0151 The Dual Orexin Receptor Antagonist Almorexant Blocks the Sleep-Disrupting Effects of Methamphetamine in Male Rhesus Monkeys

Introduction

Individuals with stimulant use disorder show a high prevalence of sleep problems. In the laboratory, stimulant drugs have been shown to affect sleep parameters in human and nonhuman primates, even when administered many hours before bedtime. Although the mechanisms underlying the relationship between stimulant use/abuse and sleep impairment remain unclear, recent research has implicated the orexin (also called “hypocretin”) system as a critical regulator of sleep-wake states. The aim of the present study was to investigate the effects of the dual orexin receptor antagonist (DORA) almorexant on the sleep-disrupting effects of methamphetamine in rhesus monkeys.

Methods

Male adult rhesus monkeys (Macaca mulatta, n=4) were fitted with primate collars to which Actiwatch monitors were attached. Actigraphy recording was conducted during baseline conditions and on the night after acute morning (9h) administration of vehicle or methamphetamine (0.03, 0.1 or 0.3 mg/kg, i.m.). During a second set of treatments, vehicle or almorexant (1, 3 or 10 mg/kg, i.m.) were administered in the evening (16:30h, 1.5h before “lights off”) following morning (9h) administration of methamphetamine (0.3 mg/kg, i.m.).

Results

Morning methamphetamine administration dose-dependently impaired sleep in rhesus monkeys, with the dose of 0.3 mg/kg significantly increasing sleep latency and decreasing sleep efficiency. Evening administration of almorexant improved both actigraphy-based sleep measures after morning administration of methamphetamine in a dose dependent manner.

Conclusion

Our findings indicate that orexin receptor systems are involved in methamphetamine-induced sleep disruption. The exact role of the two orexin receptors in this effect, alone or together, remains to be determined. This study suggests that DORAs can be effective in treating sleep impairment in individuals with methamphetamine use disorder or under stimulant prescription for other sleep and psychiatric disorders.

Support

Supported by UMMC Research Enhancement Funds.

The α2,3-selective potentiator of GABAA receptors, KRM-II-81, reduces nociceptive-associated behaviors induced by formalin and spinal nerve ligation in rats

Clinical evidence indicates that positive allosteric modulators (PAMs) of GABAA receptors have analgesic benefit in addition to efficacy in anxiety disorders. However, the utility of GABAA receptor PAMs as analgesics is compromised by the central nervous system side effects of non-selective potentiators. A selective potentiator of GABAA receptors associated with α2/3 subunits, KRM-II-81(5-(8-ethynyl-6-(pyridin-2-yl)-4H-benzo[f]imidazo[1,5-a][1,4]diazepin-3-yl)oxazole), has demonstrated anxiolytic, anticonvulsant, and antinociceptive effects in rodents with reduced motoric side effects. The present study evaluated the potential of KRM-II-81 as a novel analgesic. Oral administration of KRM-II-81 attenuated formalin-induced flinching; in contrast, diazepam was not active. KRM-II-81 attenuated nociceptive-associated behaviors engendered by chronic spinal nerve ligation (L5/L6). Diazepam decreased locomotion of rats at the dose tested in the formalin assay (10 mg/kg) whereas KRM-II-81 produced small decreases that were not dose-dependent (10-100 mg/kg). Plasma and brain levels of KRM-II-81 were used to demonstrate selectivity for α2/3- over α1-associated GABAA receptors and to define the degree of engagement of these receptors. Plasma and brain concentrations of KRM-II-81 were positively-associated with analgesic efficacy. GABA currents from isolated rat dorsal-root ganglion cultures were potentiated by KRM-II-81 with an ED50 of 32 nM. Measures of respiratory depression were reduced by alprazolam whereas KRM-II-81 was either inactive or produced effects with lower potency and efficacy. These findings add to the growing body of data supporting the idea that α2/3-selective GABAA receptor PAMs will have efficacy and tolerability as pain medications including those for neuropathic pain. Given their predicted anxiolytic effects, α2/3-selective GABAA receptor PAMs offer an additional inroad into the management of pain.

Isobolographic analysis of chlordiazepoxide and triazolam combinations in squirrel monkeys discriminating triazolam

RATIONALE

The discriminative stimulus (DS) effects of chlordiazepoxide (CDP) differ from those of other typical benzodiazepine (BZ) agonists in that CDP does not always occasion full substitution for a BZ agonist DS.

OBJECTIVES

The present study tested the hypothesis that the unusual DS effects of CDP may result from its relatively low intrinsic efficacy by examining the combinations of CDP and triazolam using isobolographic analysis in squirrel monkeys discriminating triazolam.

METHODS AND RESULTS

Squirrel monkeys were previously trained to discriminate triazolam (0.03 mg/kg, i.v.) from vehicle under a fixed-ratio 10 (FR 10) schedule of food reinforcement. CDP occasioned partial substitution for triazolam and did not alter the DS effects of triazolam, whereas single doses of triazolam enhanced the DS effects of triazolam, and bretazenil antagonized the triazolam DS. The isobolographic analysis showed that CDP and triazolam combinations resulted in additive effects in animals in which CDP substituted for triazolam, whereas infra-additive effects were obtained in animals in which CDP did not substitute for triazolam.

CONCLUSIONS

The partial substitution of CDP for triazolam and the infra-additive effects obtained in animals in which CDP did not substitute for triazolam suggest that CDP may have lower intrinsic efficacy than triazolam. However, the lack of overall effect of CDP pretreatment and the lack of shift in animals in which CDP substituted for triazolam suggest that other factors, such as differential activity at BZ receptor subtypes, may play a role in the effects of CDP.

Discriminative stimulus effects of intravenous heroin and its metabolites in rhesus monkeys: opioid and dopaminergic mechanisms

Heroin has characteristic subjective effects that contribute importantly to its widespread abuse. Drug discrimination procedures in animals have proven to be useful models for investigating pharmacological mechanisms underlying the subjective effects of drugs in humans. However, surprisingly little information exists concerning the mechanisms underlying the discriminative stimulus (DS) effects of heroin. This study characterized the DS effects of heroin in rhesus monkeys trained to discriminate i.v. heroin from saline. In drug substitution experiments, heroin, its metabolites 6-monoacetylmorphine, morphine, morphine-6-glucuronide, and morphine-3-glucuronide, and the mu-agonists fentanyl and methadone engendered dose-dependent increases in heroin-lever responding, reaching average maximums of >80% (full substitution) at doses that did not appreciably suppress response rate. In contrast, the delta-agonist SNC 80, the kappa-agonist spiradoline, and the dopamine uptake blockers/releasers cocaine, methamphetamine, and GBR 12909 did not engender heroin-like DS effects regardless of dose. In antagonism studies, in vivo apparent pA2 and pK(B) values for naltrexone combined with heroin, morphine, and 6-monoacetylmorphine (8.0-8.7) were comparable with those reported previously for naltrexone antagonism of prototypical mu-agonists. The results show that the DS effects of heroin are pharmacologically specific and mediated primarily at mu-opioid receptors. Moreover, the acetylated and glucuronated metabolites of heroin appear to play significant roles in these effects. Despite previous speculation that morphine-3-glucuronide lacks significant opioid activity, it substituted fully for heroin in our study, suggesting that it can exhibit prominent mu-agonist effects in vivo.

Similar enhancement of the discriminative stimulus effects of cocaine and GBR 12909 by heroin in squirrel monkeys

RATIONALE AND OBJECTIVES

Heroin previously was shown to engender partial cocaine-like discriminative stimulus (DS) effects in squirrel monkeys. The present study assessed the degree to which heroin modified the DS effects of cocaine and the cocaine-like DS effects of the selective dopamine transport blocker GBR 12909.

METHODS AND RESULTS

In squirrel monkeys discriminating cocaine (0.3 mg/kg) from saline, cocaine and GBR 12909 dose-dependently engendered levels of responding on the cocaine-associated lever greater than or equal to 90% (full substitution). Heroin engendered full substitution for cocaine in two monkeys, partial substitution (75%) in a third monkey, and no substitution in the fourth monkey. When administered as a pretreatment, heroin shifted the dose-response function for cocaine to the left in the three monkeys for which heroin engendered cocaine-lever responding, but did not alter the DS effects of cocaine in the fourth monkey. Heroin pretreatment also shifted the dose-response function for the cocaine-like DS effects of GBR 12909 to the left in the former three monkeys, and did not alter the effects of GBR 12909 in the fourth monkey. Isobolographic analysis of the DS effects of cocaine-heroin and GBR 12909-heroin combinations in the former three monkeys revealed that the potencies of the combinations were not different from predicted values based on dose-additive effects.

CONCLUSIONS

These findings show that heroin can enhance similarly the DS effects of cocaine and GBR 12909, suggesting that activation of dopaminergic systems underlies enhancement of the interoceptive effects of cocaine by heroin.

Modulation of cocaine and food self-administration by low- and high-efficacy D1 agonists in squirrel monkeys

RATIONALE

Dopamine D1 ligands have been proposed as candidate medications for cocaine abuse. Previous studies have shown that the ability of D1 ligands to modulate the behavioral effects of cocaine may depend on agonist efficacy.

OBJECTIVES

This study investigated the role of agonist efficacy in the ability of D1 ligands to modulate the reinforcing effects of cocaine in monkeys.

METHODS

Squirrel monkeys trained to self-administer cocaine under a second-order schedule of reinforcement were treated daily with D1 agonists varying in efficacy from low to high (SKF 83959 < SKF 77434 < or = SKF 81297 < SKF 82958) and the D1 antagonist SCH 39166.

RESULTS

D1 ligands, regardless of efficacy, produced dose-dependent reductions in responding maintained by a maximally effective dose of cocaine. Equivalent doses of each D1 ligand reduced responding for food under a similar second-order schedule, suggesting that the suppression was not specific to cocaine self-administration. When studied in combination with a range of cocaine doses, treatment with the agonists SKF 83959, SKF 77434, SKF 81297, and the antagonist SCH 39166 produced overall rightward and downward shifts in the dose-response function for cocaine self-administration. Treatment with the agonist SKF 82958, however, produced an overall suppression of responding, regardless of the dose of cocaine.

CONCLUSIONS

In contrast to a high-efficacy agonist, low-efficacy D1 ligands modulated the reinforcing effects of cocaine in a manner consistent with at least a partial antagonism of cocaine self-administration. This delineation of the efficacy-dependent profile of effects for D1 ligands should guide research into their utility as cocaine pharmacotherapies.

Zolpidem, triazolam, and diazepam decrease distress vocalizations in mouse pups: differential antagonism by flumazenil and beta-Carboline-3-carboxylate-t-butyl ester (beta-CCt)

In response to stressful events, neonatal mice emit ultrasonic vocalizations (USVs), which are suppressed by BZ agonists. The present study examined the role of the benzodiazepine/alpha1 (BZ/alpha1) receptor subtype in the suppression engendered by the BZ/alpha1-preferring agonist zolpidem and the nonselective BZ agonists triazolam and diazepam. The role of BZ receptor subtypes was explored further by conducting antagonism studies using the BZ/alpha1-preferring antagonist beta-carboline-3-carboxylate-t-butyl ester (beta-CCt), in comparison with the nonselective BZ antagonist flumazenil. Mouse pups (CFW strain) were separated from their dam and littermates at day 7, and placed for 4 min in a test chamber with reduced ambient temperature (19 +/- 1 degrees C) for recording USVs, motor incoordination (measured as a pup rolling on its back per grid cross), and body temperature. Zolpidem, triazolam, and diazepam suppressed USVs in a dose-dependent manner, concomitant with increases in incoordination and augmentation of hypothermia. These effects of the three BZ agonists were blocked by flumazenil in a manner consistent with surmountable antagonism. The ability of zolpidem, but not triazolam or diazepam, to suppress USVs and augment hypothermia was antagonized by beta-CCt, whereas the increase in motor incoordination engendered by zolpidem, triazolam, and diazepam was not sensitive to beta-CCt administration. Collectively, these results suggest that zolpidem suppresses distress USVs in mouse pups by a mechanism distinct from that of typical BZs. Furthermore, suppression of distress USVs by zolpidem may involve BZ/alpha1 receptors and a nonanxiolytic mechanism, such as hypothermia.

Discriminative stimulus effects of panadiplon (U-78875), a partial agonist at the benzodiazepine site, in pentobarbital-trained rhesus monkeys

The pentobarbital-like discriminative stimulus effects of the benzodiazepine receptor partial agonist panadiplon were assessed in rhesus monkeys trained to discriminate pentobarbital (10 mg/kg) from saline. In test sessions, pentobarbital and the benzodiazepine full agonist triazolam engendered near 100% drug-appropriate responding in all monkeys, whereas panadiplon engendered near 100% drug-appropriate responding in two of four monkeys. Panadiplon pretreatment resulted in leftward shifts in the pentobarbital dose-response function but predominantly rightward shifts of the triazolam dose-response function. These results are consistent with findings that benzodiazepine partial agonists have pentobarbital-like discriminative stimulus effects in some subjects only, and further suggest that partial agonists enhance the effects of pentobarbital but antagonize the effects of a benzodiazepine full agonist.

Triazolam discrimination in squirrel monkeys distinguishes high-efficacy agonists from other benzodiazepines and non-benzodiazepine drugs

RATIONALE

Triazolam is a high-efficacy benzodiazepine (BZ) agonist, which might be hypothesized to engender highly pharmacologically specific discriminative stimulus (DS) effects and distinguish among BZ agonists with different intrinsic efficacy.

OBJECTIVES

The pharmacological specificity of the triazolam stimulus was determined by examining the effects of conventional and atypical BZ agonists, and other ligands active at the gamma-aminobutyric acidA (GABAA) receptor complex. Receptor mechanisms underlying the DS effects of triazolam were examined further using the BZ receptor antagonist flumazenil.

METHODS AND RESULTS

Squirrel monkeys were trained to discriminate triazolam (0.03 mg/kg, i.v.) from vehicle under a fixed-ratio 10 (FR 10) schedule of food reinforcement. While the BZ agonists midazolam, diazepam, and lorazepam substituted fully for triazolam, chlordiazepoxide, oxazepam and nordiazepam produced only partial substitution, suggesting these latter compounds may have reduced intrinsic efficacy. The BZ/alpha1-preferring agonist zolpidem substituted fully for triazolam, and potencies for triazolam-like effects of BZ agonists were significantly correlated with potencies for their zolpidem-like effects (Rowlett et al. 1999). Flumazenil antagonized the DS effects of triazolam, but the slope of the Schild plot was significantly different from unity, suggesting multiple receptors may be involved in the DS effects of triazolam.

CONCLUSIONS

BZ agonists can be distinguished on the basis of substitution for triazolam and, thus, the triazolam discrimination may be a useful tool for identifying compounds of different efficacy at BZ receptors. BZ/alpha1 receptors appear to play a prominent role in the DS effects of triazolam, but the contribution of other subtypes of BZ receptors cannot be ruled out.

Using behavior to elucidate receptor mechanisms: a review of the discriminative stimulus effects of benzodiazepines

Drug discrimination procedures have been used to study receptor mechanisms of benzodiazepine (BZ) agonists with the goal of developing new therapeutic agents that retain positive effects of conventional BZ ligands yet have reduced side effects. The present review provides a synthesis of existing literature on discriminative stimulus effects of BZ agonists in order to elucidate their underlying receptor mechanisms, specifically in terms of intrinsic efficacy and receptor selectivity. The available evidence suggests that receptor selectivity is a critical determinant of the discriminative stimulus effects of BZ agonists. In particular, BZ-1 receptors appear to play a fundamental role, whereas the role of BZ-2 receptors remains elusive. In addition, data from many drug discrimination studies suggest that the conventional BZ agonist chlordiazepoxide may have reduced intrinsic efficacy compared with other BZ agonists.

A labor-supply analysis of cocaine self-administration under progressive-ratio schedules: antecedents, methodologies, and perspectives

RATIONALE

Under a progressive-ratio (PR) schedule, a subject must complete increasing fixed-ratio (FR) response requirements to obtain reinforcers. Response requirements are increased until responding stops; the final ratio completed being the "break point" and providing an index of the relative effectiveness, or value, of the reinforcer to maintain behavior.

OBJECTIVES

This review examines the historical and conceptual framework underlying the PR procedure and examines the concept of relative reinforcer value. Pharmacological analysis (based on receptor theory), and behavior analysis (based on microeconomic theory) are reviewed.

METHODS

Using a microeconomic adaptation of the reinforcement model referred to as conservation, a mathematical model of PR performance is proposed based on the curvilinear relationship between economic supply and labor. Drug consumption and instrumental responding were assumed to reflect deviations from a balance point, defined as the levels of consumption and responding under no scheduled restraint. Data sets were re-analyzed in which several response sequences were examined in rhesus monkeys maintained on PR schedules of intravenous cocaine delivery.

RESULTS

The modified conservation equation fitted the PR data accurately, and results consistent with both linear and concave labor-supply functions were obtained. These results suggest that cocaine self-administration under PR schedules conforms to labor-supply relationships characterized as inelastic (consumption is resistant to increases in schedule requirements) and unit elastic (at high response costs, consumption declines with no corresponding increase or decrease in total responding).

CONCLUSIONS

The labor-supply methodology allows for a definition of the relative value of a drug reinforcer in PR studies based on changes in consumption across response costs. Specifically, relative reinforcer value is defined in terms of changes in behavior from a balance point, rather than as a property that determines the strength of the instrumental response.

On the relationship between the dopamine transporter and the reinforcing effects of local anesthetics in rhesus monkeys: practical and theoretical concerns

RATIONALE

Drugs that are self-administered appear to vary in their potency and effectiveness as positive reinforcers. Understanding mechanisms that determine relative effectiveness of drugs as reinforcers will enhance our understanding of drug abuse.

OBJECTIVES

The hypothesis of the present study was that differences among dopamine transporter (DAT) ligands in potency and effectiveness as a positive reinforcers were related to potency and effectiveness as DA uptake inhibitors. Accordingly, self-administration of a group of local anesthetics that are DAT ligands was compared to their effects as DA uptake blockers in vitro in brain tissue.

METHODS

Rhesus monkeys were allowed to self-administer cocaine and other local anesthetics i.v. under a progressive-ratio schedule. The same compounds were compared in standard in vitro DA uptake assays using monkey caudate tissue.

RESULTS

The rank order of both potency and effectiveness as reinforcers was cocaine > dimethocaine > procaine > chloroprocaine. Tetracaine did not maintain self-administration. For inhibiting DA uptake, the potency order was cocaine > dimethocaine > tetracaine > procaine > chloro-procaine. At maximum, these compounds were equally effective in blocking DA uptake. Lidocaine did not inhibit DA uptake.

CONCLUSIONS

The potency of local anesthetics as positive reinforcers is likely related to their potency as DA uptake inhibitors. Variation in their effectiveness as positive reinforcers was not a function of differences in effectiveness as DA uptake blockers, but may be related to relative potency over the concentrations that are achieved in vivo. Effects at sodium channels may limit the reinforcing effects of local anesthetics.

Transduction of the discriminative stimulus effects of zolpidem by GABA(A)/alpha1 receptors

Zolpidem is an imidazopyridine with high affinity at gamma-aminobutyric acid(A) (GABA(A)) receptors expressing alpha1 subunits. In squirrel monkeys trained to discriminate a high dose of zolpidem (> or =3.0 mg/kg) from saline, zolpidem and another GABA(A)/alpha1 receptor-preferring agonist, zaleplon, substituted dose-dependently for zolpidem, whereas the non-selective agonists diazepam and triazolam were did not substitute at any dose tested. These findings offer the first evidence for a selective role of GABA(A)/alpha1 receptors in the interoceptive effects of high doses of zolpidem.

Regulation of drug-maintained behavior: comment on Meisch (2000)

In his article, R. A. Meisch (2000) introduces a concept termed relative persistence of behavior, a measure obtained by comparing rates of behavior under high-response costs to rates obtained under lower response costs. In this commentary, relative persistence of behavior is discussed in terms of behavioral regulation theory, in which responding is allocated in such a way as to maintain a stable balance point. Meisch's relative persistence of behavior may be analogous to the free (paired) baseline technique necessary for testing hypotheses based on behavioral regulation.

Dopamine D1- and D2-like receptor mechanisms in relapse to cocaine-seeking behavior: effects of selective antagonists and agonists

Dopaminergic mechanisms are thought to be critical in mediating relapse to cocaine-seeking behavior. This study examined the different roles of D1- and D2-like receptor mechanisms in the relapse process. Squirrel monkeys were given extended histories of i. v. cocaine self-administration under conditions in which responding was maintained jointly by response-contingent cocaine injections and a cocaine-paired visual stimulus (second-order schedule). Responding was then extinguished by substituting saline for cocaine injections and omitting presentations of the cocaine-paired stimulus. Subsequently, noncontingent priming injections of cocaine combined with restoration of the cocaine-paired stimulus induced dose-dependent reinstatement of drug-seeking behavior, with response rates approaching those maintained by active cocaine self-administration. The priming effects of cocaine were attenuated by several D1- and D2-like receptor antagonists and low efficacy agonists but not by the D3-preferring antagonists UH 232 and AJ-76. The priming effects of cocaine were mimicked by the D2-like receptor agonists R(-)-propylnorapomorphine hydrochloride (NPA) and quinpirole, less consistently by 7-OH-DPAT, and not by the D1-like receptor agonists SKF-81297 and SKF-82958, the D3-preferring agonist PD-128,907, or any low efficacy agonist. Cotreatment with NPA, PD-128,907, and 7-OH-DPAT did not alter reinstatement of drug-seeking behavior induced by a maximally effective priming dose of cocaine, whereas cotreatment with D1-like receptor agonists attenuated the priming effects of cocaine. The results suggest that D1- and D2-like receptors play fundamentally different roles in the relapse process. Although stimulation of D2-like, but probably not D3-like, receptors appears necessary for induction of relapse, either stimulation or blockade of D1-like receptors appears to be inhibitory with respect to relapse.

Discriminative-stimulus effects of zolpidem, triazolam, pentobarbital, and caffeine in zolpidem-trained humans

Six non-drug-abusing humans were trained to discriminate 15 mg zolpidem in the present experiment. After participants acquired discrimination, a range of doses of zolpidem (2.5-15.0 mg), triazolam (0.0625-0.3750 mg), pentobarbital (25-150 mg), caffeine (100-600 mg), and placebo were tested to determine whether they shared discriminative-stimulus effects with 15 mg zolpidem. The participant-rated and performance-impairing effects of zolpidem, triazolam, pentobarbital, and caffeine were assessed concurrently. Triazolam and pentobarbital dose dependently increased zolpidem-appropriate responding. Caffeine occasioned low levels of zolpidem-appropriate responding. Zolpidem, triazolam, and pentobarbital, but not caffeine, generally produced a similar constellation of participant-rated drug effects (e.g., increased scores for the Pentobarbital, Chlorpromazine, and Alcohol Group subscale on the Addiction Research Center Inventory) and dose dependently impaired performance. These results suggest that humans can reliably discriminate zolpidem. Despite its unique benzodiazepine-receptor binding profile, the discriminative-stimulus, participant-rated, and performance-impairing effects of zolpidem are similar to those of the barbiturates and benzodiazepines.

Dissociation of cocaine-antagonist properties and motoric effects of the D1 receptor partial agonists SKF 83959 and SKF 77434

Previous studies suggest that D1 receptor partial agonists may be viable candidates for development as pharmacotherapies for cocaine addiction. This study investigated the ability of the D1 receptor partial agonists SKF 83959 and SKF 77434 to modulate the behavioral effects of cocaine and compared these effects with those of the reference D1 receptor antagonist SCH 39166 and D1 receptor agonists SKF 81297 and 6-Br-APB. Squirrel monkeys were trained either to respond under a fixed-interval schedule of stimulus-shock termination or to discriminate cocaine from vehicle (procedures useful for evaluating the behavioral stimulant and subjective effects of cocaine, respectively). Additional monkeys were studied with quantitative observational techniques to evaluate the effects of the drugs on various forms of motor behavior. Like SCH 39166, but unlike SKF 81297 and 6-Br-APB, the D1 receptor partial agonists attenuated the behavioral stimulant and discriminative stimulus effects of cocaine in a dose-dependent manner, although maximum antagonism produced by SKF 77434 was not always as great as that produced by SKF 83959 or SCH 39166. In observational studies, SKF 83959 and SKF 77434 produced less severe disruptions in motor behavior than did SCH 39166 and, for SKF 83959, showed a greater separation between the dose required to antagonize the behavioral effects of cocaine and the dose that induced catalepsy (>/=33-fold). These results suggest that D1 receptor partial agonists can act as functional cocaine antagonists with less severe behavioral effects than D1 receptor antagonists. The prominent cocaine-antagonist properties and the low incidence of motoric side effects of SKF 83959 may reflect its unique binding profile at D1 as well as nondopaminergic receptors.

Cocaine-like discriminative stimulus effects of heroin in squirrel monkeys: role of active metabolites and opioid receptor mechanisms

RATIONALE AND OBJECTIVES

Opioid agonists frequently have been reported to share discriminative stimulus (DS) effects with cocaine; however, the pharmacological basis of these shared effects is not understood completely. The present study assessed the ability of heroin and its deacetylated metabolites, 6-monoacetylmorphine (6-MAM) and morphine, to engender cocaine-like DS effects and investigated the role of opioid receptor subtypes in modulating these DS effects.

METHODS

Squirrel monkeys were trained to discriminate 0.3 mg/kg cocaine (i.m.) from vehicle under a 10-response fixed-ratio schedule of food reinforcement, and responding on the drug lever was assessed after varying i.m. doses of heroin, 6-MAM, and morphine. The potential role of opioid receptor mechanisms in modulating the cocaine-like DS effects of heroin and its metabolites was assessed with the mixed mu/kappa opioid antagonist naltrexone, the delta-selective antagonist naltrindole, and the kappa-selective antagonist nor-binaltorphimine.

RESULTS

Heroin, 6-MAM, and morphine engendered dose-related increases in responding on the cocaine lever in three of four monkeys. Naltrexone shifted the dose-response functions for heroin and its metabolites to the right, and in vivo apparent pA2 analyses revealed that naltrexone antagonized the effects of the opioids in a manner consistent with mu receptor antagonism (apparent pA2 values ranging from 8.20 to 8.47). Naltrindole only minimally altered the dose-response functions of heroin, 6-MAM, and morphine, whereas nor-binaltorphimine did not block the cocaine-like DS effects of the three opioid agonists, suggesting that neither delta nor kappa receptors played a prominent role in the cocaine-like DS effects of heroin and its metabolites.

CONCLUSIONS

These results suggest that heroin and its deacetylated metabolites engendered cocaine-like DS effects in a similar fashion. Furthermore, the cocaine-like DS effects of these opioids were modulated by a predominantly mu-opioid receptor mechanism.

3'- and 4'-chloro-substituted analogs of benztropine: intravenous self-administration and in vitro radioligand binding studies in rhesus monkeys

RATIONALE

The reinforcing effects of many psychomotor stimulants have been related to increased dopaminergic neurotransmission. Drugs that block dopamine (DA) uptake have generally been found to function as positive reinforcers. Benztropine (BZT) and several of its halogenated analogs have previously been characterized as potent DA-uptake inhibitors with behavioral profiles that indicate diminished psychomotor stimulant effects relative to cocaine.

OBJECTIVES

The present experiments were designed to examine, in rhesus monkeys, the reinforcing effects of the DA-uptake inhibitor BZT and two chloro-analogs 3'-Cl-BZT and 4'-Cl-BZT, and to compare self-administration and binding profiles.

METHODS

Four rhesus monkeys self-administered cocaine i.v. under a fixed-ratio 10 (FR10) schedule until stable responding was established. Saline, and various doses of cocaine, BZT, and the BZT analogs were then made available for self-administration. Binding of these compounds to monoaminergic and cholinergic sites in monkey brain were determined using standard radioligand binding techniques.

RESULTS

Self-administration was maintained by both 3'-Cl-BZT and 4'-Cl-BZT, but not by BZT. Results suggested that 3'-Cl-BZT and 4'-Cl-BZT were weak positive reinforcers. BZT and analogs bound DA transporters (DAT) with affinities higher than that of cocaine and had affinity for muscarinic binding sites.

CONCLUSIONS

Surprisingly, high affinity at DATs was associated with weak or no reinforcing effects. The mechanism(s) that may underlie this dissociation between DAT actions and reinforcing effects remains to be established. These data support the proposal that a lead for the discovery of a pharmacotherapeutic agent for cocaine abuse may come from this group of compounds.

Discriminative stimulus effects of zolpidem in squirrel monkeys: comparison with conventional benzodiazepines and sedative-hypnotics

The present study examined whether zolpidem, an imidazopyridine with selectivity for benzodiazepine (BZ)/gamma-aminobutyric acid(A) receptors containing the alpha1-subunit, had discriminative stimulus effects similar to typical BZs and other sedative/hypnotic drugs in primates. Squirrel monkeys (Saimiri sciureus) were trained to discriminate zolpidem (1.0 mg/kg i.v.) from vehicle under a 10-response fixed-ratio schedule of food delivery. Under test conditions, zolpidem (0.1-3.0 mg/kg) increased responding on the drug lever to an average maximum of 90% of total responding. When pretreatment times were varied from 5 to 50 min, the discriminative stimulus effects of zolpidem were maximal at 5 min and near control levels 35 min after administration. Flumazenil antagonized both the discriminative stimulus and rate-decreasing effects of zolpidem in a dose-dependent and surmountable fashion (in vivo apparent pA(2) values of 7.3 and 6.6 for the discriminative stimulus and rate-suppressing effects, respectively). The BZs triazolam, midazolam, diazepam, and N-desmethyldiazepam engendered dose-related increases in drug-lever responding that reached zolpidem-like levels (90%) in the majority of monkeys tested. In contrast, lorazepam, chlordiazepoxide, and oxazepam engendered average maximums of 70% or less and substituted fully for zolpidem in one or two monkeys only. Representative barbiturates as well as drugs that bind to non-BZ sites (muscimol, baclofen, buspirone, cyproheptadine, diphenhydramine) engendered 0 to 45% of responses on the drug lever up to doses that markedly reduced response rate. These results support the view that zolpidem's selectivity for the alpha1-subunit of the BZ/gamma-aminobutyric acid(A) receptor complex confers a distinctive profile of interoceptive effects that overlaps partially with those of typical BZs but not with those of barbiturates.

Pharmacological and environmental determinants of relapse to cocaine-seeking behavior

Animal models have been developed that simulate relevant features of relapse to cocaine-seeking behavior in humans. These models have provided valuable information about pharmacological and environmental factors that precipitate reinstatement of extinguished cocaine-seeking in rats and monkeys, as well as new insights about potential pharmacotherapies for relapse prevention. Reinstatement of cocaine-seeking behavior in animals can be induced by cocaine priming or by cocaine-paired environmental stimuli: however, maximum reinstatement of drug-seeking appears to be induced when cocaine priming and cocaine-paired stimuli are combined. Drugs that share cocaine's indirect dopamine agonist properties or that act as direct agonists at D2-like dopamine receptors also induce reinstatement of cocaine-seeking behavior, whereas with some exceptions (e.g., caffeine, morphine) drugs from other pharmacological classes do not. D1-like receptor agonists block rather than mimic the priming effects of cocaine, suggesting different roles for D1- and D2-like receptor mechanisms in cocaine relapse. Although considerable overlap exists, drugs that exhibit cocaine-like discriminative stimulus and/ or reinforcing effects in other situations do not invariably induce cocaine-like reinstatement of drug-seeking and vice versa, implying that these effects are not simply different behavioral expressions of a unitary neurobiological process. Finally, recent findings with D1-like receptor agonists, partial agonists, and antagonists suggest that some of these drugs may be viable candidates for development as antirelapse pharmacotherapies.

Discriminative stimulus effects of morphine in squirrel monkeys: stimulants, opioids, and stimulant-opioid combinations

Morphine and other mu opioids mimic and/or modulate the discriminative stimulus (DS) effects of cocaine, possibly reflecting mutual stimulation of mesolimbic dopamine activity. Less is known about the capacity of cocaine and related stimulants to modulate the DS effects of morphine. The present study investigated the effects of cocaine, amphetamine, and reference drugs, administered alone and with morphine, in squirrel monkeys trained to discriminate morphine from vehicle. Additional studies determined the ability of opioid and dopamine receptor antagonists to attenuate the DS effects of morphine and the morphine-like effects of other drugs. The DS effects of morphine were mimicked by the mu-opioid agonist fentanyl but not the delta-opioid agonists SNC 80 and BW 373U86 or the kappa-opioid agonist U50,488H, and were antagonized by the opioid antagonist naltrexone but not the dopamine antagonist flupenthixol. In three of five monkeys, the DS effects of morphine also were mimicked by cocaine, amphetamine, and the dopamine transport inhibitor GBR 12909 but not the norepinephrine transport inhibitor talsupram or the serotonin transport inhibitor fluoxetine, and were antagonized by flupenthixol but not naltrexone. In this subgroup, pretreatment with cocaine or amphetamine enhanced the DS effects of morphine, whereas in the other two monkeys pretreatment with either stimulant attenuated the DS effects of morphine. The results demonstrated individual differences in morphine-like DS effects of stimulants that are mirrored by individual differences in their interactions with morphine. Furthermore, different mechanisms appear to mediate the DS effects of morphine and the morphine-like DS effects of cocaine and amphetamine.

Discriminative stimulus effects of ethyl-beta-carboline-3-carboxylate at two training doses in rats

RATIONALE AND OBJECTIVES

The role of benzodiazepine (BZ) receptor mechanisms in modulating the stimulus effects of the BZ partial inverse agonist ethyl-beta-carboline-3-carboxylate (beta-CCE) are not well understood. The purpose of the present experiments was to assess the role of BZ and non-BZ receptor stimulation in the discriminative stimulus effects of beta-CCE in rats.

METHODS

Adult male rats were trained to discriminate either a relatively high dose (10 mg/kg, n = 8) or a relatively low dose (5.0 mg/kg, n = 7) of beta-CCE from saline under a fixed-ratio 10 schedule of food presentation.

RESULTS

Under the high-dose training condition, beta-CCE engendered an increase in responding on the drug-paired lever up to 100% drug-lever responding, with no decrease in response rate. Diazepam, pentobarbital, flumazenil, (+)-amphetamine, and morphine did not share stimulus effects with 10 mg/kg beta-CCE up to doses that suppressed rate of responding. The BZ full inverse agonist dimethoxy-4-ethyl-beta-carboline-3-carboxylate also did not engender > or = 80% beta-CCE-lever responding up to doses that suppressed response rate and produced seizures in some animals. The BZ partial inverse agonists Ro 15-4513 and sarmazenil fully reproduced the stimulus effects of beta-CCE. Flumazenil antagonized the effects of beta-CCE with an in vivo apparent pA2 value of 6.1 (slope = -0.86). Under the low-dose condition, beta-CCE engendered an increase in drug-lever responding, with no changes in response rate. In contrast to the high-dose condition, diazepam, pentobarbital, and (+)-amphetamine engendered high levels of beta-CCE-lever responding (up to 77, 96, and 75%, respectively), whereas flumazenil and morphine did not engender full beta-CCE-lever responding.

CONCLUSIONS

These results indicated that the stimulus effects of the high dose of beta-CCE appeared consistent with mediation by the drug's partial inverse agonist effects at BZ receptors. The discriminative stimulus effects of beta-CCE at the lower training dose, however, appeared to be relatively non-specific.

Reinforcing and discriminative stimulus effects of the neuroactive steroids pregnanolone and Co 8-7071 in rhesus monkeys

RATIONALE AND OBJECTIVES

The present study was designed to assess possible abuse-related effects of the endogenous neuroactive steroid pregnanolone (3alpha-hydroxy-5beta-pregnan-20-one) and the orally bioavailable, water-soluble neuroactive steroid pro-drug Co 8-7071 (3alpha,21-dihydroxy-3beta-trifluoromethyl-5beta-pregnan-20- one, 21-hemisuccinate).

METHODS

Four rhesus monkeys were prepared with chronic intravenous (i.v.) catheters and trained to press a lever under a ten-response fixed-ratio (FR) schedule of methohexital injection (0.1 mg/kg per injection). Three rhesus monkeys were trained to discriminate intragastric infusions of pentobarbital (10 mg/kg) from saline infusions under a FR5 schedule of stimulus-shock termination.

RESULTS

At least two doses of pregnanolone (0.003-0.1 mg/kg per injection) maintained injections per session above saline levels in the four monkeys tested, whereas Co 8-7071 (0.01-1.0 mg/kg per injection) maintained injections per session above saline levels in two of four monkeys at relatively low levels of injections per session. In rhesus monkeys trained to discriminate pentobarbital, i.v. pregnanolone injections (0.1-1.7 mg/kg, 5-min presession) dose-dependently reproduced the discriminative stimulus effects of pentobarbital in all monkeys tested. Intravenous administration of Co 8-7071 (1-10 mg/kg, 5-min presession) resulted in a dose-dependent increase to >80% pentobarbital-appropriate responding in two of three monkeys tested. Following intragastric infusions of Co 8-7071 (1.0-30 mg/kg), > or =80% pentobarbital-appropriate responding occurred in one out of three monkeys at 10 mg/kg when administered 60 min before the session. When administered 120 min before the session, however, 10-30 mg/kg Co 8-7071 reproduced the discriminative stimulus effects of pentobarbital in each of the three monkeys tested.

CONCLUSIONS

These data demonstrate barbiturate-like abuse-related effects that differed between two pregnane steroids. Whereas pregnanolone functioned as a reinforcer, suggesting that this compound has abuse potential, Co 8-7071 did not, despite having pentobarbital-like discriminative effects.

Evaluation of the reinforcing and discriminative stimulus effects of gamma-hydroxybutyrate in rhesus monkeys

Gamma-hydroxybutyrate (GHB) is a metabolite of GABA that is present in the CNS and fulfils at least some of the criteria for a neurotransmitter. Its effects are generally similar to those of CNS depressants and include ataxia, sleep and anesthesia. It has also been suggested that GHB is a drug of abuse. The present experiment was designed to evaluate GHB in procedures predictive of abuse and dependence potential in rhesus monkeys. Three monkeys were surgically prepared with indwelling silicone venous catheters and allowed to self-administer methohexital or saline in twice-daily experimental sessions. Other groups of monkeys were trained in drug discrimination paradigms to discriminate D-amphetamine (AMPH; n = 4), pentobarbital (PB; n = 3) or triazolam (n = 3) from saline. Another group was maintained on diazepam daily and trained to discriminate flumazenil from saline (n = 2). GHB (0.01-10 mg/kg per injection) maintained self-administration marginally above saline levels at one dose (3.2 or 10 mg/kg) in two of the three monkeys tested. GHB (1.0-178 mg/kg, subcutaneously (s.c.) or intragastrically (i.g.)) did not reliably substitute as a discriminative stimulus for any of the training conditions. Taken together with previous results, the present experiment suggests that GHB has, at most, low potential for abuse.

Opioid enhancement of the discriminative stimulus effects of cocaine: evidence for involvement of mu and delta opioid receptors

Previous research in squirrel monkeys has shown enhancement of the discriminative stimulus effects of cocaine by mu-opioid agonists, but not by the delta agonist BW373U86. To examine further the role of mu and delta receptor stimulation in the ability of opioid drugs to modulate the discriminative stimulus effects of cocaine, the present study assessed the effects of cocaine alone and combined with SNC 80, a selective high-efficacy delta agonist, and fentanyl, a selective high-efficacy mu agonist. Five adult male squirrel monkeys were trained to discriminate i.m. injections of 0.3 mg/kg cocaine from saline under a fixed-ratio 10 schedule of food presentation. Cumulative doses of cocaine (0.03-1.0 mg/kg) engendered dose-related increases in drug-lever responding to a maximum of 100%, with a decrease in response rate observed at 1.0 mg/kg. Cumulative doses of SNC 80 (0.03-1.0 mg/kg) or fentanyl (0.001-0.01 mg/kg) resulted in a maximum of 22% and 48% drug-lever responding, respectively, accompanied by pronounced decreases in response rate. Administration of either SNC 80 (0.1-1.0 mg/kg) or fentanyl (0.001-0.01 mg/kg) prior to cumulative doses of cocaine produced dose-dependent leftward shifts in the cocaine dose-response function. When the selective delta antagonist naltrindole (1.0 mg/kg) was combined with SNC 80 (1.0 mg/kg) or fentanyl (0.01 mg/kg) prior to cumulative doses of cocaine, the leftward shift of the cocaine dose-response function produced by SNC 80 was blocked, whereas the leftward shift produced by fentanyl was not. By contrast, the mu antagonist naltrexone (0.3 mg/kg) blocked the cocaine-enhancing effects of fentanyl, but not of SNC 80. Combinations of SNC 80 (0.03-0.3 mg/kg) with fentanyl (0.001-0.003 mg/kg) resulted in leftward shifts in the cocaine dose-response function that were comparable in magnitude to the shifts in the cocaine dose-response function produced by either drug alone. These results suggest that opioid enhancement of the discriminative stimulus effects of cocaine is mediated independently by delta- and mu-receptor mechanisms.

Choice maintained by cocaine or food in monkeys: effects of varying probability of reinforcement

Several theoretical models of choice, including matching, maximizing and probability matching, have evolved from the experimental analysis of behavior. However, these models are based largely on results of studies involving non-drug reinforcers. The extent to which the choice to self-administer a drug of abuse conforms to these models is largely unknown. The present study was designed to examine the effects of varying probability of reinforcement of two options on choice by rhesus monkeys maintained under a discrete-trials choice procedure. One group of monkeys (n=5) was prepared with chronic indwelling venous catheters and responding was maintained by i.v. injections of cocaine (0.05 or 0.2 mg/kg per injection). In a second group, behavioral conditions were identical, except that responding was maintained by the delivery of food pellets. Two choice options were available to each group. Each option was maintained by the identical dose of cocaine or amount of food, but with varying probabilities of delivery. The percentage of trials on which monkeys chose option 1 increased with its probability of reinforcement for cocaine (0.05 or 0.2 mg/kg per injection) and this effect did not change with dose of cocaine. When probabilities of reinforcement for option 1/option 2 were 0.75/0.25, choice was exclusive for the higher probability option. Results were identical when behavior was maintained by food. The present experiment supports the conclusion that exclusive choice of the alternative with the higher probability of reinforcement can be extended to cocaine as a reinforcer. The present results also extend the known conditions under which drug-maintained responding is similar to food-maintained responding to situations involving options with different probabilities of reinforcement, i.e., probability choice. Probability matching was not observed, arguing that this model does not predict drug or food choice.

Self-administration of cocaine-heroin combinations by rhesus monkeys: antagonism by naltrexone

Low, nonreinforcing doses of heroin have been shown to shift the dose-response function of cocaine leftward in rhesus monkeys trained under a progressive-ratio schedule of i.v. drug injection. Our study sought to determine 1) whether a reciprocal enhancement of heroin self-administration would be observed when heroin was combined with low, nonreinforcing doses of cocaine, and 2) whether self-administration of cocaine-heroin combinations could be antagonized by the opioid antagonist naltrexone. Rhesus monkeys (n = 4) were prepared with i.v. catheters and trained to self-administer cocaine under a progressive-ratio schedule. The initial response requirement of this schedule was fixed-ratio 120, which doubled across the session to a maximum of 1920. Injections were separated by a 30-min time out. Cocaine dose-response functions (6.4-100 micrograms/kg/injection) for injections/session and breakpoints were monophasic, i.e., increased with dose until responding reached a maximum. Heroin dose-response functions (1.6-25 micrograms/kg/ injection) either increased to a peak and then decreased or reached an asymptote. When nonreinforcing doses of cocaine (3.2-25 micrograms/kg/injection) were combined with heroin, the heroin dose-response function was shifted to the left, without change in maximum injections/session. Pressession treatments with naltrexone (3.2-1600 micrograms/kg, i.m., 10-min presession) antagonized self-administration of heroin and heroin + cocaine combinations in a dose-dependent fashion. However, naltrexone treatment had no effect on cocaine self-administration. Antagonism by naltrexone of self-administration of heroin and heroin + cocaine was surmounted by increasing the dose of heroin either alone or in the heroin + cocaine combination. In vivo apparent pA2 and pKB analyses of these data revealed values of approximately 8.0, consistent with a role for mu opioid receptors in the self-administration of heroin and cocaine-heroin (i.e., "speedball") combinations.

Discriminative stimulus effects of benzodiazepine agonists and partial agonists in pentobarbital-trained rhesus monkeys

The present study examined the relationship between the intrinsic efficacy of benzodiazepines and their ability to reproduce the discriminative stimulus effects of pentobarbital. Rhesus monkeys (n=5) were trained to discriminate pentobarbital (10mg/kg) from saline using a discrete trials shock avoidance procedure (fixed-ratio 5). Drug-appropriate responding and response rate were assessed after intragastric administration of pentobarbital, the high efficacy benzodiazepines triazolam and clonazepam, and the low efficacy benzodiazepines imidazenil and bretazenil. Pentobarbital increased drug-appropriate responding to 100% and decreased the response rate. Triazolam and clonazepam occasioned 80% or greater drug-appropriate responding in four out of four monkeys, and decreased response rate in three out of four monkeys. The antagonist flumazenil decreased drug-appropriate responding and increased response rate after triazolam and clonazepam, consistent with antagonism of the discriminative stimulus and the rate-altering effects of these ligands. Imidazenil occasioned 80% or greater drug-appropriate responding in three out of four monkeys and suppressed the response rate in two out of four monkeys, whereas bretazenil occasioned 80% or greater drug-appropriate responding in two out of four monkeys and decreased the response rate slightly. Bretazenil, when co-administered with a dose of triazolam that produced 90-100% drug-appropriate responding, decreased responding to 0-60% but did not alter the rate-suppressing effects of this ligand. The rank order of potency (based on pmol/kg ED, values) was: triazolam > clonazepam > or = imidazenil > bretazenil. These results demonstrate differences in the pentobarbital-like discriminative stimulus and response rate-decreasing effects among benzodiazepine agonists. Specifically, full agonists engendered pentobarbital-like responding in all subjects with decreased response rates in most subjects, whereas partial agonists engendered pentobarbital-like responding and decreased response rates in subsets of subjects.

Self-administration of cocaine and heroin combinations by rhesus monkeys responding under a progressive-ratio schedule

The present study examined the reinforcing effects of cocaine and heroin, alone and combined, in rhesus monkeys (n = 4) responding under a progressive-ratio (PR) schedule. The PR schedule consisted of five components, each made up of four trials (i.e., 20 trials total), with each trial in a component having the same response requirement. The initial response requirement was fixed-ratio (FR) 120, which doubled across components to a maximum of FR1920. A trial ended with an injection or the expiration of a 15-min limited hold and the inter-trial interval was 30 min. Cocaine dose-response functions (13-400 micrograms/kg per injection) for injections/ session were monophasic, i.e., increased with dose until responding reached an asymptote or a peak. Heroin dose-response functions (1.6-100 micrograms/kg per injection) for injections/session were biphasic functions. i.e., increased to a peak and then decreased, whereas heroin dose-response functions for response rate were monophasic and reached an asymptote. When cocaine (1.6-200 micrograms/kg per injection) was combined with heroin (0.4-6.4 micrograms/kg per injection), low doses of cocaine (3.2-25 micrograms/kg per injection) and heroin (0.4-1.6 micrograms/kg per injection) that did not maintain behavior when tested alone did so when tested in combination. Combination with heroin resulted in a leftward shift in the cocaine dose-response functions, indicating that heroin increased the potency of cocaine as a reinforcer. This heroin-induced increase in cocaine's reinforcing potency may be a contributing factor to abuse of cocaine and heroin combinations (i.e., "speedballs") in humans. However, maximum injections/session for cocaine combined with heroin were not different from cocaine alone, suggesting that the reinforcing efficacy of combinations of cocaine and heroin were not higher than that of cocaine alone.

Locomotor activity and dopamine synthesis following 1 and 15 days of withdrawal from repeated apomorphine treatments

In two experiments, the effects of repeated apomorphine treatments on locomotor activity and terminal field dopamine synthesis was assessed after either a 1- or 15-day withdrawal period. In the first experiment, rats (n = 11/group) were treated with apomorphine (1.0 mg/kg, s.e.) or vehicle and tested for locomotor activity daily for 10 days. Fifteen days after the last repeated treatment, all rats received 1.0 mg/kg apomorphine and were tested for locomotor activity. Locomotor sensitization developed over the 10 day period and was still evident after the 15-day withdrawal period. In the second experiment rats (n = 11/group) were treated with apomorphine (1.0 mg/kg, s.c.) or vehicle and tested for locomotor activity daily for 10 days. Dopamine synthesis was assessed following 1 or 15 days of withdrawal by measuring dihydroxyphenylalanine (DOPA) accumulation (after DOPA decarboxylase inhibition with NSD-1015) in striatum and nucleus accumbens-olfactory tuberele. As in the first experiment, rats treated with repeated apomorphine showed locomotor sensitization over the 10 days, relative to controls. Dopamine synthesis was reliably enhanced in the striatum, but not nucleus accumbens-olfactory tuberele, following both 1- and 15-day withdrawal periods. These results indicate that enhanced basal dopamine synthesis following repeated apomorphine treatments, similar to locomotor sensitization, is a persistent phenomenon.

Discriminative stimulus effects of zolpidem in pentobarbital-trained subjects: II. Comparison with triazolam and caffeine in humans

In the present study, four non-drug-abusing humans were trained to discriminate between a hypnotic dose of pentobarbital, 100 mg, and placebo. After acquiring the pentobarbital-placebo discrimination, a range of doses of zolpidem, triazolam, pentobarbital and caffeine were tested to determine whether they shared discriminative stimulus effects with the training dose of pentobarbital, Zolpidem, a rapid-onset, short-duration, quickly eliminated imidazopyridine hypnotic agent, was tested because its discriminative stimulus effects have been shown to differ from those of classic sedative/hypnotic compounds in rodents, but not in nonhuman primates. Triazolam and caffeine were included as positive and negative controls, respectively. The subject-rated and performance-impairing effects of zolpidem, triazolam, pentobarbital and caffeine were assessed concurrently. These four subjects met the discrimination criterion (> or = 80% correct drug identifications on four consecutive seasions) in 4 to 18 (mean = 8.5) sessions, and the pentobarbital-placebo discrimination was well maintained during a test-of-novel-doses and test-of-novel-drugs phase (i.e., placebo and 100 mg pentobarbital occasioned 0-35% [mean = 17%] and 75-100% [mean = 85%] drug-appropriate responding, respectively). Zolpidem, triazolam and pentobarbital generally produced dose-related increases in pentobarbital-appropriate responding and sedative-like, subject-rated drug effects. Caffeine on average produced low levels of pentobarbital-appropriate responding, although some doses of caffeine produced maximal pentobarbital-appropriate responding in some subjects. Caffeine produced some stimulant-like (e.g., jittery, motivated, nervous and stimulated) subject-rated drug effects. Zolpidem and triazolam, and to a much lesser extent pentobarbital, but not caffeine, impaired performance. These results suggest that humans can acquire and maintain a pentobarbital-placebo discrimination, and this discrimination is pharmacologically specific. These results also suggest that despite the somewhat unique biochemical profile of zolpidem, its discriminative stimulus, subject-rated and performance-pairing effects are similar to those of classic sedative/hypnotic compounds like the barbiturates and benzodiazepines. Finally, the results observed in the present study with zolpidem, triazolam and caffeine demonstrate that the discriminative stimulus effects of drugs observed with nonhuman primates can be systematically replicated in humans.

Discriminative stimulus effects of zolpidem in pentobarbital-trained subjects: I. Comparison with triazolam in rhesus monkeys and rats

The present study compared the discriminative stimulus effects of the imidazopyridine, zolpidem, with a triazolobenzodiazepine, triazolam, in pentobarbital-trained rhesus monkeys and rats. Rhesus monkeys (n = 4), trained to discriminate pentobarbital (10 mg/kg intragastric [i.g.]) from saline under a FR 1 discrete-trials shock avoidance procedure, were given zolpidem (0.10-30 mg/kg i.g.) or triazolam (0.01-0.3 mg/kg i.g.). Both zolpidem and triazolam produced dose-dependent increases in pentobarbital-appropriate responding that reached 80% or greater at the highest doses tested. Zolpidem, but not triazolam, increased latency to respond in a dose-dependent manner. Sprague-Dawley rats (n = 12), trained to discriminate pentobarbital (8.0 mg/kg i.p.) from saline under a FR 10 schedule of food reinforcement, were given zolpidem (0.50-4.0 mg/kg i.p.; 5-, 15- and 45-min pretreatment) or triazolam (0.025-0.20 mg/kg i.p., 15-min pretreatment). Zolpidem occasioned intermediate drug-appropriate responding (maximum group mean = 48%) at the 5- and 15-min pretreatment times and no drug-appropriate responding at the 45-min pretreatment time. In contrast, triazolam occasioned > or = 80% pentobarbital-appropriate responding at 0.10 and 0.20 mg/kg. Both zolpidem and triazolam produced dose-dependent decreases in the rate of responding. The rate-decreasing effects of zolpidem were maximal at the 5-min pretreatment time and had dissipated after the 45-min pretreatment time. Further studies were conducted in rats to equate procedural variables between the monkey and rat studies. When the FR was reduced from 10 to 1, zolpidem occasioned 26 to 62% pentobarbital-appropriate responding over a dose range of 1.0 to 6.0 mg/kg i.p. After i.g. administration, zolpidem occasioned 100% drug-appropriate responding at the highest dose tested (6.0 mg/kg); however, only two of seven rats responded. Taken together, these data raise the possibility of a species difference between non-human primates and rats in the pentobarbital-like discriminative stimulus effects of zolpidem.

Assessment of benzodiazepine receptor heterogeneity in vivo: apparent pA2 and pKB analyses from behavioral studies

The purpose of this review was to establish in vivo apparent pA2 and pKB values for antagonism of the discriminative stimulus effects of benzodiazepine ligands, and to compare these values to those obtained from other behavioral procedures. Articles were chosen from the Medline data base from January 1976 to August 1995. This literature consisted of studies with flumazenil (Ro 15-1788) as the antagonist, as well as other benzodiazepine ligands (beta-carbolines, CGS 9896, CGS 8216). The dose which occasioned 50% of the maximal response (ED50) was obtained from values estimated from the graphs presented in each article. These ED50 values were used to conduct apparent pKB and apparent pA2 analyses. Apparent pA2 values for antagonism of the discriminative stimulus effects of diazepam in rats were the following (antagonist, pA2, slope): flumazenil, 4.7, -1.5; beta-CCE, 4.0, -3.0; beta-CCtB, 5.0, -2.2. The apparent pA2 value for CGS 8216 antagonism of the discriminative stimulus effects of diazepam in rats was 5.74, -2.22 (reported in Shannon and Davis 1984). The mean apparent pA2 value for flumazenil antagonism of the discriminative stimulus effects of diazepam in rhesus monkeys was 6.55, with a mean slope of -1.42. Analysis of baboon data from Ator and Griffiths (1986) revealed apparent pKB values for flumazenil antagonism of the discriminative stimulus effects of lorazepam that were lower than the pKB values for either zopiclone or CL 218,872. Analyses of the pKB data also revealed the following: no effect of route of administration (rat, PO versus IP; baboon, PO versus IM), no effect of pretreatment time (grouped into two categories: 10-20 min and 40-70 min, in rats and non-human primates), and a species effect (pKB values for rats were reliably lower than either non-human primates or pigeons, rhesus monkeys were lower than baboons). The apparent pA2 and pKB values obtained in the present review were similar across behavioral assays, except that, in squirrel monkeys, flumazenil pKB values for antagonism of benzodiazepine-induced decreases in schedule-controlled behavior were lower than pKB values obtained from drug discrimination studies. This review provides apparent pA2 values for antagonism of the discriminative stimulus effects of benzodiazepine ligands and provides evidence from pKB analyses consistent with functional heterogeneity of benzodiazepine receptors in vivo.

Zipeprol: preclinical assessment of abuse potential

Zipeprol was evaluated in a number of in vitro and in vivo assays predictive of stimulant, depressant, or opioid abuse potential. Zipeprol had affinity for mu and kappa opioid binding sites as well as sigma binding sites. However, it failed to exert opioid-like agonist actions in rodents, and did not attenuate withdrawal signs in morphine- or pentobarbital-dependent rats. Zipeprol did not substitute for either amphetamine or pentobarbital in drug discrimination assays in rhesus monkeys. On the other hand, it suppressed morphine withdrawal signs in rhesus monkeys in two assays, and it acted as a quadazocine-sensitive reinforcer in monkeys trained to self-inject alfentanil. Zipeprol also acted as a reinforcer in monkeys trained to self-inject methohexital. In a dose range of 10-18 mg/kg, zipeprol induced convulsions in monkeys. Zipeprol appears to have abuse potential and a novel spectrum of action involving both opioid and non-opioid effects.

Parametric analysis of cocaine self-administration under a progressive-ratio schedule in rhesus monkeys

The present study was designed to investigate parameters and quantitative analysis of cocaine self-administration under a progressive-ratio (PR) schedule of reinforcement, with the goal of enhancing the resolution of PR schedules for measuring reinforcing efficacy. Six rhesus monkeys were prepared with chronic intravenous catheters and trained to self-administer cocaine under a PR schedule. The schedule consisted of five components, each made up of four trials (i.e., 20 trials total). Each trial within a component had the same response requirement. Three initial response requirements were tested: fixed-ration (FR) 60, FR 120 and FR 240. The response requirements doubled in successive components to a maximum of FR 960, FR 1920 or R 3840, respectively, in the fifth component. A trial ended with an injection or the expiration of a 12- or 24-min limited hold (LH). The inter-trial interval (ITI) was 15 or 30 min. Four dependent measures were assessed: break point (last FR completed), injections/session, responses/session and response rate (responses/s). For the three initial FRs, the break point, number of injections/session, responses/session and rate increased with dose of cocaine (0.013-0.1 mg/kg per injection) at both ITI/LH values. At the ITI15/LH12, responding decreased at higher doses, i.e., the dose-response functions were biphasic. In contrast, at the ITI30/LH24, responding reached an asymptote at higher doses. In general, cocaine maintained significantly higher break points, injections/session, responses/session and rate at ITI30/LH24 than at ITI15/LH12. However, at both ITI/LHs, as initial FR was increased, injections/session at the higher doses decreased while break point, total responses/session and rate did not change. A ceiling on performance, as assessed by break point, total responses/session and response rate, may have limited the number of cocaine injections an animal could take in a session. The results of this study indicate that optimal conditions for measuring the reinforcing efficacy of cocaine were obtained at the longer ITI/LH and at initial FRs above 60.

Repeated 7-OH-DPAT treatments: behavioral sensitization, dopamine synthesis and subsequent sensitivity to apomorphine and cocaine

Male Wistar rats (250-350 g) were injected (SC) daily with the putative selective dopamine D3 receptor agonist, 7-OH-DPAT (0.01, 0.10, or 1.0 g/kg) or vehicle for 10 days. Fifteen minutes after each injection, the rats were tested for locomotor activity in photocell arenas for 20 min or 2 h. In two experiments, following this subchronic treatment, all rats received a challenge injection of apomorphine (1.0 mg/kg, SC), or cocaine (10 mg/kg, IP) on day 11, and were tested for locomotor activity. In a third experiment, dopamine synthesis in striatal and mesolimbic (nucleus accumbens-olfactory turbercle) tissue was assessed following acute or chronic 7-OH-DPAT treatments by measuring the accumulation of dihydroxyphenylalanine (DOPA) after treatment with a DOPA decarboxylase inhibitor. Major findings were as follows: a) acute 7-OH-DPAT treatment produced a dose-dependent decrease in locomotor activity; b) when tested for 2 h, the 1.0 mg/kg dose of 7-OH-DPAT produced a progressively greater increase in activity across the 10 test days (i.e., behavioral sensitization); c) subchronic treatment with 7-OH-DPAT did not result in cross-sensitization to either apomorphine or cocaine; d) acute treatment with the 1.0 mg/kg dose of 7-OH-DPAT significantly decreased dopamine synthesis in both striatal and mesolimbic regions; and e) chronic 7-OH-DPAT treatments did not affect basal dopamine synthesis in either brain region. Although the behavioral effects of 7-OH-DPAT were similar to the reported effects of the D2/D3 dopamine agonist quinpirole, the effects of repeated 7-OH-DPAT treatments differed from those of quinpirole in terms of cross-sensitization and basal dopamine synthesis. These results suggest that locomotor inhibition produced by low doses of 7-OH-DPAT is not related to dopamine autoreceptor stimulation, and the development of behavioral sensitization to high doses of 7-OH-DPAT is not due to the development of dopamine autoreceptor subsensitivity.

Cocaine-induced behavioral sensitization: effects of haloperidol and SCH 23390 treatments

The objective of this study was to determine whether the development of behavioral sensitization to cocaine could be prevented by high doses of the dopamine receptor antagonists haloperidol and SCH 23390. In two experiments, male Wistar rats were injected daily for 4 days with either cocaine (15 mg/kg, IP) or vehicle in combination with haloperidol (1.0 mg/kg, IP), SCH 23390 (0.5 mg/kg, SC), or vehicle. After the daily injections, the rats were tested for locomotor activity in photocell arenas. At 24 h after the last preexposure test session, all rats were given a challenge injection of cocaine (15 mg/kg, IP) and tested for activity. Cocaine treatments produced a greater relative increase in locomotor activity with repeated exposure compared to vehicle treatments (i.e., sensitization). Moreover, the acute activating effects of cocaine over days were blocked by both haloperidol and SCH 23390. The coadministration of haloperidol, but not SCH 23390, blocked the development of behavioral sensitization to cocaine. That is, after the cocaine challenge injection, rats pretreated with SCH 23390 and cocaine did not differ from rats preexposed only to cocaine, whereas rats pretreated with haloperidol and cocaine did not differ from rats pretreated only with vehicle. Pretreatment with haloperidol or SCH 23390 without cocaine enhanced the locomotor-activating effects of the subsequent cocaine challenge injection. These findings suggest that cocaine-induced behavioral sensitization may develop as a result of repeated dopamine D1- or D2-type receptor stimulation, and that brief dopamine antagonist treatments enhance subsequent behavioral sensitivity to cocaine.

Unit-price analysis of opioid consumption by monkeys responding under a progressive-ratio schedule of drug injection

Several reports have indicated that drug consumption in self-administration procedures is a function of the ratio of the instrumental requirement to the dose of drug, a quantity termed unit price. We evaluated three predictions from this unit-price model in a reanalysis of data on opioid self-administration in rhesus monkeys responding under a progressive-ratio schedule (Hoffmeister, 1979). We evaluated whether consumption was inversely related to unit price, and compared the goodness of fit of an equation devised by Hursh, Raslear, Shurtleff, Bauman, and Simmons (1988) to that of a linear model predicting consumption as a function of dose. We also tested whether consumption was constant when the same unit price was comprised of different combinations of dose and instrumental requirement. Consumption declined overall as unit price increased. The equation devised by Hursh et al. and the linear model based on dose fit the data equally well. Drug consumption was not uniform at a given unit price. The analyses suggest limits on the unit-price model as a characterization of drug consumption.

Ascorbate potentiates amphetamine-induced conditioned place preference and forebrain dopamine release in rats

In order to evaluate the effects of ascorbate, which is known to modulate dopamine neurotransmission, on the reinforcing effects of amphetamine, we coadministered ascorbate and amphetamine during the acquisition of conditioned place preference (CPP) in rats. Our results indicate that 100 mg/kg ascorbate potentiates the CPP induced by 0.5 mg/kg, but not 1.0 mg/kg, amphetamine. A higher dose of ascorbate (500 mg/kg) did not influence the CPP induced by either dose of amphetamine. In vitro release assays revealed that, whereas ascorbate alone (0.01-1.0 mM) did not influence striatal dopamine levels, this vitamin potentiated amphetamine-induced dopamine release in both the nucleus accumbens and neostriatum. Collectively, these results raise the possibility that ascorbate potentiates amphetamine-induced CPP by increasing the ability of this psychostimulant to release dopamine.

Repeated quinpirole treatment: locomotor activity, dopamine synthesis, and effects of selective dopamine antagonists

Repeated treatment with the non-selective dopamine agonist apomorphine results in behavioral sensitization and enhanced dopamine synthesis in dopamine projection fields. To examine the role of D2-type dopamine receptors in modulating these effects, the present experiment assessed the effects of repeated treatment with the D2-type agonist quinpirole on locomotor activity and dopamine synthesis. In the first experiment, rats were treated with vehicle or one of two doses (0.3 or 3.0 mg/kg) of quinpirole for 8 days. Daily measures of locomotor activity revealed an initial suppression of activity produced by quinpirole which dissipated over the 8 days of treatment. A trend for an increase in activity for 3.0 mg/kg quinpirole compared to vehicle was obtained on day 8. Twenty-four hours after cessation of treatment, dopamine synthesis, measured as accumulation of 3,4-dihydroxyphenylalanine (DOPA) after treatment with the DOPA decarboxylase inhibitor NSD-1015, was enhanced in the striatum, but not nucleus accumbens-olfactory tubercle (NAOT) or ventral mesencephalon (VM). In Experiment 2, rats were treated for 8 days with vehicle, 3.0 mg/kg quinpirole or the D1 antagonist SCH 23390 (0.5 mg/kg) in a two (vehicle or quinpirole) x two (vehicle or SCH 23390) design. Quinpirole-alone treatment resulted in a reduction of the locomotor suppressant effects of the drug. SCH 23390-alone and quinpirole-SCH 23390 combined treatment resulted in decreased activity compared to the vehicle control group that did not change across days. DOPA accumulation was enhanced in the striatum and NAOT after quinpirole treatment; however, SCH 23390 had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Environmental enrichment attenuates locomotor sensitization, but not in vitro dopamine release, induced by amphetamine

Rats were raised from weanling until young adulthood in either an enriched condition (EC) or isolated condition (IC). Following this, the locomotor and rewarding effects of amphetamine were determined using the conditioned place preference (CPP) paradigm. EC rats were more sensitive to the acute locomotor stimulant effect and rewarding effect of amphetamine relative to IC rats. In contrast, EC rats were less sensitive than IC rats to the locomotor sensitization effect obtained across repeated amphetamine injections. To determine the effect of environmental enrichment on alteration of brain dopamine (DA) function induced by amphetamine, the effect of amphetamine on electrically evoked release of DA and dihydroxyphenylacetic acid (DOPAC) was determined in vitro using tissue slices from the nucleus accumbens and striatum of EC and IC rats. No differences between EC and IC rats in release of DA or DOPAC were evident, suggesting that the environmentally induced difference in sensitivity to the behavioral effects of amphetamine involves a neural mechanism extrinsic to the mesolimbic and nigrostriatal terminal field regions.

Conditioned place preference using opiate and stimulant drugs: a meta-analysis

A meta-analysis was conducted on the data obtained from published articles that have used the conditioned place preference (CPP) paradigm to assess the rewarding effects of morphine, heroin, amphetamine and cocaine in rats. Using a histogram analysis of the data, significant dose-effect curves were evident with all of the drugs examined, except for cocaine. Analysis of the data also revealed that several methodological variables moderated the effect size for CPP, at least with some of the drugs examined. In particular, the following methodological variables significantly moderated CPP effect size: strain of rat used; housing condition (single or group cages); type of apparatus (2 or 3 compartments); preconditioning test (present or absent); route of drug administration; intervening saline trials (present or absent); conditioning trial duration; and drug compartment (nonpreferred, counterbalanced or white). No significant effect size differences were evident using sex, number of drug trials, or test duration as moderator variables in the analyses. These meta-analytic results may be useful to investigators for maximizing the effect size of drug-induced CPP.

Dissociation of buprenorphine-induced locomotor sensitization and conditioned place preference in rats

The locomotor and rewarding effects of the opioid mixed agonist-antagonist buprenorphine were assessed in a conditioned place preference (CPP) experiment. Separate groups of rats were given one of three doses of buprenorphine (0.3, 1.0 or 3.0 mg/kg) or saline paired with the white compartment of a CPP apparatus. The following day, all rats received saline paired with the black compartment. After six conditioning trials, rats were given free access to all compartments of the CPP apparatus. Horizontal activity data obtained during conditioning revealed increased activity (i.e., behavioral sensitization) for the three doses on trial 6. Vertical activity data revealed significant increases on trial 6 for the 1.0 and 3.0 mg/kg doses only. Significant CPP was obtained with the 0.3 mg/kg and 1.0 mg/kg doses of buprenorphine, but not with the 3.0 mg/kg dose. These data indicate that buprenorphine elicits locomotor sensitization after repeated exposures that follows a linear dose-response relationship. In contrast, these data suggest that the rewarding effects of buprenorphine follow an inverted U-shaped dose-response curve.

Age-dependent differences in the rate of recovery of striatal dopamine D1 and D2 receptors after inactivation with EEDQ

Recovery of striatal dopamine D1 and D2 binding sites in 10-, 16-, and 39-day-old rats was measured 1, 2, 4, and 8 days after irreversible antagonism with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). Ontogenetic and EEDQ-induced changes in D1 and D2 binding sites were determined by Scatchard analyses using six concentrations of either [3H]SCH 23390 or [3H]spiperone. Twenty-four hours after EEDQ (7.5 mg/kg) treatment, a significant depletion of both dopamine D1 and D2 receptors was found for all age groups; however, the magnitude of the depletion was greater in 39-day-old rats than in the two preweanling age groups. Both 16- and 39-day-old rats showed significant recovery of dopamine D1 and D2 receptors by the eighth day after EEDQ treatment, but the 16-day-old rats showed a faster recovery of dopamine D1 receptors than did the 39-day-olds. Unexpectedly, 10-day-old rats did not show any evidence of receptor recovery, as the percent control values for these animals did not change across the 8-day recovery period. Pretreatment with the dopamine D1 receptor antagonist SCH 23390 and the dopamine D2 receptor antagonist sulpiride was sufficient to protect dopamine D1 and D2 receptors from EEDQ-induced inactivation. Protein values and receptor affinity (pKd values) were not affected by EEDQ treatment at any of the ages tested. Therefore, these results indicate that the rate of dopamine receptor repopulation varies across ontogeny, with 10-day-old rats exhibiting slower recovery than older rat pups or postweanling rats.

Impaired supersensitivity to morphine following chronic naltrexone treatment in senescent rats

Chronic administration of opiate antagonists produces an increase in the density of opiate receptors, as well as an enhanced sensitivity to the analgesic and locomotor depressant effects of morphine. The present study assessed whether aging alters these regulatory processes. Young (3-4 months), middle-aged (10-11 months), and senescent (25-30 months) rats were implanted subdermally with slow-release naltrexone pellets or were given sham surgery. The pellets were removed 10 days later. Twenty-four hours after pellet removal, morphine-induced (5 mg/kg, SC) analgesia and locomotor activity were assessed. Young and middle-aged rats treated with naltrexone showed enhanced sensitivity to the analgesic and locomotor activity depressant effects of morphine relative to age-matched controls. In contrast, senescent rats treated with naltrexone did not differ from age-matched controls in their response to morphine. The density of opiate receptors labeled with 3H-naloxone was measured in the anterior striatum. Both young and senescent rats treated with naltrexone exhibited an increase in opiate receptor density relative to age-matched controls. The results indicate that senescent rats are capable of up-regulating opiate receptors following chronic naltrexone treatment but do not exhibit the corresponding functional supersensitivity to morphine.

The effect of environmental enrichment on amphetamine-stimulated locomotor activity, dopamine synthesis and dopamine release

In two separate experiments, rats were raised in either an enriched condition (EC) or impoverished condition (IC) from 21 to 60 days of age. Experiment 1 assessed amphetamine-stimulated locomotor activity and in vivo dopamine (DA) synthesis and metabolism in the nucleus accumbens (NA) and striatum (Str). In Experiment 2, amphetamine-stimulated DA release in the NA and Str was assessed in vitro. The results showed that EC rats have lower basal levels of locomotor activity than IC rats. However, in the presence of amphetamine, EC rats showed a greater increase in locomotion over IC when compared to their own controls. Concomitant with this behavioral difference, EC rats also showed an enhanced neurochemical response to amphetamine in vivo. That is, relative to IC rats, amphetamine produced a greater synthesis of DA in the Str of EC rats, as well as a greater metabolism of DA in the NA of EC rats. In the in vitro DA release experiment, EC rats had a lower concentration of tissue DA than IC. However, in contrast to the in vivo experiment, there were no significant differences between EC and IC rats in amphetamine-stimulated release of DA in vitro in either the Str or NA. The failure of amphetamine to produce differential neurochemical effects in EC and IC rats in vitro may be because this experiment eliminated either pharmacokinetic effects or neurochemical differences in brain regions outside the NA and Str.