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.     

Effects of ethanol on cocaine discrimination in rats

Ethanol and cocaine are frequently abused in combination, but little is known about how the subjective effects of the two drugs interact. The ability of ethanol and other GABA(A)-active compounds to alter the discriminative stimulus effects of cocaine was tested. Male Sprague-Dawley rats were trained to discriminate cocaine (10 mg/kg ip) from saline using either single- or cumulative-dosing methods. In single-dose testing, ethanol (0.1-0.5 g/kg) dose-dependently decreased cocaine-appropriate responding following the training dose of cocaine. Ethanol (0.5 g/kg) produced a rightward shift in the cocaine cumulative dose-effect curve. Ethanol (0.1-1.0 g/kg) failed to substitute for the discriminative stimulus effects of cocaine and the higher doses (1-2 g/kg) completely suppressed responding. Indirect GABA(A) agonists diazepam (benzodiazepine site) and pentobarbital (barbiturate site) did not block the discriminative stimulus effects of cumulative doses of cocaine. The GABA(A) antagonist pentylenetetrazol (PTZ) (10-40 mg/kg) did not substitute for cocaine. These findings suggest that ethanol can modulate the discriminative stimulus effects of cocaine, and that these effects may not be mediated by the actions of ethanol at the GABA(A) receptor.     

Effects of GABA agonists and GABA-A receptor modulators on cocaine discrimination in rhesus monkeys

RATIONALE: Dopaminergic systems thought to mediate the abuse-related effects of cocaine are under inhibitory control by GABAergic systems. These findings suggest that GABA agonists may attenuate some abuse-related effects of cocaine. OBJECTIVE: To assess the effects of GABA receptor agonists and GABA-A receptor modulators on cocaine discrimination in rhesus monkeys. METHODS: Rhesus monkeys were trained to discriminate 0.4 mg/kg cocaine from saline in a two-key, food-reinforced drug discrimination task. The effects of the GABA-A agonist muscimol, the GABA-B agonist baclofen, the barbiturate GABA-A receptor modulator pentobarbital, and the benzodiazepine GABA-A modulators triazolam and imidazenil were examined alone and as pretreatments to cocaine. For comparison, the effects of pentobarbital pretreatment on the cocaine-like discriminative stimulus effects of amphetamine were also examined. RESULTS: When administered alone, the GABA agonists and GABA-A receptor modulators produced primarily saline-appropriate responding. When administered as pretreatments to cocaine, pentobarbital attenuated the discriminative stimulus effects of cocaine in all monkeys tested, and the high efficacy benzodiazepine agonist triazolam attenuated cocaine's effects in three of five monkeys. Muscimol, baclofen and the low efficacy benzodiazepine agonist imidazenil did not alter cocaine's discriminative stimulus effects. Although pentobarbital blocked the effects of the monoamine reuptake blocker cocaine, it did not alter the cocaine-like effects of the monoamine releaser amphetamine. CONCLUSIONS: These results are consistent with the hypothesis that GABA-A receptor modulators attenuate the discriminative stimulus effects of cocaine in rhesus monkeys by decreasing the activity of dopaminergic systems. Direct GABA receptor agonists may be less effective in blocking the abuse-related effects of cocaine in rhesus monkeys.     

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.     

Acute and chronic effects of the neuroactive steroid pregnanolone on schedule-controlled responding in rhesus monkeys

This study used schedule-controlled responding to examine the acute and chronic effects of the neuroactive steroid and positive -aminobutyric acid A (GABA ) modulator pregnanolone. Pregnanolone, the positive GABA modulator triazolam, the GABA chloride channel site antagonist pentylenetetrazol (PTZ) and the -methyl-d-aspartate (NMDA) antagonist ketamine were administered to monkeys ( = 4) responding under a multiple fixed ratio (FR/FR) schedule of food presentation and stimulus shock termination (SST), before, during and after daily treatment with pregnanolone (3.2 mg/kg subcutaneously). Pregnanolone decreased responding in a dose- and time-related manner, with a duration of action of <2 h. Mutual antagonism occurred between pregnanolone and PTZ in the food component, and PTZ antagonized pregnanolone in the SST component. Daily treatment with pregnanolone increased the sensitivity to PTZ 24 h but not 2 h after daily pregnanolone administration, and daily pregnanolone treatment did not alter the sensitivity to pregnanolone, triazolam or ketamine. Baseline responding in the food component was decreased in some monkeys 24 h after daily pregnanolone administration and in all monkeys 48 h after discontinuation of daily pregnanolone treatment. These results suggest that positive GABA modulation is one mechanism by which pregnanolone decreases FR responding, and that dependence resulting from daily pregnanolone treatment is not necessarily accompanied by tolerance to pregnanolone. Failure of pregnanolone to confer tolerance under these conditions might suggest that neuroadaptations at the GABA receptor complex vary according to the site at which positive GABA modulation occurs.     

Interaction of lobeline and nicotinic receptor ligands with the discriminative stimulus properties of cocaine and amphetamine

Lobeline has high affinity for nicotinic acetylcholine receptors and inhibits the function of vesicular and plasmalemmal monoamine transporters. Moreover, lobeline has been shown to alter the neurochemical and behavioral effects of psychostimulants. The present study determined the effect of lobeline and drugs selective for nicotinic receptors on the discriminative stimulus properties of low doses of cocaine (1.6 or 5.0 mg/kg) or d-amphetamine (0.3 mg/kg) in rats, using a standard two-lever drug discrimination procedure with food reinforcement. Nicotine substituted for both amphetamine and cocaine. The nicotinic receptor antagonists mecamylamine and hexamethonium did not substitute for or block the cocaine or amphetamine stimulus. In contrast, lobeline substituted for cocaine, but did not substitute for amphetamine. In antagonism tests, lobeline doses that did not substitute for cocaine decreased responding on the cocaine-paired levers. Surprisingly, lobeline did not alter the discriminative stimulus properties of amphetamine. This research further supports the supposition that nicotine, cocaine and amphetamine produce similar, but distinct subjective states. Furthermore, the present findings suggest that lobeline has a complex mechanism of action to disrupt the behavioral effects of drugs of abuse.     

Discriminative stimulus effects of ethanol: lack of interaction with taurine

Recent microdialysis studies showed that ethanol administration increases the release of taurine in various rat brain regions, and it was suggested that this increase in extracellular concentrations of taurine might mediate some of the neurochemical effects of ethanol. Previous drug discrimination studies showed that positive modulators of the GABA(A) receptor consistently substituted for ethanol discriminative stimulus effects. Since taurine is also believed to modulate GABA(A) receptor activity, this study addressed the hypothesis that taurine mediates the discriminative stimulus effects of ethanol due to GABA(A) activation. Male Long-Evans rats were trained to discriminate water from either 1 or 2 g/kg ethanol. In a first experiment, various taurine doses (0-500 mg/kg) were tested to investigate whether taurine substitutes for ethanol. In a second experiment, rats were pretreated with either 500 mg/kg taurine or an equivalent volume of saline before testing for ethanol discrimination with various ethanol doses (0-2.0 g/kg). The results showed that taurine does not substitute for ethanol at any tested doses. In addition, taurine pretreatments failed to modify the dose-response curve for ethanol discrimination. These results demonstrate that taurine is not directly involved in mediating the discriminative stimulus effects of ethanol. It is therefore very unlikely that the brain release of taurine observed after ethanol administration is implicated in the major pharmacological effects of ethanol, i.e. positive modulation of GABA(A) receptor, that mediate its discriminative stimulus effects.     

SA 4503 attenuates cocaine-induced hyperactivity and enhances methamphetamine substitution for a cocaine discriminative stimulus

Cocaine exhibits preferential (~ 15-fold) affinity for σ₁ over σ₂ sigma receptors, and previous research has shown an interaction of σ₁ receptor-selective ligands and cocaine's behavioral effects. The present study investigated the effect of the putative sigma receptor agonist SA 4503 (1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride) on cocaine's locomotor stimulatory and discriminative stimulus properties. At doses without intrinsic activity, SA 4503 dose-dependently attenuated cocaine-induced hyperactivity in mice. This inhibition was overcome by increasing the cocaine dose. In rats trained to use cocaine as a discriminative stimulus in a drug discrimination task, doses of SA 4503 that did not substitute for the cocaine stimulus did not alter the cocaine substitution curve. However, SA 4503 potentiated the effect of methamphetamine to substitute for the cocaine stimulus. These data support a role for sigma receptors in the locomotor-activating properties of cocaine and, importantly, indicate a role for these receptors in the discriminative stimulus effects of methamphetamine. The data also suggest sigma receptors mediate the activity of different dopamine pathways responsible for the behavioral effects of psychostimulants.     

Effects of GABA(B) receptor antagonist, agonists and allosteric positive modulator on the cocaine-induced self-administration and drug discrimination

Preclinical and clinical findings indicate that a GABA(B) receptor agonist baclofen decreases cocaine use. The present study investigated the effects of the GABA(B) receptor antagonist (2S)-(+)-5,5-dimethyl-2-morpholineacetic acid (SCH 50911), the agonists baclofen and 3-aminopropyl(methyl)phoshinic acid (SKF 97541) and the allosteric positive modulator 3,5-bis(1,1-dimethylethyl-4-hydroxy-beta,beta-dimethylbenzenepropanol (CGP 7930) in cocaine-and food-maintained responding under a fixed ratio 5 schedule of reinforcement in male Wistar rats. The effects of the GABA(B) receptor ligands on cocaine (10 mg/kg)-induced discriminative stimulus in a two-lever, water-reinforced fixed ratio 20 task and on basal locomotor activity were also assessed. Baclofen (2.5-5 mg/kg), SKF 97541 (0.1-0.3 mg/kg) and CGP 7930 (30-100 mg/kg) decreased the cocaine (0.5 mg/kg/injection)-maintained responding; SCH 50911 (3-10 mg/kg) was inactive in this respect. Baclofen (5 mg/kg) and SKF 97541 (0.3 mg/kg), but not CGP 7930 or SCH 50911 attenuated the food-maintained responding. The inhibitory effects of the GABA(B) receptor agonists and the modulator were blocked by SCH 50911. SKF 97541 (0.1 mg/kg) or CGP 9730 (30-100 mg/kg) did not produce a significant shift in the cocaine (1.25-10 mg/kg) dose-response curve in a drug discrimination procedure, while baclofen (1.5 mg/kg) or SCH 50911 (10 mg/kg) attenuated the effects of separate doses of cocaine. Baclofen (5 mg/kg) and CGP 7930 (100 mg/kg) significantly reduced basal horizontal activity. We found that pharmacological stimulation of GABA(B) receptors by direct agonists or allosteric positive modulation reduces cocaine reinforcement while this property of cocaine is not related to tonic activation of GABA(B) receptors. The GABA(B) receptor stimulation-induced reduction of cocaine reinforcement was separated from its discriminative stimulus effects. Moreover, a dissociation between effects of direct GABA(B) receptor agonists and a GABA(B) allosteric positive modulator on cocaine vs. food-maintained responding was demonstrated.     

Discriminative stimulus effects of drugs acting at GABA(A) receptors: differential profiles and receptor selectivity.

The GABA(A) receptor complex contains a number of binding sites at which a variety of psychotropic drugs, including benzodiazepines, barbiturates, and some neurosteroids, act to potentiate or inhibit the effect of the transmitter. Many studies have reported that these drugs can produce discriminative stimulus actions, but the cueing effects of compounds acting at different sites to enhance the effects of GABA are not identical. The discriminative stimulus effects of benzodiazepines have been analyzed in detail, and there is also a great deal of information available on the effects of nonbenzodiazepine compounds acting at BZ(omega) recognition sites, which form part of the GABA(A) receptor complex. Of particular interest are compounds with selectivity for the BZ1(omega1) receptor subtype including zolpidem, zaleplon, and CI 218,872. BZ1(omega1)-selective drugs substitute for the discriminative stimulus produced by chlordiazepoxide only partially and at sedative doses. This is consistent with the view that sedative effects of BZ(omega) receptor agonists are mediated by the BZ1(omega1) receptor subtype, whereas the discriminative stimulus produced by chlordiazepoxide may be produced by activity at the BZ2(omega2) subtype. Analysis of this hypothesis is complicated by the variety of levels of intrinsic activity shown by different drugs.     

Effects of H(1)-antagonists on discriminative stimulus effects of cocaine and methamphetamine in rats

The effects of H(1)-antagonists, such as tripelennamine, chlorpheniramine and pyrilamine, on the discriminative stimulus effects of cocaine and methamphetamine in rats were examined. Rats were trained to discriminate between cocaine (10mg/kg, i.p.) or methamphetamine (1.0mg/kg, i.p.) and saline, with responses maintained under a fixed-ratio 10 (FR10) schedule of food delivery. In generalization tests, tripelennamine generalized to the discriminative stimulus effects of cocaine and methamphetamine, while chlorpheniramine only generalized to cocaine. These generalizations of tripelennamine and chlorpheniramine were attenuated by the dopamine receptor antagonist haloperidol. Furthermore, tripelennamine and chlorpheniramine, but not pyrilamine, potentiated the discriminative stimulus effects of cocaine and methamphetamine. These results suggest that some H(1)-antagonists possess cocaine- and/or methamphetamine-like discriminative stimulus effects which may be mediated by the dopaminergic system, and as a result the discriminative stimulus effects of cocaine and methamphetamine were potentiated by these H(1)-antagonists.     

Interaction of cocaine with positive GABA<Subscript>A</Subscript> modulators on the repeated acquisition and performance of response sequences in rats

Rationale Although positive GABA_A modulators can attenuate several cocaine-induced behavioral effects, there is a paucity of data on their interaction with cocaine on transition behavior or learning. Objectives The current study examined the effects of cocaine (3.2–32 mg/kg), pregnanolone (3.2–24 mg/kg), and lorazepam (0.1–10 mg/kg) alone and in combination in rats responding under a multiple schedule of repeated acquisition and performance. Methods In the acquisition component, subjects acquired a different three-response sequence each session, whereas in the performance component, they responded on the same three-response sequence each session. Results All three drugs produced dose-dependent rate-decreasing and error-increasing effects. Cocaine was the least effective in decreasing rates and the most effective in increasing the percentage of errors. In combination with pregnanolone (3.2 or 10 mg/kg), the rate-decreasing effects of cocaine were relatively unchanged in both components, but 3.2 mg/kg of pregnanolone enhanced its error-increasing effects and the 10-mg/kg dose produced a significant dose-dependent interaction on errors. The combination of cocaine with lorazepam (0.32 mg/kg, 70-min pretreatment) produced significantly greater rate-decreasing and error-increasing effects than cocaine alone. A 15-min pretreatment with the same dose of lorazepam enhanced the error-increasing effects of small doses and attenuated the effects of larger doses of cocaine. Combinations of pregnanolone and lorazepam produced greater rate-decreasing and error-increasing effects in both components than either drug alone. Conclusions The present data show that cocaine is more disruptive to learning in rats than pregnanolone or lorazepam, and that the disruptive effects of cocaine can be enhanced by CNS depressants.     

μ-, δ- and κ-opioid receptor agonists do not alter the discriminative stimulus effects of cocaine or d-amphetamine in rats

Opioid receptor agonists can modulate the activity of dopamine neurons and could therefore, modify the behavioral effects of drugs that act through the dopamine systems, such as d-amphetamine and cocaine. We tested the ability of agonists selective for the μ- (morphine, methadone, buprenorphine, nalbuphine and heroin), δ- (DPDPE and SCH32615), and κ- (U69593 and bremazocine) opioid receptors to alter the discriminative stimulus effects of d-amphetamine and cocaine in rats. Separate groups of male Sprague-Dawley rats were trained to discriminate between 1.0 mg/kg d-amphetamine or 10 mg/kg cocaine from saline. Rats were pretreated with vehicle or an agonist, then dose-response curves for d-amphetamine or cocaine were generated. None of the opioid agonists changed significantly the ED₅₀ values of cocaine and d-amphetamine. As a positive control, we tested for antagonism of these effects by the D1 and D2 dopamine receptor antagonists, SCH23390 and eticlopride, respectively. Both antagonists at least partially attenuated the stimulus effects of both training drugs. Our results suggest that any modulation of dopaminergic neurotransmission by the agonists tested in the present study is not sufficient to affect the stimulus effects of d-amphetamine and cocaine in rats.     

GABAergic modulation of the discriminative stimulus effects of methamphetamine

To assess whether gamma-aminobutyric acid (GABA) modulation of dopamine is important in mediation of the discriminative stimulus effects of methamphetamine, the GABA compounds chlordiazepoxide (benzodiazepine site agonist), pentobarbital (barbiturate site agonist), bicuculline and pentylenetetrazol (GABA(A) receptor antagonists) were tested in Sprague-Dawley rats trained to discriminate methamphetamine (1 mg/kg, i.p.) from saline. Each of the compounds produced modest amounts of methamphetamine-appropriate responding (20-35%) when tested alone. When tested in combination with methamphetamine, the antagonists (bicuculline and pentylenetetrazol) failed to shift the methamphetamine dose-effect curve. In contrast, chlordiazepoxide (25 mg/kg, i.p.) reduced methamphetamine-appropriate responding at each dose of methamphetamine tested, and pentobarbital (10 mg/kg, i.p.) dose-dependently decreased the discriminative stimulus effects of 1 mg/kg methamphetamine. In conclusion, GABA(A) antagonists and positive modulators likely do not produce methamphetamine-like stimulus effects. However, activation of GABA(A) receptors can interfere with the discriminative stimulus effects of methamphetamine.     

Discriminative-stimulus effects of triazolam and midazolam in rhesus monkeys

The present study characterized the discriminative-stimulus effects of triazolam and midazolam in rhesus monkeys. Six monkeys discriminated 0.1 mg/kg of triazolam from vehicle under a fixed-ratio 5 (FR 5) schedule of stimulus-shock termination (SST). Four monkeys subsequently discriminated 0.56 mg/kg of midazolam from vehicle under the same schedule of reinforcement. Benzodiazepine (BDZ) agonists midazolam and diazepam, and the barbiturate pentobarbital, substituted for triazolam, and the non-competitive N-methyl-D-aspartate (NMDA) antagonist ketamine did not. Triazolam, diazepam, lorazepam, flunitrazepam, as well as the barbiturates amobarbital and pentobarbital, substituted for midazolam, and ketamine did not. The BDZ antagonist flumazenil antagonized both the triazolam and midazolam discriminative stimuli. Bretazenil, a low-efficacy BDZ agonist, did not substitute for the midazolam discriminative stimulus in three of the monkeys and shifted the midazolam dose-effect curve to the right; in a fourth monkey, bretazenil substituted for midazolam and shifted the midazolam dose-effect curve to the left. Schild analyses with flumazenil or bretazenil, in combination with midazolam, yielded slopes that deviated significantly from unity. While clearly supporting the notion that BDZ agonists produce stimulus effects by acting at the gamma-aminobutyric acidA (GABA(A)) receptor complex, these data also suggest that the discriminative-stimulus effects of midazolam might be mediated by more than one BDZ receptor subtype.     

Involvement of dopamine D3 and D4 receptors in the discriminative stimulus properties of cocaine in the rat

The present study was carried out to determine the involvement of dopamine receptor subtypes D3 and D4, in the discriminative stimulus effects of cocaine in the rats trained to discriminate 10 mg/kg of cocaine from vehicle. The discriminative stimulus effects of cocaine (1-10 mg/kg) were dose-dependent. The dopamine D2 receptor agonist bromocriptine (1.25-20 mg/kg) and the dopamine D3 receptor agonist R(+)-7-OH-DPAT (0.0001-0.3 mg/kg) produced cocaine (10 mg/kg)-like discriminative stimulus effects. Both the dopamine D3 receptor antagonist GR103691 (1 mg/kg) and the dopamine D4 receptor antagonist L745870 (1 mg/kg) partially antagonized the discriminative stimulus effects of cocaine (10 mg/kg) and the cocaine (10 mg/kg)-like discriminative stimulus effects of R(+)-7-OH-DPAT (0.3 mg/kg). L745870 (0.001 mg/kg) inhibited the antagonistic effects of GR103691 (1 mg/kg) on the discriminative stimulus effects of cocaine (10 mg/kg), whereas the drug (0.001 mg/kg) enhanced the antagonistic effects of GR103691 (1 mg/kg) on the cocaine (10 mg/kg)-like discriminative stimulus effects of R(+)-7-OH-DPAT (0.3 mg/kg). GR103691 (1 mg/kg) in combination with L745870 (0.001 mg/kg) did not markedly affect the cocaine (10 mg/kg)-like discriminative stimulus effects of bromocriptine (20 mg/kg). These results suggest that the discriminative stimulus effects of cocaine are different from the cocaine-like discriminative stimulus effects of bromocriptine or R(+)-7-OH-DPAT, in terms of dopamine D3 and D4 receptors.     

Discriminative stimulus effects of pregnanolone in rhesus monkeys

Rationale

Neuroactive steroids and benzodiazepines can positively modulate GABA by acting at distinct binding sites on synaptic GABA_A receptors. Although these receptors are thought to mediate the behavioral effects of both benzodiazepines and neuroactive steroids, other receptors (e.g., extrasynaptic GABA_A, N-methyl-d-aspartate (NMDA), σ₁, or 5-HT₃ receptors) might contribute to the effects of neuroactive steroids, accounting for differences among positive modulators.

Objective

The current study established the neuroactive steroid pregnanolone as a discriminative stimulus to determine whether actions in addition to positive modulation of synaptic GABA_A receptors might contribute to its discriminative stimulus effects.

Methods

Four rhesus monkeys discriminated 5.6 mg/kg pregnanolone while responding under a fixed-ratio 10 schedule of stimulus-shock termination.

Results

Positive modulators acting at benzodiazepine, barbiturate, or neuroactive steroid sites produced ≥80 % pregnanolone-lever responding, whereas drugs acting primarily at receptors other than synaptic GABA_A receptors, such as extrasynaptic GABA_A, NMDA, σ₁, and 5-HT₃ receptors, produced vehicle-lever responding. Flumazenil antagonized the benzodiazepines midazolam and flunitrazepam, with Schild analyses yielding slopes that did not deviate from unity and pA₂ values of 7.39 and 7.32, respectively. Flumazenil did not alter the discriminative stimulus effects of pregnanolone.

Conclusion

While these results do not exclude the possibility that pregnanolone acts at receptors other than synaptic GABA_A receptors, they indicate a primary and possibly exclusive role of synaptic GABA_A receptors in its discriminative stimulus effects. Reported differences in the effects of benzodiazepines and neuroactive steroids are not due to differences in their actions at synaptic GABA_A receptors.     

Discriminative stimulus effects of the benzodiazepine receptor partial agonist bretazenil in pigeons and in rats

Bretazenil is a partial agonist at diazepam-sensitive (DS) GABA(A) receptors, and it also binds with high affinity to diazepaminsensitive (DI) GABA(A) receptors. A unique discriminative stimulus effect transduced by binding at DI benzodiazepine (BZ) receptors has been reported in pigeons, but has not been established in rats. Further, differences have been observed between rats and pigeons in results of drug discrimination experiments utilizing BZ receptor partial agonists. Therefore, to examine the discriminative stimulus effects of bretazenil and to explore the possibility of species differences in substitution profiles, pigeons and rats were trained to discriminate 0.3mg/kg bretazenil from vehicle. Flumazenil (0.03-1.0mg/kg) did not substitute for bretazenil in pigeons, despite full substitution of bretazenil for flumazenil in this species. Flumazenil (0.03-10.0mg/kg) also did not substitute for bretazenil in rats, despite the partial agonist effects of flumazenil in rats. Likewise, midazolam (0.3-1.0mg/kg) did not substitute for bretazenil in pigeons, despite the fact that bretazenil partially substitutes for midazolam in pigeons. However in rats, midazolam produced full, dose-dependent substitution (0.03-3.2mg/kg). Differences may result from different fractional receptor occupancy requirements for the mediation of discriminative stimulus effects through DS BZ receptors, and/or from a contribution of DI BZ receptor binding in pigeons.     

Ethanol-like discriminative stimulus effects of non-competitive n-methyl-d-aspartate antagonists

The discriminative stimulus effects of noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists were investigated in animals trained to discriminate 1.5g/kg (pigeons) or 1.25g/kg (mice) ethanol from vehicle. Key-pecking of pigeons and lever responding of mice were maintained under fixed ratio schedules of mixed grain or milk reinforcement, respectively. Phencyclidine (PCP) and ketamine dose-dependently substituted for the ethanol stimulus in both species, with PCP being about 10-fold more potent than ketamine. In pigeons, PCP and ketamine fully substituted for ethanol at doses that did not significantly alter rates of responding; with mice, complete substitution was accompanied by response rate-decreasing effects. In pigeons, the highly selective NMDA receptor/ionophore antagonist MK-801 also substituted for ethanol at a dose that was accompanied by reduced response rates. Compounds that did not substitute for the ethanol stimulus were cocaine (both species), the 5-HT(1B) receptor agonist TFMPP (pigeon), the H(1) receptor antagonist hydroxyzine (mice), and the anticonconvulsants phenytoin and ethosuximide (mice). The present data show that PCP-like drugs that are antagonists of NMDA receptor-mediated neurotransmission share common discriminative stimulus effects with ethanol.     

Pharmacological and anatomical evidence for an interaction between mGluR5- and GABA(A) alpha1-containing receptors in the discriminative stimulus effects of ethanol.

The discriminative stimulus properties of ethanol are mediated in part by positive modulation of GABA(A) receptors. Recent evidence indicates that metabotropic glutamate receptor subtype 5 (mGluR5) activity can influence GABA(A) receptor function. Therefore, the purpose of this work was to examine the potential involvement of mGluR5 in the discriminative stimulus effects of ethanol. In rats trained to discriminate ethanol (1 g/kg, intragastric gavage (i.g.)) from water, 2-methyl-6-(phenylethyl)-pyridine (MPEP) (1-50 mg/kg, i.p.) a selective noncompetitive antagonist of the mGlu5 receptor did not produce ethanol-like stimulus properties. However, pretreatment with MPEP (30 mg/kg) reduced the stimulus properties of ethanol as indicated by significant reductions in ethanol-appropriate responding, specifically at 0.5 and 1 g/kg ethanol, and a failure of ethanol test doses (1 and 2 g/kg) to fully substitute for the ethanol training dose. To test whether mGluR5 antagonism altered the GABA(A) receptor component of the ethanol stimulus, the ability of MPEP to modulate pentobarbital and diazepam substitution for ethanol was assessed. Pentobarbital substitution (1-10 mg/kg, i.p.) for ethanol was not altered by MPEP pretreatment. However, MPEP pretreatment inhibited the ethanol-like stimulus properties of diazepam (5 mg/kg, i.p.). To examine a potential anatomical basis for these pharmacological findings, expression patterns of mGluR5- and benzodiazepine-sensitive GABA(A) alpha1-containing receptors were examined by dual-label fluorescent immunohistochemistry with visualization by confocal microscopy. Results indicated that mGluR5- and GABA(A) alpha1-containing receptors were both coexpressed in limbic brain regions and colocalized on the same cells in specific brain regions including the amygdala, hippocampus, globus pallidus, and ventral pallidum. Together, these findings suggest an interaction between mGluR5- and benzodiazepine-sensitive GABA(A) receptors in mediating ethanol discrimination.