Dopaminergic and opiate agonists and antagonists differentially decrease multiple schedule responding maintained by sucrose/ethanol and sucrose

Inhibition of AMPA receptors (AMPARs) containing transmembrane AMPAR regulatory protein γ-8 with JNJ-55511118 shows preclinical efficacy in reducing chronic repetitive alcohol self-administration

BACKGROUND

A prominent therapeutic indication for alcohol use disorder (AUD) is reduction in chronic repetitive alcohol use. Glutamate α-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPARs) regulate chronic alcohol self-administration in preclinical models. Recent evidence indicates that the expression and function of AMPARs require the transmembrane AMPAR regulatory protein γ-8 (TARP γ-8). This study evaluated the preclinical efficacy of JNJ-55511118, a novel, selective, high-affinity inhibitor of TARP γ-8-bound AMPARs, in reducing chronic operant alcohol self-administration.

METHODS

Separate groups of male and female C57BL/6J mice (n = 8/sex/group) were trained to lever press for sweetened alcohol (9% v/v + sucrose 2% w/v) or sucrose only (2% w/v) in operant conditioning chambers using an FR-4 schedule of reinforcement. After a 40-day baseline, JNJ-55511118 (0, 1, and 10 mg/kg, p.o.) was administered in randomized order 1 h before self-administration sessions. Parameters of operant behavior including response rate, total reinforcers, and head entries in the drinking troughs were computer recorded.

RESULTS

During baseline, responding to alcohol, but not sucrose, was greater in female than male mice. In male mice, both doses of JNJ-55511118 decreased multiple parameters of alcohol self-administration but did not reduce behavior-matched sucrose-only self-administration. JNJ-55511118 had no effect on sweetened alcohol or sucrose self-administration in female mice. Subsequent tests of motor function showed that the lowest effective dose of JNJ-55511118 (1 mg/kg) had no effect on open-field activity in male mice.

CONCLUSIONS

This study shows for the first time that TARP γ-8-bound AMPARs regulate a behavioral pathology associated with addiction. The preclinical efficacy of JNJ-55511118 in reducing alcohol self-administration in male mice suggests that inhibition of TARP γ-8-bound AMPARs is a novel and highly significant neural target for developing medications to treat AUD and other forms of addiction.

Operant, oral alcohol self-administration: Sex differences in Sardinian alcohol-preferring rats

Sardinian alcohol-preferring (sP) rats have been selectively bred, over almost 40 years, for high alcohol preference and consumption. sP rats have served as an animal model for more than 120 published studies. With very few exceptions, however, these studies have always employed male sP rats, and little is known about alcohol-related behaviors in female sP rats. The present study was designed to fill, at least in part, this gap. Accordingly, alcohol self-administration under the fixed ratio 4 schedule of reinforcement was compared among male, intact female, and ovariectomized female sP rats. Additionally, it was investigated whether i) estrous cycle influenced alcohol self-administration, and ii) alcohol self-administration in the three sP rat groups differed in sensitivity to pharmacological manipulation. Lever-responding for alcohol was steadily higher in male than intact and ovariectomized female sP rats; conversely, because of large sex differences in rat body weight, estimated amount of self-administered alcohol (in g/kg) did not differ among the three sP rat groups or occasionally was higher in intact female than male and ovariectomized female sP rats. Blood alcohol levels derived from self-administered alcohol i) did not differ among the three sP rat groups and ii) were positively correlated with the number of lever-responses for alcohol and the estimated amount of self-administered alcohol. Treatment with the opioid receptor antagonist, naloxone (0, 0.3, 1, and 3 mg/kg, i.p. [intraperitoneally]), and the positive allosteric modulator of the GABA_B receptor, GS39783 (0, 25, 50, and 100 mg/kg, i.g. [intragastrically]), reduced alcohol self-administration with comparable potency and efficacy in the three sP rat groups. The impact of the estrous cycle on alcohol self-administration was relatively modest, limited to a tendency toward a reduction in the number of lever-responses for alcohol and the estimated amount of self-administered alcohol in estrus and metestrus. Together, these results provide the first characterization of alcohol-seeking and -taking behavior in female sP rats.

A Rat Drinking in the Dark Model for Studying Ethanol and Sucrose Consumption

Background: The intermittent access 2-bottle choice (IA2BC) and drinking in the dark (DID) models were developed for studying rodent binge-like consumption. Traditionally, IA2BC was used with rats and DID with mice. Recently, IA2BC was adapted to study mouse ethanol consumption. However, it is unknown whether DID is suitable for rats or if one rat model is more advantageous than another for studying binge-like consumption.

Methods: Male Wistar rats consumed 20% ethanol or 5% sucrose using IA2BC or DID for 12 weeks. IA2BC drinking sessions occurred on alternate days (Mondays–Fridays) and lasted 24 h, whereas DID sessions ran 4 h/day, 5 days/week (Monday–Friday). Average consumption/session, week and hour was measured. To explore DID model suitability for screening novel compounds for controlling ethanol and sucrose intake, varenicline (2 mg/kg) or vehicle was administered to DID rats.

Results: IA2BC rats consume more ethanol/session and similar amounts of ethanol/week than DID rats. While, IA2BC rats consume more sucrose/session and week than DID rats. Although IA2BC rats had more ethanol and sucrose access time, DID rats had greater ethanol and sucrose intake/hour. Varenicline significantly reduced ethanol and sucrose consumption in DID rats, consistent with previously published IA2BC studies.

Conclusions: Despite the shorter access time, the rat DID model induced higher initial intake and greater consumption/hour for both ethanol and sucrose. The shorter duration of DID sessions did not prevent detection of varenicline-induced reductions in ethanol or sucrose consumption, suggesting the DID model may be suitable for studying binge-like ethanol and sucrose consumption.

Comparison of dehydroepiandrosterone (DHEA) and pregnanolone with existing pharmacotherapies for alcohol abuse on ethanol- and food-maintained responding in male rats

The present study compared two putative pharmacotherapies for alcohol abuse and dependence, dehydroepiandrosterone (DHEA) and pregnanolone, with two Food and Drug Administration (FDA)-approved pharmacotherapies, naltrexone and acamprosate. Experiment 1 assessed the effects of different doses of DHEA, pregnanolone, naltrexone, and acamprosate on both ethanol- and food-maintained responding under a multiple fixed-ratio (FR)-10 FR-20 schedule, respectively. Experiment 2 assessed the effects of different mean intervals of food presentation on responding for ethanol under a FR-10 variable-interval (VI) schedule, whereas Experiment 3 assessed the effects of a single dose of each drug under a FR-10 VI-80 schedule. In Experiment 1, all four drugs dose-dependently decreased response rate for both food and ethanol, although differences in the rate-decreasing effects were apparent among the drugs. DHEA and pregnanolone decreased ethanol-maintained responding more potently than food-maintained responding, whereas the reverse was true for naltrexone. Acamprosate decreased responding for both reinforcers with equal potency. In Experiment 2, different mean intervals of food presentation significantly affected the number of food reinforcers obtained per session; however, changes in the number of food reinforcements did not significantly affect responding for ethanol. Under the FR-10 VI-80 schedule in Experiment 3, only naltrexone significantly decreased both the dose of alcohol presented and blood ethanol concentration (BEC). Acamprosate and pregnanolone had no significant effects on any of the dependent measures, whereas DHEA significantly decreased BEC, but did not significantly decrease response rate or the dose presented. In summary, DHEA and pregnanolone decreased ethanol-maintained responding more potently than food-maintained responding under a multiple FR-10 FR-20 schedule, and were more selective for decreasing ethanol self-administration than either naltrexone or acamprosate under that schedule. Experiment 2 showed that ethanol intake was relatively independent of the interval of reinforcement in the food-maintained component, and Experiment 3 showed that naltrexone was the most effective drug at the doses tested when the interval for food reinforcement was low and maintained under a variable-interval schedule.

Different contributions of dopamine D1 and D2 receptor activity to alcohol potentiation of brain stimulation reward in C57BL/6J and DBA/2J mice

C57BL/6J (C57) and DBA/2J (DBA) mice respond differently to drugs that affect dopamine systems, including alcohol. The current study compared effects of D1 and D2 receptor agonists and antagonists, and the interaction between D1/D2 antagonists and alcohol, on intracranial self-stimulation in male C57 and DBA mice to determine the role of dopamine receptors in the effects of alcohol on brain stimulation reward (BSR). In the initial strain comparison, dose effects on BSR thresholds and maximum operant response rates were determined for the D1 receptor agonist SKF-82958 (±-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine; 0.1-0.56 mg/kg) and antagonist SCH 23390 (+-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepinehydrochloride; 0.003-0.056 mg/kg), and the D2 receptor agonist quinpirole (0.1-3.0 mg/kg) and antagonist raclopride (0.01-0.56 mg/kg). For the alcohol interaction, SCH 23390 (0.003 mg/kg) or raclopride (0.03 mg/kg) was given before alcohol (0.6-2.4 g/kg p.o.). D1 antagonism dose-dependently elevated and SKF-82958 dose-dependently lowered BSR threshold in both strains; DBA mice were more sensitive to SKF-82958 effects. D2 antagonism dose-dependently elevated BSR threshold only in C57 mice. Low doses of quinpirole elevated BSR threshold equally in both strains, whereas higher doses of quinpirole lowered BSR threshold only in C57 mice. SCH 23390, but not raclopride, prevented lowering of BSR threshold by alcohol in DBA mice. Conversely, raclopride, but not SCH 23390, prevented alcohol potentiation of BSR in C57 mice. These results extend C57 and DBA strain differences to D1/D2 sensitivity of BSR, and suggest differential involvement of D1 and D2 receptors in the acute rewarding effects of alcohol in these two mouse strains.

Effects of the kappa opioid receptor antagonist, nor-binaltorphimine, on ethanol intake: impact of age and sex

The kappa opioid receptor (KOR) antagonist, nor-binaltorphimine (nor-BNI), was used to investigate the role of the KOR system in mediating ethanol intake. On P25 (adolescent) or P67 (adult) male and female rats were individually housed and given ad libitum access to food and water. The experimental procedure was initiated on P28 or P70: animals were given 30 min/day access to a 10% ethanol/supersaccharin solution every other day (3 baseline exposures). On the day after the final baseline test, rats were injected with nor-BNI (0, 2.5, 5, 10 mg/kg), with testing initiated 24 hr later (30-min access every other day, 3 test exposures). Nor-BNI (10 mg/kg) increased ethanol intake in adult males, whereas the same dose decreased intake in adult females, suggesting pronounced sex differences in KOR-associated mediation of ethanol intake in adulthood. There was no impact of nor-BNI in adolescent animals of either sex, suggesting that the KOR may play less of a role in modulating ethanol intake during adolescence.

Specific and nonspecific effects of naltrexone on goal-directed and habitual models of alcohol seeking and drinking

BACKGROUND

The opioid-receptor antagonist naltrexone (NTX) reduces goal-directed alcohol drinking in rats presumably by blunting alcohol reward. However, different operant conditioning behavior can be produced by different reinforcement schedules, with goal-directed operant behavior being more sensitive to changes in reward value than less flexible, habit-associated models. We tested the hypothesis that NTX more effectively reduces alcohol drinking and seeking in a goal-directed than in a habit-associated operant model, and more effectively reduces alcohol versus sucrose self-administration, consistent with diminished alcohol reward.

METHODS

Rats were trained to self-administer 10% alcohol or 1.5% sucrose in a lever-press task and then underwent a within-subject assessment of NTX (0.1 to 1 mg/kg) effects on operant behavior. A fixed-ratio (FR5) reinforcement schedule was used to model goal-directed behavior, and a variable-interval (VI30) schedule was used to model habitual behavior.

RESULTS

As predicted, NTX reduced fluid deliveries earned by the FR5-alcohol group significantly more than all other groups. However, NTX reduced lever presses during self-administration sessions in VI30-trained rats without reducing earned deliveries, due to the low contingency between rate of pressing and fluid deliveries under that schedule. Interestingly, when fluid delivery was withheld (extinction), NTX reduced reward-seeking in all rats. Finally, NTX blocked reinstatement of reward-seeking upon presentation of 0.2 ml alcohol or sucrose and associated cues in the FR5-trained but not VI30-trained rats.

CONCLUSIONS

NTX reduced goal-directed alcohol drinking compared with other operant conditions. In addition, NTX blocked reinstatement of reward-seeking in rats trained on the goal-directed FR5 reinforcement schedule but not in rats trained on the habit-like VI30 reinforcement schedule. However, NTX also exerted nonspecific effects on reward-seeking that were revealed under low-effort contingency conditions or absence of reward. Together, these data support the hypothesis that NTX is less effective in conditioning models that are more habit-associated.

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.

The acute anti-craving effect of acamprosate in alcohol-preferring rats is associated with modulation of the mesolimbic dopamine system

Acamprosate (Campral) is a drug used clinically for the treatment of alcoholism. In order to examine further the time-course and mechanism of action of acamprosate, the effect of acute and repeated acamprosate administration was examined on (i) operant ethanol self-administration and (ii) voluntary home cage ethanol consumption by alcohol-preferring Fawn-Hooded, iP and Alko Alcohol (AA) rats. Acutely, acamprosate was shown to cause a significant decrease in operant ethanol self-administration by Fawn-Hooded and alcohol-preferring iP rats in part by decreasing the motivational relevance of a specific ethanol cue; however, repeated injection of acamprosate led to tolerance to this effect. Voluntary alcohol consumption in the home cage in Fawn-Hooded and AA rats was also reduced by an acute acamprosate injection; however, again tolerance developed to repeated injections. In a separate experiment, the effect of acamprosate on markers of the dopaminergic system was examined. Interestingly, acute acamprosate was also shown to cause increased dopamine transporter density and decreased dopamine D2-like receptor density within the nucleus accumbens but not in the caudate-putamen, suggesting a link between the decreased motivational salience of the ethanol cue and altered dopaminergic signalling within the nucleus accumbens. With repeated injections of acamprosate, markers of the dopaminergic system returned to steady state levels with a similar temporal profile to the development of tolerance in the behavioural studies. Along with previous studies, our findings indicate that acamprosate modulates the mesolimbic dopaminergic system and may thereby decrease ethanol reinforcement processes; however, these effects undergo tolerance in alcohol-preferring rats and may in part explain the fact why some subjects are non-responders to chronic acamprosate treatment.

Baclofen-induced reduction of alcohol reinforcement in alcohol-preferring rats

Recent studies have demonstrated that treatment with the gamma-aminobutyric acid (GABA(B)) receptor agonist, baclofen, reduces alcohol intake in selectively bred Sardinian alcohol-preferring rats tested under the homecage two-bottle "alcohol versus water" choice regimen. This study was designed to investigate whether baclofen also reduces alcohol-reinforcing effects in Sardinian alcohol-preferring rats. To this aim, sP rats were trained to lever press for oral alcohol (15%, vol/vol) or sucrose (0.3%, wt/vol; included as alternative reinforcer to evaluate the specificity of baclofen effect on alcohol reinforcement) under a fixed ratio schedule of 4. Once steady levels of alcohol or sucrose self-administration behavior were established, the effects of acutely administered baclofen (0, 1.7, and 3 mg/kg, intraperitoneal [ip]) and naloxone (0, 1, and 3 mg/kg, ip; included as reference compound) on alcohol- or sucrose-reinforced responding were evaluated. Baclofen administration dose dependently, although not specifically, reduced alcohol-reinforced responding to an extent comparable to that of naloxone. Baclofen also produced a dose-dependent and specific delay in the onset of alcohol-reinforced responding, suggesting that it suppressed the rats' motivation to start drinking alcohol. These data are discussed in terms of adding further support to the hypothesized involvement of the GABA(B) receptor in the neural system mediating alcohol reinforcement. These data are also in agreement with the results of recent preliminary clinical studies suggesting that baclofen may have therapeutic efficacy in the treatment of alcohol dependence.

Dopamine and alcoholism: neurobiological basis of ethanol abuse

The role of the dopamine (DA) system in brain reward mechanisms and the development of substance abuse has been well established. We review earlier animal and human studies on DA and alcoholism with some relevant issues relating to those studies. The present animal and human data suggest several alterations in the DA system in the context of alcoholism. Receptor studies imply that DA D(2) receptor density and function are lower at least among type 1 alcoholics, which suggests that they could benefit from drugs that enhance DAergic activity, such as partial DA agonists. These drugs could help to restore suboptimal levels of DAergic activity by reducing both the craving for alcohol in abstinence and the euphoria subsequent to alcohol's release of DA in the nucleus accumbens (NAC), thus providing negative reinforcement for relapse.

Clozapine reduces alcohol drinking in Syrian golden hamsters

Alcohol abuse contributes substantially to the overall morbidity of schizophrenia. While typical antipsychotic medications do not limit alcohol use in patients with schizophrenia, emerging data suggest that the atypical antipsychotic clozapine does. To further elucidate the effects of these antipsychotics on alcohol use, we initiated a study in alcohol-preferring rodents. Syrian golden hamsters were given free-choice, unlimited access to alcohol. Nine days of treatment (s.c. injection) with clozapine (2-4 mg/kg/day), but not haloperidol (0.2-0.4 mg/kg/day), reduced alcohol drinking. Clozapine reduced alcohol drinking by 88% (from 11.3+/-1.7 to 1.4+/-0.2 g/kg/day) while increasing both water and food intake. Alcohol drinking gradually (during 24 days) returned toward baseline in the clozapine-treated animals when vehicle was substituted for clozapine. Further increasing the doses of haloperidol (0.6-1.0 mg/kg/day) had no effect on alcohol drinking; moreover, very low doses of haloperidol (0.025-0.1 mg/kg/day) tested in separate groups of hamsters also had no effect on alcohol drinking. This study demonstrates that clozapine, but not haloperidol, can effectively and reversibly decrease alcohol consumption in alcohol-preferring hamsters. The results are compatible with the observations that clozapine, but not haloperidol, limits alcohol use in patients with schizophrenia. These data further suggest that clozapine may serve as a prototype for developing novel treatments for alcohol abuse.

Effects of naltrexone alone and in combination with acamprosate on the alcohol deprivation effect in rats

Previous research in our laboratory has shown that responding for ethanol increases after a period of imposed deprivation during which no ethanol is available (the alcohol deprivation effect). This selective increase in responding for ethanol was blocked by chronic administration of acamprosate. In the present study the effects of naltrexone and the combination of naltrexone+acamprosate on oral ethanol self-administration were examined following an imposed period of abstinence. Male Wistar rats were trained to respond for ethanol (10% w/v) or water in a two-lever free-choice condition. After training, separate groups of rats received chronic injections (2 x /day) of saline, naltrexone, or naltrexone+acamprosate during a 5-day period of abstinence. Ethanol self-administration was tested in all groups of rats on the last day of abstinence, 30 min after the last drug injection. Responding for ethanol increased significantly following the deprivation period in animals treated with saline. Chronic administration of naltrexone and the combination naltrexone+acamprosate blocked the increased ethanol consumption following the imposed abstinence period on post-deprivation Day 1. On post-deprivation Day 2, the combination of acamprosate with naltrexone blocked the rebound increase in ethanol consumption observed in animals treated with a low dose of naltrexone. These results support the hypothesis that naltrexone and acamprosate are effective in modulating aspects of alcohol-seeking behavior, and under certain situations may be more effective in combination.

Flavor quality and ethanol concentration affect ethanol-conditioned flavor preferences

A previous report showed that outbred rats acquired preferences for a sweetened conditioned stimulus (CS) flavor paired with intragastric ethanol. To evaluate the role of sweet taste in ethanol conditioning, this study compared training with sweetened and unsweetened flavors. In Experiment 1, nondeprived rats were trained to drink one flavored solution (CS+, e.g., grape) paired with intragastric infusion of 5% ethanol and another (CS-, e.g., cherry) paired with intragastric water on alternate days. The volume of ethanol solution infused was matched to the volume of flavored solution the rats consumed. The sweet group's flavors initially contained 0.2% saccharin, reduced to 0.1%, 0.05%, and 0% over days; the plain group's flavors were unsweetened. The sweet group drank more and self-infused more ethanol during training and its preference for the CS+ over the CS- (without saccharin) exceeded that of the plain group (75% versus 62%). Experiment 2 equated total ethanol intake in rats trained with two combinations of flavor quality and ethanol concentration. The Sweet5 group drank flavors with 0.2% saccharin throughout training and tests and received 5% ethanol when they drank CS+, while the Plain10 group drank unsweetened flavors and the CS+ was paired with 10% ethanol. Despite equal daily ethanol doses, the Sweet5 group strongly preferred the CS+ (89%) while the Plain10 group avoided it (31%). The two groups continued to show opposite CS+ preference profiles even when both were tested with sweet CS flavors and 10% ethanol infusions. Thus, sweet taste contributes to the development of ethanol-conditioned flavor preferences, and this effect is not explained by a simple enhancement of ethanol intake.

Reversal of ethanol-seeking behavior by D1 and D2 antagonists in an animal model of relapse: differences in antagonist potency in previously ethanol-dependent versus nondependent rats

Mesocorticolimbic dopamine (DA) transmission has been implicated in the consummatory and, more recently, the incentive-motivational aspect of ethanol's actions. The purpose of this study was to test whether ethanol-seeking behavior induced by an ethanol-associated contextual stimulus is sensitive to antagonism of DA transmission. Male Wistar rats were trained to orally self-administer 10% ethanol and to associate olfactory discriminative stimuli with the availability of ethanol (S(+)) versus nonreward (S(-)). Ethanol-reinforced operant responding then was extinguished by withholding ethanol and the associated S(+). After reaching a predetermined extinction criterion, reinstatement tests were conducted in which the animals were presented noncontingently with only the S(+) or S(-). Exposure to the S(+) but not the S(-) reinstated responding at the previously active lever. The D1 antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH23390; 5, 10, 50 microg/kg s.c.) and the D2 antagonist eticlopride (5, 10, 50 microg/kg s.c.) dose dependently decreased the number of S(+)-induced responses and increased response latency. During a second test, conducted in the same rats, 3 weeks after withdrawal from a 12-day ethanol vapor inhalation procedure, the response-reinstating efficacy of the S(+) remained unaltered. However, the potency of both DA antagonists to inhibit the S(+)-induced drug-seeking response was significantly increased. The results confirm that ethanol-related contextual stimuli reliably elicit drug-seeking behavior and suggest that this effect requires activation of DA neurotransmission. The results also indicate that chronic ethanol exposure produces changes in D1 and D2 receptor function that lead to enhanced sensitivity to the behavioral effects of antagonists for these receptors.

A multiple schedule model of limited access drinking in the cynomolgus macaque

The present study reports on the development of a multiple schedule procedure of oral ethanol self-administration in cynomolgus macaques. Six adult cynomolgus macaques (four female, two male) were trained to self-administer ethanol and water under a 60 min, four-component multiple schedule of ethanol and water access with 1 g food pellets presented every 900 s (fixed-time 900 s). Water was available for the first and third 15 min components, ethanol in the second and fourth components. Total ethanol dose was stable at between 1-1.25 g/kg at ethanol concentration of 4%, 6% and 8%. Subsequently water was replaced with a sweetened drink (sugar-free Tang powder, General Foods). Ethanol and Tang were self-administered in similar volumes and both served as reinforcers compared with water. Acute pretreatment with 0.25 to 1.5 g/kg of intragastrically gavaged (i.g.) ethanol failed to alter ethanol or Tang self-administration significantly despite producing mean blood ethanol levels of up to 199 mg/dl when combined with self-administered ethanol. However, 1.0 g/kg i.g. ethanol administered for 15 consecutive days significantly decreased ethanol self-administration by 23%. The results suggest that ethanol self-administration under a multiple schedule is insensitive to alteration by acute ethanol pretreatment, but can be decreased by more prolonged chronic ethanol pretreatment.

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.

The role of opioid-dopamine interactions in the induction and maintenance of ethanol consumption

1. Alcohol is one of the most widely used recreational drugs, but also one of the most widely abused, causing vast economic, social and personal damage. 2. Several animal models are available to study the reinforcing mechanisms that are the basis of the abuse liability of ethanol. Innate differences in opioid or dopamine neurotransmission may enhance the abuse liability of ethanol, as indicated by animal and human studies. 3. Opioid antagonists have been shown to be effective, both experimentally and clinically, in decreasing ethanol consumption, presumably since ethanol induces the release of endogenous opioid peptides in vivo. However, ethanol may also stimulate the formation of opiate-like compounds, which could interact with opioid (or dopamine) receptors. Ethanol may cause changes in neurotransmission mediated via opioid receptors that determines whether alcohol abuse is more or less likely. 4. Ethanol appears to facilitate dopamine release by increasing opioidergic activity, disinhibiting dopaminergic neurons (by inhibition of GABAergic neurotransmission) via mu-opioid receptors in the ventral tegmental area (VTA) and delta-opioid receptors in the nucleus accumbens (NAcc). The effects of ethanol would be antagonised by presynaptic kappa-opioid receptors present on dopaminergic terminals in the NAcc. 5. Mesolimbic dopamine release induced by ethanol consumption seems to indicate ethanol-related stimuli are important, focussing attention on and enabling learning of the stimuli. However, studies indicate that there are redundant pathways, and neural pathways 'downstream' of the mesolimbic dopamine system, which also enable the reinforcing properties of ethanol to be mediated.

Endogenous opiates: 1997

This paper is the twentieth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1997 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.