How omics is revealing new roles for glia in addiction.

Experiments to study the biology of addiction have historically focused on the mechanisms through which drugs of abuse drive changes in the functioning of neurons and neural circuits. Glia have often been ignored in these studies, however, and this has left many questions in the field unanswered, particularly, surrounding how glia contribute to changes in synaptic plasticity, regulation of neuroinflammation, and functioning of neural ensembles given massive changes in signaling across the CNS. Omics methods (transcriptomics, translatomics, epigenomics, proteomics, metabolomics, and others) have expanded researchers' abilities to generate hypotheses and carry out mechanistic studies of glial cells during acquisition of drug taking, intoxication, withdrawal, and relapse to drug seeking. Here, we present a survey of how omics technological advances are revising our understanding of astrocytes, microglia, oligodendrocytes, and ependymal cells in addiction biology.

Rapid appearance of negative emotion during oral fentanyl self-administration in male and female rats.

Opioid use disorder has become an epidemic in the United States, fuelled by the widespread availability of fentanyl, which produces rapid and intense euphoria followed by severe withdrawal and emotional distress. We developed a new preclinical model of fentanyl seeking in outbred male and female rats using volitional oral self-administration (SA) that can be readily applied in labs without intravascular access. Using a traditional two-lever operant procedure, rats learned to take oral fentanyl vigorously, escalated intake across sessions, and readily reinstated responding to conditioned cues after extinction. Oral SA also revealed individual and sex differences that are essential to studying substance use risk propensity. During a behavioural economics task, rats displayed inelastic demand curves and maintained stable intake across a wide range of fentanyl concentrations. Oral SA was also neatly patterned, with distinct 'loading' and 'maintenance' phases of responding within each session. Using our software DeepSqueak, we analysed ultrasonic vocalizations (USVs), which are innate expressions of current emotional state in rats. Rats produced 50 kHz USVs during loading then shifted quickly to 22 kHz calls despite ongoing maintenance of oral fentanyl taking, reflecting a transition to negative reinforcement. Using fibre photometry, we found that the lateral habenula differentially processed drug cues and drug consumption depending on affective state, with potentiated modulation by drug cues and consumption during the negative affective maintenance phase. Together, these results indicate a rapid progression from positive to negative reinforcement occurs even within an active drug taking session, revealing a within-session opponent process.

Rapid appearance of negative emotion during oral fentanyl self-administration in male and female rats.

Opioid use disorder has become an epidemic in the United States, fueled by the widespread availability of fentanyl, which produces rapid and intense euphoria followed by severe withdrawal and emotional distress. We developed a new preclinical model of fentanyl seeking in outbred male and female rats using volitional oral self-administration that can be readily applied in labs without intravascular access. Using a traditional two lever operant procedure, rats learned to take oral fentanyl vigorously, escalated intake across sessions, and readily reinstated responding to conditioned cues after extinction. Oral self-administration also revealed individual and sex differences that are essential to studying substance use risk propensity. During a behavioral economics task, rats displayed inelastic demand curves and maintained stable intake across a wide range of fentanyl concentrations. Oral SA was also neatly patterned, with distinct "loading" and "maintenance" phases of responding within each session. Using our software DeepSqueak, we analyzed thousands of ultrasonic vocalizations (USVs), which are innate expressions of current emotional state in rats. Rats produced 50 kHz USVs during loading then shifted quickly to 22 kHz calls despite ongoing maintenance oral fentanyl taking, reflecting a transition to negative reinforcement. Using fiber photometry, we found that the lateral habenula differentially processed drug-cues and drug-consumption depending on affective state, with potentiated modulation by drug cues and consumption during the negative affective maintenance phase. Together, these results indicate a rapid progression from positive to negative reinforcement occurs even within an active drug taking session, revealing a within-session opponent process.

A cAMP-Related Gene Network in Microglia Is Inversely Regulated by Morphine Tolerance and Withdrawal.

Background: Microglia have recently been implicated in opioid dependence and withdrawal. Mu Opioid (MOR) receptors are expressed in microglia, and microglia form intimate connections with nearby neurons. Accordingly, opioids have both direct (MOR mediated) and indirect (neuron-interaction mediated) effects on microglia function. Methods: To investigate this directly, we used RNA sequencing of ribosome-associated RNAs from striatal microglia (RiboTag-Seq) after the induction of morphine tolerance and followed by naloxone precipitated withdrawal (n=16). We validated the RNA-Seq data by combining fluorescent in-situ hybridization with immunohistochemistry for microglia (n=18). Finally, we expressed and activated the Gi/o-coupled hM4Di DREADD receptor in CX3CR1-expressing cells during morphine withdrawal (n=18). Results: We detected large, inverse changes in RNA translation following opioid tolerance and withdrawal. WGCNA analysis revealed an intriguing network of cAMP-associated genes that are known to be involved in microglial motility, morphology, and interactions with neurons that were downregulated with morphine tolerance and upregulated rapidly by withdrawal. Three-dimensional histological reconstruction of microglia allowed for volumetric, visual colocalization of mRNA within individual microglia that validated our bioinformatics results. Direct activation of Gi/o-coupled DREADD receptors in CX3CR1-expressing cells exacerbated signs of opioid withdrawal rather than mimicking the effects of morphine. Conclusions: These results indicate that Gi-signaling and cAMP-associated gene networks are inversely engaged during opioid tolerance and early withdrawal, perhaps revealing a role of microglia in mitigating the consequences of opioids.

PACAP-expressing neurons in the lateral habenula diminish negative emotional valence.

The lateral habenula (LHb) is a small, bilateral, epithalamic nucleus which processes aversive information. While primarily glutamatergic, LHb neurons express genes coding for many neuropeptides, such as Adcyap1 the gene encoding pituitary adenylate cyclase-activating polypeptide (PACAP), which itself has been associated with anxiety and stress disorders. Using Cre-dependent viral vectors, we targeted and characterized these neurons based on their anatomical projections and found that they projected to both the raphe and rostromedial tegmentum but only weakly to ventral tegmental area. Using RiboTag to capture ribosomal-associated mRNA from these neurons and reanalysis of existing single cell RNA sequencing data, we did not identify a unique molecular phenotype that characterized these PACAP-expressing neurons in LHb. In order to understand the function of these neurons, we conditionally expressed hM3 Dq DREADD selectively in LHb PACAP-expressing neurons and chemogenetically excited these neurons during behavioral testing in the open field test, contextual fear conditioning, sucrose preference, novelty suppressed feeding, and conditioned place preference. We found that Gq activation of these neurons produce behaviors opposite to what is expected from the LHb as a whole-they decreased anxiety-like and fear behavior and produced a conditioned place preference. In conclusion, PACAP-expressing neurons in LHb represents a molecularly diverse population of cells that oppose the actions of the remainder of LHb neurons by being rewarding or diminishing the negative consequences of aversive events.

Stress decreases serotonin tone in the nucleus accumbens in male mice to promote aversion and potentiate cocaine preference via decreased stimulation of 5-HT1B receptors.

Stress-induced release of dynorphins (Dyn) activates kappa opioid receptors (KOR) in serotonergic neurons to produce dysphoria and potentiate drug reward; however, the circuit mechanisms responsible for this effect are not known. In male mice, we found that conditional deletion of KOR from Slc6a4 (SERT)-expressing neurons blocked stress-induced potentiation of cocaine conditioned place preference (CPP). Within the dorsal raphe nucleus (DRN), two overlapping populations of KOR-expressing neurons: Slc17a8 (VGluT3) and SERT, were distinguished functionally and anatomically. Optogenetic inhibition of these SERT+ neurons potentiated subsequent cocaine CPP, whereas optical inhibition of the VGluT3+ neurons blocked subsequent cocaine CPP. SERT+/VGluT3- expressing neurons were concentrated in the lateral aspect of the DRN. SERT projections from the DRN were observed in the medial nucleus accumbens (mNAc), but VGluT3 projections were not. Optical inhibition of SERT+ neurons produced place aversion, whereas optical stimulation of SERT+ terminals in the mNAc attenuated stress-induced increases in forced swim immobility and subsequent cocaine CPP. KOR neurons projecting to mNAc were confined to the lateral aspect of the DRN, and the principal source of dynorphinergic (Pdyn) afferents in the mNAc was from local neurons. Excision of Pdyn from the mNAc blocked stress-potentiation of cocaine CPP. Prior studies suggested that stress-induced dynorphin release within the mNAc activates KOR to potentiate cocaine preference by a reduction in 5-HT tone. Consistent with this hypothesis, a transient pharmacological blockade of mNAc 5-HT1B receptors potentiated subsequent cocaine CPP. 5-HT1B is known to be expressed on 5-HT terminals in NAc, and 5-HT1B transcript was also detected in Pdyn+, Adora2a+ and ChAT+ (markers for direct pathway, indirect pathway, and cholinergic interneurons, respectively). Following stress exposure, 5-HT1B transcript was selectively elevated in Pdyn+ cells of the mNAc. These findings suggest that Dyn/KOR regulates serotonin activation of 5HT1B receptors within the mNAc and dynamically controls stress response, affect, and drug reward.

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.

Effect of chemogenetic inhibition of lateral habenula neuronal activity on cocaine- and food-seeking behaviors in the rat.

A major problem in the treatment of cocaine addiction is high rates of relapse. Relapse is often provoked by acute reexposure to cocaine-associated cues or to cocaine itself. The lateral habenula (LHb), an epithalamic nucleus, regulates midbrain dopaminergic systems that are known to be involved in cocaine taking and seeking behaviors. However, the role of this nucleus in cocaine self-administration and reinstatement of cocaine seeking has not been entirely parsed out. We used an operant self-administration and reinstatement procedure to explore the effect of Designer Receptors Exclusively Activated by Designer Drug (DREADD)-induced transient inhibition of LHb neurons on cocaine taking and seeking. Firstly, rats were injected with adeno-associated viral vectors expressing hM4 Di (a Gi/o -coupled DREADD) into the LHb, trained to self-administer cocaine (0.75 mg/kg/infusion), and the effect of clozapine-N-oxide (an inert ligand that activates DREADDs) was assessed on cocaine self-administration. Secondly, rats were injected with hM4 Di into the LHb, trained to self-administer cocaine; the operant response was extinguished, and cue- and cocaine priming-induced reinstatement was assessed. Thirdly, we tested the generality of the effect of inhibiting LHb neurons by assessing the effect of this manipulation on food-taking and seeking. hM4 Di -induced inhibition of LHb neurons increased cocaine but not food self-administration. In contrast, this manipulation decreased reinstatement of cocaine, but not food-seeking. Taken together, our data suggest that hM4 Di - induced LHb inhibition specifically mediates taking and seeking behaviors reinforced by cocaine but not by natural reinforcers. Further, our data indicate a dissociation in the role of LHb neurons on cocaine self-administration versus reinstatement of cocaine seeking.

Sequencing the serotonergic neuron translatome reveals a new role for Fkbp5 in stress.

Serotonin is a key mediator of stress, anxiety, and depression, and novel therapeutic targets within serotonin neurons are needed to combat these disorders. To determine how stress alters the translational profile of serotonin neurons, we sequenced ribosome-associated RNA from these neurons after repeated stress in male and female mice. We identified numerous sex- and stress-regulated genes. In particular, Fkbp5 mRNA, which codes for the glucocorticoid receptor co-chaperone protein FKBP51, was consistently upregulated in male and female mice following stress. Pretreatment with a selective FKBP51 inhibitor into the dorsal raphe prior to repeated forced swim stress decreased resulting stress-induced anhedonia. Our results support previous findings linking FKBP51 to stress-related disorders and provide the first evidence suggesting that FKBP51 function may be an important regulatory node integrating circulating stress hormones and serotonergic regulation of stress responses.

Stress induces divergent gene expression among lateral habenula efferent pathways.

The lateral habenula (LHb) integrates critical information regarding aversive stimuli that shapes decision making and behavioral responses. The three major LHb outputs innervate dorsal raphe nucleus (DRN), ventral tegmental area (VTA), and the rostromedial tegmental nucleus (RMTg). LHb neurons that project to these targets are segregated and nonoverlapping, and this led us to consider whether they have distinct molecular phenotypes and adaptations to stress exposure. In order to capture a time-locked profile of gene expression after repeated forced swim stress, we used intersectional expression of RiboTag in rat LHb neurons and next-gen RNA sequencing to interrogate the RNAs actively undergoing translation from each of these pathways. The "translatome" in the neurons comprising these pathways was similar at baseline, but diverged after stress, especially in the neurons projecting to the RMTg. Using weighted gene co-expression network analysis, we found one module, which had an overrepresentation of genes associated with phosphoinositide 3 kinase (PI3K) signaling, comprising genes downregulated after stress in the RMTg-projecting LHb neurons. Reduced PI3K signaling in RMTg-projecting LHb neurons may be a compensatory adaptation that alters the functional balance of LHb outputs to GABAergic vs. monoaminergic neurons following repeated stress exposure.

Chemogenetic inhibition of lateral habenula projections to the dorsal raphe nucleus reduces passive coping and perseverative reward seeking in rats.

The lateral habenula (LHb) processes information about aversive experiences that contributes to the symptoms of stress disorders. Previously, we found that chemogenetic inhibition of rat LHb neurons reduced immobility in the forced swim test, but the downstream target of these neurons was not known. Using an intersectional viral vector strategy, we selectively transduced three different output pathways from the LHb by injecting AAV8-DIO-hM4Di into the LHb and CAV2-CRE (a retrograde viral vector) into one of the three target areas as follows: dorsal raphe nucleus (DRN), ventral tegmental area (VTA), or rostromedial tegmentum (RMTg). Using the forced swim test, we found that chemogenetic inhibition of DRN-projecting LHb neurons reduced passive coping (immobility), whereas inhibition of the other pathways did not. Chemogenetic activation of DRN-projecting neurons using hM3Dq in another cohort did not further exacerbate immobility. We next examined the impact of inhibiting DRN-projecting LHb neurons on reward sensitivity, perseverative behavior, and anxiety-like behavior using saccharin preference testing, reward-omission testing, and open-field testing, respectively. There was no effect of inhibiting any of these pathways on reward sensitivity, locomotion, or anxiety-like behavior, but inhibiting DRN-projecting LHb neurons reduced perseverative licking during reward-omission testing, whereas activating these neurons increased perseverative licking. These results support the idea that inhibiting LHb projections to the DRN provides animals with resilience during highly stressful or frustrating conditions but not under low-stress circumstances, and that inhibiting these neurons may promote persistence in active coping strategies.

Striatal Rgs4 regulates feeding and susceptibility to diet-induced obesity.

Consumption of high fat, high sugar (western) diets is a major contributor to the current high levels of obesity. Here, we used a multidisciplinary approach to gain insight into the molecular mechanisms underlying susceptibility to diet-induced obesity (DIO). Using positron emission tomography (PET), we identified the dorsal striatum as the brain area most altered in DIO-susceptible rats and molecular studies within this region highlighted regulator of G-protein signaling 4 (Rgs4) within laser-capture micro-dissected striatonigral (SN) and striatopallidal (SP) medium spiny neurons (MSNs) as playing a key role. Rgs4 is a GTPase accelerating enzyme implicated in plasticity mechanisms of SP MSNs, which are known to regulate feeding and disturbances of which are associated with obesity. Compared to DIO-resistant rats, DIO-susceptible rats exhibited increased striatal Rgs4 with mRNA expression levels enriched in SP MSNs. siRNA-mediated knockdown of striatal Rgs4 in DIO-susceptible rats decreased food intake to levels comparable to DIO-resistant animals. Finally, we demonstrated that the human Rgs4 gene locus is associated with increased body weight and obesity susceptibility phenotypes, and that overweight humans exhibit increased striatal Rgs4 protein. Our findings highlight a novel role for involvement of Rgs4 in SP MSNs in feeding and DIO-susceptibility.

The paraventricular thalamus is a critical mediator of top-down control of cue-motivated behavior in rats.

Cues in the environment can elicit complex emotional states, and thereby maladaptive behavior, as a function of their ascribed value. Here we capture individual variation in the propensity to attribute motivational value to reward-cues using the sign-tracker/goal-tracker animal model. Goal-trackers attribute predictive value to reward-cues, and sign-trackers attribute both predictive and incentive value. Using chemogenetics and microdialysis, we show that, in sign-trackers, stimulation of the neuronal pathway from the prelimbic cortex (PrL) to the paraventricular nucleus of the thalamus (PVT) decreases the incentive value of a reward-cue. In contrast, in goal-trackers, inhibition of the PrL-PVT pathway increases both the incentive value and dopamine levels in the nucleus accumbens shell. The PrL-PVT pathway, therefore, exerts top-down control over the dopamine-dependent process of incentive salience attribution. These results highlight PrL-PVT pathway as a potential target for treating psychopathologies associated with the attribution of excessive incentive value to reward-cues, including addiction.

5-HT1B Receptor-Mediated Activation of ERK1/2 Requires Both Gαi/o and ß-Arrestin Proteins.

5-HT1B receptors modulate synaptic serotonin (5-HT) levels and play a significant role in the regulation of emotional behaviors. These receptors are Gαi/o-coupled and inhibit adenylyl cyclase but have also been reported to activate MAP kinases; however, the details of signaling cascades downstream of 5-HT1B receptor activation remain unclear, particularly in neuronal cells. We generated a stable 5-HT1B receptor-expressing Neuro2A (N2A-1B) neuronal cell line and demonstrate that activation of these receptors by the selective 5-HT1B agonist CP-94253 results in activation of ERK1/2 but not of other closely related MAP kinases. Phosphoproteomics revealed four novel phosphorylation sites on the third intracellular loop of the 5-HT1B receptor, and mutations of serine-256 and serine-291 to alanine led to reduced levels of ERK1/2 phosphorylation following receptor activation. Inhibition of Gαi/o signaling with pertussis toxin, as well as MEK1/2 inhibition with U0126, also reduced 5-HT1B-mediated ERK1/2 phosphorylation. Finally, we found that knockout of either ß-arrestin 1 or ß-arrestin 2 prevented 5-HT1B-mediated phosphorylation of ERK1/2. Taken together, these results show that 5-HT1B receptor activation selectively induces ERK1/2 activation through both the Gαi subunit and ß-arrestin proteins. This work elucidates the signal transduction pathway of 5-HT1B receptors, as well as key phosphorylation sites within the receptor that modulate ERK1/2 activation, and further characterizes the intracellular mechanisms that underlie 5-HT1B receptor function.

Convergent neural connectivity in motor impulsivity and high-fat food binge-like eating in male Sprague-Dawley rats.

Food intake is essential for survival, but maladaptive patterns of intake, possibly encoded by a preexisting vulnerability coupled with the influence of environmental variables, can modify the reward value of food. Impulsivity, a predisposition toward rapid unplanned reactions to stimuli, is one of the multifaceted determinants underlying the etiology of dysregulated eating and its evolving pathogenesis. The medial prefrontal cortex (mPFC) is a major neural director of reward-driven behavior and impulsivity. Compromised signaling between the mPFC and nucleus accumbens shell (NAcSh) is thought to underlie the cognitive inability to withhold prepotent responses (motor impulsivity) and binge intake of high-fat food (HFF) seen in binge eating disorder. To explore the relationship between motor impulsivity and binge-like eating in rodents, we identified high (HI) and low impulsive (LI) rats in the 1-choice serial reaction time task and employed a rat model of binge-like eating behavior. HFF binge rats consumed significantly greater calories relative to control rats maintained on continual access to standard food or HFF. HI rats repeatedly exhibited significantly higher bingeing on HFF vs. LI rats. Next, we employed dual viral vector chemogenetic technology which allows for the targeted and isolated modulation of ventral mPFC (vmPFC) neurons that project to the NAcSh. Chemogenetic activation of the vmPFC to NAcSh pathway significantly suppressed motor impulsivity and binge-like intake for high-fat food. Thus, inherent motor impulsivity and binge-like eating are linked and the vmPFC to NAcSh pathway serves as a 'brake' over both behaviors.

DeepSqueak: a deep learning-based system for detection and analysis of ultrasonic vocalizations.

Rodents engage in social communication through a rich repertoire of ultrasonic vocalizations (USVs). Recording and analysis of USVs has broad utility during diverse behavioral tests and can be performed noninvasively in almost any rodent behavioral model to provide rich insights into the emotional state and motor function of the test animal. Despite strong evidence that USVs serve an array of communicative functions, technical and financial limitations have been barriers for most laboratories to adopt vocalization analysis. Recently, deep learning has revolutionized the field of machine hearing and vision, by allowing computers to perform human-like activities including seeing, listening, and speaking. Such systems are constructed from biomimetic, "deep", artificial neural networks. Here, we present DeepSqueak, a USV detection and analysis software suite that can perform human quality USV detection and classification automatically, rapidly, and reliably using cutting-edge regional convolutional neural network architecture (Faster-RCNN). DeepSqueak was engineered to allow non-experts easy entry into USV detection and analysis yet is flexible and adaptable with a graphical user interface and offers access to numerous input and analysis features. Compared to other modern programs and manual analysis, DeepSqueak was able to reduce false positives, increase detection recall, dramatically reduce analysis time, optimize automatic syllable classification, and perform automatic syntax analysis on arbitrarily large numbers of syllables, all while maintaining manual selection review and supervised classification. DeepSqueak allows USV recording and analysis to be added easily to existing rodent behavioral procedures, hopefully revealing a wide range of innate responses to provide another dimension of insights into behavior when combined with conventional outcome measures.

Loss of glutamate signaling from the thalamus to dorsal striatum impairs motor function and slows the execution of learned behaviors.

Parkinson's disease (PD) is primarily associated with the degeneration of midbrain dopamine neurons, but it is now appreciated that pathological processes like Lewy-body inclusions and cell loss affect several other brain regions, including the central lateral (CL) and centromedian/parafascicular (CM/PF) thalamic regions. These thalamic glutamatergic neurons provide a non-cortical excitatory input to the dorsal striatum, a major projection field of dopamine neurons. To determine how thalamostriatal signaling may contribute to cognitive and motor abnormalities found in PD, we used a viral vector approach to generate mice with loss of thalamostriatal glutamate signaling specifically restricted to the dorsal striatum (CAV2Cre-Slc17a6lox/lox mice). We measured motor function and behaviors corresponding to cognitive domains (visuospatial function, attention, executive function, and working memory) affected in PD. CAV2Cre-Slc17a6lox/lox mice were impaired in motor coordination tasks such as the rotarod and beam-walk tests compared with controls (CAV2Cre-Slc17a6+/+ mice). They did not demonstrate much cognitive impairment in the Morris water maze or a water U-maze, but had slower processing reaction times in those tests and in a two-way active avoidance task. These mice could model an aspect of bradyphrenia, the slowness of thought that is often seen in patients with PD and other neurological disorders.

Restoration of Physiological Expression of 5-HT6 Receptor into the Primary Cilia of Null Mutant Neurons Lengthens Both Primary Cilia and Dendrites.

5-HT6 (serotonin) receptors are promising targets for a variety of neuropsychiatric disorders and have been linked to several cellular signaling cascades. Endogenous 5-HT6 receptors are restricted to the primary neuronal cilium, a small sensory organelle stemming from the cell body that receives numerous extrasynaptic signals. Inhibition of 5-HT6 receptors decreases cilia length in primary neuronal cultures, but the signaling mechanisms involved are still unclear. Intense overexpression of exogenous 5-HT6 receptors increases the probability for receptors to localize outside the primary cilium and have been associated with changes in cilia morphology and dendritic outgrowth. In the present study, we explore the role of 5-HT6R rescue on neuronal morphology in primary neuronal cultures from 5-HT6R-KO mice, at the same time maintaining a more physiologic level of expression, wherein the receptor localizes to cilia in 80%-90% of neurons (similar to endogenous 5-HT6R localization). We found that rescue of 5-HT6R expression is sufficient to increase cilia length and dendritic outgrowth, but primarily in neurons in which the receptor is located exclusively in the primary cilia. Additionally, we found that expression of 5-HT6R mutants deficient in agonist-stimulated cAMP or without the predicted Fyn kinase binding domain maintained constitutive activity for stimulating cAMP and still increased the length of cilia, and that the proposed Fyn kinase domain was required for stimulating dendritic outgrowth. These findings highlight the complexity of 5-HT6R function and localization, particularly with the use of exogenous overexpression, and provide greater understanding and potential mechanisms for 5-HT6R drug therapies.