Effects of training dose on discrimination and cross-generalization of chlordiazepoxide, pentobarbital and ethanol in the rat

Discriminative Stimulus Effects of Abused Inhalants

Inhalants are a loosely organized category of abused compounds defined entirely by their common route of administration. Inhalants include volatile solvents, fuels, volatile anesthetics, gasses, and liquefied refrigerants, among others. They are ubiquitous in modern society as ingredients in a wide variety of household, commercial, and medical products. Persons of all ages abuse inhalants but the highest prevalence of abuse is in younger adolescents. Although inhalants have been shown to act upon a host of neurotransmitter receptors, the stimulus effects of the few inhalants which have been trained or tested in drug discrimination procedures suggest that their discriminative stimulus properties are mediated by a few key neurotransmitter receptor systems. Abused volatile solvent inhalants have stimulus effects that are similar to a select group of GABAA positive modulators comprised of benzodiazepines and barbiturates. In contrast the stimulus effects of nitrous oxide gas appear to be at least partially mediated by uncompetitive antagonism of NMDA receptors. Finally, volatile anesthetic inhalants have stimulus effects in common with both GABAA positive modulators as well as competitive NMDA antagonists. In addition to a review of the pharmacology underlying the stimulus effects of inhalants, the chapter also discusses the scientific value of utilizing drug discrimination as a means of functionally grouping inhalants according to their abuse-related pharmacological properties.

Comparing the discriminative stimulus effects of modulators of GABAA receptors containing α4-δ subunits with those of gaboxadol in rats

RATIONALE

Gaboxadol is a selective agonist at γ-aminobutyric acidA (GABAA) receptors that contain α4-δ subunits, and it produces anxiolytic and sedative effects. Although adverse effects preclude its clinical use, its mechanism of action suggests that those receptors might provide novel therapeutic targets, particularly for modulators of those GABAA receptor subtypes, by retaining therapeutic effects of gaboxadol and not adverse effects.

OBJECTIVES

The current study compared discriminative stimulus effects of gaboxadol with those of modulators acting at GABAA receptors containing α4-δ subunits.

MATERIALS

Eight rats discriminated 5.6 mg/kg gaboxadol from vehicle while responding under a fixed - ratio 10 schedule for food. Modulators acting at GABAA receptors containing α4-δ subunits (pregnanolone, ethanol, and flumazenil) and receptors that do not contain those subunits (midazolam) were studied alone; pregnanolone and ethanol were also combined with gaboxadol. In addition, gaboxadol was studied in separate groups discriminating 0.32 mg/kg midazolam, 3.2 mg/kg pregnanolone, or 0.75 g/kg ethanol from vehicle.

RESULTS

Gaboxadol produced ≥80 % gaboxadol-lever responding and did not alter rates. No other drug produced, on average, ≥80 % drug-lever responding up to doses that decreased rates, although 1.78 mg/kg midazolam produced 32 % gaboxadol-lever responding. Ethanol and pregnanolone did not enhance the effects of gaboxadol. Rats discriminating midazolam, pregnanolone, or ethanol from vehicle responded predominantly on the vehicle lever after receiving gaboxadol.

CONCLUSIONS

Drugs that modulate GABAA receptors containing α4-δ subunits neither mimicked nor enhanced the discriminative stimulus effects of gaboxadol, indicating that at least some effects of gaboxadol are not shared with modulators of that GABAA receptor subtype.

Drug effects on multiple and concurrent schedules of ethanol- and food-maintained behaviour: context-dependent selectivity

BACKGROUND AND PURPOSE

Drugs that more potently or effectively reduce ethanol-maintained behaviour versus an alternative are considered selective and are considered promising pharmacotherapies for alcoholism. Such results are often obtained using separate groups or multiple schedules where ethanol and the alternative are available alone or sequentially. Recently, we observed that when ethanol and food were available sequentially under a multiple schedule, fluvoxamine and varenicline were selective; yet this selectivity disappeared when ethanol and food were concurrently available.

EXPERIMENTAL APPROACH

We examined the generality of these findings by comparing doses of several drugs required to decrease ethanol- and food-maintained responding under a multiple schedule and under a concurrent schedule. Effects were determined for chlordiazepoxide, 2,5-dimethoxy-4-iodoamphetamine (DOI), meta-chlorophenylpiperazine (mCPP), morphine, naltrexone and d-amphetamine.

KEY RESULTS

Under the multiple schedule, ED50 values for decreases in ethanol-maintained responding were significantly different and lower than ED50 s for decreases in food-maintained responding (demonstrating selectivity) for each drug except for chlordiazepoxide (which was equipotent) and naltrexone (which did not affect responding). However, this selectivity vanished or even inverted under the concurrent schedule, such that ED50 values for decreasing ethanol- and food-maintained responding were not different (or, following DOI, the ED50 for food-maintained responding was lower than for ethanol-maintained responding).

CONCLUSIONS AND IMPLICATIONS

Results are consistent with those seen following fluvoxamine and varenicline administration, and suggest that selectivity is assay-dependent. These results indicate the need for careful interpretation of selective drug effects, especially when obtained in situations where ethanol or the alternative is the only programmed reinforcement available.

Effects of positive and negative modulators of the γ-aminobutyric acid A receptor complex on responding under a differential-reinforcement-of-low-rate schedule of reinforcement in rats

Relatively little is known about the behavioral effects of the neurosteroids compared with other drugs that modulate the γ-aminobutyric acid A (GABAA) receptor complex. This study examined the acute effects of pregnanolone and dehydroepiandrosterone (DHEA) in male rats responding under a differential-reinforcement-of-low-rate schedule of reinforcement. For comparison, three positive modulators of the GABAA receptor (lorazepam, ethanol, and pentobarbital), one negative modulator (β-CCM), and one neutral modulator (flumazenil) were tested. Pregnanolone was also administered in combination with DHEA to test for antagonism between these substances. Pregnanolone, lorazepam, and pentobarbital produced increases in responding at intermediate doses, and ethanol and pentobarbital produced decreases in responding at the highest doses tested. However, all four drugs dose-dependently decreased reinforced responding by decreasing inter-response times. DHEA, β-CCM, and flumazenil did not increase responding at intermediate doses or decrease reinforced responding. DHEA did not competitively antagonize the disruptive effects of pregnanolone. In summary, pregnanolone and DHEA produced effects on differential-reinforcement-of-low-rate responding that are similar to other positive and negative GABAA modulators, respectively, and do not produce these effects through a single binding site.

Cross-substitution of Δ9-tetrahydrocannabinol and JWH-018 in drug discrimination in rats

Synthetic indole-derived cannabinoids, originally developed to probe cannabinoid CB1 and CB2 receptors, have become widely abused for their marijuana-like intoxicating properties. The present study examined the effects of indole-derived cannabinoids in rats trained to discriminate Δ(9)-tetrahydrocannabinol (Δ(9)-THC) from vehicle. In addition, the effects of Δ(9)-THC in rats trained to discriminate JWH-018 from vehicle were assessed. Adult male Sprague-Dawley rats were trained to discriminate 3mg/kg Δ(9)-THC or 0.3mg/kg JWH-018 from vehicle. JWH-018, JWH-073, and JWH-210 fully substituted in Δ(9)-THC-trained rats and Δ(9)-THC substituted in JWH-018-trained rats. In contrast, JWH-320, an indole-derived cannabinoid without affinity for CB1 receptors, failed to substitute for Δ(9)-THC. Pre-treatment with 1mg/kg rimonabant significantly reduced responding on the JWH-018-associated lever in JWH-018-trained rats. These results support the conclusion that the interoceptive effects of Δ(9)-THC and synthetic indole-derived cannabinoids show a large degree of overlap, which is predictive of their use for their marijuana-like intoxicating properties. Characterization of the extent of pharmacological differences among structural classes of cannabinoids, and determination of their mechanisms remain important goals.

Role of training dose in drug discrimination: a review

Drug discrimination has been an important technique in behavioural pharmacology for at least 40 years. The characteristics of drug-produced discriminative stimuli are influenced by behavioural and pharmacological variables, including the doses used to establish discriminations. This review covers studies on the effects of varying the training dose of a drug in a search for general principles that are applicable across different drug classes and methodological approaches. With respect to quantitative changes, relationships between training dose and the rate of acquisition or magnitude of stimulus control were found for most drug classes. Acquisition accelerated with dose up to a point beyond which drug-induced impairments of performance had a deleterious impact. Sensitivity to the training drug as measured by ED(50) values typically increased when the training dose was reduced. Qualitative changes were more complex and appeared to fall into three categories: (a) changes in profiles of generalization between partial and full agonists; (b) reduced specificity of some discriminations at small training doses; and (c) changes in the relative salience of actions mediated through different neurotransmitter systems or from central and peripheral sites. Three-lever discrimination procedures incorporating 'drug versus drug' or 'dose versus dose' contingencies enabled detection of more subtle differences than the simple 'drug versus no drug' approach when applied to the opioid, hallucinogen and barbiturate classes of drugs. These conclusions have implications for the interpretation of data from studies that use either within-subject or between-subject designs for studying the discriminative stimulus effects of drugs.

Neurosteroid Binding Sites on the GABA(A) Receptor Complex as Novel Targets for Therapeutics to Reduce Alcohol Abuse and Dependence

Despite the prevalence of alcohol abuse and dependence in the US and Europe, there are only five approved pharmacotherapies for alcohol dependence. Moreover, these pharmacotherapeutic options have limited clinical utility. The purpose of this paper is to present pertinent literature suggesting that both alcohol and the neurosteroids interact at the GABA_A receptor complex and that the neurosteroid sites on this receptor complex could serve as new targets for the development of novel therapeutics for alcohol abuse. This paper will also present data collected by our laboratory showing that one neurosteroid in particular, dehydroepiandrosterone (DHEA), decreases ethanol intake in rats under a variety of conditions. In the process, we will also mention relevant studies from the literature suggesting that both particular subtypes and subunits of the GABA_A receptor play an important role in mediating the interaction of neurosteroids and ethanol.

Effects of pregnanolone and dehydroepiandrosterone on ethanol intake in rats administered ethanol or saline during adolescence

BACKGROUND

Adolescent alcohol use may contribute to long-term changes in the receptors and neuroactive steroids that may mediate its effects and to subsequent alcohol abuse and dependence as an adult. Therefore, in this study, ethanol preference and intake as an adult were examined after adolescent ethanol or saline administration. In addition, ethanol intake in the same groups was examined after administration of 2 neuroactive steroids with modulatory effects at GABA(A) receptors.

METHODS

Two groups of male Long-Evans rats were administered 15 intraperitoneal (i.p.) injections of either ethanol (2 g/kg, 20% v/v) or saline between postnatal days 35 and 63. Starting on postnatal day 75, both groups were trained to consume 10% ethanol using a saccharin-fading procedure, and ethanol intake and preference were measured after a series of manipulations involving food deprivation, changes in the duration of access to ethanol, and changes in the concentrations of ethanol presented. Following these manipulations, pregnanolone (1 to 10 mg/kg) and dehydroepiandrosterone (DHEA, 1 to 100 mg/kg) were administered prior to preference sessions with an 18% ethanol solution.

RESULTS

Adult ethanol preference and intake did not differ significantly in subjects treated with either saline or ethanol as adolescents during training, the substitution of other ethanol concentrations (3.2 to 32%), ad-lib feeding, or moderate food deprivation. Pregnanolone administration altered the intake of both adolescent-treated groups after the first injection of 3.2 mg/kg and after repeated injections with 10 mg/kg, a dose that produced sedation. In contrast, multiple doses of DHEA consistently decreased intake of an 18% ethanol concentration in both groups after repeated injections and 3 doses of DHEA (10, 32, and 56 mg/kg) administered with various ethanol concentrations dose-dependently shifted the ethanol-concentration curves for the volume and dosage of ethanol consumed downward.

CONCLUSIONS

These results indicate that chronic intermittent ethanol (CIE) administration of 2 g/kg during adolescence did not alter preference or overall consumption of ethanol in outbred rats trained to drink ethanol as an adult under the conditions tested, and that DHEA may be more effective than pregnanolone at significantly decreasing ethanol consumption.

Relative potency and effectiveness of flunitrazepam, ethanol, and beta-CCE for disrupting the acquisition and retention of response sequences in rats

Despite the knowledge that gamma-aminobutyric acid(A) modulators can affect learning and memory, their capacity for disrupting each of these complex processes is rarely compared, and often mistakenly assumed to occur with identical potency. For these reasons, the effects of flunitrazepam (0.056-3.2 mg/kg), ethanol (0.25-1.5 g/kg), and ethyl-beta-carboline-3-carboxylate (beta-CCE; 1-17.8 mg/kg) were compared in groups of rats responding under baselines that assessed learning and memory separately. The first baseline was a multiple schedule of repeated acquisition and performance of tandem response sequences, whereas the second baseline was a retention or memory procedure where a tandem response sequence was acquired and then retested after a 30-min delay. Under both procedures, responding was maintained under a second-order fixed-ratio-2 schedule of food reinforcement, and incorrect responding (errors) produced a 5-s timeout. With regard to the effects of the three drugs on sequence acquisition (learning), all three drugs dose dependently decreased the overall response rate and increased the percentage of errors. Both flunitrazepam and beta-CCE affected accuracy more potently than response rate, whereas ethanol was equipotent in affecting these two dependent measures. With regard to the effects of these drugs on sequence retention (memory), both flunitrazepam and ethanol dose dependently decreased retention at doses that had little or no effect on sequence acquisition under the multiple schedule, whereas beta-CCE decreased retention and sequence acquisition similarly at the doses tested. Together, these data show that drugs with differing capacities for altering the function of gamma-aminobutyric acid(A) receptors differ in their capacity for disrupting the acquisition and retention of response sequences and that positive modulation of this receptor complex may be more predictive of disruptions in memory than disruptions in learning.

Overlapping, but not identical, discriminative stimulus effects of the neuroactive steroid pregnanolone and ethanol

Many behavioral effects of neuroactive steroids are mediated by GABA(A) receptors; however, other receptors might be involved. Ethanol has a complex mechanism of action, and many of the same receptors have been implicated in the effects of neuroactive steroids and ethanol. The goal of this study was to determine whether actions of neuroactive steroids and ethanol at multiple receptors result in similar discriminative stimulus effects. Rats discriminated 5.6 mg/kg of pregnanolone while responding under a fixed-ratio 20 schedule of food presentation. Pregnanolone, flunitrazepam and pentobarbital produced >80% pregnanolone-lever responding. In contrast, neither morphine nor the negative GABA(A) modulator beta-CCE substituted for pregnanolone up to doses that markedly decreased response rates. Ethanol substituted only in some rats; in other rats, ethanol produced <20% pregnanolone-lever responding up to rate-decreasing doses. Thus, substitution of positive GABA(A) modulators, and not morphine or beta-CCE, for pregnanolone in all rats suggests that positive modulation of GABA(A) receptors is important in the discriminative stimulus effects of pregnanolone. Although pregnanolone might have actions at other receptors, in addition to actions at GABA(A) receptors, substitution of ethanol for pregnanolone only in some rats suggests that the mechanisms of action of pregnanolone and ethanol overlap, but are not identical.

Inhaled toluene vapor as a discriminative stimulus

Few studies exist exploring the discriminative stimulus effects of inhalants and none that have trained an interoceptive discrimination using the inhaled route. This study was designed to assess if it was possible to train an inhaled toluene discrimination. The second objective was to determine whether the discrimination was based on interoceptive or exteroceptive stimulus effects. Eight B6SJLF1/J mice were trained to discriminate 10 min of exposure to 6000 ppm inhaled toluene vapor from air, using a standard food-reinforced operant procedure. Toluene vapor produced robust, concentration-dependent, discriminative stimulus effects, with concentrations of 4000 ppm and higher producing full substitution. Substitution of inhaled toluene vapor for the training condition was exposure-time dependent. A minimum of 7 min of exposure to 6000 ppm was required to produce complete substitution. Injected intraperitoneal toluene produced dose-dependent full substitution for inhaled toluene vapor. Both inhaled and intraperitoneal ethylbenzene produced similar levels of partial substitution for 6000 ppm toluene vapor. Inhaled isoflurane vapor produced no substitution for toluene vapor. These results show that a toluene vapor discrimination can be successfully trained in mice and the discrimination is selective for toluene compared to ethylbenzene and isoflurane. The results also suggest that the discrimination was likely to have been based primarily on interoceptive rather than exteroceptive stimulus effects.

Discriminative stimulus effects of flumazenil: perceptual masking by baclofen, and lack of substitution with gamma-hydroxybutyrate and its precursors 1,4-butanediol and gamma-butyrolactone

Pigeons trained to discriminate 0.1 mg/kg flumazenil, proposed as an in-vivo model to study interactions with diazepam-insensitive gamma-aminobutyric acid (GABA)A receptors, were tested with various GABAergic and non-GABAergic compounds. As a result of its pharmacological selectivity, the model was suitable for further examining previously reported flumazenil-like effects of gamma-hydroxybutyrate (GHB). Flumazenil and the GABAA negative modulator Ro 15-4513 produced 82-100% flumazenil-appropriate responding. Diazepam and the direct-acting GABAA agonists muscimol and 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridine-3-ol (THIP) produced 38-64% flumazenil-appropriate responding. GHB, its precursors 1,4-butanediol (1,4-BD) and gamma-butyrolactone (GBL), and the GABAB agonists baclofen and SKF97541 produced 0-24% flumazenil-appropriate responding. Baclofen shifted the flumazenil dose-response curve to the right and down, possibly involving perceptual masking of the discriminative stimulus effects of flumazenil by agonist activity at GABAB receptors. These masking effects of baclofen were blocked by the GABAB antagonist CGP35348. When CGP35348 was given together with GHB to block its GABAB agonist effects, GHB did not produce flumazenil-appropriate responding. Conceivably, effects of GHB at non-GABAB receptors (e.g. diazepam-sensitive GABAA receptors and GHB receptors) may interfere with the expression of its flumazenil-like discriminative stimulus effects. The asymmetric substitution between GHB and flumazenil is consistent with the hypothesis that the discriminative stimulus effects of GHB consist of several components, not all of which are mimicked by flumazenil.

Ethanol substitutes for the discriminative stimulus effects of m-chlorophenylpiperazine

Rats were trained to discriminate the 5-HT agonist m-chlorophenylpiperazine (mCPP, 1 mg/kg, i.p.) from saline. Ethanol (0.1 to 1 g/kg, i.p.) partially substituted for the discriminative stimulus effects of 1 mg/kg mCPP. Methysergide (10 mg/kg, i.p.), a 5-HT(1/2) receptor antagonist, blocked the ability of ethanol (1 g/kg) to substitute for mCPP. The largest dose of ethanol markedly reduced response rate. These findings suggest an important role of serotonin receptors in mediating the discriminative stimulus effects of ethanol.

Stimulus properties of nicotine, amphetamine, and chlordiazepoxide as positive features in a pavlovian appetitive discrimination task in rats

Recent experiments from our laboratory have demonstrated that drug states can signal when environmental cues will be followed by rewarding outcomes (ie Pavlovian conditioning). However, little is known about the generality of this approach and whether it can be used for studying the pharmacological properties of drug states. Accordingly, the present experiments tested the pharmacological specificity of nicotine (0.4 mg/kg), amphetamine (1 mg/kg), and chlordiazepoxide (CDP, 5 mg/kg) in this Pavlovian drug discrimination procedure. Following drug administration, presentation of a conditional stimulus (CS) was followed by brief access to sucrose. When saline was administered, the same CS was presented but sucrose was withheld. In substitution tests, rats in each condition received varying doses of all training drugs and caffeine. Anticipatory food seeking developed during the CS on drug sessions but not on saline sessions for all drug features (ie drug state-specific conditional response (CR)). In generalization tests, this CR decreased as a function of decreases in the training dose. Median effective doses (ED50s) were calculated for nicotine (0.054 mg/kg), amphetamine (0.26 mg/kg), and CDP (2.48 mg/kg). No compound tested substituted for the CDP training drug. Partial substitution was evident between nicotine and amphetamine; CDP did not substitute for either of these drug features. Caffeine fully substituted for nicotine (ED50 = 15.45 mg/kg) and amphetamine (ED50 = 3.70 mg/kg), but not for CDP. These results are consistent with the hypothesis that drug states can occasion appetitive Pavlovian CRs in a pharmacologically specific manner.

Combined discriminative stimulus effects of midazolam with other positive GABAA modulators and GABAA receptor agonists in rhesus monkeys

RATIONALE

Interactions among compounds at GABA(A) receptors might have important implications for the therapeutic and other effects of positive GABA(A) modulators (e.g. benzodiazepines).

OBJECTIVES

This study examined whether a midazolam discriminative stimulus is modified by GABA(A) agonists that act at sites other than benzodiazepine sites.

METHODS

Rhesus monkeys discriminating midazolam (0.32 mg/kg SC) received direct-acting GABA(A) receptor agonists (e.g. muscimol and gaboxadol), an indirect-acting GABA(A) receptor agonist (progabide), ethanol, another benzodiazepine (triazolam), a barbiturate (pentobarbital), or a neuroactive steroid (pregnanolone) alone and in combination with midazolam.

RESULTS

When administered alone, triazolam (0.1 mg/kg), pentobarbital (17.8 mg/kg) and pregnanolone (5.6 mg/kg) occasioned high levels of midazolam lever responding, ethanol (1-3 g/kg) occasioned intermediate levels of midazolam lever responding, and muscimol (0.32-1 mg/kg), gaboxadol (3.2-10 mg/kg) and progabide (10-32 mg/kg) occasioned low levels of midazolam lever responding. When combined with less-than-fully effective doses of midazolam, progabide (32 mg/kg) and ethanol (1 g/kg), but not muscimol and gaboxadol, enhanced the midazolam discriminative stimulus. Triazolam, pregnanolone and pentobarbital increased the potency of midazolam to occasion midazolam lever responding and the effects of these combinations were additive.

CONCLUSIONS

Direct-acting GABA(A) receptor agonists are qualitatively different from positive GABA(A) modulators in rhesus monkeys trained to discriminate midazolam. Although GABA(A) receptor agonists and modulators can enhance the actions of benzodiazepines at the GABA(A) receptor complex, the same drugs do not necessarily modify the discriminative stimulus effects of benzodiazepines. These results underscore the importance of the mechanism by which drugs alter Cl(-) flux at the GABA(A) receptor complex as a determinant not only of drug action but also of drug interaction and whether any particular drug enhances the behavioral effects of a benzodiazepine.

Effects of abused inhalants and GABA-positive modulators in dizocilpine discriminating inbred mice

There is in vitro evidence that some of the effects of abused volatile solvents may be produced by actions at the NMDA receptor. In addition, some solvents produce phencyclidine-like discriminative stimulus effects. The major goal of the present study was to further compare abused solvents to NMDA antagonists by testing them in two strains of mice trained to discriminate 0.17 mg/kg of the very selective uncompetitive NMDA antagonist, dizocilpine, from saline and contrast those results with several GABA(A)-positive modulators, PCP and ethanol. The results indicated that the discriminative stimulus produced by 0.17 mg/kg dizocilpine was highly specific in both mouse strains. PCP produced 91% dizocilpine-lever responding in C57BL/6J mice, but only 56% dizocilpine-lever responding in DBA/2J mice. Pentobarbital, midazolam and ethanol produced at least some overlap in discriminative stimulus effects with dizocilpine in one or both mouse strains. In contrast, toluene, 1,1,1-trichloroethane (TCE), xylene and methoxyflurane produced saline-appropriate responding almost exclusively. These data indicate that, at least under the specific conditions tested, abused volatile solvents do not have substantial dizocilpine-like discriminative stimulus effects in either C57BL/6J or DBA/2J mice, providing little support that NMDA antagonism plays a central role in the production of this abuse-related effect.

A comparison of the discriminative stimulus effects of delta(9)-tetrahydrocannabinol and O-1812, a potent and metabolically stable anandamide analog, in rats

Efforts to determine whether Delta(9)-tetrahydrocannabinol (Delta(9)-THC) and anandamide elicit similar discriminative stimulus effects have yielded conflicting results. The difficulty in establishing a discriminative cue to anandamide may be due to its metabolic instability. Rats were trained to discriminate either Delta(9)-THC or O-1812, a metabolically stable anandamide analog, from vehicle to avoid this issue. O-1812 and Delta(9)-THC substituted for each other; however, both drugs were more potent in the O-1812-trained rats. Further, O-1812 only substituted for Delta(9)-THC at response rate decreasing doses. The CB(1) antagonist, SR141716A, blocked the discriminative stimulus effects of both drugs but augmented their rate effects. O-1839, a VR(1) agonist, failed to substitute for either cannabinoid. These results suggest that the discriminative stimulus effects of Delta(9)-THC and O-1812 are similar, but subtle differences also exist.

Discriminative stimulus effects of gamma-hydroxybutyrate in pigeons: role of diazepam-sensitive and -insensitive GABA(A) and GABA(B) receptors

gamma-Hydroxybutyrate (GHB) is an emerging drug of abuse with multiple mechanisms of action. This study is part of an effort to examine the role of GHB, GABA(A), and GABA(B) receptors in the discriminative stimulus (DS) effects of GHB. In pigeons trained to discriminate 100 mg/kg GHB from saline, GHB and its precursors gamma-butyrolactone and 1,4-butanediol produced 80 to 100% GHB-appropriate responding, whereas other compounds such as morphine, naltrexone, cocaine, and haloperidol produced no more than 34%. Compounds interacting with GABA receptors produced different maximal levels of GHB-appropriate responding. For example, the GABA(A) agonist muscimol produced 3%; the GABA(A)-positive modulators diazepam, pentobarbital, and ethanol, and the GABA(B) agonist baclofen produced levels ranging from 54 to 73%; and the benzodiazepine antagonist flumazenil and inverse agonist Ro 15-4513 (ethyl 8-azido-6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-alpha]-[1,4]-benzodiazepine-3-carboxylate) both produced 96%. The putative GHB receptor antagonist (2E)-(5-hydroxy-5,7,8,9-tetrahydro-6H-benzo[a][7]annulen-6-ylidene ethanoic acid (NCS-382) produced 70% GHB-appropriate responding. The GABA(B) antagonist (3-aminopropyl)(diethoxymethyl)phosphinic acid (CGP 35348) completely blocked the GHB-like DS effects of NCS-382 and baclofen at a dose of 56 mg/kg. CGP 35348 also blocked the DS effects of GHB, but incompletely and only at a dose of 560 mg/kg. Together, these results are consistent with a role for diazepam-sensitive and -insensitive GABA(A) and GABA(B) receptors in the DS effects of GHB. Together with previous findings, the present results suggest that diazepam-insensitive GABA(A) receptors are more prominently involved in the DS effects of GHB in pigeons than in rats, whereas GABA(B) receptors are less prominently involved. Exploring the role of GHB receptors with NCS-382 is hampered by its GABA(B) receptor-mediated, GHB-like agonist activity.

Four-choice drug discrimination in pigeons

(+)Amphetamine was added as a training stimulus for pigeons previously trained to discriminate among pentobarbital, morphine and saline using a three-choice procedure. Pigeons quickly learned the four-choice drug discrimination. Generalization from the training drugs was similar to that established with simpler drug discriminations; pentobarbital generalized to chlordiazepoxide, morphine generalized to methadone, and (+)amphetamine generalized to cocaine and methamphetamine. Low doses of phencyclidine generalized to saline, while higher doses partially generalized to pentobarbital and (+)amphetamine. When dose-response curves were redetermined with a cumulative-dosing procedure, the same pattern of generalization occurred as under single-dose procedures. Dose-response curves were quantal under both the single-dose and the cumulative-dosing procedures. The four-choice procedure offers some important advantages for studying the discriminative stimulus effects of drugs that interact with multiple receptor subtypes and for studying drug mixtures.

Generalisation of ethanol with drug mixtures containing a positive modulator of the GABA(A) receptor and an NMDA antagonist

Ethanol is thought to produce its discriminative stimulus effect by actions on two or more neurotransmitter systems. To test this idea further, rats were trained to discriminate mixtures of two drugs from vehicle in two-lever procedures with food reinforcers presented on a tandem variable-interval fixed ratio schedule. After drug-appropriate responding with the training mixtures reached 85%, generalisation to ethanol was examined in extinction tests. Rats trained to discriminate a mixture of chlordiazepoxide (5.0 mg/kg, s. c.) plus dizocilpine (0.08 mg/kg, i.p.) yielded a mean of 76% drug-appropriate responding when tested with ethanol (3.0 g/kg, i.g. ). However, when rats were trained with an 8.0 mg/kg dose of pentobarbitone in a mixture with 0.08 mg/kg of dizocilpine, the same dose of ethanol produced only 33% drug-appropriate responding. After retraining with pentobarbitone (12 mg/kg) plus dizocilpine (0.04 mg/kg), ethanol (3.0 g/kg, i.g.) produced 75% drug-appropriate responding. Pentobarbitone and dizocilpine administered alone produced full, dose-related generalisation, but there was no generalisation to (+)-amphetamine (0.025-0.8 mg/kg, s.c.). Thus, ethanol substituted for mixtures in which the GABA(A)-modulatory component had equal or greater salience than the NMDA-antagonist component. Doses of ethanol that generalised with the drug mixtures always reduced overall rates of responding as compared with control rates. Nevertheless, these data provide further support for the hypothesis that ethanol produces a compound stimulus comprised of elements resembling the effects of positive modulators of GABA(A) receptors and those of NMDA antagonists.

Effects of benzodiazepine receptor ligands and ethanol in rats trained to discriminate pregnanolone

Although GABA(A) receptor positive modulators share many behavioral effects, subtle differences have been detected among their discriminative stimulus effects. The purpose of the present study was to determine the extent of shared discriminative stimulus effects of pregnanolone with various benzodiazepine receptor ligands and with ethanol. Naive male Sprague-Dawley rats were trained to discriminate the endogenous neuroactive steroid pregnanolone (5.6 or 8.0 mg/kg) from vehicle. The benzodiazepine receptor agonists, triazolam and lorazepam, the benzodiazepine receptor partial agonist, bretazenil, the benzodiazepine1 (BZ1) receptor subtype selective agonists, zolpidem and zaleplon and ethanol were tested. Triazolam, lorazepam and bretazenil substituted for pregnanolone. Lorazepam, but not triazolam or bretazenil, decreased response rates at the highest dose tested. Zaleplon completely substituted for pregnanolone with no effect on response rates. Zolpidem substituted for pregnanolone only at a dose that severely disrupted response rates. Ethanol partially substituted for pregnanolone and decreased response rates. The results are consistent with GABA(A) receptor mediation of the discriminative stimulus effects of pregnanolone. The effects on response rates suggest subtle differentiation among the GABA(A) receptor-mediated cues.

Influence of training history on ethanol discrimination in rats

The compound stimulus hypothesis of ethanol discrimination predicts that a history of training to discriminate drugs that mimic individual elements of the ethanol stimulus should attenuate stimulus control by other stimulus elements (associative blocking). Rats were trained initially to discriminate either chlordiazepoxide (5 mg / kg s.c., n = 10) or dizocilpine (0.08 mg / kg i.p., n = 10) from vehicle in two-lever procedures with food reinforcers presented on a tandem variable-interval fixed ratio schedule. Control rats received 'sham training' (vehicle injections only, n = 9). All subjects were then trained to discriminate ethanol (1.5 g / kg intragastrically (i.g.)) until discrimination accuracy reached 95%. Chlordiazepoxide (1.25-10.0 mg / kg s.c.) produced more drug-appropriate responding in rats with a previous history of training to discriminate chlordiazepoxide than in either of the other two groups, but stimulus control by dizocilpine was not attenuated. Equivalent results were obtained in rats with a previous history of training to discriminate dizocilpine. Ethanol (0.375-3.0 g / kg i.g.) produced similar dose-related increases in drug-appropriate responding in all three groups. Thus, previous discrimination training modified the characteristics of ethanol discrimination in a way that may be explained by persistence of the original discriminations. The lack of evidence for associative blocking contrasts with results of previous experiments on the discrimination of compound stimuli produced by administering drug mixtures. The findings provide limited support for the hypothesis that ethanol produces a compound stimulus that includes elements of positive modulation of gamma-aminobutyric acid (GABA(A)) receptors and of N-methyl-D-aspartate (NMDA) antagonism.

Opiate delta-2-receptor antagonist naltriben does not alter discriminative stimulus effects of ethanol

The ability of a selective 2-opiate receptor antagonist, naltriben, to modulate ethanol discrimination was investigated in a rat model using a drug discrimination procedure. Rats were trained to discriminate ethanol (1.25 g/kg, IP) from saline on a fixed-ratio schedule, FR10. Once rats had acquired the ethanol-saline discrimination, ethanol dose-response tests were conducted with 15-min pretest injections. Following the characterization of the ethanol dose-response curve, the effect of naltriben on ethanol's discriminative stimulus was assessed by administering naltriben (0. 032-5.6 mg/kg, IP) 15 min before the ethanol administration. In the present study, naltriben did not have any modulatory effect on ethanol discrimination, suggesting that either Delta(2)-opiate receptors are not involved in the formation of ethanol's discriminative stimulus or the antagonism of Delta(2)-opiate receptors is not sufficient to alter ethanol's compound discriminative stimulus.

Drug discrimination under two concurrent fixed-interval fixed-interval schedules

Pigeons were trained to discriminate 5.0 mg/kg pentobarbital from saline under a two-key concurrent fixed-interval (FI) 100-s FI 200-s schedule of food presentation, and later tinder a concurrent FI 40-s FI 80-s schedule, in which the FI component with the shorter time requirement reinforced responding on one key after drug administration (pentobarbital-biased key) and on the other key after saline administration (saline-biased key). After responding stabilized under the concurrent FI 100-s FI 200-s schedule, pigeons earned an average of 66% (after pentobarbital) to 68% (after saline) of their reinforcers for responding under the FI 100-s component of the concurrent schedule. These birds made an average of 70% of their responses on both the pentobarbital-biased key after the training dose of pentobarbital and the saline-biased key after saline. After responding stabilized under the concurrent FI 40-s FI 80-s schedule, pigeons earned an average of 67% of their reinforcers for responding under the FI 40 component after both saline and the training dose of pentobarbital. These birds made an average of 75% of their responses on the pentobarbital-biased key after the training dose of pentobarbital, but only 55% of their responses on the saline-biased key after saline. In test sessions preceded by doses of pentobarbital, chlordiazepoxide, ethanol, phencyclidine, or methamphetamine, the dose-response curves were similar under these two concurrent schedules. Pentobarbital, chlordiazepoxide, and ethanol produced dose-dependent increases in responding on the pentobarbital-biased key as the doses increased. For some birds, at the highest doses of these drugs, the dose-response curve turned over. Increasing doses of phencyclidine produced increased responding on the pentobarbital-biased key in some, but not all, birds. After methamphetamine, responding was largely confined to the saline-biased key. These data show that pigeons can perform drug discriminations under concurrent schedules in which the reinforcement frequency under the schedule components differs only by a factor of two, and that when other drugs are substituted for the training drugs they produce dose-response curves similar to the curves produced by these drugs under other concurrent interval schedules.

Anesthesia is a risk factor for drug and alcohol craving and relapse in ex-abusers

Relapse to cocaine, heroin, and alcohol is a common occurrence in ex-abusers of these substances. Although there are many potential causes for relapse, we postulate that one cause in some people may be re-exposure in an anesthesia setting to a drug similar to the formerly abused drug. We hypothesize, for example, that opioids given during and after surgery may reinstate craving for, and initiate subsequent seeking of, heroin in former abusers. There are a substantial number of animal studies and some human studies documenting a reinstatement phenomenon in which an experimenter-administered psychoactive drug can precipitate drug-seeking behavior in 'abstinent' animals and humans. There is concern amongst health professionals and patients alike on this issue, and we discuss possible avenues of research, both preclinical and clinical, to explore the validity of our hypothesis.

Relationship of components of an alcohol interoceptive stimulus to induction of desire for alcohol in social drinkers

The ability of a low (0.2 g/kg) oral dose of ethanol to provide a drug discriminative stimulus was studied in young healthy human volunteers, who were social drinkers. Seventeen of 24 subjects acquired the discrimination following 10 trials in which they received aliquots of ethanol or of placebo drink (tonic water mixed with Tabasco sauce). In generalization studies, in which the dose of ethanol was varied, discrimination performance was dose dependent; doses greater than 0.05 g/kg gave rise to significant ethanol-appropriate responding. Concurrent estimates of the subjective effects of doses administered as discriminative stimuli revealed that two factors--taste and light-headedness--were associated with discrimination: at the training dose, 0.2 g/kg, although both the factors taste and light-headedness were significantly increased, only taste predicted discrimination performance. At lower doses, taste did not contribute to discrimination, but the subjective rating light-headedness correlated significantly with discrimination accuracy. Post hoc analyses of the influence of the amount of alcohol regularly drunk by the volunteers, on discrimination performance suggested light-headedness correlated with discriminative performance only in social drinkers drinking more than 20 units per week. In a second experiment, groups of "high" (mean 40 units per week) and "low" (mean 10 units per week) social drinkers were prospectively identified. Discrimination performance of 0.2 g/kg ethanol in orange juice vs. orange juice vehicle indicated that both groups were able to perform the discrimination following a single training trial, and that generalization curves over the range 0.05-0.2 g/kg were dose dependent, and not different between the groups. At the lowest dose, discrimination performance was predicted by taste, stimulation, and light-headedness in the "high" group, but not in the "low" group. The ability of these ethanol doses to induce feelings of craving for ethanol were assessed in parallel, using the Desire for Alcohol Questionnaire (DAQ). "High" drinkers showed higher desire for ethanol on all factors of the DAQ except the "positive negative reinforcement" factor, and sampling ethanol tended to increase desire in these measures. However, at each dose, the induction of feelings of desire for ethanol showed a negative correlation with discrimination performance. These findings are discussed in the context of the ability of animals and humans to use several components of drug-induced stimuli in the performance of drug discrimination, and the role of such discriminative stimuli in priming of ethanol drinking.

High-dose discrimination training with midazolam: context determines generalization profile

In previous work, greater differentiation among ligands for the benzodiazepine site was found in rats trained to discriminate among vehicle, 0.32, and 3.2 mg/kg midazolam than in animals trained to discriminate a single midazolam dose from vehicle (i.e., virtually all test drugs occasioned low-dose midazolam-appropriate responding, but most did not occasion high-dose midazolam-appropriate responding even at high test doses). A possibility was that merely training with 3.2 mg/kg-midazolam (not previously studied) would result in greater selectivity than training with lower midazolam doses. In the present study, rats were trained to discriminate 3.2 mg/kg i.p. midazolam from no drug under a two-lever, food-maintained, procedure; and drugs from the previous three-lever studies were tested. Triazolam, bretazenil, clonazepam, lorazepam, midazolam, zolpidem, chlordiazepoxide, pentobarbital, and flurazepam all dose-dependently occasioned >80% responding on the midazolam-appropriate lever in roughly that order of potency. Only triazolam had occasioned midazolam 3.2 mg/kg-appropriate responding in the previous work. The greater differentiation among these drugs in the dose-vs.-dose procedure likely was due to a training dose context rather than to the high training dose per se.

Strategies for understanding the pharmacological effects of ethanol with drug discrimination procedures

Ethanol appears to produce a stimulus complex, or compound cue, composed of distinct components that are mediated by different receptor systems. In ethanol vs. water discriminations, it appears that ethanol produces a redundant stimulus complex such that separate, receptor-mediated activity can serve as the basis for the discrimination. These discriminations have been termed redundant, because multiple features of the cue could serve as the basis of the discrimination. In ethanol vs. water discriminations, one common feature is the asymmetrical generalizations between components of the ethanol cue and ethanol. There is also evidence for overshadowing of one component by other components of the ethanol stimulus complex. It appears possible to transfer the basis of the ethanol cue from a redundant cue to a conditional cue with specific training procedures. When the discriminative stimulus effects of ethanol are juxtoposed with those of one component of the ethanol complex, as in ethanol vs. water vs. pentobarbital discriminations, the ethanol discrimination shifts to a conditional basis. The ability to antagonize an ethanol discrimination may be dependent upon whether the discrimination is based on redundant component stimuli or conditional presence of all component stimuli.

Drug mixtures and ethanol as compound internal stimuli

Drug discrimination methods that entail training with mixtures of drugs may shed light on polydrug abuse and on the actions of single drugs that interact with more than one receptor. In AND-discrimination procedures (drug A + drug B vs. vehicle), mixtures are discriminated primarily on the basis of their component drugs: these discriminations may be useful for testing interactions between component drugs in mixtures. The role of training dose, overshadowing and associative blocking in AND-discriminations have been investigated. For example, after prior training with midazolam, it was possible to demonstrate associative blocking of the nicotine element of the mixture stimulus, and vice versa. Using the AND-OR discriminations (drug A + drug B vs. drug A or drug B) increased pharmacological specificity considerably, and these procedures may be valuable for determining whether the effects of a novel mixture are similar to the combined effects of the training drugs. Ethanol is an example of a single drug that may produce a compound cue; rats trained to discriminate ethanol from water generalize (asymmetrically) to GABA(A) enhancers such as chlordiazepoxide (CDP) or pentobarbitone, to NMDA antagonists such as dizocilpine (MK-801), and to some serotonin agonists, such as trifluoromethylphenylpiperazine (5-HT(1B/2C)). In addition, rats trained to discriminate mixtures of either CDP or pentobarbitone plus MK-801 generalize to ethanol. A previous history of training with MK-801 or CDP (prior to ethanol discrimination training) enhanced the MK-801-like and CDP-like effects of ethanol respectively, but associative blocking of proposed elements in the ethanol stimulus was not seen. These studies provide some support for the multielement concept of ethanol discrimination but also suggest that rules governing three-component stimuli (such as those putatively produced by ethanol) may differ from those for the two-component mixtures of drugs studied previously.

Drug discrimination under a concurrent fixed-ratio fixed-ratio schedule

Pigeons were trained to discriminate 5.0 mg/kg pentobarbital from saline under a two-key concurrent fixed-ratio 10 fixed-ratio 40 schedule of food presentation, in which the fixed-ratio component with the lower response requirement was programmed to reinforce responding on one key after drug administration (pentobarbital-biased key) and on the other key after saline administration (saline-biased key). After responding stabilized, pigeons averaged 98% of their responses on the pentobarbital-biased key during training sessions preceded by pentobarbital, and they averaged 90% of their responses on the saline-biased key during training sessions preceded by saline. In test sessions preceded by doses of pentobarbital, chlordiazepoxide, or ethanol, pigeons switched from responding on the saline-biased key at low doses to responding on the pentobarbital-biased key at higher doses (the dose-response curve was quantal). High doses of phencyclidine produced responding on both keys, whereas pigeons responded almost exclusively on the saline-biased key after all doses of methamphetamine. These and previous experiments using concurrent reinforcement schedules to study drug discrimination illustrate that the schedule of reinforcement is an important determinant of the shape of dose-effect curves in drug-discrimination experiments.

Increased specificity of ethanol's discriminative stimulus effects in an ethanol-pentobarbital-water discrimination in rats

Ethanol's modulation of a number of receptor systems results in a heterogeneous discriminative stimulus complex. A previous study found that these heterogeneous discriminative stimulus effects were seemingly diminished when rats were trained to discriminate ethanol (2.0 g/kg) from pentobarbital (10.0 mg/kg). The present experiment was designed to extend these findings by using a lower training dose of ethanol (1.0 g/kg). Adult male Long-Evans rats (n = 7) discriminated pentobarbital (10.0 mg/kg; intragastric (i.g.)) from ethanol (1.0 g/kg; i.g.) from water (2.3 ml; i.g.) in a 3 lever, food-reinforced task. Substitution tests were conducted following intraperitoneal (i.p.) administration of GABA(A) positive modulators, noncompetitive NMDA antagonists, 5-HT1 agonists and isopropanol. The GABA(A) positive modulators diazepam, midazolam and allopregnanolone completely substituted for pentobarbital. Isopropanol completely substituted for ethanol, while the NMDA antagonists dizocilpine and phencyclidine partially substituted for ethanol. The 5-HT agonists RU 24969 and CGS 12066B did not result in complete substitution for ethanol or pentobarbital, although RU 24969 resulted in partial pentobarbital substitution. These data replicate and extend the previous findings that discriminating ethanol from pentobarbital attenuates the ethanol-like effects of GABA(A) positive modulators, NMDA antagonists and 5-HT1 agonists and results in a more specific ethanol cue. The outcome appears to be a conditional basis for the ethanol discrimination, where a full ethanol-like effect is produced only by drugs with pharmacological activity similar to the heterogenous effects of ethanol (e.g. other alcohols).

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.

Discriminative stimulus function of ethanol: role of GABAA receptors in the nucleus accumbens

Because the subjective effects of drugs may be related to abuse potential, this study was conducted to assess the involvement of GABAA receptor systems in the nucleus accumbens (N Acc) in the discriminative stimulus effects of ethanol. Male Long-Evans rats were trained to discriminate between intraperitoneal (IP) injections of ethanol (1 g/kg) and saline under a fixed-ratio 10 schedule of sucrose (10% w/v) reinforcement. When performance during training sessions met the accuracy criteria (> 80% correct responding for five consecutive days), an ethanol generalization curve was determined. The rats were then surgically implanted with bilateral stainless-steel guide cannulae aimed at the N Acc. Intra-accumbens (IA) substitution test sessions were conducted during which the direct GABAA agonist muscimol (0.01, 0.04, 0.10, and 0.40 micrograms/microliter; IA) was administered in combination with saline (IP). The direct GABAA antagonist bicuculline (0.03, 0.10, and 0.30 micrograms/microliter; IA) was administered in combination with the training dose of ethanol (1 g/kg, ip). At 10-min postinjection, IA muscimol partially substituted for IP ethanol. However, at 15-min postinjection, muscimol (0.10 microgram/microliter; IA) fully substituted for IP ethanol. Bicuculline attenuated the discriminative stimulus properties of IP ethanol, but only at doses that significantly decreased response rate. At 10-min postinjection, muscimol (0.01 and 0.04 micrograms/microliter) potentiated (> 80% ethanol lever responding) the discriminative stimulus properties of a dose of ethanol (0.5 g/kg) that alone produced only partial generalization. These data suggest that ethanol discrimination is mediated centrally and demonstrate that infusions of the GABAA agonist muscimol in the N Acc are sufficient to produce the stimulus effects corresponding to a 1.0 g/kg training dose of ethanol. When taken together with data showing that GABAA receptor activation in the N Acc potentiates the termination of ethanol self-administration, these data suggest that ethanol's discriminative stimulus function may influence its reinforcement function.

Comparison of the stimulus properties of ethanol and the Ca2+ channel antagonist nimodipine in rats

A variety of L-type Ca2+ channel antagonists, including the dihydropyridine derivative nimodipine, have been shown to be effective in reducing ethanol intake and preference in animal models of alcoholism. The behavioral mechanism involved in the anti-alcohol effects of nimodipine are, however, not clear yet. The aim of the present study was to investigate the possibility that the effects of nimodipine on ethanol intake are based on stimulus substitution. Therefore, rats were trained to discriminate ethanol (12.5% w/v, 1000 mg/kg i.p.) from saline in a two-lever food-reinforced drug discrimination procedure (dose range of ethanol tested: 125-1000 mg/kg i.p., ED50 value: 488 mg/kg). In cross-generalization tests with nimodipine (0.15-15 mg/kg i.p.), stimulus substitution was not noted. In addition, a cross-familiarization conditioned taste aversion paradigm was utilized. In rats, 1000 mg/kg i.p. ethanol was used as the reference drug producing a conditioned taste aversion. Effects of preexposure to ethanol (500-1500 mg/kg i.p.) and nimodipine (7.5-30 mg/kg i.p.) on the magnitude of the ethanol-induced conditioned taste aversion were investigated as an index for stimulus similarity between preexposure and reference drug. Preexposure to both ethanol and nimodipine prevented the development of a conditioned taste aversion. Contrary to the drug discrimination results, these latter findings suggest that there may be similarities between the stimulus properties of nimodipine and ethanol. Moreover, the apparent discrepancy between the results obtained in drug discrimination and cross-familiarization conditioned taste aversion suggests that different stimulus properties of ethanol control behavior in both procedures. The finding that, under particular conditions, ethanol and nimodipine appear to share common stimulus properties needs to be further evaluated, as this may be related to the reported anti-alcohol effects of nimodipine and other Ca2+ channel antagonists.

GABAergic transmission in the nucleus accumbens is involved in the termination of ethanol self-administration in rats

Long-Evans rats (n = 12) were trained to lever-press on a fixed-ratio 4 schedule of reinforcement with ethanol (10% v/v) presented as the reinforcer. After implantation of bilateral stainless-steel guide cannulae aimed at the nucleus accumbens, site-specific microinjections of muscimol (1-30 ng) and bicuculline (1-10 ng) were tested for effects on ethanol-reinforced responding. Baseline response patterns were characterized by initial high rates that terminated abruptly after approximately 20 min. Muscimol administration in the nucleus accumbens decreased the total number of ethanol-reinforced responses and obtained reinforcers. Bicuculline also decreased ethanol-reinforced responses and reinforcers at the highest dose tested. When a dose of bicuculline (1 ng) that was ineffective by itself was coadministered with an effective dose of muscimol (10 ng), the muscimol-induced decreases in responding were blocked. Analysis of response patterns showed that muscimol decreased ethanol self-administration by terminating responding, normally lasting 20 min, after approximately 10 min with no changes in local response rate. Bicuculline decreased total responding by producing parallel, but nonsignificant, changes in time course and response rate. These data suggest that GABAergic transmission in the nucleus accumbens is involved in the termination, but not the onset or maintenance of ethanol self-administration. The specificity of this effect gives emphasis to the importance of measuring behavioral parameters, as well as products of behavior (such as intake volume) in the study of ethanol self-administration.

Empirical evidence that the state dependence and drug discrimination paradigms can generate different outcomes

The study compared the outcomes generated by the State Dependence and Drug Discrimination paradigms with ethanol in the rat. Food-deprived rats learned to complete a fixed-ratio 10 schedule of bar presses for food within 120 s while treated with 320- to 1250-mg/kg doses of ethanol. Subsequent tests of recall of this response with saline failed to generate any evidence that transfer was hampered following the drug-to-saline state change. In contrast, each of 14 rats learned to discriminate 1250 mg/kg ethanol from saline in a Drug Discrimination procedure that also required the animals to press one of two levers for food according to a fixed- ratio 10 schedule. The results offer the first empirical evidence to demonstrate directly that the State Dependence and Drug Discrimination paradigms can generate different outcomes in otherwise identical experimental conditions.

Involvement of benzodiazepine/GABA-A receptor complex in ethanol-induced state-dependent learning in rats

State-dependent learning (SDL) induced by ethanol (EtOH) was investigated on the step-through passive avoidance task in rats. Pretraining injection of EtOH dose-dependently reduced step-through latency in the test session 24 h after the training. Injection of EtOH (1.0 g/kg) before both the training and test sessions, however, failed to reduce the latency. These results show that EtOH produces SDL. The failure of learning performance in SDL (dissociation in SDL) induced by EtOH was blocked by bicuculline, Ro15-4513 and picrotoxin injected before the training session. The success of learning performance in SDL (non-dissociation in SDL) induced by EtOH was also blocked by bicuculline, Ro15-4513 and picrotoxin injected before the test session. The antagonism of Ro15-4513 against EtOH was blocked by flumazenil. In the substitution test, pretest injection of EtOH produced non-dissociation in SDL in the both of pretraining diazepam-and muscimol-treated rats. On the other hand, neither pretest injection of diazepam nor muscimol produced non-dissociation in the pretraining EtOH-treated rats: asymmetrical cross-substitution between EtOH and diazepam and between EtOH and muscimol was observed. These results suggest that the EtOH-induced SDL is partially mediated by the benzodiazepine (BDZ)/GABA-A receptor complex.

Symmetrical generalization between the discriminative stimulus effects of gamma-hydroxybutyric acid and ethanol: occurrence within narrow dose ranges

Gamma-hydroxybutyric acid (GHB) has been shown to reduce ethanol consumption and suppress ethanol withdrawal syndrome both in laboratory animals and humans. The present study was designed to assess the similarity between the discriminative stimulus effects, or subjective feelings, of GHB and ethanol using a T-maze, food-reinforced drug discrimination procedure. Three groups of rats were trained to discriminate ethanol (1.0 or 2.0 g/kg; p.o.) or GHB (300 mg/kg; p.o.) from water. In the 1.0 g/kg ethanol-trained rats, substitution for ethanol was an inverted U-shape function of GHB dose, with only 300 mg/kg GHB resulting in complete substitution for ethanol. No dose of GHB elicited selection of ethanol-appropriate arm higher than 10% in the 2.0 g/kg ethanol-trained group. In the 300 mg/kg GHB-trained rats, complete substitution for GHB occurred only at the dose of 1.0 g/kg ethanol. Doses of ethanol lower or higher than 1.0 g/kg did not substitute for GHB. The results of the present study indicate that symmetrical generalization between ethanol and GHB occurred within narrow dose ranges. They are discussed in terms of common neurotransmitter systems involved in the mediation of GHB and ethanol effects.

Defeat engenders pentylenetetrazole-appropriate responding in rats: antagonism by midazolam

Defeat and the threat of defeat by an aggressive conspecific is stressful and may engender an anxiety- or fear-like state in animals; the present experiment investigated whether defeat generalized to the discriminative stimulus properties of PTZ and how benzodiazepine receptors were involved in this generalization. Separate groups of male Long-Evans rats (Rattus norvegicus) were trained to discriminate 20 mg/kg pentylenetetrazole (PTZ) or 0.4 mg/kg midazolam (MDZ) from saline in a two-choice drug-discrimination task. After establishing stimulus control, PTZ- and MDZ-trained rats were exposed to an aggressive conspecific which resulted in defeat, as defined by the display of defensive and submissive postures as well as audible and ultrasonic vocalizations. Administration of saline after defeat resulted in greater than 80% PTZ lever selection in 15 out of 25 PTZ-trained rats; this effect was attenuated through pretreatment with MDZ (1 mg/kg). Furthermore, short-term defeat substitution for the PTZ discriminative stimulus was not accompanied by long-term changes in the post-defeat generalization curves for PTZ and MDZ when compared to pre-defeat generalization curves. Nor did defeat alter the antagonism of PTZ by diazepam (2.5 mg/kg) or MDZ by flumazenil (10 mg/kg). In order further to characterize the necessary features for defeat substitution for the PTZ discriminative stimulus, exposure to a threatening conspecific was also attempted by PTZ-trained rats protected from physical contact with a wire mesh cage. In these tests, saline continued to engender greater than 50% PTZ lever responding in 15 of 25 rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Discriminative stimulus effects of melatonin in the rat

To provide initial information on the potential mechanisms underlying the discriminative stimulus effects of melatonin, rats were trained to discriminate melatonin (150 mg/kg, IP) from saline in a two-choice discrete-trial avoidance paradigm. Stimulus generalization curves for melatonin were steep; complete generalization with melatonin occurred at 100-150 mg/kg. Triazolam generalized completely with melatonin (n = 7). Flurazepam generalized completely with melatonin in only two out of six rats; however, partial generalization was produced in the remaining four animals. The melatonin-appropriate responding produced by triazolam was antagonized completely (in six out of seven rats) by 0.3-10 mg/kg flumazenil (Ro 15-1788). In contrast, the dose of flumazenil sufficient to block completely the melatonin-like discriminative effects of triazolam failed to block the stimulus effects of the training dose of melatonin. Pentobarbital produced primarily melatonin-appropriate responding, with complete generalization with melatonin in five out of seven rats. Diphenhydramine generalized completely with melatonin in two out of seven rats; however, little or no partial generalization was observed in the remaining five rats. These results suggest that melatonin may produce its discriminative effects through sites on the GABAA-benzodiazepine receptor complex distinct from the benzodiazepine binding sites.

Discriminative stimulus effects of alpidem, a new imidazopyridine anxiolytic

Alpidem in an imidazopyridine derivative which binds selectively to the omega 1 (BZ1) receptor subtype. It is active in some, but not all, behavioural tests sensitive to benzodiazepine anxiolytics and has clinical anti-anxiety effects. However, in a previous study, it was shown that alpidem did not substitute for chlordiazepoxide in rats trained to discriminate this benzodiazepine. The present experiments were carried out to investigate the discriminative stimulus properties of alpidem in greater detail. In the first experiment rats learned to discriminate a dose of 10 mg/kg alpidem from saline. Acquisition of the discrimination was long and performance unstable. Chlordiazepoxide, clorazepate and zolpidem substituted only partially for alpidem but the effects of the training dose of alpidem were blocked by 10 mg/kg flumazenil. The second experiment established stimulus control more rapidly to a dose of 30 mg/kg alpidem. Alpidem induced dose-related stimulus control, and dose-related and complete substitution for alpidem was produced by zolpidem, abecarnil, CL 218,872, triazolam and suriclone. Partial substitution occurred with chlordiazepoxide, clorazepate and pentobarbital. In most cases, high levels of substitution were produced only by doses which greatly reduced response rates even though the training dose of alpidem produced only modest decreases in rates. Ethanol, buspirone and bretazenil produced very little substitution for alpidem and both flumazenil and bretazenil antagonised the effects of alpidem. In two further experiments alpidem was found to substitute for the stimulus produced by zolpidem (2 mg/kg) but not for that produced by ethanol (1.5 g/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Discriminative stimulus effects of presynaptic GABA agonists in pentobarbital-trained rats

The discriminative stimulus effects of indirect-acting GABAergic drugs were compared to those of pentobarbital (PB) and midazolam in rats trained to discriminate 5 mg/kg PB from saline under a two-lever fixed-ratio 32 schedule of food reinforcement. PB and midazolam produced dose-dependent substitution for the training dose of PB with response rate reduction only at doses above those producing full substitution. Valproic acid, an antiepileptic drug and GABA transaminase inhibitor, substituted for PB but only at a dose that produced response rate suppression. Vigabatrin, an irreversible GABA transaminase inhibitor, failed to substitute for PB, but did produce a dose-dependent decrease in response rates. The GABA uptake inhibitors, 1-[2-[bis[4-(trifluoromethyl)phenyl]-methoxy]ethyl]-1,2,5,6- tetrahydro-3-pyridinecarboxylic acid (CI-966) and (R(-)-N-[4,4-bis(3-methylthien-2-yl)but-3-enyl] nipecotic acid HCl (tiagabine), produced no greater than 40% PB-lever responding. Aminooxyacetic acid (AOAA), which is described as a nonselective presynaptic GABA agonist, yielded a maximum of 43% PB-lever responding. These results indicate that the discriminative stimulus effects of the indirect GABAA agonists, PB and midazolam, although similar to one another, differ from those of presynaptic GABAergic drugs. Differences in the discriminative stimulus properties of GABA transaminase inhibitors and uptake inhibitors also exist, indicating that not all presynaptic GABA agonists have similar behavioral profiles. These results contribute to a further understanding of the similarities and differences in the behavioral effects of drugs that enhance GABAergic neurotransmission.

Ipsapirone and 8-OH-DPAT reduce ethanol preference in rats: involvement of presynaptic 5-HT1A receptors

The selective serotonin(5-HT)1A receptor agonists 8-OH-DPAT and ipsapirone were tested in selectively inbred Wistar rats, with high preference [70-90%: defined as the ratio of ethanol (EtOH) to total fluid intake] for EtOH (10% v/v) over water in a two-bottle free choice situation. Rats were injected shortly before the overnight test session (8:00 P.M.-8:00 A.M.). EtOH and water consumption were determined in 20-min intervals; food consumption after the session. 8-OH-DPAT (ED50: 2.4 mg/kg, SC) and ipsapirone (ED50: 12.5 mg/kg, SC) reduced EtOH preference in a dose-dependent manner. In addition, 8-OH-DPAT increased total fluid intake, whereas ipsapirone enhanced total food intake. The EtOH preference reduction was time-dependent and reached a maximum within the second 4 h after application of 8-OH-DPAT (-73%) and ipsapirone (-72%). The preference reducing effect of ipsapirone (20 mg/kg, PO) was completely blocked by the nonselective 5-HT1A antagonist spiperone (0.05 mg/kg, SC). Local application of 8-OH-DPAT (10 micrograms, 0.5 microliters) into the dorsal raphe nucleus (DRN, a brain area rich in somatodendritic 5-HT1A autoreceptors), reduced the EtOH preference significantly as compared to the saline injection in the same animal (-12%, 8:00-12:00 P.M.). Only marginal effects on ingestion behavior were observed after microinjection into the nucleus accumbens. Reduction of brain 5-HT levels by pretreatment with the 5-HT synthesis inhibitor pCPA (2 x 150 mg/kg, IP) resulted in a short lasting, marked reduction (-54%) and a long lasting, small attenuation of the EtOH preference. Total food consumption was strongly decreased but returned soon to normal; total fluid intake was only slightly decreased. The EtOH preference reducing effect of ipsapirone (5 and 20 mg/kg, SC) was attenuated in pCPA-pretreated rats. The present data suggest that 5-HT1A receptor ligands reduce EtOH preference via stimulation of 5-HT1A receptors in the DRN. The possibility of additional mechanism(s) is discussed.

Substitution of the 5-HT1 agonist trifluoromethylphenylpiperazine (TFMPP) for the discriminative stimulus effects of ethanol: effect of training dose

The role of the ethanol training dose on the ability of the selective 5-HT1 agonist TFMPP (m-trifluoromethylphenylpiperazine) to produce ethanol-like discriminative stimulus effects was evaluated in three groups of rats trained to discriminate 1.0 g/kg (n = 5), 1.5 g/kg (n = 6) or 2.0 g/kg (n = 7) ethanol (IG) from water using a two-lever procedure with food reinforcement available under a fixed ratio 20 (FR 20) schedule. Ethanol generalization gradients were comparable in the three groups, indicating few potency differences in the ethanol stimulus across training dose. However, the ability of TFMPP (0.1-1.7 mg/kg; IP) to substitute for ethanol was dependent on the training dose. TFMPP resulted in partial substitution in the 1.0 g/kg group, complete substitution for 1.5 g/kg group and no substitution in the 2.0 g/kg ethanol training group. The results indicate a serotonergic component to the discriminative stimulus effects of an intermediate dose of ethanol that is not prominent as the dose of ethanol is raised. These data add further support for the hypothesis that ethanol produces a mixed discriminative cue, the components of which are not uniformly amplified when the dose of ethanol is increased.

Pentobarbital-like discriminative stimulus effects of direct GABA agonists in rats

The discriminative stimulus effects of direct and indirect-acting GABAergic drugs were investigated in rats trained to discriminate 5 mg/kg pentobarbital (PB) from saline under a two-lever fixed ratio (FR) 32 schedule of food reinforcement. PB and diazepam produced dose-dependent substitution for the training dose of PB with response rate reduction only at doses above those producing full substitution. Muscimol, thiomuscimol and 4,5,6,7-tetrahydroisoxazolo [5,4-c]-pyridin-3-ol (THIP) produced intermediate levels of pentobarbital-lever responding (40-60%), accompanied by dose-dependent decreases in rates of responding following THIP and muscimol administration. The GABAA agonist progabide and its metabolite 4-([(4-chlorophenyl) (5-fluoro-2-hydroxyphenyl)methylene]amino)] butyric acid (SL 75102) also partially substituted for PB, producing means of 39-73% PB-lever responding. The GABAB agonist, baclofen, completely failed to substitute for PB even at doses that decreased rates of responding. These results show that the discriminative stimulus effects of indirect GABAA agonists, PB and diazepam, although similar to one another, differ from those of direct GABAA receptor agonists, which produced only partial substitution for PB. The GABAB agonist, baclofen, can be distinguished by lacking any ability to substitute for PB. These results contribute to a further understanding of the similarities and differences in the behavioral effects of different types of GABA agonists.

Drug discrimination analysis of ethanol as an N-methyl-D-aspartate receptor antagonist

Ethanol has been shown to antagonize N-methyl-D-aspartate (NMDA) receptor-mediated neurotransmission in a number of in vitro systems. Drug discrimination procedures in rats were used to evaluate ethanol as an antagonist of NMDA discrimination and for its ability to produce discriminative stimulus effects similar to those of competitive and noncompetitive NMDA antagonists. Ethanol (300-1500 mg/kg i.p.) failed to antagonize the stimulus effects of 30 mg/kg NMDA, nor did it substitute fully for either the competitive antagonist NPC 12626 nor the noncompetitive antagonist phencyclidine (PCP). A maximum average of 55.4% PCP-lever responding provided evidence for partial substitution in this model. The effects of ethanol on NMDA discrimination are distinct from those previously reported for competitive NMDA antagonists but similar to those of noncompetitive antagonists. On the other hand, ethanol can be distinguished from both competitive and PCP-like noncompetitive NMDA antagonists using drug discrimination procedures.

Effects of ethanol and toluene on fixed-ratio performance in short sleep and long sleep mice

This study examined the effects of ethanol (EtOH) and toluene on fixed-ratio (FR) responding in mice selectively bred for sensitivity to the effects of EtOH on sleep time. Although the more sensitive long sleep (LS) mice showed greater EtOH-induced impairment in a motor performance task than did the less sensitive short sleep (SS) mice, changes in FR performance in the two lines did not differ in response to EtOH, regardless of route (oral or intraperitoneal) or time (40 vs. 60 min pre-session) of administration. These results emphasize the importance of considering task variables in determination of the behavior of different genotypes. In contrast to results with EtOH, the volatile inhalant toluene produced different effects on FR responding in the selected lines, with SS mice being more sensitive than LS mice.

Assessment of the stimulus properties of anxiolytic drugs by means of the conditioned taste aversion procedure

The conditioned taste aversion (CTA) procedure has recently been described as a more rapid alternative to two-lever operant procedures in drug discrimination research. We trained different groups of rats to discriminate the benzodiazepine chlordiazepoxide (CDP, 20 mg/kg) or the 5-hydroxytryptamine1A (5-HT1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (0.4 mg/kg) from saline by means of the CTA procedure. The results were in agreement with findings from two-lever operant drug discrimination procedures. However, discrimination training took 40 sessions in the case of CDP and 72 sessions for 8-OH-DPAT, which is comparable to results obtained with two-lever operant procedures. Dose-response curves were determined and generalization tests were performed for different benzodiazepine and nonbenzodiazepine anxiolytics. Baseline behavior deteriorated in the course of generalization and substitution testing, thus preventing further generalization testing. Our experience is that the use of the CTA procedure in drug discrimination research does not have sufficient advantages over traditionally used procedures to replace the latter.