Intravenous heroin and ethanol self-administration by alcohol-preferring AA and alcohol-avoiding ANA rats

Samidorphan, an opioid receptor antagonist, attenuates drug-induced increases in extracellular dopamine concentrations and drug self-administration in male Wistar rats

Opioid receptors modulate neurochemical and behavioral responses to drugs of abuse in nonclinical models. Samidorphan (SAM) is a new molecular entity that binds with high affinity to human mu- (μ), kappa- (κ), and delta- (δ) opioid receptors and functions as a μ-opioid receptor antagonist with partial agonist activity at κ- and δ-opioid receptors. Based on its in vitro profile, we hypothesized that SAM would block key neurobiological effects of drugs of abuse. Therefore, we assessed the effects of SAM on ethanol-, oxycodone-, cocaine-, and amphetamine-induced increases in extracellular dopamine (DA_ext) in the nucleus accumbens shell (NAc-sh), and ethanol and cocaine self-administration behavior in rats. In microdialysis studies, administration of SAM alone did not result in measurable changes in NAc-sh DA_ext when given across a large range of doses. However, SAM markedly decreased average and maximal increases in NAc-sh DA_ext produced by each of the drugs of abuse tested. In behavioral studies, SAM attenuated fixed-ratio ethanol self-administration and progressive ratio cocaine self-administration. These results highlight the potential of SAM to counteract the neurobiological and behavioral effects of several drugs of abuse with differing mechanisms of action.

The effects of combined intravenous cocaine and ethanol self-administration on the behavioral and amino acid profile of young adult rats

Under paradigms of combined intravenous cocaine and ethanol self-administration, the effects on behavior have been poorly explored. Numerous studies have found sex differences in amino acids profile and behavioral responses to each drug, yet few have focused on the interactions between cocaine and ethanol. The main objective of this work was to explore the acquisition and maintenance of intravenous self-administration behavior with a combination of cocaine and ethanol in male and female young adult rats. Likewise, the amino acids profile in blood plasma was quantified 48 hours after the last self-administration session. Male and female 52 days old Wistar rats were randomly assigned to one of 3 groups: i) saline control, ii) cocaine (1 mg/kg bodyweight/injection) and iii) cocaine and ethanol (1 mg + 133 mg/kg bodyweight/ injection). After 24 self-administration sessions carried out on a fixed-ratio-1 schedule, with a limit of 15 doses per session, 14 plasma amino acids were quantified by mean Capillary Electrophoresis technique. The curve of cocaine and ethanol combined self-administration was similar to that associated with cocaine administration alone, with females acquiring self-administration criterion before males. The self-administration of cocaine and ethanol altered the plasma concentration and relative ratios of the amino acid L-Tyrosine. In our intravenous self-administration model, females appeared more vulnerable to acquire abusive consumption of the cocaine and ethanol combination, which altered plasma L-Tyrosine levels.

Intravenous self-administration of alcohol in rats-problems with translation to humans

Alcohol is consumed orally by humans, and oral self-administration has been successfully modeled in laboratory animals. Over the last several years, attempts have been made to develop a procedure for the reliable intravenous (IV) self-administration of alcohol in rodents. IV self-administration would provide a better tool for investigating neurobiological mechanisms of alcohol reinforcement and dependence because confounding factors associated with oral self-administration, such as variations in orosensory sensitivity to alcohol and/or its absorption, are avoided. A review of the literature shows that rats, mice and non-human primates can initiate and maintain IV self-administration of alcohol. However, there are 50- to 100-fold interspecies differences in the reported alcohol infusion doses required. Most surprising is that the infusion dose (1-2 mg/kg) that reliably maintains IV alcohol self-administration in rats results in total alcohol intakes of only 20-25 mg/kg/hour, which are unlikely to have significant pharmacological effects. The evidence to support IV self-administration of such low doses of alcohol in rats as well as the potential biological mechanisms underlying such self-administration are discussed. The minute amounts of alcohol shown to reliably maintain IV self-administration behavior in rats challenge the relationship between their blood alcohol levels and the rewarding and reinforcing effects of alcohol.

Nicotine Increases Alcohol Intake in Adolescent Male Rats

Background: Use of alcohol and tobacco, the two most concurrently abused drugs, typically first occurs during adolescence. Yet, there have been no systematic analyses of ethanol (EtOH) and nicotine (Nic) interactions during adolescence. Recent animal studies report that kappa-opioid (KOR) receptor activation mediates age differences in drug reinforcement. Our hypothesis is that concurrent self-administration of EtOH and Nic will be greater in adolescent rats because of age differences in KOR function. Furthermore, exposure to alcohol and nicotine during adolescence has been reported to increase EtOH intake in adulthood. We performed a longitudinal animal study and hypothesized adolescent rats allowed to self-administer nicotine would drink more alcohol as adults. Methods: Adolescent, postnatal day (P)32, and adult (P90) male and female Sprague-Dawley rats were allowed to self-administer EtOH, Nic, or a combination of both, EtOH+Nic, in an intravenous self-administration paradigm. The role of KOR was pharmacologically evaluated with the KOR antagonist, norbinaltorphamine (norBNI) and with the KOR agonist, U50,488H. Alcohol drinking was subsequently evaluated with male rats in a drinking in the dark (DID), 2-bottle choice test. Results: Concurrent Nic increased EtOH intake in adolescent males, but not in adults or females. Pharmacological blockade of KOR with norBNI robustly increased EtOH+Nic self-administration in adult male rats, but had no effect with female rats. Lastly, in our longitudinal study with male rats, we found prior self-administration of Nic or EtOH+Nic during adolescence increased subsequent oral EtOH intake, whereas prior self-administration of EtOH alone in adults increased subsequent EtOH drinking. Conclusions: There are major age- and sex-differences in the reinforcing effects of EtOH+Nic. Adolescent males are sensitive to the reinforcing interactions of the two drugs, whereas this effect is inhibited by KOR activation in male adults. Nicotine self-administration in adolescent males also increased subsequent oral EtOH intake. These findings suggest that brain mechanisms underlying the reinforcing effects of EtOH and nicotine are both age- and sex-dependent, and that tobacco or e-cigarette use may increase the vulnerability of teenage boys to alcohol abuse.

Experimental Models on Effects of Psychostimulants

Psychostimulants are a diverse group of substances that cause an increase in psychomotor activity at least in part through their actions on catecholaminergic systems including the dopaminergic mesolimbic pathways. Animal models used to study addiction are based on the psychomotor stimulant theory of addiction. The basics of this theory are that the reinforcing effects and the addition liabilities of the drugs can be predicted from their ability to induce psychomotor activation. This approach focuses on the ability of the drugs to directly control the animal's behavior and to induce psychomotor stimulation, and is consistent with the behavioral definition of addiction and behavioral sensitization. Animal experiments have the advantage over clinical studies of lower variation and fewer confounding effects.

Low copulatory activity in selectively bred Sardinian alcohol-nonpreferring (sNP) relative to alcohol-preferring (sP) rats

BACKGROUND

There is a growing consensus that similar neural mechanisms are involved in the reinforcing properties of natural rewards, like food and sex, and drugs of abuse. Rat lines selectively bred for high and low oral alcohol intake and preference have been useful for understanding factors contributing to excessive alcohol intake and may constitute proper animal models for investigating the neurobiological basis of natural rewarding stimuli.

METHODS

The present study evaluated copulatory behavior in alcohol and sexually naïve Sardinian alcohol-preferring (sP) and -nonpreferring (sNP) male rats in three consecutive copulatory behavior tests.

RESULTS

The main finding was that, under the conditions used in this study, sNP rats were sexually inactive relative to sP rats. To gain more information about the sexual behavior in sP rats, Wistar rats were included as an external reference strain. Only minor differences between sP and Wistar rats were revealed.

CONCLUSIONS

The reason behind the low copulatory activity of sNP rats remains to be elucidated, but may in part be mediated by innate differences in brain transmitter systems. The comparison between sP and Wistar rats may also suggest that the inherent proclivity to excessive alcohol drinking in sP rats may mainly be dependent on its anxiolytic properties, as previously proposed, and not changes in the reward system.

Intravenous alcohol self-administration in the P rat

Alcohol consumption produces a complex array of effects that can be divided into two types: the explicit pharmacological effects of ethanol (which can be temporally separate from time of intake) and the more temporally "relevant" effects (primarily olfactory and taste) that bridge the time from intake to onset of the pharmacological effects. Intravenous (IV) self-administration of ethanol limits the confounding "non-pharmacological" effects associated with oral consumption, allows for controlled and precise dosing, and bypasses first order absorption kinetics, allowing for more direct and better-controlled assessment of alcohol's effect on the brain. IV ethanol self-administration has been reliably demonstrated in mouse and human experimental models; however, models of IV self-administration have been historically problematic in the rat. An operant multiple-schedule study design was used to elucidate the role of each component of a compound IV-ethanol plus oral-sucrose reinforcer. Male alcohol-preferring P rats had free access to both food and water during all IV self-administration sessions. Animals were trained to press a lever for orally delivered 1% sucrose (1S) on a fixed ratio 4 schedule, and then surgically implanted with an indwelling jugular catheter. Animals were then trained to respond on a multiple FR4-FR4 schedule composed of alternating 2.5-min components across 30-min sessions. For the multiple schedule, two components were used: an oral 1S only and an oral 1S plus IV 20% ethanol (25 mg/kg/injection). Average total ethanol intake was 0.47 ± 0.04 g/kg. We found significantly higher earning of sucrose-only reinforcers and greater sucrose-lever error responding relative to the compound oral-sucrose plus IV-ethanol reinforcer. These response patterns suggest that sucrose, not ethanol, was responsible for driving overall responding. The work with a compound IV ethanol-oral sucrose reinforcer presented here suggests that the existing intravenous ethanol self-administration methodology cannot overcome the aversive properties of ethanol via this route in the rat.

Roux-en-Y gastric bypass increases intravenous ethanol self-administration in dietary obese rats

Roux-en-Y gastric bypass surgery (RYGB) is an effective treatment for severe obesity. Clinical studies however have reported susceptibility to increased alcohol use after RYGB, and preclinical studies have shown increased alcohol intake in obese rats after RYGB. This could reflect a direct enhancement of alcohol’s rewarding effects in the brain or an indirect effect due to increased alcohol absorption after RGYB. To rule out the contribution that changes in alcohol absorption have on its rewarding effects, here we assessed the effects of RYGB on intravenously (IV) administered ethanol (1%). For this purpose, high fat (60% kcal from fat) diet-induced obese male Sprague Dawley rats were tested ∼2 months after RYGB or sham surgery (SHAM) using both fixed and progressive ratio schedules of reinforcement to evaluate if RGYB modified the reinforcing effects of IV ethanol. Compared to SHAM, RYGB rats made significantly more active spout responses to earn IV ethanol during the fixed ratio schedule, and achieved higher breakpoints during the progressive ratio schedule. Although additional studies are needed, our results provide preliminary evidence that RYGB increases the rewarding effects of alcohol independent of its effects on alcohol absorption.

Effects of the combination of metyrapone and oxazepam on intravenous nicotine self-administration in rats

RATIONALE

Despite increased education regarding its dangers, cigarette smoking remains a significant public health concern due to serious associated health consequences such as cancer and respiratory and cardiovascular diseases. Most smokers fail in their attempts to quit smoking, and current pharmacological interventions have relatively low levels of efficacy and are associated with significant adverse events. We have previously reported that combinations of metyrapone and oxazepam, administered at doses that were ineffective when delivered singly, resulted in dose-related decreases in cocaine self-administration in rats while not affecting food-maintained responding during the same sessions.

OBJECTIVES

The current study was designed to test the effects of the administration of a metyrapone:oxazepam combination on nicotine self-administration in rats.

METHODS

Several dose combinations of metyrapone (12.5, 25 or 50 mg/kg) and oxazepam (5 or 10 mg/kg) were tested in rats trained to intravenously (IV) self-administer nicotine (0.03 mg/kg/infusion) during 1-h self-administration sessions using both fixed-ratio and progressive-ratio (PR) schedules of reinforcement.

RESULTS

The administration of low doses of metyrapone and oxazepam in combination significantly decreased IV nicotine self-administration in rats. At the lowest doses of 12.5 mg/kg of metyrapone and 5 mg/kg of oxazepam, the drugs alone did not decrease IV nicotine self-administration, but the combination was effective. Varenicline was also tested using the fixed-ratio schedule, and reductions in nicotine intake were similar to those seen with the moderate dose of the combination.

CONCLUSIONS

The results of this study suggest a potential utility of the combination of metyrapone and oxazepam for smoking cessation in humans.

Effects of the specific α4β2 nAChR antagonist, 2-fluoro-3-(4-nitrophenyl) deschloroepibatidine, on nicotine reward-related behaviors in rats and mice

RATIONALE

Alleviating addiction to tobacco products could prevent millions of deaths. Investigating novel compounds selectively targeting α4β2 nAChRs hypothesized to have a key role in the rewarding effects of nicotine may be a useful approach for future treatment.

OBJECTIVES

The present study was designed to evaluate 2-fluoro-3-(4-nitrophenyl) deschloroepibatidine (4-nitro-PFEB), a potent competitive antagonist of neuronal α4β2 nAChRs, in several animal models related to nicotine reward: drug discrimination, intracranial self-stimulation (ICSS), conditioned place preference, and limited access to self-administration.

METHODS

Long Evans rats were trained in a two-lever discrimination procedure to discriminate 0.4 mg/kg nicotine (s.c.) from saline. Male Sprague-Dawley rats were stereotaxically implanted with electrodes and trained to respond for direct electrical stimulation of the medial forebrain bundle. ICR mice were evaluated using an unbiased place preference paradigm, and finally, male Wistar rats were implanted with intrajugular catheters and tested for nicotine self-administration under limited access (1 h/day).

RESULTS

4-Nitro-PFEB attenuated the discriminative stimulus effects of nicotine, but alone did not produce nicotine-like discriminative stimulus effects. Nicotine-induced facilitation of ICSS reward thresholds was reversed by 4-nitro-PFEB, which alone had no effect on thresholds. 4-Nitro-PFEB also blocked the conditioned place preference produced by nicotine, but alone had no effect on conditioned place preference. Finally, 4-nitro-PFEB dose-dependently decreased nicotine self-administration.

CONCLUSIONS

These results support the hypothesis that neuronal α4β2 nAChRs play a key role in mediating the rewarding effects of nicotine and further suggest that targeting α4β2 nAChRs may yield a potential candidate for the treatment of nicotine dependence.

Substituted heteroaromatic compounds: effect on nicotine self-administration in rats

RATIONALE

Certain compounds that nonselectively inhibit a prominent human nicotine-metabolizing enzyme (i.e., human cytochrome P-450 2A6, hCYP 2A6) showed inhibition of smoking in humans. However, a comprehensive examination of hCYP 2A6 inhibitors to decrease nicotine self-administration in rats has not been reported.

OBJECTIVES

We tested substituted heteroaromatic compounds designed to selectively inhibit hCYP 2A6 in a model system to (a) examine selective hCYP 2A6 inhibitors to decrease cotinine formation in vivo in rats administered with nicotine and (b) examine their efficacy to decrease nicotine self-administration in rats.

METHODS

Rats were trained to IV self-administer nicotine in 1-h sessions. Nicotine self-administration was carried out at a unit dose of 0.03 mg/kg/infusion in 0.1 ml/s. Pretreatment with substituted heteroaromatic test compounds (0.5-25 mg/kg, i.p., 30 min prior to nicotine self-administration sessions) resulted in dose-dependent decreases of nicotine self-administration. Using operant conditioning techniques, nicotine- vs. food-reinforced responding was evaluated for compounds 10 and 11.

RESULTS

Compounds 10 and 11 selectively decreased nicotine self-administration with estimated ED(50) values 4 and 2.8 mg/kg, respectively. Of the test compounds examined, none showed significant affinity for mammalian α4β2- or α7-neuronal nicotinic acetylcholine (nAChR) receptors and none were inhibitors of the human dopamine transporter (hDAT); thus, neither the endogenous nAChRs nor DAT apparently plays a role in decreasing nicotine self-administration for this series of compounds.

CONCLUSION

The results indicate that chemical analogs of nicotine can play a role in nicotine self-administration harm reduction but a non-nAChR and a non-hDAT mechanism are likely involved.

Cocaine self-administration in Warsaw alcohol high-preferring (WHP) and Warsaw alcohol low-preferring (WLP) rats

Individuals prone to drug self-administration may be vulnerable not only to a single drug reinforcer but to a variety of drug reinforcers. It has been shown that two thirds of alcoholics regularly use drugs other than ethanol (alcohol). Up to 30% of alcohol-dependent patients report concurrent misuse of cocaine. The aim of the present study was to investigate intravenous cocaine self-administration in selectively bred, alcohol-preferring WHP (Warsaw high-preferring) and non-preferring WLP (Warsaw low-preferring) rats. It was hypothesized that WHPs could be more prone to cocaine self-administration in comparison to WLPs. Rats from both lines were allowed to nose-poke for cocaine infusions (0.33 mg/kg/infusion) under the FR-1, FR-2, and FR-3 schedule of reinforcement. Dose-response curves were assessed with increasing doses of cocaine (0.03, 0.1, 0.33, 1.0mg/kg/infusion). The WHP and WLP rats did not differ in cocaine self-administration. Both groups quickly acquired nose-poke responding for cocaine, presented a similar response profile when the schedule of reinforcement was increased from FR-1 to FR-3, and similar sensitivity to cocaine in the dose-response test. The present results may indicate that the selective breeding of alcohol-preferring WHP and alcohol non-preferring WLP rats did not lead to differences in cocaine's rewarding effects as assessed in the self-administration procedure.

A critical evaluation of a nicotine vaccine within a self-administration behavioral model

(S)-Nicotine is a psychostimulant legal drug responsible for causing addiction to tobacco smoking. Tobacco smoking has been irrevocably linked to a number of serious diseases and at present is considered the leading cause of preventable death in the United States. Despite well-documented adverse medical consequences, nicotine addiction has historically been one of the hardest to break. Current therapies have offered limited success and show high rates of relapse, emphasizing the need to engineer alternative therapies to aid nicotine cessation. The current study presents a protein-based immunopharmacotherapy approach for the treatment of nicotine addiction. Immunopharmacotherapy aims to use highly specific antibodies to blunt passage of drug into the brain thus minimizing reinforcing effects on the reward pathways of the central nervous system. Generation of a successful vaccine heavily relies on appropriate optimization of hapten design, immunogenic carrier and adjuvant. Modification of a classical nicotine hapten in conjugation with three distinct carrier proteins allowed for priming of a nicotine vaccine able to elicit significant amounts of nicotine-specific antibodies. Increased self-administration with use of a high drug dose (0.03 mg/kg/infusion; approximately 2 cigarettes in human) was observed in the vaccinated versus control animals suggesting a compensatory pattern and possibly reduced passage of nicotine to the brain. These results support the hypothesis that proper optimization of vaccine formulations could lead to successful nicotine vaccines for human use.

Role of protein kinase C epsilon (PKCvarepsilon) in the reduction of ethanol reinforcement due to mGluR5 antagonism in the nucleus accumbens shell

RATIONALE

The type 5 metabotropic glutamate receptor (mGluR5) and the epsilon isoform of protein kinase C (PKCepsilon) regulate ethanol intake, and we have previously demonstrated that mGluR5 receptor antagonism reduces ethanol consumption via a PKCepsilon-dependent mechanism.

OBJECTIVES

We explored the potential neuroanatomical substrates of regulation of ethanol reinforcement by this mGluR5-PKCepsilon signaling pathway by infusing selective inhibitors of these proteins into the shell or core region of the nucleus accumbens (NAc).

METHODS

Male Wistar rats were trained to self-administer ethanol intravenously and received intra-NAc infusions of vehicle or the selective mGluR5 antagonist 3-((2-methyl-1,3-thiazol-4-yl)ethynyl)pyridine (MTEP) alone and in combination with a PKCepsilon translocation inhibitor (epsilonV1-2) or a scrambled control peptide (svarepsilonV1-2). The effects of intra-NAc MTEP on food-reinforced responding and open-field locomotor activity were also determined.

RESULTS

MTEP (1 microg/microl) had no effect on ethanol or food reinforcement or locomotor activity when infused into either region. MTEP (3 microg/microl) reduced ethanol reinforcement when infused into the NAc shell but not the core, and this effect was reversed by epsilonV1-2 (1 microg/microl) but not sepsilonV1-2 (1 microg/microl). In both regions, this concentration of MTEP did not alter food-reinforced responding or locomotor activity, and infusion of epsilonV1-2 alone did not alter ethanol reinforcement. MTEP (10 microg/microl) reduced locomotor activity when infused into the shell; therefore, this concentration was not further tested on responding for ethanol or food.

CONCLUSIONS

Blockade of mGluR5 receptors in the NAc shell reduces ethanol reinforcement via a PKCepsilon-dependent mechanism.

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.

Reinstatement of ethanol-seeking behavior following intravenous self-administration in Wistar rats

BACKGROUND

In animal models of alcoholism, subjects are traditionally trained to self-administer ethanol via the oral route. However, ethanol is also self-administered intravenously (IV), a paradigm which offers several advantages over oral self-administration methods, including immediate delivery to the bloodstream, more rapid onset of pharmacological effects, and elimination of the need to utilize tastants or sweeteners to mask the aversive orosensory properties of ethanol. However, no studies to date have examined reinstatement of ethanol-seeking behavior in animals with a history of IV ethanol self-administration.

METHODS

Male Wistar rats were implanted with indwelling jugular vein catheters and trained to self-administer ethanol IV (1% v/v solution, equivalent to 1 mg/kg) in an operant lever-pressing paradigm in twice daily 1 hour sessions. Each IV delivery of ethanol was paired with presentation of a light-tone complex stimulus. After stabilization of response patterns, IV self-administration behavior was subjected to extinction procedures. Next, animals were exposed to the three types of stimuli known to reinstate ethanol-seeking behavior: presentation of ethanol-associated cues, a priming dose of ethanol (0.5 g/kg i.p.), or exposure to stress via administration of the anxiogenic compound yohimbine (2.5 mg/kg i.p.) or its corresponding vehicle.

RESULTS

During the maintenance phase of self-administration, animals exhibited significantly more presses on the lever that delivered the ethanol solution than the inactive lever, indicating that IV ethanol functioned as a positive reinforcer. Following extinction, it was found that ethanol-seeking behavior could be reinstated by all three types of stimuli (cues, ethanol priming, and yohimbine). Vehicle injection did not affect responding on either lever.

CONCLUSIONS

Ethanol serves as a reinforcer when self-administered IV, and following extinction, ethanol-seeking behavior can be reinstated by ethanol-associated cues, ethanol priming, or a pharmacological stressor. Thus, reinstatement of ethanol-seeking behavior in animals with a history of IV ethanol self-administration may be a novel animal model of relapse.

The alcohol-preferring AA and alcohol-avoiding ANA rats: neurobiology of the regulation of alcohol drinking

The AA (alko, alcohol) and ANA (alko, non-alcohol) rat lines were among the earliest rodent lines produced by bidirectional selection for ethanol preference. The purpose of this review is to highlight the strategies for understanding the neurobiological factors underlying differential alcohol-drinking behavior in these lines. Most early work evaluated functioning of the major neurotransmitter systems implicated in drug reward in the lines. No consistent line differences were found in the dopaminergic system either under baseline conditions or after ethanol challenges. However, increased opioidergic tone in the ventral striatum and a deficiency in endocannabinoid signaling in the prefrontal cortex of AA rats may comprise mechanisms leading to increased ethanol consumption. Because complex behaviors, such as ethanol drinking, are not likely to be controlled by single factors, system-oriented molecular-profiling strategies have been used recently. Microarray based expression analysis of AA and ANA brains and novel data-mining strategies provide a system biological view that allows us to formulate a hypothesis on the mechanism underlying selection for ethanol preference. Two main factors appear active in the selection: a recruitment of signal transduction networks, including mitogen-activated protein kinases and calcium pathways and involving transcription factors such as Creb, Myc and Max, to mediate ethanol reinforcement and plasticity. The second factor acts on the mitochondrion and most likely provides metabolic flexibility for alternative substrate utilization in the presence of low amounts of ethanol.

Short and Prolonged Periods of Maternal Separation and Voluntary Ethanol Intake in Male and Female Ethanol-Preferring AA and Ethanol-Avoiding ANA Rats

BACKGROUND

Genetic as well as environmental factors can affect the propensity for psychopathology and/or drug dependence. Maternal separation represents an animal experimental model that is useful in studies of effects of early life experiences. The authors have established a protocol for short and prolonged periods of maternal separation to study adult neurochemistry, behavior, and ethanol intake and have previously reported alterations in ethanol intake in Wistar rats and ethanol-preferring rats. The aim of the current study was to more thoroughly study how early life experiences affect an inherited propensity for high and low ethanol intake, respectively, in male and female ethanol-preferring AA (Alko alcohol) and ethanol-avoiding ANA (Alko, Non-Alcohol) rats.

METHODS

AA and ANA pups were assigned to one of three different rearing conditions: 15 min (MS15) or 360 min (MS360) of daily maternal separation in litters or normal animal facility rearing (AFR) during postnatal days 1 to 21. In adulthood, voluntary ethanol intake was investigated using the two-bottle free choice paradigm.

RESULTS

In male ethanol-preferring AA rats, MS15 resulted in a lower intake and fewer high-preferring animals at 8% and 10% ethanol compared with MS360 rats. The male MS360 rats had a higher ethanol intake at 8% and 10% ethanol in comparison with AFR rats. In contrast, the female AA MS15 and MS360 rats had a lower ethanol intake and a lower preference for the 10% ethanol solution compared with the female AA AFR rats. In male and female ANA rats, no major separation-induced effects were found.

CONCLUSIONS

The current results show that genetic inheritance can be affected by environmental manipulations in AA rats with an inherent high ethanol intake. The findings in female ethanol-preferring AA rats give further evidence of a differential outcome of maternal separation in male and female rats, as previously shown.

The role of mesolimbic dopamine in the development and maintenance of ethanol reinforcement

The neurobiological processes by which ethanol seeking and consumption are established and maintained are thought to involve areas of the brain that mediate motivated behavior, such as the mesolimbic dopamine system. The mesolimbic dopamine system is comprised of cells that originate in the ventral tegmental area (VTA) and project to several forebrain regions, including a prominent terminal area, the nucleus accumbens (NAcc). The NAcc has been subdivided into core and shell subregions. Both areas receive converging excitatory input from the cortex and amygdala and dopamine input from the VTA, with the accumbal medium spiny neuron situated to integrate the signals. Although forced ethanol administration enhances dopamine activity in the NAcc, conclusions regarding the role of mesolimbic dopamine in ethanol reinforcement cannot be made from these experiments. Behavioral experiments consistently show that pharmacological manipulations of the dopamine transmission in the NAcc alter responding for ethanol, although ethanol reinforcement is maintained after lesions of the accumbal dopamine system. Additionally, extracellular dopamine increases in the NAcc during operant self-administration of ethanol, which is consistent with a role of dopamine in ethanol reinforcement. Behavioral studies that distinguish appetitive responding from ethanol consumption show that dopamine is important in ethanol-seeking behavior, whereas neurochemical studies suggest that accumbal dopamine is also important during ethanol consumption before pharmacological effects occur. Cellular studies suggest that ethanol alters synaptic plasticity in the mesolimbic system, possibly through dopaminergic mechanisms, and this may underlie the development of ethanol reinforcement. Thus, anatomical, pharmacological, neurochemical, cellular, and behavioral studies are more clearly defining the role of mesolimbic dopamine in ethanol reinforcement.

Differential behavioural sensitization to intermittent morphine treatment in alcohol-preferring AA and alcohol-avoiding ANA rats: role of mesolimbic dopamine

Alcohol-preferring AA (Alko Alcohol) and alcohol-avoiding ANA (Alko Non-Alcohol) rats have well-documented differences in their voluntary ethanol consumption and brain opioidergic systems. The aim of the present study was to investigate whether these rat lines differ in their susceptibility to morphine-induced behavioural and neurochemical sensitization. The rats were given 15 injections of morphine (10 mg/kg, s.c.) or saline every other day. Locomotor activity and release of dopamine in the nucleus accumbens were monitored after a challenge with additional morphine injections (10 mg/kg) 1 and 5 weeks after withdrawal from the repeated treatment. Morphine increased locomotion more in the previously morphine-treated rats than in the saline-treated controls. Furthermore, AA rats were more sensitive to this effect of morphine than ANA rats. Accumbal morphine-induced dopamine release was significantly higher in the morphine-treated AA than ANA rats after the first challenge injection 1 week from withdrawal, but no differences were observed after the second challenge. The brain and plasma concentrations of morphine were similar among the lines suggesting that the differences in the effects of morphine cannot be explained in terms of differential pharmacokinetics of morphine in these lines. These data show that AA rats are more susceptible to morphine-induced behavioural sensitization than ANA rats. Furthermore, it suggests that mesolimbic dopamine has at best only a transient role in the expression of opioid-induced behavioural sensitization. The relationship between the mechanisms underlying the differential sensitivity of these rat lines to the effects of repeated morphine and voluntary ethanol drinking remains to be determined.

Intravenous ethanol/cocaine self-administration initiates high intake of intravenous ethanol alone

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.

Marinobufagenin (MBG) suppression of ethanol-seeking behavior is associated with inhibition of brain cortex Na/K-ATPase in mice

In the present study the hypothesis was tested that sodium pump ligands (SPL) can modulate alcohol-seeking behavior and that this effect is related to changes in Na/K-ATPase activity in the central nervous system. Mice were tested for initiation of ethanol intravenous self-administration (IVSA) following i.p. pretreatment with vehicle or the endogenous SPL, marinobufagenin (MBG). Drug- and experimentally-naive mice acquired IVSA of 2% ethanol during a single 30-min session. MBG was found to dose-dependently attenuate (1.25-2.5 microg/kg) initiation of ethanol IVSA producing a decrease in the ratio and in the difference between operant responses of response-dependent and yoked animals as well as a decrease in percentage of mice demonstrating ethanol-seeking behavior. Attenuation of the reinforcing effect of ethanol resulting from MBG was associated with brain levels of this steroid capable of concurrently inhibiting Na/K-ATPase in the brain cortex. We hypothesize that endogenous digitalis-like factors could modulate the reinforcing effect of ethanol.

Effects of cocaine on responding for ethanol or sucrose under a progressive ratio schedule

Progressive ratio (PR) schedules have been increasingly used to study motivation for self-administered drugs of abuse, such as psychostimulants and ethanol. In these and other studies, the breaking point (BP) is thought to be a measure of the motivation of the animal to work for a particular reward. Ethanol, a highly abused drug, maintains only low BPs. The present experiment was designed to examine if the low BP achieved by animals working for ethanol could be increased by the administration of a psychostimulant. A group working for a sucrose reinforcer was included for comparison. Rats previously trained to lever press under a PR schedule for 0.1 ml aliquots of 10% ethanol or 5% sucrose reinforcers were dosed once a week with cocaine (0, 5 and 15 mg/kg intraperitoneally) 30 min prior to their daily operant session using a Latin square design. Vehicle and 5 mg/kg cocaine had no effect on BP for any reinforcer, but 15 mg/kg cocaine produced a significantly higher BP (P<0.05) for animals working for either ethanol or sucrose. The same doses of cocaine decreased consumption of, and preference for, a 5% sucrose solution. These results indicate that, although cocaine administration does not increase sucrose preference, it may increase BP values in PR schedules. It is therefore unlikely that the increases in BP reflect cocaine-induced increased motivation, and they may be due to cocaine's stimulant or other properties. These data reinforce opinions that PR schedules may be unsuitable for assessing the effects of experimental manipulations on motivation for drugs with stimulant actions.

Neurobiological processes in alcohol addiction

This article represents the proceedings of a symposium at the ISBRA Meeting in Yokohama, Japan. The chairs were A. D. Lê and K. Kiianmaa. The presentations were (1) Alcohol reward and aversion, by C. L. Cunningham; (2) The role of sensitization of neuronal mechanisms in ethanol self-administration, by J. A. Engel, M. Ericson, and B. Söderpalm; (3) Alcohol self-administration in dependent animals: Neurobiological mechanisms, by G. F. Koob, A. J. Roberts, and F. Weiss; (4) Stress and relapse to alcohol, by A. D. Lê; (5) Alcohol-preferring AA and alcohol-avoiding ANA rats differ in locomotor activation induced by repeated morphine injections, by P. Hyytiä, S. Janhunen, J. Mikkola, P. Bäckström, and K. Kiianmaa; and (6) Initial sensitivity and acute functional tolerance to the hypnotic effects of ethanol in mice genetically selected for mild and severe ethanol withdrawal convulsions, by I. Ponomarev and J. C. Crabbe.

Taste reactivity in alcohol-preferring AA and alcohol-avoiding ANA rats

Alcohol-preferring rats (Alko alcohol or AA) were tested for taste reactivity to water, sucrose, quinine, and a range of alcohol concentrations (5-40%) both before and after a period of continuous alcohol access. The alcohol-avoiding line of rats (Alko nonalcohol or ANA) was also tested for comparison. It was found that AA rats displayed greater ingestive reactivity to alcohol compared to ANA rats both before and after a three-week period of continuous access to 10% alcohol (during which time AA rats drank significantly more alcohol than ANA rats). AA rats also made significantly more ingestive responses to a 0.3 M sucrose solution and a 0.0005 M quinine solution. Differences between AA rats and ANA rats in aversive reactivity appeared only after the alcohol consumption tests; AA rats made significantly fewer aversive responses to the 30% and 40% concentrations after continuous alcohol access. AA rats also made significantly more aversive responses to the quinine solution. The results suggest that line differences between AA rats and ANA rats in the reactivity response to alcohol solutions have been selected in association with the original selection phenotype of alcohol consumption.

Effects of calcium channel blockade on intravenous self-administration of ethanol in rats

In the present study the involvement of voltage-operated calcium channels (VOCCs) in the acquisition and maintenance of operant i.v. ethanol (EtOH) self-administration was investigated in rats. Rats readily learned to self-administer EtOH (unit dose range: 0.5-4% v/v) within five daily 2-h sessions, when infusions were made contingent upon nose-poking in a hole containing infrared sensors. Response rate was related to the EtOH concentration in an inverted U-shaped manner, the maximal rate and intake being observed at a unit dose of 1% v/v (0.27 mg EtOH/infusion). Self-administration of EtOH appeared to be behaviorally specific, as responding in the reinforced hole did not coincide with increased responding in a nonreinforced hole. Daily treatment with the dihydropyridine VOCC blocker nimodipine (2.5-20 mg/kg, i.p., t-15 min) dose-dependently attenuated acquisition of EtOH self-administration; the 5 mg/kg dose resulting in a partial, and the 10 and 20 mg/kg doses in a complete prevention of i.v. self-administration behavior. The effects of nimodipine (2.5-5.0 mg/kg) were considered to be relatively specific, as an inhibition of the reinforced responding could be demonstrated in the absence of a significant effect on nonreinforced responding. When tested in rats showing stable self-administration behavior (unit dose: 1% v/v EtOH), nimodipine showed biphasic dose-response effects; with 2.5 and 5 mg/kg resulting in a mild increase, and 10 and 20 mg/kg resulting in a decrease of self-administration behavior, respectively. The present study suggests that blockade of VOCCs attenuates the reinforcing stimulus effects of EtOH; and, as such, the data may offer an explanation for the previously reported EtOH intake-reducing effects of dihydropyridine calcium channel ligands obtained in two-bottle choice paradigms. Dihydropyridine derivatives, such as nimodipine, may therefore offer an interesting approach to the pharmacotherapy of alcoholism.

Enhanced sensitivity of the nucleus accumbens proenkephalin system to alcohol in rats selectively bred for alcohol preference

Evidence suggests that alcohol-induced activation of the endogenous opioid system is part of a neurobiological mechanism that may be functionally involved in alcohol reinforcement and high alcohol drinking behavior. We postulate that a genetic predisposition toward alcohol drinking is accompanied by increased responsiveness of the opioid system to alcohol. To test this hypothesis, the present study compared the effect of an acute alcohol challenge on enkephalin gene expression in discrete brain regions which are high in preproenkephalin (PPENK) mRNA content and/or are important in mediating alcohol reward in rats selectively bred for alcohol preference (P) or nonpreference (NP). PPENK mRNA content was measured by in situ hybridization performed with a 36 base oligonucleotide probe for PPENK mRNA and was quantified using a computerized image-analysis system. Blood alcohol concentration (BAC) and rate of alcohol elimination following alcohol infusion were similar in P and NP rats. P and NP rats did not differ in basal content of PPENK mRNA in any of the brain areas examined prior to onset of infusion. An intragastric (I.G.) infusion of alcohol (2.5 g/kg b.wt) produced a significant increase in PPENK mRNA in the nucleus accumbens (both shell and core) of P but not NP rats at 1 h after the onset of infusion which coincided with the time at which peak BAC was attained. In contrast, at 8 h after the onset of the alcohol infusion, when BAC was falling toward baseline, PPENK mRNA was decreased in the nucleus accumbens of both P and NP rats and in the anterior striatum and amygdala of NP rats. The results suggest that enhanced responsiveness of the enkephalinergic system to alcohol is associated with, and may be functionally involved in, mediating high alcohol drinking behavior.

More vasopressin mRNA in the paraventricular hypothalamic nucleus of alcohol-preferring rats and high alcohol-drinking rats selectively bred for high alcohol preference

Both the selectively bred alcohol-preferring (P) and high alcohol-drinking (HAD) rats exhibit alcohol preference, and develop tolerance to alcohol more quickly than their counterparts, the alcohol-nonpreferring (NP) and low alcohol-drinking (LAD) rats, respectively. It has been shown that the P rats retain developed tolerance longer than do NP rats, and alcohol drinking increases concurrently with the development of tolerance. Although alcohol preference and tolerance are fundamental elements of alcoholism, the exact mechanisms underlying these two phenotypes in P and HAD rats are not well understood. Recent studies have suggested that arginine vasopressin (AVP) may be involved in modulation of alcohol tolerance. Accordingly, this study was designed to examine whether the AVP mRNA level in the hypothalamus differs in rats that have been selectively bred for alcohol preference and nonpreference. A 35S-AVP antisense oligodeoxynucleotide probe was used for in situ hybridization to localize AVP mRNA in the paraventricular hypothalamic nucleus (PVN) and supraoptic nucleus (SON), two major sites for AVP synthesis in the hypothalamus. Quantitative autoradiography demonstrated that P rats had higher levels of AVP mRNA in the PVN than NP rats. Similarly, higher levels of AVP mRNA were also found in the PVN of HAD rats, compared with LAD rats. The AVP mRNA levels in the SON were similar in the alcohol-preferring and alcohol-nonpreferring rat lines. Basal plasma AVP levels were higher in NP rats than in P rats as determined by radioimmunoassay, whereas plasma AVP levels were not significantly different between HAD and LAD rats. The results suggest that increased AVP gene expression in the PVN may contribute to alcohol preference and the development of alcohol tolerance.