Levo-tetrahydropalmatine attenuates cocaine self-administration and cocaine-induced reinstatement in rats

Rationale Levo-tetrahydropalmatine (l-THP) is an alkaloid constituent of plants from the botanical genera Corydalis and Stephania and is contained in many traditional Chinese herbal preparations. In addition to its low-affinity antagonism of D2 dopamine (DA) receptors, we report that l-THP functions as a higher-affinity antagonist at D1 DA receptors and interacts with D3 DA receptors, suggesting that it may be effective for the treatment of drug addiction. Accordingly, l-THP has been reported to reduce heroin craving and relapse in recovering addicts. Objective This study investigated the effects of l-THP (3.75, 7.5, and 15.0 mg/kg, i.p.) on cocaine self-administration (SA) and cocaine-induced reinstatement. Materials and methods Rats were trained to self-administer cocaine and food by pressing separate response levers during sessions consisting of a multiple schedule of alternating 30-min FR4 cocaine and 15-min FR4 food reinforcement. During the cocaine components of each session, the available cocaine dose varied such that rats had access to low and high dose ranges in varying sequence on alternating days. After SA, cocaine-reinforced responding was extinguished, and effects of l-THP on cocaine-induced reinstatement (10 mg/kg, i.p.) were examined. Results l-THP produced a rightward and downward shift in the dose–response curve for cocaine SA and attenuated cocaine-induced reinstatement. l-THP also reduced food-reinforced responding and locomotor activity. However, reductions in cocaine SA were found at doses that failed to alter food-reinforced responding, and significant effects were not observed on food responding during reinstatement. Conclusions These findings suggest that l-THP is potentially useful for treating cocaine addiction. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Antinociceptive effects of tramadol in co-administration with metamizol after single and repeated administrations in rats

Opiate addiction is a chronic recrudescent disorder characterized by a high rate of relapse. Levo-tetrahydropalmatine (l-THP) is an alkaloid substance extracted from Corydalis and Stephania and is contained in a number of traditional Chinese herbal preparations. Compared to other dopamine receptor antagonists, l-THP has lower affinity for D2 receptors than for D1 receptors, and a recent study showed that l-THP also binds to D3 receptors, possibly functioning as an antagonist. The unique pharmacological profile of l-THP suggests that l-THP may be effective for the treatment of opiate addiction. In this study, we investigated the effects of l-THP on heroin self-administration and reinstatement triggered by a priming injection of heroin in abstinent rats trained to stably self-administer heroin under an extinction/reinstatement protocol, and found that l-THP (2.5 and 5 mg/kg, i.p.) decreased heroin self-administration on the fixed-ratio 1 schedule and dose-dependently (1.25, 2.5 and 5 mg/kg, i.p.) inhibited heroin-induced reinstatement of heroin-seeking behavior. Importantly, l-THP (1.25 and 2.5 mg/kg, i.p.) did not affect locomotion, indicating that the observed effects of l-THP on reinstatement do not appear to be due to motor impairments. The present results demonstrated that dopamine receptor antagonist l-THP attenuates heroin self-administration and heroin-induced reinstatement.     

l-Isocorypalmine reduces behavioral sensitization and rewarding effects of cocaine in mice by acting on dopamine receptors

Background

We previously reported isolation of l-isocorypalmine (l-ICP), a mono-demethylated analog of l-tetrahydropalmatine (l-THP), from the plant Corydalis yanhusuo. Here we characterized its in vitro pharmacological properties and examined its effects on cocaine-induced behaviors in mice.

Methods

Receptor binding, cAMP and [³⁵S]GTPγS assays were used to examine pharmacological actions of l-ICP in vitro. Effects of l-ICP on cocaine-induced locomotor hyperactivity and sensitization and conditioned place preference (CPP) in mice were investigated. HPLC was employed to analyze metabolites of l-ICP in mouse serum.

Results

Among more than 40 targets screened, l-ICP and l-THP bound only to dopamine (DA) receptors. l-ICP was a high-affinity partial agonist of D1 and D5 receptors and a moderate-affinity antagonist of D2, D3 and D4 receptors, whereas l-THP bound to only D1 and D5 receptors, with lower affinities than l-ICP. At 10 mg/kg (i.p.), l-ICP inhibited spontaneous locomotor activity for a shorter time than l-THP. Pretreatment with l-ICP reduced cocaine-induced locomotor hyperactivities. Administration of l-ICP before cocaine once a day for 5 days reduced cocaine-induced locomotor sensitization on days 5 and 13 after 7 days of withdrawal. Pretreatment with l-ICP before cocaine daily for 6 days blocked cocaine-induced CPP, while l-ICP itself did not cause preference or aversion. HPLC analysis showed that l-ICP was the main compound in mouse serum following i.p. injection of l-ICP.

Conclusions

l-ICP likely acts as a D1 partial agonist and a D2 antagonist to produce its in vivo effects and may be a promising agent for treatment of cocaine addiction.     

The effects of the novel DA D3 receptor antagonist SR 21502 on cocaine reward, cocaine seeking and cocaine-induced locomotor activity in rats

Rationale

There is a focus on developing D3 receptor antagonists as cocaine addiction treatments.

Objective

We investigated the effects of a novel selective D3 receptor antagonist, SR 21502, on cocaine reward, cocaine-seeking, food reward, spontaneous locomotor activity and cocaine-induced locomotor activity in rats.

Methods

In Experiment 1, rats were trained to self-administer cocaine under a progressive ratio (PR) schedule of reinforcement and tested with vehicle or one of three doses of SR 21502. In Experiment 2, animals were trained to self-administer cocaine under a fixed ratio schedule of reinforcement followed by extinction of the response. Then, animals were tested with vehicle or one of the SR 21502 doses on cue-induced reinstatement of responding. In Experiment 3, animals were trained to lever press for food under a PR schedule and tested with vehicle or one dose of the compound. In Experiments 4 and 5, in separate groups of animals, the vehicle and three doses of SR 21502 were tested on spontaneous or cocaine (10 mg/kg, IP)-induced locomotor activity, respectively.

Results

SR 21502 produced significant, dose-related (3.75, 7.5 and 15 mg/kg) reductions in breakpoint for cocaine self-administration, cue-induced reinstatement (3.75, 7.5 and 15 mg/kg) and cocaine-induced locomotor activity (3.75, 7.5 and 15 mg/kg) but failed to reduce food self-administration and spontaneous locomotor activity.

Conclusions

SR 21502 decreases cocaine reward, cocaine-seeking and locomotor activity at doses that have no effect on food reward or spontaneous locomotor activity. These data suggest SR 21502 may selectively inhibit cocaine’s rewarding, incentive motivational and stimulant effects.     

Effects of 14-day treatment with the schedule III anorectic phendimetrazine on choice between cocaine and food in rhesus monkeys

Background

The clinical utility of monoamine releasers such as phenmetrazine or d-amphetamine as candidate agonist medications for cocaine dependence is hindered by their high abuse liability. Phendimetrazine is a clinically available schedule III anorectic that functions as a prodrug for phenmetrazine and thus may have lower abuse liability. This study determined the effects of continuous 14-day treatment with phendimetrazine on cocaine vs. food choice in rhesus monkeys (N = 4).

Methods

Responding was maintained under a concurrent schedule of food delivery (1-g pellets, fixed-ratio 100 schedule) and cocaine injections (0–0.1 mg/kg/injection, fixed-ratio 10 schedule). Cocaine choice dose–effect curves were determined daily before and during 14-day periods of continuous intravenous treatment with saline or (+)-phendimetrazine (0.32–1.0 mg/kg/h). Effects of 14-day treatment with (+)-phenmetrazine (0.1–0.32 mg/kg/h; N = 5) and d-amphetamine (0.032–0.1 mg/kg/h; N = 6) were also examined for comparison.

Results

During saline treatment, food was primarily chosen during availability of low cocaine doses (0, 0.0032, and 0.01 mg/kg/injection), and cocaine was primarily chosen during availability of higher cocaine doses (0.032 and 0.1 mg/kg/injection). Phendimetrazine initially decreased overall responding without significantly altering cocaine choice. Over the course of 14 days, tolerance developed to rate decreasing effects, and phendimetrazine dose-dependently decreased cocaine choice (significant at 0.032 mg/kg/injection cocaine). Phenmetrazine and d-amphetamine produced qualitatively similar effects.

Conclusions

These results demonstrate that phendimetrazine can produce significant, though modest, reductions in cocaine choice in rhesus monkeys. Phendimetrazine may be especially suitable as a candidate medication for human studies because of its schedule III clinical availability.     

Efficacy of buspirone for attenuating cocaine and methamphetamine reinstatement in rats

Background

There are no approved pharmacotherapies for preventing psychomotor stimulant relapse. The operant reinstatement model has been suggested as a screen for identifying candidate medications. The present study examined if the anxiolytic buspirone could attenuate reinstatement of extinguished responding in Long–Evans rats that previously self-administered intravenous cocaine or methamphetamine.

Methods

Rats were trained in 2-h daily sessions to self-administer 0.5 mg/kg cocaine or 0.1 mg/kg methamphetamine infusions followed by 12 days of instrumental extinction. Reinstatement was evoked by 17 mg/kg i.p. cocaine primes or response-contingent cocaine-paired cues in cocaine-reinforced rats, and by 1 mg/kg i.p. methamphetamine primes or response-contingent methamphetamine-paired cues in methamphetamine-reinforced rats.

Results

Buspirone (1 and 3 mg/kg) significantly (p < 0.05) attenuated cocaine cue but not cocaine prime reinstatement. Buspirone (1 and 3 mg/kg) also significantly attenuated methamphetamine cue reinstatement. Buspirone (3 mg/kg) significantly attenuated methamphetamine prime reinstatement. During all reinstatement tests, 3 mg/kg buspirone reduced levels of inactive lever pressing relative to those of vehicle, significantly so during the cocaine cue-induced reinstatement tests.

Conclusions

Given the complexity of buspirone's neuropharmacology consisting of serotonin 5-HT_1A receptor partial agonist activity, and dopamine D2, D3 and D4 receptor antagonist effects, it is uncertain which of these activities or their combination is responsible for the present results. Overall, these results suggest that buspirone may reduce the likelihood of relapse to cocaine and methamphetamine use under some conditions, although this speculation must be interpreted with caution given buspirone's similar potency to attenuate inactive-lever responding.     

The preferential dopamine D3 receptor antagonist S33138 inhibits cocaine reward and cocaine-triggered relapse to drug-seeking behavior in rats

We have previously reported that selective dopamine (DA) D₃ receptor antagonists are effective in a number of animal models of drug addiction, but not in intravenous drug self-administration, suggesting a limited ability to modify drug reward. In the present study, we evaluated the actions of S33138, a novel partially selective D₃ receptor antagonist, in animal models relevant to drug addiction. S33138, at doses of 0.156 or 0.625 mg/kg (i.p.), attenuated cocaine-enhanced brain-stimulation reward (BSR), and the highest dose tested (2.5 mg/kg) produced a significant aversive-like rightward shift in BSR rate-frequency reward functions. Further, S33138 produced biphasic effects on cocaine self-administration, i.e., a moderate dose (2.5 mg/kg, p.o.) increased, while a higher dose (5 mg/kg, p.o.) inhibited, cocaine self-administration. The increase in cocaine self-administration likely reflects a compensatory response to a partial reduction in drug reward after S33138. In addition, S33138 (0.156-2.5 mg/kg, p.o.) also dose-dependently inhibited cocaine-induced reinstatement of drug-seeking behavior. The reduction in cocaine-enhanced BSR and cocaine-triggered reinstatement produced by lower effective doses (e.g., 0.156 or 0.625 mg/kg) of S33138 is unlikely due to impaired locomotion, as lower effective doses of S33138 decreased neither Y_max levels in the BSR paradigm, rotarod performance, nor locomotion. However, the higher doses (2.5 or 5 mg/kg) of S33138 also significantly inhibited sucrose self-administration and rotarod performance, suggesting non-D₃ receptor-mediated effects on non-drug reward and locomotion. These data suggest that lower doses of S33138 interacting essentially with D₃ receptors have pharmacotherapeutic potential in treatment of cocaine addiction, while higher doses occupying D₂ receptors may influence locomotion and non-drug reward.     

Oxytocin modulates behavioural adaptation to repeated treatment with cocaine in rats.

Behavioural adaptation to and the effects of the neurohypophyseal peptide, oxytocin, on repeated treatment with cocaine were investigated in rats. The content of immunoreactive oxytocin in the plasma, hypothalamus and different limbic structures in the brain were also studied after treatment with cocaine, identical to that used in the behavioural experiment. Repeated administration of cocaine (7.5 mg/kg, s.c.) produced a behavioural tolerance to the stereotyped sniffing-inducing effect of the challenge doses (1.875, 3.75 and 7.5 mg/kg, s.c.) of cocaine on the fifth day, which was demonstrated by parallel shifting of the dose-response and time-effect curves of the test doses of cocaine. The development of tolerance was inhibited by pretreatment with oxytocin (0.05 micrograms, (s.c.), administered before each daily injection of cocaine. A smaller dose of oxytocin (0.005 micrograms, s.c.) had no effect in this model. A decreased amount of immunoreactive oxytocin was detected in the plasma, in the hypothalamus and in the hippocampus, after repeated treatment with cocaine. Replacement of oxytocin by local injection (100 pg) into the ventral hippocampus, before each daily administration of cocaine, prevented the development of tolerance to cocaine. These results suggest that endogenous oxytocin, localized in limbic-forebrain areas, may have an important regulatory role in the development of behavioural changes induced by the repeated administration of cocaine.     

Effect of l-tetrahydropalmatine on dopamine release and metabolism in the rat striatum

In vivo voltammetric measurements of striatal extracellular DOPAC concentrations and of striatal DA release in combination with post-mortem analysis of striatal catechols were performed in the rat to study the effects on the nigro-striatal neurons of a Chinese neuroleptic, l-tetrahydropalmatine (l-THP), which is known to block post-synaptic dopaminergic receptors. l-THP injected at doses that cause sedation in rats and mice (5–10 mg/kg) induced a marked increase in post-mortem DOPAC levels (+250%), while no significant effect was observed on post-mortem DA levels. The extracellular DOPAC concentration was increased to 155±9% of the control value after l-THP administration (1 mg/kg, IP). Further, an injection of l-THP (1 mg/kg, IP) induced an increase in extracellular DA concentration (220% of the basal value), reversed the decrease in extracellular DA concentration induced by apomorphine (0.05 mg/kg, SC) and enhanced the effects of the electrical stimulation of the nigro-striatal pathway. These results confirm that l-THP acts on the nigro-striatal neurons as a dopaminergic antagonist that can block both post-and presynaptic receptors.Visiting scholar from Shanghai, Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 200031, China     

Antagonism of the discriminative stimulus effects of cocaine at two training doses by dopamine D2-like receptor antagonists

RATIONALE: The relative contributions of different dopamine receptor subtypes to the discriminative stimulus effects of cocaine may be influenced by the training dose of cocaine. Substitution tests with dopamine receptor agonists have suggested that the role of dopamine D2-like receptors is diminished relative to that of D1-like receptors at a training dose of 3 mg/kg cocaine compared with a training dose of 10 mg/kg. OBJECTIVES: To test whether dopamine D2-like receptor antagonists were differentially effective at attenuating cocaine's discriminative stimulus effects at different training doses, and to test for the first time an antagonist that is selective for the dopamine D2 receptor within the D2-like receptor subfamily. METHODS: Rats were trained to press one lever after receiving cocaine and another after receiving saline (maintaining >95% drug-appropriate responding). Three dopamine D2-like receptor antagonists (haloperidol, raclopride and L-741,626) were tested in rats trained at 3 mg/kg or 10 mg/kg cocaine. At the lower training dose, the D1-like receptor antagonist SCH 39166 was also tested. RESULTS: The antagonists were not differentially effective between training groups: they all produced parallel, rightward shifts in cocaine's dose-effect function, indicating surmountable antagonism. CONCLUSIONS: The results demonstrate that D2-like receptor antagonists with different affinities for the various D2-like receptors can antagonise the discriminative stimulus effects of cocaine at two training doses. Importantly, antagonism by L-741,626 implies that stimulation of D2 receptors alone (not D3 or D4 receptors) is sufficient to mediate cocaine's discriminative stimulus effects. Finally, the claim that D1-like receptors are preferentially involved at low training doses of cocaine is only consistent with the current findings if indirect stimulation of D2 receptors by low doses of cocaine remains necessary for the expression of the D1-like receptor-mediated effect.     

Cocaine abuse versus cocaine dependence: Cocaine self-administration and pharmacodynamic response in the human laboratory

Cocaine has high abuse liability but only a subset of individuals who experiment with it develop dependence. The DSM-IV (APA. Diagnostic and Statistical Manual of Mental Disorders DSM-IV-R. American Psychiatric Association, Washington, DC, 2000) provides criteria for diagnosing cocaine abuse and cocaine dependence as distinct disorders- the latter characterized by additional symptoms related to loss of control over drug use. In this study, two groups of cocaine users (n = 8/group), matched on demographic factors and length of cocaine use history and meeting criteria for either cocaine abuse (CocAb) or cocaine dependence (CocDep), were compared on (1) measures related to impulsivity and sensation seeking, (2) response to experimenter-administered cocaine (0, 12.5, 25 and 50 mg/70 kg, i.v.), and (3) cocaine self-administration using a Relapse Choice and a Progressive Ratio Procedure (0, 12.5 and 25 mg/70 kg, i.v.). Groups did not differ on impulsivity or sensation seeking scores. After experimenter-administered cocaine, the CocAb group reported feeling more suspicious and observers rated them significantly higher on unpleasant effects (e.g., irritability, difficulty concentrating). In contrast, the CocDep group reported significantly greater desire for cocaine, which was sustained over the course of the study, and gave higher street value estimates for cocaine (p < 0.05). While cocaine self-administration was dose-related and generally comparable across the two procedures, the CocDep users chose to take significantly more cocaine than the CocAb users. These data suggest that, while regular long-term users of cocaine with cocaine abuse or dependence diagnoses cannot be distinguished by trait measures related to impulsivity, they do exhibit significant differences with regard to cocaine-directed behavior and response to cocaine administration.     

Behavioral interactions produced by co-administration of 7-OH-DPAT with cocaine or apomorphine in the rat

 Previous research from our laboratory suggests that low doses (<0.1 mg/kg) of the dopamine (DA) D₃-preferring agonist 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) attenuate conditioned place preference (CPP) produced by the indirect DA agonist d-amphetamine, but enhance d-amphetamine-induced stereotypic behaviors. This study further examined the effects of 7-OH-DPAT on behaviors produced by the indirect DA agonist, cocaine, and the non-selective direct DA agonist, apomorphine. To examine whether 7-OH-DPAT would alter cocaine and apomorphine dose-response curves for motor behaviors and CPP, 0.1 mg/kg 7-OH-DPAT was co-administered with 0–30 mg/kg cocaine and 0–3 mg/kg apomorphine. To establish place conditioning, drug injections were paired with one of two distinctly different compartments, whereas saline injections were paired with the other compartment. Locomotion, sniffing, oral stereotypy, and headbobbing were measured following acute and repeated drug administration during conditioning, and place conditioning was assessed 24 h following the last conditioning day.7-OH-DPAT enhanced cocaine- and apomorphine-induced stereotypies following repeated administration. 7-OH-DPAT also attenuated cocaine-CPP, but potentiated apomorphine-CPP. Furthermore, 7-OH-DPAT attenuated locomotion produced by high doses of apomorphine. The attenuation of cocaine-CPP by 7-OH-DPAT likely involves stimulation of D₂/D₃ autoreceptors in the mesolimbic pathway, whereas the potentiation of apomorphine-CPP likely involves stimulation of D₂/D₃ postsynaptic receptors. Furthermore, it is suggested that attenuation of apomorphine-induced locomotion by 7-OH-DPAT likely involves stimulation of postsynaptic D₃ receptors in the mesolimbic pathway. Thus, if postsynaptic D₃ receptors are involved in mediating CPP and locomotion, then stimulation of D₃ receptors may facilitate CPP but inhibit locomotion. Received: 20 May 1998 / Final version: 17 September 1998     

Lower reinforcing strength of the phenyltropane cocaine analogs RTI-336 and RTI-177 compared to cocaine in nonhuman primates

Drugs that inhibit brain dopamine transporters (DAT) have been developed as potential agonist medications for cocaine abuse and dependence. Because the mechanism of action of such drugs is similar to cocaine, one concern regarding their use is the abuse potential of the medications themselves. The present study compared the reinforcing strength of cocaine (0.003-0.3 mg/kg) and two 3-phenyltropane analogs of cocaine, RTI-336 (3β-(4-chlorophenyl)-2β-[3-(4′-methylphenyl)isoxazol-5-yl]tropane hydrochloride; 0.003-0.1 mg/kg) and RTI-177 (3β-(4-chlorophenyl)-2β-[3-phenylisoxazol-5-yl]tropane hydrochloride; 0.003-0.1 mg/kg), using a progressive-ratio (PR) schedule in rhesus monkeys (n = 4). PR schedules of reinforcement are frequently used to measure reinforcing strength of drugs. Earlier research using limited-access conditions reported that cocaine was a stronger reinforcer than either RTI-336 or RTI-177. Because the 3-phenyltropanes have longer durations of action, one purpose of the present study was to examine reinforcing strength using longer experimental sessions. Under these conditions, cocaine functioned as a reinforcer in all monkeys, and RTI-336 and RTI-177 functioned as a reinforcer in three of four subjects. Consistent with their documented slower onset of neurochemical and pharmacological effects, RTI-336 and RTI-177 were weaker reinforcers, resulting in fewer injections than cocaine. On average, the potencies of the two RTI compounds were not different than that of cocaine. These results support the view that slow-onset DA-selective uptake inhibitors have lower abuse liability than cocaine. In addition, the present findings suggest that changes in PR session length can influence potency comparisons between drugs, but not measures of reinforcing strength.     

Anorexic activity of cocaine and coca extract in naive and cocaine tolerant rats

Dose response curves for reducing limited access food consumption were determined for cocaine HC1 IP, cocaine HC1 PO, and whole Erythroxylum coca extract PO. The ED₅₀'s for cocaine HC1 in drug naive rats were 19.6 mg/kg (IP) and 34.6 mg/kg (PO). When the dose of E. coca extract was expressed in terms of cocaine HC1 content, the ED₅₀ was 52.6 mg/kg (PO). When dose response curves were determined in rats that had received cocaine (45 mg/kg, PO) for 30 days, a shift to the right in the cocaine HC1 curve (an ED₅₀ of 98.4 mg/kg PO) indicated tolerance. However, the shift to the right was less for E. coca extract than for cocaine HCI. Although the anorexic activity of E. coca extract was less than that of an equivalent amount of cocaine in naive rats it was often more than that of equivalent doses of cocaine HC1 in tolerant rats. Interaction with other constituents of E. coca extract appears to alter the potency of the cocaine content of the extract in different directions in naive and tolerant rats.     

Discriminative stimulus properties of cocaine,norcocaine, and N-allylnorcocaine

A discriminative stimulus paradigm was employed to train eight male and female Wistar rats to discriminate 5.0 mg/kg cocaine HCl from 2.0 ml/kg saline. Subjects responded in a two bar operant chamber on an FR 30 schedule for food reinforcement. All sessions followed a 10 minute pretreatment with either saline, the training dose of cocaine, four probe doses of cocaine HCl (1.0, 2.5, 7.5, 10 mg/kg), four probe doses of norcocaine (1.0, 2.5, 5.0, 7.5 mg/kg) or four probe doses of N-allylnorcocaine (5.0, 7.5, 10, 20 mg/kg). All probe doses were tested using an extinction procedure. The three highest doses of cocaine generalized to cocaine while the 1.0 mg/kg dose of cocaine generalized to saline. The two highest doses of norcocaine generalized to cocaine while the 2.5 mg/kg dose of norcocaine resulted in 57% responding on the cocaine lever with the 1.0 mg/kg dose generalizing to saline. Only the highest dose of N-allylnorcocaine was found to generalize to cocaine with the intermediate doses resulting in an intermediate level of responding occurring on the cocaine lever. The 5.0 mg/kg dose of N-allylnorcocaine generalized to saline.     

Differential effects of 7-OH-DPAT on amphetamine-induced stereotypy and conditioned place preference

 Low doses of the dopamine D₃-preferring agonist 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) produce a behavioral profile that is opposite to that produced by the psychomotor stimulants cocaine and amphetamine. For example, low doses of 7-OH-DPAT produce conditioned place aversion and hypolocomotion, whereas psychomotor stimulants produce conditioned place preference (CPP) and hyperlocomotion. In experiment 1, the effects of low doses of 7-OH-DPAT (0.01–0.1 mg/kg) on d-amphetamine-induced (1 mg/kg) motor behaviors and CPP were assessed. In experiment 2, the effects of 0.1 mg/kg 7-OH-DPAT on d-amphetamine (0–10 mg/kg) dose-response curves for the same behaviors were examined. During conditioning, drug injections were paired with a distinct compartment, whereas saline injections were paired with another compartment. Locomotion and headbobbing were measured following acute and repeated drug administration during conditioning and place conditioning was assessed 24 h following the last conditioning day. In experiment 1, d-amphetamine-induced locomotion was dose-dependently decreased by 7-OH-DPAT following repeated administration, which was probably due to the emergence of headbobbing, a behavior not observed with d-amphetamine alone. d-Amphetamine-CPP was not altered by co-administration of 0–0.03 mg/kg 7-OH-DPAT, but was attenuated by co-administration of 0.1 mg/kg 7-OH-DPAT. In experiment 2, 7-OH-DPAT co-administered with low doses of d-amphetamine (0–0.5 mg/kg) produced a decrease in locomotion following acute administration. However, 7-OH-DPAT produced sensitization of locomotion at the 0.5 mg/kg dose of d-amphetamine and an increase in headbobbing at the 0.5–10 mg/kg doses of d-amphetamine following repeated administration. In contrast, d-amphetamine-CPP was attenuated by co-administration of 7-OH-DPAT. These findings suggest that 0.1 mg/kg 7-OH-DPAT attenuates the reinforcing effects of d-amphetamine despite enhancing stereotypic behaviors. Received: 14 May 1997 / Final version: 22 January 1998     

Immunopharmacotherapeutic manifolds and modulation of cocaine overdose

Cocaine achieves its psychostimulant, reinforcing properties through selectively blocking dopamine transporters, and this neurobiological mechanism impedes the use of classical receptor-antagonist pharmacotherapies to outcompete cocaine at CNS sites. Passive immunization with monoclonal antibodies (mAb) specific for cocaine circumvents this problem as drug is sequestered in the periphery prior to entry into the brain. To optimize an immunopharmacotherapeutic strategy for reversing severe cocaine toxicity, the therapeutic properties of mAb GNC92H2 IgG were compared to those of its engineered formats in a mouse overdose model. Whereas the extended half-life of an IgG justifies its application to the prophylactic treatment of addiction, the rapid, thorough biodistribution of mAb-based fragments, including F(ab′)₂, Fab and scFv, may correlate to accelerated scavenging of cocaine and reversal of toxicity. To test this hypothesis, mice were administered the anti-cocaine IgG (180 mg/kg, i.v.) or GNC92H2-based agent after receiving an LD₅₀ cocaine dose (93 mg/kg, i.p.), and the timeline of overdose symptoms was recorded. All formats lowered the rate of lethality despite the > 100-fold molar excess of drug to antibody binding capacity. However, only F(ab′)₂-92H2 and Fab-92H2 significantly attenuated the progression of premorbid behaviors, and Fab-92H2 prevented seizure generation in a percentage of mice. The calculation of serum half-life of each format demonstrated that the pharmacokinetic profile of Fab-92H2 (elimination half-life, t_1/2 ~ 100 min) best approximated that of cocaine. These results not only confirm the importance of highly specific and tight drug binding by the mAb, but also highlight the benefit of aligning the pharmacokinetic and pharmacodynamic properties of the immunopharmacotherapeutic with the targeted drug.     

Cocaine-induced locomotor activity and cocaine discrimination in dopamine D2 receptor mutant mice

RATIONALE: Dopamine (DA) D2-like antagonists block several effects of cocaine, including its locomotor stimulant and interoceptive discriminative-stimulus effects. Because these compounds generally lack selectivity among the D2-like DA receptors, the specific roles of the subtypes remain unclear. OBJECTIVES: DA D2 receptor knockout (DA D2R KO), heterozygous (HET), and wild-type (WT) mice were used to study the role of D2 DA receptors in the effects of cocaine. Some effects of the relatively selective DA D2-like antagonist raclopride were also studied to further assess the role of D2 receptors. METHODS: DA D2R KO, HET, and WT mice were treated with cocaine (1-10 mg/kg) or vehicle, and their horizontal locomotor activity was assessed. The mice were also trained to discriminate i.p. injections of saline from cocaine (10 mg/kg) using a two-response key, fixed-ratio-20 response, food-reinforcement procedure. A range of doses of cocaine (1.0-17 mg/kg) was administered before 15-min test sessions. RESULTS: Both DA D2R KO and HET mice showed reduced levels of horizontal activity relative to WT mice. Cocaine dose dependently stimulated activity in each genotype, with the highest level of activity induced in the DA D2R WT mice. All three genotypes acquired the discrimination of 10 mg/kg cocaine; tested doses of 1.0-10.0 mg/kg produced dose-related increases in the number of cocaine-appropriate responses. Raclopride, at inactive to fully active doses (0.1-1.0 mg/kg), did not fully substitute for cocaine. Raclopride dose dependently shifted the cocaine dose-effect curve to the right in DA D2R WT and HET mice. However, in DA D2R KO mice, raclopride was inactive as an antagonist. CONCLUSIONS: The present data indicate an involvement of D2 DA receptors in the locomotor-stimulating effects and the interoceptive discriminative-stimulus effects of cocaine in WT subjects. However, the D2 receptor is not necessary for the effects, suggesting redundant dopaminergic mechanisms for the discriminative-stimulus interoceptive effects of cocaine.     

The stereotypy-inducing effects of N-substituted benztropine analogs alone and in combination with cocaine do not account for their blockade of cocaine self-administration

Rationale

Previous studies have demonstrated that several N-substituted 4′, 4″-diF-benztropine (BZT) analogs with high dopamine transporter affinity selectively decreased cocaine self-administration without affecting food-maintained behavior in rats.

Objectives

The present study examined if the decreases in cocaine self-administration are due to competition from excess behavioral activity (hyperlocomotion or stereotypy) induced by the BZT analogs alone or in combination with cocaine.

Results

Pretreatments with the typical dopamine uptake inhibitor methylphenidate [1.0, 3.2, and 10 mg/kg, intraperitoneally (i.p.)] dose-dependently shifted the cocaine self-administration dose–effect curve (0, 0.032, 0.1, 0.32, and 1.0 mg/kg/injection) leftward. The shift in the dose–effect curve was obtained at doses of methylphenidate that, when administered alone, also decreased food-maintained behavior and increased locomotor activity and stereotypy. In contrast, the N-substituted BZT analogs, JHW 007 (1.0, 3.2, and 10 mg/kg, i.p.), AHN 1-055 (10 mg/kg), and, AHN 2-005 (10 mg/kg), as previously reported, decreased the maximum for the cocaine self-administration dose–effect curve, and did so at doses that were virtually without effects on food-maintained behavior. Further, the BZT analogs alone had minimal effects on locomotor activity and stereotypies and did not appreciably change the effects of cocaine on these measures when administered in combination with cocaine.

Conclusions

The present results suggest that the decrease in cocaine self-administration produced by the N-substituted BZT analogs is due to an antagonism of the reinforcing effects of cocaine rather than due to interference from competing behavioral overstimulation, and further supports the development of N-substituted BZT analogs as medications to treat cocaine abuse.     

Preexposure to cocaine attenuates aversions induced by both cocaine and fluoxetine: Implications for the basis of cocaine-induced conditioned taste aversions

Although cocaine-induced conditioned taste aversions (CTA) are well documented, little is known about the basis for cocaine's aversive effects. To address the role of serotonin (5-HT) in cocaine-induced aversions, the present experiments used the cross-drug preexposure design in which the effects of exposure to fluoxetine, a selective 5-HT reuptake inhibitor (SSRI) with 5-HT transporter (SERT) inhibitory properties, were examined on aversions induced by cocaine (a nonselective monoamine transport inhibitor) and the effects of cocaine preexposure were examined on fluoxetine-induced aversions. Prior to these assessments, a fluoxetine dose-response function (3.2, 5.6, 10, and 18 mg/kg) was established in male Sprague-Dawley rats to determine a dose of fluoxetine that would induce intermediate aversions that were comparable to those induced by 18 mg/kg cocaine (Experiment 1). Other groups of rats were then exposed to fluoxetine prior to aversion conditioning with cocaine (Experiment 2) and with cocaine prior to aversion conditioning with fluoxetine (Experiment 3). All drugs were administered subcutaneously (cocaine 18 mg/kg; fluoxetine 10 mg/kg). Although there was no effect of fluoxetine preexposure on either cocaine- or fluoxetine-induced aversions, preexposure to cocaine significantly attenuated aversions induced by itself and by fluoxetine. These results were discussed in terms of the possible role 5-HT might play in the mediation of aversions induced by cocaine.