Protracted withdrawal from cocaine self-administration flips the switch on 5-HT(1B) receptor modulation of cocaine abuse-related behaviors

Neurolipidomics in schizophrenia: A not so well-oiled machine

Most patients with schizophrenia (SCZ) do not exhibit violent behaviors and are more likely to be victims rather than perpetrators of violent acts. However, a subgroup of forensic detainees with SCZ exhibit tendencies to engage in criminal violations. Although numerous models have been proposed, ranging from substance use, serotonin transporter gene, and cognitive dysfunction, the molecular underpinnings of violence in SCZ patients remains elusive. Lithium and clozapine have established anti-aggression properties and recent studies have linked low cholesterol levels and ultraviolet (UV) radiation with human aggression, while vitamin D3 reduces violent behaviors. A recent study found that vitamin D3, omega-3 fatty acids, magnesium, and zinc lower aggression in forensic population. In this review article, we take a closer look at aryl hydrocarbon receptor (AhR) and the dysfunctional lipidome in neuronal membranes, with emphasis on cholesterol and vitamin D3 depletion, as sources of aggressive behavior. We also discuss modalities to increase the fluidity of neuronal double layer via membrane lipid replacement (MLR) and natural or synthetic compounds. This article is part of the Special Issue on "Personality Disorders".

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.

5-HT1B receptor agonist enhances breakpoint for cocaine on a progressive ratio (PR) schedule during maintenance of self-administration in female rats but reduces breakpoint for sucrose

Background: Previous research showed that the 5-HT_1B receptor agonist CP94253 enhanced cocaine reinforcement rate during maintenance of daily self-administration (SA), but inhibited reinforcement rate after 21 days of abstinence in male rats. Here we examined whether female rats show similar effects of CP94253 during maintenance as males across estrous cycle phases.

Methods: Female rats trained on a fixed ratio 5 (FR5) cocaine reinforcement schedule were tested for the effects of CP94253 (5.6 mg/kg, s.c.) on cocaine reinforcement rate during each phase of the estrous cycle, with access to either low (0.075 and 0.1875) or high (0.375 and 0.75) cocaine doses available for 1 h sequentially in descending dose order. Other female and male rats trained on a progressive ratio (PR) schedule of cocaine or sucrose reinforcement were tested for CP94253 (0, 3.2, 5.6, and 10 mg/kg, s.c.) effects on reinforcement rate in 3-h sessions. CP94253 effects on responding during sucrose cue-reactivity were also examined post-abstinence.

Results: Regardless of sex, CP94253 enhanced breakpoints on the PR schedule during maintenance of cocaine SA but attenuated breakpoints for sucrose reinforcement and decreased responding during sucrose cue-reactivity. FR results showed that CP94253 attenuated cocaine reinforcement rate during all estrous cycle phases except metestrus.

Conclusions: Overall, we suggest that CP94253 increased incentive motivation for cocaine during maintenance of SA in female and male rats, yet decreased motivation for sucrose. We also suggest that 5-HT_1BRs modulate motivation similarly across sexes except when females are in metestrus.

Brain systems in cocaine abstinence-induced anxiety-like behavior in rodents: A review

Cocaine use disorder (CUD) is a significant public health issue that generates substantial personal, familial, and economic burdens. Still, there are no FDA-approved pharmacotherapies for CUD. Cocaine-dependent individuals report anxiety during withdrawal, and alleviation of anxiety and other negative affective states may be critical for maintaining drug abstinence. However, the neurobiological mechanisms underlying abstinence-related anxiety in humans or anxiety-like behavior in rodents are not fully understood. This review summarizes investigations regarding anxiety-like behavior in mice and rats undergoing cocaine abstinence, as assessed using four of the most common anxiety-related assays: the elevated plus (or its derivative, the elevated zero) maze, open field test, light-dark transition test, and defensive burying task. We first summarize available evidence that cocaine abstinence generates anxiety-like behavior that persists throughout protracted abstinence. Then, we examine investigations concerning neuropeptide, neurotransmitter, and neuromodulator systems in cocaine abstinence-induced anxiety-like behavior. Throughout, we discuss how differences in sex, rodent strain, cocaine dose and dosing strategy and abstinence duration interact to generate anxiety-like behavior.

Serotonin receptors contribute to dopamine depression of lateral inhibition in the nucleus accumbens

Dopamine modulation of nucleus accumbens (NAc) circuitry is central to theories of reward seeking and reinforcement learning. Despite decades of effort, the acute dopamine actions on the NAc microcircuitry remain puzzling. Here, we dissect out the direct actions of dopamine on lateral inhibition between medium spiny neurons (MSNs) in mouse brain slices and find that they are pathway specific. Dopamine potently depresses GABAergic transmission from presynaptic dopamine D2 receptor-expressing MSNs (D2-MSNs), whereas it potentiates transmission from presynaptic dopamine D1 receptor-expressing MSNs (D1-MSNs) onto other D1-MSNs. To our surprise, presynaptic D2 receptors mediate only half of the depression induced by endogenous and exogenous dopamine. Presynaptic serotonin 5-HT1B receptors are responsible for a significant component of dopamine-induced synaptic depression. This study clarifies the mechanistic understanding of dopamine actions in the NAc by showing pathway-specific modulation of lateral inhibition and involvement of D2 and 5-HT1B receptors in dopamine depression of D2-MSN synapses.

5-HT1B receptor agonist attenuates cocaine self-administration after protracted abstinence and relapse in rats

BACKGROUND

The 5-HT_1B receptor (5-HT_1BR) agonist, CP94253, enhances cocaine intake during maintenance of self-administration (SA) but attenuates intake after 21 days of forced abstinence in male rats.

AIMS

We examined whether CP94253 attenuates cocaine intake in female rats after a period of abstinence, and if these attenuating effects persist or revert to enhancing cocaine intake during resumption (i.e. relapse) of daily cocaine SA.

METHODS

Male and female rats trained to lever press on a fixed ratio 5 schedule of cocaine reinforcement underwent ⩾21 days of forced abstinence. They were then tested for the effects of CP94253 (5.6 mg/kg, SC) or vehicle on cocaine SA. During the test session, rats had 1-h access to the training dose of cocaine (0.75 mg/kg, IV) followed by 1-h access to a lower cocaine dose (0.075 mg/kg, IV). Rats then resumed cocaine SA for 15 days to mimic relapse and were retested as done previously. Subsequently, rats underwent abstinence again (21-60 days) and were tested for CP94253 effects on locomotion and cue reactivity (i.e. responding for light/tone cues previously paired with cocaine infusions).

RESULTS

Regardless of sex, CP94253 decreased cocaine intake after abstinence and during resumption of SA and decreased cue reactivity while having no effect on locomotion.

CONCLUSIONS

CP94253 decreases cocaine intake and cocaine seeking in both males and females even after resumption of cocaine SA. These findings suggest that the inhibitory effects of CP94253 observed after abstinence are long-lasting, and therefore, 5-HT_1BR agonists may have clinical efficacy as anti-relapse medications for cocaine use disorders.

A role for reward valuation in the serotonergic modulation of impulsivity

RATIONALE

Impulsive behavior is a deleterious component of a number of mental health disorders but has few targeted pharmacotherapies. One contributing factor to the difficulty in understanding the neural substrates of disordered impulsivity is the diverse presentations of impulsive behavior. Defining the behavioral and cognitive processes which contribute to different subtypes of impulsivity is important for understanding the neural underpinnings of dysregulated impulsive behavior.

METHODS

Using a mouse model for disordered impulsivity, our goal was to identify behavioral and cognitive processes that are associated with increased impulsivity. Specifically, we were interested in the facets of impulsivity modulated by serotonin signaling. We used mice lacking the serotonin 1B receptor (5-HT_1BR) and measured different types of impulsivity as well as goal-directed responding, extinction, habitual-like behavior, cue reactivity, and reward reactivity.

RESULTS

Mice lacking expression of 5-HT_1BR had increased levels of impulsive action, goal-directed responding, and motivation, with no differences seen in rate of extinction, development of habitual behavior, delay discounting, or effort-based discounting. Interestingly, mice lacking 5-HT_1BR expression also showed an overall increase in the choice of higher value rewards, increased hedonic responses to sweet rewards, and responded more for cues that predict reward. We developed a novel paradigm to demonstrate that increasing anticipated reward value could directly increase impulsive action. Furthermore, we found that 5-HT_1BR KO-induced impulsivity could be ameliorated by decreasing the reward value relative to controls, suggesting that the increased 5-HT_1BR-associated impulsive action may be a result of increased reward valuation.

CONCLUSIONS

Taken together, these data show that the effects of serotonin on impulsive action are mediated through the modulation of hedonic value, which may alter the reward representations that motivate action. Overall, this data supports a role for reward value as an important substrate in impulsive action which may drive clinically relevant increases in impulsivity.

Dual Effect of 5-HT1B/1D Receptors on Dopamine Neurons in Ventral Tegmental Area: Implication for the Functional Switch After Chronic Cocaine Exposure

BACKGROUND

Serotonin (5-HT) 1B/1D receptor (5-HT_1B/1DR) agonists undergo an abstinence-induced switch in their effects on cocaine-related behaviors, which may involve changes in modulation of dopamine (DA) neurons in the ventral tegmental area (VTA). However, it is unclear how 5-HT_1B/1DRs affect VTA DA neuronal function and whether modulation of these neurons mediates the abstinence-induced switch after chronic cocaine exposure.

METHODS

We examined the ability of 5-HT_1B/1DRs to modulate D₂ autoreceptors (D₂ARs) and synaptic transmission in the VTA by slice recording and single unit recording in vivo in naïve mice and in mice with chronic cocaine treatment.

RESULTS

We report a bidirectional modulation of VTA DA neuronal firing through the interaction of VTA 5-HT_1B/1DRs and D₂ARs. In both VTA slices and the VTA of anesthetized mice, the 5-HT_1B/1DR agonist CP94253 decreased DA neuronal firing rate and evoked excitatory postsynaptic currents to DA neurons in slice. Paradoxically, CP94253 decreased quinpirole-induced inhibition of DA neurons by reducing D₂AR-mediated G protein-gated inwardly rectifying potassium current. This manifested decreased GABA_A (gamma-aminobutyric acid A) receptor-mediated evoked inhibitory postsynaptic currents in slice, resulting in disinhibition of DA neurons, in opposition to the 5-HT_1B/1DR-induced inhibition. This dual effect was verified in chronic cocaine-treated and mild stress-treated, male mice on days 1 and 20 posttreatment.

CONCLUSIONS

This study revealed dual effects of CP94253 on VTA DA neurons that are dependent on D₂AR sensitivity, with anti-inhibition under normal D₂AR sensitivity and inhibition under low D₂AR sensitivity. These dual effects may underlie the ability of CP94253 to both enhance and inhibit cocaine-induced behaviors.

Preclinical support for the therapeutic potential of zolmitriptan as a treatment for cocaine use disorders

Serotonin 1B receptor (5-HT_1BR) agonists enhance cocaine intake in rats during daily self-administration but attenuate cocaine intake after prolonged abstinence. Here we investigated whether the less selective but clinically available 5-HT_1D/1BR agonist, zolmitriptan, produces similar effects. Male and free-cycling female Sprague-Dawley rats were trained to lever press for cocaine (0.75 mg/kg, i.v.) or sucrose (45 mg pellet) reinforcement until performance rates stabilized. Rats then received zolmitriptan (3.0, 5.6, and 10 mg/kg, s.c.) prior to testing for its effects on response and reinforcement rates. Under cocaine testing conditions, rats had access to the training dose for the first hour followed by a lower cocaine dose (0.075 mg/kg, i.v.) for the second hour. Zolmitriptan decreased cocaine intake at both cocaine doses and in both sexes even without a period of abstinence and without altering sucrose intake. A separate group of rats underwent identical training procedures and were tested for effects of the selective 5-HT_1B and 5-HT_1D receptor antagonists, SB224289 (3.2, 5.6, and 10 mg/kg, s.c.) and BRL15572 (0.3, 1.0, and 3.0 mg/kg, i.p.), respectively, alone or in combination with zolmitriptan (5.6 mg/kg, s.c.) under identical cocaine testing procedures as above. The zolmitriptan-induced decrease in cocaine intake was reversed by SB224289 and to a lesser extent by BRL15572, suggesting that the effects of zolmitriptan involve both 5-HT_1B and 5-HT_1D receptors. Neither zolmitriptan, SB224289, or BRL15572 altered locomotor activity at the doses effective for modulating cocaine intake. These findings suggest that zolmitriptan has potential for repurposing as a treatment for cocaine use disorders.

Selective Regulation of 5-HT1B Serotonin Receptor Expression in the Striatum by Dopamine Depletion and Repeated L-DOPA Treatment: Relationship to L-DOPA-Induced Dyskinesias

Dopamine and serotonin in the basal ganglia interact in a bidirectional manner. On the one hand, serotonin (5-HT) receptors regulate the effects of dopamine agonists on several levels, ranging from molecular signaling to behavior. These interactions include 5-HT receptor-mediated facilitation of dopamine receptor-induced gene regulation in striatal output pathways, which involves the 5-HT1B receptor and others. Conversely, there is evidence that dopamine action by psychostimulants regulates 5-HT1B receptor expression in the striatum. To further investigate the effects of dopamine and agonists on 5-HT receptors, we assessed the expression of 5-HT1B and other serotonin receptor subtypes in the striatum after unilateral dopamine depletion by 6-OHDA and subsequent treatment with L-DOPA (5 mg/kg; 4 weeks). Neither dopamine depletion nor L-DOPA treatment produced significant changes in 5-HT2C, 5-HT4, or 5-HT6 receptor expression in the striatum. In contrast, the 6-OHDA lesion caused a (modest) increase in 5-HT1B mRNA levels throughout the striatum. Moreover, repeated L-DOPA treatment markedly further elevated 5-HT1B expression in the dopamine-depleted striatum, an effect that was most robust in the sensorimotor striatum. A minor L-DOPA-induced increase in 5-HT1B expression was also seen in the intact striatum. These changes in 5-HT1B expression mimicked changes in the expression of neuropeptide markers (dynorphin, enkephalin mRNA) in striatal projection neurons. After repeated L-DOPA treatment, the severity of L-DOPA-induced dyskinesias and turning behavior was positively correlated with the increase in 5-HT1B expression in the associative, but not sensorimotor, striatum ipsilateral to the lesion, suggesting that associative striatal 5-HT1B receptors may play a role in L-DOPA-induced behavioral abnormalities.

Neural Circuits Associated with 5-HT1B Receptor Agonist Inhibition of Methamphetamine Seeking in the Conditioned Place Preference Model

5-HT_1B receptors (5-HT_1BRs) modulate psychostimulant reward and incentive motivation in rodents. Here we investigated the effects of the 5-HT_1BR agonist CP94253 (10 mg/kg, IP) on the acquisition and expression of methamphetamine (Meth) conditioned place preference (CPP) in C57BL/6 male mice. We subsequently examined the potential brain regions involved in CP94253 effects using FOS as a marker of neural activity. In the acquisition experiment, mice received the agonist 30 min before each of the Meth injections given during conditioning. In the expression experiment, mice that had acquired Meth-CPP were given either saline or CP94253 and were tested for CPP 30 min later. We found that CP94253 attenuated the expression of Meth-CPP, but had no effect on acquisition. Mice expressing Meth-CPP had elevated numbers of FOS⁺ cells in the ventral tegmental area (VTA) and basolateral amygdala (BlA) and reduced FOS⁺ cells in the central amygdala (CeA) compared to saline controls. CP94253 given before the expression test, but not acutely in drug-naive mice, enhanced FOS⁺ cells in the VTA, the nucleus accumbens (NAc) shell and core, and the dorsomedial striatum and reversed the Meth-conditioned changes in FOS in the BlA and CeA. Approximately 50-70% of FOS⁺ cells in the NAc and VTA were GABAergic regardless of group. By contrast, we did not observe FOS-labeling in dopamine neurons in the VTA. The findings suggest that CP94253 attenuates the motivational effects of the Meth-associated environment and highlight the amygdala, VTA, NAc, and dorsomedial striatum as potential regions involved in this effect.

Depression-like symptoms of withdrawal in a genetic mouse model of binge methamphetamine intake

Binge methamphetamine (MA) users have higher MA consumption, relapse rates and depression-like symptoms during early periods of withdrawal, compared with non-binge users. The impact of varying durations of MA abstinence on depression-like symptoms and on subsequent MA intake was examined in mice genetically prone to binge-level MA consumption. Binge-level MA intake was induced using a multiple-bottle choice procedure in which mice were offered one water drinking tube and three tubes containing increasing concentrations of MA in water, or four water tubes (control group). In two studies, depression-like symptoms were measured using a tail-suspension test and a subsequent forced-swim test, after forced abstinence of 6 and 30 hours from a 28-day course of chronic MA intake. An additional study measured the same depression-like symptoms, as well as MA intake, after prolonged abstinence of 1 and 2 weeks. MA high drinking mice and one of their progenitor strains DBA/2J escalated their MA intake with increasing MA concentration; however, MA high drinking mice consumed almost twice as much MA as DBA/2J mice. Depression-like symptoms were significantly higher early after MA access was withdrawn, compared to levels in drug-naïve controls, with more robust effects of MA withdrawal observed in MA high drinking than DBA/2J mice. When depression-like symptoms were examined after 1 or 2 weeks of forced abstinence in MA high drinking mice, depression-like symptoms dissipated, and subsequent MA intake was high. The MA high drinking genetic mouse model has strong face validity for human binge MA use and behavioral sequelae associated with abstinence.

Amphetamine Self-Administration and Its Extinction Alter the 5-HT1B Receptor Protein Levels in Designated Structures of the Rat Brain

Manipulation of the serotonin (5-HT)_1B receptors can modify the behavioral effects of amphetamine including its reinforcing properties. Focus of this study was to examine changes in 5-HT_1B receptor protein expression in several brain structures linked to substance drug disorder in different stages of amphetamine addiction—single session of amphetamine self-administration, 20 consecutive days of amphetamine self-administration, and 3 and 14 days of extinction from chronic drug intake. “Yoked” procedure was employed to set apart pharmacological and motivational effects of amphetamine intoxication. Immunohistofluorescence was performed on brain slices containing the following regions: nucleus accumbens (NAc) shell and core, globus pallidum (GP) lateral and ventral, hippocampus (HIP), substantia nigra (SN), and ventral tegmental area (VTA). Single amphetamine session decreased the amount of 5-HT_1B receptors in SN, VTA, and HIP in active and yoked rats. On the contrary, 20 days of chronic amphetamine exposure triggered elevation of 5-HT_1B receptors exclusively in animals that voluntarily administered the drug in NAc core, GP ventral, and HIP. Furthermore, 14-day (but not 3-day) extinction from amphetamine increased the 5-HT_1B receptor expression in ventral and lateral GP, HIP, and SN. This study is the first to demonstrate that exposure to amphetamine and its extinction alter the expression of 5-HT_1B receptors in various rat brain regions, and those changes seem to be transient and region specific. Importantly, since increased expression of 5-HT_1B receptor after chronic amphetamine self-administration was limited only to active group of animals, we suggest that 5-HT_1B receptor is linked to motivational aspect of addiction.

Restoration of Kv7 Channel-Mediated Inhibition Reduces Cued-Reinstatement of Cocaine Seeking

Cocaine addicts display increased sensitivity to drug-associated cues, due in part to changes in the prelimbic prefrontal cortex (PL-PFC). The cellular mechanisms underlying cue-induced reinstatement of cocaine seeking remain unknown. Reinforcement learning for addictive drugs may produce persistent maladaptations in intrinsic excitability within sparse subsets of PFC pyramidal neurons. Using a model of relapse in male rats, we sampled >600 neurons to examine spike frequency adaptation (SFA) and afterhyperpolarizations (AHPs), two systems that attenuate low-frequency inputs to regulate neuronal synchronization. We observed that training to self-administer cocaine or nondrug (sucrose) reinforcers decreased SFA and AHPs in a subpopulation of PL-PFC neurons. Only with cocaine did the resulting hyperexcitability persist through extinction training and increase during reinstatement. In neurons with intact SFA, dopamine enhanced excitability by inhibiting Kv7 potassium channels that mediate SFA. However, dopamine effects were occluded in neurons from cocaine-experienced rats, where SFA and AHPs were reduced. Pharmacological stabilization of Kv7 channels with retigabine restored SFA and Kv7 channel function in neuroadapted cells. When microinjected bilaterally into the PL-PFC 10 min before reinstatement testing, retigabine reduced cue-induced reinstatement of cocaine seeking. Last, using cFos-GFP transgenic rats, we found that the loss of SFA correlated with the expression of cFos-GFP following both extinction and re-exposure to drug-associated cues. Together, these data suggest that cocaine self-administration desensitizes inhibitory Kv7 channels in a subpopulation of PL-PFC neurons. This subpopulation of neurons may represent a persistent neural ensemble responsible for driving drug seeking in response to cues.SIGNIFICANCE STATEMENT Long after the cessation of drug use, cues associated with cocaine still elicit drug-seeking behavior, in part by activation of the prelimbic prefrontal cortex (PL-PFC). The underlying cellular mechanisms governing these activated neurons remain unclear. Using a rat model of relapse to cocaine seeking, we identified a population of PL-PFC neurons that become hyperexcitable following chronic cocaine self-administration. These neurons show persistent loss of spike frequency adaptation, reduced afterhyperpolarizations, decreased sensitivity to dopamine, and reduced Kv7 channel-mediated inhibition. Stabilization of Kv7 channel function with retigabine normalized neuronal excitability, restored Kv7 channel currents, and reduced drug-seeking behavior when administered into the PL-PFC before reinstatement. These data highlight a persistent adaptation in a subset of PL-PFC neurons that may contribute to relapse vulnerability.

The 5-HT1B receptor - a potential target for antidepressant treatment

Major depressive disorder (MDD) is the leading cause of disability worldwide. The serotonin hypothesis may be the model of MDD pathophysiology with the most support. The majority of antidepressants enhance synaptic serotonin levels quickly, while it usually takes weeks to discern MDD treatment effect. It has been hypothesized that the time lag between serotonin increase and reduction of MDD symptoms is due to downregulation of inhibitory receptors such as the serotonin 1B receptor (5-HT1BR). The research on 5-HT1BR has previously been hampered by a lack of selective ligands for the receptor. The last extensive review of 5-HT1BR in the pathophysiology of depression was published 2009, and based mainly on findings from animal studies. Since then, selective radioligands for in vivo quantification of brain 5-HT1BR binding with positron emission tomography has been developed, providing new knowledge on the role of 5-HT1BR in MDD and its treatment. The main focus of this review is the role of 5-HT1BR in relation to MDD and its treatment, although studies of 5-HT1BR in obsessive-compulsive disorder, alcohol dependence, and cocaine dependence are also reviewed. The evidence outlined range from animal models of disease, effects of 5-HT1B receptor agonists and antagonists, case-control studies of 5-HT1B receptor binding postmortem and in vivo, with positron emission tomography, to clinical studies of 5-HT1B receptor effects of established treatments for MDD. Low 5-HT1BR binding in limbic regions has been found in MDD patients. When 5-HT1BR ligands are administered to animals, 5-HT1BR agonists most consistently display antidepressant-like properties, though it is not yet clear how 5-HT1BR is best approached for optimal MDD treatment.

Effects of a 5-HT1B Receptor Agonist on Locomotion and Reinstatement of Cocaine-Conditioned Place Preference after Abstinence from Repeated Injections in Mice

5-HT_1B receptors (5-HT_1BRs) modulate behavioral effects of cocaine. Here we examined the effects of the 5-HT_1BR agonist 5-propoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-pyrrolo[3,2-b]pyridine (CP94253) on spontaneous and cocaine-induced locomotion and on cocaine-primed reinstatement of conditioned place preference (CPP) in male mice given daily repeated injections of either saline or cocaine (15 mg/kg, IP) for 20 days. In the locomotor activity experiment, testing occurred both 1 and 20 days after the final injection. In the CPP experiment, mice underwent conditioning procedures while receiving the last of their daily injections, which were given either during or ≥2 h after CPP procedures. The CPP procedural timeline consisted of baseline preference testing (days 12–13 of the chronic regimen), conditioning (days 14–19, 2 daily 30-min sessions separated by 5 h), CPP test (day 21), extinction (days 22–34; no injections), CPP extinction test (day 35), and reinstatement test (day 36). Mice that had not extinguished received additional extinction sessions prior to reinstatement testing on day 42. On test days, mice were pretreated with either saline or CP94253 (10 mg/kg, IP). Testing began 30 min later, immediately after mice were primed with either saline or cocaine (5 mg/kg for locomotion; 15 mg/kg for reinstatement). We found that CP94253 increased spontaneous locomotion in mice receiving repeated injections of either saline or cocaine when tested 1 day after the last injection, but had no effect on spontaneous locomotion after 20 days abstinence from repeated injections. Surprisingly, cocaine-induced locomotion was sensitized regardless of whether the mice had received repeated saline or cocaine. CP94253 attenuated expression of the sensitized locomotion after 20 days abstinence. A control experiment in noninjected, drug-naïve mice showed that CP94253 had no effect on spontaneous or cocaine-induced locomotion. Mice reinstated cocaine-CPP when given a cocaine prime, and CP94253 pretreatment attenuated cocaine reinstatement.The findings suggest that stress from repeated saline injections and/or co-housing with cocaine-injected mice may cross-sensitize with cocaine effects on locomotion and that CP94253 attenuates these effects, as well as reinstatement of cocaine-CPP. This study supports the idea that 5-HT_1BR agonists may be useful anti-cocaine medications.

The 5-HT1B serotonin receptor regulates methylphenidate-induced gene expression in the striatum: Differential effects on immediate-early genes

Drug combinations that include a psychostimulant such as methylphenidate (Ritalin) and a selective serotonin reuptake inhibitor such as fluoxetine are indicated in several medical conditions. Co-exposure to these drugs also occurs with "cognitive enhancer" use by individuals treated with selective serotonin reuptake inhibitors. Methylphenidate, a dopamine reuptake inhibitor, by itself produces some addiction-related gene regulation in the striatum. We have demonstrated that co-administration of selective serotonin reuptake inhibitors potentiates these methylphenidate-induced molecular effects, thus producing a more "cocaine-like" profile. There is evidence that the 5-HT1B serotonin receptor subtype mediates some of the cocaine-induced gene regulation. We thus investigated whether the 5-HT1B receptor also modifies methylphenidate-induced gene regulation, by assessing effects of a selective 5-HT1B receptor agonist (CP94253) on immediate-early gene markers ( Zif268, c- Fos, Homer1a) in adolescent male rats. Gene expression was measured by in situ hybridization histochemistry. Our results show that CP94253 (3, 10 mg/kg) produced a dose-dependent potentiation of methylphenidate (5 mg/kg)-induced expression of Zif268 and c- Fos. This potentiation was widespread in the striatum and was maximal in lateral (sensorimotor) sectors, thus mimicking the effects seen after cocaine alone, or co-administration of fluoxetine. However, in contrast to fluoxetine, this 5-HT1B agonist did not influence methylphenidate-induced expression of Homer1a. CP94253 also potentiated methylphenidate-induced locomotor activity. These findings indicate that stimulation of the 5-HT1B receptor can enhance methylphenidate (dopamine)-induced gene regulation. This receptor may thus participate in the potentiation induced by fluoxetine (serotonin) and may serve as a pharmacological target to attenuate methylphenidate + selective serotonin reuptake inhibitor-induced "cocaine-like" effects.

Preclinical Evidence That 5-HT1B Receptor Agonists Show Promise as Medications for Psychostimulant Use Disorders

BACKGROUND

5-HT1B receptor agonists enhance cocaine intake during daily self-administration sessions but decrease cocaine intake when tested after prolonged abstinence. We examined if 5-HT1B receptor agonists produce similar abstinence-dependent effects on methamphetamine intake.

METHODS

Male rats were trained to self-administer methamphetamine (0.1 mg/kg, i.v.) on low (fixed ratio 5 and variable ratio 5) and high (progressive ratio) effort schedules of reinforcement until intake was stable. Rats were then tested for the effects of the selective 5-HT1B receptor agonist, CP 94,253 (5.6 or 10 mg/kg), or the less selective but clinically available 5-HT1B/1D receptor agonist, zolmitriptan (10 mg/kg), on methamphetamine self-administration both before and after a 21-day forced abstinence period during which the rats remained in their home cages.

RESULTS

The inverted U-shaped, methamphetamine dose-response function for intake on the fixed ratio 5 schedule was shifted downward by CP 94,253 both before and after abstinence. The CP 94,253-induced decrease in methamphetamine intake was replicated in rats tested on a variable ratio 5 schedule, and the 5-HT1B receptor antagonist SB 224,289 (10 mg/kg) reversed this effect. CP 94,253 also attenuated methamphetamine intake on a progressive ratio schedule both pre- and postabstinence. Similarly, zolmitriptan attenuated methamphetamine intake on a variable ratio 5 schedule both pre- and postabstinence, and the latter effect was sustained after each of 2 more treatments given every 2 to 3 days prior to daily sessions.

CONCLUSIONS

Unlike the abstinence-dependent effect of 5-HT1B receptor agonists on cocaine intake reported previously, both CP 94,253 and zolmitriptan decreased methamphetamine intake regardless of abstinence. These findings suggest that 5-HT1B receptor agonists may have clinical efficacy for psychostimulant use disorders.

Serotonin receptors in depression: from A to B

The role of serotonin in major depressive disorder (MDD) is the focus of accumulating clinical and preclinical research. The results of these studies reflect the complexity of serotonin signaling through many receptors, in a large number of brain regions, and throughout the lifespan. The role of the serotonin transporter in MDD has been highlighted in gene by environment association studies as well as its role as a critical player in the mechanism of the most effective antidepressant treatments – selective serotonin reuptake inhibitors. While the majority of the 15 known receptors for serotonin have been implicated in depression or depressive-like behavior, the serotonin 1A (5-HT _1A) and 1B (5-HT _1B) receptors are among the most studied. Human brain imaging and genetic studies point to the involvement of 5-HT _1A and 5-HT _1B receptors in MDD and the response to antidepressant treatment. In rodents, the availability of tissue-specific and inducible knockout mouse lines has made possible the identification of the involvement of 5-HT _1A and 5-HT _1B receptors throughout development and in a cell-type specific manner. This, and other preclinical pharmacology work, shows that autoreceptor and heteroreceptor populations of these receptors have divergent roles in modulating depression-related behavior as well as responses to antidepressants and also have different functions during early postnatal development compared to during adulthood.

Regulation of nucleus accumbens transcript levels in mice by early-life social stress and cocaine

Much interest has been piqued regarding the quality of one's environment at early ages in modulating the susceptibility to drug addiction in adulthood. However, the molecular mechanisms that are engaged during early trauma and mediate the risk for drug addiction are poorly understood. In rodents, exposure to early-life stress alters the rewarding effects of cocaine, amphetamine, and morphine in adulthood. Recently, we demonstrated that the exposure of juvenile mice to social threat (Social Stress, S-S) promoted cocaine-seeking behavior and relapse of cocaine-seeking after periods of withdrawal, compared with unhandled controls (UN) and with juvenile mice that experienced only daily isolation in a novel environment (no social stress, NS-S). Interestingly, while the exposure to NS-S slightly increased cocaine-seeking behavior compared with UN, the same was not sufficient to promote cocaine reinstatement. In this study, we examined the long-term transcriptional changes that are induced by S-S compared to NS-S and linked the increased susceptibility of S-S mice to cocaine reinstatement. To this end, we performed genome-wide RNA sequencing analysis in the nucleus accumbens (NAC), which revealed that 89 transcripts were differentially expressed between S-S and NS-S mice. By Gene Ontology classification, these hits were enriched in genes that mediate cell proliferation, neuronal differentiation, and neuron/forebrain development. Eleven of these genes have been reported to be involved in substance use disorders, and the remaining genes are novel candidates in this area. We characterized 4 candidates with regard to their significant neurobiological relevance (ZIC1, ZIC2, FABP7, and PRDM12) and measured their expression in the NAC by immunohistochemistry. These findings provide insights into novel molecular mechanisms in NAC that might be associated with the risk of relapse in cocaine-dependent individuals.

Attenuated methamphetamine-induced locomotor sensitization in serotonin transporter knockout mice is restored by serotonin 1B receptor antagonist treatment

Repeated administration of methamphetamine (METH) enhances acute locomotor responses to METH administered in the same context, a phenomenon termed as 'locomotor sensitization'. Although many of the acute effects of METH are mediated by its influences on the compartmentalization of dopamine, serotonin systems have also been suggested to influence the behavioral effects of METH in ways that are not fully understood. The present experiments examined serotonergic roles in METH-induced locomotor sensitization by assessing: (a) the effect of serotonin transporter (SERT; Slc6A4) knockout (KO) on METH-induced locomotor sensitization; (b) extracellular monoamine levels in METH-treated animals as determined by in-vivo microdialysis; and (c) effects of serotonin (5-HT) receptor antagonists on METH-induced behavioral sensitization, with focus on effects of the 5-HT1B receptor antagonist SB 216641 and a comparison with the 5-HT2 receptor antagonist ketanserin. Repeated METH administration failed to induce behavioral sensitization in homozygous SERT KO (SERT-/-) mice under conditions that produced substantial sensitization in wild-type or heterozygous SERT KO (SERT+/-) mice. The selective 5-HT1B antagonist receptor SB 216641 restored METH-induced locomotor sensitization in SERT-/- mice, whereas ketanserin was ineffective. METH-induced increases in extracellular 5-HT (5-HTex) levels were substantially reduced in SERT-/- mice, although SERT genotype had no effect on METH-induced increases in extracellular dopamine. These experiments demonstrate that 5-HT actions, including those at 5-HT1B receptors, contribute to METH-induced locomotor sensitization. Modulation of 5-HT1B receptors might aid therapeutic approaches to the sequelae of chronic METH use.

Fluoxetine potentiation of methylphenidate-induced gene regulation in striatal output pathways: potential role for 5-HT1B receptor

Drug combinations that include the psychostimulant methylphenidate plus a selective serotonin reuptake inhibitor (SSRI) such as fluoxetine are increasingly used in children and adolescents. For example, this combination is indicated in the treatment of attention-deficit/hyperactivity disorder and depression comorbidity and other mental disorders. Such co-exposure also occurs in patients on SSRIs who use methylphenidate as a cognitive enhancer. The neurobiological consequences of these drug combinations are poorly understood. Methylphenidate alone can produce gene regulation effects that mimic addiction-related gene regulation by cocaine, consistent with its moderate addiction liability. We have previously shown that combining SSRIs with methylphenidate potentiates methylphenidate-induced gene regulation in the striatum. The present study investigated which striatal output pathways are affected by the methylphenidate + fluoxetine combination, by assessing effects on pathway-specific neuropeptide markers, and which serotonin receptor subtypes may mediate these effects. Our results demonstrate that a 5-day repeated treatment with fluoxetine (5 mg/kg) potentiates methylphenidate (5 mg/kg)-induced expression of both dynorphin (direct pathway marker) and enkephalin (indirect pathway). These changes were accompanied by correlated increases in the expression of the 5-HT1B, but not 5-HT2C, serotonin receptor in the same striatal regions. A further study showed that the 5-HT1B receptor agonist CP94253 (3-10 mg/kg) mimics the fluoxetine potentiation of methylphenidate-induced gene regulation. These findings suggest a role for the 5-HT1B receptor in the fluoxetine effects on striatal gene regulation. Given that 5-HT1B receptors are known to facilitate addiction-related gene regulation and behavior, our results suggest that SSRIs may enhance the addiction liability of methylphenidate by increasing 5-HT1B receptor signaling.

Serotonin/dopamine interactions in a hyperactive mouse: reduced serotonin receptor 1B activity reverses effects of dopamine transporter knockout

Knockout (KO) mice that lack the dopamine transporter (SL6A3; DAT) display increased locomotion that can be attenuated, under some circumstances, by administration of drugs that normally produce psychostimulant-like effects, such as amphetamine and methylphenidate. These results have led to suggestions that DAT KO mice may model features of attention deficit hyperactivity disorder (ADHD) and that these drugs may act upon serotonin (5-HT) systems to produce these unusual locomotor decreasing effects. Evidence from patterns of brain expression and initial pharmacologic studies led us to use genetic and pharmacologic approaches to examine the influence of altered 5-HT_1B receptor activity on hyperactivity in DAT KO mice. Heterozygous 5-HT_1B KO and pharmacologic 5-HT_1B antagonism both attenuated locomotor hyperactivity in DAT KO mice. Furthermore, DAT KO mice with reduced, but not eliminated, 5-HT_1B receptor expression regained cocaine-stimulated locomotion, which was absent in DAT KO mice with normal levels of 5-HT_1B receptor expression. Further experiments demonstrated that the degree of habituation to the testing apparatus determined whether cocaine had no effect on locomotion in DAT KO or reduced locomotion, helping to resolve differences among prior reports. These findings of complementation of the locomotor effects of DAT KO by reducing 5-HT_1B receptor activity underscore roles for interactions between specific 5-HT receptors and dopamine (DA) systems in basal and cocaine-stimulated locomotion and support evaluation of 5-HT_1B antagonists as potential, non-stimulant ADHD therapeutics.

Reductions in brain 5-HT1B receptor availability in primarily cocaine-dependent humans

BACKGROUND

Preclinical evidence implicates the serotonin receptor 5-hydroxytryptamine 1B (5-HT1B) in the effects of cocaine. This study explores 5-HT1B in humans by examining receptor availability in vivo in subjects whose primary addiction is cocaine dependence (CD) using positron emission tomography.

METHODS

Study participants included 14 medically healthy subjects with CD (mean age = 41 ± 6 years) who were compared with 14 age-matched healthy control subjects (mean age = 41 ± 8 years) with no past or current history of cocaine or other illicit substance abuse. Participants underwent magnetic resonance imaging followed by positron emission tomography with the highly selective 5-HT1B tracer, [(11)C]P943, for purposes of quantifying regional binding potential. Voxel-based morphometry and gray matter masking also were employed to control for potential partial volume effects.

RESULTS

The [(11)C]P943 positron emission tomography imaging data in nine candidate regions (amygdala, anterior cingulate cortex, caudate, frontal cortex, hypothalamus, pallidum, putamen, thalamus, and ventral striatum) showed significant or nearly significant reductions of regional binding potential in subjects with CD in three regions: anterior cingulate (-16%, p < .01), hypothalamus (-16%, p = .03), and frontal cortex (-7%, p = .08). Voxel-based morphometry showed significant gray matter reductions in the frontal cortex of subjects with CD. After gray matter masking, statistically significant reductions in the [(11)C]P943 regional binding potential were either retained (anterior cingulate, -14%, p = .01; hypothalamus, -20%, p < .01) or achieved (frontal cortex, -14%, p < .01). Whole-brain voxel-wise parameter estimation confirmed these results. Secondary analyses were also significant in some regions for years of cocaine and daily tobacco use.

CONCLUSIONS

The reductions found in this study suggest that 5-HT1B receptors may contribute to the etiology or expression of CD and potentially represent a target for medication development.

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.

Enhanced self-administration of the CB1 receptor agonist WIN55,212-2 in olfactory bulbectomized rats: evaluation of possible serotonergic and dopaminergic underlying mechanisms

Depression has been associated with drug consumption, including heavy or problematic cannabis use. According to an animal model of depression and substance use disorder comorbidity, we combined the olfactory bulbectomy (OBX) model of depression with intravenous drug self-administration procedure to verify whether depressive-like rats displayed altered voluntary intake of the CB1 receptor agonist WIN55,212-2 (WIN, 12.5 μg/kg/infusion). To this aim, olfactory-bulbectomized (OBX) and sham-operated (SHAM) Lister Hooded rats were allowed to self-administer WIN by lever-pressing under a continuous [fixed ratio 1 (FR-1)] schedule of reinforcement in 2 h daily sessions. Data showed that both OBX and SHAM rats developed stable WIN intake; yet, responses in OBX were constantly higher than in SHAM rats soon after the first week of training. In addition, OBX rats took significantly longer to extinguish the drug-seeking behavior after vehicle substitution. Acute pre-treatment with serotonin 5HT1B receptor agonist, CGS-12066B (2.5-10 mg/kg), did not significantly modify WIN intake in OBX and SHAM Lister Hooded rats. Furthermore, acute pre-treatment with CGS-12066B (10 and 15 mg/kg) did not alter responses in parallel groups of OBX and SHAM Sprague Dawley rats self-administering methamphetamine under higher (FR-2) reinforcement schedule with nose-poking as operandum. Finally, dopamine levels in the nucleus accumbens (NAc) of OBX rats did not increase in response to a WIN challenge, as in SHAM rats, indicating a dopaminergic dysfunction in bulbectomized rats. Altogether, our findings suggest that a depressive-like state may alter cannabinoid CB1 receptor agonist-induced brain reward function and that a dopaminergic rather than a 5-HT1B mechanism is likely to underlie enhanced WIN self-administration in OBX rats.

Pharmacological evidence for an abstinence-induced switch in 5-HT1B receptor modulation of cocaine self-administration and cocaine-seeking behavior

Studies examining serotonin-1B (5-HT_1B) receptor manipulations on cocaine self-administration and cocaine-seeking behavior initially seemed discrepant. However, we recently suggested based on viral-mediated 5-HT_1B-receptor gene transfer that the discrepancies are likely due to differences in the length of abstinence from cocaine prior to testing. To further validate our findings pharmacologically, we examined the effects of the selective 5-HT_1B receptor agonist CP 94,253 (5.6 mg/kg, s.c.) on cocaine self-administration during maintenance and after a period of protracted abstinence with or without daily extinction training. We also examined agonist effects on cocaine-seeking behavior at different time points during abstinence. During maintenance, CP 94,253 shifted the cocaine self-administration dose–effect function on an FR5 schedule of reinforcement to the left, whereas following 21 days of abstinence CP 94,253 downshifted the function and also decreased responding on a progressive ratio schedule of reinforcement regardless of extinction history. CP 94,253 also attenuated cue-elicited and cocaine-primed drug-seeking behavior following 5 days, but not 1 day, of forced abstinence. The attenuating effects of CP 94,253 on the descending limb of the cocaine dose–effect function were blocked by the selective 5-HT_1B receptor antagonist SB 224289 (5 mg/kg, i.p.) at both time points, indicating 5-HT_1B receptor mediation. The results support a switch in 5-HT_1B receptor modulation of cocaine reinforcement from facilitatory during self-administration maintenance to inhibitory during protracted abstinence. These findings suggest that the 5-HT_1B receptor may be a novel target for developing medication for treating cocaine dependence.

Serotonin at the nexus of impulsivity and cue reactivity in cocaine addiction

Cocaine abuse and addiction remain great challenges on the public health agendas in the U.S. and the world. Increasingly sophisticated perspectives on addiction to cocaine and other drugs of abuse have evolved with concerted research efforts over the last 30 years. Relapse remains a particularly powerful clinical problem as, even upon termination of drug use and initiation of abstinence, the recidivism rates can be very high. The cycling course of cocaine intake, abstinence and relapse is tied to a multitude of behavioral and cognitive processes including impulsivity (a predisposition toward rapid unplanned reactions to stimuli without regard to the negative consequences), and cocaine cue reactivity (responsivity to cocaine-associated stimuli) cited as two key phenotypes that contribute to relapse vulnerability even years into recovery. Preclinical studies suggest that serotonin (5-hydroxytryptamine; 5-HT) neurotransmission in key neural circuits may contribute to these interlocked phenotypes well as the altered neurobiological states evoked by cocaine that precipitate relapse events. As such, 5-HT is an important target in the quest to understand the neurobiology of relapse-predictive phenotypes, to successfully treat this complex disorder and improve diagnostic and prognostic capabilities. This review emphasizes the role of 5-HT and its receptor proteins in key addiction phenotypes and the implications of current findings to the future of therapeutics in addiction. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Differential effect of viral overexpression of nucleus accumbens shell 5-HT1B receptors on stress- and cocaine priming-induced reinstatement of cocaine seeking

5-HT1B receptors are densely expressed on terminals of medium spiny neurons projecting from the nucleus accumbens shell (NAccSh) to the ventral tegmental area, where 5-HT1B receptors modulate GABA release directly, and firing of dopaminergic neurons indirectly. While interactions between NAccSh 5-HT1B receptors and stress have been reported in early stages of psychostimulant-induced neuroadaptations, specifically psychomotor sensitization, the effect of this interaction on later stages of drug seeking is currently unknown. Here, we examined the effect of herpes simplex virus (HSV)-mediated overexpression of NAccSh 5-HT1B receptors on reinstatement of cocaine seeking induced by exposure to stress or a cocaine prime. Rats were trained to self-administer cocaine (0.75 mg/kg/infusion) and the operant response was extinguished. Rats were then injected with viral vector expressing 5-HT1B and green fluorescent protein (GFP) or GFP alone into the NAccSh. The effect of 5-HT1B receptor overexpression was assessed on reinstatement induced by intermittent footshock (0.5 mA for 15 min) or a cocaine prime (10mg/kg, ip). Results indicate that NAccSh 5-HT1B receptor overexpression had no effect on footshock reinstatement while significantly decreasing cocaine priming-induced reinstatement. We also found that NAccSh overexpression of 5-HT1B receptors had no effect on saccharin intake following social defeat stress. These results suggest that the efficacy of pharmacological agents targeting 5-HT1B receptors for the treatment of cocaine relapse will depend largely on the nature of the reinstating stimulus. Taken together with previous results, it appears that NAccSh 5-HT1B receptors influence stress responses in early, but not in the later stages of psychostimulant-induced neuroadaptations.

Dopamine D3 and 5-HT1B receptor dysregulation as a result of psychostimulant intake and forced abstinence: Implications for medications development

Addiction to psychostimulants, including cocaine and amphetamine, is associated with dysregulation of dopamine and serotonin (5-HT) neurotransmitter systems. Neuroadaptations in these systems vary depending on the stage of the drug taking-abstinence-relapse cycle. Consequently, the effects of potential treatments that target these systems may vary depending on whether they are given during abstinence or relapse. In this review, we discuss evidence that dopamine D3 receptors (D3Rs) and 5-HT1B receptors (5-HT1BRs) are dysregulated in response to both chronic psychostimulant use and subsequent abstinence. We then review findings from preclinical self-administration models which support targeting D3Rs and 5-HT1BRs as potential medications for psychostimulant dependence. Potential side effects of the treatments are discussed and attention is given to studies reporting positive treatment outcomes that depend on: 1) whether testing occurs during self-administration versus abstinence, 2) whether escalation of drug self-administration has occurred, 3) whether the treatments are given repeatedly, and 4) whether social factors influence treatment outcomes. We conclude that D3/D2 agonists may decrease psychostimulant intake; however, side effects of D3/D2R full agonists may limit their therapeutic potential, whereas D3/D2R partial agonists have fewer undesirable side effects. D3-selective antagonists may not reduce psychostimulant intake during relapse, but nonetheless, may decrease motivation for seeking psychostimulants with relatively few side-effects. 5-HT1BR agonists provide a striking example of treatment outcomes that are dependent on the stage of the addiction cycle. Specifically, these agonists initially increase cocaine's reinforcing effects during maintenance of self-administration, but after a period of abstinence they reduce psychostimulant seeking and the resumption of self-administration. In conclusion, we suggest that factors contributing to dysregulation of monoamine systems, including drug history, abstinence, and social context, should be considered when evaluating potential treatments to better model treatment effects in humans. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Associations of the 5-hydroxytryptamine (serotonin) receptor 1B gene (HTR1B) with alcohol, cocaine, and heroin abuse

Abnormal serotonergic pathways are implicated in numerous neuropsychiatric disorders including alcohol and drug dependence (abuse). The human 5-hydroxytryptamine (serotonin) receptor 1B, encoded by the HTR1B (5-HT1B) gene, is a presynaptic serotonin autoreceptor that plays an important role in regulating serotonin synthesis and release. Although there was evidence of associations of the HTR1B gene variants in the etiologies of substance use disorders, negative findings were also reported. To clarify the roles of commonly reported single nucleotide polymorphisms (SNPs) of the HTR1B gene underlying alcohol and drug dependence (abuse), we performed a meta-analysis based on the available genotype data from individual candidate gene-based association studies. Evidence of association was found between the functional SNP -161A>T (rs130058) and alcohol, cocaine, and heroin dependence (e.g., P = 0.03 and odds ratio (OR) = 1.2 (1.02, 1.42) in the combined European, Asian, African, and Hispanic populations). SNP -261T>G (rs11568817) also showed evidence of association but with different directions in Europeans and non-Europeans (e.g., P = 0.0018 with OR = 1.42 (1.14, 1.76) and P = 0.01 with ORs = 0.5 (0.3, 0.85), respectively). This meta-analysis supports the associations of HTR1B -261T>G and -161A>T with alcohol and drug abuse and further investigations are warranted in larger samples.