Increased expression of 5-HT6 receptors in the nucleus accumbens blocks the rewarding but not psychomotor activating properties of cocaine

The nucleus accumbens in reward and aversion processing: insights and implications

The nucleus accumbens (NAc), a central component of the brain's reward circuitry, has been implicated in a wide range of behaviors and emotional states. Emerging evidence, primarily drawing from recent rodent studies, suggests that the function of the NAc in reward and aversion processing is multifaceted. Prolonged stress or drug use induces maladaptive neuronal function in the NAc circuitry, which results in pathological conditions. This review aims to provide comprehensive and up-to-date insights on the role of the NAc in motivated behavior regulation and highlights areas that demand further in-depth analysis. It synthesizes the latest findings on how distinct NAc neuronal populations and pathways contribute to the processing of opposite valences. The review examines how a range of neuromodulators, especially monoamines, influence the NAc's control over various motivational states. Furthermore, it delves into the complex underlying mechanisms of psychiatric disorders such as addiction and depression and evaluates prospective interventions to restore NAc functionality.

Cilia loss on distinct neuron populations differentially alters cocaine-induced locomotion and reward

BACKGROUND

Neuronal primary cilia are being recognized for their role in mediating signaling associated with a variety of neurobehaviors, including responses to drugs of abuse. They function as signaling hubs, enriched with a diverse array of G-protein coupled receptors (GPCRs), including several associated with motivation and drug-related behaviors. However, our understanding of how cilia regulate neuronal function and behavior is still limited.

AIMS

The objective of the current study was to investigate the contributions of primary cilia on specific neuronal populations to behavioral responses to cocaine.

METHODS

To test the consequences of cilia loss on cocaine-induced locomotion and reward-related behavior, we selectively ablated cilia from dopaminergic or GAD2-GABAergic neurons in mice.

RESULTS

Cilia ablation on either population of neurons failed to significantly alter acute locomotor responses to cocaine at a range of doses. With repeated administration, mice lacking cilia on GAD2-GABAergic neurons showed no difference in locomotor sensitization to cocaine compared to wild-type (WT) littermates, whereas mice lacking cilia on dopaminergic neurons exhibited reduced locomotor sensitization to cocaine at 10 and 30 mg/kg. Mice lacking cilia on GAD2-GABAergic neurons showed no difference in cocaine conditioned place preference (CPP), whereas mice lacking cilia on dopaminergic neurons exhibited reduced CPP compared to WT littermates.

CONCLUSIONS

Combined with previous findings using amphetamine, our results show that behavioral effects of cilia ablation are cell- and drug type-specific, and that neuronal cilia contribute to modulation of both the locomotor-inducing and rewarding properties of cocaine.

Impact of 5-HT6 Receptor Subcellular Localization on Its Signaling and Its Pathophysiological Roles

The serotonin (5-HT)₆ receptor still raises particular interest given its unique spatio-temporal pattern of expression among the serotonin receptor subtypes. It is the only serotonin receptor specifically expressed in the central nervous system, where it is detected very early in embryonic life and modulates key neurodevelopmental processes, from neuronal migration to brain circuit refinement. Its predominant localization in the primary cilium of neurons and astrocytes is also unique among the serotonin receptor subtypes. Consistent with the high expression levels of the 5-HT₆ receptor in brain regions involved in the control of cognitive processes, it is now well-established that the pharmacological inhibition of the receptor induces pro-cognitive effects in several paradigms of cognitive impairment in rodents, including models of neurodevelopmental psychiatric disorders and neurodegenerative diseases. The 5-HT₆ receptor can engage several signaling pathways in addition to the canonical Gs signaling, but there is still uncertainty surrounding the signaling pathways that underly its modulation of cognition, as well as how the receptor's coupling is dependent on its cellular compartmentation. Here, we describe recent findings showing how the proper subcellular localization of the receptor is achieved, how this peculiar localization determines signaling pathways engaged by the receptor, and their pathophysiological influence.

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.

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.

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.

Regulators of G-protein signaling 2 and 4 differentially regulate cocaine-induced rewarding effects

There is a need to identify new therapeutic targets for the treatment of cocaine addiction due to the rise in cocaine abuse and deaths due to cocaine overdose. Regulator of G protein signaling (RGS) proteins such as RGS2 and RGS4 are widely distributed in brain regions that play a role in drug reward. Importantly, RGS2 and RGS4 negatively regulate G-protein coupled receptor signaling pathways of monoaminergic neurotransmitters that play a role in the rewarding effects of cocaine by enhancing the rate of hydrolysis of Gα-bound guanine nucleotide triphosphate. Thus, the objective of this study was to investigate the effects of cocaine on conditioned place preference (CPP) and locomotor activity in mice that lacked either RGS2 or RGS4 (i.e. knockout (KO) mice) and their wildtype (WT) littermates. Moreover recent studies have reported influence of sex on RGS functioning and hence studies were conducted in both male and female mice. Cocaine-induced CPP was attenuated in male, but not female RGS4 KO mice compared to respective RGS4 WT mice. Cocaine-induced CPP was not influenced by deletion of RGS2 in either male or female mice. Similarly, cocaine-induced locomotor activity was not influenced by deletion of either RGS2 or RGS4 irrespective of sex. Together, the data indicate that the rewarding effects of cocaine were attenuated in the absence of RGS4 expression, but not in the absence of RGS2 expression in a sex-dependent manner. Importantly, these data suggest that RGS4 can serve as a potential target for medications that can be used to treat cocaine addiction.

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.

Blockade of the 5-HT transporter contributes to the behavioural, neuronal and molecular effects of cocaine

BACKGROUND AND PURPOSE

The psychostimulant cocaine induces complex molecular, cellular and behavioural responses as a consequence of inhibiting presynaptic dopamine, noradrenaline and 5-HT transporters. To elucidate 5-HT transporter (SERT)-specific contributions to cocaine action, we evaluated cocaine effects in the SERT Met172 knock-in mouse, which expresses a SERT coding substitution that eliminates high-affinity cocaine recognition.

EXPERIMENTAL APPROACH

We measured the effects of SERT Met172 on cocaine antagonism of 5-HT re-uptake using ex vivo synaptosome preparations and in vivo microdialysis. We assessed SERT dependence of cocaine actions behaviourally through acute and chronic locomotor activation, sensitization, conditioned place preference (CPP) and oral cocaine consumption. We used c-Fos, quantitative RT-PCR and RNA sequencing methods for insights into cellular and molecular networks supporting SERT-dependent cocaine actions.

KEY RESULTS

SERT Met172 mice demonstrated functional insensitivity for cocaine at SERT. Although they displayed wild-type levels of acute cocaine-induced hyperactivity or chronic sensitization, the pattern of acute motor activation was different, with a bias toward thigmotaxis. CPP was increased, and a time-dependent elevation in oral cocaine consumption was observed. SERT Met172 mice displayed relatively higher levels of neuronal activation in the hippocampus, piriform cortex and prelimbic cortex (PrL), accompanied by region-dependent changes in immediate early gene expression. Distinct SERT-dependent gene expression networks triggered by acute and chronic cocaine administration were identified, including PrL Akt and nucleus accumbens ERK1/2 signalling.

CONCLUSION AND IMPLICATIONS

Our studies reveal distinct SERT contributions to cocaine action, reinforcing the possibility of targeting specific aspects of cocaine addiction by modulation of 5-HT signalling.

5-HT6 receptor blockade regulates primary cilia morphology in striatal neurons

The 5-HT₆ receptor has been implicated in a variety of cognitive processes including habitual behaviors, learning, and memory. It is found almost exclusively in the brain, is expressed abundantly in striatum, and localizes to neuronal primary cilia. Primary cilia are antenna-like, sensory organelles found on most neurons that receive both chemical and mechanical signals from other cells and the surrounding environment; however, the effect of 5-HT₆ receptor function on cellular morphology has not been examined. We confirmed that 5-HT₆ receptors were localized to primary cilia in wild-type (WT) but not 5-HT₆ knockout (5-HT₆KO) in both native mouse brain tissue and primary cultured striatal neurons then used primary neurons cultured from WT or 5-HT₆KO mice to study the function of these receptors. Selective 5-HT₆ antagonists reduced cilia length in neurons cultured from wild-type mice in a concentration and time-dependent manner without altering dendrites, but had no effect on cilia length in 5-HT₆KO cultured neurons. Varying the expression levels of heterologously expressed 5-HT₆ receptors affected the fidelity of ciliary localization in both WT and 5-HT₆KO neurons; overexpression lead to increasing amounts of 5-HT₆ localization outside of the cilia but did not alter cilia morphology. Introducing discrete mutations into the third cytoplasmic loop of the 5-HT₆ receptor greatly reduced, but did not entirely eliminate, trafficking of the 5-HT₆ receptor to primary cilia. These data suggest that blocking 5-HT₆ receptor activity reduces the length of primary cilia and that mechanisms that regulate trafficking of 5-HT₆ receptors to cilia are more complex than previously thought.

Striatal 5-HT6 Receptors Regulate Cocaine Reinforcement in a Pathway-Selective Manner

The nucleus accumbens (NAc) in the ventral striatum integrates many neurochemical inputs including dopamine and serotonin projections from midbrain nuclei to modulate drug reward. Although D1 and D2 dopamine receptors are differentially expressed in the direct and indirect pathway medium spiny neurons (dMSNs and iMSNs, respectively), 5-HT6 receptors are expressed in both pathways, more strongly than anywhere else in the brain, and are an intriguing target for neuropsychiatric disorders. In the present study, we used viral vectors utilizing dynorphin or enkephalin promoters to drive expression of 5-HT6 receptors or green fluorescent protein (GFP) selectively in the dMSNs or iMSNs of the NAc shell. Rats were then trained to self-administer cocaine. Increased 5-HT6 receptor expression in dMSNs did not change any parameter of cocaine self-administration measured. However, increasing 5-HT6 receptors in iMSNs reduced the amount of cocaine self-administered under fixed-ratio schedules, especially at low doses, increased the time to the first response and the length of the inter-infusion interval, but did not alter motivation as measured by progressive ratio 'break point' analysis. Modeling of cocaine pharmacokinetics in NAc showed that increased 5-HT6 receptors in iMSNs reduced the rat's preferred tissue cocaine concentration at each dose. Finally, increased 5-HT6 receptors in iMSNs facilitated conditioned place preference for a low dose of cocaine. We conclude that 5-HT6 receptors in iMSNs of NAcSh increase the sensitivity to the reinforcing properties of cocaine, particularly at low doses, suggesting that these receptors may be a therapeutic target for the treatment of cocaine addiction.

Deconstructing 5-HT6 receptor effects on striatal circuit function

Medium spiny neurons (MSNs) constitute 95% of neurons in the dorsal striatum subdivided into direct (striatonigral) and indirect (striatopallidal) pathways. Whereas D1 and D2 receptors and several neuropeptides, including dynorphin and enkephalin, are differentially expressed in these neurons, 5-hydroxytryptamine 6 receptors (5-HT6) are expressed in both pathways. Previous results demonstrate that concurrent 5-HT6 receptor overexpression in MSNs of both pathways in the dorsomedial striatum (DMS) interferes with instrumental learning and that 5-HT6 overexpression in the dorsolateral striatum (DLS) relieves rats from inflexible habitual behaviors. We hypothesized that 5-HT6 receptor-mediated co-activation of both pathways interferes with the differential activation/inhibition of direct/indirect pathways by dopamine. To test this idea, we cloned novel viral vectors to selectively overexpress 5-HT6 receptors in direct or indirect pathway MSNs to deconstruct their role in modulating instrumental learning and habitual responding. We found that increasing 5-HT6 receptor expression in either direct or indirect pathway MSNs of the posterior DMS selectively enhanced or impaired initial acquisition of a discrete instrumental learning task respectively, though all rats were ultimately able to learn the task. In a separate set of experiments, 5-HT6 receptor overexpression in indirect pathway MSNs of the DLS facilitated behavioral flexibility in rats overtrained on a repetitive pressing task using a variable interval schedule of reinforcement, during an omission contingency training session and subsequent probe testing. Together these findings further the notion that 5-HT6 signaling causes balanced activation of opposing MSN pathways by serotonin in sub-regions of the dorsal striatum allowing for more reflective modalities of behavior.

Psychostimulants: Basic and Clinical Pharmacology

Substance use disorder, and particularly psychostimulant use disorder, has considerable socioeconomic burden globally. The psychostimulants include several chemical classes, being derivatives of benzoylecgonine, phenethylamine, phenylpropanolamine, or aminoaryloxazoline. Psychostimulant drugs activate the brain reward pathways of the mesoaccumbal system, and continued use leads to persistent neuroplastic and dysfunctional changes of a variety of structures involved in learning and memory, habit-forming learning, salience attribution, and inhibitory control. There are a variety of neurochemical and neurobehavioral changes in psychostimulant addiction, for example, dopaminergic, glutamatergic, serotonergic (5-HT-ergic), and γ-amino butyric acid (GABA) changes have all noted. In this chapter, we will review pharmacological changes associated with psychostimulant use and abuse in humans and animals, and on the basis of the best characterized and most widely abused psychostimulants (amphetamines, cocaine) discuss why use transitions into abuse and review basic science and clinical strategies that might assist in treating psychostimulant abuse.

The role of serotonin in drug use and addiction

The use of psychoactive drugs is a wide spread behaviour in human societies. The systematic use of a drug requires the establishment of different drug use-associated behaviours which need to be learned and controlled. However, controlled drug use may develop into compulsive drug use and addiction, a major psychiatric disorder with severe consequences for the individual and society. Here we review the role of the serotonergic (5-HT) system in the establishment of drug use-associated behaviours on the one hand and the transition and maintenance of addiction on the other hand for the drugs: cocaine, amphetamine, methamphetamine, MDMA (ecstasy), morphine/heroin, cannabis, alcohol, and nicotine. Results show a crucial, but distinct involvement of the 5-HT system in both processes with considerable overlap between psychostimulant and opioidergic drugs and alcohol. A new functional model suggests specific adaptations in the 5-HT system, which coincide with the establishment of controlled drug use-associated behaviours. These serotonergic adaptations render the nervous system susceptible to the transition to compulsive drug use behaviours and often overlap with genetic risk factors for addiction. Altogether we suggest a new trajectory by which serotonergic neuroadaptations induced by first drug exposure pave the way for the establishment of addiction.

The role of serotonin in memory: interactions with neurotransmitters and downstream signaling

Serotonin, or 5-hydroxytryptamine (5-HT), is found to be involved in many physiological or pathophysiological processes including cognitive function. Seven distinct receptors (5-HT1-7), each with several subpopulations, have been identified for serotonin, which are different in terms of localization and downstream signaling. Because of the development of selective agonists and antagonists for these receptors as well as transgenic animal models of cognitive disorders, our understanding of the role of serotonergic transmission in learning and memory has improved in recent years. A large body of evidence indicates the interplay between serotonergic transmission and other neurotransmitters including acetylcholine, dopamine, γ-aminobutyric acid (GABA) and glutamate, in the neurobiological control of learning and memory. In addition, there has been an alteration in the density of serotonergic receptors in aging and Alzheimer's disease, and serotonin modulators are found to alter the process of amyloidogenesis and exert cognitive-enhancing properties. Here, we discuss the serotonin-induced modulation of various systems involved in mnesic function including cholinergic, dopaminergic, GABAergic, glutamatergic transmissions as well as amyloidogenesis and intracellular pathways.

Serotonin receptor of type 6 (5-HT6) in human prefrontal cortex and hippocampus post-mortem: an immunohistochemical and immunofluorescence study

Given the paucity of data on the distribution of serotonin (5-HT) receptors of type 6 (5-HT(6)) in the human brain, the aim of this study was to investigate their distribution in postmortem human prefrontal cortex, striatum and hippocampus by either immunohistochemical or immunofluorescence techniques. The brain samples were obtained from 6 subjects who had died for causes not involving primarily or secondarily the CNS. The 5-HT(6) receptor distribution was explored by the [(125)I]SB-258585 binding to brain membranes followed by immunohistochemical and immunofluorescence evaluations. A specific [(125)I]SB-258585 binding was detected in all the regions under investigation, whilst the content in the hippocampus and cortex being about 10-30 times lower than in the striatum. Immunohistochemistry and double-label immunofluorescence microscopy experiments, carried out in the prefrontal cortex and hippocampus only, since data in the striatum were already published, showed the presence of 5-HT(6) receptors in both pyramidal and glial cells of prefrontal cortex, while positive cells were mainly pyramidal neurons in the hippocampus. The heterogeneous distribution of 5-HT(6) receptors provides a preliminary explanation of how they might regulate different functions in different brain areas, such as, perhaps, brain trophism in the cortex and neuronal firing in the hippocampus. This study, taking into account all the limitations due to the postmortem model used, represents the starting point to explore the 5-HT(6) receptor functionality and its sub-cellular distribution.

5-HT(1B) autoreceptor regulation of serotonin transporter activity in synaptosomes

Serotonin-1B (5-HT(1B) ) autoreceptors are located in serotonin (5-HT) terminals, along with serotonin transporters (SERT), and play a critical role in autoregulation of serotonergic neurotransmission and are implicated in disorders of serotonergic function, particularly emotional regulation. SERT modulates serotonergic neurotransmission by high-affinity reuptake of 5-HT. Alterations in SERT activity are associated with increased risk for depression and anxiety. Several neurotransmitter receptors are known to regulate SERT K(m) and V(max) , and previous work suggests that 5-HT(1B) autoreceptors may regulate 5-HT reuptake, in addition to modulating 5-HT release and synthesis. We used rotating disk electrode voltammetry to investigate 5-HT(1B) autoreceptor regulation of SERT-mediated 5-HT uptake into synaptosomes. The selective 5-HT(1B) antagonist SB224289 decreased SERT activity in synaptosomes prepared from wild-type but not 5-HT(1B) knockout mice, whereas SERT uptake was enhanced after pretreatment with the selective 5-HT(1B) agonist CP94253. Furthermore, SERT activity varies as a function of 5-HT(1B) receptor expression-specifically, genetic deletion of 5-HT(1B) decreased SERT function, while viral-mediated overexpression of 5-HT(1B) autoreceptors in rat raphe neurons increased SERT activity in rat hippocampal synaptosomes. Considered collectively, these results provide evidence that 5-HT(1B) autoreceptors regulate SERT activity. Because SERT clearance rate varies as a function of 5-HT(1B) autoreceptor expression levels and is modulated by both activation and inhibition of 5-HT(1B) autoreceptors, this dynamic interaction may be an important mechanism of serotonin autoregulation with therapeutic implications.

Cocaine hydrolase gene therapy for cocaine abuse

Rapid progress in the past decade with re-engineering of human plasma butyrylcholinesterase has led to enzymes that destroy cocaine so efficiently that they prevent or interrupt drug actions in the CNS even though confined to the blood stream. Over the same time window, improved gene-transfer technology has made it possible to deliver such enzymes by endogenous gene transduction at high levels for periods of a year or longer after a single treatment. This article reviews recent advances in this field and considers prospects for development of a robust therapy aimed at aiding recovering drug users avoid addiction relapse.

Increased expression of the 5-HT6 receptor by viral mediated gene transfer into posterior but not anterior dorsomedial striatum interferes with acquisition of a discrete action-outcome task

Serotonin plays a role in reinforcement learning; however, it is not known which serotonin receptors mediate these effects. Serotonin 6 (5-HT(6)) receptors are abundant in the striatum, a brain area that is involved in reinforcement learning. We previously found that 5-HT(6) receptors in the dorsomedial striatum (DMS) affect reinforcement learning or consolidation over several days. We use viral-mediated gene transfer to discern the role that 5-HT(6) receptors play in mediating post-synaptic responses in anterior versus posterior DMS. Male Long-Evans rats were used to study learning acquisition during a single session of 100 trials on a fixed interval of 20 seconds. In a discrete action-outcome learning task, rats had 10 seconds to press a lever to induce lever retraction and sucrose pellet delivery. In another group of rats, the task had a lever that was continuously extended but only active every 20 seconds, allowing for repetitive, mostly non-reinforced, lever pressing. Results demonstrate that increased expression of 5-HT(6) receptors in the posterior DMS interferes with earning sucrose pellets in only the former task. We take this to indicate that 5-HT(6) receptor signaling in the posterior DMS interferes with acquisition of discrete action-outcome responding.

Increased expression of 5-HT₆ receptors in dorsolateral striatum decreases habitual lever pressing, but does not affect learning acquisition of simple operant tasks in rats

Serotonin-6 (5-HT(6)) receptors are densely expressed in the dorsolateral striatum (DLS), a brain region linked to habits. Medications acting on the serotonergic system, including 5-HT(6) receptors, can diminish habitual and repetitive behaviors associated with clinical syndromes such as obsessive-compulsive disorder, and may have implications for addiction as well. To examine the role of 5-HT(6) receptors in the acquisition and persistence of habitual behavior, we manipulated 5-HT(6) receptor expression in the DLS with herpes simplex virus vectors in combination with different behavioral procedures; control rats received a vector expressing enhanced green fluorescent protein. In one set of experiments, rats were tested under conditions that favor the acquisition of either discrete action-outcome responding or repetitive responding; increased 5-HT(6) receptor expression in DLS did not alter learning in either paradigm. In the next experiment, rats were over-trained on fixed- then variable-interval schedules, resulting in an escalation of lever pressing over sessions far in excess of that necessary to receive sucrose pellets. After training, rats received viral vector infusion into the DLS. Subsequently, half of each group underwent an omission contingency training session in which they received reinforcement for refraining from pressing the lever, while the other half served as yoked controls. A probe session under extinction conditions was performed the following day. Only rats that received both the 5-HT(6) vector and omission contingency training showed reduced lever pressing during the probe session. These results suggest that increasing 5-HT(6) receptor signaling in the DLS facilitates behavioral flexibility in the face of changing contingencies.

Serotonin 1B autoreceptors originating in the caudal dorsal raphe nucleus reduce expression of fear and depression-like behavior

BACKGROUND

Serotonin 1B (5-HT(1B)) autoreceptors regulate release of serotonin from terminals of dorsal raphe nucleus (DRN) projections. Expression of 5-HT(1B) in the DRN inversely correlates with behavioral measures of emotion, and viral-mediated overexpression of 5-HT(1B) receptors in the middle DRN inversely reduces measures of fear and anxiety in unstressed rats. Because the caudal subregion of the DRN is important in translating stress into emotional dysregulation, we explored behavioral functions of 5-HT(1B) autoreceptors in the caudal DRN.

METHODS

We manipulated 5-HT(1B) autoreceptor function in rats using either viral-mediated gene transfer into the caudal DRN or systemic injections of the 5-HT(1B) agonist 3-(1,2,5,6-tetrahydro-4-pyridyl)-5-propoxypyrrolo[3,2-b]pyridine (CP-94253). Rats were tested in forced swim test, open field test, and contextual fear conditioning.

RESULTS

Overexpression of 5-HT(1B) in the caudal DRN increased swimming in the forced swim test. It did not alter locomotion or thigmotaxis in the open field test but did reduce conditioned freezing. Freezing was reduced when 5-HT(1B) overexpression was present only during testing but not training. The CP-94253 exerted an inverted U-shaped dose response curve on conditioned freezing, with most pronounced effects seen at 1 mg/kg. At this dose, CP-94253 administered before a fear retention test reduced freezing both during that session and in subsequent drug-free testing, but only when drug was paired with re-exposure to the fear context.

CONCLUSIONS

The 5-HT(1B) autoreceptors originating in the caudal DRN regulate behavioral expression of helplessness and fear. Because systemic pharmacologic treatment with a 5-HT(1B) agonist facilitates reductions in fear, 5-HT(1B) receptors may be a target for the treatment of certain anxiety disorders.

5-HT receptors and reward-related behaviour: a review

The brain's serotonin (5-HT) system is key in the regulation of reward-related behaviours, from eating and drinking to sexual activity. The complexity of studying this system is due, in part, to the fact that 5-HT acts at many receptor subtypes throughout the brain. The recent development of drugs with greater selectivity for individual receptor subtypes has allowed for rapid advancements in our understanding of this system. Use of these drugs in combination with animal models entailing selective reward measures (i.e. intracranial self-stimulation, drug self-administration, conditioned place preference) have resulted in a greater understanding of the pharmacology of reward-related processing and behaviour (particularly regarding drugs of abuse). The putative roles of each 5-HT receptor subtype in the pharmacology of reward are outlined and discussed here. It is concluded that the actions of 5-HT in reward are receptor subtype-dependent (and thus should not be generalized) and that all studied subtypes appear to have a unique profile which is determined by content (e.g. receptor function, localization - both throughout the brain and within the synapse) and context (e.g. type of behavioural paradigm, type of drug). Given evidence of altered reward-related processing and serotonergic function in numerous neuropsychiatric disorders, such as depression, schizophrenia, and addiction, a clearer understanding of the role of 5-HT receptor subtypes in this context may lead to improved drug development and therapeutic approaches.

Overexpression of 5-HT(1B) mRNA in nucleus accumbens shell projection neurons differentially affects microarchitecture of initiation and maintenance of ethanol consumption

Serotonin 1B (5-HT(1B)) heteroreceptors on nucleus accumbens shell (NAcSh) projection neurons have been shown to enhance the voluntary consumption of alcohol by rats, presumably by modulating the activity of the mesolimbic reward pathway. The present study examined whether increasing 5-HT(1B) receptors expressed on NAcSh projection neurons by means of virus-mediated gene transfer enhances ethanol consumption during the initiation or maintenance phase of drinking and alters the temporal pattern of drinking behavior. Animals received stereotaxic injections of viral vectors expressing either 5-HT(1B) receptor and green fluorescent protein (GFP) or GFP alone. Home cages equipped with a three-bottle (water and 6 and 12% ethanol) lickometer system recorded animals' drinking behaviors continuously, capturing either initiation or maintenance of drinking behavior patterns. Overexpression of 5-HT(1B) receptors during initiation increased consumption of 12% ethanol during both forced-access and free-choice consumption. There was a shift in drinking pattern for 6% ethanol with an increase in number of drinking bouts per day, although the total number of drinking bouts for 12% ethanol was not different. Finally, increased 5-HT(1B) expression induced more bouts with very high-frequency licking from the ethanol bottle sippers. During the maintenance phase of drinking, there were no differences between groups in total volume of ethanol consumed; however, there was a shift toward drinking bouts of longer duration, especially for 12% ethanol. This suggests that during maintenance drinking, increased 5-HT(1B) receptors facilitate longer drinking bouts of more modest volumes. Taken together, these results indicate that 5-HT(1B) receptors expressed on NAcSh projection neurons facilitate ethanol drinking, with different effects during initiation and maintenance of ethanol-drinking behavior.

5-HT6 antagonism attenuates cue-induced relapse to cocaine seeking without affecting cocaine reinforcement

Re-exposure to drug-related cues elicits drug-seeking behaviour and relapse in humans even after months of abstinence. Similarly, in laboratory rats, drug-associated stimuli reinstate cocaine seeking after prolonged withdrawal periods, thus providing a model to study mechanisms underlying cocaine relapse. 5-HT6 receptors (5-HT6Rs) are abundantly expressed in brain areas such as the nucleus accumbens and prefrontal cortex, which are critically involved in cocaine reinforcement and relapse. Nevertheless, the role of 5-HT6Rs in relapse mechanisms has not been investigated. We report here that the 5-HT6R antagonists SB-271046 and Ro-04-6790 significantly attenuate cue-induced cocaine seeking. However, effective doses of both 5-HT6R antagonists did not affect cocaine self-administration. This indicates that 5-HT6Rs are specifically involved in the secondary reinforcing properties of cocaine, leaving primary reinforcement and ability to perform an operant response unaffected. As such, 5-HT6Rs may represent a novel target for the prevention of relapse to cocaine seeking.

Behavioral evidence for the significance of serotoninergic (5-HT) receptors in cocaine addiction

Cocaine addiction has somatic, psychological, psychiatric, socio-economic and legal implications in the developed world. Presently, there is no medication approved for the treatment of cocaine addiction. In recent years, data from the literature (pre-clinical studies and clinical trials) have provided several lines of evidence that serotonin (5-HT) and 5-HT receptors play a modulatory role in the mechanisms of action of cocaine. Here we review the contribution of 5-HT receptor subtypes to cocaine sensitization, discrimination, conditioned place preference, self-administration, reinstatement of seeking behavior and withdrawal symptoms in laboratory animals. Additionally, the consequences of chronic cocaine exposure on particular 5-HT receptor-assigned functions in pre-clinical studies are presented.

Differences between nicotine and cocaine-induced conditioned place preferences

In previous studies, we found differences between nicotine and cocaine-induced changes in the levels of neurotransmitters in various brain areas, which suggested differences in their reward - preference mechanisms. The present study was based on the idea that drug preference is modulated by a number of different factors, among them several neurotransmitters and their receptors, and antagonists of specific receptors will influence preference. We also assumed that the factors (components of reward mechanisms) involved are different in the case of different drugs. We compared the inhibition of nicotine preference with cocaine preference. We assayed preference as conditioned place preference (CPP) and measured CPP inhibition by receptor subtype antagonists using mice. In general, induced CPP of cocaine was stronger than of nicotine as shown by more time spent in the nonpreferred area after conditioning with cocaine. We measured inhibition by four antagonists: mecamylamine, atropine, SCH23390, and phentolamine: antagonists respectively of nicotinic, and muscarinic acetylcholine, dopamine D1, and alpha noradrenergic receptors. The inhibition by the antagonists of cocaine CPP was lower in most instances than that of nicotine CPP. Atropine and SCH23390 inhibited nicotine and cocaine CPP approximately to the same degree, while the inhibition by mecamylamine and phentolamine of nicotine CPP was 100%; that of cocaine was 20% and 0, respectively. We conclude that several receptor systems and transmitters play a role in drug preference, some represent essential elements or circuits, some may be only required partially or their role can be partially substituted. The composition of such systems is different for different drugs - in the present study, some of the components influencing CPP are different for nicotine as opposed to cocaine.

Pairing mild stress with increased serotonin-1B receptor expression in the nucleus accumbens increases susceptibility to amphetamine

Both serotonin-1B (5-HT(1B)) receptors and stress modulate the behavioral and neurobiological effects of psychostimulant drugs. In order to examine how these factors interact to influence the development of behaviors associated with addiction, we used viral-mediated gene transfer to transiently increase expression of 5-HT(1B) receptors in the nucleus accumbens (NAc) shell along with exposure to repeated mild stress (novelty + saline injection) in rats. Once the viral-mediated increases in gene expression had dissipated, the resulting effects of this 5-HT(1B)/stress pairing on the acute locomotor response to amphetamine and on the development of psychomotor sensitization were examined. We report that the increasing expression of 5-HT(1B) receptors on the terminals of NAc shell neurons that project to the ventral tegmental area and repeatedly exposing rats to mild stress subsequently enhance the acute locomotor-activating effects of amphetamine. In addition, the development of psychomotor sensitization (both locomotor activity and stereotypy components) is facilitated. These results suggest that serotonin signaling through NAc 5-HT(1B) heteroreceptors can interact with stress to increase susceptibility to the enduring forms of drug-induced plasticity that are associated with addiction.

Acquisition of and withdrawal from cocaine self-administration regulates 5-HT mRNA expression in rat striatum

This study investigated how different stages of cocaine self-administration in rats affect the expression of two serotonin receptors in dorsal and ventral striatum, the 5-HT(1B) and 5-HT(6) subtypes, which have both been implicated in mediating some aspects of cocaine-related behaviors. In the first experiment, rats were trained to work for saccharin (oral) or cocaine (i.v.) reinforcers. We found that continuous access to cocaine for 23 days did not change the level of 5-HT(1B) mRNA expression compared to control animals receiving saccharin. However, a single cocaine session, given either by self-administration or non-contingently, increased 5-HT(1B) mRNA in dorsal striatum, whereas forced abstinence for two weeks after cocaine reduced 5-HT(1B) mRNA expression in the same subregion. 5-HT(6) mRNA was not changed by any of these treatments. A follow-up experiment investigated the effects of limited versus extended access to cocaine as well as forced abstinence, and we found that 14 days of forced abstinence significantly reduced 5-HT(1B) mRNA throughout the dorsal and ventral striatum compared to no withdrawal. These results suggest that the influence of 5-HT(1B) receptors in striatal projection neurons may be increased during cocaine acquisition and reduced after forced abstinence and may therefore be targets for pharmacological intervention in addiction.