the T-maze delayed alternation task, TRPM2 deficiency significantly reduced choice accuracy. These results indicate that TRPM2-deficient mice shows behavioral inflexibility. Meanwhile, social avoidance induced by repeated social defeat stress was significantly attenuated in TRPM2-deficient mice, suggesting that TRPM2 deficiency confers stress resiliency. Our findings indicate that TRPM2 plays an essential role in maintaining behavioral flexibility but it increases susceptibility to stress.

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

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.