Differential concentration-response curves for oral ethanol self-administration in C57BL/6J and BALB/cJ mice

Shortening time for access to alcohol drives up front-loading behavior, bringing consumption in male rats to the level of females

Background

Incentives to promote drinking (“happy hour”) can encourage faster rates of alcohol consumption, especially in women. Sex differences in drinking dynamics may underlie differential health vulnerabilities relating to alcohol in women versus men. Herein, we used operant procedures to model the happy hour effect and gain insight into the alcohol drinking dynamics of male and female rats.

Methods

Adult male and female Wistar rats underwent operant training to promote voluntary drinking of 10% (w/v) alcohol (8 rats/sex). We tested how drinking patterns changed after manipulating the effort required for alcohol (fixed ratio, FR), as well as the length of time in which rats had access to alcohol (self-administration session length). Rats were tested twice within the 12 h of the dark cycle, first at 2 h (early phase of the dark cycle, “early sessions”) and then again at 10 h into the dark cycle (late phase of the dark cycle, “late sessions”) with an 8-h break between the two sessions in the home cage.

Results

Adult females consumed significantly more alcohol (g/kg) than males in the 30-min sessions with the FR1 schedule of reinforcement when tested late in the dark cycle. Front-loading of alcohol was the primary factor driving higher consumption in females. Changing the schedule of reinforcement from FR1 to FR3 reduced total consumption. Notably, this manipulation had minimal effect on front-loading behavior in females, whereas front-loading behavior was significantly reduced in males when more effort was required to access alcohol. Compressing drinking access to 15 min to model a happy hour drove up front-loading behavior, generating alcohol drinking patterns in males that were similar to patterns in females (faster drinking and higher intake).

Conclusions

This strategy could be useful for exploring sex differences in the neural mechanisms underlying alcohol drinking and related health vulnerabilities. Our findings also highlight the importance of the time of testing for detecting sex differences in drinking behavior.

Voluntary alcohol drinking is higher in adult female rats compared to adult male rats. This sex difference is most pronounced in the later phase of the dark cycle, and when the operant effort is minimal (when 1 lever press gives 1 reward: fixed ratio 1, FR1).

Higher alcohol intake in females is primarily due to “front-loading”, or the rapid consumption of alcohol within the first 5 min of access.

Increasing the effort required to obtain alcohol from FR1 to FR3 dampens front-loading drinking behavior, resulting in similar levels of total intake in males and females.

Compressing the time of access to 15 min drives up front-loading to such a degree that rats end up consuming more alcohol in total than they do in 30-min sessions. In males, this increase in drinking is large enough that it eliminates the sex difference in total alcohol intake.

Genetic Relationships Between Ethanol-Induced Conditioned Place Aversion and Other Ethanol Phenotypes in 15 Inbred Mouse Strains

The genetic relationships between different behaviors used to index the aversive effects of ethanol are unknown. To address this issue, ethanol-induced conditioned place aversion (CPA) was tested in a genetically diverse panel of 15 inbred mouse strains. Mice were exposed to an unbiased place conditioning procedure using ethanol doses of 0, 2, or 4 g/kg; all injections were given immediately after 5-min exposure to distinctive tactile cues. There were dose-dependent effects of ethanol on CPA and on the change in pre-injection activity rates between the first and last conditioning trials. Most strains (80%) developed CPA, demonstrating the generalizability of this behavior. Moreover, genotype had significant effects on CPA magnitude and locomotor activity rates. Strain means from this study and previously published studies were then used to examine genetic correlations. These analyses showed significant genetic correlations between CPA and ethanol intake/preference, conditioned taste aversion, and drug withdrawal (but not blood ethanol concentration or conditioned place preference), supporting the idea of commonality in the genes underlying CPA and each of these behaviors. The overall pattern of findings is consistent with previous data suggesting that genetic differences in sensitivity to ethanol's aversive effects play a role in determining strain differences in ethanol drinking. The broader implication is that individuals who are more sensitive to the aversive effects of ethanol may be protected from developing the excessive drinking behaviors characteristic of alcohol use disorders.

Mouse strain differences in punished ethanol self-administration

Determining the neural factors contributing to compulsive behaviors such as alcohol-use disorders (AUDs) has become a significant focus of current preclinical research. Comparison of phenotypic differences across genetically distinct mouse strains provides one approach to identify molecular and genetic factors contributing to compulsive-like behaviors. Here we examine a rodent assay for punished ethanol self-administration in four widely used inbred strains known to differ on ethanol-related behaviors: C57BL/6J (B6), DBA/2J (D2), 129S1/SvImJ (S1), and BALB/cJ (BALB). Mice were trained in an operant task (FR1) to reliably lever-press for 10% ethanol using a sucrose-fading procedure. Once trained, mice received a punishment session in which lever pressing resulted in alternating ethanol reward and footshock, followed by tests to probe the effects of punishment on ethanol self-administration. Results indicated significant strain differences in training performance and punished attenuation of ethanol self-administration. S1 and BALB showed robust attenuation of ethanol self-administration after punishment, whereas behavior in B6 was attenuated only when the punishment and probe tests were conducted in the same contexts. By contrast, D2 were insensitive to punishment regardless of context, despite receiving more shocks during punishment and exhibiting normal footshock reactivity. Additionally, B6, but not D2, reduced operant self-administration when ethanol was devalued with a bitter tastant. B6 and D2 showed devaluation of sucrose self-administration, and punished suppression of sucrose seeking was context dependent in both the strains. While previous studies have demonstrated avoidance of ethanol in D2, particularly when ethanol is orally available from a bottle, current findings suggest this strain may exhibit heightened compulsive-like self-administration of ethanol, although there are credible alternative explanations for the phenotype of this strain. In sum, these findings offer a foundation for future studies examining the neural and genetic factors underlying AUDs.

Strain commonalities and differences in response-outcome decision making in mice

The ability to select between actions that are more vs. less likely to be reinforced is necessary for survival and navigation of a changing environment. A task termed "response-outcome contingency degradation" can be used in the laboratory to determine whether rodents behave according to such goal-directed response strategies. In one iteration of this task, rodents are trained to perform two food-reinforced behaviors, then the predictive relationship between one instrumental response and the associated outcome is modified by providing the reinforcer associated with that response non-contingently. During a subsequent probe test, animals can select between the two trained responses. Preferential engagement of the behavior most likely to be reinforced is considered goal-directed, while non-selective responding is considered a failure in response-outcome conditioning, or "habitual." This test has largely been used with rats, and less so with mice. Here we compiled data collected from several cohorts of mice tested in our lab between 2012 and 2015. Mice were bred on either a C57BL/6 or predominantly BALB/c strain background. We report that both strains of mice can use information acquired as a result of instrumental contingency degradation training to select amongst multiple response options the response most likely to be reinforced. Mice differ, however, during the training sessions when the familiar response-outcome contingency is being violated. BALB/c mice readily generate perseverative or habit-like response strategies when the only available response is unlikely to be reinforced, while C57BL/6 mice more readily inhibit responding. These findings provide evidence of strain differences in response strategies when an anticipated reinforcer is unlikely to be delivered.

Previous ethanol experience enhances synaptic plasticity of NMDA receptors in the ventral tegmental area

Alcohol addiction (alcoholism) is one of the most prevalent substance abuse disorders worldwide. Addiction is thought to arise, in part, from a maladaptive learning process in which enduring memories of drug experiences are formed. However, alcohol (ethanol) generally interferes with synaptic plasticity mechanisms in the CNS and thus impairs various types of learning and memory. Therefore, it is unclear how powerful memories associated with alcohol experience are formed during the development of alcoholism. Here, using brain slice electrophysiology in mice, we show that repeated in vivo ethanol exposure (2 g/kg, i.p., three times daily for 7 d) causes increased susceptibility to the induction of long-term potentiation (LTP) of NMDA receptor (NMDAR)-mediated transmission in mesolimbic dopamine neurons, a form of synaptic plasticity that may drive the learning of stimuli associated with rewards, including drugs of abuse. Enhancement of NMDAR plasticity results from an increase in the potency of inositol 1,4,5-trisphosphate (IP(3)) in producing facilitation of action potential-evoked Ca(2+) signals, which is critical for LTP induction. This increase in IP(3) effect, which lasts for a week but not a month after ethanol withdrawal, occurs through a protein kinase A (PKA)-dependent mechanism. Corticotropin-releasing factor, a stress-related neuropeptide implicated in alcoholism and other addictions, further amplifies the PKA-mediated increase in IP(3) effect in ethanol-treated mice. Finally, we found that ethanol-treated mice display enhanced place conditioning induced by the psychostimulant cocaine. These data suggest that repeated ethanol experience may promote the formation of drug-associated memories by enhancing synaptic plasticity of NMDARs in dopamine neurons.

The influence of selection for ethanol withdrawal severity on traits associated with ethanol self-administration and reinforcement

BACKGROUND

Several meta-analyses indicate that there is an inverse genetic correlation between ethanol preference drinking and ethanol withdrawal severity, but limited work has characterized ethanol consumption in 1 genetic animal model, the Withdrawal Seizure-Prone (WSP) and-Resistant (WSR) mouse lines selected for severe or mild ethanol withdrawal, respectively.

METHODS

We determined whether line differences existed in: (i) operant self-administration of ethanol during sucrose fading and under different schedules of reinforcement, followed by extinction and reinstatement of responding with conditioned cues and (ii) home cage drinking of sweetened ethanol and the development of an alcohol deprivation effect (ADE).

RESULTS

Withdrawal Seizure-Prone-1 mice consumed more ethanol than WSR-1 mice under a fixed ratio (FR)-4 schedule as ethanol was faded into the sucrose solution, but this line difference dissipated as the sucrose was faded out to yield an unadulterated 10% v/v ethanol solution. In contrast, WSR-1 mice consumed more ethanol than WSP-1 mice when a schedule was imposed that procedurally separated appetitive and consummatory behaviors. After both lines achieved the extinction criterion, reinstatement was serially evaluated following oral ethanol priming, light cue presentation, and a combination of the 2 cues. The light cue produced maximal reinstatement of responding in WSP-1 mice, whereas the combined cue was required to produce maximal reinstatement of responding in WSR-1 mice. There was no line difference in the home cage consumption of a sweetened ethanol solution over a period of 1 month. Following a 2-week period of abstinence, neither line developed an ADE.

CONCLUSIONS

Although some line differences in ethanol self-administration and reinstatement were identified between WSP-1 and WSR-1 mice, the absence of consistent divergence suggests that the genes underlying these behaviors do not reliably overlap with those that govern withdrawal severity.

Ethanol drinking in rodents: is free-choice drinking related to the reinforcing effects of ethanol?

Many studies have used voluntary ethanol consumption by animals to assess the influence of genetic and environmental manipulations on ethanol drinking. However, the relationship between home cage ethanol consumption and more formal assessments of ethanol-reinforced behavior using operant and instrumental conditioning procedures is not always clear. The present review attempted to evaluate whether there are consistent correlations between mouse and rat home cage ethanol drinking on the one hand, and either operant oral self-administration (OSA), conditioned taste aversion (CTA), or conditioned place preference (CPP) with ethanol on the other. We also review literature on intravenous ethanol self-administration (IVSA). To collect data, we evaluated a range of genetic manipulations that can change both genes and ethanol drinking behavior including selective breeding, transgenic and knockout models, and inbred and recombinant inbred strain panels. For a genetic model to be included in the analysis, there had to be published data resulting in differences on home cage drinking and data for at least one of the other behavioral measures. A consistent, positive correlation was observed between ethanol drinking and OSA, suggesting that instrumental behavior is closely genetically related to consummatory and ingestive behavior directed at ethanol. A negative correlation was observed between CTA and drinking, suggesting that ethanol's aversive actions may limit oral consumption of ethanol. A more modest, positive relationship was observed between drinking and CPP, and there were not enough studies available to determine a relationship with IVSA. That some consistent outcomes were observed between widely disparate behavioral procedures and genetic populations may increase confidence in the validity of findings from these assays. These findings may also have important implications when researchers decide which phenotypes to use in measuring alcohol-reward relevant behaviors in novel animal models.

Increased Drinking During Withdrawal From Intermittent Ethanol Exposure Is Blocked by the CRF Receptor Antagonist d-Phe-CRF(12?41)

BACKGROUND

Studies in rodents have determined that intermittent exposure to alcohol vapor can increase subsequent ethanol self-administration, measured with operant and 2-bottle choice procedures. Two key procedural factors in demonstrating increased alcohol intake are the establishment of stable alcohol self-administration before alcohol vapor exposure and the number of bouts of intermittent vapor exposure. The present studies provide additional behavioral validation and initial pharmacological validation of this withdrawal-associated drinking procedure.

METHODS

Studies at 2 different sites (Portland and Scripps) examined the effect of intermittent ethanol vapor exposure (3 cycles of 16 hours of ethanol vapor+8 hours air) on 2-hour limited access ethanol preference drinking in male C57BL/6 mice. Separate studies tested 10 or 15% (v/v) ethanol concentrations, and measured intake during the circadian dark. In one study, before measuring ethanol intake after the second bout of intermittent vapor exposure, mice were tested for handling-induced convulsions (HICs) indicative of physical dependence on ethanol. In a second study, the effect of bilateral infusions of the corticotropin-releasing factor (CRF) receptor antagonist D-Phe-CRF(12-41) (0.25 microg/0.5 microL) into the central nucleus of the amygdala (CeA) on ethanol intake was compared in vapor-exposed animals and air controls.

RESULTS

Intermittent ethanol vapor exposure significantly increased ethanol intake by 30 to 40%, and the mice had higher blood ethanol concentrations than controls. Intra-amygdala infusions of D-Phe-CRF(12-41) significantly decreased the withdrawal-associated increase in ethanol intake without altering ethanol consumption in controls. Following the second bout of intermittent vapor exposure, mice exhibited an increase in HICs, when compared with their own baseline scores or the air controls.

CONCLUSIONS

Intermittent alcohol vapor exposure significantly increased alcohol intake and produced signs of physical dependence. Initial pharmacological studies suggest that manipulation of the CRF system in the CeA can block this increased alcohol intake.

Evidence for the role of dopamine D3 receptors in oral operant alcohol self-administration and reinstatement of alcohol-seeking behavior in mice

The present study examined the effects of the acute intraperitoneal (i.p.) administration of the selective dopamine (DA) D(3) receptor antagonist SB-277011A (10, 20 or 30 mg/kg i.p.) on the oral operant self-administration of alcohol in male C57BL/6N mice. These effects were compared with those of naltrexone (0.5, 1 and 2 mg/kg i.p.) and acamprosate (100, 200 and 400 mg/kg i.p.). Compared with vehicle, the acute administration of SB-277011A (10 or 20 mg/kg) did not significantly alter the operant self-administration of alcohol, whereas the 30 mg/kg dose significantly reduced alcohol intake (g/kg), the number of reinforcers, and the number of active lever presses. The oral self-administration of alcohol was not significantly altered by the acute administration of either naltrexone or acamprosate, compared with vehicle-treated mice. SB-277011A, naltrexone and acamprosate were also tested in a model of drug/cue-triggered reinstatement of alcohol-seeking behavior. In this model, neither naltrexone (2 mg/kg) nor acamprosate (400 mg/kg) prevented relapse to alcohol-seeking behavior. In contrast, SB-277011A significantly reduced reinstatement of alcohol seeking in a dose-dependent manner. Provided these results can be extrapolated to humans, they suggest that selective DA D(3) receptor antagonists may be useful in the pharmacotherapeutic management of alcohol intake and prevention of relapse to alcohol-seeking behavior.

A procedure to produce high alcohol intake in mice

RATIONALE

While prolonged access to ethanol (EtOH), or deprivations, or their combination have occasionally been shown to yield high levels of voluntary self-administration, in almost all cases, rodents do not self-administer alcohol to the degree that they will develop substantial, intoxicating blood alcohol levels and then continue to self-administer at these levels.

OBJECTIVES

The purpose of the present series of experiments was to modify a fluid restriction procedure to demonstrate consistent, high EtOH consumption.

METHODS

Male and female mice from an alcohol preferring inbred strain (C57BL/6J; B6) as well as from a genetically heterogeneous strain (WSC) were given varying periods of access to fluid, ranging from 90 min to 10 h per day, for 12-21 days. Every 3rd or 4th day, separate groups of mice were offered a 5, 7 or 10% EtOH solution for either 10 min or 30 min, followed by water for the remainder of the time.

RESULTS

In all studies, stable high EtOH doses were consumed by both B6 and WSC mice across the EtOH sessions, exceeding 2 g/kg in a 30-min session. Mean blood EtOH concentration exceeded 1 mg/ml (i.e. 100 mg%), with values in individual animals ranging from 0.6 mg/ml to 3.4 mg/ml. Notably, mice receiving 10 h of fluid/day continued to consume 2 g/kg doses of EtOH. While this procedure did not produce subsequent preference for EtOH in WSC mice, consumption remained high in some animals.

CONCLUSIONS

These data indicate that scheduling fluid intake produces high, stable EtOH consumption and BEC in male and female B6 and WSC mice.

Neuropeptide-Y Y5 Receptors Modulate the Onset and Maintenance of Operant Ethanol Self-Administration

BACKGROUND

Neuropeptide Y (NPY) is the most abundant and widely distributed peptide in the mammalian central nervous system and increases feeding behavior at NPY Y1 or Y5 receptor subtypes. Recent pharmacological and mutant mouse data indicate that NPY activity at its receptors can influence ethanol self-administration, although the direction and strength of this influence are not clear.

METHODS

Effects of the novel NPY Y5 receptor antagonist L-152,804 on the onset and maintenance of operant self-administration were examined in male C57BL/6J mice, which were trained to self-administer ethanol (10% v/v) versus water via the sucrose substitution method during 16 hr overnight sessions. After 4 months of baseline responding, mice were injected with L-152,804 (0, 10, 30, or 60 mg/kg, intraperitoneally) before operant sessions. Potential locomotor effects of L-152,804 and possible interaction with the sedative properties of ethanol also were examined.

RESULTS

All three doses of L-152,804 significantly delayed the onset of ethanol-reinforced responding relative to vehicle injection. L-152,804 produced no effect on the total number of ethanol- or water-reinforced responses per 16 hr session. However, L-152,804 selectively modulated the temporal distribution of ethanol-reinforced responding depending on the dose (10 and 60 mg/kg) and time point measured in a manner consistent with blockade of ethanol reinforcement. Additional experiments determined that L-152,804 (10 or 60 mg/kg) did not alter spontaneous locomotor activity or influence the sedative effects of ethanol (4 g/kg).

CONCLUSIONS

These results indicate that blockade NPY Y5 receptor activity modulates the onset and maintenance of ethanol self-administration. For this reason, NPY-Y5 receptor antagonists may be useful in medical management of alcohol abuse and alcoholism.

Blood alcohol concentrations after scheduled access in high-alcohol-preferring mice

Development of procedures yielding substantial blood alcohol concentrations during voluntary access to an alcohol solution in mice is necessary to further characterize genetic and neurobiologic mechanisms underlying alcohol self-administration. Although, in experimental situations, some populations of mice readily drink an alcohol solution, results from previous studies have not typically revealed high blood alcohol concentrations after voluntary access, probably because of the high alcohol metabolism rate in mice. Toward development of a murine drinking model, 36 selectively bred high-alcohol-preferring mice of both sexes were subjected to a 30-min scheduled-access procedure by using saccharin fading to gradually introduce an alcohol solution. Mice had ad libitum access to food and water 24 h a day. The alcohol solution was available 1 h after the start of the dark part of the cycle for 30 min per day, 5 days per week. After complete removal of saccharin from the drinking tubes, mice consistently drank 1.4 g/kg of a 10% [volume/volume (vol./vol.)] alcohol solution in 30 min. Analysis of tail blood samples, taken immediately after the end of the 30-min access period, indicated blood alcohol concentrations were tightly correlated with alcohol intakes (range, 6-130 mg/dl; average, nearly 60 mg/dl). A concentration-response function of 10%, 12%, 15%, 18%, and 21% (vol./vol.) alcohol solutions indicated an inverted U-shaped relation between alcohol intake and alcohol concentration, with peak intake of greater than 1.75 g/kg per 30 min when a 15% alcohol solution was available. No sex differences were seen. These findings indicate the utility of this procedure in obtaining pharmacologically relevant blood alcohol concentrations after voluntary oral self-administration of an alcohol solution in mice.

Animal models of motivation for drinking in rodents with a focus on opioid receptor neuropharmacology

Ethanol, like other drugs of abuse, has motivating properties that can be developed as animal models of self-administration. A major strength of the operant approach where an animal must work to obtain ethanol is that it reduces confounds due to palatability and controls for nonspecific malaise-inducing effects. In the domain of opioid peptide systems, limited access paradigms have good predictive validity. In addition, animal models of excessive drinking-either environmentally or genetically induced-also appear sensitive to blockade or inactivation of opioid peptide receptors. Ethanol availability can be predicted by cues associated with positive reinforcement, and these models are sensitive to the administration of opioid antagonists. Perhaps most exciting are the recent results suggesting that the key element in opioid peptide systems that is important for the positive reinforcing effects of ethanol is the mu-opioid receptor. How exactly ethanol modulates mu-receptor function will be a major challenge of future research. Nevertheless, the apparently critical role of the mu receptor in ethanol reinforcement refocuses the neuropharmacology of ethanol reinforcement in the opioid peptide domain and opens a novel avenue for exploring medications for treating alcoholism.

HPA axis function and drug addictive behaviors: insights from studies with Lewis and Fischer 344 inbred rats

Much research supports a link between stress and its concomitant hypothalamic-pituitary-adrenal (HPA) axis responses with behavioral sensitivity to psychoactive drugs. Our research demonstrates that Lewis inbred rats more readily acquire drug self-administration than Fischer 344 (F344) inbred rats and, compared to this strain, Lewis rats have hyporesponsive HPA axis responses to stress exposure. This association appears to conflict with investigations using outbred rats and suggests that the relationship between drug sensitivity and HPA axis responsiveness is more complicated than originally thought. It is essential to better understand this relationship because of its relevance to vulnerability and relapse to drug abuse. Thus, this paper reviews the literature in which these two inbred strains have been compared. We discuss strain differences in HPA axis function, in characteristics of the mesolimbic dopamine system, and in behaviors thought to reflect emotionality. Strain differences in unconditioned and conditioned effects of psychoactive drugs are then reviewed. Next, we discuss the possible role of sex and gonadal hormones on responsiveness to psychoactive drugs in these strains. Finally, a comparison of results obtained from these strains to three other comparator groups (e.g., high and low responders) suggests that a non-monotonic relationship between behavioral sensitivity to drugs and HPA axis responsiveness can explain much of the discrepancies in the literature.

Delta-opioid and 5-HT3 receptor antagonist effects on ethanol reward and discrimination in C57BL/6 mice

The effects of the receptor antagonists MDL 72222 (MDL, 5-HT3) and naltrindole (delta-opioid) on ethanol reward and its discrimination were examined in ethanol-preferring C57BL/6 (C57) mice. MDL attenuated lever responding for 12% ethanol delivered on a fixed-ratio 8 reinforcement schedule at a dose that did not influence responding for water reward, thus confirming a previous report that ICS 205-930 reduced ethanol reward for Long-Evans rats. Our study in combination with the reduced ethanol consumption reported for C57 mice injected with odansetron indicates that 5-HT3 receptor systems are involved in mediating behavior directed toward obtaining ethanol as well as its consumption. By attenuating the rewarding effects of ethanol or of ethanol conditioned cues (e.g., the operant environment), 5-HT3 antagonists may be useful in the treatment of alcohol abuse. The 5-HT3 antagonist effects in this study are comparable with the effects of naltrexone on ethanol reward in C57 mice, although higher doses were required to reduce operant responding for ethanol reward. In contrast to the 5-HT3 antagonist and naltrexone effects, naltrindole, an antagonist with greater specificity for the delta-opioid receptor, was without effect on ethanol reward. This result and recent reports for rats and monkeys suggests that the general antagonists might be more efficacious in attenuating ethanol reward. Both MDL and naltrindole produced only slight reductions in the ethanol discriminative cue, suggesting that the rewarding and discriminative effects of ethanol are not likely mediated by identical neural mechanisms as previously suggested.

Operant-self-administration of ethanol in mice prenatally exposed to cocaine

Prenatal drug exposure may affect postnatal response to the reinforcing effects of a broad array of drugs. This study investigated the effects of prenatal cocaine exposure on operant self-administration of ethanol. Eighteen male, C57BL/6J mice were divided into three groups. The first had been prenatally exposed to 30 mg/kg of cocaine twice per day while the other groups were offspring of mothers which were either pair fed and saline intubated or untreated. Once adults, the mice were trained and subsequently tested for reinforcement from ethanol. The prenatal cocaine group responded less than the two control groups, with the largest decrease during a progressive ratio schedule of reinforcement. There were no differences in responding as a function of ethanol concentrations. These findings suggest that prenatal exposure to cocaine may not affect reinforcement per se, but may decrease motivation, drive state or propensity to work for ethanol.

Operant ethanol reward in C57BL/6 mice: influence of gender and procedural variables

Food-deprived C57BL/6 (C57) mice of either sex responded for oral ethanol rewards delivered on ratio schedules of reinforcement, thus extending to female C57 mice effects previously reported only for male members of the strain. Lever responding for ethanol reward was influenced by thirst motivation (post- vs. preprandial tests), time of access to ethanol reward, ethanol concentration, and reinforcement schedule. A particularly high response output for 12% ethanol delivered on a PR2 schedule (e.g., approximately 1400/15 min test session) indicates its efficacy as a reinforcer for C57 mice. Estimated consumption of ethanol differed from lever responding when reward access time was relatively long (10 s) and response demand of the reinforcement schedule was low, but paralleled lever responding when reward access time was restricted (3 s) and response demands were greater. Gender influenced lever responding for ethanol reward and its consumption, the difference depending upon reward access time and reinforcement schedule. When the response demands were low and the reward access time long, females tended to respond more than males for ethanol reward; with greater response demands and shorter reward access time, males tended to respond more. In conjunction with our companion report, the present study helps define the behavioral conditions under which ethanol is rewarding for C57 mice and establish the conditions under which ethanol reward differs for male and female mice.

Ethanol consumption by C57BL/6 mice: influence of gender and procedural variables

Both sexes of C57BL/6 (C57) mice consumed substantial quantities of ethanol without food or water deprivation whether access was continuous or limited. Food deprivation increased the amount of ethanol consumed, and the amount consumed depended upon when the animals were tested with reference to their daily food allotment. Ethanol consumption was greater if the mice were tested postprandially, high thirst motivation, rather than preprandially (approximately 10 vs. approximately 4.5 g/kg/30 min). Preference for ethanol over water, however, was greater when mice were under low thirst motivation (i.e., tested preprandially or with water available during the test). Compared to males, female mice consumed more of a high-ethanol concentration solution (10%) when access was continuous or limited to the first hour of the dark (active) phase of the circadian cycle. Also, in contrast to males, female mice exhibited increased ethanol consumption across days of drinking experience. Finally, although ethanol consumption under the food deprivation conditions of this experiment did not differ according to sex, females had higher blood ethanol concentrations than male C57 mice, a finding not previously reported for rodents but common to humans.

Genetics of alcoholism: rapid development of a new high-ethanol-preferring (HEP) strain of female and male rats

A genetically based animal model of alcoholism has been developed in a relatively short period of 3 years. The new strain is characterized by an intense preference for ethanol over water as well as unique behavioral, neurochemical and other attributes. This new strain, termed high-ethanol-preferring (HEP) rats, was derived initially from selective cross-breeding of a variant strain of female Harlan Sprague-Dawley (SD) rats with the outbred Wistar line of male ethanol-preferring (P) rats. In this study, drinking patterns of both genders were obtained over 10 days by presenting water and ethanol in concentrations ranging from 3% to 30%. To expedite the development of the new strain, only three to five female and male rats served as breeders, which were chosen from all litters on the basis of their maximum g/kg intake integrated with proportion of ethanol to total fluid values. Profiles of intake of preferred concentrations of ethanol were obtained over 24 h of unlimited access as well as during 2-h intervals of limited access to ethanol. Levels of blood ethanol were measured in both female and male HEP animals during bouts of ethanol drinking in the limited access paradigm. By the sixth generation of HEP rats, ethanol consumption of the females often exceeded that of any other rat genetically bred to drink ethanol (e.g., at a concentration of 15.7%, 10.3 g/kg per day). Seven additional characteristics are notable: 1) the HEP rats prefer ethanol in the presence of a nutritious chocolate drink or nonnutrient sweetened solution (aspartame); 2) high levels of blood ethanol are associated with their drinking; 3) females drink significantly greater g/kg amounts of ethanol than HEP males and prefer a higher percent concentration of ethanol; 4) the drinking of ethanol by the female HEP animals does not fluctuate during the estrous cycle; 5) neurochemical assays show differential profiles of 5-HT, dopamine, and their metabolites in different regions of the brain; 6) measures of activity using the elevated plus maze, open field, and cork gnawing reveal differences between genders of HEP rats and SD rats; and 7) the HEP animals are without phenotypically expressed abnormalities. Finally, one cardinal principle derived from this study revealed that the breeding strategy to develop high-ethanol-drinking rats centers on the use of multiple solutions of ethanol whereby the intakes of ethanol in concentration of 9% through 20% dictate the ultimate selection of breeding pairs over successive F generations. Further, it is concluded that because of an intense rise in ethanol drinking of the F1 generation of female HEP rats well above that of the parental SD female breeders, the complex genotypic characteristic of the male P rat is predominantly responsible for evoking ethanol drinking in female offspring.

Mouse strain differences in oral operant ethanol reinforcement under continuous access conditions

The present experiment examined ethanol self-administration in C57BL/6J (C57) and DBA/2J (DBA) mice using a continuous access operant procedure. Adult male C57 and DBA mice were initially trained to perform a lever press response to obtain access to 10% w/v sucrose solution. Subsequently, the mice were placed in operant chambers on a continuous (23 hr/day) basis with access to food (FR1), 10% v/v ethanol (FR4), and water from a sipper tube. C57 mice displayed greater rates of responding on the ethanol-associated lever compared with DBA mice. Responding on the food lever was the same in both strains, but DBA mice consumed greater amounts of water. C57 mice consistently displayed both prandial and nonprandial episodes (bouts) of ethanol responding. DBA mice did not respond for ethanol in bouts. Following 50 consecutive sessions, ethanol concentration was altered every 5 days. Response patterns were determined using 0, 5, 10, 20, and 30% v/v ethanol concentrations. C57 mice displayed concentration-dependent responding on the ethanol lever showing that ethanol was functioning as an effective reinforcer in this strain. In contrast, responding on the ethanol lever by DBA mice did not change as a function of ethanol concentration. Saccharin (0.2% w/v) was subsequently added to the ethanol mixture, and responding was examined at 0, 5, 10, and 20% ethanol concentrations. Overall, ethanol lever responding was increased in both strains. As before, C57 mice showed higher levels of ethanol responding, compared with DBA mice. C57 mice also showed higher responding for saccharin alone. These results are consistent with findings that suggest orally administered ethanol is a more effective reinforcer in C57 mice than in DBA mice. Furthermore, C57 mice engage in ethanol-reinforced responding over a broader range of conditions than DBA mice.

Genotype- and gender-dependent hepatic alcohol dehydrogenase (ADH) activity in developing mice

Ethanol is metabolized in human and rodent liver primarily by the enzyme alcohol dehydrogenase (ADH). Hepatic ADH activity in adult males and females of seven inbred strains of mice was determined to examine genotype- and sex-dependent variability among the strains and the level of sexual dimorphism in each of the strains. ADH activity varied considerably among the strains, which could be categorized into high-activity strains C57BL/6, C57Brcd, and Swiss, and relatively low-activity strains C3H, CBA, and DBA. Adult Swiss, AKR, C57BL/6, and DBA females exhibited significantly higher levels of hepatic ADH than their male counterparts, whereas no gender differences were seen in C3H, CBA, and C57Brcd. Young mice of high and/or low ADH activity strains viz. C57BL/6 and C3H did not exhibit gender differences in ADH activity at weanling but the enzyme levels increased by sixth week in females and remained higher thereafter. The progeny of a high-activity strain with sexual dimorphism (C57BL/6) and a low-activity strain lacking gender difference (C3H) exhibited intermediate levels of ADH and age-dependent sexual dimorphism.

The discrimination and durability of an ethanol cue in young and mid-aged female mice

Young adult (6 months) and mid-aged (12 months) C57BL/6 mice both learned to discriminate ethanol (ETOH, 1.0 g/kg) although criterion performance occurred later for mid-aged mice. ETOH discrimination improved with increasing dose (0.25-1.0 g/kg) and the dose-response function was unaffected by age. The ETOH cue had declined by 40 min postinjection for young mice not unlike a previous report for young rats. In contrast, the ETOH cue remained discriminable at 40 min for mid-aged mice, an effect perhaps due to their slower rate of ETOH metabolism and accountable for the previously reported reduction in ethanol consumption by mid-aged mice. Retention tests and reacquisition training both indicated that the ETOH cue can be retained by both age groups for at least 60 days without discrimination training or food deprivation. The present study suggests that the ethanol discriminative cue in mid-aged mice does not differ from that in young adult mice in potency but is more long lasting, the latter perhaps being related to their reduced ethanol consumption. Of significance from a therapeutic perspective, is that the ETOH cue remained discriminable for 2 months in both age groups (i.e., approximately 1/12 of their total life span).

Ethanol self-administration and motor deficits in adults C57BL/6J mice exposed prenatally to cocaine

Daily injections of 10 mg/kg cocaine on gestation days 12-18 did not alter maternal weight gain or offspring birth weight, viability, growth, or adult weight compared to saline controls. Adult male and female offspring were food deprived and trained to lever press for ethanol. Responding on an FR2 schedule and ethanol intake (g/kg) were recorded as measures of the reinforcing effects of ethanol. Lever press duration was used to assess motor performance. Results demonstrate that C57 mice will work for and consume large quantities of ethanol and that prenatal cocaine exposure increased the amounts ingested by both male and female mice. Prenatal-cocaine-exposed males also exhibited motor deficits as indicated by longer response duration times compared to controls. The consumption of large amounts of ethanol exacerbated the motor impairment in prenatal-cocaine males and revealed such deficits in cocaine females. The present results demonstrate that maternal cocaine exposure, at doses having no observable effect upon pregnancy, birth, or offspring growth, can increase the consumption of ethanol and enhance its motor impairing effects on fully mature offspring.

Inbred mouse strains vary in oral self-selection of nicotine

Inbred mouse strains differ in sensitivity to a first dose of nicotine and in the development of tolerance to nicotine. The experiments reported here used six inbred mouse strains (A, BUB, C3H, C57BL/6, DBA/2, ST/b) that differ in sensitivity to an acute challenge dose of nicotine to determine whether differences in oral self-selection of nicotine exist. Animals were presented with solutions containing nicotine or vehicle (water or 0.2% saccharin) and their daily intake of the two fluids was measured for 4 days starting with a 10 micrograms/ml nicotine solution. This was followed by sequential 4-day testing with 20, 35, 50, 65, 80, 100, 125, 160 and 200 micrograms/ml nicotine solutions. The strains differed dramatically in their self-selection of nicotine and in maximal daily dose (mg/kg); the rank order of the strains was C57BL/6 > DBA > BUB > A > or = C3H > or = ST/b for both the tap water and 0.2% saccharin choice experiments. Correlations between nicotine consumption and sensitivity to nicotine, as measured by a battery of behavioral and physiological responses, were also calculated. Strain differences in nicotine intake were highly correlated with sensitivity to nicotine-induced seizures. As sensitivity to nicotine-induced seizures increases, oral self-selection of nicotine decreases. This finding may suggest that this toxic action of nicotine serves to limit intake.

Ethanol self-administration is genetically independent of locomotor stimulation in fast and slow mice

One substance abuse hypothesis proposes that rewarding effects of drugs are causally related to their psychostimulant effects. We examined this hypothesis by comparing operant self-administration of ethanol in mice selectively bred for either high (Fast) or low (Slow) locomotor stimulation response to ethanol. Mice were trained to lever press for ethanol using postprandial induction and were then tested over a range of conditions to determine the degree of self-administration. There were no significant differences between Fast and Slow mice in either the amount of work produced to obtain ethanol or the amount of ethanol consumed. In general, none of the groups of mice showed robust ethanol-reinforced behavior. This is in contrast with C57BL/6J mice tested concurrently, which showed substantial ethanol-reinforced behavior. Further analysis revealed individual differences in responding within each of the selected lines. However, there was no systematic pattern within or between groups for these individual differences, suggesting that the genes mediating ethanol-reinforced behavior are segregating in a manner independent from genes mediating the locomotor stimulant response to ethanol, and thus, the mechanistic processes mediating reinforcement from ethanol are distinct from those that influence the psychomotor stimulant response to this drug.

Localization of genes influencing ethanol-induced conditioned place preference and locomotor activity in BXD recombinant inbred mice

Genetic differences in ethanol's ability to induce conditioned place preference were studied in 20 BXD Recombinant Inbred (RI) mouse strains and in the C57BL/6J and DBA/2J progenitor strains. Male mice from each strain were exposed to a Pavlovian conditioning procedure in which a distinctive floor stimulus (CS+) was paired four times with ethanol (2 g/kg). A different floor stimulus (CS-) was paired with saline. Control mice were injected only with saline. Floor preference testing without ethanol revealed significant genetic differences in conditioned place preference, with some strains spending nearly 80% time on the ethanol-paired floor while others spent only 50% (i.e., no preference). Control mice showed genetic differences in unconditioned preference for the floor cues, but unconditioned preference was not genetically correlated with conditioned preference. There were also substantial genetic differences in ethanol-stimulated activity, but contrary to psychomotor stimulant theory, ethanol-induced activity on conditioning trials was not positively correlated with strength of conditioned place preference. However, there was a significant negative genetic correlation (r = -0.42) between test session activity and preference. Quantitative trait loci (QTL) analyses showed strong associations (P < 0.01) between conditioned place preference and marker loci on chromosomes 4, 8, 9, 18 and 19. Weaker associations (0.01 < P < 0.05) were identified on several other chromosomes. Analysis also yielded several significant QTL for unconditioned preference, ethanol-stimulated activity, and sensitization. Overall, these data support the conclusion that genotype influences ethanol-induced conditioned place preference, presumably via genetic differences in sensitivity to ethanol's rewarding effects. Moreover, several chromosomal regions containing candidate genes of potential relevance to ethanol-induced conditioned place preference have been identified.

Selective serotonin reuptake inhibitors: effects of chronic treatment on ethanol-reinforced behavior in mice

Several lines of evidence suggest that aspects of ethanol drinking are mediated, at least in part, by serotonergic (5-HT) neurotransmitter systems. Ethanol-preferring animals show decreases in serotonin function and receptor densities. In addition, serotonin uptake inhibitors have been shown to decrease ethanol consumption in animal models and in humans. However, the time course of these effects and their duration remain undetermined. In the present studies, C57BL/6J male mice were treated with one of three selective 5-HT reuptake inhibitors (SSRIs): fluoxetine, sertraline, or paroxetine. All three drugs produced initial decreases in operant lever pressing behavior for ethanol followed by a return to baseline on subsequent days. Immediately following 14 days of this initial treatment, subsequent treatment with higher SSRI doses was ineffective in decreasing ethanol-reinforced behavior. However, after a several week "washout period," SSRI pretreatment again produced an initial decrease in responding for ethanol, again followed by a return to baseline. Thus, suppression of ethanol drinking may be related to immediate changes in 5-HT function following treatment with SSRIs, and tolerance to this effect appears to develop rapidly.

Opioid operant self-administration, analgesia, stimulation and respiratory depression in mu-deficient mice

It is commonly thought that mu-receptors play an important role in the reinforcing effects of opioids. In the present study, inbred strains widely divergent in CNS opiate receptor densities were used to investigate the influence of genetic variation in receptor concentration on opioid-reinforced behavior. In particular, the CXBK/ByJ mice were used as an investigative tool because of their significantly lower number of CNS mu opioid receptors. The behavioral pharmacology of opioids in the mu-deficient CXBK/ByJ mice was compared to other commonly used inbred mouse strains, C57BL/6J and BALB/cJ, and the opiate receptor rich CXBH/ByJ mice. Operant opioid reinforced behavior, opioid-induced locomotor stimulation, analgesia and respiratory depression were investigated in all four inbred strains. To assess the acquisition and maintenance of opioid reinforced behavior, oral self-administration of the potent benzimidazole opioid, etonitazene, was determined using an operant fixed-ratio schedule of reinforcement (FR 8). Acquisition of etonitazene-reinforced behavior was established in all four strains including the mu-deficient CXBK/ByJ mice. However, there were significant genetic differences in the amount of drug intake during the maintenance of opioid-reinforced behavior and extinction behavior following vehicle substitution. For example, drug intake was significantly greater in the BK versus BH mice during the maintenance phase and an extinction burst was seen in the BH but not the BK mice following vehicle substitution. Thus, mu-receptor density may not account for individual variability in the acquisition of opioid-reinforced behavior under these conditions. Sensitivity to etonitazene-induced respiratory depression, stimulation of locomotor activity and analgesia were unrelated to drug intake during self-administration sessions across these four inbred strains.(ABSTRACT TRUNCATED AT 250 WORDS)

Operant ethanol-reinforced behavior in P, NP, HAD, and LAD rats bred for high versus low ethanol preference

These studies examined the reinforcing effects of ethanol in rats selectively bred for high versus low ethanol drinking in a two-bottle choice preference task, namely the Preferring (P), Non-Preferring (NP), High Alcohol Drinking (HAD), and Low Alcohol Drinking (LAD) rats. The results substantiate findings suggesting that genetic factors are significant in determining whether ethanol will come to serve as a reinforcer. P rats exhibited high levels of responding for ethanol compared with the water vehicle, NP and HAD rats exhibited more moderate levels of responding for ethanol, and the behavior of LAD rats suggested that ethanol served only inconsistently as a reinforcer for these rats. Overall, the results suggest the existence of distinct, biologically influenced components of ethanol drinking behavior. Preference appears to measure an inherent facilitative factor allowing animals to initiate ethanol drinking. The operant chamber paradigm appears to measure factors related to whether and to what extent ethanol will serve as a positive reinforcer following conditioned exposure to the drug. Although preferring animals generally find ethanol reinforcing there seems to be little quantitative relationship between degree of preference and whether ethanol will serve as a reinforcer. Lack of preference does not seem to be predictive of lack of reinforcement. Thus, it appears that preference for ethanol and reinforcement from ethanol are somewhat overlapping, but distinct factors that contribute to ethanol drinking. These results suggest the existence of multiple components of behavior mediated by multiple mechanisms that contribute to ethanol drinking.

Regulation of operant ethanol-reinforced behavior is genetically independent of regulation of withdrawal severity in WSP and WSR mice

Selectively bred withdrawal seizure prone (WSP1 and WSP2) and withdrawal seizure resistant (WSR1 and WSR2) mice were used to test the extent to which severity of ethanol withdrawal response is predictive of the reinforcing effects of ethanol. Mice were systematically introduced to ethanol under a fixed ratio 1 (FR 1) schedule using adjunctive drinking methods. There were no significant differences in ethanol consumption between the lines during training. Subsequently, responding for ethanol concentrations of 8%, 0% (vehicle control), and 8% retest under a FR 1 schedule in the absence of food induction was measured. Group data showed that ethanol did not serve as a reinforcer in the test phase within any of the four lines, and there were no significant line differences in rate of responding, intake, or blood ethanol concentrations (BEC). Mice were next tested for responding for ethanol under a FR 4 schedule. Again, ethanol did not serve as a reinforcer for any of the four groups, and there were no significant differences between the lines. However, further analysis showed that there were individual differences in responding within each group, some animals were apparently reinforced by ethanol, while others showed no reinforcement and some appeared to avoid ethanol. There was no systematic pattern within or between groups for these individual differences in responding. Thus, both the group results as well as the behavior patterns of individual animals are consistent with the conclusion that genes regulating rewarding effects of ethanol appear to be segregating randomly across groups and are independent of genes mediating ethanol withdrawal severity.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic animal models of alcohol and drug abuse

Behavioral and pharmacological responses of selectively bred and inbred rodent lines have been analyzed to elucidate many features of drug sensitivity and the adverse effects of drugs, the underlying mechanisms of drug tolerance and dependence, and the motivational states underlying drug reward and aversion. Genetic mapping of quantitative trait loci (QTLs) has been used to identify provisional chromosomal locations of genes influencing such pharmacological responses. Recent advances in transgenic technology, representational difference analysis, and other molecular methods now make feasible the positional cloning of QTLs that influence sensitivity to drugs of abuse. This marks a new period of synthesis in pharmacogenetic research, in which networks of drug-related behaviors, their underlying pharmacological, physiological, and biochemical mechanisms, and particular genomic regions of interest are being identified.

Genetic models in the study of alcoholism and substance abuse mechanisms

1. Vulnerability to substance abuse is an important emerging issue. Some related factors are the relationship between propensity to self-administer a drug and neurosensitivity to that drug; similarities and differences between various models of drug seeking behavior; and the commonality of drug-seeking behavior across drugs and genotypes, that is, whether reinforcement from and abuse of alcohol and other drugs define variations within a single behavioral phenomenon, or whether reinforcement and abuse must be individually defined for each substance involved. 2. Findings related to these issues are now being obtained from the areas of pharmacogenetics and operant drug self-administration. 3. The results indicate that reinforcement from alcohol and other drugs is only moderately related to preference for alcohol and other drugs. In addition, neurosensitivity to drugs appears to have little influence on whether that drug will come to serve as a positive reinforcer for any given individual or animal. Indeed, the critical factor appears to be the individual organism's innate propensity to find a particular drug reinforcing. 4. Initial findings also show that genotypic patterns of reinforcement from ethanol appear to correlate highly with patterns of reinforcement from cocaine and opiates. 5. From these findings it is concluded that there exist important genetic determinants of drug reinforced behavior; reinforcement is an important and independent effect of several psychoactive drugs; and drug seeking behaviors maintained by ethanol, cocaine and opiates may have at least some common biological determinants.

Molecular Mechanisms Associated with Cocaine Effects

Cocaine has been shown to be a highly addictive and toxic drug. It produces these effects and a variety of other physiological and behavioral effects through its interactions with several distinct central nervous system receptor sites. We present the results of a series of studies that utilized multiple site analyses to elucidate which cocaine binding sites influence the reinforcing and toxic effects of cocaine and with what proportion of influence. The nature of cocaine interactions with monoamine transporters is also discussed, especially with the dopamine transporter, which has been shown to be the cocaine binding site that is primarily associated with the reinforcing effects of cocaine. We also provide evidence that vulnerability to both the toxic and addictive effects of cocaine may be significantly influenced by genetic differences in both humans and animals. In view of the fact that cocaine is commonly abused in a polydrug situation, we present the results of both behavioral and biochemical experiments which suggest that common biochemical pathways may mediate the reinforcing or addictive properties of drugs of abuse. Finally, we discuss research on the biochemical mechanisms associated with effects of ethanol, particularly those which may also influence cocaine self-administration and speculate on pharmacotherapeutic strategies for concurrent abuse of cocaine and ethanol.

Genetic differences in the rewarding and activating effects of morphine and ethanol

The influence of genotype on the rewarding and locomotor activating effects of morphine and ethanol was examined in the place conditioning paradigm. Two inbred mouse strains (C57BL/6J and DBA/2J) were exposed to a differential conditioning procedure in which each mouse received four pairings of a distinctive floor stimulus with IP injection of morphine (0, 2.5, 5 or 10 mg/kg) or ethanol (0, 1, 2, 3 or 4 g/kg). A different floor stimulus was paired with saline. Conditioning trials lasted 30 min and each experiment concluded with a floor preference test in the absence of drug. In accord with previous studies, morphine evoked a dose-dependent increase in activity during conditioning that was greater in C57BL/6J mice than in DBA/2J mice. In contrast, ethanol produced a dose-dependent increase in activity that was greater in DBA/2J than in C57BL/6J mice. Both strains showed conditioned place preference with morphine, but only the DBA/2J strain showed conditioned place preference with ethanol. No conditioned place aversion was seen. With both drugs, stronger place preference conditioning was obtained in DBA/2J mice, supporting the general conclusion that sensitivity to drug reward is influenced by genotype. The fact that the same genotype is more sensitive to the rewarding effects of two different drugs supports theories postulating commonality in the biological mechanisms of drug reward. Although the outcome of the ethanol study supports predictions of the psychomotor stimulant theory of addiction concerning the relationship between drug-induced activation and reward, the outcome of the morphine study does not.(ABSTRACT TRUNCATED AT 250 WORDS)

Orally delivered cocaine functions as a positive reinforcer in C57BL/6J mice

Cocaine serves as a reinforcer across several routes of administration and species. However, whether orally delivered cocaine serves as a positive reinforcer has not been systematically established. We determined the extent to which contingent access to orally delivered cocaine would maintain lever pressing behavior in C57BL/6J mice who had a prior history of operant ethanol-reinforced behavior. The findings presented in this report demonstrate that orally delivered cocaine can serve as a reinforcer of operant behavior. A drug substitution procedure where cocaine was substituted for gradually decreasing ethanol concentrations was successful in inducing pharmacologically significant intakes of cocaine under a fixed ratio (FR) schedule of drug access. When ethanol was removed, responding for cocaine continued. As FR size was increased, proportionate increases in responding occurred except at the highest FR value. Responding maintained by cocaine significantly exceeded responding maintained by vehicle, with the mice typically consuming 6-10 mg/kg cocaine per 30-min session. The utilization of inbred strains and the procedures followed in the present studies should prove useful in determining the extent of both genetic and environmental influences on various behavioral effects of cocaine and their mechanisms of action.

Effects of feeding condition on orally delivered ethanol as a reinforcer for rhesus monkeys

Twenty adult male rhesus monkeys could drink either or both an 8%-ethanol (w/v) solution or water during daily 3-h experimental sessions. Subjects were initially tested under ad lib feeding conditions, having received daily access to large amounts of food for approximately 8 months (Phase 1). The ethanol solution was concurrently available with its water vehicle under fixed-ratio schedules of mouth contacts with a drinking spout in which emission of 8 responses produced a liquid delivery (FR 8). Under these conditions ethanol served as a reinforcer for some subjects--it maintained higher response rates than water--but not for others. In the next experimental condition (Phase 2), subjects were again tested for ethanol self-administration under FR 8 schedules with water concurrently available, but while receiving daily food rations that maintained their body weights at less than what they had been under ad lib feeding conditions. Under these conditions all subjects self-administered ethanol at appreciable levels, but water-maintained responding also was at appreciable levels for some subjects. Increases in ethanol self-administration following institution of the reduced-feeding conditions in Phase 2 generally were inversely related to self-administration levels under the free-feeding conditions of Phase 1 (the greatest proportional increases in ethanol intake occurred with those monkeys that self-administered the least amount of ethanol in Phase 1). In Phase 3, the requirements of the concurrently operating fixed-ratio schedules delivering ethanol and water were increased to 16 responses (FR 16).(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic approaches to studying drug abuse: correlates of drug self-administration

Some important issues in substance abuse are the relationship between propensity to self-administer a drug and neurosensitivity to that drug; similarities and differences between various models of drug-seeking behavior; and the commonality of drug-seeking behavior across drugs and genotypes. Findings related to these issues are now emerging from the areas of pharmacogenetics and operant drug self-administration. Ethanol has been readily established as a positive reinforcer in AA (Alcohol Accepting), P (Preferring) and LEWIS rats, as well as C57BL/6J and LS/Ibg mice. In low ethanol preferring F344 and NP (Non-Preferring) rats, ethanol maintains significant but low levels of responding. Ethanol does not maintain lever-pressing behavior in BALB/cJ or SS/Ibg mice, and is avoided in DBA/2J mice. This pattern of reinforcement from ethanol is only moderately correlated with ethanol preference, and is not correlated with neurosensitivity to ethanol, at least as measured by duration of loss of the righting reflex (LORR). However, these genotypic patterns of reinforcement from ethanol do appear to correlate highly with patterns of reinforcement from cocaine and opiates. From these findings it is concluded that: 1) there exist important genetic determinants of drug reinforced behavior; 2) ethanol preference is not a highly accurate measure of reinforcement from ethanol; 3) sensitivity to ethanol as measured by LORR and self-administration of this drug are not highly genetically correlated; and 4) drug-seeking behaviors maintained by ethanol, cocaine and opiates may have at least some common biological determinants.

Genetically determined alcohol preference and cyclic AMP binding proteins in mouse brain

Free-choice consumption of alcohol by mice with differing phenotypic alcohol preferences caused uniformly large decreases in brain cyclic AMP-dependent protein kinase activity toward an exogenous substrate (histone 2b) but the effect of alcohol on brain cyclic AMP binding activity was strain-specific. Furthermore, particulate kinase phosphorylating activity toward an endogenous protein (kinase regulatory subunit, RII) was altered by alcohol consumption in a strain-specific manner. The changes in cyclic AMP binding and phosphorylating activity appeared to result from phenotypic differences in the brain's response to alcohol. Thus, low preference animals were sensitive to alcohol and showed a large decrease in cyclic AMP binding and an increase in phosphorylation of regulatory subunit in response to alcohol. In contrast, high preference strain had only a small decrease in cyclic AMP binding and a decrease in phosphorylation, even though these animals consumed a significantly larger dose of alcohol. These data suggest that changes in cyclic AMP binding and/or phosphorylation of kinase regulatory subunit may be phenotypic markers of alcohol preference in inbred mice.

Analysis of spontaneous alcohol drinking in rhesus monkeys

This analysis aims at determining to what extent spontaneous alcohol drinking in adult male rhesus monkeys (Macaca mulatta) represents ethanol-directed behaviour. It is shown that in a condition of free access to an ethanol/water solution (2 percent v/v) and drinking water, alcohol drinking was initiated in all subjects (n = 4) within a few days, without any specific induction procedure. Relationship between drinking behaviour and ethanol concentrations was studied in 8 subjects by use of a concurrent 3-bottle-design. 2 bottles containing ethanol solution (concentrations 2.4; 4.8; 8.16; 16.32 percent v/v), 1 bottle contained drinking water. When ethanol concentrations in the solutions increased, consumption of ethanol solutions decreased, of drinking water increased, and of total water decreased. Net ethanol intake from a certain solution was influenced by its concentration and the concentration of the concurrently available solution. After an initial increase, total net ethanol intake remained relatively constant. Consumed amounts of ethanol (on the average 2-6 ml.kg-1 per day) could lead to notable blood ethanol levels. Drinking from ethanol solution was not just an alternative for ingesting water. The observed alcohol drinking is interpreted as resulting from a central reinforcement of ethanol intake and avoidance of negative, potentially harmful effects of ethanol.

Ethanol self-administration in ALKO rats: I. Effects of selection and concentration

The ALKO Alcohol Accepting (AA) rats and Alcohol Non-Accepting (ANA) rats were used to study the relationship between ethanol preference in a two-bottle choice paradigm and oral ethanol self-administration in an operant conditioning paradigm. Ethanol served as a positive reinforcer in the preferring AA line. Under a continuous reinforcement schedule (FR 1), ethanol deliveries were consistently greater than water (vehicle) deliveries and varied as an orderly inverted U-shaped function of ethanol concentration. Conversely, in nonpreferring ANA animals, ethanol did not serve as a reinforcer. Findings obtained with the AA rats were compared with those obtained with Sprague-Dawley rats. Sprague-Dawley rats maintained higher levels of responding and greater ethanol intake, relative to AA rats, at all concentrations of ethanol tested. The data are consistent with evidence that genotype is a critical factor in determining the extent to which ethanol serves as a reinforcer. The results also suggest that ethanol preference and the maintenance of ethanol reinforced behavior under operant conditions appear to have some common mechanisms. However, since the selection for ethanol preference in AA rats apparently did not maximize the maintenance of ethanol-reinforced behavior in an operant paradigm, ethanol drinking behavior in the preference paradigm may not be completely generalizable to that observed in the operant model.

Ethanol self-administration in ALKO rats: II. Effects of selection and fixed-ratio size

The ethanol-reinforced behavior of ALKO Alcohol Accepting (AA) rats was studied as a function of fixed-ratio (FR) size (1, 2, 4, 8 and 16) across several ethanol concentrations (8, 16 and 32% w/v). A comparison was also made with ethanol-reinforced behavior of Sprague-Dawley (SD) rats. Ethanol, previously shown to serve as a reinforcer in AA and SD rats, maintained responding in these animals under conditions of intermittent reinforcement. SD rats exhibited higher response rates than AA rats to obtain 8% ethanol concentrations as fixed-ratio size was varied in these experiments. These results confirm that, for AA rats, the selection for preference in a two-bottle choice situation did not include the selection for those biological factors which maximize ethanol-reinforced behavior under conditions of intermittent reinforcement. These experiments suggest the existence of distinct, biologically influenced components of ethanol drinking behavior, and have demonstrated several similarities as well as differences between two-bottle choice preference tests and operant studies of ethanol-reinforced behavior.

Serotonin and ethanol preference

This chapter brings together evidence indicating the involvement of serotonin (5-HT) in ethanol preference using data mainly obtained from selectively bred alcohol-preferring and alcohol-nonpreferring lines of rodents. Although several laboratories have established rodent lines that will consume large quantities of ethanol daily, only one line thus far has been established that satisfied all the criteria for an animal model of alcoholism and that would be suitable for studying the biological basis of ethanol preference. This is the P line of alcohol-preferring rats that: (1) freely consumes 5-9 g ethanol/kg body wt/day; (2) drinks sufficient alcohol to produce intoxicating blood alcohol concentrations; (3) works to obtain alcohol; (4) self-administers ethanol for its CNS pharmacological effects; and (5) develops chronic tolerance to and dependence on alcohol with free-choice drinking. Relative to the NP line of alcohol-nonpreferring rats, the P rat has lower 5-HT levels in several CNS regions, including some, such as the nucleus accumbens, hypothalamus, and frontal cortex, which are involved in the brain reward circuitry. Furthermore, both acute and chronic ethanol administration have effects on the 5-HT pathway from the dorsal raphe nucleus to the nucleus accumbens in the P rat. Pharmacological studies have demonstrated that fluoxetine, a serotonin uptake inhibitor, reduced the oral consumption or intragastric self-administration of alcohol in the P rats. In addition, administration of a 5-HT1B agonist also attenuated the oral intake of ethanol by P rats. It is hypothesized that the serotonergic pathway from the dorsal raphe nucleus to the nucleus accumbens is involved in the reinforcing actions of alcohol in the P line of rats.

Genetic tools in the study of drug self-administration

Recent studies have shown that large genetic differences exist in the extent to which orally delivered ethanol will come to serve as a positive reinforcer under operantly defined conditions. In addition, these studies suggest that a significant correlation exists between results from two-bottle choice studies of ethanol drinking and operant self-administration studies of ethanol functioning as a reinforcer. The present paper reports further genetic influences on ethanol self-administration which were found using Long Sleep and Short Sleep mice, bred selectively for high and low duration of loss of the righting reflex in responses to ethanol, respectively. It was possible to establish ethanol as a reinforcer in Long Sleep mice but not in Short Sleep mice. These results indicate that neurosensitivity to ethanol may determine the absolute amount of ethanol consumption but is not highly related to the ability of ethanol to serve as a positive reinforcer. In addition, this paper presents genetic correlations which indicate that (a) ethanol preference and self-administration are highly correlated across genotype; (b) sensitivity to ethanol and self-administration of this drug are not highly genetically correlated; (c) ethanol is not self-administered in operant studies solely for its caloric value; and (d) there exist important genetic determinants of drug reinforced behavior.

Fixed-ratio schedules of oral ethanol self-administration in inbred mouse strains

Previous studies of ethanol reinforcement in BALB/cJ and C57BL/6J mice have shown that over a range of concentrations oral ethanol appeared to serve as a reinforcer only for the C57BL/6J mice. In the previous studies BALB/cJ mice maintained rates of responding for ethanol that only slightly exceeded the rates maintained by the vehicle, water. However, the quantity of ethanol consumed with the continuous reinforcement schedule (fixed ratio one) may have led to pharmacologically significant effects, given the high sensitivity to ethanol of this genotype. The present study tested whether and to what extent ethanol would maintain responding under increasing fixed ratio size in these two strains of mice at ethanol concentrations of 0%, 8%, and 16% (w/v). For the C57BL/6J mice, as fixed-ratio size increased from 1 to 2, 4, and 8, there were almost directly proportional increases in response rate at ethanol concentrations of 8% and 16% (w/v), but not at 0%. Post-session blood ethanol levels confirmed intake of pharmacologically significant quantities. The volume consumed per unit of body weight decreased as fixed-ratio size increased. For the BALB/cJ mice, at no condition did ethanol maintain responding at levels that significantly exceeded vehicle maintained responding. BALB/cJ mice did not differ from C57BL/6J mice as fixed-ratio size was increased during vehicle conditions. These results, along with earlier findings, demonstrate that ethanol can serve as a reinforcer for C57BL/6J mice but not in BALB/cJ mice over a range of schedule conditions. They further support the conclusion that genotype is an important determinant of ethanol reinforced behavior.