NLX-112, a highly selective 5-HT1A receptor biased agonist, does not exhibit misuse potential in male rats or macaques.

NLX-112 (a.k.a. F13640 or befiradol) exhibits nanomolar affinity, exceptional selectivity and biased agonism at serotonin 5-HT1A receptors. NLX-112 displays robust analgesic activity in a number of rodent models of pain, and is currently developed as a treatment for l-DOPA-induced dyskinesia (LID) in Parkinson's disease (PD) patients. Noteworthy, PD patients can suffer from comorbid chronic pain, thus necessitating the use of analgesic drugs, such as opioids, which have potential for misuse. Additionally, dopamine agonists used to treat PD can produce cocaine-like effects in preclinical assays of misuse potential. The present study investigated whether NLX-112 possesses misuse potential of its own using two behavioural assays routinely used for this purpose: intracranial self-stimulation (ICSS) in rats, and cocaine discrimination in macaque monkeys. In rats, low doses of NLX-112 (0.03 and 0.1 mg/kg p.o.) did not alter ICSS frequency-rate curves, while higher doses (0.3 and 1.0 mg/kg) shifted the curve to the right and flattened it, i.e., reduced ICSS. As expected, cocaine (10 mg/kg i.p.) shifted the curve to the left, i.e., facilitated ICSS, but NLX-112 (0.03 and 0.1 mg/kg p.o.) did not further enhance cocaine-induced facilitation of ICSS. In monkeys trained to discriminate cocaine (0.4 mg/kg i.m.) from saline, NLX-112 (0.01-0.1 mg/kg p.o.) did not substitute for cocaine. Taken together, these results suggest that NLX-112, at doses displaying anti-dyskinetic activity in rat, marmoset and macaque models of LID, is free from misuse potential. From a translational perspective, this is a desirable property for a compound destined to be used in PD patients, who can suffer from comorbid chronic pain necessitating the use of potentially misused analgesic drugs.

Behavioral metabolomics analysis identifies novel neurochemical signatures in methamphetamine sensitization.

Behavioral sensitization has been widely studied in animal models and is theorized to reflect neural modifications associated with human psychostimulant addiction. While the mesolimbic dopaminergic pathway is known to play a role, the neurochemical mechanisms underlying behavioral sensitization remain incompletely understood. In this study, we conducted the first metabolomics analysis to globally characterize neurochemical differences associated with behavioral sensitization. Methamphetamine (MA)-induced sensitization measures were generated by statistically modeling longitudinal activity data for eight inbred strains of mice. Subsequent to behavioral testing, nontargeted liquid and gas chromatography-mass spectrometry profiling was performed on 48 brain samples, yielding 301 metabolite levels per sample after quality control. Association testing between metabolite levels and three primary dimensions of behavioral sensitization (total distance, stereotypy and margin time) showed four robust, significant associations at a stringent metabolome-wide significance threshold (false discovery rate, FDR <0.05). Results implicated homocarnosine, a dipeptide of GABA and histidine, in total distance sensitization, GABA metabolite 4-guanidinobutanoate and pantothenate in stereotypy sensitization, and myo-inositol in margin time sensitization. Secondary analyses indicated that these associations were independent of concurrent MA levels and, with the exception of the myo-inositol association, suggest a mechanism whereby strain-based genetic variation produces specific baseline neurochemical differences that substantially influence the magnitude of MA-induced sensitization. These findings demonstrate the utility of mouse metabolomics for identifying novel biomarkers, and developing more comprehensive neurochemical models, of psychostimulant sensitization.

Preclinical evaluation of the abuse potential of Pitolisant, a histamine H3 receptor inverse agonist/antagonist compared with Modafinil.

BACKGROUND AND PURPOSE: Pitolisant, a histamine H3 receptor inverse agonist/antagonist is currently under Phase III clinical trials for treatment of excessive daytime sleepiness namely in narcoleptic patients. Its drug abuse potential was investigated using in vivo models in rodents and monkeys and compared with those of Modafinil, a psychostimulant currently used in the same indications. EXPERIMENTAL APPROACH: Effects of Pitolisant on dopamine release in the nucleus accumbens, on spontaneous and cocaine-induced locomotion, locomotor sensitization were monitored. It was also tested in three standard drug abuse tests i.e. conditioned place preference in rats, self-administration in monkeys and cocaine discrimination in mice as well as in a physical dependence model. KEY RESULTS: Pitolisant did not elicit any significant changes in dopaminergic indices in rat nucleus accumbens whereas Modafinil increased dopamine release. In rodents, Pitolisant was without any effect on locomotion and reduced the cocaine-induced hyperlocomotion. In addition, no locomotor sensitization and no conditioned hyperlocomotion were evidenced with this compound in rats whereas significant effects were elicited by Modafinil. Finally, Pitolisant was devoid of any significant effects in the three standard drug abuse tests (including self-administration in monkeys) and in the physical dependence model. CONCLUSIONS AND IMPLICATIONS: No potential drug abuse liability for Pitolisant was evidenced in various in vivo rodent and primate models, whereas the same does not seem so clear in the case of Modafinil.

Current evidence supporting a role of cannabinoid CB1 receptor (CB1R) antagonists as potential pharmacotherapies for drug abuse disorders.

Since the discovery of the cannabinoid CB1 receptor (CB1R) in 1988, and subsequently of the CB2 receptor (CB2R) in 1993, there has been an exponential growth of research investigating the functions of the endocannabinoid system. The roles of CB1Rs have been of particular interest to behavioral pharmacologists because of their selective presence within the central nervous system (CNS) and because of their association with brain-reward circuits involving mesocorticolimbic dopamine systems. One potential role that has become of considerable recent focus is the ability of CB1Rs to modulate the effects of drugs of abuse. Many drugs of abuse elevate dopamine levels, and the ability of CB1R antagonists or inverse agonists to attenuate these elevations has suggested their potential application as pharmacotherapies for treating drug abuse disorders. With the identification of the selective CB1R antagonist, SR141716, in 1994, and its subsequent widespread availability, there has been a rapid expansion of research investigating its ability to modulate the effects of drugs of abuse. The preliminary clinical reports of its success in retarding relapse in tobacco users have accelerated this expansion. This report critically reviews preclinical and clinical studies involving the ability of CB1R antagonists to attenuate the effects of drugs of abuse, while providing an overview of the neuroanatomical and neurochemical points of contact between the endocannabinoid system and systems mediating abuse-related effects.

Modulation of the discriminative stimulus effects of mu opioid agonists in rats: I. Effects of dopamine D2/3 antagonists.

Mu opioid receptor agonists such as morphine stimulate the release of dopamine (DA) in various brain regions. These increases in DA are thought to be involved in some of the behavioral effects of mu agonists. The present study was designed to examine the modulatory actions of two D2/3 antagonists (nafadotride and eticlopride), the D2/3 partial agonist BP897, the D1/2 antagonist flupenthixol, and the D1 antagonist SCH23390 on the discriminative stimulus effects of the mu partial agonist nalbuphine and the higher-efficacy mu agonists heroin, methadone and morphine, in rats trained to discriminate heroin from water. Both nafadotride and eticlopride attenuated the effects of the mu agonists, whereas BP897 was effective against nalbuphine and partially effective against morphine. Flupenthixol attenuated the heroin-like discriminative stimulus effects of heroin and morphine, although not as completely as nafadotride or eticlopride. SCH23390 was least effective and produced little attenuation. These results demonstrate that the discriminative stimulus effects of mu agonists in rats are more readily attenuated by drugs that block D2-like, rather than D1-like, receptors.

Modulation of the discriminative stimulus effects of mu opioid agonists in rats: II. Effects of dopamine D2/3 agonists.

Dopamine (DA) D2/3 receptor agonists have been shown to attenuate the behavioral effects of mu opioid agonists. This study was designed to examine the modulatory actions of the D2/3 agonists quinelorane, quinpirole and (+/-)-2-dipropylamino-7-hydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide (7-OH-DPAT) on the discriminative stimulus effects of the higher-efficacy mu agonists heroin, methadone and morphine, as well as the lower-efficacy agonist nalbuphine, in rats trained to discriminate heroin from water. All three D2/3 agonists attenuated the heroin-like discriminative stimulus effects of morphine, methadone and nalbuphine, whereas quinpirole and 7-OH-DPAT, but not quinelorane, effectively attenuated the discriminative stimulus effects of heroin. Each D2/3 agonist administered alone occasioned water-appropriate responding and decreased rates of responding. These results extend previous findings, which demonstrated that activation of D2/3 receptors attenuates the antinociceptive effects of mu agonists, to now include their discriminative stimulus effects as well. The exact nature of this modulation of opioid effects by dopamine agonists is unclear, and may include neurochemical interactions as well as psychological mechanisms such as perceptual masking.

The selective glycine antagonist gavestinel lacks phencyclidine-like behavioral effects.

Gavestinel [GV150526A; ( E)-3[(phenylcarbamoil)ethenyl]-4,6-dichloroindole-2-carboxylic acid sodium salt] is a selective antagonist at the strychnine-insensitive glycine site of the -methyl-D-aspartate (NMDA) receptor. It was tested for its ability to substitute for phencyclidine (PCP) in rats and rhesus monkeys trained to discriminate PCP from saline, under a two-lever fixed-ratio (FR) food reinforcement schedule, and for its ability to maintain responding in rhesus monkeys trained to self-administer PCP under a FR reinforcement schedule. No PCP-lever responding was observed after gavestinel (1-56 mg/kg i.p.) administration to rats discriminating PCP (2.0 mg/kg i.p.) from saline. The highest dose of gavestinel (100 mg/kg i.p.) tested eliminated responding. Likewise, no PCP-lever responding was observed after gavestinel (1-30 mg/kg s.c.) administration to rhesus monkeys discriminating PCP (0.08 or 0.1 mg/kg i.m.) from saline; the highest dose of gavestinel (30 mg/kg s.c.) tested reduced response rates to approximately 50% of those observed after its vehicle ( -cyclodextrin in 0.9% saline). Gavestinel (0.1-1 mg/kg per i.v. infusion) was not self-administered by rhesus monkeys that reliably self-administered PCP (0.0056 or 0.01 mg/kg per i.v. infusion). Infusion rates at the highest dose were typically lower than those for vehicle or saline, suggesting behavioral activity. Together, these results suggest that at behaviorally active doses gavestinel is not PCP-like and is likely to have low abuse liability.

Competitive and noncompetitive NMDA antagonist effects in rats trained to discriminate lever-press counts.

The glutamate activated N-methyl-D-aspartate (NMDA) receptor may play a role in short-term memory processing. Among the evidence for this is that NMDA antagonists can impair accuracy in fixed consecutive number (FCN) tasks. This study was designed to further characterize this effect by examining NMDA antagonists differing in their cellular mechanisms of action. Rats were trained to respond under an FCN operant schedule, which required eight presses on one lever (counting lever) before one press at an alternate lever (reinforcement lever) would produce food reinforcement. The effects of three noncompetitive [MK-801 (0.01-0.56 mg/kg); phencyclidine (0.3-3.0 mg/kg); memantine (1-10 mg/kg)] and two competitive [SDZ EAA 494 (0.3-3.0 mg/kg) and NPC 17742 (2.0-16 mg/kg)] NMDA antagonists were analyzed. MK-801 and phencyclidine decreased accuracy at doses not reducing response rates. Memantine, and both of the competitive antagonists, also reduced accuracy, but did so only at doses that markedly reduced response rates. These results suggest that both the affinity and the site bound on the NMDA glutamate receptor by antagonists can determine their effects on FCN performance. Subsequent studies investigated whether SCH 23390, a dopamine D1 receptor antagonist, and NMDA could modulate the effects by phencyclidine and SDZ EAA 494, respectively, on FCN performance.

Opioid-NMDA receptor interactions may clarify conditioned (associative) components of opioid analgesic tolerance.

Recent evidence suggests that acute administration of opioid analgesic drugs (such as morphine or heroin) produces delayed hyperalgesia. This hyperalgesic response is likely to result from hyperactivation of NMDA receptors triggered by stimulation of opioid receptors and may mediate acute tolerance. In support of this hypothesis, blockade of NMDA receptors attenuates opioid-induced delayed hyperalgesia and prolongs the duration of antinociceptive activity of morphine. Furthermore, the NMDA receptor-induced hyperalgesia is likely an unconditioned response to opioid receptor stimulation that becomes spatiotemporally associated with environmental cues accompanying repeated opioid exposure. This hypothesis conforms to the traditional Pavlovian requirement for conditioned and unconditioned responses to be qualitatively similar. In support of the role of NMDA receptor hyperactivation in morphine tolerance, NMDA receptor antagonists have been shown to block development of analgesic tolerance induced by repeated exposures to morphine. The view of the conditioned nature of opioid tolerance may be significantly extended by assuming that upon repeated drug administration an early-onset effect of a drug may become a predictive stimulus for a later-onset effect and, consequentially, it may become empowered to elicit the later-onset effect itself. Such 'intra-drug' conditioning hypothesis is well in line with the current experimental evidence but further studies will be needed to verify it directly.

Structure-activity relationship studies of 4-[2-(diphenylmethoxy)ethyl]-1-benzylpiperidine derivatives and their N-analogues: evaluation of O-and N-analogues and their binding to monoamine transporters.

In our effort to develop a pharmacotherapy for the treatment of cocaine addiction, we embarked on synthesizing novel molecules targeting the dopamine transporter (DAT) molecule in the brain as DAT has been implicated strongly in the reinforcing effect of cocaine. Our previously developed DAT-selective piperidine analogue, 4-[2-(diphenylmethoxy)ethyl]-1-benzylpiperidine, was the basis for our current structure-activity relationship (SAR) studies exploring the significance of the contribution of the benzhydryl O- and N-atoms in these molecules in interacting with the DAT. Thus, we replaced the benzhydryl O-atom with an N-atom, altered the location of the benzhydryl N-atom to an adjacent position, and in one other occasion converted the benzhydryl O-ether linkage into an oxime-type derivative. Furthermore, we also evaluated the important contribution of the piperidine N-atom to binding by altering its pK(a) value chemically. Novel analogues were tested for potency in inhibiting [3H]WIN 35,428, [3H]citalopram, and [3H]nisoxetine binding at the DAT, serotonin transporter (SERT), and norepinepherine transporter (NET). [3H]DA was used to measure DA reuptake inhibition. The results indicated that the benzhydryl O- and N-atoms are exchangeable for the most part. On the other hand, an enhanced interaction with the SERT was observed when the benzhydryl N-atom moved to an adjacent position (21a; DAT (IC(50)) = 19.7, SERT (IC(50)) = 137 nM, NET (IC(50)) = 1111 nM). In either cases, further alkylation of the N-atom reduced the activity for the transporter. The presence of a powerful electron-withdrawing cyano group in compound 5d expectedly produced the most potent and selective ligand for the DAT (DAT (IC(50)) = 3.7 nM, DAT/SERT = 615). Selected compounds were further analyzed in the dopamine reuptake inhibition assay. Preliminary behavioral assessment of some of the selected compounds in mice indicated that these compounds are much less stimulating when compared with cocaine at comparable doses. In drug-discrimination studies these selected compounds incompletely generalized from the cocaine stimulus in mice trained to discriminate 10 mg/kg cocaine from vehicle.

The D3R partial agonist, BP 897, attenuates the discriminative stimulus effects of cocaine and D-amphetamine and is not self-administered.

Growing attention has been directed towards the potential involvement of the dopamine D3 receptor (D3R) in modulating effects of psychomotor stimulants. BP 897 (N-[4-[4-(2-methoxyphenyl)-1-piperazinyl]butyl]-2-naphthylcarboxamide; aka BP 4.897 and DO897) is amongst the most selective partial agonists for the D3R receptor thus far reported. BP 897 was tested for its ability to support self-administration in rhesus monkeys (0.3-30 microg/kg) and for its ability to produce cocaine- and D-amphetamine-like discriminative stimulus effects in mice (0.01-17 mg/kg i.p.). BP 897 was not self-administered above vehicle and saline levels in any of the four monkeys tested, and produced less than 30% generalization from either the cocaine or D-amphetamine stimulus. When BP 897 was administered before administrations of cocaine or D-amphetamine, percent drug-lever selections were reduced. These results suggest that BP 897 has a profile of activity suitable for consideration as a potential treatment for cocaine dependency disorders.

Cocaine-like discriminative stimulus effects of novel cocaine and 3-phenyltropane analogs in the rat.

RATIONALE: Cocaine dependence is a major health concern and there are no effective pharmacotherapies currently available. Although cocaine is an indirect DA agonist that binds to all three monoamine transporters, there is much evidence implicating a greater role for the dopamine (DAT) than norepinephrine (NET) and serotonin (SERT) transporters in the behavioral effects of cocaine. As such, several groups have developed compounds that exhibit high affinity and selectivity for the DAT. OBJECTIVE: The present investigation examined the cocaine-like discriminative stimulus effects in rats of novel cocaine analogs (RTI 12, 13, 15) and 3-phenyltropane analogs (RTI 111, 112, 113, 114, 117 120, 121, 123, 134 and 152) of which several exhibit high affinity (e.g., <7 nM) and selectivity for the DAT. RESULTS: During dose-effect testing all drugs produced 75-100% cocaine-lever responding. Analyses indicated that the potency of the compounds to produce cocaine-like discriminative stimulus effects was correlated with their affinity for the DAT and the NET but not SERT. Due to the extremely large concentrations (e.g., 10,000-31,024 nM) needed to occupy the NET in vitro, it is doubtful if the doses administered had meaningful NET activity. The selectivity at the DAT, relative to the other transporters, was not indicative of the potency with which these drugs substituted for cocaine. CONCLUSIONS: The cocaine-like discriminative stimulus of the RTI compounds tested appear to be mediated by the DAT, however the extent to which the NET is involved remains unclear. Additionally, several of the RTI compounds had properties consistent with those thought desirable in a pharmacotherapeutic for cocaine dependence.

Effects of N-methyl-D-aspartate receptor antagonists on reinstatement of cocaine-seeking behavior by priming injections of cocaine or exposures to cocaine-associated cues in rats.

The reinstatement of extinguished cocaine self-administration behavior was studied in rats pretreated with N-methyl-D-aspartate receptor antagonists. Rats were trained to self-administer intravenous cocaine (0.32 mg/kg/infusion) during five consecutive daily sessions that were followed by five consecutive daily extinction sessions, during which cocaine was unavailable and cocaine-associated cues (sound and light) were absent. Neither the competitive N-methyl-D-aspartate receptor antagonist D-CPPene (0.3-3 mg/kg) nor the low-affinity N-methyl-D-aspartate receptor channel blocker memantine (1-10 mg/kg) reinstated extinguished responding. Priming injections of intravenous cocaine (Experiment 1), and exposures to cocaine-associated stimuli (buzzer and light; Experiment 2) engendered responding on the reinforced lever in excess of that on the non-reinforced lever. In Experiment 1, administration of D-CPPene or memantine prior to the priming injection of cocaine eliminated the difference between reinforced-lever and non-reinforced-lever response rates. For both D-CPPene and memantine, however, this effect was largely due to increased responding upon the non-reinforced lever rather than to decreased reinforced-lever responding. In Experiment 2, D-CPPene, but not memantine, abolished in a dose-dependent manner the selective increase in reinforced-lever over non-reinforced-lever responding that was induced by exposures to cocaine-related stimuli. This effect of D-CPPene was not due to increased non-reinforced-lever responding. These data help define the boundaries within which N-methyl-D-aspartate receptor antagonists can prevent reinstatement of cocaine-seeking behavior (e.g. type of antagonist used and reinstatement procedure).

Effects of NMDA receptor antagonists on cocaine-conditioned motor activity in rats.

NMDA receptor antagonists have been reported to affect learned behaviors conditioned with abused drugs, with the outcome dependent, in part, on the class of NMDA receptor antagonist used. The present study tested the ability of various site-selective NMDA receptor antagonists to modify cocaine-conditioned motor activity. Two procedures were used for independently assessing drug effects on spontaneous activity and expression of cocaine-conditioned behavior. In the conditioning experiments, rats were administered i.p. injections of cocaine (30 mg/kg) or saline paired with distinctive environments. Spontaneous horizontal activity was dose-dependently enhanced by dizocilpine (0.03-0.3 mg/kg) and memantine (1-30 mg/kg), but not by D-CPPene (3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphonic acid; SDZ EAA 494; 1-10 mg/kg), ACEA-1021 (5-nitro-6,7-dichloro-1,4-dihydro-2, 3-quinoxalinedione; 3-56 mg/kg), or eliprodil (3-30 mg/kg). Higher doses of memantine, D-CPPene (1-10 mg/kg), eliprodil (3-30 mg/kg), or ACEA-1021 reduced vertical activity. Following five cocaine-environment pairings, rats displayed significant increases in motor activity when exposed to the cocaine-paired environment. The following antagonists were administered prior to the conditioning test: dizocilpine (MK-801; 0.03-0.1 mg/kg), memantine (1-10 mg/kg), D-CPPene (0.3-3 mg/kg), ACEA-1021 (3-10 mg/kg), and eliprodil (1-10 mg/kg). Of these, memantine, ACEA-1021 and, to the lesser degree, eliprodil attenuated expression of cocaine-conditioned motor activity at doses that did not significantly affect spontaneous motor activity. These results show that cocaine-conditioned behaviors can be selectively modulated by some, but not all, NMDA receptor antagonists.

Effects of the cannabinoid CB(1) receptor antagonist, SR141716A, after Delta(9)-tetrahydrocannabinol withdrawal.

Rats were trained to lever press according to variable interval 10 s schedules during daily experimental sessions composed of six 3 min food reinforcement periods and were treated twice daily for 6 days with either vehicle or escalating regimens of Delta(9)-tetrahydrocannabinol. On days 7 and 8, the rats were challenged with vehicle and cumulative doses of SR141716A (N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4, -dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxyamide hydrochloride), a cannabinoid CB(1) receptor antagonist, up to 3 and 9 mg/kg, respectively. Response rates increased during Delta(9)-tetrahydrocannabinol withdrawal and towards those of the vehicle treatment group suggesting a waning of the direct effects of Delta(9)-tetrahydrocannabinol. SR141716A reduced response rates but only in rats pre-treated with Delta(9)-tetrahydrocannabinol. These data suggest that dependence upon Delta(9)-tetrahydrocannabinol was induced and SR141716A precipitated withdrawal.

N-Methyl-D-aspartate receptor antagonists and the development of tolerance to the discriminative stimulus effects of morphine in rats.

Several reports have indicated that N-methyl-D-aspartate (NMDA) receptor antagonists prevent the development of analgesic tolerance to opiates. Some effects of opiates, such as their discriminative stimulus effects, are known to be more resistant to tolerance induction. In this study, adult male Long-Evans rats were trained to discriminate 3.2 mg/kg of s.c. morphine from water (vehicle) using a standard, two-lever fixed ratio 10 schedule of food reinforcement. Subsequently, repeated morphine treatment (20 mg/kg; 14 days b.i.d.) was administered, which induced tolerance-like rightward shifts in the dose-effect curves for both morphine's discriminative stimulus and response rate-suppressing effects. Withdrawal-induced, response rate reductions indicative of behavioral dependence appeared as well. Separate groups were then treated repeatedly with a combination of morphine or its vehicle and one of the following competitive or noncompetitive NMDA antagonists: dizocilpine (0.1 mg/kg i.p.), 3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphonic acid (D-CPPene; 3 and 5.6 mg/kg i.p.), eliprodil (17.3 mg/kg i.p.), or R(+)-3-amino-1-hydroxy-2-pyrrolidone [(+)-HA-966; 10 mg/kg i.p.]. The development of tolerance to morphine's stimulus effects was attenuated by eliprodil and the higher dose of D-CPPene, but not by dizocilpine, the lower dose of D-CPPene, nor R(+)-3-amino-1-hydroxy-2-pyrrolidone. All antagonists prevented the induction of tolerance to morphine's response rate effects. Dizocilpine and D-CPPene (5.6 mg/kg) appeared to prevent the induction of behavioral dependence as well. NMDA antagonists can prevent tolerance to the discriminative stimulus effects of morphine, and perhaps to its behavioral dependence effects, but their site of action on the NMDA receptor complex confers a different ability to do so.

Effects of test conditions on the outcome of place conditioning with morphine and naltrexone in mice.

Drug administration during test trials can increase the expression of place conditioning, offering an opportunity to determine the specificity of this enhanced response. Prior to training, Swiss-Webster mice spent similar durations in each of the distinctive compartments of a two-compartment box during three 900-s tests. During a 4-day conditioning period, daily injections of morphine (5-20 mg/kg, SC) or vehicle were differentially paired with one of two compartments of the box using an unbiased place conditioning procedure. Post-conditioning tests were conducted 2 and 3 days after the last conditioning day. Mice pre-treated during post-conditioning tests with vehicle did not show significant preference for the morphine-paired compartment when conditioned with morphine. Pretreatment with morphine (2.5-30 mg/kg, SC) led to a dose-dependent increase in time spent in the morphine-paired compartment. Post-conditioning tests in other groups of mice were conducted with heroin (0.1-3 mg/kg), fentanyl (0.01-0.3 mg/kg), cocaine (10-30 mg/kg) and pentobarbital (10-30 mg/kg), and results suggested that none of the tested drugs facilitated the expression of the morphine-conditioned place preference. In another experiment, naltrexone (0.1-10 mg/kg, SC) was administered as the conditioning drug. When tested with naltrexone (0.1-10 mg/kg), there was a dose-dependent avoidance of the naltrexone-paired compartment. Overall, the present data indicated that: (1) failure to exhibit place preference or place aversion when tested in a drug-free state does not imply the failure of conditioning procedure; and (2) effects of the morphine cue reinstatement during the post-conditioning tests appeared to be related to the unique pharmacological profile of the morphine stimulus.

Separation of the locomotor stimulant and discriminative stimulus effects of cocaine by its C-2 phenyl ester analog, RTI-15.

During a routine evaluation of several analogs of cocaine, we observed that the C-2 phenyl ester, RTI-15, appeared to suppress motor activity in rats. We subsequently examined RTI-15 for its cocaine-like stimulus effects as well as for its locomotor activity effects. RTI-15 dose-dependently generalized from the cocaine stimulus in rats trained to discriminate 10 mg/kg cocaine from saline with complete substitution (> or = 80% cocaine-lever responding) occurring at 24 mg/kg. During automated locomotor activity tests in mice, cocaine (3-60 mg/kg) dose-dependently increased activity counts and movement time across the entire 1 h test session. RTI-15, however, had little affect on activity counts and movement time from 10-30 mg/kg, and decreased these measures at 60 mg/kg, the highest dose tested. These results indicate that while changing the C-2 methyl ester of cocaine to a C-2 phenyl ester increases dopamine-transporter selectivity, it dissociates its locomotor activity effects from its discriminative stimulus effects suggesting that the underlying mechanisms mediating these effects are not identical.

Interactions between N-methyl-D-aspartate receptor antagonists and the discriminative stimulus effects of morphine in rats.

NMDA receptor antagonists have previously been reported to alter some pharmacological and behavioral effects of acute and chronic opioid administration. The present study assessed the interactions of NMDA antagonists with the discriminative stimulus properties of morphine. Adult male Long Evans rats were trained to discriminate 3.2 mg/kg of s.c. morphine from water under a two-lever fixed-ratio 10 schedule of food reinforcement. During test sessions. I.p. injections of the noncompetitive NMDA receptor antagonist dizocilpine (0.03-0.2 mg/kg), the competitive antagonists NPC 17742 (1-16 mg/kg), and SDZ 220-581 (0.1-3 mg/kg), the polyamine site antagonist eliprodil (3-17.3 mg/kg), the glycine-site partial agonist (+)-HA-966 (3-56 mg/kg), and the nonselective glutamate antagonist kynurenic acid (30-150 mg/kg) were coadministered with s.c. morphine (1-3.2 mg/kg; interaction tests) or water (generalization tests). In generalization tests, none of the compounds completely substituted for morphine. Concurrent administration of morphine and NMDA antagonists did not greatly alter the discriminative stimulus properties of morphine. Various doses of NPC 17742, SDZ 220-581, or (+)-HA-966 somewhat increased levels of morphine-appropriate lever selection, whereas some attenuation of morphine-lever selection was obtained when morphine was coadministered with eliprodil. These results show that NMDA antagonists have minimal interactions with the discriminative stimulus effects of morphine.

Evaluation of the discriminative stimulus and reinforcing effects of dihydroetorphine.

These experiments examined whether dihydroetorphine (DHE) could serve as a reinforcer in rhesus monkeys and evoke the discriminative stimulus effects of heroin (HER) in rats, two procedures useful in predicting the overall abuse potential of compounds. Rhesus monkeys were trained to self-administer i.v. HER (10 micrograms/kg for monkeys M-BA, M-NI, and M-HO; 3 micrograms/kg for monkey M-PO) during daily; 2-h experimental sessions under FR 10 Timeout 4 min schedules. VEH and doses of HER (1-30 micrograms/kg), codeine (COD; 30-1000 micrograms/kg), and DHE (1-100 ng/kg) were then substituted for the HER maintenance doses. Results indicated that DHE served as a reinforcer. The dose of DHE that maintained peak numbers of infusions was 171 and 8571 times smaller than those maintaining peak numbers of infusions of heroin and codeine, respectively. Additionally, male Sprague-Dawley rats were trained to discriminate 0.3 mg/kg HER s.c. from vehicle (VEH) in an FR 10 (fixed-ratio 10), food-reinforced, operant procedure. During tests, HER, morphine (MOR), and DHE dose-dependently evoked heroin-lever responding with ED50s of 0.055, 0.74, and 0.00033 mg/kg, respectively. These results indicate that DHE is self-administered by rhesus monkeys, and potently produces the discriminative stimulus effects of HER in rats, and suggest that DHE would have a substantial potential for abuse.