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Ultrasonic vocalizations (USVs) have been established as an animal model of emotional status and are often utilized in drug abuse studies as motivational and emotional indices. Further USV functionality has been demonstrated in our recent work showing accurate identification of selectively-bred high versus low alcohol-consuming male rats ascertained exclusively from 22 to 28 kHz and 50–55 kHz FM USV acoustic parameters. With the hypothesis that alcohol-sensitive sex differences could be revealed through USV acoustic parameters, the present study examined USVs and alcohol consumption in male and female selectively bred high-alcohol drinking (HAD-1) rats. For the current study, we examined USV data collected during a 12-week experiment in male and female HAD-1 rats. Experimental phases included Baseline (2 weeks), 4-h EtOH Access (4 weeks), 24-h EtOH Access (4 weeks) and Abstinence (2 weeks). Findings showed that both male and female HAD-1 rats spontaneously emitted a large number of 22–28 kHz and 50–55 kHz FM USVs and that females drank significantly more alcohol compared to males over the entire course of the experiment. Analyses of USV acoustic characteristics (i.e. mean frequency, duration, bandwidth and power) revealed distinct sex-specific phenotypes in both 50–55 kHz FM and 22–28 kHz USV transmission that were modulated by ethanol exposure. Moreover, by using a linear combination of these acoustic characteristics, we were able to develop binomial logistic regression models able to discriminate between male and female HAD-1 rats with high accuracy. Together these results highlight unique emotional phenotypes in male and female HAD-1 rats that are differentially modulated by alcohol experience.

Excessive alcohol (ethanol) consumption negatively impacts social, emotional, as well as cognitive function and well-being. Thus, identifying behavioral and/or biological predictors of excessive ethanol consumption is important for developing prevention and treatment strategies against alcohol use disorders (AUDs). Sex differences in alcohol consumption patterns are observed in humans, primates, and rodents. Selectively bred high alcohol-drinking rat lines, such as the “HAD-1” lines are recognized animal models of alcoholism. The present work examined sex differences in alcohol consumption, object recognition, and exploratory behavior in male and female HAD-1 rats. Naive male and female HAD-1 rats were tested in an object recognition test (ORT) prior to a chronic 24 h intermittent ethanol access procedure for five weeks. Object recognition parameters measured included exploratory behavior, object investigation, and time spent near objects. During the initial training trial, rearing, active object investigation and amount of time spent in the object-containing section was significantly greater in female HAD-1 rats compared to their male counterparts. During the subsequent testing trial, time spent in the object-containing section was greater in female, compared to male, rats; but active object investigation and rearing did not statistically differ between females and males. In addition, female HAD-1 rats consumed significantly more ethanol than their male counterparts, replicating previous findings. Moreover, across all animals there was a significant positive correlation between exploratory behavior in ORT and ethanol consumption level. These results indicate there are significant sex differences in cognitive performance and alcohol consumption in HAD-1 rats, which suggests neurobiological differences as well.

Heightened emotional states increase impulsive behaviors such as excessive ethanol consumption in humans. Though positive and negative affective states in rodents can be monitored in real-time through ultrasonic vocalization (USV) emissions, few animal studies have focused on the role of emotional status as a stimulus for initial ethanol drinking. Our laboratory has recently developed reliable, high-speed analysis techniques to compile USV data during multiple-hour drinking sessions. Since High Alcohol Drinking (HAD-1) rats are selectively bred to voluntarily consume intoxicating levels of alcohol, we hypothesized that USVs emitted by HAD-1 rats would reveal unique emotional phenotypes predictive of alcohol intake and sensitive to alcohol experience. In this study, male HAD-1 rats had access to water, 15% and 30% EtOH or water only (i.e., Controls) during 8 weeks of daily 7-hr drinking-in-the-dark (DID) sessions. USVs, associated with both positive (i.e., 50–55 kHz frequency-modulated or FM) and negative (i.e., 22–28 kHz) emotional states, emitted during these daily DID sessions were examined. Findings showed basal 22–28 kHz USVs were emitted by both EtOH-Naïve (Control) and EtOH-experienced rats, alcohol experience enhanced 22–28 kHz USV emissions, and USV acoustic parameters (i.e., mean frequency in kHz) of both positive and negative USVs were significantly suppressed by chronic alcohol experience. These data suggest that negative affective status initiates and maintains excessive alcohol intake in selectively bred HAD-1 rats and support the notion that unprovoked emissions of negative affect-associated USVs (i.e., 22–28 kHz) predict vulnerability to excessive alcohol intake in distinct rodent models.

Background: Exposure to endocrine-disrupting chemicals (EDCs) during gestation influences development of the F1 generation offspring and can result in disease and dysfunction in adulthood. Limited evidence suggests consequences on the F2 generation, exposed as germ cells within the F1 fetus. These F2s provide a unique window into the programming effects of EDCs. Objective: This study assessed intergenerational effects of EDC exposure on adult physiology and behavior in Sprague-Dawley rats. Methods: Pregnant rats were exposed to either a polychlorinated biphenyl (PCB) mixture, Aroclor 1,221 (A1221), the fungicide vinclozolin (VIN), or the vehicle (VEH) (6% dimethylsulfoxide in sesame oil) alone. A1221 is weakly estrogenic, while VIN is antiandrogenic, enabling us to compare different classes of EDCs. The F1 male and female offspring were bred to generate the paternal- and maternal-lineage F2 generation. This F2 generation was assessed for physiological outcomes, ultrasonic vocalizations (USVs), and sexual behavior in adulthood. Results: Each EDC caused phenotypic effects in a sex- and lineage-dependent manner. The most robustly affected group was the paternal-lineage males. F2 VIN paternal male descendants had increased body weight throughout the lifespan, lower concentrations of circulating estradiol, and lower adrenal and testicular indices. Both VIN and A1221 paternal-lineage males also exhibited the greatest number of changes in the characteristics of USVs in response to an opposite-sex animal and changes in sexual behaviors in a mating test. Conclusion: Exposure of rats to EDCs at the germ cell stage led to differences in the physiological and behavioral phenotype later in life, especially in males. This finding has implications for multigenerational physiological and reproductive health in wildlife and humans. https://doi.org/10.1289/EHP3550

Background Emotional states are often thought to drive excessive alcohol intake and influence the development of alcohol use disorders. To gain insight into affective properties associated with excessive alcohol intake, we utilized ultrasonic vocalization (USV) detection and analyses to characterize the emotional phenotype of selectively bred alcohol-preferring (P) rats; an established animal model of excessive alcohol intake. USVs emitted by rodents have been convincingly associated with positive (50-55 kHz frequency-modulated [FM]) and negative (22-28 kHz) affective states. Therefore, we hypothesized that 50-55 and 22-28 kHz USV emission patterns in P rats would reveal a unique emotional phenotype sensitive to alcohol experience. Methods 50-55 kHz FM and 22-28 kHz USVs elicited from male P rats were assessed during access to water, 15 and 30% EtOH (v/v). Ethanol (EtOH; n = 12) or water only (Control; n = 4) across 8 weeks of daily drinking-in-the-dark (DID) sessions. Results Spontaneous 22-28 kHz USVs are emitted by alcohol-naive P rats and are enhanced by alcohol experience. During DID sessions when alcohol was not available (e.g., “EtOH OFF” intervals), significantly more 22-28 kHz than 50-55 kHz USVs were elicited, while significantly more 50-55 kHz FM than 22-28 kHz USVs were emitted when alcohol was available (e.g., “EtOH ON” intervals). In addition, USV acoustic property analyses revealed chronic effects of alcohol experience on 22-28 kHz USV mean frequency, indicative of lasting alcohol-mediated alterations to neural substrates underlying emotional response. Conclusions Our findings demonstrate that acute and chronic effects of alcohol exposure are reflected in changes in 22-28 and 50-55 kHz FM USV counts and acoustic patterns. These data support the notion that initiation and maintenance of alcohol intake in P rats may be due to a unique, alcohol-responsive emotional phenotype and further suggest that spontaneous 22-28 kHz USVs serve as behavioral markers for excessive drinking vulnerability.

Excessive alcohol consumption has a vast, negative impact on society. Rodent models have been successful in furthering our understanding of the biological underpinnings that drive alcohol consumption. Rodents emit ultrasonic vocalizations (USVs) that are each composed of several acoustic characteristics (e.g., frequency, duration, bandwidth, power). USVs reflect neurotransmitter activity in the ascending limb of the mesolimbic dopaminergic and cholinergic neurotransmitter systems and serve as noninvasive, real-time biomarkers of dopaminergic and cholinergic neurotransmission in the limbic system. In the present study, we recorded spontaneously emitted USVs from alcohol-naive Long-Evans (LE) rats and then measured their alcohol intake. We compared the USV acoustic characteristics and alcohol consumption data from these LE rats with previously published data from selectively bred high-alcohol (P and HAD-1) and low-alcohol (NP and LAD-1) drinking lines from studies with the same experimental method. Predictive analytic techniques were applied simultaneously to this combined data set and revealed that (a) USVs emitted by alcohol-naive rats accurately discriminated among high-alcohol consuming, LE, and low-alcohol consuming rat lines, and (b) future alcohol consumption in these same rat lines was reliably predicted from the USV data collected in an alcohol-naive state. To our knowledge, this is the first study to show that alcohol consumption is predicted directly from USV profiles of alcohol-naive rats. Because USV acoustic characteristics are sensitive to underlying neural activity, these findings suggest that baseline differences in mesolimbic cholinergic and dopaminergic tone could determine the propensity for future alcohol consumption in rodents.

Ultrasonic vocalizations (USVs) are mediated through specific dopaminergic and cholinergic neural pathways and serve as real-time measures of positive and negative emotional status in rodents. Although most USV studies focus primarily on USV counts, each USV possesses a number of characteristics shown to reflect activity in the associated neurotransmitter system. In the present study, we recorded spontaneously emitted USVs from alcohol-naive high alcohol drinking (HAD-1) and low alcohol drinking (LAD-1) rats. Using our recently developed WAAVES algorithm, we quantified four acoustic characteristics (mean frequency, duration, power, and bandwidth) from each 22–28 kHz and 50–55 kHz frequency-modulated (FM) USV. This rich USV representation allowed us to apply advanced statistical techniques to identify the USV acoustic characteristics that distinguished HAD-1 from LAD-1 rats. Linear mixed models (LMM) examined the predictability of each USV characteristic in isolation and linear discriminant analysis (LDA), and binomial logistic regression examined the predictability of linear combinations of the USV characteristics as a group. Results revealed significant differences in acoustic characteristics between HAD-1 and LAD-1 rats in both 22–28 kHz and 50–55 kHz FM USVs. In other words, these rats selectively bred for high- and low-alcohol consumption can be identified as HAD-1 or LAD-1 rats with high classification accuracy (approximately 92–100%) exclusively based on their emitted 22–28 kHz and 50–55 kHz FM USV acoustic characteristics. In addition, acoustic characteristics of 22–28 kHz and 50–55 kHz FM USVs emitted by alcohol-naive HAD-1 and LAD-1 rats significantly correlate with their future alcohol consumption. Our current findings provide novel evidence that USV acoustic characteristics can be used to discriminate between alcohol-naive HAD-1 and LAD-1 rats, and may serve as biomarkers in rodents with a predisposition for, or against, excessive alcohol intake.

Background Negative emotional status and adverse emotional events increase vulnerability to alcohol abuse. Ultrasonic vocalizations (USVs) emitted by rats are a well-established model of emotional status that can reflect positive or negative affective responses in real time. Most USV studies assess counts, yet each USV is a multidimensional data point characterized by several acoustic characteristics that may provide insight into the neurocircuitry underlying emotional response. Methods USVs emitted from selectively bred alcohol-naive and alcohol-experienced alcohol-preferring and nonpreferring rats (P and NP rats) were recorded during 4-hour sessions on alternating days over 4 weeks. Linear mixed modeling (LMM) and linear discriminant analysis (LDA) were applied to USV acoustic characteristics (e.g., frequency, duration, power, and bandwidth) of negative affect (22 to 28 kilohertz [kHz])- and positive (50 to 55 kHz) affect-related USVs. Results Hundred percent separation between alcohol-naive P and NP rats was achieved through a linear combination (produced by LDA) of USV acoustic characteristics of 22- to 28-kHz USVs, whereas poor separation (36.5%) was observed for 50- to 55-kHz USVs. 22- to 28-kHz LDA separation was high (87%) between alcohol-experienced P and NP rats, but was poor for 50- to 55-kHz USVs (57.3%). USV mean frequency and duration were the highest weighted characteristics in both the naive and experienced 22- to 28-kHz LDA representations suggesting that alcohol experience does not alter the representations. LMM analyses of 22- to 28-kHz USV acoustic characteristics matched the LDA results. Poor LDA separation was observed between alcohol-naive and alcohol-experienced P rats for both 22- to 28-kHz and 50- to 55-kHz USVs. Conclusions Advanced statistical analysis of negative affect-associated USV data predicts future behaviors of excessive alcohol drinking and alcohol avoidance in selectively bred rats. USV characteristics across rat lines reveal affect-related motivation to consume alcohol and may predict neural pathways mediating emotional response. Further characterization of these differences could delineate particular neurocircuitry and methods to ameliorate dysregulated emotional states often observed in human alcohol abusers.

There is a concern that hot environments enhance adverse effects of 3,4-methylenedioxymethamphetamine (MDMA or "Ecstasy"). In this study, long-term (4-weeks) daily MDMA self-administration sessions and an MDMA Challenge test were conducted with rats under normal and high thermal conditions (23° or 32°C). During MDMA self-administration sessions, activity and body temperature were increased by heat or MDMA experience, while MDMA self-administration rates increased with experience, but were comparable between thermal conditions. At the MDMA Challenge test (3.0 mg/kg, i.v.), in vivo microdialysis showed that nucleus accumbens serotonin (NAcc 5-HT) and dopamine (DA) responses were significantly increased in both thermal conditions. In the heated environment, MDMA-stimulated 5-HT responses and core temperature (but not DA) were significantly greater than at room temperature. Though the heated environment did not acutely boost MDMA intake, exaggerated NAcc 5-HT responses to MDMA may result in 5-HT depletion; a condition associated with Ecstasy use escalation and neural dysfunctions altering mood and cognition.

In drug dependence studies, rats are often tested daily with short breaks (such as weekends) spent untested in their home cages. Research on alcohol models has suggested that breaks from continuous testing can transiently enhance self-administration (termed the “alcohol deprivation effect”). The present study explored whether the salience of cocaine-access cues is increased after skipping weekend cocaine and cue exposures. Ultrasonic vocalizations (USVs) of the 50-kHz class are emitted by rats exposed to intravenous cocaine and have been shown to increase with repeated drug exposure at the same dose level (sensitization). The present study found that over the course of several weeks of cocaine self- or yoked-administration pre-drug cues signaling forthcoming access or delivery of cocaine elicited marked amounts of anticipatory 50-kHz USVs, and that weekend deprivation from cues and cocaine exaggerated further the level of calling (more calls on Mondays compared to Fridays). Anticipatory USVs extinguished less rapidly when weekend access to unreinforced cues was denied. The results may have clinical implications, in that intermittently avoiding cues or context may enhance drug cue salience and resistance to extinction.

Ultrasonic vocalization (USV) in the 50-kHz range occurs in rats immediately upon first-time exposure to cocaine or amphetamine, and rapidly increases with repetitive drug exposure at the same dose. This sensitized positive-affect response to these drugs of abuse is persistent in that the peak level of USVs again appears when the drug is reintroduced after several weeks of drug discontinuation. The present study explored whether with enough experience USVs might be elicited, and gradually escalate, in anticipation of impending drug delivery. Rats were trained to self-administer (SA) cocaine intravenously by lever pressing 5 days per week for 4 weeks. Yoked rats received experimenter-delivered cocaine matching that of SA rats. USVs and locomotor activity were recorded during each 10-min period prior to 60-min drug access sessions. Extinction trials in which drug access was denied were then carried out over an additional 4-week period. After about a week of cocaine experience, both the SA and Yoked groups began to progressively increase USVs when placed in an environment that predicted forthcoming drug exposure. Extinction of anticipatory calls and locomotion occurred over days after drug access ended. USVs may be a useful model for specifically investigating the neural basis of drug anticipation and aid in developing and assessing new addiction treatment strategies for reducing craving and relapse.

Short 50-kilohertz (kHz) range frequency-modulated ultrasonic vocalizations (USVs) produced by rats and mice are unconditionally elicited by drugs of abuse or electrical stimulation that increase dopamine activity in the nucleus accumbens, and it has been suggested that they reflect “positive affect” or incentive motivational states associated with appetitive behavior. The repeated administration of amphetamine is known to not only produce “psychomotor” sensitization, but also to facilitate a number of appetitive behaviors, including conditioned drug pursuit behavior. We were interested, therefore, in whether amphetamine-induced 50-kHz USVs would also increase with repeated drug exposure. USV recordings were made during 5-min sessions immediately after a saline infusion, and again 4-5 hours later, after 1.0 mg/kg intravenous amphetamine exposure. These sessions took place every other day over a 5-day period. A challenge dose of 1.0 mg/kg amphetamine was administered 2 weeks later to determine whether sensitization would persist. The initial amphetamine infusion increased 50-kHz USVs relative to the saline infusion. This effect was enhanced over trials and during the amphetamine challenge two weeks later. Classification of 50-kHz range call types revealed that complex frequency-modulated trill calls were sensitized by amphetamine, but not flat 50-kHz calls. It is possible that 50-kHz USV recordings could provide a potentially valuable behavioral measure of sensitization linked to enhanced incentive salience and increased tendency to self-administer drugs of abuse.

Cocaine is known to enhance nucleus accumbens dopamine (NAcc DA), to serve as a positive reinforcer and to produce negative effects, such as anxiety. The influence of diazepam on cocaine intake, cocaine-stimulated behavioral activity and NAcc DA was investigated using self-administration and experimenter-administered intravenous (i.v.) cocaine. In Experiment 1, rats were pretreated with diazepam (0.25 mg/kg) or saline (0.1 ml) 30 min prior to 20 daily 1-hour cocaine (0.75 mg/kg/injection) self-administration sessions. Cocaine intake increased for all animals across sessions, but was highest in diazepam-pretreated animals. Diazepam rats also self-administered their first cocaine injection of each session faster than controls. Experiment 2 utilized in vivo microdialysis to assess NAcc DA levels before and after experimenter-administered i.v. cocaine injections (0.75 mg/kg/injection x 2; 10-min interval) in diazepam- and saline-pretreated rats. Group differences were not revealed across basal and cocaine-stimulated NAcc DA assessments, indicating that diazepam did not decrease NAcc DA during cocaine self-administration. Findings that diazepam enhances cocaine self-administration and decreases cocaine response latency support the notion that cocaine-induced anxiety limits voluntary cocaine intake. It is further suggested that individual variations in cocaine-induced aversive effects may determine whether cocaine use is avoided or repeated.

MDMA (3,4-methylenedioxymethamphetamine), also known as ecstasy, is a popular drug often taken in environments rich in audio and visual stimulation, such as clubs and dance parties. The present experiments were conducted to test the notion that auditory stimulation influences the rewarding effects of MDMA. In Experiment 1, a conditioned place preference (CPP) procedure was conducted in which rats received MDMA (1.5 mg/kg, s.c.) in a distinctive environment accompanied by music (65–75 dB), white noise (70 dB), or no added sound. Animals were pretreated with saline on alternating days in an alternate environment. Results revealed CPP in animals exposed to white noise during MDMA trials. For Experiment 2, rats from Experiment 1 had access to operant levers that delivered intravenous MDMA (0.5 mg/kg/inj) or saline (0.1 ml) on alternate days in the presence or absence of the same types of auditory stimuli as previously experienced. After three each of MDMA and non-reinforced (saline) sessions, animals were tested for NAcc DA and 5-HT responses to MDMA (1.5 mg/kg) or saline under the same stimulus conditions. Findings revealed that NAcc DA and 5-HT increased after an MDMA injection, and both DA and 5-HT were significantly highest in animals exposed to music during the test session. These results indicate that paired sensorial stimuli can engage the same systems activated during drug use and enhance neurochemical and behavioral responses to MDMA administration.

Uncertainty and errors in predicting natural rewards influence associative learning and dopamine activity. The present study was conducted to determine the influence of cue-induced cocaine uncertainty, certainty and prediction error on nucleus accumbens dopamine (NAcc DA) in rats. For Certainty training, distinctive sensory cues were present during cocaine availability and alternate cues were paired with non-reinforced (saline) operant sessions. For Uncertainty training, all cues were equally associated with both cocaine and non-reinforcement. After training, animals self-administered cocaine or saline in the presence of conditioned cues while NAcc DA responses were assessed using in vivo microdialysis. Findings revealed cocaine-stimulated NAcc DA increased significantly less in Certainty — compared to Uncertainty-trained animals, and cocaine-paired cues in the absence of cocaine (Negative Prediction Error) resulted in a significant depression of baseline NAcc DA. These findings provide support for enhanced DA activity during cocaine uncertainty or the development of conditioned cocaine tolerance in subjects certain of a cocaine outcome.

Dopamine increases in the nucleus accumbens after ethanol administration in rats, but the contributions of the core and shell subregions to this response are unclear. The goal of this study was to determine the effect of various doses of intravenous (i.v.) ethanol infusions on dopamine in these two subregions of the nucleus accumbens. Male Long-Evans rats were infused with either acute i.v. ethanol (0.5, 1.0, 1.5 g/kg), repeated i.v. ethanol (four 1.0 g/kg infusions resulting in a cumulative dose of 4.0 g/kg), or saline as a control for each condition. Dopamine and ethanol were measured in dialysate samples from each experiment. The in vivo extraction fraction for ethanol of probes was determined using i.v. 4-methylpyrazole, and was used to estimate peak brain ethanol concentrations after the infusions. The peak brain ethanol concentrations after the 0.5, 1.0, and 1.5 g/kg ethanol infusions were estimated to be 20, 49, and 57 mM, respectively. A significant dopamine increase was observed for the 0.5 g/kg ethanol group when collapsed across subregions. However, both the 1.0 g/kg and 1.5 g/kg ethanol infusions produced significant increases in dopamine levels in the shell that were significantly higher than those in the core. An ethanol dose-response effect on dopamine in the shell was observed when saline controls, 0.5, 1.0, and 1.5 g/kg groups were compared. For the cumulative-dosing study, the first, second, and fourth infusions resulted in significant increases in dopamine in the shell. However, these responses were not significantly different from one another. The results of this study show that the shell has a stronger response than the core to intravenous ethanol, and that dopamine in the shell increases in a dose-dependent manner between 0.5 -1.0 g/kg doses, but that the response to higher ethanol doses reaches a plateau.

Reproductive function involves an interaction of three regulatory levels: hypothalamus, pituitary, and gonad. The primary drive upon this system comes from hypothalamic gonadotropin-releasing hormone (GnRH) neurosecretory cells, which receive afferent inputs from other neurotransmitter systems in the central nervous system to result in the proper coordination of reproduction and the environment. Here, we hypothesized that the recreational drug (±)-3,4-methylenedioxymethamphetamine (MDMA; ‘ecstasy’), which acts through several of the neurotransmitter systems that affect GnRH neurons, suppresses the hypothalamic-pituitary-gonadal reproductive axis of male rats. Adult male Sprague-Dawley rats self-administered saline or MDMA either once (acute) or for 20 days (chronic) and were euthanized 7 days following the last administration. We quantified hypothalamic GnRH mRNA, serum luteinizing hormone concentrations, and serum testosterone levels as indices of hypothalamic, pituitary, and gonadal functions, respectively. The results indicate that the hypothalamic and gonadal levels of the hypothalamic-pituitary-gonadal axis are significantly altered by MDMA, with GnRH mRNA and serum testosterone levels suppressed in rats administered MDMA compared to saline. Furthermore, our finding that hypothalamic GnRH mRNA levels are suppressed in the context of low testosterone concentrations suggests that the central GnRH neurosecretory system may be a primary target of inhibitory regulation by MDMA usage.

Upregulation of cyclin-dependent kinase 5 (Cdk5) after chronic cocaine administration has led to speculation that Cdk5 plays an important role in drug addiction. However, as Cdk5 involvement is implicated in a variety of neural events, including neuronal development, synaptic plasticity and learning, a specific role in drug abuse is yet to be determined. The present study utilized cocaine self-administration and food-reinforced operant procedures to assess possible relationships between cocaine intake, food-reinforced operant responding, behavioral activity, and Cdk5 levels in the nucleus accumbens (NAcc), ventral tegmental area (VTA), and prefrontal cortex (PFC) in rats. In Experiment 1, animals undergoing daily cocaine self-administration (1-hr/30 days) or food-reinforced operant sessions (20-min/30 days) showed significant between-group differences in operant responding and behavioral activity, but no significant differences in NAcc, VTA or PFC Cdk5 levels compared to a handled control group. In Experiment 2, animals that had self-administered cocaine in ten daily 1-hr sessions (Short-Access Cocaine) showed significantly greater NAcc Cdk5 expression compared to an Unhandled Control group, and no evidence of cocaine-induced behavioral sensitization. Animals given 4-hr daily access to cocaine over the same number of sessions (Long-Access Cocaine) showed significantly enhanced cocaine-reinforced responding and locomotor activation by the end of the sessions, but no significant differences in Cdk5 expression compared to Control animals. These findings suggest that overexpression of Cdk5 may be a transient adaptation to cocaine experience that subsides with increased cocaine exposure and does not correspond with measures of cocaineinduced behavioral sensitization.

Dopamine (DA) responses in the nucleus accumbens (NAcc) and dorsal striatum (DS) are commonly associated with different aspects of cocaine effects. Enhanced NAcc DA has been most convincingly linked with the positive reinforcing effects of cocaine, while DS DA is thought to mediate cocaine-induced motoric effects. Though several studies have shown NAcc DA enhancement following cocaine self-administration, very little work has examined the effects of cocaine self-administration on DS DA. In this study, DA levels in the NAcc and DS, and locomotor responses to a single self-administered cocaine injection (1.5mg/kg) were assessed in operant-trained, drug-naïve Sprague-Dawley rats. Locomotor activity, NAcc and DS DA levels increased significantly over baseline activity immediately after cocaine injection. However, while basal and cocaine-stimulated NAcc DA concentrations (nM) were significantly greater than DS DA levels, the magnitude of response was statistically comparable between brain regions. These findings indicate that, though both the NAcc and DS are importantly involved in the dopaminergic response to self-administered cocaine in drug-naïve rats, basal DA differences in dialysis data are obscured by statistical conversions to baseline percentages.

The involvement of the dopaminergic pathway from the ventral tegmental area (VTA) to the nucleus accumbens (NAcc) in the reinforcing properties of many drugs of abuse is well established. Though the prefrontal cortex (PFC) exhibits significant influence over activity in this pathway, its role in drug abuse is less defined. The present experiment investigated the impact of PFC D1 activity on cocaine self-administration (0.25, 0.75 mg/kg/inj) under progressive (PR) and fixed ratio (FR) schedules of reinforcement by assessing immediate and delayed effects of bilateral intra-PFC infusions of a D1 agonist (SKF 38393; 0.23 μg/side) and antagonist (SCH 23390; 0.25 μg/side). Immediately following infusion of dopaminergic agents or vehicle, no significant changes in self-administration occurred under any tested condition. However, 24 h after intra-PFC antagonist treatment, significantly lower PR breakpoints were observed for low (0.25 mg/kg), but not moderate (0.75 mg/kg) unit doses of self-administered cocaine. Locomotor activity levels during these assessments were unaffected by intra-PFC treatments. On an FR-3 schedule of reinforcement, the 0.25 cocaine unit dose elicited higher total cocaine intake and hyperlocomotor activation during a shorter session, but intra-PFC treatment had no significant effects on the number of reinforced responses or behavioral activity. The observation of decreased cocaine breakpoints after intra-PFC DA antagonist treatment reflects decrements in cocaine reinforcement efficacy. This finding corresponds temporally with previous work showing increased NAcc DA levels after similar treatment. Current findings demonstrate that transient changes in PFC DA neurotransmission can specifically influence reinforced behaviors without affecting overall behavioral activation.

This is a commentary on article Barker DJ. Making WAAVES in the vocalization community, but how big is the splash? J Neurosci Methods. 2014 Jan 15;221:228-9.

Cocaine reinforcement is strongly associated with increased nucleus accumbens dopamine (NAcc DA). The involvement of medial prefrontal cortex (mPFC) DA in cocaine reward is less defined, but substantial evidence indicates that increased mPFC DA may suppress NAcc DA levels. Using in vivo microdialysis, NAcc or mPFC DA was determined in cocaine-naive rats after a self-administered cocaine injection (3.0 mg/kg). Extracellular levels of NAcc DA were dramatically enhanced 10 min post-cocaine injection, but dropped significantly at each subsequent assessment. mPFC DA also increased significantly, but to a lesser extent than observed in the NAcc. Findings of prominent DA increases in both the NAcc and mPFC terminals during the test session indicate that NAcc DA responses do not appear to be inhibited by increased mPFC DA during cocaine self-administration.

The dopaminergic pathway from the ventral tegmental area (VTA) to the nucleus accumbens (NAcc) is well known to be involved in the reinforcing properties of many drugs of abuse. The medial prefrontal cortex (mPFC) has been shown to exhibit significant influence over activity in this pathway, and has also been implicated in drug abuse. The present experiment investigated the ability of D1 activity in the mPFC to influence accumbal dopamine levels. NAcc dopamine (DA) was monitored before, immediately after, and 24 h following mPFC infusion of a D1 agonist (SKF 38393), D1 antagonist (SCH 23390), or a vehicle solution. Immediately following infusion of dopaminergic agents or vehicle, no significant changes in accumbal DA were observed. However, 24 h following infusion of the antagonist but not the agonist, significant elevations of accumbal DA were observed. Since elevated NAcc DA was only observed 24 h after treatment, these results provide evidence that long-term neural adaptations can be induced by transient neuropharmacological treatment.

Adult rats often produce 50-kHz ultrasonic vocalizations (USVs), particularly the frequency-modulated varieties, in appetitive situations. These calls are thought by some to reflect positive affective states and the reinforcing value of drugs such as amphetamine and cocaine. The objective of this study was to determine whether the number of unconditioned 50-kHz USVs elicited by amphetamine predicts the development and/or magnitude of drug-conditioned motivation. In three experiments, we recorded USVs before and after injections of 1 mg/kg amphetamine (i.v. or i.p.) administered once per session. Rats were categorized as “high callers” or “low callers” according to individual differences in the number of 50-kHz USVs elicited by their first amphetamine injection. We examined the conditioned appetitive behavior and conditioned place preference (CPP) that emerged in high and low callers after repeated pairings of amphetamine with specific contexts. We also examined whether amphetamine-induced calling was affected by treatment within an unfamiliar (test chamber) versus familiar (home cage) context. Within an unfamiliar environment, the high callers consistently produced more amphetamine-induced 50-kHz USVs than the low callers. Compared to the low callers, high callers showed significantly greater amphetamine CPP as well as enhanced conditioned 50-kHz USVs and locomotor activity during anticipation of amphetamine. Individual differences were stable when amphetamine was administered in test chambers, but when it was administered in home cages, low callers showed an increase in 50-kHz calling that matched the high callers. These findings suggest that individual differences in drug-induced USVs can reveal environment-sensitive traits involved in drug-related appetitive motivation.

3,4-Methylenedioxymethamphetamine (MDMA, or “Ecstasy”) is a popular recreational drug. However, its exposure is often limited to the weekends in a highly stimulating environment. The goal of this study was to investigate the behavioral domains of working and recognition memory within a model of “weekend” Ecstasy use. Rats self-administered MDMA during 2-h sessions on two consecutive days followed by five drug-free days. Coupling this intermittent dosing schedule with a novel object recognition task, we found non-spatial memory impaired after only two “weekends” of self-administered MDMA. Our findings suggest that MDMA at recreational use levels can disrupt memory processes.

Subtypes of 50-kHz ultrasonic vocalizations (USVs) in rats are thought to reflect positive affect and occur with cocaine or amphetamine delivery. In contexts predicting forthcoming cocaine, pre-drug anticipatory USVs are initially minimal during daily sessions but gradually escalate over several weeks, presumably as the animal learns to expect and look forward to impending drug access. To gain more insight into motivational aspects of cocaine intake in animal models of drug dependence studies, it is important to compare experience-dependent changes in lever response rate, USVs, and locomotion during cocaine conditioning and extinction trials. To address whether cocaine-induced increases in lever responding and locomotor activity correspond with USV production. The study also determined whether short-term cocaine and context deprivation effects could be detected during conditioning or extinction. Rats underwent 20 days of 60-min sessions of self- or yoked administration of cocaine (0.75 mg/kg/infusion, i.v.), followed by 19 days of extinction training (8 weeks total, weekends off). Lever responding for cocaine and cocaine-induced locomotor activity increased across conditioning sessions. In contrast, the number of frequency modulated 50-kHz USVs evoked in response to cocaine infusion decreased with cocaine experience, suggesting perhaps tolerance to the rewarding properties of the drug. In addition, USVs but not lever pressing or locomotion are affected after brief periods of drug and/or drug context abstinence. Except for initial drug exposure, increased cocaine seeking during cocaine delivery could reflect either enhanced drug motivation or the development of drug tolerance, but not enhanced positive affect.

Animal models of reinforcement have proven to be useful for understanding the neurobiological mechanisms underlying drug addiction. Operant drug self-administration and conditioned place preference (CPP) procedures are expansively used in animal research to model various components of drug reinforcement, consumption, and addiction in humans. For this study, we used a novel approach to studying drug reinforcement in rats by combining traditional CPP and self-administration methodologies. We assembled an apparatus using two Med Associate operant chambers, sensory stimuli, and a Plexiglas-constructed neutral zone. These modifications allowed our experiments to encompass motivational aspects of drug intake through self-administration and drug-free assessment of drug/cue conditioning strength with the CPP test. In our experiments, rats self-administered cocaine (0.75 mg/kg/inj, i.v.) during either four (e.g., the "short-term") or eight (e.g., the "long-term") alternating-day sessions in an operant environment containing distinctive sensory cues (e.g., olfactory and visual). On the alternate days, in the other (differently-cued) operant environment, saline was available for self-infusion (0.1 ml, i.v.). Twenty-four hours after the last self-administration/cue-pairing session, a CPP test was conducted. Consistent with typical CPP findings, there was a significant preference for the chamber associated with cocaine self-administration. In addition, in animals undergoing the long-term experiment, a significant positive correlation between CPP magnitude and the number of cocaine-reinforced lever responses. In conclusion, this apparatus and approach is time and cost effective, can be used to examine a wide array of topics pertaining to drug abuse, and provides more flexibility in experimental design than CPP or self-administration methods alone.

Drug self-administration procedures are commonly used to study behavioral and neurochemical changes associated with human drug abuse, addiction and relapse. Various types of behavioral activity are commonly utilized as measures of drug motivation in animals. However, a crucial component of drug abuse relapse in abstinent cocaine users is "drug craving", which is difficult to model in animals, as it often occurs in the absence of overt behaviors. Yet, it is possible that a class of ultrasonic vocalizations (USVs) in rats may be a useful marker for affective responses to drug administration, drug anticipation and even drug craving. Rats vocalize in ultrasonic frequencies that serve as a communicatory function and express subjective emotional states. Several studies have shown that different call frequency ranges are associated with negative and positive emotional states. For instance, high frequency calls ("50-kHz") are associated with positive affect, whereas low frequency calls ("22-kHz") represent a negative emotional state. This article describes a procedure to assess rat USVs associated with daily cocaine self-administration. For this procedure, we utilized standard single-lever operant chambers housed within sound-attenuating boxes for cocaine self-administration sessions and utilized ultrasonic microphones, multi-channel recording hardware and specialized software programs to detect and analyze USVs. USVs measurements reflect emotionality of rats before, during and after drug availability and can be correlated with commonly assessed drug self-administration behavioral data such lever responses, inter-response intervals and locomotor activity. Since USVs can be assessed during intervals prior to drug availability (e.g., anticipatory USVs) and during drug extinction trials, changes in affect associated with drug anticipation and drug abstinence can also be determined. In addition, determining USV changes over the course of short- and long-term drug exposure can provide a more detailed interpretation of drug exposure effects on affective functioning.

Ventral tegmental area (VTA) stimulation produced conditioned place preferences for stimulation-paired environments the magnitudes of which were dose-dependently reduced by systemic application of the dopamine antagonist, haloperidol (0.0, 0.15, 0.3 mg/kg). Bilateral microinjections of cis-flupenthixol (FLU) into the nucleus accumbens (0.0, 1.0, 5.0 or 10.0 micrograms) also resulted in reductions in the size of stimulation-induced place preferences as well as reductions in the magnitude of the hyperlocomotor response to 1.5 mg/kg (s.c.) D-amphetamine. Comparable microinjections of FLU into the medial prefrontal cortex (PFC) produced diametrically opposite effects: the size of VTA stimulation-induced place preferences was either unaffected (1.0 and 5.0 microgram groups) or slightly increased (10 micrograms group) and amphetamine-stimulated hyperlocomotion was dose-dependently potentiated. These behavioral findings suggest a dopamine-mediated modulatory role for the PFC over reward relevant elements within the nucleus accumbens.

A Conditioned Place Preference test procedure [Ettenberg and Duvauchelle (13)] was used to investigate the effects of dopamine antagonist challenge on the rewarding properties of medial prefrontal cortex (MPFC) electrical stimulation. Rats exhibited strong preferences for the side of a two-compartment test apparatus in which they experienced sessions of experimenter-administered 0.5-s trains of MPFC sine-wave 60-Hz stimulation. Pretreatment with the neuroleptic dopamine antagonist drug, haloperidol (0.0, 0.15, or 0.3 mg/kg IP), resulted in a dose-dependent reduction in the magnitude of observed place preferences. Preference tests were conducted 24 hours after drug-conditioning trials and, hence, were not subject to motoric or other nonspecific actions of the neuroleptic treatments. In a control experiment, haloperidol did not block the place aversions produced by dorsomedial tegmental stimulation. Animals can, therefore, recall place-associations formed in the presence of haloperidol, a result which challenges “state-dependent learning” explanations of the drug's actions. Together, these results are consistent with the view that dopamine neurotransmission is involved in the rewarding consequences of electrical stimulation in the medial prefrontal cortex.

Hyperthermia and hyperlocomotor activity are commonly reported acute effects of high dose, experimenter-delivered 3,4-methylenedioxymethamphetamine (MDMA). The current investigation was performed to determine short- to long-term physiological and behavioral changes induced by moderate intake MDMA self-administration. In the present study, rats self-administered MDMA (approx. 2.0-7.0 mg/kg/day) across 20 days during daily 2-h operant sessions. Locomotor activity was assessed during MDMA self-administration sessions and core temperatures were recorded before and after each session. Findings of the first several sessions showed core temperatures significantly decreased after MDMA self-administration compared to baseline and to a control group that self-administered saline during operant sessions. As sessions proceeded, the MDMA-induced hypothermic response diminished and core temperatures normalized, then increased during the last few sessions. Also, locomotor activity during MDMA self-administration sessions was initially equivalent to saline level activity, but increased by day 8 to significantly greater levels. Our findings demonstrate experience-dependent changes after voluntary administration of MDMA that are clearly observable in temperature regulation and behavioral activity.

In the present study the effects of a neurotoxic regimen of (+)-MDMA (20 mg/kg × 4, s.c.) in 4- and 10-week-old C57Bl/6J mice during treatment and 7 days post-treatment were examined. Rectal temperatures monitored between (+)-MDMA injections (30 min post-injection/2 h intervals) revealed hyperthermic responses in both age groups, with the magnitude of the response significantly greater in older mice. Seven days post-treatment, immunoblot analyses of the vesicular monoamine transporter 2 (VMAT2), and tyrosine hydroxylase (TH) revealed significant reductions (−37 and −58%, respectively) in the older animals, but not in the younger group, compared to age-matched controls. Dopamine transporter (DAT) expression was significantly reduced in both 4- and 10-week-old animals (26 and 69.7%, respectively). (+)-MDMA-treated animals also exhibited significantly lower levels of striatal dopamine, and 3,4-dihydroxyphenylacetic acid than controls, again the effect being more pronounced in the older animals. Although both age groups showed evidence of (+)-MDMA-induced toxicity, our data revealed that older animals exhibited a greater hyperthermic response to (+)-MDMA and were also are more susceptible to subsequent dopaminergic damage than the younger animals

The selective serotonergic neurotoxicity of 3,4-methylenedioxyamphetamine (MDA) and 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) depends on their systemic metabolism. We have recently shown that inhibition of brain endothelial cell gamma-glutamyl transpeptidase (gamma-GT) potentiates the neurotoxicity of both MDMA and MDA, indicating that metabolites that are substrates for this enzyme contribute to the neurotoxicity. Consistent with this view, glutathione (GSH) and N-acetylcysteine conjugates of alpha-methyl dopamine (alpha-MeDA) are selective neurotoxicants. However, neurotoxic metabolites of MDMA or MDA have yet to be identified in brain. Using in vivo microdialysis coupled to liquid chromatography-tandem mass spectroscopy and a high-performance liquid chromatography-coulometric electrode array system, we now show that GSH and N-acetylcysteine conjugates of N-methyl-alpha-MeDA are present in the striatum of rats administered MDMA by subcutaneous injection. Moreover, inhibition of gamma-GT with acivicin increases the concentration of GSH and N-acetylcysteine conjugates of N-methyl-alpha-MeDA in brain dialysate, and there is a direct correlation between the concentrations of metabolites in dialysate and the extent of neurotoxicity, measured by decreases in serotonin (5-HT) and 5-hydroxyindole acetic (5-HIAA) levels. Importantly, the effects of acivicin are independent of MDMA-induced hyperthermia, since acivicin-mediated potentiation of MDMA neurotoxicity occurs in the context of acivicin-mediated decreases in body temperature. Finally, we have synthesized 5-(N-acetylcystein-S-yl)-N-methyl-alpha-MeDA and established that it is a relatively potent serotonergic neurotoxicant. Together, the data support the contention that MDMA-mediated serotonergic neurotoxicity is mediated by the systemic formation of GSH and N-acetylcysteine conjugates of N-methyl-alpha-MeDA (and alpha-MeDA). The mechanisms by which such metabolites access the brain and produce selective serotonergic neurotoxicity remain to be determined.

The nucleus accumbens, a major component of the ventral striatum, and the dorsal striatum are primary targets of the mesolimbic dopamine pathway, which is a pathway that plays a critical role in reward and addiction. The shell compartment of the nucleus accumbens and the ventromedial striatum, in particular, receive extensive afferent projections from the ventral tegmental area, which is the major afferent source of the mesolimbic pathway [Prog Brain Res 99 (1993) 209; J Neurosci 7 (1987) 3915]. The present study focused on striatal cholinergic interneurons as potential key neurons involved in the neural basis of drug reinforcement. The main finding of this study is that cholinergic interneurons located in the shell compartment of the nucleus accumbens and the ventromedial striatum were activated, as measured by Fos labeling, following a 1 h session of the self-administration of cocaine in rats. A direct correlation existed between the percent of cholinergic interneurons that were activated and the amount of cocaine that was self-administered. The greatest amount of administered cocaine (approximately 10 mg/kg) resulted in the activation of approximately 80% of the cholinergic neurons. No such correlation existed in the group of animals that self-administered saline. In addition, activation was not found in the core compartment of the nucleus accumbens or the dorsolateral striatum, which receive extensive innervation from the substantia nigra and thus are more closely tied to the motor effects of the drug. In conclusion, cocaine-driven neuronal activation was specific to the shell compartment of the nucleus accumbens ( R 2 =0.9365) and the ventromedial striatum ( R 2 =0.9059). These findings demonstrate that cholinergic interneurons are involved in the initial stage of cocaine intake and that these neurons are located in areas of the nucleus accumbens and dorsal striatum that are more closely tied to the rewarding and hedonic effects rather than the motor effects of cocaine intake.

Evidence suggests that ethanol (EtOH) preexposure influences the rewarding valence of subsequent EtOH use. This study was conducted to determine if EtOH preexposure through EtOH/cocaine self-administration facilitates the motivational effects of EtOH alone. Rats self-administered intravenous (iv) EtOH/cocaine combinations (EtOH/Cocaine Fading group; EtOH 125.0 mg/kg/inj+Cocaine 0.1-0.75 mg/kg/inj) during a preexposure period. Consequently, these rats self-administered intravenous EtOH alone (62.5, 125.0, 250.0 and 500.0 mg/kg/inj) significantly more than a control group with prior cocaine self-administration experience (0.1-0.75 mg/kg/inj). In addition, at equal EtOH intake levels, locomotor activity was significantly enhanced in the EtOH/Cocaine Fading group but not the Cocaine Control animals (P=.01). The amount of EtOH self-administered in the EtOH/Cocaine Fading group during 1-h sessions (approximately 0.5-2.0 g/kg) corresponded with blood alcohol levels (BAL) ranging from 44 to 221 mg/dl. The highest BALs reported here have not previously been demonstrated after voluntary EtOH intake through any route of administration. These data suggest that preexposure to EtOH during EtOH/cocaine self-administration sessions modified neural substrates underlying both the reinforcing and locomotor responses to EtOH alone. Further studies utilizing intravenous EtOH self-administration will allow identification of various long-term behavioral and neural consequences of voluntary high EtOH intake.

In vivo microdialysis, behavioral activity assessments, and a conditioned place preference (CPP) test were used to investigate dopaminergic correlates of cocaine-conditioned behaviors. Over 12 days, rats were given either intravenous cocaine (4.2 mg/kg) or saline (6 cocaine and 6 saline infusions) daily in distinctively different environments. The following day, rats were tested in the cocaine- and saline-paired environments; 48 hr later, CPP was determined. The cocaine-associated environment elicited greater nucleus accumbens dopamine (NAcc DA) levels, hyperactivity, and place preference, though the emergence of DA increases was not in synchrony with peak behavioral activation. Although conditioned behavioral effects after repeated cocaine are well documented, direct evidence of increased NAcc DA in response to a cocaine-paired environment has not been previously reported. Discrepancies with previous work are attributed to a number of methodological differences.

In two conditioning experiments, identical procedures (previously shown to produce place preferences for a cocaine-paired environment) were used to assess dopaminergic and behavioral activity correlates of cocaine reward conditioning and sensitization. In these experiments, animals received repeated injections of intravenous cocaine (4.2 mg/kg×6) or saline (0.2 ml×6) on alternating days. One group in each of these experiments (‘Cocaine Cues’) occupied a consistent distinctive environment during cocaine treatments and testing sessions. For the other conditioned group (‘Novel’), all procedures were the same, except that the last cocaine injection was administered while animals were occupying a novel environment. During day 1 and day 6 of the cocaine treatment, behavioral activity was assessed in experiment 1 and in vivo microdialysis procedures were conducted in experiment 2. Over the course of the conditioning sessions, cocaine-induced behavioral activity (locomotion and rearing) increased significantly in the Cocaine Cues group, but not in the Novel group. In addition, cocaine-induced increases in NAcc dopamine levels were significantly greater when cocaine-experienced animals were tested in a cocaine-paired environment compared to equally experienced and cocaine-naive animals tested in a novel environment. Context-dependent behavioral sensitization is a well-documented phenomenon. The observation of a corresponding enhancement of dopamine efflux in lieu of a lengthy withdrawal period is uncommon, but can be attributed to methodological differences across studies. The present study uniquely demonstrates concurrent context-dependent potentiation of behavioral and dopaminergic responses to cocaine occurring in conjunction with cocaine reward.

A reward-relevant relationship between dopamine projection regions of the ventral tegmental area (VTA) was investigated through the use of brain stimulation reward (BSR) thresholds. Using a rate-free method, changes in VTA BSR thresholds were determined after intracranial injections of the dopamine D1 antagonist, SCH 23390 into the prefrontal cortex (PFC), or the nucleus accumbens (NAcc). Reward thresholds assessed immediately after the infusion of SCH 23390 into the NAcc (0.5 microgram/0.5 microliter/side) were significantly higher than those assessed just after saline infusions, indicating a drug-induced attenuation of the rewarding effects of the brain stimulation. The effects of this dose subsided when tested 24 h later. Conversely, intra-PFC infusions of SCH 23390 at the same dose (0.5 microgram/0.5 microliter/side) resulted in lowered BSR thresholds when rats were tested immediately after infusion. In addition, animals tested 24 h after receiving the lowest dose (0.125 microgram/0.5 microliter/side) demonstrated a robust delayed threshold-lowering effect. These immediate and delayed effects of the intra-PFC dopamine antagonist demonstrate a facilitation of VTA BSR and are consistent with the view that PFC dopamine serves a modulatory role over important reward elements within the NAcc. The deferred effects of intra-prefrontal cortex DA receptor blockade on brain stimulation reward thresholds may reflect adaptive responses of subcortical structures to changes in PFC dopamine neurotransmission. It has been suggested that neural adjustments of this type may underlie long term changes in central nervous system functioning brought about by disease, drug use or behavioral conditioning.

The relative reinforcing value of cocaine/heroin combination ("speedball") was compared in the rat using a progressive-ratio (PR) reinforcement schedule. The initial training for all rats was a combined dose of 18 microg/kg/inj of heroin (H) plus 300 microg/kg/inj of cocaine (C). Break points for the training dose and individual component doses were determined for half and double the training dose. Of the three doses of each treatment, only C yielded the expected monotonic increase in break point as a function of dose. Also, break points for C (300 and 600 microg/kg/inj) was greater than for the combination of C and H (18 H/300 C and 36 H/600 C microg/kg/inj), suggesting a greater reward value for C alone. The doses for these three drug treatments that produced saline level break points were then determined. At these lower doses, significant break points were obtained with the H/C combination at which the respective doses of H or C had break points identical to those of saline. These lower dose results indicate that the combination is clearly synergistic and that the discrepancy with doses at the opposite end of the dose response curve suggest that the PR schedule is vulnerable to drug-induced motor effects.

The role of dopamine neurotransmission in opioid reward was investigated using a rate-independent measure for determining brain stimulation reward (BSR) thresholds. Intra-accumbens infusions of the mu- and delta-specific peptides, D-Ala2, N-Me-Phe4, Gly-ol5-Enkephalin and D-Pen2, D-Pen5-Enkephalin caused significant lowering of BSR thresholds. The dopamine D1/D2 antagonist, cis-flupenthixol, blocked these effects at a dose that did not significantly alter thresholds when given alone. These data suggest both mu- and delta-opioid potentiation of BSR is dopamine dependent.

A rate-free method of determining brain-stimulation reward thresholds was used to identify the rewarding effects of the δ-opioid receptor and μ-opioid receptor agonist peptides, [ d -Pen 2 , d -Pen 5 ]enkephalin (DPDPE) and [ d -Ala 2 -MePhe 4 -Gly(ol) 5 ]enkephalin (DAMGO). The nucleus accumbens-delivered opioid receptor agonists produced marked lowering of the threshold for ventral tegmental area brain-stimulation reward. No change in baseline thresholds was seen after peripheral administration of the nonpeptide δ-opioid receptor antagonist, naltrindole. However, an unexpected finding was that naltrindole blocked the threshold-lowering effects of both DPDPE and DAMGO. These data demonstrate nucleus accumbens activation of δ- and μ-opioid receptors and ventral tegmental area brain-stimulation reward share common brain substrates. In addition, the interference of both δ- and μ-opioid receptor mediated reward by naltrindole may have implications for therapeutic use.

This report describes a simple surgical method of exiting an implanted intravenous catheter through the skin via a nonsutured site. The preparation, which makes use of a biopsy punch, avoids placing the catheter exiting point through the incision used for subcutaneously fixing the catheter platform on the back of the animal.

Electrical stimulation in the vicinity of the cell bodies of the locus coeruleus (LC) has been shown to support self-stimulation behaviors in rats. In the present study, a Conditioned Place Test, sensitive to both rewarding and aversive qualities of brain stimulation, was employed to determine (a) whether rewarding locus coeruleus stimulation would result in place preferences and (b) if so, whether dopamine receptor antagonism would affect the development of such place preferences. Animals were pretreated with pimozide (0.0, 0.5 or 1.0 mg/kg) prior to exposure to two distinctive environments only one of which was paired with locus coeruleus stimulation. Rats that received vehicle injections prior to stimulation/place pairings developed strong preferences for the stimulation-paired environment while those animals pretreated with 0.5 mg/kg pimozide showed no reliable shift in preference from baseline performance. Additionally, animals injected with the 1.0 mg/kg dose of pimozide exhibited mild place aversions to the stimulation-paired environment. It is hypothesized that dopamine neurotransmission is important for the rewarding effects of locus coeruleus stimulation without which such stimulation appears to be aversive.