Established sensitization of ethanol-induced locomotor activity is not reversed by psilocybin or the 5-HT2A receptor agonist TCB-2 in male DBA/2J mice.

RATIONALE: Psychedelic drugs, which share in common 5-HT2A receptor agonist activity, have shown promise in treating alcohol-use disorders (AUDs). Repeated exposure to ethanol (EtOH) induces molecular and behavioural changes reflective of neuroadaptations that may contribute to addiction. Psychedelic drugs can induce neuroplasticity also, raising the possibility that their potential clinical effects in AUD may involve an action to reverse or offset effects of long-term changes induced by EtOH. This possibility was examined by investigating whether psilocybin, or the 5-HT2A receptor agonist TCB-2, counteracted established sensitization of EtOH-induced locomotor activity. METHODS: Male DBA/2J mice received repeated injections of 2.2 g/kg EtOH to induce a sensitized locomotor activity response. In two experiments separate groups of mice were then injected with psilocybin (0, 0.3 and 1 kg/kg) or TCB-2 (0, 1 and 3 mg/kg) on 5 consecutive days. Next, mice were challenged with 1.8 g/kg EtOH and locomotor activity measured for 15 min. RESULTS: Relative to naïve controls, previously sensitized mice showed enhanced locomotor activity to the challenge dose. Despite reducing locomotor activity in their own right psilocybin and TCB-2 did not alter the strength of this sensitized response. CONCLUSION: Psilocybin and TCB-2 at behaviourally effective doses did not reverse sensitization of EtOH-induced activity. This suggests that mechanisms involved in mediating short-term reductions in EtOH intake by psilocybin or TCB-2 may not involve a capacity of these drugs to offset enduring changes in behaviour and any underlying neural adaptations induced by repeated intermittent exposure to EtOH.

Effects of psilocybin, the 5-HT2A receptor agonist TCB-2, and the 5-HT2A receptor antagonist M100907 on visual attention in male mice in the continuous performance test.

RATIONALE: Neuropsychiatric disorders such as depression are characterized in part by attention deficits. Attention is modulated by the serotonin (5-HT) neurotransmitter system. The 5-HT2A agonist and hallucinogen psilocybin (PSI) is a promising treatment for disorders characterized by attention changes. However, few studies have investigated PSI's direct effect on attention. OBJECTIVE: Using the rodent continuous performance task (CPT), we assessed PSI's effect on attention. We also evaluated the impact of 5-HT2A receptor agonist TCB-2 and antagonist M100907 for comparative purposes. METHODS: In the CPT, mice learned to distinguish visual targets from non-targets for milkshake reward. Performance was then tested following injections of PSI (0.3, 1, and 3 mg/kg), TCB-2 (0.3, 1, and 3 mg/kg), or M100907 (0.1, 0.3, and 1 mg/kg). Subsequently, drug effects were then evaluated using a more difficult CPT with variable stimulus durations. Mice were then tested on the CPT following repeated PSI injections. Drug effects on locomotor activity were also measured. RESULTS: In the CPT, all three drugs reduced hit and false alarm rate and induced conservative responding. PSI also reduced target discrimination. These effects were seen primarily at doses that also significantly reduced locomotor activity. No drug effects were seen on the more difficult CPT or following repeated PSI injections. CONCLUSIONS: Psilocybin, TCB-2, and M100907 impaired performance of the CPT. However, this may be in part due to drug-induced locomotor changes. The results provide little support for the idea that psilocybin alters visual attention, or that 5-HT2A receptors modulate this process.

Effects of 5-HT2C receptor stimulation in male mice on behaviour and Fos expression: Feeding, reward and impulsivity.

Serotonin modulates many motivated behaviours via multiple receptor subtypes. Agonists at 5-HT2C receptors have potential for treating behavioural problems associated with obesity and drug use. In this work we examined the impact of the 5-HT2C receptor agonist lorcaserin on several motivated behaviours related to feeding, reward and waiting impulsivity, and on neuronal activation in key brain areas mediating those behaviours. In male C57BL/6J mice effects of lorcaserin (0.2, 1 and 5 mg/kg) were examined on feeding, and on operant responding for a palatable reward. Feeding was reduced only at 5 mg/kg, whereas operant responding was reduced at 1 mg/kg. At a much lower dose range lorcaserin 0.05-0.2 mg/kg also reduced impulsive behaviour measured as premature responding in the 5-choice serial reaction time (5-CSRT) test, without affecting attention or ability to perform the task. Lorcaserin induced Fos expression in brain regions related to feeding (paraventricular nucleus and arcuate nucleus), reward (ventral tegmental area), and impulsivity (medial prefrontal cortex, VTA) although these effects did not show the same differential sensitivity to lorcaserin as the behavioural measures. These results indicate a broad profile of action of 5-HT2C receptor stimulation on brain circuitry and on motivated behaviours, but with clear evidence of differential sensitivity across behavioural domains. This is exemplified by the fact that impulsive behaviour was reduced at a much lower dose range than was feeding behaviour. Along with previous work, and some clinical observations, this work supports the idea that 5-HT2C agonists may be useful for behavioural problems associated with impulsivity.

Behavioural phenotypes of intrinsic motivation in schizophrenia determined by cluster analysis of objectively quantified real-world performance.

Intrinsic motivation deficits are a prominent feature of schizophrenia that substantially impacts functional outcome. This study used cluster analysis of innate real-world behaviours captured during two open-field tasks to dimensionally examine heterogeneity in intrinsic motivation in schizophrenia patients (SZ) and healthy controls (HC). Wireless motion capture quantified participants' behaviours aligning with distinct aspects of intrinsic motivation: exploratory behaviour and effortful activity in the absence of external incentive. Cluster analysis of task-derived measures identified behaviourally differentiable subgroups, which were compared across standard clinical measures of general amotivation, cognition, and community functioning. Among 45 SZ and 47 HC participants, three clusters with characteristically different behavioural phenotypes emerged: low exploration (20 SZ, 19 HC), low activity (15 SZ, 8 HC), and high exploration/activity (10 SZ, 20 HC). Low performance in either dimension corresponded with similar increased amotivation. Within-cluster discrepancies emerged for amotivation (SZ > HC) within the low exploration and high performance clusters, and for functioning (SZ < HC) within all clusters, increasing from high performance to low activity to low exploration. Objective multidimensional characterization thus revealed divergent behavioural expression of intrinsic motivation deficits that may be conflated by summary clinical measures of motivation and overlooked by unidimensional evaluation. Deficits in either aspect may hinder general motivation and functioning particularly in SZ. Multidimensional phenotyping may help guide personalized remediation by discriminating between intrinsic motivation impairments that require amelioration versus unimpaired tendencies that may facilitate remediation.

Effects of pimavanserin and lorcaserin on alcohol self-administration and reinstatement in male and female rats.

The neurotransmitter serotonin (5-hydroxytryptamine; 5-HT) modulates fundamental motivational processes, and the neurochemical and behavioural effects of drugs of abuse. Recently, attention has focused on the role of 5-HT acting via 5-HT2A and 5-HT2C receptor sub-types in this regard. We examined the impact of manipulating 5-HT2A and 5-HT2C receptor mediated function on several aspects of alcohol self-administration and alcohol-seeking behaviour in male and female rats. Specifically, experiments investigated the effect of the 5-HT2A inverse agonist/antagonist pimavanserin, and the 5-HT2C receptor agonist lorcaserin on these behaviours. In male and female rats trained to respond for alcohol reinforcement on fixed ratio (FR) and progressive ratio (PR) schedules of reinforcement pimavanserin (0.3, 1 and 3 mg/kg) had no consistent effect on responding. Lorcaserin (0.25, 0.5 and 1 mg/kg) reduced these behaviours in both sexes. Following extinction of responding for alcohol, alcohol-seeking was reinstated by cues previously paired with alcohol. Pimavanserin (1 mg/kg) and lorcaserin (0.5 mg/kg) significantly reduced this reinstatement. In a two-bottle 24 h intermittent access procedure pimavanserin had no significant effects, but lorcaserin reduced alcohol consumption in both sexes at 1, 4 and 24 h after access to alcohol was allowed. Finally, as determined using in vivo microdialysis, alcohol increased, and lorcaserin (0.5 mg/kg) reduced, extracellular levels of DA in the NAc in male rats. In rats treated with lorcaserin prior to alcohol injection the net effect was that DA levels were not changed compared to those measured in control rats. These results suggest that blocking 5-HT2A receptor activity has a very limited action to reduce alcohol-seeking. Activating 5-HT2C receptors had a broader behavioural profile to reduce alcohol self-administration, alcohol drinking and alcohol seeking. These effects may partly result from a blunting of the effect of alcohol on mesolimbic DA release.

The serotonin 2C receptor agonist lorcaserin, alone and in combination with the opioid receptor antagonist naltrexone, attenuates binge-like ethanol drinking.

The serotonin (5-HT) system has been implicated in the pathophysiology of alcohol (ethanol; EtOH) use disorders. Lorcaserin, a 5-HT2C receptor agonist, attenuates drug self-administration in animal models. We investigated the effects of lorcaserin on EtOH intake using the drinking-in-the-dark (DID) procedure, an animal model of binge-like drinking. We compared the effects of lorcaserin to those of the Food and Drug Administration (FDA)-approved drug naltrexone and examined the effects of combining lorcaserin and naltrexone. To examine whether effects were specific for EtOH, we examined the effects of lorcaserin and naltrexone, administered alone and in combination, on saccharin intake. Adult male C57BL/6J mice received EtOH access (20% v/v) for 2 h in the home-cage during the first 3 days of the DID procedure, beginning 3 h into the dark cycle. On day 4, mice were injected with lorcaserin, naltrexone, or a combination of lorcaserin and naltrexone prior to a 4-h EtOH access. Intake was measured at 2 and 4 h. Lorcaserin reduced EtOH intake in a dose-dependent fashion over the 2- and 4-h measurement periods. Naltrexone also reduced EtOH intake when administered alone, with dose-dependent effects being more pronounced over 2 h rather than the full 4-h session. Combining lorcaserin and naltrexone reduced binge-like EtOH drinking to a greater extent than either drug alone. A similar pattern of results was obtained for saccharin intake. These results suggest that lorcaserin and naltrexone can have additive effects on binge-like EtOH drinking. They also support continued research into the therapeutic potential of lorcaserin for alcohol use disorders.

Stone size on endoscopic view as a predictor of successful stone retrieval during flexible ureteroscopy: an in vitro analysis.

PURPOSE: Basketing plays an important role during flexible ureteroscopy, but it can be time-consuming, especially when fragments are too large to pass through the ureteral access sheath. We aim to present the optimal on-screen, endoscopic stone size that predicts successful basketing through various access sheaths. METHODS: A tipless basket, individually extended to 5 mm from multiple ureteroscopes: (Flex-Xc, Karl Storz; Flex-X2s, Karl Storz; LithoVue, Boston Scientific; or URF-P6R, Olympus) and via differently sized access sheaths (10-12 Fr through 13-15 Fr), was used in retrieval attempts of various artificial stone sizes (2 mm through 5 mm). A relative endoscopic stone size was recorded as the stone's maximum diameter on endoscopic view compared to the total image diameter. RESULTS: Basketing of stones up to 2.5 mm, yielding relative endoscopic stone sizes of 0.38 (Flex-Xc), 0.30 (Flex-X2s), 0.32 (LithoVue), and 0.34 (URF-P6R), was successful using all access sheaths. Only the 12-14 Fr and greater sheaths allowed for successful basketing of 3 mm stones. Larger stones did not successfully pass through any of the access sheaths. CONCLUSION: Successful stone retrieval can be predicted by estimating the stone's size on screen, which is influenced by the type of flexible ureteroscope and access sheath. In our testing, stones of approximately one-third of the screen size passed successfully in all cases.

Prenatal disruption of D1R-SynGAP complex causes cognitive deficits in adulthood.

γ-aminobutyric acid (GABA)-ergic interneurons are essential for the physiological function of the mammalian central nervous system. Dysregulated GABAergic interneuron function has been implicated in the pathophysiology of a number of neurodevelopmental disorders including schizophrenia and autism spectrum disorder. Tangential migration is an important process to ensure the proper localization of GABAergic interneurons. Previously we found that disrupting the interaction between dopamine D1 receptor (D1R) and synaptic Ras GTPase- activating protein (SynGAP) using an interfering peptide (TAT-D1Rpep) during embryonic development impaired tangential migration. Here, we assessed the effects of prenatal disruption of D1R-SynGAP complex with the TAT-D1Rpep on the expression of several behaviours during adulthood. Mice with prenatal D1R-SynGAP disruption exhibited transiently reduced locomotor activity, abnormal sensorimotor gating, impaired sociability and deficits in visual discrimination associative learning compared to their control counterparts. Our findings reinforce the importance of GABAergic interneuron migration in the manifestation of normal motor, sensory, and cognitive behaviours of animals during adulthood.

The pharmacological stressor yohimbine, but not U50,488, increases responding for conditioned reinforcers paired with ethanol or sucrose.

RATIONALE: Environmental stimuli paired with alcohol can function as conditioned reinforcers (CRfs) and trigger relapse to alcohol-seeking. In animal models, pharmacological stressors can enhance alcohol consumption and reinstate alcohol-seeking, but it is unknown whether stress can potentiate the conditioned reinforcing properties of alcohol-paired stimuli. OBJECTIVES: We examined whether the pharmacological stressors, the α-2 adrenoreceptor antagonist yohimbine (vehicle, 1.25, 2.5 mg/kg; IP) and the K-opioid receptor agonist U50,488 (vehicle, 1.25, 2.5 mg/kg; SC), increase responding for conditioned reinforcement, and if their effects interact with the nature of the reward (alcohol vs. sucrose). We subsequently examined the effects of yohimbine (vehicle, 1.25, 2.5 mg/kg; IP) on responding for sensory reinforcement. METHODS: Male Long-Evans underwent Pavlovian conditioning, wherein a tone-light conditioned stimulus (CS) was repeatedly paired with 12% EtOH or 21.7% sucrose. Next, tests of responding for a CRf were conducted. Responding on the CRf lever delivered the CS, whereas responding on the other lever had no consequences. In a separate cohort of rats, the effects of yohimbine on responding just for the sensory reinforcer were examined. RESULTS: Both doses of yohimbine, but not U50,488, increased responding for conditioned reinforcement. This enhancement occurred independently of the nature of the reward used during Pavlovian conditioning. Yohimbine-enhanced responding for a CRf was reproducible and stable over five tests; it even persisted when the CS was omitted. Both doses of yohimbine also increased responding for sensory reinforcement. CONCLUSIONS: Yohimbine increases operant responding for a variety of sensory and conditioned reinforcers. This enhancement may be independent of its stress-like effects.

Serotonin transporter protein in autopsied brain of chronic users of cocaine.

RATIONALE: The long-held speculation that the brain serotonin system mediates some behavioral effects of the psychostimulant cocaine is supported in part by the high affinity of cocaine for the serotonin transporter (SERT) and by reports that the serotonin transporter (SERT), estimated by SERT binding, is increased in brain of human chronic cocaine users. Excessive SERT activity and consequent synaptic serotonin deficiency might cause a behavioral (e.g., mood) abnormality in chronic users of the drug. OBJECTIVE AND METHODS: Previous studies focused on changes in SERT binding, which might not necessarily reflect changes in SERT protein. Therefore, we compared levels of SERT protein, using a quantitative Western blot procedure, in autopsied brain (striatum, cerebral cortices) of chronic human cocaine users (n = 9), who all tested positive for the drug/metabolite in brain, to those in control subjects (n = 15) and, as a separate drug of abuse group, in chronic heroin users (n = 11). RESULTS: We found no significant difference in protein levels of SERT or the serotonin synthesizing enzyme tryptophan hydroxylase-2 among the control and drug abuse groups. In the cocaine users, no significant correlations were observed between SERT and brain levels of cocaine plus metabolites, or with levels of serotonin or its metabolite 5-hydroxyindoleacetic acid. CONCLUSION: Our postmortem data suggest that a robust increase in striatal/cerebral cortical SERT protein is not a common characteristic of chronic, human cocaine users.

Median raphe serotonin neurons promote anxiety-like behavior via inputs to the dorsal hippocampus.

Anxiety disorders may be mediated in part by disruptions in serotonin (5-hydroxytryptamine, 5-HT) system function. Behavioral measures of approach-avoidance conflict suggest that serotonin neurons within the median raphe nucleus (MRN) promote an anxiogenic state, and some evidence indicates this may be mediated by serotonergic signaling within the dorsal hippocampus. Here, we test this hypothesis using an optogenetic approach to examine the contribution of MRN 5-HT neurons and 5-HT innervation of the dorsal hippocampus (dHC) to anxiety-like behaviours in female mice. Mice expressing the excitatory opsin ChR2 were generated by crossing the ePet-cre serotonergic cre-driver line with the conditional Ai32 ChR2 reporter line, resulting in selective expression of ChR2 in 5-HT neurons. Electrophysiological recordings confirmed that this approach enabled reliable optogenetic stimulation of MRN 5-HT neurons, and this stimulation produced downstream 5-HT release in the dHC as measured by in vivo microdialysis. Optogenetic stimulation of the MRN elicited behavioral responses indicative of an anxiogenic effect in three behavioural tests: novelty-suppressed feeding, marble burying and exploration on the elevated-plus maze. These effects were shown to be behaviourally-specific. Stimulation of 5-HT terminals in the dHC recapitulated the anxiety-like behaviour in the novelty-suppressed feeding and marble burying tests. These results show that activation of 5-HT efferents from the MRN rapidly induces expression of anxiety-like behaviour, in part via projections to the dHC. These findings reveal an important neural circuit implicated in the expression of anxiety in female mice.

Clozapine reliably increases the motivation for food: parsing the role of the 5-HT2c and H1 receptors.

RATIONALE AND OBJECTIVES: Although clozapine is effective in treating schizophrenia, it is associated with adverse side effects including weight gain and metabolic syndrome. Despite this, the role of clozapine on feeding behaviour and food intake has not been thoroughly characterised. Clozapine has a broad pharmacological profile, with affinities for several neurotransmitter receptors, including serotonin (5-hydroxytriptamine, 5-HT) and histamine. Given that the serotonin 5-HT2C receptor and histaminergic H1 receptor are involved in aspects of feeding behaviour, the effect of clozapine on feeding may be linked to its action at these receptors. METHODS: We assessed, in rats, the effect of acute and subchronic administration of clozapine on responding for food under a progressive ratio (PR) schedule under conditions of food restriction and satiety. We also examined the effect of antagonists of the serotonin 5-HT2C and histaminergic H1 receptors on the same schedule. Clozapine reliably increased responding for food, even when rats had ad libitum access to food. The effect of clozapine on responding for food was reproduced by combined (but not individual) antagonism of the serotonin 5-HT2C and histaminergic H1 receptors. CONCLUSION: These findings show that clozapine enhances the motivation to work for food, that this effect is stable over repeated testing, and is independent of hunger state of the animal. This effect may relate to a combined action of clozapine at the serotonin 5-HT2C and histaminergic H1 receptors.

Effects of exposure to chronic uncertainty and a sensitizing regimen of amphetamine injections on locomotion, decision-making, and dopamine receptors in rats.

Gambling disorder (GD) is a behavioral addiction that may be linked to alterations in dopamine (DA) systems. Gambling involves chronic exposure to uncertain reward, which can sensitize the activity of DA systems. Here we explored how combinations of Pavlovian and instrumental uncertainty impact DA sensitization and risky decision-making. Experiment 1: 40 rats underwent 66 uncertainty exposure (UE) sessions during which they responded for saccharin. Animal responding was reinforced according to a fixed or variable (FR/VR) ratio schedule that turned on a conditioned stimulus (CS; light), which predicted saccharin on 50% or 100% of trials. Animals responded under one of the four conditions: FR-CS100% (no uncertainty), VR-CS100%, FR-CS50%, and VR-CS50% (maximal uncertainty). DA sensitization was inferred from an enhanced locomotor response to d-amphetamine (d-AMPH; 0.5 mg/kg) challenge. The rat gambling task (rGT) was used to assess decision-making. Experiment 2: 24 rats received 5 weeks of sensitizing d-AMPH or saline doses, followed by locomotor activity and rGT testing. Experiment 3: Effects of UE and a sensitizing d-AMPH regimen on DA D1, D2, and D3 receptor binding were assessed in 44 rats using autoradiography. Compared to FR-CS100%, VR-CS100% and VR-CS50% rats displayed a greater locomotor response to d-AMPH, and VR-CS50% rats demonstrated riskier decision-making. Chronic d-AMPH-treated rats mirrored the effects of VR-CS50% groups on these two indices. Both VR-CS50% and d-AMPH-treated groups had increased striatal DA D2 receptor binding. These results suggest that chronic uncertainty exposure, similar to exposure to a sensitizing d-AMPH regimen, sensitized the function of DA systems and increased risky decision-making.

Lorcaserin: A review of its preclinical and clinical pharmacology and therapeutic potential.

The selective 5-HT2C receptor agonist lorcaserin, in conjunction with lifestyle modification, was approved by the FDA in 2012 for weight management. It has been marketed in the US as Belviq® since 2013. This article provides a review of the preclinical and clinical pharmacology of lorcaserin, including its pharmacokinetic and safety profiles. Preclinical studies with lorcaserin initially focused on simple measures of food intake and body weight gain, but have now expanded to include studies on its effects on appetitive aspects of feeding behaviour and models of binge-eating. A significant number of studies have also shown that lorcaserin alters behaviours related to drug use and addiction, in rodents and non-human primates. Potential clinically-relevant effects of lorcaserin have also been reported in models of pain and seizure-like activity. Not surprisingly, the majority of clinical work with lorcaserin has focused on its effects on weight gain, and on physiological processes related to energy intake. However, results of clinical trials and experimental laboratory studies involving lorcaserin are now appearing which describe effects on a range of other behaviours and physiological functions. These include smoking cessation, cocaine self-administration, and behavioural and brain responses to food cues. All of this work suggests that lorcaserin may have therapeutic potential for a variety of disorders and conditions beyond obesity. Based on clinical experience, including the outcomes from several, large, well-powered clinical obesity trials at the approved 10mg BID dose both pre and post approval, a priori concerns about cardiac valvulopathy have largely been allayed. However, as with any recently approved first-in-class pharmacotherapy, there may be yet-unknown risks, as well as benefits, associated with use of lorcaserin. Nonetheless, the current safety profile and an expanding post approval safety data base should encourage further experimental laboratory-based and clinical trial-based research with lorcaserin in targeted populations to investigate its full therapeutic potential.

Disruption of SynGAP-dopamine D1 receptor complexes alters actin and microtubule dynamics and impairs GABAergic interneuron migration.

Disruption of γ-aminobutyric acid (GABA)-ergic interneuron migration is implicated in various neurodevelopmental disorders, including autism spectrum disorder and schizophrenia. The dopamine D1 receptor (D1R) promotes GABAergic interneuron migration, which is disrupted in various neurological disorders, some of which are also associated with mutations in the gene encoding synaptic Ras-guanosine triphosphatase-activating protein (SynGAP). Here, we explored the mechanisms underlying these associations and their possible connection. In prenatal mouse brain tissue, we found a previously unknown interaction between the D1R and SynGAP. This D1R-SynGAP interaction facilitated D1R localization to the plasma membrane and promoted D1R-mediated downstream signaling pathways, including phosphorylation of protein kinase A and p38 mitogen-activated protein kinase. These effects were blocked by a peptide (TAT-D1Rpep) that disrupted the D1R-SynGAP interaction. Furthermore, disrupting this complex in mice during embryonic development resulted in pronounced and selective deficits in the tangential migration of GABAergic interneurons, possibly due to altered actin and microtubule dynamics. Our results provide insights into the molecular mechanisms regulating interneuron development and suggest that disruption of the D1R-SynGAP interaction may underlie SYNGAP1 mutation-related neurodevelopmental disorders.

Adolescent exposure to Δ9-tetrahydrocannabinol delays acquisition of paired-associates learning in adulthood.

RATIONALE AND OBJECTIVES: Adolescence is a sensitive period of brain development, during which there may be a heightened vulnerability to the effects of drug use. Despite this, the long-term effects of cannabis use during this developmental period on cognition are poorly understood. METHODS: We exposed adolescent rats to escalating doses of Δ9-tetrahydrocannabinol (THC)-the primary psychoactive component of cannabis-or vehicle solution during postnatal days (PND) 35-45, a period of development that is analogous to human adolescence (THC doses: PND 35-37, 2.5 mg/kg; PND 38-41, 5 mg/kg; PND 42-45, 10 mg/kg). After a period of abstinence, in adulthood, rats were tested on an automated touchscreen version of a paired-associates learning (PAL) task to assess their ability to learn and recall object-location associations. Prepulse inhibition (PPI) of the startle response was also measured at three time points (5 days, 4 months, and 6 months after exposure) to assess sensorimotor gating, the ability to filter out insignificant sensory information from the environment. RESULTS: Compared to rats exposed to vehicle alone, rats exposed to THC during adolescence took longer to learn the PAL task when tested in adulthood, even when trials contained visually identical stimuli that differed only in location. Despite this, no differences were observed later in testing, when trials contained visually distinct stimuli in different locations. Rats exposed to THC also displayed impairments in sensorimotor gating, as measured by prepulse inhibition of the startle response, though this deficit did appear to decrease over time. CONCLUSION: Taken together, THC exposure during adolescence produces long-term deficits in associative learning and sensorimotor gating, though the impact of these deficits seems to diminish with time. Thus, adolescence may represent a period of neurocognitive development that is vulnerable to the harms of cannabis use, though the stability of such harms is uncertain.

Nicotine enhances responding for conditioned reinforcement via α4ß2 nicotinic acetylcholine receptors in the ventral tegmental area, but not the nucleus accumbens or the prefrontal cortex.

Nicotine enhances the conditioned reinforcing properties of reward-paired cues. We investigated the role of the ventral tegmental area (VTA), nucleus accumbens (NAcc), and prelimbic (PrL) and infralimbic (IL) cortices in mediating this enhancement. Male Long-Evans rats were implanted with bilateral guide cannulae aimed at either the VTA, NAcc, PrL or IL cortex. Next, rats underwent 12 sessions of Pavlovian conditioning. Each session consisted of 30 trials wherein a 5 s conditioned stimulus (CS) was paired with water (0.05 ml). Tests of responding for conditioned reinforcement were conducted during which presentation of the CS, now acting as a conditioned reinforcer (CRf), was contingent upon pressing one of two levers (CRf lever). Pressing the other lever had no consequences (NCRf lever). To determine if nicotinic acetylcholine receptors (nAChRs) in the VTA, NAcc, PrL cortex, or IL cortex mediate nicotine-enhanced responding for a CRf, the α4ß2 nAChR antagonist Dihydro-Beta-Erythroidine (DHßE; 10 nmol/0.5 µL) was infused into the respective areas prior to a systemic nicotine injection (0.2 mg/kg; SC). DHßE infused into the VTA, NAcc, or IL cortex, but not PrL cortex, attenuated nicotine-enhanced responding for a CRf. Next, to confirm that nAChRs in the VTA, NAcc, or IL cortex mediate this enhancement, nicotine (8, 16, or 32 nmol/0.5 µL) was infused into the respective areas. Nicotine infused into the VTA, but not NAcc or IL cortex, enhanced responding for a CRf. These findings suggest that nicotine primarily acts on α4ß2 nAChRs in the VTA to potentiate the conditioned reinforcing properties of reward-related cues.

Dorsal raphe serotonin neurons inhibit operant responding for reward via inputs to the ventral tegmental area but not the nucleus accumbens: evidence from studies combining optogenetic stimulation and serotonin reuptake inhibition.

The monoamine neurotransmitter serotonin (5-hydroxytryptamine; 5-HT) exerts an inhibitory influence over motivation, but the circuits mediating this are unknown. Here, we used an optogenetic approach to isolate the contribution of dorsal raphe nucleus (DRN) 5-HT neurons and 5-HT innervation of the mesolimbic dopamine (DA) system to motivated behavior in mice. We found that optogenetic stimulation of DRN 5-HT neurons enhanced downstream 5-HT release, but this was not sufficient to inhibit operant responding for saccharin, a measure of motivated behavior. However, combining optogenetic stimulation of DRN 5-HT neurons with a low dose of the selective serotonin reuptake inhibitor (SSRI) citalopram synergistically reduced operant responding. We then examined whether these effects could be recapitulated if optogenetic stimulation specifically targeted 5-HT terminals in the ventral tegmental area (VTA) or nucleus accumbens (NAc) of the mesolimbic DA system. Optogenetic stimulation of 5-HT input to the VTA combined with citalopram treatment produced a synergistic decrease in responding for saccharin, resembling the changes produced by targeting 5-HT neurons in the DRN. However, this effect was not observed when optogenetic stimulation targeted 5-HT terminals in the NAc. Taken together, these results suggest that DRN 5-HT neurons exert an inhibitory influence over operant responding for reward through a direct interaction with the mesolimbic DA system at the level of the VTA. These studies support an oppositional interaction between 5-HT and DA systems in controlling motivation and goal-directed behavior, and have important implications for the development and refinement of treatment strategies for psychiatric disorders such as depression and addiction.

Preclinical evidence for combining the 5-HT2C receptor agonist lorcaserin and varenicline as a treatment for nicotine dependence.

Varenicline, a nicotinic acetylcholine receptor partial agonist, is used to treat nicotine dependence. Lorcaserin, a 5-HT2C receptor agonist has been approved in some countries to treat obesity. Based on preclinical and preliminary clinical evidence, lorcaserin may have potential to treat nicotine dependence. These experiments examined in rats the effects of combining varenicline (0.5 or 1 mg/kg) and lorcaserin (0.3, 0.6 and 1 mg/kg) on nicotine self-administration, reinstatement of nicotine seeking, responding for food and impulsive action. Both drugs alone reduced nicotine self-administration. Combining varenicline and 0.6 mg/kg lorcaserin reduced responding to a greater extent than either drug alone. In a relapse model, extinguished nicotine seeking was reinstated by a priming injection of nicotine and nicotine-associated cues. Reinstatement was reduced by varenicline (1 mg/kg) and by lorcaserin (0.3 mg/kg). Combining lorcaserin (0.3 mg/kg) with varenicline (0.5 or 1 mg/kg) reduced reinstatement to a greater degree than either drug alone. Both drugs had minimal effects on responding for food, alone or in combination. In the five-choice serial reaction time test, varenicline (0.5 or 1 mg/kg) increased impulsivity, measured as increased premature responding. This effect was reduced by lorcaserin (0.3 mg/kg). Plasma levels of varenicline or lorcaserin were not altered by co-administration of the other drug. Varenicline and lorcaserin have additive effects on nicotine self-administration, and on nicotine seeking. Lorcaserin prevents impulsivity induced by varenicline. This pattern of effects suggests that co-administration of varenicline and lorcaserin has potential as a treatment for nicotine dependence that may exceed the value of either drug alone.

Clozapine reduces nicotine self-administration, blunts reinstatement of nicotine-seeking but increases responding for food.

People with schizophrenia display significantly higher rates of smoking than the general population, which may be due to an interaction between nicotine and antipsychotic medication. While the conventional antipsychotic drug haloperidol sometimes increases cigarette smoking in patients with schizophrenia, there is some evidence suggesting that clozapine, an atypical antipsychotic drug, may reduce nicotine use in these patients. However, the effects of antipsychotic drugs like clozapine on aspects of nicotine self-administration and reinstatement have not been systematically examined. In the current study, we assessed the effect of clozapine on nicotine self-administration under fixed ratio and progressive ratio schedules of reinforcement, as well as reinstatement of nicotine-seeking following a period of abstinence. To determine the specificity of its effect on nicotine reward, we also tested the effect of clozapine on responding for food reinforcement under fixed ratio and progressive ratio schedules. For comparison, we also examined the effects of haloperidol, a first-generation antipsychotic drug, under some of the same behavioral conditions as clozapine. We show that clozapine inhibits nicotine self-administration and reinstatement of nicotine-seeking but also increases the amount of effort that rats will exert for food reward. In contrast, haloperidol at a wide range of doses attenuated responding for nicotine and food reward, suggestive of a non-specific reduction in reinforcer efficacy. These results show the potential utility of clozapine as a smoking cessation treatment for patients with schizophrenia, in addition to its antipsychotic properties.