Progressive Ratio Schedule Research Articles

Transient impact of chronic social stress on effort-based reward motivation in non-food restricted mice: involvement of corticosterone.

Chronic stress has been connected to a reduced effort and motivational deficits. To study effort-based motivation in rodents, operant conditioning is often employed. However, caloric restriction is typically imposed simultaneously. Since caloric restriction is a stressor in its own right, this procedure interferes with data interpretation. Here, we investigate whether chronic social defeat stress (CSD), lasting 10 consecutive days, would alter effort-based reward motivation in mice trained under ad libitum food conditions. Utilizing operant FED3 boxes in home cages, mice were trained within eight days to nose poke for palatable food. After training completion, operant memory was retained for at least 16 days, and mice demonstrated sustained effort, as assessed with a progressive ratio schedule, to obtain reward pellets. Directly after CSD exposure (10th day), mice exhibited reduced effort for palatable food rewards, but also displayed reduced nose poking in general. The effects of CSD on effort were short-lived, with no lasting impact on effort-based reward motivation one week post-stress. As corticosterone (CORT) levels were increased at day 10 of CSD, but not at day 17, we hypothesized that CORT might mediate the acute effects of CSD on effort-based reward motivation. Indeed, CORT administration [100 μg/ml], supplied via the drinking water, mirrored the CSD-induced CORT spike and temporarily reduced reward motivation. Our findings emphasize that CSD does not result in long-term deficits in reward motivation, suggesting a resilient adaptive response in mice under unrestricted feeding conditions. This study underscores the necessity of considering temporal dynamics of stress impacts and highlights the modulating effects of CORT. These insights contribute to a deeper understanding of the resilience mechanisms in motivational impairments and pave the way for further research into factors facilitating this resilience.

Selective dopamine D3 receptor partial agonist (±)VK4-40 reduces the reinforcing strength of d-amphetamine but not cocaine in rhesus monkeys responding under a progressive-ratio schedule of reinforcement

Selective dopamine D3 receptor partial agonist (±)VK4-40 reduces the reinforcing strength of d-amphetamine but not cocaine in rhesus monkeys responding under a progressive-ratio schedule of reinforcement

The Relative Reinforcing Value of Menthol Among Young Adult Cigarette Smokers: Results From a Behavioral Choice Task.

Menthol cigarettes are associated with experimentation and progression to regular use. Although reinforcement processes likely underlie menthol's appeal, the reinforcing value of menthol cigarettes remains unknown. This study examined the relative reinforcing value (RRV) of menthol versus nonmenthol cigarettes in young adult menthol (n = 54) and nonmenthol (n = 53) smokers, and differences in menthol's RRV by race, ethnicity, and sexual orientation. Overnight abstinent participants completed a choice task assessing willingness to "work" to click targets on a computer screen to earn menthol or nonmenthol cigarette puffs. A progressive ratio schedule was used where the menthol target had to be clicked progressively more times, over 10 trials, to earn a menthol cigarette puff, while clicks for the nonmenthol target were fixed across trials. RRV for menthol was defined by the breakpoint, or the highest trial (out of to 10) completed for a menthol cigarette puff. Number of clicks for menthol and nonmenthol puffs were also examined. Menthol smokers worked harder for menthol versus nonmenthol cigarette puffs (breakpoint = 9.17; ~1236 clicks vs. 24 clicks). Breakpoint was higher among Hispanic (6.49) versus NH White (4.83) and NH non-White smokers (4.43). In exploratory analyses of interactions of menthol preference with race and ethnicity, nonmenthol Hispanic smokers worked harder for menthol cigarette puffs versus NH non-White and NH White nonmenthol smokers. Menthol cigarettes are highly reinforcing for young adult menthol and Hispanic smokers. A menthol ban may reduce addiction risk among younger individuals and some minoritized groups of smokers. This study provides evidence of the greater relative reinforcing value of menthol compared to nonmenthol cigarettes among young adults who had a cigarette flavor preference, suggesting increased addiction risk of menthol cigarettes. Young adult menthol smokers and Hispanic (vs. non-Hispanic) smokers worked harder to earn menthol (vs. nonmenthol) cigarette puffs. Findings add to the evidence base supporting the U.S. Food and Drug Administration's (FDA) intent to ban menthol in cigarettes. Further, prevention messaging campaigns and cessation programs should take into account the reinforcing value of menthol in cigarettes, especially in vulnerable and at-risk populations.

Responsiveness and Reliability of a Sipping Device to Measure Motivation in Normal-Weight Individuals and Bariatric Surgery Patients.

There is an urgent need to measure the motivation to taste a sweet fluid in order to determine the influence of sweet tastes on the potential choices and consumption of beverages in patients with obesity. Current methods utilize either survey instruments or arbitrary operant tasks. The sipometer enables the participant to utilize an actual ingestive behavioral response to measure motivation during access to beverages on either ad libitum (AL) or progressive time ratio (PR) schedules. We determined the sipometer's responsiveness and reliability as a test of change in motivation for sweet tastes after bariatric surgery. Participants (58 patients and 28 controls, BMI: 18.5-24.9 kg/m2) sham-consumed an aspartame-sweetened (S) and non-sweetened (N) beverage under AL and PR schedules at a pre-surgery/baseline and a 3-month and 24-month visit (patients only). Cumulative pressure (CumPres), a measure of effort, was the sum of the pressures exerted during sipping under each condition. Baseline CumPres for PRS was higher than ALS and ALN in patients (p < 0.03) and higher than PRN in controls (p = 0.009). At 3 months, CumPres did not differ amongst conditions in patients, but CumPres for PRS was higher than all other conditions in controls (p < 0.0005). There were no baseline group differences; however, patients' CumPres for PRS was lower than controls' at 3 months (p = 0.002). Patients' CumPres for PRS decreased non-significantly between the baseline and 3 months but increased at 24 months compared to 3 months (p = 0.025) and was no different from baseline. Controls' CumPres for PRS increased at 3 months (p = 0.0359), but CumPres for all conditions was correlated between visits (p's < 0.038). The sipometer is a reliable and sensitive measure of motivation to consume sweet beverages and may reflect changes in post-operative energy intake.

Unveiling the power of optimism: Exploring behavioral and neuromolecular correlates of alcohol seeking and drinking in rats with biased judgement

Alcohol use disorder (AUD) is a common psychiatric condition with substantial global mortality. Despite extensive research into its pathophysiology, the cognitive predispositions driving alcohol dependence are less understood. This study explores whether biased cognition, specifically traits of optimism and pessimism, predicts susceptibility to alcohol-seeking behaviors using an animal model.Rats were initially tested for judgement bias through Ambiguous Cue Interpretation tests. Those identified as ‘optimistic' or ‘pessimistic' were further examined for their tendency to escalate alcohol intake using the intermittent access 2-bottle choice (2BC) paradigm. Additionally, we assessed how judgement bias influenced the development of compulsive alcohol-seeking behavior in a Seeking-Taking (ST) and Seeking-Taking Punishment tasks, alcohol-seeking motivation in the Progressive Ratio Schedule of Reinforcement paradigm, the speed of extinction, and reinstatement after abstinence. Neurochemical analyses were conducted to investigate trait-specific differences in neurotransmitter-related gene expression and receptor densities in the brain. We used TaqMan Gene Expression Array Cards to analyze expression levels of genes linked to serotonergic, dopaminergic, glutamatergic, and GABAergic pathways, and alcohol metabolism in various brain regions. Receptor densities for 5-HT1A, 5-HT2A, and D2 were measured using autoradiography analysis.Behaviorally, ‘optimistic' rats showed significantly lower alcohol consumption in the 2BC paradigm compared to ‘pessimistic' rats. This lowered intake correlated with decreased monoamine oxidase-A (Maoa) expression in the medial prefrontal cortex (mPFC) and increased metabotropic glutamate receptor 2 (Grm2) expression in the amygdala (Amy). Additionally, we observed significant interactions between judgement bias and alcohol intake in the expression of several genes in the mPFC, nucleus accumbens (Nacc), orbitofrontal cortex (OFC), and Amy, as well as in 5-HT2A receptor binding in the Nacc.Overall, these results suggest that optimism is linked to lower alcohol consumption and related neurochemical changes, indicating a potential cognitive mechanism in AUD risk.

Kappa opioid receptor mediated operant performance in male and female rats

Anhedonia and avolition are emotions frequently endorsed by individuals with stress related disorders. Kappa opioid receptor (KOR) activation can induce negative emotions and recent clinical evidence suggests that KOR antagonism can alleviate anhedonia in a transdiagnostic cohort of patients. However, the behavioral consequences of KOR activation and antagonism in modulating motivation, as assessed by schedule-controlled behavioral performance without preexisting conditions (stress or substance use), have not been formally assessed. To address this gap in the literature, this report utilized male and female Sprague Dawley rats to (1) evaluate the impact of the selective KOR agonist U50,488, on the performance of animals responding for sucrose pellets under a progressive ratio (PR) schedule and (2) determine the effects of the short-acting KOR antagonist LY2444296 alone and on U50,488 mediated reductions in PR performance. Overall, U50,488 5 mg/kg significantly reduced the breakpoint and number of rewards obtained by animals. This occurred in the absence of motor impairment and independent of evidence for satiation. LY2444296 did not alter PR performance when administered alone but effectively blocked the deficits induced by U50,488. To further delineate the behavioral alterations that underlie these reductions in responding, a more detailed analysis was conducted on PR performance in the first 15 min of the session, the period of time when animals obtained the most reinforcers. During this period, U50,488 increased the length of the post-reinforcement pause and reduced the running rate on PR schedules. These changes in behavior produced by acute activation of KORs are consistent with a reduction of effort-related motivation in rodents. These data contribute to the understanding of how KORs modulate motivation, which is critical to future efforts to evaluate performance in the context of stress and assess how KOR antagonists alleviate anhedonic behaviors associated with stress.

Sex Differences in the Development of an Opioid Addiction–Like Phenotype: A Focus on the Telescoping Effect

BackgroundWomen develop addiction and drug-related health consequences after fewer years of drug use than men; this accelerated time course, or telescoping effect, has been observed clinically for multiple drugs, including opioids. Preclinical studies indicate that this is a biologically based phenomenon; however, these studies have focused exclusively on cocaine, and none have considered health effects. MethodsIn this study, we used a rat (Sprague Dawley) model to determine sex differences in the time course for the development of an opioid addiction–like phenotype, as defined by the development of physical dependence (withdrawal-induced weight loss) and an increase in motivation for fentanyl (under a progressive-ratio schedule). Effects were determined following either 10 days (optimized, experiment 1) or 3 days (threshold, experiment 2) of extended-access fentanyl self-administration (24 hours/day, fixed ratio 1, 2- to 5-minute trials/hour) or following short-access fentanyl self-administration (subthreshold, experiment 3; fixed ratio 1, up to 40 infusions/day). Opioid-related adverse health effects were also determined (experiment 4). ResultsMotivation for fentanyl was similarly increased in males and females following 10 days of extended-access self-administration (experiment 1), was transiently increased in females, but not males, following 3 days of extended-access self-administration (experiment 2) and was not increased in either sex following short-access self-administration (experiment 3). Females developed fentanyl-associated adverse health effects more readily than males (experiment 4), with particularly robust differences during extended-access self-administration and withdrawal. ConclusionsAs with findings in humans, female rats developed opioid addiction–like features and adverse health consequences more readily than male rats. These data provide support for a biologically based telescoping effect in females for opioids, particularly for opioid-related adverse health consequences.

Negative allosteric modulation of CB1 cannabinoid receptor signalling decreases intravenous morphine self-administration and relapse in mice.

The endocannabinoid system interacts with the reward system to modulate responsiveness to natural reinforcers, as well as drugs of abuse. Previous preclinical studies suggested that direct blockade of CB1 cannabinoid receptors (CB1R) could be leveraged as a potential pharmacological approach to treat substance use disorder, but this strategy failed during clinical trials due to severe psychiatric side effects. Alternative strategies have emerged to circumvent the side effects of direct CB1 binding through the development of allosteric modulators. We hypothesized that negative allosteric modulation of CB1R signalling would reduce the reinforcing properties of morphine and decrease behaviours associated with opioid misuse. By employing intravenous self-administration in mice, we studied the effects of GAT358, a functionally-biased CB1R negative allosteric modulator (NAM), on morphine intake, relapse-like behaviour and motivation to work for morphine infusions. GAT358 reduced morphine infusion intake during the maintenance phase of morphine self-administration under a fixed ratio 1 schedule of reinforcement. GAT358 also decreased morphine-seeking behaviour after forced abstinence. Moreover, GAT358 dose dependently decreased the motivation to obtain morphine infusions under a progressive ratio schedule of reinforcement. Strikingly, GAT358 did not affect the motivation to work for food rewards in an identical progressive ratio task, suggesting that the effect of GAT358 in decreasing opioid self-administration was reward specific. Furthermore, GAT58 did not produce motor ataxia in the rotarod test. Our results suggest that CB1R NAMs reduced the reinforcing properties of morphine and could represent a viable therapeutic route to safely decrease misuse of opioids.

PKMζ alters oxycodone-taking in a dose- and sex-dependent manner

Opioid use disorder involves disruptions to glutamate homeostasis and dendritic spine density in the reward system. PKMζ is an atypical isoform of protein kinase C that is expressed exclusively in neurons and plays a role in postsynaptic glutamate signaling and dendritic spine maturation. As opioid use leads to alterations in glutamate transmission and dendritic spine density, we hypothesized that PKMζ deletion would alter opioid-taking behaviors. The current study examined two doses of oxycodone self-administration in male and female mice with constitutive deletion of PKMζ compared to wildtype controls. At a dose of 0.25 mg/kg/infusion, PKMζ deletion significantly potentiated oxycodone self-administration in both male and female mice. However, increases in motivation for oxycodone, as indicated by increased breakpoint on a progressive ratio schedule, were only seen in male PKMζ knockout mice and not females. When we examined a lower dose of oxycodone, 0.125 mg/kg/infusion, PKMζ knockout led to increases in oxycodone self-administration only in female mice. Additionally, female PKMζ knockout mice exhibited higher breakpoints on a progressive ratio schedule at this dose compared to all other groups. In addition to the self-administration studies, we also examined locomotor sensitization in response to experimenter administered oxycodone. PKMζ KO decreased oxycodone induced locomotion in males and potentiated oxycodone sensitization in females. Together, these results suggest that PKMζ acts to dampen oxycodone taking in both sexes, but females may be more sensitive to its effects.

The positive reinforcing effects of cocaine and opposite-sex social contact: roles of biological sex and estrus.

Preclinical studies report that drug use and social contact mutually influence the reinforcing effects of one another. Most of these studies have used same-sex dyads exclusively, and the role of factors related to biological sex and hormonal fluctuations are not well understood. The purpose of this study was to examine the reinforcing effects of cocaine and social contact with an opposite-sex partner in male and female rats, and how these effects are modulated by ovarian hormones. Male and female rats were trained in a nonexclusive choice procedure in which cocaine and social contact with an opposite-sex partner were simultaneously available on concurrent progressive ratio schedules of reinforcement. To examine the effects of ovarian hormones related to estrous cycling, Experiment 1 used naturally cycling, gonadally intact females, whereas Experiment 2 used ovariectomized females, and estrus was artificially induced with exogenous hormones. In both experiments, cocaine and social contact functioned as robust reinforcers, and there were no significant effects of biological sex or estrus status of the females. The positive reinforcing effects of both cocaine and social contact increased as a function of cocaine dose, indicating that contingent cocaine administration increases the reinforcing effects of social contact. These data suggest that cocaine use among opposite-sex partners may enhance factors that contribute to social bonding.

Response-contingent cocaine increases the reinforcing effectiveness of social contact.

Epidemiological studies report a high concordance rate of drug use within groups, suggesting an interplay between drug reinforcement and social cohesion. Preclinical studies reveal that (a) contingent access to a social partner increases cocaine intake and (b) experimenter-delivered cocaine increases the reinforcing effects of social contact. The purpose of this study was to determine if response-contingent cocaine increases the reinforcing effectiveness of social contact. Male rats were implanted with intravenous catheters and trained on a fixed ratio (FR1) schedule for 30-s access to a social partner. The reinforcing effectiveness of social contact was then determined using a progressive ratio (PR) schedule. After the PR test, rats were divided into two groups in which each response on an FR1 schedule produced social access and either response-contingent cocaine (0.5 mg/kg/infusion) or saline. After 9 days, the reinforcing effectiveness of social contact in the absence of infusions was determined again on the PR schedule. The cocaine and saline reinforcers were then switched between groups and the latter procedures were repeated. Recent exposure to response-contingent cocaine increased the reinforcing effectiveness of social contact on the PR schedule. This effect was transient, and the reinforcing effectiveness of social contact returned to baseline levels once response-contingent cocaine was replaced with saline. These data indicate that recent exposure to response-contingent cocaine transiently increases the reinforcing effectiveness of social contact and suggest that cocaine use may strengthen social cohesion by increasing the reinforcing effects of social contact with other individuals. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Inbred rat heredity and sex affect oral oxycodone self-administration and augmented intake in long sessions: correlations with anxiety and novelty-seeking.

Oxycodone abuse begins with prescription oral oxycodone, yet vulnerability factors determining abuse are largely undefined. We evaluated genetic vulnerability in a rat model of oral oxycodone self-administration (SA): increasing oxycodone concentration/session (0.025-0.1mg/ml; 1,4,16-h) followed by extinction and reinstatement. Active licks and oxycodone intake were greater in females than males during 4-h and 16-h sessions (p< 0.001). Each sex increased intake during 16-h vs 4-h sessions (p<2e-16), but a subset of strains dramatically augmented intake at 16-h (p=0.0005). Heritability (h 2) of active licks/4-h at increasing oxycodone dose ranged from 0.30-0.53. Under a progressive ratio schedule, breakpoints were strain-dependent (p<2e-16). Cued reinstatement was greater in females (p<0.001). Naive rats were assessed by elevated plus maze (EPM), open field (OF) and novel object interaction (NOI). We correlated these behaviors with 28 parameters of oxycodone SA. Anxiety-defining EPM traits were most associated with SA in both sexes, whereas more OF and NOI traits were SA-associated in males. Sex and heredity are major determinants of motivation to take and seek oxycodone; intake augments dramatically during extended access in specific strains; and pleiotropic genes affect anxiety and multiple SA parameters.

Effects of self- and experimenter-administered cocaine on subsequent ethanol drinking in rhesus monkeys

BackgroundOne possible reason for the lack of FDA-approved pharmacotherapies to treat cocaine use disorder (CUD) is that, although cocaine is typically used in combination with alcohol, it is studied in isolation in preclinical studies. A better understanding of the cocaine-alcohol interactions that promote polysubstance use (PSU) will improve animal models of CUD and hasten pharmacotherapy development. We used a rhesus monkey model of cocaine-alcohol PSU to investigate one possible mechanism: that alcohol is used to mitigate negative effects associated with termination of cocaine use. MethodsIn 6 adult male rhesus monkeys, the relationship between self-administered cocaine intake and oral ethanol intake 2hours later was examined during self-administration of cocaine (0.0003–0.3mg/kg per injection, i.v.) under a fixed-ratio 30 schedule (FR30) or a progressive-ratio (PR) schedule. Next, ethanol consumption was measured 0–120minutes after experimenter-administered cocaine (0.3–1.7mg/kg, i.v.). ResultsSelf-administered cocaine intake under both FR30 and PR schedules was unrelated to oral ethanol intakes 2hours later. When cocaine was administered non-contingently, cocaine decreased ethanol intake as well as intake of a non-alcoholic solution in monkeys who never consumed ethanol (n=4) in a time- and dose-dependent manner. ConclusionsTaken together, the results do not provide evidence for cocaine-induced increases in ethanol consumption. By extension, the results do not support the hypothesis that cocaine users drink alcohol to counteract negative effects that occur after terminating use. This finding implies either that such effects do not exist or that such effects exist but are unaffected by ethanol.

Assessment of the antinociceptive, respiratory-depressant, and reinforcing effects of the low pKa fluorinated fentanyl analogs, FF3 and NFEPP

RationaleRecent studies report that fentanyl analogs with relatively low pKa values produce antinociception in rodents without other mu opioid-typical side effects due to the restriction of their activity to injured tissue with relatively low pH values. However, it is unclear if and to what degree these compounds may produce mu opioid-typical side effects (respiratory depression, reinforcing effects) at doses higher than those required to produce antinociception. ObjectivesThe present study compared the inflammatory antinociceptive, respiratory-depressant, and reinforcing effects of fentanyl and two analogs of intermediate (FF3) and low (NFEPP) pKa values in terms of potency and efficacy in male and female Sprague-Dawley rats. MethodsNociception was produced by administration of Complete Freund's Adjuvant into the hind paw of subjects, and antinociception was measured using an electronic Von Frey test. Respiratory depression was measured using whole-body plethysmography. Reinforcing effects were measured in self-administration using a progressive-ratio schedule of reinforcement. The dose ranges tested for each drug encompassed no effect to maximal effects. ResultsAll compounds produced full effects in all measures but varied in potency. FF3 and fentanyl were equipotent in antinociception and self-administration, but FF3 was less potent than fentanyl in respiratory depression. NFEPP was less potent than fentanyl in every measure. The magnitude of potency difference between antinociception and other effects was greater for FF3 than for NFEPP or fentanyl, indicating that FF3 had the widest margin of safety when relating antinociception to respiratory-depressant and reinforcing effects. ConclusionsLow pKa fentanyl analogs possess potential as safer analgesics, but determining the optimal degree of difference for pKa relative to fentanyl will require further study due to some differences between the current results and findings from prior work with these analogs.

Neuroplasticity-related genes correlate with individual differences in distinct phases of oxycodone self-administration in male rats

Opioid use disorder (OUD) is a chronic condition associated with long-lasting molecular and behavioral changes. Animals with prolonged access to opioids develop behaviors similar to human OUD. Identifying associated molecular changes can provide insight to underpinnings that lead to or maintain OUD. In pilot studies, we identified several miRNA targets that are altered by the administration of oxycodone. We selected mir182 for follow up as it was recently shown to be dysregulated in plasma of men administered oxycodone. In addition, mir182 is increased in reward-related brain regions of male rats following exposure to various addictive substances. The present study utilizes a long-access oxycodone self-administration paradigm to examine changes in mir182 and its mRNA targets associated with neuroplasticity, which may be involved in the maintenance of OUD-like phenotype in rats. Male rats were trained to self-administer oxycodone (0.1 mg/kg/infusion, i. v.) for 6 h daily sessions for 12 days. Each animal had a yoked saline control that received matched saline infusions. Animals were then tested on a progressive ratio schedule to measure motivation to obtain a single infusion of oxycodone. Drug seeking was measured following 28 days of forced abstinence using a 90-min cued/test. RTqPCR was utilized to measure mir182 and mRNA targets related to neuroplasticity (wnt3, plppr4, pou3f3, tle4, cacna2d, and bdnf) from the nucleus accumbens. Data revealed that animals responded on a continuum for oxycodone. When divided into two groups termed high- and low responders, animals diverged during self-administration acquisition and maintained differences in behavior and gene expression throughout the study. mir182 was upregulated in the nucleus accumbens of both high and low responders and negatively correlated with tle4, which showed a strong negative correlation with reinstatement behavior. mRNA target levels were correlated with behaviors associated with increased severity of OUD behavior in male rats.

Validation of the extinction probe trial as a measure of motivation in male and female Long Evans rats.

Ethanol self-administration is governed by appetitive and consummatory behaviors. The sipper model procedurally separates these behaviors by training rats to meet a response requirement within 20 min to obtain continuous access to a sipper tube for an additional 20 min. Variations of this paradigm have been developed to quantify appetitive strength by evaluating lever presses during an extinction probe trial (EPT) or by deriving a break point (BP) from a progressive ratio (PR) schedule of reinforcement. However, no study has assessed the relationship between these tasks, within subjects, in both sexes. Male and female rats (n = 16) were trained to meet a response requirement of 20 to access a slightly sweetened ethanol solution (10% ethanol + 1% sucrose). Two EPTs, during which no operant behavior was reinforced, were interleaved between 18 reinforced sessions. Next, rats completed an across-session PR schedule, where the response requirement increased each session. BP was defined as the highest completed response requirement. We then replicated the methodology in the same subjects responding for a 3% sucrose solution. Finally, the experiment was replicated in a separate cohort of rats (n = 24) trained to a response requirement of 4 to earn access to the ethanol solution and paradigm order (EPT vs. PR) was counterbalanced. We report strong, positive correlations between average EPT lever presses and BP across all experiments. No sex differences were observed in appetitive behaviors. However, the two cohorts revealed mixed results when assessing sex differences in consummatory measures. This study further validates the EPT as a measure of motivation and suggests that similar levels of motivation exist to procure alcohol in males and females. The findings complement the literature showing that appetitive and consummatory processes are distinct and thus should be independently assessed in self-administration paradigms.

Cannabinoid type-1 receptor signaling in dopaminergic Engrailed-1 expressing neurons modulates motivation and depressive-like behavior.

The endocannabinoid system comprises highly versatile signaling functions within the nervous system. It is reported to modulate the release of several neurotransmitters, consequently affecting the activity of neuronal circuits. Investigations have highlighted its roles in numerous processes, including appetite-stimulating characteristics, particularly for palatable food. Moreover, endocannabinoids are shown to fine-tune dopamine-signaled processes governing motivated behavior. Specifically, it has been demonstrated that excitatory and inhibitory inputs controlled by the cannabinoid type 1 receptor (CB1) regulate dopaminergic neurons in the mesocorticolimbic pathway. In the present study, we show that mesencephalic dopaminergic (mesDA) neurons in the ventral tegmental area (VTA) express CB1, and we investigated the consequences of specific deletion of CB1 in cells expressing the transcription factor Engrailed-1 (En1). To this end, we validated a new genetic mouse line EN1-CB1-KO, which displays a CB1 knockout in mesDA neurons beginning from their differentiation, as a tool to elucidate the functional contribution of CB1 in mesDA neurons. We revealed that EN1-CB1-KO mice display a significantly increased immobility time and shortened latency to the first immobility in the forced swim test of adult mice. Moreover, the maximal effort exerted to obtain access to chocolate-flavored pellets was significantly reduced under a progressive ratio schedule. In contrast, these mice do not differ in motor skills, anhedonia- or anxiety-like behavior compared to wild-type littermates. Taken together, these findings suggest a depressive-like or despair behavior in an inevitable situation and a lack of motivation to seek palatable food in EN1-CB1-KO mice, leading us to propose that CB1 plays an important role in the physiological functions of mesDA neurons. In particular, our data suggest that CB1 directly modifies the mesocorticolimbic pathway implicated in depressive-like/despair behavior and motivation. In contrast, the nigrostriatal pathway controlling voluntary movement seems to be unaffected.

Co-targeting the kappa opioid receptor and dopamine transporter reduces motivation to self-administer cocaine and partially reverses dopamine system dysregulation

Cocaine disrupts dopamine (DA) and kappa opioid receptor (KOR) system activity, with long-term exposure reducing inhibiton of DA uptake by cocaine and increasing KOR system function. Single treatment therapies have not been successful for cocaine use disorder; therefore, this study focuses on a combination therapy targeting the dopamine transporter (DAT) and KOR. Sprague Dawley rats self-administered 5 days of cocaine (1.5 mg/kg/inf, max 40 inf/day, FR1), followed by 14 days on a progressive ratio (PR) schedule (0.19 mg/kg/infusion). Behavioral effects of individual and combined administration of phenmetrazine and nBNI were then examined using PR. Additionally, ex vivo fast scan cyclic voltammetry was then used to assess alterations in DA and KOR system activity in the nucleus accumbens before and after treatments. Chronic administration of phenmetrazine as well as the combination of phenmetrazine and nBNI—but not nBNI alone—significantly reduced PR breakpoints. In addition, the combination of phenmetrazine and nBNI partially reversed cocaine-induced neurodysregulations of the KOR and DA systems, indicating therapeutic benefits of targeting the DA and KOR systems in tandem. These data highlight the potential benefits of the DAT and KOR as dual-cellular targets to reduce motivation to administer cocaine and reverse cocaine-induced alterations of the DA system.

β2* nAChR sensitivity modulates acquisition of cocaine self-administration in male rats

Signaling through nicotinic acetylcholine receptors (nAChRs) plays a role in cocaine reward and reinforcement, suggesting that the cholinergic system could be manipulated with therapeutics to modulate aspects of cocaine use disorder (CUD). We examined the interaction between nAChRs and cocaine reinforcement by expressing a hypersensitive β2 nAChR subunit (β2Leu9′Ser) in the ventral tegmental area of male Sprague Dawley rats. Compared to control rats, β2Leu9′Ser rats acquired (fixed ratio) intravenous cocaine self-administration faster and with greater likelihood. By contrast, β2Leu9′Ser rats were approximately equivalent to controls in their intake of cocaine on a progressive ratio schedule of reinforcement, suggesting differential effects of cholinergic signaling depending on experimental parameters. Like progressive ratio cocaine SA, β2Leu9′Ser rats and controls did not differ significantly in food SA assays, including acquisition on a fixed ratio schedule or in progressive ratio sessions. These results highlight the specific role of high-affinity, heteropentameric β2* (β2-containing) nAChRs in acquisition of cocaine SA, suggesting that mesolimbic acetylcholine signaling is active during this process.

Assessment of the inbred C57BL/6 and outbred CD1 mouse strains using a progressive ratio schedule during development

Inbred strains have a genetic similarity of at least 98.6% compared to their outbred counterparts. Several studies have shown that inbred C57BL/6 mice and outbred ICR (CD1) mice differ in locomotion, cognitive flexibility, and aggression. However, their performance in operant paradigms is not well understood. A progressive ratio (PR) schedule of reinforcement is a method of quantitative estimation of the incentive state of an animal for a reward by increasing response requirements for reinforcer delivery, which is relevant to assess the breakpoint (amount of response effort an animal is willing to invest for a single unit of reward). This study tested male and female C57BL/6 and CD1 mice with an open field to analyze locomotion. Then, we used conditioning chambers with a PR3 schedule for ten consecutive days (P30-P40). PR performance was measured with the breakpoint, and the mathematical principles of reinforcement (MPR) were used to estimate motivation, impulsivity, and motor skills to manipulate the operandum. We found that CD1 mice showed higher locomotor activity than C57BL/6 independently of sex. CD1 mice had a higher breakpoint. However, male CD1 mice gradually increased breakpoint until the last session. In the MPR model, CD1 mice showed decreased fixed paused parameter (impulsivity) than C57BL/6, independent of sex. Our data suggest that the higher breakpoint in CD1 strain may partially be related to impulsivity. Therefore, the MPR model can help identify factors that affect performances, such as motivation, impulsivity, and motor skills during a PR in adolescent CD1 and C57BL/6 mice. These findings are essential to characterize the differences in the behavioral performance between C57BL/6 and CD1 strains and their potential as animal models.