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

Characterization of DREADD receptor expression and function in rhesus macaques trained to discriminate ethanol

Circuit manipulation has been a staple technique in neuroscience to identify how the brain functions to control complex behaviors. Chemogenetics, including designer receptors exclusively activated by designer drugs (DREADDs), have proven to be a powerful tool for the reversible modulation of discrete brain circuitry without the need for implantable devices, thereby making them especially useful in awake and unrestrained animals. This study used a DREADD approach to query the role of the nucleus accumbens (NAc) in mediating the interoceptive effects of 1.0 g/kg ethanol (i.g.) in rhesus monkeys (n = 7) using a drug discrimination procedure. After training, stereotaxic surgery was performed to introduce an AAV carrying the human muscarinic 4 receptor DREADD (hM4Di) bilaterally into the NAc. The hypothesis was that decreasing the output of the NAc by activation of hM4Di with the DREADD actuator, clozapine-n-oxide (CNO), would potentiate the discriminative stimulus effect of ethanol (i.e., a leftward shift the ethanol dose discrimination curve). The results showed individual variability shifts of the ethanol dose-response determination under DREADD activation. Characterization of the expression and function of hM4Di with MRI, immunohistochemical, and electrophysiological techniques found the selectivity of NAc transduction was proportional to behavioral effect. Specifically, the proportion of hM4Di expression restricted to the NAc was associated with the potency of the discriminative stimulus effects of ethanol. Together, these experiments highlight the NAc in mediating the interoceptive effects of ethanol, provide a framework for validation of chemogenetic tools in primates, and underscore the importance of robust within-subjects examination of DREADD expression for interpretation of behavioral findings.

Low-dose alcohol: Interoceptive and molecular effects and the role of dentate gyrus in rats

Alcohol abuse and dependence are world-wide health problems. Most research on alcohol use focuses on the consequences of moderate to high levels of alcohol. However, even at low concentrations, alcohol is capable of producing effects in the brain that can ultimately affect behavior. The current studies seek to understand the effects of low-dose alcohol (blood alcohol levels of ≤10mM). To do so, these experiments utilize a combination of behavioral and molecular techniques to (1) assess the ability of the interoceptive effects of a low dose of alcohol to gain control over goal-tracking behavior in a Pavlovian discrimination task, (2) determine brain regional differences in cellular activity via expression of immediate early genes (IEGs), and (3) assess the role of the dentate gyrus in modulating sensitivity to the interoceptive effects of a low dose of alcohol. Here, we show that intragastric administration of a dose of 0.8 g/kg alcohol produces blood alcohol levels ≤10mM in both male and female Long-Evans rats and can readily be trained as a Pavlovian interoceptive drug cue. In rats trained on this procedure, this dose of alcohol also modulates expression of the IEGs c-Fos and Arc in brain regions known to modulate expression of alcohol interoceptive effects. Finally, pharmacological inactivation of the dentate gyrus with GABA agonists baclofen and muscimol disrupted the ability of a low dose of alcohol to serve as an interoceptive cue. Together, these findings demonstrate behavioral and molecular consequences of low-dose alcohol.

Cross-Species Translational Findings in the Discriminative Stimulus Effects of Ethanol

The progress on understanding the pharmacological basis of ethanol's discriminative stimulus effects has been substantial, but appears to have plateaued in the past decade. Further, the cross-species translational efforts are clear in laboratory animals, but have been minimal in human subject studies. Research findings clearly demonstrate that ethanol produces a compound stimulus with primary activity through GABA and glutamate receptor systems, particularly ionotropic receptors, with additional contribution from serotonergic mechanisms. Further progress should capitalize on chemogenetic and optogenetic techniques in laboratory animals to identify the neural circuitry involved in mediating the discriminative stimulus effects of ethanol. These infrahuman studies can be guided by in vivo imaging of human brain circuitry mediating ethanol's subjective effects. Ultimately, identifying receptors systems, as well as where they are located within brain circuitry, will transform the use of drug discrimination procedures to help identify possible treatment or prevention strategies for alcohol use disorder.

Ovarian hormones and the heterogeneous receptor mechanisms mediating the discriminative stimulus effects of ethanol in female rats

Past studies have suggested that progesterone-derived ovarian hormones contribute to the discriminative stimulus effects of ethanol, particularly via progesterone metabolites that act at γ-aminobutyric acid type A (GABA(A)) receptors. It is unknown whether loss of ovarian hormones in women, for example, after menopause, may be associated with altered receptor mediation of the effects of ethanol. The current study measured the substitution of allopregnanolone, pregnanolone, pentobarbital, midazolam, dizocilpine, TFMPP, and RU 24969 in female sham and ovariectomized rats trained to discriminate 1.0 g/kg ethanol from water. The groups did not differ in the substitution of GABA(A)-positive modulators (barbiturates, benzodiazepines, neuroactive steroids) or the N-methyl-D-aspartate receptor antagonist dizocilpine. Similarly, blood-ethanol concentration did not differ between the groups, and plasma adrenocorticotropic hormone, progesterone, pregnenolone, and deoxycorticosterone were unchanged 30 min after administration of 1.0 g/kg ethanol or water. However, substitution of neuroactive steroids and RU 24969, a 5-hydroxytryptamine (5-HT)(1A/1B) receptor agonist, was lower than observed in previous studies of male rats, and TFMPP substitution was decreased in ovariectomized rats. Ovarian hormones appear to contribute to 5-HT receptor mediation of the discriminative stimulus effects of ethanol in rats.

Discriminative stimulus effects of pregnanolone in rats: role of training dose in determining mechanism of action

RATIONALE

Positive γ-aminobutyric acid(A) (GABA(A)) modulators acting at different binding sites often produce similar behavioral effects; however, their effects are not identical. Actions of neuroactive steroids at other receptors, in addition to GABA(A) receptors, might account for some differences between neuroactive steroids and other positive modulators, like benzodiazepines.

OBJECTIVE

Multiple mechanisms of other drugs (e.g., ethanol) have been elucidated by comparing their discriminative stimulus effects across different training doses; the current study used that approach to examine the mechanisms of action of the neuroactive steroid pregnanolone.

METHODS

Separate groups of rats (n = 6-8/group) discriminated pregnanolone from vehicle while responding under a fixed-ratio 10 schedule of food presentation. Two groups initially discriminated 3.2 mg/kg; once stimulus control was established, the training dose was systematically decreased to 1.33 mg/kg in one group and increased to 7.5 mg/kg in the other group. Other rats discriminated either 1.33 or 7.5 mg/kg without training at another dose.

RESULTS

Stimulus control was established in 24-28 sessions in all groups. Positive GABA(A) modulators produced ≥80 % pregnanolone-lever responding, regardless of training dose; rank-order potency was flunitrazepam > midazolam > pregnanolone = pentobarbital. Ethanol produced some drug-lever responding (42 %) only in rats discriminating 1.33 mg/kg, whereas the N-methyl-D: -aspartate receptor antagonist ketamine and the serotonin receptor agonist 1-(m-chlorophenyl)-biguanide occasioned predominantly vehicle-lever responding in all rats.

CONCLUSIONS

There was little difference in discriminative stimulus effects of pregnanolone across different training conditions, confirming a predominant, if not exclusive, role of GABA(A) receptors in these effects of pregnanolone.

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.

Dissociations between motor timing, motor coordination, and time perception after the administration of alcohol or caffeine

RATIONALE

The impacts of psychoactive drugs on timing have usefully informed theories of timing and its substrates.

OBJECTIVES

The objectives of the study are to test the effects of alcohol and caffeine on the explicit timing involved in tapping with the implicit timing observed in the coordinated picking up of an object, and with the temporal discrimination.

MATERIALS AND METHODS

Participants in the "alcohol" experiment (N = 16) received placebo, "low" (0.12 g/kg or 0.14 g/kg for women/men, respectively) or "high" (0.37 g/kg or 0.42 g/kg, respectively) doses of alcohol, and those in the "caffeine" experiment (N = 16) received placebo, 200 or 400 mg caffeine. Time production variability was measured by repetitive tapping of specified intervals, and sources of variance attributable to central timer processes and peripheral motor implementation were dissociated. The explicit timing in tapping was compared with the implicit timing in the coordinated picking up of an object. Time perception was measured as discrimination thresholds for intervals of similar duration. Drug effects on reaction time were also measured.

RESULTS

For tapping, alcohol significantly increased timer variability, but not motor variability; it did not affect coordination timing in the grip-lift task. Conversely, for time perception, the low dose of alcohol improved temporal discrimination. Caffeine produced no effects on any of the timing tasks, despite significantly reducing reaction times.

CONCLUSIONS

The effects of alcohol argue against a common clock process underlying time interval perception and production in the range below 1 s. In contrast to reaction time measures, time perception and time production appear relatively insensitive to caffeine.

Differentiating the discriminative stimulus effects of gamma-hydroxybutyrate and ethanol in a three-choice drug discrimination procedure in rats

Anecdotal reports indicate that GHB produces subjective effects similar to those of ethanol. However, recent investigations comparing the discriminative stimulus effects of GHB to those of ethanol suggest that the subjective effects of these substances may differ considerably. To explore further potential differences between GHB and ethanol, 16 male Sprague-Dawley rats were trained in a three-lever drug discrimination procedure to discriminate ethanol (1.0 g/kg, experiment 1; 1.5 g/kg, experiment 2) and GHB (300 mg/kg) from vehicle. Dose-response functions determined with both training compounds revealed a clear dissociation between the discriminative stimulus effects of these drugs. As expected, the GHB precursors gamma-butyrolactone and 1,4-butanediol produced full substitution for GHB. In addition, the GABA(B) receptor agonist baclofen substituted for GHB, whereas the benzodiazepine flunitrazepam and the NMDA receptor antagonist ketamine engendered greater responding on the ethanol-lever. GHB's discriminative stimulus effects were blocked by the GABA(B) receptor antagonist CGP-35348 but only partially blocked by the putative GHB receptor antagonist NCS 382. These findings are consistent with previous reports of GHB's discriminative stimulus effects in two-choice drug discrimination procedures and provide additional evidence that these effects are distinct from those of ethanol.

Effects of gamma-hydroxybutyric acid and flunitrazepam on ethanol intake in male rats

Both gamma-hydroxybutyric acid (GHB) and flunitrazepam are often used illicitly in combination with ethanol. Nevertheless, the effects that these and other drugs of abuse have on the reinforcing effects of ethanol remain inconclusive. To test the effects of GHB and flunitrazepam on contingent ethanol intake, twelve male Long-Evans rats were trained to orally consume ethanol using a saccharin-fading procedure. After training, all animals preferentially consumed ethanol instead of water at each of five ethanol concentrations (0-32%) when tested with a two-bottle preference test in the homecage. Animals then received a noncontingent dose of ethanol (0.32, 0.56, 1, and 1.33 g/kg), flunitrazepam (0.032, 0.1, and 0.32 mg/kg), or GHB (100, 180, 320, and 560 mg/kg) prior to each subject's daily access to ethanol (18% v/v). Noncontingent doses of ethanol decreased ethanol intake, however, the subjects consumed enough ethanol to maintain a consistent total ethanol dose in g/kg. Flunitrazepam did not affect ethanol intake at any dose tested, whereas GHB only affected intake at the highest dose (560 mg/kg), a dose that also produced sedation. These data suggest that there are perceptible or qualitative differences between GHB, flunitrazepam, and ethanol in terms of their capacity for modulating oral ethanol intake in outbred rats.

Aversive stimulus properties of the 5-HT2C receptor agonist WAY 161503 in rats

Serotonin2C (5-HT2C) receptors may influence motivation and reward through effects on the mesocorticolimbic dopamine (DA) system. Previous work from this laboratory indicated that 5-HT2C receptor stimulation does not induce place conditioning when animals are tested in a drug-free state, but does result in decreased locomotor activity and increased frequency thresholds for electrical self-stimulation of the ventral tegmental area (VTA). The present study was conducted to determine whether the 5-HT2C receptor agonist WAY 161503 may induce place conditioning in a state-dependent manner and also whether this compound will induce gustatory avoidance conditioning in the conditioned taste aversion (CTA) paradigm. The effects of the 5-HT2C receptor agonist WAY 161503 in the place conditioning and CTA (two-bottle choice test) paradigms were assessed in male Sprague-Dawley rats. Administration of WAY 161503 (3.0 mg/kg) induced a state-dependent conditioned place aversion and a CTA to saccharin. The differential state dependency of 5-HT2C receptor agonists' effects in place conditioning (state dependent) and CTA (non-state dependent) is consistent with the activation of different brain systems in these two paradigms. The state-dependent effects in place conditioning underscore the need to include controls for state dependency in studies of 5-HT receptor related compounds.

Involvement of 5-HT1B receptors within the ventral tegmental area in ethanol-induced increases in mesolimbic dopaminergic transmission

Evidence suggests that 5-hydroxytriptamine-1B (5-HT1B) receptors play a role in modifying ethanol's reinforcing effects and voluntary intake, and that 5-HT1B receptors within the ventral tegmental area (VTA) are involved in regulation of mesolimbic dopaminergic neuronal activity. Since increased mesolimbic dopaminergic transmission has been implicated in ethanol's reinforcing properties, this study was designed to assess the involvement of VTA 5-HT1B receptors in mediating the stimulatory effects of ethanol on VTA dopaminergic neurons. Dual-probe microdialysis was performed in freely moving adult Sprague-Dawley rats with one probe within the VTA and the other within the ipsilateral nucleus accumbens (NACC). Dopamine (DA) levels in dialysates from both areas, as the index of the activity of mesolimbic DA neurons, were measured simultaneously. The results showed that intraperitoneal injection of ethanol at the doses of 1 and 2 g/kg increased extracellular DA concentrations in both the VTA and the NACC, suggesting increased DA neuronal activity. These ethanol-induced increases of the DA release in the VTA and the NACC were significantly attenuated by intra-tegmental infusion of SB 216641 (a 5-HT(1B) receptor antagonist), but not BRL 15572 (a 5-HT(1D/1A) receptor antagonist) or WAY 100635 (a 5-HT1A receptor antagonist). Administration of ethanol at the same doses did not significantly alter extracellular levels of GABA in the VTA. The results also showed that intra-tegmental infusion of CP 94253, a 5-HT1B receptor agonist, significantly prolonged the effects of ethanol on NACC DA. The results suggest that blockade and activation of VTA 5-HT1B receptors attenuates and potentiates the neurochemical effects of ethanol, respectively, and support the suggestion that VTA 5-HT(1B) receptors may be involved in part in mediating the activating effects of ethanol on mesolimbic DA neurons.

Effects of 5,7-dihydroxytryptamine lesion of the dorsal raphe nucleus on ethanol discrimination in the rat

It has been shown that ethanol produces a complex interoceptive cue in rodents with distinct GABAergic, glutamatergic, and serotonergic (5-hydroxytryptamine, 5-HT) components. The present study aimed to examine the contribution of the 5-HT system originating in the dorsal raphe nucleus (DRN) to the discriminative stimulus effects of ethanol in male Wistar rats. Therefore, selective lesions of 5-HT neurons in the DRN were induced by microinfusions of 5,7-dihydroxytryptamine. The DRN- and sham-lesioned rats were trained to discriminate ethanol (1.0 g/kg) from saline in a standard two-lever drug discrimination procedure. Acquisition of ethanol discrimination and discrimination performance after consumption of lower doses of ethanol did not differ between the groups. In substitution tests, diazepam (0.5-2.5 mg/kg), a nonselective benzodiazepine receptor agonist, partially generalized from the ethanol cue in both groups. In contrast, m-chlorophenylpiperazine (0.1-0.9 mg/kg), a mixed 5-HT(1B/2C) receptor agonist, did not mimic the ethanol cue. The drug decreased response rates in both groups, but this effect was more evident in the sham-lesioned group. A 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propyloamino)-tetraline (0.05-0.4 mg/kg), did not produce significant increase in ethanol-appropriate responding in either group. These results may indicate that 5-HT neurons of the DRN are not critically involved in ethanol discrimination in the rat.

Discriminative stimulus effects of ethanol in rats using a three-choice ethanol???midazolam???water discrimination

Three-choice discrimination procedures are used to characterize how similar the discriminative stimulus effects of two drugs are in relation to each other. This procedure has suggested similarities between ethanol and ligands that positively modulate the gamma-aminobutyric acid type A (GABAA) receptor complex. As an extension to these studies, male Long-Evans rats were trained to discriminate midazolam (3.0 mg/kg, i.p.) from ethanol (1.0 g/kg, i.g.) from water (2.3 ml, i.g.) in a three-lever, food reinforced task. Substitution tests were conducted following administration of GABAA-positive modulators, noncompetitive N-methyl-D-aspartate (NMDA) antagonists, 5-HT1B agonists and isopropanol. Among the GABAA-positive modulators, diazepam was the only drug that completely substituted for midazolam; both pentobarbital and the neurosteroid allopregnanolone showed partial midazolam substitution. The NMDA antagonist dizocilpine substituted for ethanol, while phencyclidine showed no substitution for either ethanol or midazolam. The serotonin agonists tested also showed no substitution for either ethanol or midazolam. Isopropanol was the only other drug that completely substituted for ethanol. These data extend previous findings from an ethanol-pentobarbital-water discrimination and further define training conditions that result in a conditional basis for the ethanol discrimination where only those drugs with pharmacological heterogeneous effects similar to ethanol produce a full ethanol-like effect.

Tolerance to the discriminative stimulus effects of ethanol following chronic inhalation exposure to ethanol in C57BL/6J mice

A significant consequence of chronic ethanol (EtOH) exposure is the development of tolerance. The present study was designed to investigate tolerance to the discriminative stimulus properties of EtOH following chronic EtOH exposure. Adult male C57BL/6J mice were trained to discriminate EtOH (1.00 g/kg; i.p.) from saline, using a food-reinforced two-lever operant task. Following acquisition and establishment of criterion discrimination performance, a series of generalization tests were conducted to generate a baseline EtOH dose-response curve with a calculated ED50 dose of 0.42 g/kg. Mice were then placed into control (air) or EtOH inhalation chambers for 64 h. In Experiment 1, discriminative stimulus generalization tests with the EtOH ED50 dose conducted 24 h following chronic EtOH (or air) exposure did not yield significantly different EtOH responding, although a trend towards reduced sensitivity to the EtOH cue (tolerance) was evident. In Experiment 2, a cumulative dosing procedure (ED50=0.37 g/kg) was employed, yielding a baseline EtOH dose-response function with a calculated ED50 dose of 0.37 g/kg. At 24 h following chronic EtOH exposure, re-determination of the EtOH dose-response curve revealed a significant shift to the right, with more than a twofold increase in the ED50 value (ED50=1.09 g/kg) compared to the control air exposure condition (ED50=0.49 g/kg). This apparent tolerance to the EtOH cue dissipated in chronic EtOH-exposed mice tested 48 h following the inhalation treatment (ED50=0.51 g/kg). These results demonstrate tolerance to the discriminative stimulus effects of EtOH in C57BL/6J mice following chronic EtOH exposure in inhalation chambers.

Involvement of 5-HT1B receptors within the ventral tegmental area in regulation of mesolimbic dopaminergic neuronal activity via GABA mechanisms: a study with dual-probe microdialysis

This study was designed to assess the involvement of 5-HT1B receptors within the ventral tegmental area (VTA) in the regulation of mesolimbic dopaminergic transmission. Dual-probe microdialysis was performed in freely moving adult Sprague-Dawley rats with one probe within the VTA and the other within the ipsilateral nucleus accumbens (NACC). Drugs were administered into the VTA via retrograde dialysis. Dialysates from both the VTA and the NAC were collected for determination of dopamine (DA) and gamma-aminobutyric acid (GABA) by high-performance liquid chromatography with electrochemical detection. Intra-tegmental infusion of CP 93129 (20, 40, and 80 microM), a 5-HT1B receptor agonist, increased extracellular DA concentrations in a concentration-dependent manner not only in the NACC but also in the VTA, indicating increased mesolimbic DA neuron activity. Administration of CP 93129 at 80 microM into the VTA also significantly decreased extracellular GABA concentrations in this region. Co-infusion of the 5-HT1B receptor antagonist SB 216641 (10 microM), but not the 5-HT1A receptor antagonist WAY 100635 (10 microM) or the 5-HT1D/1A receptor antagonist BRL 15572 (10 microM), antagonized not only the effects of intra-tegmental CP 93129 (80 microM) on VTA DA and NAC DA but also on VTA GABA. The results suggest that activation of VTA 5-HT1B receptors increases mesolimbic DA neuron activities. The increased DA neuron activity may be associated, at least in part, with the 5-HT1B receptor-mediated inhibition of VTA GABA release.

Serotonin 5-HT2 Receptors and Alcohol: Reward, Withdrawal and Discrimination

This article represents the proceedings of a symposium at the 2003 Research Society on Alcoholism meeting in Fort Lauderdale, Florida. The organizer was Karl J. Buck, and the chairperson was Mark S. Brodie. The presentations were (1) The Multiple PDZ Domain Protein May Mediate Genetic Differences in Ethanol Withdrawal Severity Via Interaction With 5-HT2 Receptors, by Matthew T. Reilly and Kari J. Buck; (2) The Ionic Mechanism of Serotonin Potentiation of Ethanol Excitation of Ventral Tegmental Area Neurons, by Mark S. Brodie; and (3) 5-HT(2C) Receptor Agonists in the Discriminative Stimulus Effects of Ethanol, by Laura M. Rogers, Ken Szeliga, and Kathleen Grant.

Brain ethanol concentrations and ethanol discrimination in rats: effects of dose and time

RATIONALE

In drug discrimination procedures, the substitution pattern for ethanol of various receptor ligands is dependent upon ethanol training dose, presumably reflecting functionally different concentrations of ethanol in the brain. The discriminative stimulus effects of ethanol are also time-dependent, although very few studies have investigated the time course of ethanol discriminations.

OBJECTIVES

The present study investigated the relationship between brain ethanol concentrations (BrEC), as measured by intracranial microdialysis of the nucleus accumbens, and the time course of ethanol discriminative effects.

METHODS

Two groups of rats were trained to discriminate either 1.0 or 2.0 g/kg ethanol from water following a 30-min post-ethanol interval. Following training, the time course of the discriminative stimulus was assessed using a series of abbreviated testing trials at 20-min intervals for 5 h after the administration of various ethanol doses (0, 0.5, 1.0 and 2.0 g/kg). The rats were then fitted with microdialysis probes and the time course of BrECs were determined under conditions similar to the behavioral assessments.

RESULTS

BrECs were significantly above zero at 4 min post-gavage and attained peak concentrations of 16 mmol/l, 24 mmol/l and 42 mmol/l at 9 min, 16 min and 95 min after IG administration of 0.5, 1.0 and 2.0 g/kg ethanol, respectively. BrECs were similar in ethanol-naive and ethanol-trained rats, indicating a lack of pharmacokinetic tolerance under these discrimination procedures. The discriminative stimulus effects of ethanol were dose- and time-dependent, with a threshold concentration of approximately 12 mmol/l achieved at 5 min after 1.0 g/kg ethanol gavage in rats trained to discriminate 1.0 g/kg ethanol. Acute tolerance to the discriminative stimulus effects of ethanol was evident from BrECs 2-5 h post-ethanol gavage.

CONCLUSIONS

Ethanol given intragastrically results in a rapid increase in BrEC, independent of ethanol exposure history. The discriminative stimulus effects of ethanol trained at 30 min post-gavage reflect a specific range of BrEC, and depend on the training dose. These data suggest that qualitatively different stimulus effects of ethanol reflect both different ranges of BrEC, as well as within dose acute tolerance to the discriminative stimulus effects.

Manipulation of operant responding for an ethanol-paired conditioned stimulus in the rat by pharmacological alteration of the serotonergic system

It is becoming increasingly clear that environmental stimuli play a critical role in the maintenance of drug taking behaviour. This has led to investigations into the neural mechanisms by which environmental stimuli can come to control behaviour using paradigms such as conditioned reinforcement. The majority of this work has involved the use of food-paired conditioned stimulus rodent paradigms. Relatively few studies have attempted to investigate the neuropharmacology of behaviour maintained by presentation of a stimulus paired with ethanol drinking. Several lines of research support an important role for brain serotonin (5-HT) neurotransmitter systems in the control of alcohol drinking behaviour. The aim of the present study was, initially, to establish a procedure in which rats respond for an ethanol-paired conditioned stimulus, and second, to study the effects of a range of serotonergic compounds previously shown to be effective in reducing oral ethanol self-administration, on responding for this conditioned stimulus. Results showed that the 5-HT releaser d-fenfluramine, the selective serotonin reuptake inhibitor fluoxetine, the 5-HT1A receptor agonist 8-hydroxy-2[di-n-propylamino]tetralin, the partial 5-HT1A receptor agonist buspirone, and the 5-HT1B/5-HT2C receptor agonist 1-(3-trifluoromethylphenyl)piperazine, but not the 5-HT2A/5-HT2C receptor agonist 1-(2,5-dimethoxy-4-iodophenylaminopropane)-2, selectively reduced responding on a lever leading to presentation of an ethanol paired conditioned stimulus. In addition the non-specific D1/D2 dopamine receptor antagonist haloperidol was active in this paradigm. Results are consistent with involvement of the dopaminergic and 5-HT systems, in particular activation of 5-HT1A and 5-HT1B receptor subtypes, in mediation of the conditioned or secondary reinforcing properties of ethanol.

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.

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.

Effect of 5-HT(1B) receptor ligands on self-administration of ethanol in an operant procedure in rats

Recent evidence suggests that 5-HT(1B) receptor activation modifies ethanol's reinforcing, intoxicating and discriminative stimulus effects. The present study further explored the role played by 5-HT(1A/1B) receptors by examining their influence on oral ethanol self-administration. Male Wistar rats were trained on an FR 4 schedule to obtain a reinforcer of 0.1 12% w/v ethanol solution. Once responding was stable, the effect of the 5-HT(1A/1B) agonist RU24969 alone and in combination with the 5-HT(1B) antagonist GR127935 or the 5-HT(1A) antagonists (+) WAY100135 and (+) WAY100635 was assessed. The effect of RU24969 on ethanol's pharmacokinetic profile and on operant oral saline self-administration was also examined to assess if alterations in oral ethanol self-administration were due to nonspecific effects on level pressing. For comparison, we examined the effect of another 5-HT(1A/1B) agonist, CGS12066B, on oral ethanol self-administration. Both RU24969 (0.1 to 1 mg/kg) and CGS12066B (0.1 to 1 mg/kg) significantly suppressed oral ethanol self-administration. Administration of GR127935 (1 mg/kg), significantly reversed the effects elicited by RU24969, whereas neither WAY100635 (1 mg/kg) nor (+)WAY100135 (1 mg/kg) had any effect. The effects of lower doses of RU24969 on oral ethanol self-administration were selective as oral saline self-administration and blood ethanol levels were not altered by these doses. These data demonstrate that 5-HT(1B) receptor activation suppresses oral ethanol self-administration. These studies provide further evidence that 5-HT(1B) receptors play a modulatory role in ethanol's behavioral effects.

Characterization of the discriminative stimulus effects of gamma-hydroxybutyric acid as a means for unraveling the neurochemical basis of gamma-hydroxybutyric acid actions and its similarities to those of ethanol

The present paper reviews the drug discrimination studies, both from the literature and from this laboratory, conducted to investigate the pharmacological profile of the discriminative stimulus effects of gamma-hydroxybutyric acid. Collectively, the results of these studies suggest that: (1) the discriminative stimulus effects of gamma-hydroxybutyric acid are composed of different cues, each one being the effect of gamma-hydroxybutyric acid on a specific receptor system; (2) the proportion of each component cue varies as the training dose of gamma-hydroxybutyric acid is increased; (3) the gamma-aminobutyric acid B-mediated cue is a major ingredient of the mixed stimulus of gamma-hydroxybutyric acid, but it is more prominent at high training doses than at low training doses of gamma-hydroxybutyric acid; and (4) positive modulation of the gamma-aminobutyric acid A receptor is a relevant part of the discriminative stimulus effects of low gamma-hydroxybutyric acid doses. Finally, data indicating symmetrical generalization between the discriminative stimulus effects of a specific range of doses of gamma-hydroxybutyric acid and those of ethanol are discussed in regard to their further support of the hypothesis that gamma-hydroxybutyric acid may exert its antialcohol effects through a substitution mechanism.

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.

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.

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.

Effects of moderate alcohol consumption on the central nervous system

The concept of moderate consumption of ethanol (beverage alcohol) has evolved over time from considering this level of intake to be nonintoxicating and noninjurious, to encompassing levels defined as "statistically" normal in particular populations, and the public health-driven concepts that define moderate drinking as the level corresponding to the lowest overall rate of morbidity or mortality in a population. The various approaches to defining moderate consumption of ethanol provide for a range of intakes that can result in blood ethanol concentrations ranging from 5 to 6 mg/dl, to levels of over 90 mg/dl (i.e., approximately 20 mM). This review summarizes available information regarding the effects of moderate consumption of ethanol on the adult and the developing nervous systems. The metabolism of ethanol in the human is reviewed to allow for proper appreciation of the important variables that interact to influence the level of exposure of the brain to ethanol once ethanol is orally consumed. At the neurochemical level, the moderate consumption of ethanol selectively affects the function of GABA, glutamatergic, serotonergic, dopaminergic, cholinergic, and opioid neuronal systems. Ethanol can affect these systems directly, and/or the interactions between and among these systems become important in the expression of ethanol's actions. The behavioral consequences of ethanol's actions on brain neurochemistry, and the neurochemical effects themselves, are very much dose- and time-related, and the collage of ethanol's actions can change significantly even on the rising and falling phases of the blood ethanol curve. The behavioral effects of moderate ethanol intake can encompass events that the human or other animal can perceive as reinforcing through either positive (e.g., pleasurable, activating) or negative (e.g., anxiolysis, stress reduction) reinforcement mechanisms. Genetic factors and gender play an important role in the metabolism and behavioral actions of ethanol, and doses of ethanol producing pleasurable feelings, activation, and reduction of anxiety in some humans/animals can have aversive, sedative, or no effect in others. Research on the cognitive effects of acute and chronic moderate intake of ethanol is reviewed, and although a number of studies have noted a measurable diminution in neuropsychologic parameters in habitual consumers of moderate amounts of ethanol, others have not found such changes. Recent studies have also noted some positive effects of moderate ethanol consumption on cognitive performance in the aging human. The moderate consumption of ethanol by pregnant women can have significant consequences on the developing nervous system of the fetus. Consumption of ethanol during pregnancy at levels considered to be in the moderate range can generate fetal alcohol effects (behavioral, cognitive anomalies) in the offspring. A number of factors--including gestational period, the periodicity of the mother's drinking, genetic factors, etc.--play important roles in determining the effect of ethanol on the developing central nervous system. A series of recommendations for future research endeavors, at all levels, is included with this review as part of the assessment of the effects of moderate ethanol consumption on the central nervous system.

Studies on the role of nicotinic acetylcholine receptors in the discriminative and aversive stimulus properties of ethanol in the rat

The role of the nicotinic acetylcholine receptor (nAChR) in the discriminative and aversive stimulus effects of ethanol was studied in rats. In the operant drug discrimination procedure the rats were trained to discriminate between 1.0 g/kg ethanol and saline under the FR10 schedule of sweetened milk reinforcement. Neither the nAChR agonist, nicotine (0.1-0.6 mg/kg) nor the nAChR antagonist, mecamylamine (3.0-6.0 mg/kg) substituted for the ethanol stimulus. Moreover, mecamylamine (0.5-6.0 mg/kg) did not antagonise the ethanol stimulus. The cross-familiarisation conditioned taste aversion procedure was used as an alternative method to study stimulus resemblance between ethanol and nicotine. Six daily injections of nicotine (0.6 mg/kg) significantly decreased a subsequent ethanol-induced taste aversion conditioning. The aversive stimulus effects of ethanol were investigated with the conditioned taste aversion (CTA) paradigm. Mecamylamine (1.0-3.0 mg/kg) did not attenuate an ethanol-induced CTA. These results suggest that: (1) nAChRs are not primarily involved in the discriminative stimulus effects of ethanol when studied with the operant drug discrimination test; (2) nAChRs are not critically involved in the ethanol-induced CTA.

Analysis of the 5-HT2 receptor ligands dimethoxy-4-indophenyl-2-aminopropane and ketanserin in ethanol discriminations

Previous studies have suggested a modulatory role of the 5-HT2 receptor system in the behavioral effects of ethanol. The present study examined the discriminative stimulus effects of the 5-HT2A/2C agonist (-)-dimethoxy-4-indophenyl-2-aminopropane (DOI) and the 5-HT2A antagonist ketanserin in rats trained to discriminate either 1.5 g/kg of ethanol from water (intragastrically, n = 7) or 2.0 g/kg of ethanol from water (intragastrically, n = 8). In substitution tests, neither DOI (0.3 to 1.0 mg/kg, i.p.) nor ketanserin (3.0 to 17.0 mg/kg, i.p.) produced discriminative stimulus effects similar to either training dose of ethanol, although decreases in rates of responding were significant at the highest doses tested. Likewise, when given in combination with ethanol, neither 5-HT2 ligand shifted the ethanol-dose response determination in either the 1.5 or 2.0 g/kg ethanol training groups. DOI in combination with ethanol did not alter rates of responding, whereas ketanserin in combination with ethanol significantly decreased response rates. Thus, the 5-HT2A receptor ligands do not appreciably affect the discriminative stimulus effects of ethanol, in contrast to previous results with 5-HT1B ligands.

Discrimination of ethanol in rats: effects of nicotine, diazepam, CGP 40116, and 1-(m-chlorophenyl)-biguanide

The drug discrimination paradigm was used to evaluate the role of certain ligand-gated ion channels in the discriminative stimulus properties of ethanol. Rats were trained to discriminate ethanol (1.0 g/kg) from saline vehicle under the FR10 schedule of sweetened milk reinforcement. The discrimination of lower ethanol doses was enhanced by either the GABA(A) receptor positive modulator, diazepam (0.5 mg/kg), or nicotinic acetylcholine receptor agonist, nicotine (0.3 mg/kg). Neither diazepam nor nicotine produced any effect on the rate of responding. Both the NMDA receptor competitive antagonist, CGP 40116 (0.5 mg/kg) and the 5-HT) receptor agonist, 1-(m-chlorophenyl)-biguanide (5.0 mg/kg) enhanced the cueing properties of lower ethanol doses, but these effects were associated with a significant reduction in the response rate. The ethanol-like stimulus effects produced by diazepam or CGP 40116 were not influenced by 0.3 mg/kg nicotine. In contrast, CGP 40116 moderately enhanced the ethanol-like stimulus effects of diazepam. The present results show that: 1) pretreatment with nicotine, diazepam, CGP 40116 or 1-(m-chlorophenyl)-biguanide enhance the ethanol discrimination; 2) neither the GABA(A) nor the NMDA receptor complex alone is critically involved in the nicotine-induced enhancement of the ethanol discrimination; 3) NMDA receptor competitive antagonist and GABAergic benzodiazepine derivative may produce moderate additive effects in rats trained to discriminate ethanol.

Fluoxetine's effects on ethanol's rewarding, aversive and stimulus properties

These experiments examined the influence of fluoxetine on ethanol-induced conditioned place preference, ethanol-induced conditioned taste aversion, and ethanol discrimination. In the place conditioning experiment, male Swiss-Webster mice received 4 pairings of a distinctive floor cue with 2 g/kg ethanol, 10 mg/kg fluoxetine + ethanol, or fluoxetine alone. A different floor was paired with saline. During conditioning ethanol produced locomotor stimulation. Fluoxetine + ethanol resulted in greater levels of locomotor activity during conditioning trials 2-4. Fluoxetine alone also caused increases in activity. Floor preference testing revealed conditioned place preference in groups receiving ethanol. Fluoxetine did not change the magnitude of ethanol-induced conditioned place preference nor produced place conditioning alone. In the taste conditioning procedure, mice received 1-h access to 0.2 M NaCl solution followed by injections of 0, 5 or 10 mg/kg fluoxetine and 0 or 2.5 g/kg ethanol. Ethanol produced reductions in NaCl intake. Fluoxetine (10 mg/kg) enhanced the development of ethanol-conditioned taste aversion but did not cause taste aversion alone. In the ethanol discrimination experiment, mice were trained to respond for 10% sucrose on an FR20 schedule following injections of either 1 g/kg ethanol or saline. Following acquisition, 10 mg/kg fluoxetine pretreatment enhanced ethanol-appropriate responding at a dose of ethanol (0.5 g/kg) below the training dose. These results indicate enhancement of serotonergic activity influences ethanol aversion and discrimination but not ethanol reward.

Studies on the role of 5-HT3 receptors in the mediation of the ethanol interoceptive cue

The drug discrimination test was used to evaluate the role of 5-HT3 receptors in the mediation of the stimulus properties of ethanol in rats trained to discriminate between ethanol (1.0 g/kg, 10% v/v, i.p.) and saline vehicle. Rats trained to discriminate between a lower dose of ethanol (0.5 g/kg i.p.) failed to attain discrimination criteria after 20 weeks (100 sessions) of training. None of the doses of 5-HT3 receptor antagonists (0.001, 0.01, 0.1, 1.0, 10.0 mg/kg of tropisetron or ondansetron) administered i.p. 30 min before ethanol, antagonized the discriminative stimulus properties of ethanol. Furthermore, none of the centrally (1, 10, 35 micrograms per rat) or i.p. (0.1, 1.0, 2.5, 5.0, 10.0 mg/kg) administered doses of 5-HT3 receptor agonist, 1-(m-chlorophenyl)-biguanide, could replace the ethanol discriminative cue. These results suggest that 5-HT3 receptors are not primarily involved in the mediation of the stimulus properties of ethanol.

T-maze and food reinforcement: an inexpensive drug discrimination procedure

The present study provides a detailed description of a novel drug discrimination procedure employing a T-maze and food reinforcement. Three groups of rats (n = 5) were trained to run one of the two arms of the maze after the i.g. administration of either 1.0, 1.5 or 2.0 g/kg ethanol (chosen as training drug) and the opposite arm after administration of water. Only correct trials were rewarded. All the rats learned the discrimination task. Substitution tests with different doses of ethanol in all the three groups and with the non-competitive NMDA antagonist, dizocilpine (MK-801), in the 2.0 g/kg ethanol-trained group provided a pharmacological validation of the paradigm. Advantages and disadvantages of this procedure are discussed.

Ethanol self-administration restores withdrawal-associated deficiencies in accumbal dopamine and 5-hydroxytryptamine release in dependent rats

Basal forebrain dopamine (DA) and 5-HT neurotransmission has been implicated in the mediation of the acute reinforcing actions of ethanol. Neuroadaptation theories predict that compensatory changes in neurochemical systems that are activated by alcohol acutely may underlie symptoms of withdrawal after chronic administration. To test this hypothesis, the release of DA and 5-HT was monitored by microdialysis in the nucleus accumbens of dependent male Wistar rats at the end of a 3-5 week ethanol (8.7% w/v) liquid diet regimen, during 8 hr of withdrawal, and during renewed availability of ethanol involving (1) the opportunity to operantly self-administer ethanol (10% w/v) for 60 min, followed by (2) unlimited access to the ethanol-liquid diet. Results were compared to control groups pair-fed with ethanol-free liquid diet and trained to self-administer either ethanol or water. In nondependent rats, operant ethanol self-administration increased both DA and 5-HT release in the NAC. Withdrawal from the chronic ethanol diet produced a progressive suppression in the release of these transmitters over the 8 hr withdrawal period. Self-administration of ethanol reinstated and maintained DA release at prewithdrawal levels but failed to completely restore 5-HT efflux. 5-HT levels recovered rapidly, however, within 1 hr of reexposure to ethanol liquid diet. These findings suggest that deficits in accumbal monoamine release may contribute to the negative affective consequences ethanol withdrawal and, thereby, motivate ethanol-seeking behavior in dependent subjects.

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.

Chronic alcoholization alters the expression of 5-HT1A and 5-HT1B receptor subtypes in rat brain

The expression of central 5-HT1A and 5-HT1B receptors was studied in several brain areas of rats subjected to a 2-week period of chronic alcoholization, followed by 18 h withdrawal. Quantitative autoradiography indicated that the ethanol treatment provoked an increase (approximately +30%) in the labeling by [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) and [3H]N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane carboxamide ([3H]WAY-100635) of 5-HT1A autoreceptors in the dorsal raphe nucleus, accompanied by a concomitant decrease in the labeling of postsynaptic 5-HT1A receptors in the hippocampus (approximately -20%), anterior (approximately -30%) and posterior (approximately -32%) cortices. These changes were associated with a tendency toward an increase and decrease in 5-HT1A mRNA levels in the anterior raphe area and hippocampus, respectively, suggesting that the changes observed are due to modifications in 5-HT1A receptor protein synthesis. The autoradiographic labeling of 5-HT1B receptors by serotonin-O-carboxymethylglycyl[125I]iodotyrosinamide ([125I]GTI) was found to increase (+55%) in the globus pallidus of alcoholized rats. Interestingly, a significant increase (+57%) in 5-HT1B receptor mRNA levels was observed in the striatum, which contains cell bodies of neurons projecting into the globus pallidus. These data suggest that altered sensitivity of chronically alcoholized rats to 5-HT1A and 5-HT1B receptor ligands may result from alcohol-induced changes in the transcription of the genes encoding these receptors.

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.

The role of 5-HT3 receptors in drug dependence

Since the discovery of serotonin receptor subtypes in 1957, the classification of serotonin receptors now includes 5-HT1 through 5-HT7 receptors, with further subtypes of receptors in each family. Unique among this expanding group of 5-HT receptor subtypes is the 5-HT3 receptor, which is the only known 5-HT receptor that directly gates an ion channel. The channel conducts primarily Na+ and K+, resulting in rapid depolarization followed by a rapid desensitization. The immediate consequence of neuronal depolarization resulting from 5-HT3 receptor activation is the release of stored neurotransmitter. The subsequent release of stored neurotransmitter, particularly dopamine in the mesolimbic pathways, suggest a potentially important role for this receptor system in neuronal circuitry involved in drug abuse. The following review broadly covers the structure, function and distribution of the 5-HT3 receptor system in the CNS and data addressing the potential role of this receptor system in modulating the effects of a wide variety of abused drugs. Most of the evidence indicates an association between the ability of 5-HT3 antagonists to decrease mesolimbic dopamine levels and to attenuate the psychomotor stimulant effects of drugs. However 5-HT3 receptor antagonists are less robust at attenuating other drug effects that are believed to be related to their abuse liability, such as discriminative stimulus and reinforcing effects. The one exception may be ethanol, which directly potentiates the effects of 5-HT at the 5-HT3 receptor channel complex. In addition to the implications of an interaction with the mesolimbic dopaminergic system, the ability of 5-HT3 receptor antagonists to function as anxiolytics suggest they could be useful pharmacotherapies during drug withdrawal. However, further studies are needed since currently available 5-HT3 receptor antagonists do not have uniform behavioral effects, may interact with other receptor systems, and have atypical dose-response effects.

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.