Amphetamine-induced enhancement of ethanol consumption: role of central catecholamines

Methamphetamine self-administration reduces alcohol consumption and preference in alcohol-preferring P rats

Subclinical levels of polysubstance use are a prevalent and understudied phenomenon. Alcohol is a substance commonly co‐used with other substances of other drug classes. These studies sought to determine the consumption effects of combining alcohol drinking and methamphetamine (MA) self‐administration. Male alcohol‐preferring P rats had continuous access to a two‐bottle alcohol drinking procedure in the home cage. Control rats remained alcohol naïve. Rats were also surgically implanted with intra‐jugular catheters and trained to self‐administer saline (control) or MA in daily 2‐hour sessions. We first measured the acquisition and maintenance of MA intake in alcohol‐consuming or control rats. MA intake was initially enhanced by alcohol consumption on a fixed ratio 1 schedule of reinforcement, but this effect did not prevail as the difficulty of the schedule (FR5 and progressive ratio) was increased. We next measured both alcohol consumption and preference before, during and after MA (or saline) self‐administration. MA self‐administration significantly reduced alcohol intake and preference ratios, a robust effect that persisted across several experimental variations. Interestingly, alcohol consumption rebounded following the cessation of MA self‐administration. The effects of MA self‐administration were specific to alcohol intake because it did not alter total fluid consumption or consumption of sucrose. MA self‐administration did not impact blood‐alcohol concentrations or alcohol‐induced loss of righting reflex suggesting no effect of MA intake on the alcohol metabolism or sensitivity. Together, the results suggest that MA intake disrupts alcohol consumption and preferences but not the reverse in alcohol‐preferring P rats.

Pharmacologic dissociation between impulsivity and alcohol drinking in high alcohol preferring mice

BACKGROUND

Impulsivity is genetically correlated with, and precedes, addictive behaviors and alcoholism. If impulsivity or attention is causally related to addiction, certain pharmacological manipulations of impulsivity and/or attention may affect alcohol drinking, and vice versa. The current studies were designed to explore the relationship among impulsivity, drinking, and vigilance in selectively bred High Alcohol Preferring (HAP) mice, a line that has previously demonstrated both high impulsivity and high alcohol consumption. Amphetamine, naltrexone, and memantine were tested in a delay discounting (DD) task for their effects on impulsivity and vigilance. The same drugs and doses were also assessed for effects on alcohol drinking in a 2-bottle choice test.

METHODS

HAP mice were subjected to a modified version of adjusting amount DD using 0.5-second and 10-second delays to detect decreases and increases, respectively, in impulsive responding. In 2 experiments, mice were given amphetamine (0.4, 0.8, or 1.2 mg/kg), naltrexone (3 and 10 mg/kg), and memantine (1 and 5 mg/kg) before DD testing. Another pair of studies used scheduled access, 2-bottle choice drinking to assess effects of amphetamine (0.4, 1.2, or 3.0 mg/kg), naltrexone (3 and 10 mg/kg), and memantine (1 and 5 mg/kg) on alcohol consumption.

RESULTS

Amphetamine dose-dependently reduced impulsivity and vigilance decrement in DD, but similar doses left alcohol drinking unaffected. Naltrexone and memantine decreased alcohol intake at doses that did not affect water drinking but had no effects on impulsivity or vigilance decrement in the DD task.

CONCLUSIONS

Contrary to our hypothesis, none of the drugs tested here, while effective on either alcohol drinking or impulsivity, decreased both behaviors. These findings suggest that the genetic association between drinking and impulsivity observed in this population is mediated by mechanisms other than those targeted by the drugs tested in these studies.

Effects of cocaine-induced sensitization on ethanol drinking: sex and strain differences

Sensitization induced by repeated drug exposure has been proposed to increase 'wanting' the drug and to facilitate the transition from moderate to excessive drug intake. The present study examined the effects of cocaine-induced sensitization on ethanol-drinking behavior in male and female rats from different strains. In experiment 1, rats were pretreated with six injections of saline or cocaine (10 mg/kg, i.p.), spaced by 3-day intervals, and were subsequently allowed access to ethanol intake in an unrestricted free-choice procedure. In experiment 2, rats had acquired ethanol-drinking behavior and were exposed to the sensitizing treatment described previously or were left undisturbed. Subsequently, all animals again had access to ethanol. Whatever the sex and strain concerned, sensitized and control animals did not differ in either the acquisition or the maintenance of ethanol-drinking behavior, suggesting that cocaine-induced behavioral sensitization does not modify ethanol intake. The present results also confirm the sex- and strain-dependent character of alcohol intake and of the 'alcohol deprivation effect'.

Quinpirole- and amphetamine-induced hyperdipsia: influence of fluid palatability and behavioral cost

Daily administration of moderate doses of amphetamine or of the dopaminergic D2 agonist quinpirole is associated with the development of excessive, non-regulatory drinking. Here we compared the influence of manipulating fluid palatability and behavioral cost on the development of this drinking augmentation. Experiment 1 was based on the phenomenon of contrafreeloading (CFL): animals work for a resource even though the same resource is freely available. The effects of 15 daily injections of amphetamine (1.0 and 1.7 mg/kg i.p. ) or quinpirole (0.1 and 0.56 mg/kg i.p.) were evaluated in mildly water-deprived rats. For the first 6 days the rats obtained water by lever pressing (FR3) only; over the following 9 days water was also freely available (CFL). Initially, 0.56 mg/kg quinpirole reduced lever pressing for water. A complete recover of responding was then obtained, and was followed by a progressive increment in the amount water obtained by lever pressing during the CFL phase (from 10 to 50%). Amphetamine did not affect percent CFL, but at the highest dose (1.7 mg/kg) reduced total water intake during the last 3 days of treatment. In experiment 2 the rats had free access to two bottles, one of which contained tap water, and the other contained either an ethanol (6%) or a sucrose (5%) solution. After habituation to this regimen, the rats received 10 daily i.p. injections of vehicle, amphetamine (1.0 or 3 mg/kg), or quinpirole (0.1 or 0.56 mg/kg). Quinpirole 0.56 mg/kg enhanced daily fluid intake under both sucrose and ethanol conditions, but selectively reduced ethanol preference. The higher amphetamine dose reduced fluid intake and sucrose preference. In conclusion, chronic exposure to a dopaminergic D2 agonist, but not to amphetamine, produced an increment of drinking that was resistant to manipulation of either palatability or the behavioral cost of the fluid.

Lack of effect of dopamine D2 blockade on ethanol intake in selected and unselected strains of rats

Previous research has suggested that brain catecholamines may be involved in regulating ethanol intake. This study was designed to look more specifically at dopamine (DA) and whether DA D2 receptor blockade with the antagonist pimozide would alter ethanol consumption in rats. Subjects were male Maudsley Reactive and Wistar rats, the former previously shown to consume larger amounts of ethanol than the latter. Both strains were screened for ethanol intake by presentation of ethanol solutions (free choice with water) in increasing steps from 2% to 10% (v/v) on an alternate-day schedule. Following the screening period, animals were switched to a schedule of everyday presentation of the 10% (v/v) ethanol solution (free choice with water) for 10 baseline days. Animals were then divided into high and low drinking levels according to whether their mean baseline ethanol intake (g/kg) fell within +/- 0.5 SD of the mean intake of their group (Maudsley Reactives: mean = 2.55 g/kg, low drinkers < 1.63, high drinkers > 3.47; Wistars: mean = 2.17 g/kg, low drinkers < 1.53, high drinkers > 2.82). The animals were assigned to one of five treatment groups for 5 subsequent days where they received IP injections of pimozide (0.08, 0.24, or 0.48 mg/kg), tartaric acid, or saline. Following the treatment period, ethanol consumption was recorded for 5 posttreatment days. No significant differences due to treatment were observed for either intake or preference of ethanol across treatments, drinking groups, or strains. The results obtained in the present study suggested that interference in DA neurotransmission through administration of the D2 antagonist pimozide does not significantly alter ethanol consumption in either MR or Wistar animals.

Effect of ethanol on extracellular dopamine in nucleus accumbens: comparison between Lewis and Fischer 344 rat strains

The purpose of this study was to examine the differential effects of intraperitoneal ethanol on the mesoaccumbens dopamine (DA) system in Fischer 344 and Lewis rat strains, utilizing microdialysis in awake animals. At the lowest dose tested (0.5 g/kg), there were no changes in extracellular DA in the nucleus accumbens in either strain. There was a differential response to the intermediate dose of 1 g/kg ethanol, with an 84% increase in extracellular DA in the Fischer, but no change in Lewis rats. The highest dose administered (2 g/kg) did not induce significant increases in DA in either strain. These data demonstrate that the mesoaccumbens DA systems of Fischer and Lewis rat strains differ in their susceptibility to activation by ethanol, and suggest that the higher alcohol preference of Lewis rats is not associated with an enhanced DAergic response to acute experimental administration of ethanol.

Alcohol Self-Administration: Role of Mesolimbic Dopamine

It appears clear that ethanol reinforcement, like that of many abused drugs, utilizes the mesolimbic DA pathways. From the data presented on microinjection of DA agonists and antagonists, it would seem that only part of the regulatory process controlling ethanol drinking is directly involved with this pathway. Once drinking has begun, the DA antagonist raclopride results in a rapid termination of drinking. This appears to be a blocking effect of what may be conditioned reinforcement resulting from prior ethanol reinforcement initiation procedures. Microinjection of the DA agonists d-amphetamine and quinpirole prolonged drinking, with little signs of normal termination apparent in the 30-min session in many animals. This appeared to be the result of interference with normal termination processes. While it remains to be demonstrated that oral ethanol consumption results in the release of DA in the nucleus accumbens, evidence from prior work and the present studies support a role for the mesolimbic DA system in ethanol reinforcement.

Opioidergic, serotonergic, and dopaminergic manipulations and rats' intake of a sweetened alcoholic beverage

Groups of rats were maintained on a daily regimen of 22 h of water deprivation followed by a 2-h opportunity to take either water or a sweetened ethanol solution (ES). In one experiment, it was shown that previous morphine (M) dependence had no effect on initial daily intakes of fluids. After stable ES intakes were achieved, a variety of pharmacological manipulations were assessed for their effects on intake of the ES. Nalmefene, an opioid antagonist, dose-relatedly decreased intakes of ES, and was effective across days of injections. Fluoxetine (FX), a serotonergic reuptake inhibitor, also reduced ES intakes dose relatedly, and across days of injections, but the reduction was not as great as that seen with opioid antagonists. A small dose of M increased ES intakes when given in combination with an ineffective dose of FX, just as it does by itself. However, M had no effect on ES intakes in combination with an effective dose of FX. Pimozide (PIM), a dopaminergic antagonist, dose-relatedly decreased intakes of ES and water, and responding for positively reinforcing intracranial stimulation (ICS). When given in combination, M blunted PIM's reduction of ES intake, but had no effect on PIM's ability to decrease either intake of water or responding for ICS. Amphetamine did not reliably affect rats' intakes of ES across a range of doses. The data, in addition to previous work, lead to the idea that endogenous opioid systems are more salient, with respect to intake of alcoholic beverages, than the other tested neurotransmitter systems. Furthermore, the collective data suggest that a long-lasting opioid antagonist may be an effective pharmacological adjunct to other treatments for alcohol abuse and alcoholism.

Anatomical "circuitry" in the brain mediating alcohol drinking revealed by THP-reactive sites in the limbic system

The involvement of aldehyde adducts in the etiology of alcoholism continues to be supported by a number of experimental findings. These metabolites are synthesized endogenously from a condensation reaction of a biogenic aldehyde with a catechol- or indole-amine and act in the brain to augment or suppress the drinking of ethyl alcohol. When given by the intracerebroventricular route in an animal which does not prefer alcohol, certain tetrahydro-isoquinolines and beta-carbolines can augment significantly the voluntary intake of alcohol even in aversive concentrations. This paper describes the historical background and current status of the "Multiple Metabolite" theory of alcoholism. The recent identification of anatomical structures in the limbic-midbrain, limbic-forebrain of the Sprague-Dawley rat, which mediate changes in the intake of alcohol induced by tetrahydropapaveroline (THP) is also described. When injected in a low dose of 25 ng in a specific site, over a 3-day period, THP induces persistent increases in the intake of alcohol even in aversive concentrations. These THP-reactive sites comprise the substantia nigra, reticular formation, medial lemniscus, zona incerta, medial forebrain bundle, nucleus accumbens, olfactory tubercle, lateral septal nucleus, preoptic area, stria terminalis, and rostral hippocampus. A higher dose of 250 ng THP microinjected at homologous loci tends to inhibit the rat's self-selection of alcohol or exert no effect on drinking. Morphological mapping of histologically identified sites sensitive to THP revealed a distinct "circuitry" of neuronal structures overlapping both dopaminergic and enkephalinergic pathways. This "circuit" extends from the tegmental-nigral area of the midbrain rostrally to structures within the limbic-forebrain. When a THP-reactive structure, the N. accumbens, was lesioned by either of two neurotoxins, 6-hydroxydopamine or 5.7-dihydroxytryptamine, the rats' preference for alcohol increased sharply. This suggests that impairment of transmitter release, denervation supersensitivity or other perturbation of receptor function within this and other structures play a part in the aberrant drinking of alcohol. It is envisaged that a dopamine-enkephalin link underlies the mechanism for the onset, maintenance and permanency of alcohol preference generated by an aldehyde adduct. Finally, the "Two-Channel, Brain Metabolite" theory of alcoholism proposes that the transitory presence of an endogenously formed aldehyde adduct within cells of the brain causes a permanent perturbation of normal receptor processes and transmitter activity within synapses of specific structures of the limbic system. This theory thus explains the nature of the rewarding properties of alcohol as well as its complex addictive liability which is physiologically irreversible.

Isoquinolines, beta-carbolines and alcohol drinking: involvement of opioid and dopaminergic mechanisms

Two classes of amine-aldehyde adducts, the tetrahydroisoquinoline (TIQ) and beta-carboline (THBC) compounds, have been implicated in the mechanism in the brain underlying the addictive drinking of alcohol. One part of this review focuses on the large amount of evidence unequivocally demonstrating not only the corporeal synthesis of the TIQs and THBCs but their sequestration in brain tissue as well. Experimental studies published recently have revealed that exposure to alcohol enhances markedly the endogenous formation of condensation products. Apart from their multiple neuropharmacological actions, certain adducts when delivered directly into the brain of either the rat or monkey, to circumvent the brain's blood-barrier system, can evoke an intense and dose-dependent increase in the voluntary drinking of solutions of alcohol even in noxious concentrations. That the abnormal intake of alcohol is related functionally to opioid receptors in the brain is likely on the basis of several distinct lines of evidence which include: the attenuation of alcohol drinking by opioid receptor antagonists; binding of a TIQ to opiate receptors in the brain; and marked differences in enkephalin values in animals genetically predisposed to the ingestion of alcohol. Finally, it is proposed that the dopaminergic reward pathways which traverse the meso-limbic-forebrain systems of the brain more than likely constitute an integrative anatomical substrate for the adduct-opioid cascade of neuronal events which promote and sustain the aberrant drinking of alcohol.

Inhibition of brain dopa-decarboxylase by RO 4-4602 infused ICV blocks alcohol drinking induced in rats by cyanamide

Following the stereotaxic implantation of chronic cannulae for intracerebroventricular (ICV) infusion, rats were given an alcohol preference test to establish their preferred concentration in comparison with water. After alcohol was removed, 15 mg/kg cyanamide was then injected subcutaneously for 4 days in order to maximize volitional intake of single solutions of alcohol, which in these animals ranged from 7 to 15%. The L-dopa-decarboxylase inhibitor benserazide (Ro 4-4602) injected subcutaneously twice daily in doses of 50-100 mg/kg failed to alter the rats' alcohol consumption either in terms of g/kg or proportional values. However, when given ICV twice daily in concentrations of 10 ng-2.0 micrograms per 5.0 microliters volume, benserazide attenuated the rats' alcohol drinking significantly. This reduction occurred in a dose-dependent manner in terms of both absolute and proportional intakes of alcohol. Pre-treatment of the animals with 1.0 microgram benserazide given ICV, when alcohol was removed from the test situation, did not abolish the subsequent ingestion of alcohol but its peripheral administration (50 mg/kg) enhanced drinking. These results suggest that the interference with the metabolic pathway of dopamine or serotonin synthesis, possibly through the mechanism of reduced formation of aldehyde adducts in the brain, markedly alters the pattern of voluntary drinking in the rat. Alternatively, benserazide could act by its central inhibition of aldehyde dehydrogenase, which in turn would concomitantly elevate levels of acetaldehyde and thereby reduce alcohol drinking.

Stress-related effects of ethanol in mammals

The interactions of ethanol and stress are complex; some experimental models of stress cause increases in voluntary consumption of ethanol by animals, but others do not. Conversely, low doses relieve the effects of some stressors but not of others, while high doses of ethanol give rise to stress, and tolerances can develop to both effects. Recent studies of the molecular mechanisms of action of ethanol and benzodiazepines have not yet revealed fully the basis of the anti-stress effects of either. Clinical and epidemiological studies suggest that both the stress-relieving and -inducing actions of ethanol are important in human disease.