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

Nicotine-induced transcriptional changes and mitochondrial dysfunction in the ventral tegmental area revealed by single-nucleus transcriptomics

Nicotine is widely recognized as the primary contributor to tobacco dependence. Previous studies have indicated that molecular and behavioral responses to nicotine are primarily mediated by ventral tegmental area (VTA) neurons, and accumulating evidence suggests that glia play prominent roles in nicotine addiction. However, VTA neurons and glia have yet to be characterized at the transcriptional level during the progression of nicotine self-administration. Here, a male mouse model of nicotine self-administration was established and the timing of three critical phases (pre-addiction, addicting, and post-addiction phase) was characterized. Single-nucleus RNA sequencing (snRNA-seq) in the VTA at each phase was performed to comprehensively classify specific cell subtypes. Adaptive changes occurred during the addicting and post-addiction phases, with the addicting phase displaying highly dynamic neuroplasticity that profoundly impacted the transcription in each cell subtype. Furthermore, significant transcriptional changes in energy metabolism-related genes were observed, accompanied by notable structural alterations in neuronal mitochondria during the progression of nicotine self-administration. The results provide insights into mechanisms underlying the progression of nicotine addiction, serving as important resource for identifying potential molecular targets for nicotine cessation.

Approach/Avoidance Behavior to Novel Objects is Correlated with the Serotonergic and Dopaminergic Systems in the Brown Rat (Rattus norvegicus)

The brown rat (Rattus norvegicus) is known to show three types of behavioral responses to novel objects. Whereas some rats are indifferent to novel objects, neophobic and neophilic rats show avoidance and approach behavior, respectively. Here, we compared the dopaminergic, serotonergic, and noradrenergic systems immunohistochemically among these rats. Trapped wild rats and laboratory rats were first individually exposed to the novel objects in their home cage. Wild rats were divided into neophobic and indifferent rats depending on their behavioral responses. Similarly, laboratory rats were divided into neophilic and indifferent rats. Consistent with the behavioral differences, in the paraventricular nucleus of the hypothalamus, Fos expression in corticotropin-releasing hormone-containing neurons was higher in the neophobic rats than in the indifferent rats. In the anterior basal amygdala, the neophobic rats showed higher Fos expression than the indifferent rats. In the posterior basal amygdala, the neophobic and neophilic rats showed lower and higher Fos expressions than the indifferent rats, respectively. When we compared the neuromodulatory systems, in the dorsal raphe, the number of serotonergic neurons and Fos expression in serotonergic neurons increased linearly from neophobic to indifferent to neophilic rats. In the ventral tegmental area, Fos expression in dopaminergic neurons was higher in the neophilic rats than in the indifferent rats. These results demonstrate that approach/avoidance behavior to novel objects is correlated with the serotonergic and dopaminergic systems in the brown rat. We propose that the serotonergic system suppresses avoidance behavior while the dopaminergic system enhances approach behavior to novel objects.

Optimization of AAV vectors for transactivator-regulated enhanced gene expression within targeted neuronal populations

Adeno-associated virus (AAV) vectors are potential tools for cell-type-selective gene delivery to the central nervous system. Although cell-type-specific enhancers and promoters have been identified for AAV systems, there is limited information regarding the effects of AAV genomic components on the selectivity and efficiency of gene expression. Here, we offer an alternative strategy to provide specific and efficient gene delivery to a targeted neuronal population by optimizing recombinant AAV genomic components, named TAREGET (TransActivator-Regulated Enhanced Gene Expression within Targeted neuronal populations). We established this strategy in oxytocinergic neurons and showed that the TAREGET enabled sufficient gene expression to label long-projecting axons in wild-type mice. Its application to other cell types, including serotonergic and dopaminergic neurons, was also demonstrated. These results demonstrate that optimization of AAV expression cassettes can improve the specificity and efficiency of cell-type-specific gene expression and that TAREGET can renew previously established cell-type-specific promoters with improved performance.

Optogenetic activation of dorsal raphe serotonin neurons induces brain-wide activation

Serotonin is a neuromodulator that affects multiple behavioral and cognitive functions. Nonetheless, how serotonin causes such a variety of effects via brain-wide projections and various receptors remains unclear. Here we measured brain-wide responses to optogenetic stimulation of serotonin neurons in the dorsal raphe nucleus (DRN) of the male mouse brain using functional MRI with an 11.7 T scanner and a cryoprobe. Transient activation of DRN serotonin neurons caused brain-wide activation, including the medial prefrontal cortex, the striatum, and the ventral tegmental area. The same stimulation under anesthesia with isoflurane decreased brain-wide activation, including the hippocampal complex. These brain-wide response patterns can be explained by DRN serotonergic projection topography and serotonin receptor expression profiles, with enhanced weights on 5-HT1 receptors. Together, these results provide insight into the DR serotonergic system, which is consistent with recent discoveries of its functions in adaptive behaviors.

Aggression modulator: Understanding the multifaceted role of the dorsal raphe nucleus

Aggressive behavior is instinctively driven behavior that helps animals to survive and reproduce and is closely related to multiple behavioral and physiological processes. The dorsal raphe nucleus (DRN) is an evolutionarily conserved midbrain structure that regulates aggressive behavior by integrating diverse brain inputs. The DRN consists predominantly of serotonergic (5-HT:5-hydroxytryptamine) neurons and decreased 5-HT activity was classically thought to increase aggression. However, recent studies challenge this 5-HT deficiency model, revealing a more complex role for the DRN 5-HT system in aggression. Furthermore, emerging evidence has shown that non-5-HT populations in the DRN and specific neural circuits contribute to the escalation of aggressive behavior. This review argues that the DRN serves as a multifaceted modulator of aggression, acting not only via 5-HT but also via other neurotransmitters and neural pathways, as well as different subsets of 5-HT neurons. In addition, we discuss the contribution of DRN neurons in the behavioral and physiological aspects implicated in aggressive behavior, such as arousal, reward, and impulsivity, to further our understanding of DRN-mediated aggression modulation.

Requisite role of dorsal raphé in contextual cocaine-memory reconsolidation

Memory reconsolidation is a process by which labile drug memories are restabilized in long-term memory stores, permitting their enduring control over drug-seeking behaviors. In the present study, we investigated the involvement of the dorsal raphé nuclei (DRN) in cocaine-memory reconsolidation. Sprague-Dawley rats (male, female) were trained to self-administer cocaine in a distinct environmental context to establish contextual drug memories. They then received extinction training in a different context. Next, the rats were re-exposed to the cocaine-predictive context for 15 min to reactivate their cocaine memories or remained in their home cages (no-reactivation control). Memory reactivation was sufficient to increase c-Fos expression, an index of neuronal activation, in the DRN, but not in the median raphé nuclei, during reconsolidation, compared to no reactivation. To determine whether DRN neuronal activity was necessary for cocaine-memory reconsolidation, rats received intra-DRN baclofen plus muscimol (BM; GABAB/A agonists) or vehicle microinfusions immediately after or 6 h after a memory reactivation session conducted with or without lever access. The effects of DRN functional inactivation on long-term memory strength, as indicated by the magnitude of context-induced cocaine seeking, were assessed 72 h later. Intra-DRN BM treatment immediately after memory reactivation with or without lever access attenuated subsequent context-induced cocaine-seeking behavior, independent of sex. Conversely, BM treatment in the adjacent periaqueductal gray (PAG) immediately after memory reactivation, or BM treatment in the DRN 6 h after memory reactivation, did not alter responding. Together, these findings indicate that the DRN plays a requisite role in maintaining cocaine-memory strength during reconsolidation.

Integrative Research of Neuropharmacology and Informatics Pharmacology for Mental Disorder

Mental illness poses a huge social burden, accounting for approximately 14% of all deaths. Depression, a major component of mental illness, affects approximately 300 million people worldwide, mainly in developed countries, and is not only a major social burden but also a cause of suicide. The social burden of depression is estimated to increase further in developing countries, and overcoming it is a pressing issue for all countries, including Japan. Although clinical evidence has demonstrated the efficacy of serotonergic neurotransmission enhancers in the treatment of depression, the full picture of their therapeutic effects has not yet been fully elucidated. In this review, we show that the hyperactivity of serotonin neurons, especially those in the dorsal raphe nucleus, is commonly induced by various antidepressants within a period corresponding to the onset of their clinical efficacy. We established quantitative prediction methods for pharmacological activity using only chemical structures to translate the biological understanding of mental disorders, including major depressive disorders, into clinically effective therapeutics. Our method exhibited better performance than the previously reported methods of quantitative prediction, while targeting a larger number of proteins. Our article suggests the importance of integrative neuropharmacology and informatics-based pharmacology studies to understand the biological basis of mental disorders and facilitate drug development for these disorders.

The States of Different 5-HT Receptors Located in the Dorsal Raphe Nucleus Are Crucial for Regulating the Awakening During General Anesthesia

General anesthesia is widely used in various clinical practices due to its ability to cause loss of consciousness. However, the exact mechanism of anesthesia-induced unconsciousness remains unclear. It is generally thought that arousal-related brain nuclei are involved. 5-Hydroxytryptamine (5-HT) is closely associated with sleep arousal. Here, we explore the role of the 5-HT system in anesthetic awakening through pharmacological interventions and optogenetic techniques. Our data showed that exogenous administration of 5-hydroxytryptophan (5-HTP) and optogenetic activation of 5-HT neurons in the dorsal raphe nucleus (DR) could significantly shorten the emergence time of sevoflurane anesthesia in mice, suggesting that regulation of the 5-HT system using both endogenous and exogenous approaches could mediate delayed emergence. In addition, we first discovered that the different 5-HT receptors located in the DR, known as 5-HT autoreceptors, are essential for the regulation of general anesthetic awakening, with 5-HT1A and 5-HT2A/C receptors playing a regulatory role. These results can provide a reliable theoretical basis as well as potential targets for clinical intervention to prevent delayed emergence and some postoperative risks.

μ-opioid receptor agonists and psychedelics: pharmacological opportunities and challenges

Opioid misuse and opioid-involved overdose deaths are a massive public health problem involving the intertwined misuse of prescription opioids for pain management with the emergence of extremely potent fentanyl derivatives, sold as standalone products or adulterants in counterfeit prescription opioids or heroin. The incidence of repeated opioid overdose events indicates a problematic use pattern consistent with the development of the medical condition of opioid use disorder (OUD). Prescription and illicit opioids reduce pain perception by activating µ-opioid receptors (MOR) localized to the central nervous system (CNS). Dysregulation of meso-corticolimbic circuitry that subserves reward and adaptive behaviors is fundamentally involved in the progressive behavioral changes that promote and are consequent to OUD. Although opioid-induced analgesia and the rewarding effects of abused opioids are primarily mediated through MOR activation, serotonin (5-HT) is an important contributor to the pharmacology of opioid abused drugs (including heroin and prescription opioids) and OUD. There is a recent resurgence of interest into psychedelic compounds that act primarily through the 5-HT2A receptor (5-HT 2A R) as a new frontier in combatting such diseases (e.g., depression, anxiety, and substance use disorders). Emerging data suggest that the MOR and 5-HT2AR crosstalk at the cellular level and within key nodes of OUD circuitry, highlighting a major opportunity for novel pharmacological intervention for OUD. There is an important gap in the preclinical profiling of psychedelic 5-HT2AR agonists in OUD models. Further, as these molecules carry risks, additional analyses of the profiles of non-hallucinogenic 5-HT2AR agonists and/or 5-HT2AR positive allosteric modulators may provide a new pathway for 5-HT2AR therapeutics. In this review, we discuss the opportunities and challenges associated with utilizing 5-HT2AR agonists as therapeutics for OUD.

Inhibition of SK Channels in VTA Affects Dopaminergic Neurons to Improve the Depression-Like Behaviors of Post-Stroke Depression Rats

Purpose

This study aimed to investigate the effect of small-conductance calcium-activated potassium channels (SK channels) on the dopaminergic (DA) neuron pathways in the ventral tegmental area (VTA) during the pathogenesis of post-stroke depression (PSD) and explore the improvement of PSD by inhibiting the SK channels.

Patients and Methods

Four groups of Sprague-Dawley rats were randomly divided: Control, PSD, SK channel inhibitor (apamin) and SK channel activator (CyPPA) groups. In both control and CyPPA groups, sham surgery was performed. In the other two groups, middle cerebral arteries were occluded. The behavioral indicators related to depression in different groups were compared. Immunofluorescence was used to measure the activity of DA neurons in the VTA, while qRT-PCR was used to assess the expression of SK channel genes.

Results

The results showed that apamin treatment improved behavioral indicators related to depression compared to the PSD group. Furthermore, the qRT-PCR analysis revealed differential expression of the KCNN1 and KCNN3 subgenes of the SK channels in each group. Immunofluorescence analysis revealed an increase in the expression of DA neurons in the VTA of the PSD group, which was subsequently reduced upon apamin intervention.

Conclusion

This study suggests that SK channel activation following stroke contributes to depression-related behaviors in PSD rats through increased expression of DA neurons in the VTA. And depression-related behavior is improved in PSD rats by inhibiting the SK channels. The results of this study provide a new understanding of PSD pathogenesis and the possibility of developing new strategies to prevent PSD by targeting SK channels.

The role of lateral habenula NMDA receptors in tramadol-induced conditioning

The role of the lateral habenula (LHb) as a hub for receiving and relaying signals from the limbic system to serotonergic, dopaminergic, and norepinephrinergic regions in the brainstem makes this area a critical region in the control of reward and addiction. Behavioral evidence reveals the vital role of the LHb in negative symptoms during withdrawal. In this investigation, we study the role of the LHb N-Methyl D-Aspartate receptor (NMDAR) in the modulation of tramadol reward. Male adult Wistar rats were used in this study. The effect of intra-LHb micro-injection of NMDAR agonist (NMDA, 0.1, 0.5, 2 µg/rat) and antagonist (D-AP5, 0.1, 0.5, 1 µg/rat) was evaluated in conditioned place preference (CPP) paradigm. The obtained results showed that intra-LHb administration of NMDA induced place aversion dose-dependently, while blockade of NMDAR in the LHb using D-AP5 micro-injection led to an increased preference score in the CPP task. Co-administration of NMDA (0.5 µg/rat) with tramadol (4 mg/kg) reduced preference score, while co-administration of D-AP5 (0.5 µg/rat) with a non-effective dose of tramadol (1 mg/kg) potentiate the rewarding effect of tramadol. LHb receives inputs from the limbic system and projects to the monoaminergic nuclei in the brainstem. It has been declared that NMDAR is expressed in LHb, and as obtained data revealed, these receptors could modulate the rewarding effect of tramadol. Therefore, NMDA receptors in the LHb might be a new target for modulating tramadol abuse.

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-HT^MRN→IPN), as a key mediator of reward and aversive stimuli. Moreover, 5-HT receptors, including 5-HT_2A 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-HT^MRN→IPN, in the processing of reward and aversive stimuli.

Diff-ATAC-STARR-Seq: A Method for Genome-Wide Functional Screening of Enhancer Activity in Vivo

Transcriptional regulatory elements, including promoters and enhancers, play a key role in the cell-type specific regulation of the transcriptome. Application of rapidly evolving genetic tools, such as optogenetic/chemogenetic actuators and fluorescent reporters to elucidate the function of cell subtypes in vivo necessitates cell-type specific promoters or enhancers. In this context, methods for genome-wide functional screening of cis-regulatory elements, including enhancers, are of utmost importance. In this study, we describe a novel method for genome-wide functional screening of enhancer activity in vivo with minimal handling. Application of the method to cells from different brain structures and subsequent differential analysis allow identification of active enhancers in the target tissue or brain structures. To demonstrate proof of concept, we applied this method to samples from the dorsal raphe nucleus (DRN) and the medial prefrontal cortex of the mouse brain and successfully identified six enhancers with highly biased activity towards the dorsal raphe nucleus. Considering that these two structures consist of largely similar cell types whereas serotonin and dopamine neurons exist only in the DRN, our results confirm the validity of this method in identifying cell-type specific and brain-structure specific enhancers. Overall, this method will be helpful in identifying cis-regulatory elements suitable for cell-type specific manipulations.

Brainstem glucose metabolism predicts reward dependence scores in treatment-resistant major depression

BACKGROUND

It has been suggested that individual differences in temperament could be involved in the (non-)response to antidepressant (AD) treatment. However, how neurobiological processes such as brain glucose metabolism may relate to personality features in the treatment-resistant depressed (TRD) state remains largely unclear.

METHODS

To examine how brainstem metabolism in the TRD state may predict Cloninger's temperament dimensions Harm Avoidance (HA), Novelty Seeking (NS), and Reward Dependence (RD), we collected ¹⁸fluorodeoxyglucose positron emission tomography (¹⁸FDG PET) scans in 40 AD-free TRD patients. All participants were assessed with the Temperament and Character Inventory (TCI). We applied a multiple kernel learning (MKL) regression to predict the HA, NS, and RD from brainstem metabolic activity, the origin of respectively serotonergic, dopaminergic, and noradrenergic neurotransmitter (NT) systems.

RESULTS

The MKL model was able to significantly predict RD but not HA and NS from the brainstem metabolic activity. The MKL pattern regression model identified increased metabolic activity in the pontine nuclei and locus coeruleus, the medial reticular formation, the dorsal/median raphe, and the ventral tegmental area that contributed to the predictions of RD.

CONCLUSIONS

The MKL algorithm identified a likely metabolic marker in the brainstem for RD in major depression. Although ¹⁸FDG PET does not investigate specific NT systems, the predictive value of brainstem glucose metabolism on RD scores however indicates that this temperament dimension in the TRD state could be mediated by different monoaminergic systems, all involved in higher order reward-related behavior.

Effect of Co-Treatment of Olanzapine with SEP-363856 in Mice Models of Schizophrenia

Olanzapine is a commonly used drug in the treatment of schizophrenia, but its clinical application has been restricted by metabolic-related side effects. In order to mitigate the weight gain side effects caused by olanzapine, other drugs with different targets were selected for combined use and evaluated in animal models of schizophrenia. SEP-363856 is a novel psychotropic agent which is under phase III clinical trials for schizophrenia treatment. The aim of the research was to evaluate whether co-administration of olanzapine and SEP-363856 exerts synergistic anti-schizophrenic effects in the apomorphine (APO)-induced climbing test, the MK-801-induced hyperactivity test, and the Morris water maze test, and therefore reduces the weight gain side effects induced by olanzapine. Through isobolographic analysis, the results showed a synergistic interaction in the climbing test; the experimental ED30 (3 mg/kg) was significantly smaller (p < 0.05) than the theoretical ED30 (5 mg/kg). Additionally, such potentiating effects appeared additive in the MK-801 challenge experiment. Co-treatment with an effective dose of olanzapine and a low dose of SEP-363856 reversed MK-801-induced cognitive impairment symptoms in mice. Moreover, combination treatment with olanzapine and SEP-363856 controls sustained weight gain in mice with chronic exposure to olanzapine. These results support further clinical trials to test the effectiveness of co-treatment of olanzapine and SEP-363856 for controlling symptoms and weight gain in patients with schizophrenia during antipsychotic treatments.

Aversive motivation and cognitive control

Aversive motivation plays a prominent role in driving individuals to exert cognitive control. However, the complexity of behavioral responses attributed to aversive incentives creates significant challenges for developing a clear understanding of the neural mechanisms of this motivation-control interaction. We review the animal learning, systems neuroscience, and computational literatures to highlight the importance of experimental paradigms that incorporate both motivational context manipulations and mixed motivational components (e.g., bundling of appetitive and aversive incentives). Specifically, we postulate that to understand aversive incentive effects on cognitive control allocation, a critical contextual factor is whether such incentives are associated with negative reinforcement or punishment. We further illustrate how the inclusion of mixed motivational components in experimental paradigms enables increased precision in the measurement of aversive influences on cognitive control. A sharpened experimental and theoretical focus regarding the manipulation and assessment of distinct motivational dimensions promises to advance understanding of the neural, monoaminergic, and computational mechanisms that underlie the interaction of motivation and cognitive control.

Dorsal Raphe Nucleus Serotoninergic Neurons Mediate Morphine Rewarding Effect and Conditioned Place Preference

Morphine rewarding properties are the main reasons for drug-craving in behaviors occurring during morphine addiction. It has been suggested that morphine addiction relies on signals to the mesolimbic dopamine system, although the mechanisms outside the dopaminergic system are still unclear. Notably, the role of the dorsal raphe nucleus (DRN) serotoninergic (5-hydroxytryptamine, 5-HT) system remains unexplored. Using in vivo electrophysiological and optogenetic approaches, we found that morphine treatment increased DRN 5-TH neurons firing rate and optogenetic activation of DRN 5-HT neurons induced a rewarding effect, indicating that morphine reward is related to DRN 5-HT neurons. Accordingly, optogenetic inhibition of DRN 5-HT neurons following morphine injection reversed conditioned place preference (CPP) during chronic morphine treatment. These findings aid our understanding of the new functions of the DRN 5-HT neurons for morphine rewarding effect and provide a potential approach for the treatment of morphine addiction.

Serotonergic regulation of the dopaminergic system: Implications for reward-related functions

Serotonin is a critical neuromodulator involved in development and behavior. Its role in reward is however still debated. Here, we first review classical studies involving electrical stimulation protocols and pharmacological approaches. Contradictory results on the serotonergic' involvement in reward emerge from these studies. These differences might be ascribable to either the diversity of cellular types within the raphe nuclei or/and the specific projection pathways of serotonergic neurons. We continue to review more recent work, using optogenetic approaches to activate serotonergic cells in the Raphe to VTA pathway. From these studies, it appears that activation of this pathway can lead to reinforcement learning mediated through the excitation of dopaminergic neurons by serotonergic neurons co-transmitting glutamate. Finally, given the importance of serotonin during development on adult emotion, the effect of abnormal early-life levels of serotonin on the dopaminergic system will also be discussed. Understanding the interaction between the serotonergic and dopaminergic systems during development and adulthood is critical to gain insight into the specific facets of neuropsychiatric disorders.

The role of the anterior pretectal nucleus in pain modulation: A comprehensive review

Descending pain modulation involves multiple encephalic sites and pathways that range from the cerebral cortex to the spinal cord. Behavioral studies conducted in the 1980s revealed that electrical stimulation of the pretectal area causes antinociception dissociation from aversive responses. Anatomical and physiological studies identified the anterior pretectal nucleus and its descending projections to several midbrain, pontine, and medullary structures. The anterior pretectal nucleus is morphologically divided into a dorsal part that contains a dense neuron population (pars compacta) and a ventral part that contains a dense fiber band network (pars reticulata). Connections of the two anterior pretectal nucleus parts are broad and include prominent projections to and from major encephalic systems associated with somatosensory processes. Since the first observation that acute or chronic noxious stimuli activate the anterior pretectal nucleus, it has been established that numerous mediators participate in this response through distinct pathways. Recent studies have confirmed that at least two pain inhibitory pathways are activated from the anterior pretectal nucleus. This review focuses on rodent anatomical, behavioral, molecular, and neurochemical data that have helped to identify mediators of the anterior pretectal nucleus and pathways related to its role in pain modulation.

Serotonergic inhibition of responding for conditioned but not primary reinforcers

Serotonin is widely implicated as a modulator of brain reward function. However, laboratory studies have not yielded a consensus on which specific reward-related processes are influenced by serotonin and in what manner. Here we explored the role of serotonin in cue-reward learning in mice. In a first series of experiments, we found that acute administration of the serotonin reuptake inhibitors citalopram, fluoxetine, or duloxetine all reduced lever pressing reinforced on an FR1 schedule with presentation of a cue that had been previously paired with delivery of food. However, citalopram had no effect on responding that was reinforced with both cue and food on an FR1 schedule. Furthermore, citalopram did not affect nose poke responses that produced no auditory, visual, or proprioceptive cues but were reinforced with food pellets on a progressive ratio schedule. We next performed region-specific knock out of tryptophan hydroxylase-2 (Tph2), the rate-limiting enzyme in serotonin synthesis. Viral delivery of Cre recombinase was targeted to dorsal or median raphe nuclei (DRN, MRN), the major sources of ascending serotonergic projections. MRN but not DRN knockouts were impaired in development of cue-elicited approach during Pavlovian conditioning; both groups were subsequently hyper-responsive when lever pressing for cue presentation. The inhibitory effect of citalopram was attenuated in DRN but not MRN knockouts. Our findings are in agreement with prior studies showing serotonin to suppress responding for conditioned reinforcers. Furthermore, these results suggest an inhibitory role of MRN serotonin neurons in the initial attribution of motivational properties to a reward-predictive cue, but not in its subsequent maintenance. In contrast, the DRN appears to promote the reduction of motivational value attached to a cue when it is presented repeatedly in the absence of primary reward.

Medial prefrontal cortical control of reward- and aversion-based behavioral output: Bottom-up modulation

How does the brain guide our actions? This is a complex issue, where the medial prefrontal cortex (mPFC) plays a crucial role. The mPFC is essential for cognitive flexibility and decision making. These functions are related to reward- and aversion-based learning, which ultimately drive behavior. Though, cortical projections and modulatory systems that may regulate those processes in the mPFC are less understood. How does the mPFC regulate approach-avoidance behavior in the case of conflicting aversive and appetitive stimuli? This is likely dependent on the bottom-up neuromodulation of the mPFC projection neurons. In this review, we integrate behavioral-, pharmacological-, and viral-based circuit manipulation data showing the involvement of mPFC dopaminergic, noradrenergic, cholinergic, and serotoninergic inputs in reward and aversion processing. Given that an incorrect balance of reward and aversion value could be a key problem in mental diseases such as substance use disorders, we discuss outstanding questions for future research on the role of mPFC modulation in reward and aversion.

Rhythms, Reward, and Blues: Consequences of Circadian Photoperiod on Affective and Reward Circuit Function

Circadian disruptions, along with altered affective and reward states, are commonly associated with psychiatric disorders. In addition to genetics, the enduring influence of environmental factors in programming neural networks is of increased interest in assessing the underpinnings of mental health. The duration of daylight or photoperiod is known to impact both the serotonin and dopamine systems, which are implicated in mood and reward-based disorders. This review first examines the effects of circadian disruption and photoperiod in the serotonin system in both human and preclinical studies. We next highlight how brain regions crucial for the serotoninergic system (i.e., dorsal raphe nucleus; DRN), and dopaminergic (i.e., nucleus accumbens; NAc and ventral tegmental area; VTA) system are intertwined in overlapping circuitry, and play influential roles in the pathology of mood and reward-based disorders. We then focus on human and animal studies that demonstrate the impact of circadian factors on the dopaminergic system. Lastly, we discuss how environmental factors such as circadian photoperiod can impact the neural circuits that are responsible for regulating affective and reward states, offering novel insights into the biological mechanisms underlying the pathophysiology, systems, and therapeutic treatments necessary for mood and reward-based disorders.

THE INTRIGUING LIGAND-DEPENDENT AND LIGAND-INDEPENDENT ACTIONS OF THE GROWTH HORMONE SECRETAGOGUE RECEPTOR ON REWARD-RELATED BEHAVIORS

The growth hormone secretagogue receptor (GHSR) is a G-protein-coupled receptor (GPCR) highly expressed in the brain, and also in some peripheral tissues. GHSR activity is evoked by the stomach-derived peptide hormone ghrelin and abrogated by the intestine-derived liver-expressed antimicrobial peptide 2 (LEAP2). In vitro, GHSR displays ligand-independent actions, including a high constitutive activity and an allosteric modulation of other GPCRs. Beyond its neuroendocrine and metabolic effects, cumulative evidence shows that GHSR regulates the activity of the mesocorticolimbic pathway and modulates complex reward-related behaviors towards different stimuli. Here, we review current evidence indicating that ligand-dependent and ligand-independent actions of GHSR enhance reward-related behaviors towards appetitive stimuli and drugs of abuse. We discuss putative neuronal networks and molecular mechanisms that GHSR would engage to modulate such reward-related behaviors. Finally, we briefly discuss imaging studies showing that ghrelin would also regulate reward processing in humans. Overall, we conclude that GHSR is a key regulator of the mesocorticolimbic pathway that influences its activity and, consequently, modulates reward-related behaviors via ligand-dependent and ligand-independent actions.

Serotonin in Health and Disease

The International Journal of Molecular Sciences Special Issue "Serotonin in health and diseases" covers several aspects of the multiple and still mysterious functions of serotonin (5-hydroxytryptamine; 5-HT). 5-HT is neurotransmitter acting in the central nervous system (CNS), blood factor, and neurohormone controlling the function of several peripheral organs. Beyond its widespread implication in physiology, the 5-HT system is involved in numerous diseases of the CNS (e.g., depression, anxiety, schizophrenia, obsessive-compulsive disorders, addiction, Parkinson's disease) and peripheral organs (e.g., gastrointestinal disorders, cardiac arrhythmia, hypertension). The Special Issue includes 14 articles dealing with molecular and cellular effects of 5-HT in periphery and CNS, from functional aspects in lower animals to clinical practices. Beyond physiology, the Special Issue also covers the influence of 5-HT and its receptors in the mechanism of action of psychoactive molecules including antipsychotics, antidepressants, and drug of abuse. The recent progress made on the function and dysfunction of the 5-HT system will certainly increase the understanding of the widespread role of 5-HT ultimately leading to better apprehend its targeting in human diseases.