Control of Amygdala Circuits by 5-HT Neurons via 5-HT and Glutamate Cotransmission

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.

Serotonergic neuromodulation of synaptic plasticity

Synaptic plasticity constitutes a fundamental process in the reorganization of neural networks that underlie memory, cognition, emotional responses, and behavioral planning. At the core of this phenomenon lie Hebbian mechanisms, wherein frequent synaptic stimulation induces long-term potentiation (LTP), while less activation leads to long-term depression (LTD). The synaptic reorganization of neuronal networks is regulated by serotonin (5-HT), a neuromodulator capable of modify synaptic plasticity to appropriately respond to mental and behavioral states, such as alertness, attention, concentration, motivation, and mood. Lately, understanding the serotonergic Neuromodulation of synaptic plasticity has become imperative for unraveling its impact on cognitive, emotional, and behavioral functions. Through a comparative analysis across three main forebrain structures-the hippocampus, amygdala, and prefrontal cortex, this review discusses the actions of 5-HT on synaptic plasticity, offering insights into its role as a neuromodulator involved in emotional and cognitive functions. By distinguishing between plastic and metaplastic effects, we provide a comprehensive overview about the mechanisms of 5-HT neuromodulation of synaptic plasticity and associated functions across different brain regions.

Effects of the suppression of 5-HT1A receptors in the left, right, or bilateral basolateral amygdala on memory consolidation in chronic stress in male rats

The serotonin-1A receptors (5-HT1A) in the two cerebral hemispheres are differentially involved in memory. The distribution of 5-HT1A receptors in the left and right amygdala is different. Furthermore, evidence shows that the 5-HT1A receptors in the left and right amygdala work differently in memory function. The basolateral amygdala (BLA) also regulates hippocampal long-term potentiation (LTP) during stress. However, which BLA structure in each hemisphere underlies such lateralized function is unclear. The present research investigated the possible involvement of 5-HT1A lateralization in the BLA on stress-induced memory impairment. 5-HT1A receptor antagonist (Way-100-635) was injected into the left, right, or bilateral BLA twenty minutes before chronic restraint stress (CRS) for 14 consecutive days. Results indicated that suppression of 5HT1A-receptors in the BLA plays an essential role in reducing the acquisition of passive avoidance in the shuttle box test and spatial memory in the Barnes maze test in the stress animals. This decrease was significant in the CRS animals with left and bilateral suppressed 5HT1A-receptors in the BLA. Field potential recording results showed that the left, right, and bilateral injection of Way-100-635 into the BLA significantly reduced the slope and amplitude of fEPSP in the CA1 area of the hippocampus in stressed rats. No significant difference was observed in neuronal arborization in the CA1 area of the hippocampus. In conclusion, the 5-HT1A receptor in the left and right sides of BLA nuclei play a different role in memory consolidation in the hippocampus under stress.

Evidence for a Role of 5-HT-glutamate Co-releasing Neurons in Acute Stress Mechanisms

A major subpopulation of midbrain 5-hydroxytryptamine (5-HT) neurons expresses the vesicular glutamate transporter 3 (VGLUT3) and co-releases 5-HT and glutamate, but the function of this co-release is unclear. Given the strong links between 5-HT and uncontrollable stress, we used a combination of c-Fos immunohistochemistry and conditional gene knockout mice to test the hypothesis that glutamate co-releasing 5-HT neurons are activated by stress and involved in stress coping. Acute, uncontrollable swim stress increased c-Fos immunoreactivity in neurons co-expressing VGLUT3 and the 5-HT marker tryptophan hydroxylase 2 (TPH2) in the dorsal raphe nucleus (DRN). This effect was localized in the ventral DRN subregion and prevented by the antidepressant fluoxetine. In contrast, a more controllable stressor, acute social defeat, had no effect on c-Fos immunoreactivity in VGLUT3-TPH2 co-expressing neurons in the DRN. To test whether activation of glutamate co-releasing 5-HT neurons was causally linked to stress coping, mice with a specific deletion of VGLUT3 in 5-HT neurons were exposed to acute swim stress. Compared to wildtype controls, the mutant mice showed increased climbing behavior, a measure of active coping. Wildtype mice also showed increased climbing when administered fluoxetine, revealing an interesting parallel between the behavioral effects of genetic loss of VGLUT3 in 5-HT neurons and 5-HT reuptake inhibition. We conclude that 5-HT-glutamate co-releasing neurons are recruited by exposure to uncontrollable stress. Furthermore, natural variation in the balance of 5-HT and glutamate co-released at the 5-HT synapse may impact stress susceptibility.

Serotonin modulates excitatory synapse maturation in the developing prefrontal cortex

Serotonin (5-HT) imbalances in the developing prefrontal cortex (PFC) are linked to long-term behavioral deficits. However, the synaptic mechanisms underlying 5-HT-mediated PFC development are unknown. We found that chemogenetic suppression and enhancement of 5-HT release in the PFC during the first two postnatal weeks decreased and increased the density and strength of excitatory spine synapses, respectively, on prefrontal layer 2/3 pyramidal neurons in mice. 5-HT release on single spines induced structural and functional long-term potentiation (LTP), requiring both 5-HT2A and 5-HT7 receptor signals, in a glutamatergic activity-independent manner. Notably, LTP-inducing 5-HT stimuli increased the long-term survival of newly formed spines ( ≥ 6 h) via 5-HT7 Gαs activation. Chronic treatment of mice with fluoxetine, a selective serotonin-reuptake inhibitor, during the first two weeks, but not the third week of postnatal development, increased the density and strength of excitatory synapses. The effect of fluoxetine on PFC synaptic alterations in vivo was abolished by 5-HT2A and 5-HT7 receptor antagonists. Our data describe a molecular basis of 5-HT-dependent excitatory synaptic plasticity at the level of single spines in the PFC during early postnatal development.

Lateralization of the 5-HT1A receptors in the basolateral amygdala in metabolic and anxiety responses to chronic restraint stress

Behavioral and functional studies describe hemispheric asymmetry in anxiety and metabolic behaviors in responses to stress. However, no study has reported serotonergic receptor (the 5-HT1A receptor) lateralization in the basolateral amygdala (BLA) in vivo on anxiety and metabolic behaviors under stress. In the present study, the effect of unilateral and bilateral suppression of the 5-HT1A receptor in the BLA on anxiety, and metabolic responses to chronic restraint stress was assessed. Male Wistar rats 7 days after cannulation into the BLA received chronic restraint stress for 14 consecutive days. 20 minutes before induction of stress, WAY-100-635 (selective 5-HT1A antagonist) or sterile saline (vehicle) was administered either uni- or bi-laterally into the BLA. Behavioral (elevated plus maze; EPM, and open field test), and metabolic parameter studies were performed. Results showed that stress causes a significant increase in weight gain compared to control. In the non-stress condition, the left and bilaterally, and in the stress condition the right, left, and both sides, inhibition of 5-HT1A in the BLA reduced weight gain. In the restraint stress condition, only inhibition of the 5-HT1A receptor in the left BLA led to decreased food intake compared to the control group. In stress conditions, inhibition of the 5-HT1A receptor on the right, left, and bilateral BLA increased water intake compared to the stress group. Inhibition of the 5-HT1A receptor on the left side of the BLA by WAY-100-635 induced anxiety-like behaviors in stressed rats. Similarly, WAY-100-635 on the left BLA effectively caused anxiety-like behaviors in both EPM and open field tests in the control animals. In conclusion, it seems that 5-HT1A receptors in the left BLA are more responsible for anxiety-like behaviors and metabolic changes in responses to stress.

Synthetic surprise as the foundation of the psychedelic experience

Psychedelic agents, such as LSD and psilocybin, induce marked alterations in consciousness via activation of the 5-HT2A receptor (5-HT2ARs). We hypothesize that psychedelics enforce a state of synthetic surprise through the biased activation of the 5-HTRs system. This idea is informed by recent insights into the role of 5-HT in signaling surprise. The effects on consciousness, explained by the cognitive penetrability of perception, can be described within the predictive coding framework where surprise corresponds to prediction error, the mismatch between predictions and actual sensory input. Crucially, the precision afforded to the prediction error determines its effect on priors, enabling a dynamic interaction between top-down expectations and incoming sensory data. By integrating recent findings on predictive coding circuitry and 5-HT2ARs transcriptomic data, we propose a biological implementation with emphasis on the role of inhibitory interneurons. Implications arise for the clinical use of psychedelics, which may rely primarily on their inherent capacity to induce surprise in order to disrupt maladaptive patterns.

Mu Opioid Receptor-Expressing Neurons in the Dorsal Raphe Nucleus Are Involved in Reward Processing and Affective Behaviors

BACKGROUND

Mu opioid receptors (MORs) are key for reward processing, mostly studied in dopaminergic pathways. MORs are also expressed in the dorsal raphe nucleus (DRN), which is central for the modulation of reward and mood, but MOR function in the DRN remains underexplored. Here, we investigated whether MOR-expressing neurons of the DRN (DRN-MOR neurons) participate in reward and emotional responses.

METHODS

We characterized DRN-MOR neurons anatomically using immunohistochemistry and functionally using fiber photometry in responses to morphine and rewarding/aversive stimuli. We tested the effect of opioid uncaging on the DRN on place conditioning. We examined the effect of DRN-MOR neuron optostimulation on positive reinforcement and mood-related behaviors. We mapped their projections and selected DRN-MOR neurons projecting to the lateral hypothalamus for a similar optogenetic experimentation.

RESULTS

DRN-MOR neurons form a heterogeneous neuronal population essentially composed of GABAergic (gamma-aminobutyric acidergic) and glutamatergic neurons. Calcium activity of DRN-MOR neurons was inhibited by rewarding stimuli and morphine. Local photo-uncaging of oxymorphone in the DRN produced conditioned place preference. DRN-MOR neuron optostimulation triggered real-time place preference and was self-administered, promoted social preference, and reduced anxiety and passive coping. Finally, specific optostimulation of DRN-MOR neurons projecting to the lateral hypothalamus recapitulated the reinforcing effects of total DRN-MOR neuron stimulation.

CONCLUSIONS

Our data show that DRN-MOR neurons respond to rewarding stimuli and that their optoactivation has reinforcing effects and promotes positive emotional responses, an activity which is partially mediated by their projections to the lateral hypothalamus. Our study also suggests a complex regulation of DRN activity by MOR opioids, involving mixed inhibition/activation mechanisms that fine-tune DRN function.

Mechanisms of SSRI Therapy and Discontinuation

SSRIs are one of the most widely used drug therapies in primary care and psychiatry, and central to the management of the most common mental health problems in today's society. Despite this, SSRIs suffer from a slow onset of therapeutic effect and relatively poor efficacy as well as adverse effects, with recent concerns being focused on a disabling SSRI discontinuation syndrome. The mechanism underpinning their therapeutic effect has long shifted away from thinking that SSRIs act simply by increasing 5-HT in the synapse. Rather, a current popular view is that increased 5-HT is just the beginning of a series of complex downstream signalling events, which trigger changes in neural plasticity at the functional and structural level. These changes in plasticity are then thought to interact with neuropsychological processes to enhance re-learning of emotional experiences that ultimately brings about changes in mood. This compelling view of SSRI action is underpinning attempts to understand fast-acting antidepressants, such as ketamine and psychedelic drugs, and aid the development of future therapies. An important gap in the theory is evidence that changes in plasticity are causally linked to relevant behavioural effects. Also, predictions that the SSRI-induced neural plasticity might have applicability in other areas of medicine have not yet been borne out. In contrast to the sophisticated view of the antidepressant action of SSRIs, the mechanism underpinning SSRI discontinuation is little explored. Nevertheless, evidence of rebound increases in 5-HT neuron excitability immediately on cessation of SSRI treatment provide a starting point for future investigation. Indeed, this evidence allows formulation of a mechanistic explanation of SSRI discontinuation which draws on parallels with the withdrawal states of other psychotropic drugs.

Sex-specific modulation of the medial prefrontal cortex by glutamatergic median raphe neurons

A substantial proportion of raphe neurons are glutamatergic. However, little is known about how these glutamatergic neurons modulate the forebrain. We investigated how glutamatergic median raphe nucleus (MRN) input modulates the medial prefrontal cortex (mPFC), a critical component of fear circuitry. We show that vesicular glutamate transporter 3 (VGLUT3)-expressing MRN neurons activate VGLUT3- and somatostatin-expressing neurons in the mPFC. Consistent with this modulation of mPFC GABAergic neurons, activation of MRN (VGLUT3) neurons enhances GABAergic transmission in mPFC pyramidal neurons and attenuates fear memory in female but not male mice. Serotonin plays a key role in MRN (VGLUT3) neuron-mediated GABAergic plasticity in the mPFC. In agreement with these female-specific effects, we observed sex differences in glutamatergic transmission onto MRN (VGLUT3) neurons and in mPFC (VGLUT3) neuron-mediated dual release of glutamate and GABA. Our results demonstrate a cell type-specific modulation of the mPFC by MRN (VGLUT3) neurons and reveal a sex-specific role of this neuromodulation in mPFC synaptic plasticity.

Altered neurotransmission in stress-induced depressive disorders: The underlying role of the amygdala in depression

Depression is the second leading cause of disability in the world population, for which currently available pharmacological therapies either have poor efficacy or have some adverse effects. Accumulating evidence from clinical and preclinical studies demonstrates that the amygdala is critically implicated in depressive disorders, though the underlying pathogenesis mechanism needs further investigation. In this literature review, we overviewed depression and the key role of Gamma-aminobutyric acid (GABA) and Glutamate neurotransmission in depression. Notably, we discussed a new cholecystokinin-dependent plastic changes mechanism under stress and a possible antidepressant response of cholecystokinin B receptor (CCKBR) antagonist. Moreover, we discussed the fundamental role of the amygdala in depression, to discuss and understand the pathophysiology of depression and the inclusive role of the amygdala in this devastating disorder.

Role of 5-HT1A receptors in the basolateral amygdala on 3,4-methylenedioxymethamphetamine-induced prosocial effects in mice

3,4-methylenedioxymethamphetamine (MDMA), a recreational drug, induces euphoric sensations and psychosocial effects, such as increased sociability and empathy. Serotonin, also called 5-hydroxytryptamine (5-HT), is a neurotransmitter that has been associated with MDMA-induced prosocial effects. However, the detailed neural mechanisms remain elusive. In the present study, we investigated whether 5-HT neurotransmission in the medial prefrontal cortex (mPFC) and the basolateral nucleus of amygdala (BLA) is involved in MDMA-induced prosocial effects using the social approach test in male ICR mice. Systemic administration of (S)-citalopram, a selective 5-HT transporter inhibitor, before administration of MDMA failed to suppress MDMA-induced prosocial effects. On the other hand, systemic administration of the 5-HT_1A receptor antagonist WAY100635, but not 5-HT_1B, 5-HT_2A, 5-HT_2C, or 5-HT₄ receptor antagonist, significantly suppressed MDMA-induced prosocial effects. Furthermore, local administration of WAY100635 into the BLA but not into the mPFC suppressed MDMA-induced prosocial effects. Consistent with this finding, intra-BLA MDMA administration significantly increased sociability. Together, these results suggest that MDMA induces prosocial effects through the stimulation of 5-HT_1A receptors in the BLA.

Serotonergic current responses of neurons in rat oculomotor neural integrators

The prepositus hypoglossi nucleus (PHN) and the interstitial nucleus of Cajal (INC) are involved in controlling horizontal and vertical gaze, respectively. Previous studies have shown that PHN neurons exhibit depolarized or hyperpolarized responses to serotonin (5-hydroxytryptamine, 5-HT). However, serotonergic modulation of INC neurons has not been examined. Furthermore, the relationship between 5-HT-induced responses and neuron types based on neurotransmitter phenotypes has not been clarified. In this study, we investigated 5-HT-induced current responses in PHN and INC neurons and the distributions of distinct current responses in different neuron types, using whole cell recordings of wild-type and transgenic rat brain stem slices. Local application of 5-HT to the cell soma confirmed that slow inward (SI) and slow outward (SO) currents were mediated by 5-HT₂ and 5-HT_1A receptors, respectively. Furthermore, fast inward (FI) currents that were mediated by 5-HT₃ receptors were observed. These three current responses were observed in both PHN and INC neurons. Analyses of the distributions of the three current responses revealed that fluorescently identified glutamatergic and inhibitory neurons in the PHN showed high proportions of SI and SO currents, respectively, whereas glutamatergic and inhibitory neurons in the INC showed mainly SO currents. When PHN and INC neurons were characterized on the basis of firing patterns, the proportions of the currents depended on the firing patterns. The different distributions of 5-HT-induced currents suggest distinct serotonergic modulation modes specific to horizontal and vertical gaze control.NEW & NOTEWORTHY Serotonergic modulation of vertical gaze control (interstitial nucleus of Cajal, INC) is less understood than that of horizontal gaze control (prepositus hypoglossal nucleus, PHN). Here, we report 5-HT-induced fast inward currents in addition to the previously reported slow inward and outward currents. The distributions of these currents in INC neurons based on neurotransmitter phenotypes differ from those in PHN neurons. These results suggest distinct serotonergic modulation modes in horizontal and vertical gaze control centers.

Constitutive depletion of brain serotonin differentially affects rats' social and cognitive abilities

Central serotonin appears a promising transdiagnostic marker of psychiatric disorders and a modulator of some of their key behavioral symptoms. In adult male Tph2 ^-/- rats, constitutively lacking central serotonin, we tested individual's cognitive, social and non-social abilities and characterized group's social organization under classical and ethological testing conditions. Using unsupervised machine learning, we identified the functions most dependent on serotonin. Although serotonin depletion did not affect cognitive performances in classical testing, in the home-cage it induced compulsive aggression and sexual behavior, hyperactive and hypervigilant stereotyped behavior, reduced self-care and exacerbated corticosterone levels. This profile recalled symptoms of impulse control and anxiety disorders. Serotonin appeared essential for behavioral adaptation to dynamic social environments. Our animal model challenges the essential role of serotonin in decision-making, flexibility, impulsivity, and risk-taking. These findings highlight the importance of studying everyday life functions within the dynamic social living environment to model complexity in animal models.

Serotonin regulation of behavior via large-scale neuromodulation of serotonin receptor networks

Although we understand how serotonin receptors function at the single-cell level, what role different serotonin receptors play in regulating brain-wide activity and, in turn, human behavior, remains unknown. Here, we developed transcriptomic-neuroimaging mapping to characterize brain-wide functional signatures associated with specific serotonin receptors: serotonin receptor networks (SRNs). Probing SRNs with optogenetics-functional magnetic resonance imaging (MRI) and pharmacology in mice, we show that activation of dorsal raphe serotonin neurons differentially modulates the amplitude and functional connectivity of different SRNs, showing that receptors' spatial distributions can confer specificity not only at the local, but also at the brain-wide, network level. In humans, using resting-state functional MRI, SRNs replicate established divisions of serotonin effects on impulsivity and negative biases. These results provide compelling evidence that heterogeneous brain-wide distributions of different serotonin receptor types may underpin behaviorally distinct modes of serotonin regulation. This suggests that serotonin neurons may regulate multiple aspects of human behavior via modulation of large-scale receptor networks.

Distinct serotonergic pathways to the amygdala underlie separate behavioral features of anxiety

Anxiety-like behaviors in mice include social avoidance and avoidance of bright spaces. Whether these features are distinctly regulated is unclear. We demonstrate that in mice, social and anxiogenic stimuli, respectively, increase and decrease serotonin (5-HT) levels in basal amygdala (BA). In dorsal raphe nucleus (DRN), 5-HT∩vGluT3 neurons projecting to BA parvalbumin (DRN^{5-HT∩vGluT3}-BA^PV) and pyramidal (DRN^{5-HT∩vGluT3}-BA^Pyr) neurons have distinct intrinsic properties and gene expression and respond to anxiogenic and social stimuli, respectively. Activation of DRN^{5-HT∩vGluT3}→BA^PV inhibits 5-HT release via GABA_B receptors on serotonergic terminals in BA, inducing social avoidance and avoidance of bright spaces. Activation of DRN^{5-HT∩vGluT3}→BA neurons inhibits two subsets of BA^Pyr neurons via 5-HT1A receptors (HTR1A) and 5-HT1B receptors (HTR1B). Pharmacological inhibition of HTR1A and HTR1B in BA induces avoidance of bright spaces and social avoidance, respectively. These findings highlight the functional significance of heterogenic inputs from DRN to BA subpopulations in the regulation of separate anxiety-related behaviors.

Autism spectrum disorders pathogenesis: Toward a comprehensive model based on neuroanatomic and neurodevelopment considerations

Autism spectrum disorder (ASD) involves alterations in neural connectivity affecting cortical network organization and excitation to inhibition ratio. It is characterized by an early increase in brain volume mediated by abnormal cortical overgrowth patterns and by increases in size, spine density, and neuron population in the amygdala and surrounding nuclei. Neuronal expansion is followed by a rapid decline from adolescence to middle age. Since no known neurobiological mechanism in human postnatal life is capable of generating large excesses of frontocortical neurons, this likely occurs due to a dysregulation of layer formation and layer-specific neuronal migration during key early stages of prenatal cerebral cortex development. This leads to the dysregulation of post-natal synaptic pruning and results in a huge variety of forms and degrees of signal-over-noise discrimination losses, accounting for ASD clinical heterogeneities, including autonomic nervous system abnormalities and comorbidities. We postulate that sudden changes in environmental conditions linked to serotonin/kynurenine supply to the developing fetus, throughout the critical GW7 - GW20 (Gestational Week) developmental window, are likely to promote ASD pathogenesis during fetal brain development. This appears to be driven by discrete alterations in differentiation and patterning mechanisms arising from in utero RNA editing, favoring vulnerability outcomes over plasticity outcomes. This paper attempts to provide a comprehensive model of the pathogenesis and progression of ASD neurodevelopmental disorders.

Divergent properties and independent regulation of striatal dopamine and GABA co-transmission

Substantia nigra pars compacta (SNc) dopamine neurons play a key role in regulating the activity of striatal circuits within the basal ganglia. In addition to dopamine, these neurons release several other transmitters, including the major inhibitory neurotransmitter γ-aminobutyric acid (GABA). Both dopamine and GABA are loaded into SNc synaptic vesicles by the vesicular monoamine transporter 2 (VMAT2), and co-release of GABA provides strong inhibition to the striatum by directly inhibiting striatal medium spiny projection neurons (MSNs) through activation of GABAA receptors. Here, we found that despite both dopamine and GABA being co-packaged by VMAT2, the properties of transmission, including Ca2+ sensitivity, release probability, and requirement of active zone scaffolding proteins, differ between the two transmitters. Moreover, the extent by which presynaptic neuromodulators inhibit co-transmission also varied. Differences in modulation and the mechanisms controlling release allow for independent regulation of dopamine and GABA signals despite both being loaded via similar mechanisms.

Anxiety and Gene Expression Enhancement in Mice Exposed to Glyphosate-Based Herbicide

Growing evidence demonstrates that serotonin (5-HT) depletion increases activity in the amygdala and medial prefrontal cortex (mPFC), ultimately leading to anxiety behavior. Previously, we showed that glyphosate-based herbicides (GBHs) increased anxiety levels and reduced the number of serotoninergic fibers within the mPFCs and amygdalas of exposed mice. However, the impact of this 5-HT depletion following GBH exposure on neuronal activity in these structures is still unknown. In this study, we investigated the effects of GBH on immediate early gene (IEG) activation within the mPFCs and amygdalas of treated mice from juvenile age to adulthood and its subsequent effects on anxiety levels. Mice were treated for subchronic (6 weeks) and chronic (12 weeks) periods with 250 or 500 mg/kg/day of GBH and subjected to behavioral testing using the open field and elevated plus maze paradigms. Then, we analyzed the expression levels of c-Fos and pCREB and established the molecular proxies of neuronal activation within the mPFC and the amygdala. Our data revealed that repeated exposure to GBH triggers anxiogenic behavior in exposed mice. Confocal microscopy investigations into the prelimbic/infralimbic regions of the mPFC and in basolateral/central nuclei of the amygdala disclosed that the behavioral alterations are paralleled by a robust increase in the density and labelling intensity of c-Fos- and pCREB-positive cells. Taken together, these data show that mice exposed to GBH display the hyperactivation of the mPFC–amygdala areas, suggesting that this is a potential mechanism underlying the anxiety-like phenotype.

Colocalization of different neurotransmitter transporters on synaptic vesicles is sparse except for VGLUT1 and ZnT3

Vesicular transporters (VTs) define the type of neurotransmitter that synaptic vesicles (SVs) store and release. While certain mammalian neurons release multiple transmitters, it is not clear whether the release occurs from the same or distinct vesicle pools at the synapse. Using quantitative single-vesicle imaging, we show that a vast majority of SVs in the rodent brain contain only one type of VT, indicating specificity for a single neurotransmitter. Interestingly, SVs containing dual transporters are highly diverse (27 types) but small in proportion (2% of all SVs), excluding the largest pool that carries VGLUT1 and ZnT3 (34%). Using VGLUT1-ZnT3 SVs, we demonstrate that the transporter colocalization influences the SV content and synaptic quantal size. Thus, the presence of diverse transporters on the same vesicle is bona fide, and depending on the VT types, this may act to regulate neurotransmitter type, content, and release in space and time.

A Role for Serotonin in Modulating Opposing Drive and Brake Circuits of Impulsivity

Impulsivity generally refers to a deficit in inhibition, with a focus on understanding the neural circuits which constitute the “brake” on actions and gratification. It is likely that increased impulsivity can arise not only from reduced inhibition, but also from a heightened or exaggerated excitatory “drive.” For example, an action which has more vigor, or is fueled by either increased incentive salience or a stronger action-outcome association, may be harder to inhibit. From this perspective, this review focuses on impulse control as a competition over behavioral output between an initially learned response-reward outcome association, and a subsequently acquired opposing inhibitory association. Our goal is to present a synthesis of research from humans and animal models that supports this dual-systems approach to understanding the behavioral and neural substrates that contribute to impulsivity, with a focus on the neuromodulatory role of serotonin. We review evidence for the role of serotonin signaling in mediating the balance of the “drive” and “brake” circuits. Additionally, we consider parallels of these competing instrumental systems in impulsivity within classical conditioning processes (e.g., extinction) in order to point us to potential behavioral and neural mechanisms that may modulate the competing instrumental associations. Finally, we consider how the balance of these competing associations might contribute to, or be extracted from, our experimental assessments of impulsivity. A careful understanding of the underlying behavioral and circuit level contributions to impulsivity is important for understanding the pathogenesis of increased impulsivity present in a number of psychiatric disorders. Pathological levels of impulsivity in such disorders are likely subserved by deficits in the balance of motivational and inhibitory processes.

At the heart of the interoception network: Influence of the parasubthalamic nucleus on autonomic functions and motivated behaviors

The parasubthalamic nucleus (PSTN), a small nucleus located on the lateral edge of the posterior hypothalamus, has emerged in recent years as a highly interconnected node within the network of brain regions sensing and regulating autonomic function and homeostatic needs. Furthermore, the strong integration of the PSTN with extended amygdala circuits makes it ideally positioned to serve as an interface between interoception and emotions. While PSTN neurons are mostly glutamatergic, some of them also express neuropeptides that have been associated with stress-related affective and motivational dysfunction, including substance P, corticotropin-releasing factor, and pituitary adenylate-cyclase activating polypeptide. PSTN neurons respond to food ingestion and anorectic signals, as well as to arousing and distressing stimuli. Functional manipulation of defined pathways demonstrated that the PSTN serves as a central hub in multiple physiologically relevant networks and is notably implicated in appetite suppression, conditioned taste aversion, place avoidance, impulsive action, and fear-induced thermoregulation. We also discuss the putative role of the PSTN in interoceptive dysfunction and negative urgency. This review aims to synthesize the burgeoning preclinical literature dedicated to the PSTN and to stimulate interest in further investigating its influence on physiology and behavior.

In vivo electrophysiological study of the targeting of 5-HT3 receptor-expressing cortical interneurons by the multimodal antidepressant, vortioxetine

The antidepressant vortioxetine has high affinity for the ionotropic 5-HT₃ receptor (5-HT₃ R) as well as other targets including the 5-HT transporter. The procognitive effects of vortioxetine have been linked to altered excitatory:inhibitory balance in cortex. Thus, vortioxetine purportedly inhibits cortical 5-HT₃ R-expressing interneurons (5-HT₃ R-INs) to disinhibit excitatory pyramidal neurons. The current study determined for the first time, the effect of vortioxetine on the in vivo firing of putative 5-HT₃ R-INs whilst simultaneously recording pyramidal neuron activity using cortical slow-wave oscillations as a readout. Extracellular single unit and local field potential recordings were made in superficial layers of the prefrontal cortex of urethane-anaesthetised rats. 5-HT₃ R-INs were identified by a short-latency excitation evoked by electrical stimulation of the dorsal raphe nucleus (DRN). Juxtacellular-labelling found such neurons had the morphological and immunohistochemical properties of 5-HT₃ R-INs; basket cell or bipolar cell morphology, expression of 5-HT₃ R-IN markers, and parvalbumin-immunonegative. Vortioxetine inhibited the short-latency DRN-evoked excitation of 5-HT₃ R-INs and simultaneously decreased cortical slow wave oscillations, indicative of pyramidal neuron activation. Likewise, the 5-HT₃ R antagonist ondansetron inhibited the short-latency DRN-evoked excitation of 5-HT₃ R-INs. However unlike vortioxetine, ondansetron did not decrease cortical slow-wave oscillations suggesting a dissociation between this effect and inhibition of 5-HT₃ R-INs. The 5-HT reuptake inhibitor escitalopram had no consistent effect on any electrophysiological parameter measured. Overall, the current findings suggest that vortioxetine simultaneously inhibits (DRN-evoked) 5-HT₃ R-INs and excites pyramidal neurons, thereby changing the excitatory:inhibitory balance in cortex. However, under the current experimental conditions these two effects were dissociable with only the former likely involving a 5-HT₃ R-mediated mechanism.

Structural, functional, and behavioral significance of sex and gonadal hormones in the basolateral amygdala: A review of preclinical literature

The basolateral amygdala (BLA) is intimately involved in the development of neuropsychiatric disorders such as anxiety and alcohol use disorder (AUD). These disorders have clear sex biases, with women more likely to develop an anxiety disorder and men more likely to develop AUD. Preclinical models have largely confirmed these sex-specific vulnerabilities and emphasize the effects of sex hormones on behaviors influenced by the BLA. This review will discuss sex differences in BLA-related behaviors and highlight potential mechanisms mediated by altered BLA structure and function, including the composition of GABAergic interneuron subpopulations, glutamatergic pyramidal neuron morphology, glutamate/GABA neurotransmission, and neuromodulators. Further, sex hormones differentially organize dimorphic circuits during sensitive developmental periods (organizational effects) and initiate more transient effects throughout adulthood (activational effects). Current literature indicates that estradiol and allopregnanolone, a neuroactive progestogen, generally reduce BLA-related behaviors through a variety of mechanisms, including activation of estrogen receptors or facilitation of GABA_A-mediated inhibition, respectively. This enhanced GABAergic inhibition may protect BLA pyramidal neurons from the excitability associated with anxiety and alcohol withdrawal. Understanding sex differences and the effects of sex hormones on BLA structure and function may help explain sex-specific vulnerabilities in BLA-related behaviors and ultimately improve treatments for anxiety and AUD.

A New Player in the Hippocampus: A Review on VGLUT3+ Neurons and Their Role in the Regulation of Hippocampal Activity and Behaviour

Glutamate is the most abundant excitatory amino acid in the central nervous system. Neurons using glutamate as a neurotransmitter can be characterised by vesicular glutamate transporters (VGLUTs). Among the three subtypes, VGLUT3 is unique, co-localising with other "classical" neurotransmitters, such as the inhibitory GABA. Glutamate, manipulated by VGLUT3, can modulate the packaging as well as the release of other neurotransmitters and serve as a retrograde signal through its release from the somata and dendrites. Its contribution to sensory processes (including seeing, hearing, and mechanosensation) is well characterised. However, its involvement in learning and memory can only be assumed based on its prominent hippocampal presence. Although VGLUT3-expressing neurons are detectable in the hippocampus, most of the hippocampal VGLUT3 positivity can be found on nerve terminals, presumably coming from the median raphe. This hippocampal glutamatergic network plays a pivotal role in several important processes (e.g., learning and memory, emotions, epilepsy, cardiovascular regulation). Indirect information from anatomical studies and KO mice strains suggests the contribution of local VGLUT3-positive hippocampal neurons as well as afferentations in these events. However, further studies making use of more specific tools (e.g., Cre-mice, opto- and chemogenetics) are needed to confirm these assumptions.

Neuronal pericellular baskets: neurotransmitter convergence and regulation of network excitability

A pericellular basket is a presynaptic configuration of numerous axonal boutons outlining a target neuron soma and its proximal dendrites. Recent studies show neurochemical diversity of pericellular baskets and suggest that neurotransmitter usage together with the dense, soma-proximal boutons may permit strong input effects on different timescales. Here we review the development, distribution, neurochemical phenotypes, and possible functions of pericellular baskets. As an example, we highlight pericellular baskets formed by projections of certain Pet1/Fev neurons of the serotonergic raphe nuclei. We propose that pericellular baskets represent convergence sites of competition or facilitation between neurotransmitter systems on downstream circuitry, especially in limbic brain regions, where pericellular baskets are widespread. Study of these baskets may enhance our understanding of monoamine regulation of memory, social behavior, and brain oscillations.

The dorsal raphe nucleus in the control of energy balance

Energy balance is orchestrated by an extended network of highly interconnected nuclei across the central nervous system. While much is known about the hypothalamic circuits regulating energy homeostasis, the 'extra-hypothalamic' circuits involved are relatively poorly understood. In this review, we focus on the brainstem's dorsal raphe nucleus (DRN), integrating decades of research linking this structure to the physiologic and behavioral responses that maintain proper energy stores. DRN neurons sense and respond to interoceptive and exteroceptive cues related to energy imbalance and in turn induce appropriate alterations in energy intake and expenditure. The DRN is also molecularly differentiable, with different populations playing distinct and often opposing roles in controlling energy balance. These populations are integrated into the extended circuit known to regulate energy balance. Overall, this review summarizes the key evidence demonstrating an important role for the DRN in regulating energy balance.

5-HT7 receptors enhance inhibitory synaptic input to principal neurons in the mouse basal amygdala

The basal amygdala (BA) has been implicated in encoding fear and its extinction. The level of serotonin (5-HT) in the BA increases due to arousal and stress related to aversive stimuli. The effects of 5-HT₇ receptor (5-HT₇R) activation and blockade on the activity of BA neurons have not yet been investigated. In the present study, a transgenic mouse line carrying green fluorescent protein (GFP) reporter gene was used to identify neurons that express the 5-HT₇R. GFP immunoreactivity was present mainly in cells that also expressed GAD67 or parvalbumin (PV), the phenotypic markers for GABAergic interneurons. Most cells showing GFP fluorescence demonstrated firing patterns characteristic of BA inhibitory interneurons. Activation of 5-HT₇Rs resulted in a depolarization and/or occurrence of spontaneous spiking activity of BA interneurons that was accompanied by an increase in the mean frequency and mean amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs) recorded from BA principal neurons. These effects were blocked by a specific 5-HT₇R antagonist, SB269970 and were absent in slices from 5-HT₇R knockout mice. Activation of 5-HT₇Rs also decreased the mean frequency of spontaneous excitatory postsynaptic currents (sEPSCs) recorded from BA principal neurons, which was blocked by the GABA_A receptor antagonist picrotoxin. Neither inhibitory nor excitatory miniature postsynaptic currents (mIPSCs/mEPSCs) were affected by 5-HT₇R activation. These results show that in the BA 5-HT₇Rs stimulate an activity-dependent enhancement of inhibitory input from local interneurons to BA principal neurons and provide insights about the possible involvement of BA serotonergic receptors in neuronal mechanisms underlying fear memory.

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.

Visual Hallucinations and the Role of Medications in Parkinson's Disease: Triggers, Pathophysiology, and Management

Visual hallucinations, which are part of the syndrome of Parkinson's disease (PD) psychosis, affect patients' quality of life and increase the likelihood of residential aged-care placement. The association between visual hallucinations and dopaminergic and other medications that are necessary for the symptomatic management of motor and other symptoms of PD is a common clinical dilemma. While dopaminergic medications have long been associated with PD psychosis, a clear causal link has not been established, and other neurotransmitter systems, particularly noradrenaline, serotonin, and acetylcholine, are implicated and important. A diverse range of demographic and disease-related risk factors, some being modifiable, highlight the complexity of potential underlying pathophysiological processes but also broaden practical options for prevention and treatment that can be multifaceted and individualized. The investigators reviewed the clinical features and epidemiology of visual hallucinations and PD, explored the pathological evidence for dysfunction of multiple neurotransmitter systems that may be relevant to these phenomena, and addressed the potential of medications commonly used in PD to either trigger or treat these symptoms.

Sucrose Consumption Alters Serotonin/Glutamate Co-localisation Within the Prefrontal Cortex and Hippocampus of Mice

The overconsumption of sugar-sweetened food and beverages underpins the current rise in obesity rates. Sugar overconsumption induces maladaptive neuroplasticity to decrease dietary control. Although serotonin and glutamate co-localisation has been implicated in reward processing, it is still unknown how chronic sucrose consumption changes this transmission in regions associated with executive control over feeding—such as the prefrontal cortex (PFC) and dentate gyrus (DG) of the hippocampus. To address this, a total of 16 C57Bl6 mice received either 5% w/v sucrose or water as a control for 12 weeks using the Drinking-In-The-Dark paradigm (n = 8 mice per group). We then examined the effects of chronic sucrose consumption on the immunological distribution of serotonin (5-HT), vesicular glutamate transporter 3 (VGLUT3) and 5-HT⁺/VGLUT3⁺ co-localised axonal varicosities. Sucrose consumption over 12 weeks decreased the number of 5-HT^–/VGLUT3⁺ and 5-HT⁺/VGLUT3⁺ varicosities within the PFC and DG. The number of 5-HT⁺/VGLUT3^– varicosities remained unchanged within the PFC but decreased in the DG following sucrose consumption. Given that serotonin mediates DG neurogenesis through microglial migration, the number of microglia within the DG was also assessed in both experimental groups. Sucrose consumption decreased the number of DG microglia. Although the DG and PFC are associated with executive control over rewarding activities and emotional memory formation, we did not detect a subsequent change in DG neurogenesis or anxiety-like behaviour or depressive-like behaviour. Overall, these findings suggest that the chronic consumption of sugar alters serotonergic neuroplasticity within neural circuits responsible for feeding control. Although these alterations alone were not sufficient to induce changes in neurogenesis or behaviour, it is proposed that the sucrose consumption may predispose individuals to these cognitive deficits which ultimately promote further sugar intake.

5-HT neurons of the medullary raphe contribute to respiratory control in toads

Air-breathing vertebrates undergo respiratory adjustments when faced with disturbances in the gas composition of the environment. In mammals, the medullary raphe nuclei are involved in the neuronal pathway that mediates the ventilatory responses to hypoxia and hypercarbia. We investigate whether the serotoninergic neurons of the medullary raphe nuclei of toads (Rhinella diptycha) play a functional role in respiratory control during resting conditions (room air), hypercarbia (5% CO₂), and hypoxia (5% O₂). The raphe nuclei were located and identified based on the location of the serotoninergic neurons in the brainstem. We then lesioned the medullary raphe (raphe pallidus, obscurus and magnus) with anti-SERT-SAP and measured ventilation in both control and lesioned groups and we observed that serotonin (5-HT) specific chemical lesions of the medullary raphe caused reduced respiratory responses to both hypercarbia and hypoxia. In summary, we report that the serotoninergic neurons of the medullary raphe of the cururu toad Rhinella diptycha participate in the chemoreflex responses during hypercarbia and hypoxia, but not during resting conditions. This current evidence in anurans, together with the available data in mammals, brings insights to the evolution of brain sites, such as the medullary raphe, involved in the ventilatory chemoreflex in vertebrates.

Oxytocinergic Modulation of Threat-Specific Amygdala Sensitization in Humans Is Critically Mediated by Serotonergic Mechanisms

BACKGROUND

Overarching conceptualizations propose that the complex social-emotional effects of oxytocin (OXT) in humans are partly mediated by interactions with other neurotransmitter systems. Recent animal models suggest that the anxiolytic effects of OXT are critically mediated by the serotonin (5-HT) system, yet direct evidence in humans is lacking.

METHODS

To determine the role of 5-HT in OXT-induced attenuation of amygdala threat reactivity and sensitization/desensitization, we conducted a parallel-group, randomized, placebo-controlled, double-blind experiment during which 121 healthy subjects underwent a transient decrease in 5-HT signaling via acute tryptophan depletion or the corresponding placebo-control protocol before the administration of intranasal OXT or placebo intranasal spray, respectively. Mean and repetition-dependent changes in threat-specific amygdala reactivity toward threatening stimuli (angry faces) as assessed by functional magnetic resonance imaging served as the primary outcome.

RESULTS

No main or interaction effects of treatment on amygdala threat reactivity were observed, yet OXT switched bilateral amygdala threat sensitization to desensitization, and this effect was significantly attenuated during decreased central 5-HT signaling via pretreatment with acute tryptophan depletion.

CONCLUSIONS

The present findings provide the first evidence for a role of OXT in threat-specific amygdala desensitization in humans and suggest that these effects are critically mediated by the 5-HT system. OXT may have a therapeutic potential to facilitate amygdala desensitization, and adjunct upregulation of 5-HT neurotransmission may facilitate OXT's anxiolytic potential.

Somatostatin interneurons activated by 5-HT2A receptor suppress slow oscillations in medial entorhinal cortex

Serotonin (5-HT) is one of the major neuromodulators present in the mammalian brain and has been shown to play a role in multiple physiological processes. The mechanisms by which 5-HT modulates cortical network activity, however, are not yet fully understood. We investigated the effects of 5-HT on slow oscillations (SOs), a synchronized cortical network activity universally present across species. SOs are observed during anesthesia and are considered to be the default cortical activity pattern. We discovered that (±)3,4-methylenedioxymethamphetamine (MDMA) and fenfluramine, two potent 5-HT releasers, inhibit SOs within the entorhinal cortex (EC) in anesthetized mice. Combining opto- and pharmacogenetic manipulations with in vitro electrophysiological recordings, we uncovered that somatostatin-expressing (Sst) interneurons activated by the 5-HT_2A receptor (5-HT_2AR) play an important role in the suppression of SOs. Since 5-HT_2AR signaling is involved in the etiology of different psychiatric disorders and mediates the psychological effects of many psychoactive serotonergic drugs, we propose that the newly discovered link between Sst interneurons and 5-HT will contribute to our understanding of these complex topics.

Leveraging VGLUT3 Functions to Untangle Brain Dysfunctions

Vesicular glutamate transporters (VGLUTs) were long thought to be specific markers of glutamatergic excitatory transmission. The discovery, two decades ago, of the atypical VGLUT3 has thoroughly modified this oversimplified view. VGLUT3 is strategically expressed in discrete populations of glutamatergic, cholinergic, serotonergic, and even GABAergic neurons. Recent reports show the subtle, but critical, implications of VGLUT3-dependent glutamate co-transmission and its roles in the regulation of diverse brain functions and dysfunctions. Progress in the neuropharmacology of VGLUT3 could lead to decisive breakthroughs in the treatment of Parkinson's disease (PD), addiction, eating disorders, anxiety, presbycusis, or pain. This review summarizes recent findings on VGLUT3 and its vesicular underpinnings as well as on possible ways to target this atypical transporter for future therapeutic strategies.

Basolateral Amygdala Serotonin 2C Receptor Regulates Emotional Disorder-Related Symptoms Induced by Chronic Methamphetamine Administration

Globally, methamphetamine (MA) is the second most abused drug, with psychotic symptoms being one of the most common adverse effects. Emotional disorders induced by MA abuse have been widely reported both in human and animal models; however, the mechanisms underlying such disorders have not yet been fully elucidated. In this study, a chronic MA administration mouse model was utilized to elucidate the serotonergic pathway involved in MA-induced emotional disorders. After 4 weeks of MA administration, the animals exhibited significantly increased depressive and anxious symptoms. Molecular and morphological evidence showed that chronic MA administration reduced the expression of the 5-hydroxytryptamine (5-HT) rate-limiting enzyme, tryptophan hydroxylase 2, in the dorsal raphe and the concentrations of 5-HT and its metabolite 5-hydroxyindoleacetic acid in the basolateral amygdala (BLA) nuclei. Alterations in both 5-HT and 5-HT receptor levels occurred simultaneously in BLA; quantitative polymerase chain reaction, western blotting, and fluorescence analysis revealed that the expression of the 5-HT2C receptor (5-HT_2CR) increased. Neuropharmacology and virus-mediated silencing strategies confirmed that targeting 5-HT_2CR reversed the depressive and anxious behaviors induced by chronic MA administration. In the BLA, 5-HT_2CR-positive cells co-localized with GABAergic interneurons. The inactivation of 5-HT_2CR ameliorated impaired GABAergic inhibition and decreased BLA activation. Thus, herein, for the first time, we report that the abnormal regulation of 5-HT_2CR is involved in the manifestation of emotional disorder-like symptoms induced by chronic MA use. Our study suggests that 5-HT_2CR in the BLA is a promising clinical target for the treatment of MA-induced emotional disorders.

Serotonergic modulation of basolateral amygdala nucleus in the extinction of reward-driven learning: The role of 5-HT bioavailability and 5-HT1A receptor

Serotonin (5-HT) neurotransmission has been associated with reward-related behaviour. Moreover, the serotonergic system modulates the basolateral amygdala (BLA), a structure involved in reward encoding, and reward prediction error. However, the role played by 5-HT on BLA during a reward-driven task has not been fully elucidated. In this paper, we investigated whether serotonergic modulation of the BLA is involved in reward-driven learning. To this end, we trained Long Evans rats in an operant conditioning task, and examined the effects of fluoxetine treatment (a selective serotonin reuptake inhibitor, 10 mg/kg) in combination with BLA lesions with NMDA (20 mg/mL) on extinction learning. We also investigated whether intra-BLA injection of the serotonergic 5-HT_1A receptor agonist 8-OH DPAT, or antagonist WAY-100635, alters extinction performance. We found that fluoxetine treatment strongly accelerated extinction learning, while BLA lesions partially reverted this effect and slightly impaired consolidation of extinction. Stimulation and inhibition of 5-HT_1A receptors in BLA induced opposite effects to those of fluoxetine, impairing or accelerating extinction performance, respectively. Our findings suggest that 5-HT modulates reward-driven learning, and 5-HT_1A receptors located in the BLA are relevant for extinction.

Endocannabinoid-serotonin systems interaction in health and disease

Endocannabinoid (eCB) and serotonin (5-HT) neuromodulatory systems work both independently and together to finely orchestrate neuronal activity throughout the brain to strongly sculpt behavioral functions. Surprising parallelism between the behavioral effects of 5-HT and eCB activity has been widely reported, including the regulation of emotional states, stress homeostasis, cognitive functions, food intake and sleep. The distribution pattern of the 5-HT system and the eCB molecular elements in the brain display a strong overlap and several studies report a functional interplay and even a tight interdependence between eCB/5-HT signaling. In this review, we examine the available evidence of the interaction between the eCB and 5-HT systems. We first introduce the eCB system, then we describe the eCB/5-HT crosstalk at the neuronal and synaptic levels. Finally, we explore the potential eCB/5-HT interaction at the behavioral level with the implication for psychiatric and neurological disorders. The precise elucidation of how this neuromodulatory interaction dynamically regulates biological functions may lead to the development of more targeted therapeutic strategies for the treatment of depressive and anxiety disorders, psychosis and epilepsy.

CB1/5-HT/GABA interactions and food intake regulation

Despite historically the serotonergic, GABAergic, and cannabinoid systems have been shown to play a crucial role in the central regulation of eating behavior, interest in the study of the interactions of these neurotransmission systems has only now been investigated. Current evidence suggests that serotonin may influence normal and pathological eating behavior in significantly more complex ways than was initially thought. This knowledge has opened the possibility of exploring the potential clinical utility of new therapeutic strategies more effective and safer than the current approaches to treat pathological eating behavior. Furthermore, the nature and complexity of the interactions between these neurotransmitter systems have provided a better understanding of the pathophysiological mechanisms not only of eating behavior and eating disorders but also of some of the comorbidities associated with modulation of cortical circuits, which are involved in high order cognitive processes. Accordingly, in the present chapter, the clinical and experimental findings of the interactions between serotonin, GABA, and cannabinoids are synthesized, emphasizing the pharmacological, neurophysiological, and neuroanatomical aspects that could potentially improve the current therapeutic approaches against pathological eating behavior.

Phasic Activation of Dorsal Raphe Serotonergic Neurons Increases Pupil Size

Transient variations in pupil size (PS) under constant luminance are coupled to rapid changes in arousal state,1-3 which have been interpreted as vigilance,4 salience,5 or a surprise signal.6-8 Neural control of such fluctuations presumably involves multiple brain regions5,9-11 and neuromodulatory systems,3,12,13 but it is often associated with phasic activity of the noradrenergic system.9,12,14,15 Serotonin (5-HT), a neuromodulator also implicated in aspects of arousal16 such as sleep-wake transitions,17 motivational state regulation,18 and signaling of unexpected events,19 seems to affect PS,20-24 but these effects have not been investigated in detail. Here we show that phasic 5-HT neuron stimulation causes transient PS changes. We used optogenetic activation of 5-HT neurons in the dorsal raphe nucleus (DRN) of head-fixed mice performing a foraging task. 5-HT-driven modulations of PS were maintained throughout the photostimulation period and sustained for a few seconds after the end of stimulation. We found no evidence that the increase in PS with activation of 5-HT neurons resulted from interactions of photostimulation with behavioral variables, such as locomotion or licking. Furthermore, we observed that the effect of 5-HT on PS depended on the level of environmental uncertainty, consistent with the idea that 5-HT could report a surprise signal.19 These results advance our understanding of the neuromodulatory control of PS, revealing a tight relationship between phasic activation of 5-HT neurons and changes in PS.

Resilience to fear: The role of individual factors in amygdala response to stressors

Resilience to stress is an adaptive process that varies individually. Resilience refers to the adaptation, or the ability to maintain or regain mental health, despite being subject to adverse situation. Resilience is a dynamic concept that reflects a combination of internal individual factors, including age and gender interacting with external factors such as social, cultural and environmental factors. In the last decade, we have witnessed an increase in the prevalence of anxiety disorders, including post-traumatic stress disorder. Given that stress in unavoidable, it is of great interest to understand the neurophysiological mechanisms of resilience, the individual factors that may contribute to susceptibility and promote efficacious approaches to improve resilience. Here, we address this complex question, attempting at defining clear and operational definitions that may allow us to improve our analysis of behavior incorporating individuality. We examine how individual perception of the stressor can alter the outcome of an adverse situation using as an example, the fear-conditioning paradigm and discuss how individual differences in the reward system can contribute to resilience. Given the central role of the endocannabinoid system in regulating fear responses and anxiety, we discuss the evidence that polymorphisms in several molecules of this signaling system contribute to different anxiety phenotypes. The endocannabinoid system is highly interconnected with the serotoninergic and dopaminergic modulatory systems, contributing to individual differences in stress perception and coping mechanisms. We review how the individual variability in these modulatory systems can be used towards a multivariable assessment of stress risk. Incorporating individuality in our research will allow us to define biomarkers of anxiety disorders as well as assess prognosis, towards a personalized clinical approach to mental health.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Sex-Specific Role for Dopamine Receptor D2 in Dorsal Raphe Serotonergic Neuron Modulation of Defensive Acoustic Startle and Dominance Behavior

Brain networks underlying states of social and sensory alertness are normally adaptive, influenced by serotonin and dopamine (DA), and abnormal in neuropsychiatric disorders, often with sex-specific manifestations. Underlying circuits, cells, and molecules are just beginning to be delineated. Implicated is a subtype of serotonergic neuron denoted Drd2-Pet1, distinguished by expression of the type-2 DA receptor (Drd2) gene, inhibited cell-autonomously by DRD2 agonism in slice, and, when constitutively silenced in male mice, affects levels of defensive and exploratory behaviors (Niederkofler et al., 2016). Unknown has been whether DRD2 signaling in these Pet1 neurons contributes to their capacity for shaping defensive behaviors. To address this, we generated mice in which Drd2 gene sequences were deleted selectively in Pet1 neurons. We found that Drd2^Pet1-CKO males, but not females, demonstrated increased winning against sex-matched controls in a social dominance assay. Drd2^Pet1-CKO females, but not males, exhibited blunting of the acoustic startle response, a protective, defensive reflex. Indistinguishable from controls were auditory brainstem responses (ABRs), locomotion, cognition, and anxiety-like and depression-like behaviors. Analyzing wild-type Drd2-Pet1 neurons, we found sex-specific differences in the proportional distribution of axonal collaterals, in action potential (AP) duration, and in transcript levels of Gad2, important for GABA synthesis. Drd2^Pet1-CKO cells displayed sex-specific differences in the percentage of cells harboring Gad2 transcripts. Our results suggest that DRD2 function in Drd2-Pet1 neurons is required for normal defensive/protective behaviors in a sex-specific manner, which may be influenced by the identified sex-specific molecular and cellular features. Related behaviors in humans too show sex differences, suggesting translational relevance.

Investigation of the Role of Stimulation and Blockade of 5-HT7 Receptors in Ketamine Anesthesia

Although several pieces of evidence have indicated the ability of the serotonin-7 receptor (5-HTR₇) to modulate N-methyl-_D-aspartate receptor (NMDAR) activation, the possible impact on ketamine anesthesia has not been examined directly. The purpose of the present study is thus to investigate the possible role of the 5-HTR₇ in ketamine anesthesia using a 5-HTR₇ agonist and/or antagonist. The influence of a 5-HTR₇ agonist/antagonist on ketamine anesthesia for behavioral impact was assessed by testing potential anesthetic parameters. Its functional impact was assessed by mRNA expression with real-time PCR and immunostaining in the hippocampus and prefrontal cortex of mice. Two different doses of ketamine-high and low-were administered to induce anesthesia. In the high-dose ketamine-applied group in particular, the administration of both the 5-HTR₇ agonist and antagonist intensified the anesthetic effect of ketamine. The reflection of the change in anesthesia parameters to 5-HTR₇ expression was observed as an increase in the hippocampus and a decrease in the prefrontal cortex in the anesthetized groups by stimulation of 5-HTR₇. It is noteworthy that the results of NMDAR expressions are parallel to the results of the 5-HTR₇ expressions of both the hippocampus and the prefrontal cortex. The 5-HTR₇ may play a role in ketamine anesthesia. It may act through NMDAR in ketamine anesthesia, depending on the parallelism between both receptors.

A discrete serotonergic circuit regulates vulnerability to social stress

Exposure to social stress and dysregulated serotonergic neurotransmission have both been implicated in the etiology of psychiatric disorders. However, the serotonergic circuit involved in stress vulnerability is still unknown. Here, we explored whether a serotonergic input from the dorsal raphe (DR) to ventral tegmental area (VTA) influences vulnerability to social stress. We identified a distinct, anatomically and functionally defined serotonergic subpopulation in the DR that projects to the VTA (5-HTDR→VTA neurons). Moreover, we found that susceptibility to social stress decreased the firing activity of 5-HTDR→VTA neurons. Importantly, the bidirectional manipulation of 5-HTDR→VTA neurons could modulate susceptibility to social stress. Our findings reveal that the activity of 5-HTDR→VTA neurons may be an essential factor in determining individual levels of susceptibility to social stress and suggest that targeting specific serotonergic circuits may aid the development of therapies for the treatment of stress-related disorders.

The Wiring Logic of an Identified Serotonergic Neuron That Spans Sensory Networks

Serotonergic neurons project widely throughout the brain to modulate diverse physiological and behavioral processes. However, a single-cell resolution understanding of the connectivity of serotonergic neurons is currently lacking. Using a whole-brain EM dataset of a female Drosophila, we comprehensively determine the wiring logic of a broadly projecting serotonergic neuron (the CSDn) that spans several olfactory regions. Within the antennal lobe, the CSDn differentially innervates each glomerulus, yet surprisingly, this variability reflects a diverse set of presynaptic partners, rather than glomerulus-specific differences in synaptic output, which is predominately to local interneurons. Moreover, the CSDn has distinct connectivity relationships with specific local interneuron subtypes, suggesting that the CSDn influences distinct aspects of local network processing. Across olfactory regions, the CSDn has different patterns of connectivity, even having different connectivity with individual projection neurons that also span these regions. Whereas the CSDn targets inhibitory local neurons in the antennal lobe, the CSDn has more distributed connectivity in the LH, preferentially synapsing with principal neuron types based on transmitter content. Last, we identify individual novel synaptic partners associated with other sensory domains that provide strong, top-down input to the CSDn. Together, our study reveals the complex connectivity of serotonergic neurons, which combine the integration of local and extrinsic synaptic input in a nuanced, region-specific manner.SIGNIFICANCE STATEMENT All sensory systems receive serotonergic modulatory input. However, a comprehensive understanding of the synaptic connectivity of individual serotonergic neurons is lacking. In this study, we use a whole-brain EM microscopy dataset to comprehensively determine the wiring logic of a broadly projecting serotonergic neuron in the olfactory system of Drosophila Collectively, our study demonstrates, at a single-cell level, the complex connectivity of serotonergic neurons within their target networks, identifies specific cell classes heavily targeted for serotonergic modulation in the olfactory system, and reveals novel extrinsic neurons that provide strong input to this serotonergic system outside of the context of olfaction. Elucidating the connectivity logic of individual modulatory neurons provides a ground plan for the seemingly heterogeneous effects of modulatory systems.

Reward Contributions to Serotonergic Functions

The brain serotonin systems participate in numerous aspects of reward processing, although it remains elusive how exactly serotonin signals regulate neural computation and reward-related behavior. The application of optogenetics and imaging techniques during the last decade has provided many insights. Here, we review recent progress on the organization and physiology of the dorsal raphe serotonin neurons and the relationships between their activity and behavioral functions in the context of reward processing. We also discuss several interesting theories on serotonin's function and how these theories may be reconciled by the possibility that serotonin, acting in synergy with coreleased glutamate, tracks and calculates the so-called beneficialness of the current state to guide an animal's behavior in dynamic environments.

A single-cell transcriptomic and anatomic atlas of mouse dorsal raphe Pet1 neurons

Among the brainstem raphe nuclei, the dorsal raphe nucleus (DR) contains the greatest number of Pet1-lineage neurons, a predominantly serotonergic group distributed throughout DR subdomains. These neurons collectively regulate diverse physiology and behavior and are often therapeutically targeted to treat affective disorders. Characterizing Pet1 neuron molecular heterogeneity and relating it to anatomy is vital for understanding DR functional organization, with potential to inform therapeutic separability. Here we use high-throughput and DR subdomain-targeted single-cell transcriptomics and intersectional genetic tools to map molecular and anatomical diversity of DR-Pet1 neurons. We describe up to fourteen neuron subtypes, many showing biased cell body distributions across the DR. We further show that P2ry1-Pet1 DR neurons - the most molecularly distinct subtype - possess unique efferent projections and electrophysiological properties. These data complement and extend previous DR characterizations, combining intersectional genetics with multiple transcriptomic modalities to achieve fine-scale molecular and anatomic identification of Pet1 neuron subtypes.

Central 5-HT receptors and their function; present and future

Since our review of central 5-HT receptors and their function twenty years ago, no new 5-HT receptor has been discovered and there is little evidence that this situation will change in the near future. Nevertheless, over this time significant progress has been made in our understanding of the properties of these receptors and in the clinical translation of this information, and some of these developments are highlighted herein. Such highlights include extensive mapping of 5-HT receptors in both animal and human brain, culminating in readily-accessible brain atlases of 5-HT receptor distribution, as well as emerging data on how 5-HT receptors are distributed within complex neural circuits. Also, a range of important pharmacological and genetic tools have been developed that allow selective 5-HT receptor manipulation, in cells through to whole organism models. Moreover, unexpected complexity in 5-HT receptor function has been identified including agonist-dependent signalling that goes beyond the pharmacology of canonical 5-HT receptor signalling pathways set down in the 1980s and 1990s. This new knowledge of 5-HT signalling has been extended by the discovery of combined signalling of 5-HT and co-released neurotransmitters, especially glutamate. Another important advance has been the progression of a large number of 5-HT ligands through to experimental medicine studies and clinical trials, and some such agents have already become prescribed therapeutic drugs. Much more needs to be discovered and understood by 5-HT neuropharmacologists, not least how the diverse signalling effects of so many 5-HT receptor types interact with complex neural circuits to generate neurophysiological changes which ultimately lead to altered cognitions and behaviour. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.