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

Adolescent and adult rats differ in the amnesic effects of acute ethanol in two hippocampus-dependent tasks: Trace and contextual fear conditioning

Experience-produced deficits in trace conditioning and context conditioning have been useful tools for examining the role of the hippocampus in learning. It has also been suggested that learning in these tasks is especially vulnerable to neurotoxic effects of alcohol during key developmental periods such as adolescence. In five experiments we systematically examined the presence and source of age-dependent vulnerability to the memory-disrupting effects of acute ethanol in trace conditioning and contextual fear conditioning. In Experiment 1a pre-training ethanol disrupted trace conditioning more strongly in adolescent (postnatal day, PD30-35) than adult rats (PD65-75). In Experiment 1b when pre-training ethanol was accompanied by pre-test ethanol no deficit in trace conditioning was observed in adolescents, suggesting that state-dependent retrieval failure mediated ethanol's disruption of trace conditioning at this age. Experiment 2a and b examined the effect of ethanol pretreatment on context conditioning. Here, adult but not adolescent rats were impaired in conditioned freezing to context cues. Experiment 2c explored state-dependency of this effect. Pre-training ethanol continued to disrupt context conditioning in adults even when ethanol was also administered prior to test. Collectively these findings reveal clear age-dependent and task-dependent vulnerabilities in ethanol's disruptive effects on hippocampus-dependent memory. Adolescents were more disrupted by ethanol in trace conditioning than adults, and adults were more disrupted by ethanol in context conditioning than adolescents. We suggest that adolescents may be more susceptible to changes in internal state (state-dependent retrieval failure) than adults and that ethanol disrupted performance in trace and context conditioning through different mechanisms. Relevance of these findings to theories of hippocampus function is discussed.

Effects of vigabatrin, an irreversible GABA transaminase inhibitor, on ethanol reinforcement and ethanol discriminative stimuli in mice

We tested the hypothesis that the irreversible γ-amino butyric acid transaminase inhibitor, γ-vinyl γ-amino butyric acid [vigabatrin (VGB)], would reduce ethanol reinforcement and enhance the discriminative-stimulus effect of ethanol, effectively reducing ethanol intake. The present studies used adult C57BL/6J (B6) mice in well-established operant, two-bottle choice consumption, locomotor activity, and ethanol discrimination procedures to comprehensively examine the effects of VGB on ethanol-supported behaviors. VGB dose-dependently reduced operant responding for ethanol and ethanol consumption for long periods of time. Importantly, a low dose (200 mg/kg) of VGB was selective for reducing ethanol responding without altering the intake of food or water reinforcement. Higher VGB doses (>200mg/kg) reduced ethanol intake, but also significantly increased water consumption and, more modestly, increased food consumption. Although not affecting locomotor activity on its own, VGB interacted with ethanol to reduce the stimulatory effects of ethanol on locomotion. Finally, VGB (200 mg/kg) significantly enhanced the discriminative-stimulus effects of ethanol as evidenced by significant leftward and upward shifts in ethanol generalization curves. Interestingly, VGB treatment was associated with slight increases in blood ethanol concentrations. The reduction in ethanol intake by VGB appears to be related to the ability of VGB to potentiate the pharmacological effects of ethanol.

The effect of age on the discriminative stimulus effects of ethanol and its GABA(A) receptor mediation in cynomolgus monkeys

RATIONALE

Excessive alcohol consumption is less common among aged compared to young adults, with aged adults showing greater sensitivity to many behavioral effects of ethanol.

OBJECTIVES

This study compared the discriminative stimulus effects of ethanol in young and middle-aged adult cynomolgus monkeys (Macaca fascicularis) and its γ-aminobutyric acid (GABA)(A) receptor mediation.

METHODS

Two male and two female monkeys trained to discriminate ethanol (1.0 g/kg, i.g.; 60-min pre-treatment interval) from water at 5-6 years of age (Grant et al. in Psychopharmacology 152:181-188, 2000) were re-trained in the current study more than a decade later (19.3 ± 1.0 years of age) for a within-subjects comparison. Also, four experimentally naïve middle-aged (mean ± SEM, 17.0 ± 1.5 years of age) female monkeys were trained to discriminate ethanol for between-subjects comparison with published data from young adult naïve monkeys.

RESULTS

Two of the naïve middle-aged monkeys attained criterion performance, with weak stimulus control and few discrimination tests, despite greater blood-ethanol concentration 60 min after 1.0 g/kg ethanol in middle-aged compared to young adult female monkeys (Green et al. in Alcohol Clin Exp Res 23:611-616, 1999). The efficacy of the GABA(A) receptor positive modulators pentobarbital, midazolam, allopregnanolone, pregnanolone, and androsterone to substitute for the discriminative stimulus effects of 1.0 g/kg ethanol was maintained from young adulthood to middle age.

CONCLUSIONS

The data suggest that 1.0 g/kg ethanol is a weak discriminative stimulus in naive middle-aged monkeys. Nevertheless, the GABA(A) receptor mechanisms mediating the discriminative stimulus effects of ethanol, when learned as a young adult, appear stable across one third of the primate lifespan.

Discriminative Stimulus Effects of Ethanol in Mice Lacking the γ-Aminobutyric Acid Type A Receptor δ Subunit

BACKGROUND

Genetically altered mice have been used to examine gene contributions to ethanol phenotypes. Recently, mice with a targeted deletion of the delta subunit of the gamma-aminobutyric acid (GABA)A receptor have been generated. These mice display decreased sensitivity to neuroactive steroids and altered responses to some behavioral effects of ethanol. Given the application of drug discrimination to characterize receptor-mediated stimulus effects of ethanol and given the data showing altered ethanol responses in mice lacking the delta subunit of the GABAA receptor, these mice were characterized in an ethanol-discrimination procedure. It has been shown that neurosteroids will substitute for the discriminative stimulus effects of ethanol, and this study aimed to determine whether the substitution patterns of neuroactive steroids or other GABAA-positive modulators would be altered in these mice.

METHODS

Twelve adult delta +/+ and delta-/- mice were trained to discriminate between ethanol 1.5 g/kg and saline in daily 15-min food-reinforced operant sessions. Once the discrimination was trained, substitution tests with ethanol, pentobarbital, midazolam, androsterone, alphaxalone, pregnanolone, morphine, zolpidem, and MK-801 were conducted.

RESULTS

Both delta+/+ and delta-/- mice acquired ethanol discrimination in a similar number of days. Ethanol, midazolam, alphaxalone, pregnanolone, and MK-801 fully substituted (>80%) for ethanol in both delta+/+ and delta-/- mice. Pentobarbital fully substituted for ethanol in delta-/- mice but only partially substituted (74%) for ethanol in delta+/+ mice. Androsterone, zolpidem, and morphine did not substitute for ethanol in either delta+/+ or delta-/- mice. There were no significant differences in the response rate-suppressing effects of any of the compounds between delta+/+ and delta-/- mice.

CONCLUSIONS

The training dose of ethanol resulted in substitution of five GABAA receptor ligands, indicating a robust GABAA mediation of ethanol's discriminative stimulus effects. Deletion of the delta subunit of the GABAA receptor does not alter the acquisition of an ethanol/saline discrimination or the substitution patterns of GABAA-positive modulators. Therefore, the delta subunit is not necessary in the mediation of ethanol-like effects of any of the GABAA ligands tested, including sensitivity to ethanol, barbiturate, benzodiazepine, and neurosteroid discriminative stimulus effects.

Differential Neurosensitivity to the Discriminative Stimulus Properties of Ethanol in C57BL/6J and C3H/He Mice

BACKGROUND

A large body of evidence suggests that the interoceptive cue associated with ethanol intoxication is complex and dependent on a number of environmental and biological factors. Despite the fact that mice have been widely used to study genetic influences on sensitivity to various actions of ethanol, few studies have used mice to examine sensitivity to the discriminative stimulus effects of ethanol. The purpose of this study was to compare sensitivity to the discriminative stimulus effects of ethanol in two inbred mouse strains, namely C57BL/6J and C3H/He mice.

METHODS

Adult male C57BL/6J and C3H/He mice were trained to discriminate between ethanol and saline using a two-lever food reinforcement operant procedure. Once criterion discrimination performance was achieved, dose-response functions were determined from generalization tests. Additional experiments were conducted to determine whether differences in discrimination performance were related to differential blood/brain ethanol levels in the two mouse strains.

RESULTS

A greater proportion of C57BL/6J mice acquired the discrimination and required fewer trials to achieve criterion performance compared with C3H/He mice with a 1.0 g/kg ethanol training dose. This deficit in acquisition was overcome when the training dose was increased to 2.0 g/kg for C3H/He mice. In a second experiment, a 1.5 g/kg training dose of ethanol was used for both strains. Again, a greater proportion of C57BL/6J mice acquired the discrimination and required fewer training trials to achieve criterion performance compared with C3H/He mice. Blood ethanol levels did not differ between the strains after administration of the 1.5 g/kg training dose. However, blood and brain ethanol levels did differ between the strains after doses of ethanol were administered that produced equivalent discrimination performance.

CONCLUSIONS

Results indicate that ethanol discrimination was more readily acquired and maintained in C57BL/6J mice than C3H/He mice. Ethanol dose-response functions generated from generalization tests also clearly demonstrated greater sensitivity to the discriminative stimulus properties of ethanol in C57BL/6J mice compared with the C3H/He strain. This differential sensitivity to the interoceptive cue produced by ethanol does not seem to be related to learning or pharmacokinetic differences between the two inbred strains.

A strain-specific alteration of proteomic expression in mouse liver fructose 1,6-bisphosphatase isoforms by alcohol

To study alcohol-related metabolism across inbred mouse strains, liver tissues from C57BL/6J (B6, an alcohol-preferring mouse) and DBA/2J (D2, an alcohol-avoiding strain) mice were analyzed for proteomic expression patterns over time after a single-dose of alcohol (1.5 g/kg ingestion). Despite no significant difference in the elimination rate of blood ethanol, two-dimensional electrophoresis gel images of liver proteins showed that proteins in B6 mice exhibited faster response and more quantitative (spot numbers) and qualitative (spot densities) changes than in D2 mice. Among the differentially expressed metabolic enzymes, four variants (alpha, beta, gamma and delta) of fructose 1,6-bisphosphatase (FBPase), a key regulatory gluconeogenic enzyme, showed remarkable changes in expression with time across the strains. The degree of spot alteration in alpha- and gamma-variants of FBPase in B6 mice was much higher than in D2 mice, while the beta- and delta-forms were not changed as much. Mass spectrometry (MS) analysis showed that the 1714.9 +/- 1 mass peak from the alpha- and gamma-variants of FBPase was much stronger than that of the beta- and delta-variants in both strains regardless of spot density. This MS peak contains 2-ANHAPFETDISTLTR-16, located at the N-terminal of FBPase, where the N-terminal alanine was found to be trimethylated. Thus, we propose this N-terminal fragment as a potential site for enzyme modification in response to ethanol, allowing for differences in two-dimensional gel spot intensity of variants of FBPase in the two mouse strains.

Chronic Ethanol Consumption by C57BL/6 Mice Promotes Tolerance to Its Interoceptive Cues and Increases Extracellular Dopamine, an Effect Blocked by Naltrexone

BACKGROUND

C57BL/6 (B6) mice voluntarily consume ethanol. Although preingestive factors might be accountable, the fact that B6 mice voluntarily consume sufficient ethanol to set the conditions for an ethanol-deprivation effect suggest that post-ingestive pharmacological induced changes also occur. In this study, we determined the amounts of ethanol voluntarily consumed by B6 mice and associated blood ethanol levels (BEL), the effects of this consumption on extracellular dopamine (DA) and how this was altered by naltrexone, as well as on its interoceptive discriminative cues.

METHODS

In experiment 1, the amounts of 12% ethanol consumed at 2, 4, and 6 hr into the active phase of the circadian cycle and associated BEL were determined. In experiment 2, dialysate samples were collected for 1 hr to establish basal DA levels. Mice were then injected with saline or naltrexone (6 mg/kg) and given access to water and 12% ethanol or to water only, and samples were collected at 20-min intervals for the next 2 hr. In experiment 3, mice were trained to discriminate ethanol's interoceptive cues via operant techniques, and half were given 3 weeks access to ethanol and water, the other half water only. Ethanol-consuming and water control mice were again tested for their ability to discriminate the drug's interoceptive cues.

RESULTS

Mice ingested nearly 6 g/kg of ethanol and attained BEL near 100 mg/100 mL by 6 hr into the active phase. Ethanol intake at 2-hr into the dark phase was approximately 2.5 g/kg, and increased DA to approximately 100% above basal levels. Naltrexone reduced ethanol consumption and blocked the DA increase. Ethanol consumption for 3 weeks attenuated its discriminative cues.

CONCLUSIONS

B6 mice voluntarily consume sufficient ethanol (1) to produce intoxicating BEL; (2) to increase DA levels in nucleus accumbens, an effect blocked by naltrexone; and (3) to attenuate its discriminative cues.

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

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

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

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

Discriminative stimulus effects of ethanol: neuropharmacological characterization

Generally, compounds discriminated by animals possess psychotropic effects in animals and humans. As with many other drugs of abuse, strength of the ethanol discriminative stimulus is dose related. The majority of studies show that doses close to 1.0 g/kg are close to the minimum at which the discrimination can be learned easily. Substitution studies suggest that anxiolytic, sedative, atactic, and myorelaxant effects of ethanol all play an important role in the formation of its intercoeptive stimulus. Low doses of ethanol produce more excitatory cues, similar to amphetamine-like subjective stimuli, whereas higher doses produce rather sedative/hypnotic stimuli similar to those elicited by barbiturates. Substitution studies have shown that the complete substitution for ethanol may be exerted by certain GABA-mimetic drugs acting through different sites within the GABA(A)-benzodiazepine receptor complex (e.g., diazepam, pentobarbital, certain neurosteroids), gamma-hydroxybutyrate, and antagonists of the glutamate NMDA receptor. Among the NMDA receptor antagonists both noncompetitive (e.g., dizocilpine) and competitive antagonists (e.g., CGP 40116) are capable of substituting for ethanol. Further, some antagonists of strychnine-insensitive glycine modulatory sites among the NMDA receptor complex (e.g., L-701,324) dose-dependently substitute for the ethanol discriminative stimulus. On the other hand, neither GABA-benzodiazepine antagonists nor NMDA receptor agonists produce contradictory effects (i.e., reduce the ethanol discriminative stimulus). There is influence of a particular training dose of ethanol on the substitution pattern of different compounds. For example, 5-HT(1B/2C) agonists substitute for intermediate (1.0 g/kg) but not higher (2.0 g/kg) ethanol training doses. Discrimination studies with ethanol and drugs acting on NMDA and GABA receptors consistently indicate asymmetrical generalization. For example, ethanol is able to generalize to barbiturates and benzodiazepines, but neither the benzodiazepine nor barbiturate response generalizes to ethanol. Only a few drugs are able to antagonize, at least to some extent, the discriminative stimulus of ethanol (e.g., partial inverse GABA-benzodiazepine receptor antagonist Ro 15-4513 and the opioid antagonist naloxone). The ethanol stimulus effect may be increased (i.e., stronger recognition) by N-cholinergic drugs (nicotine), dopaminergic drugs (apomorphine), and 5-HT3 receptor agonists (m-chlorophenylbiguanide). Thus, the ethanol stimulus is composed of the several components, with the NMDA receptor and GABA(A) receptor complex being of particular importance. This suggests that a drug mixture may be more capable of substituting for ethanol (or block its stimulus) than a single compound. The ability of drugs to substitute for the ethanol discriminative stimulus is frequently, although not preclusively, associated with the reduction of voluntary ethanol consumption. The examples of positive correlation are gamma-hydroxybutyrate, possibly memantine and certain serotonergic drugs such as fluoxetine. However, it remains uncertain to what extent the discriminative stimulus of ethanol can be seen as relevant in the understanding of the complex mechanisms of dependence.