Functional observational battery comparing effects of ethanol, 1,1,1-trichloroethane, ether, and flurothyl

Several recent reports have demonstrated that acute solvent exposure in animals produces a profile of neurobehavioral effects similar to that of classical CNS depressant drugs such as the barbiturates and ethanol. The present investigation further delineated the behavioral pharmacology of three solvents [1,1,1-trichloroethane (TCE), ether, and flurothyl] using a functional observational battery (FOB) composed of 21 qualitative and quantitative measures of behavior. The profiles of acute effects produced by TCE and ether were similar to one another and similar to the profile of effects produced by the IP administration of ethanol. This profile of depressant effects included changes in posture, decreased arousal, disturbances in gait, decreased forelimb grip strength, increased landing foot splay, and impaired psychomotor coordination. Flurothyl exposure also produced dose-related effects on many of the measures in the FOB; however, unlike the depressant vapors, flurothyl did not affect measures of muscle tone and equilibrium such as forelimb grip strength and landing foot splay, or measures of sensorimotor reactivity, including the touch response and tail pinch response. In addition, flurothyl produced handling-induced convulsions in some mice. Recovery from the acute effects of these vapors was rapid and began within minutes of removal from the exposure chamber. These results provide further evidence that exposure to certain solvents produces a profile of reversible effects qualitatively similar to that produced by depressant drugs and alcohol, and that the FOB can be used to compare and contrast profiles of depressant and excitatory effects of inhalants.

Dexfenfluramine lacks amphetamine-like abuse potential

1. The amphetamine-like abuse potential of dexfenfluramine (dFEN) was evaluated using drug discrimination and self-administration procedures. 2. Male Fischer rats were trained to discriminate either dFEN (1.0 mg/kg) or d-amphetamine (dAMP; 1.0 mg/kg) from saline in a two-choice discrete-trial avoidance paradigm. 3. In dAMP-trained rats, dFEN (0.5-4.0 mg/kg) engendered almost exclusively saline-appropriate responding. In dFEN-trained rats, dAMP (1.0-4.0 mg/kg) engendered entirely saline-appropriate responding in 3 of 6 rats and intermediate levels of dFEN-appropriate responding in the remaining animals. 4. Potential reinforcing effects of dFEN were also evaluated in 3 male rhesus monkeys trained to self-administer cocaine (i.v.) during daily 60 min sessions under a fixed-ratio (FR)-10 schedule. 5. Various doses of dFEN (30-1000 micrograms/kg/infusion) and dAMP (10 micrograms/kg/infusion) were substituted for cocaine in 4 consecutive daily sessions. In all subjects, dFEN maintained rates of self-administration within the range of rates maintained by saline and considerably below those maintained by cocaine and dAMP. Furthermore, the within-session distribution of responding with dFEN resembled that produced by saline. 6. Taken together, these results strongly suggest that dFEN will not have amphetamine-like abuse potential in humans.

Evaluation of the cocaine-like discriminative stimulus effects and reinforcing effects of modafinil

Modafinil [(diphenyl-methyl)sulphinyl-2-acetamide] is a novel psychostimulant drug which is effective in the treatment of narcolepsy and idiopathic hypersomnia. It also has neuroprotective effects in animal models of striatal neuropathology. Although the cellular mechanisms of action of modafinil are poorly understood, it has been shown to have a profile of pharmacological effects that differs considerably from that of amphetamine-like stimulants. There is some evidence that modafinil has central alpha 1-adrenergic agonist effects. In the present study modafinil was evaluated for cocaine-like discriminative stimulus effects in rats and for reinforcing effects in rhesus monkeys maintained on intravenous cocaine self-administration. Modafinil, l-ephedrine and d-amphetamine all produced dose dependent increases in cocaine-lever responding, with maximal levels of 67%, 82% and 100%, respectively. Modafinil produced full substitution in four out of the six rats tested while the highest levels of substitution were associated with substantial response rate decreasing effects. Little evidence was obtained that the discriminative stimulus effects of modafinil were produced by alpha 1-adrenergic activation, based upon results of tests performed in combination with prazosin. In the self-administration procedure, modafinil and l-ephedrine functioned as reinforcers in rhesus monkeys. The reinforcing and discriminative stimulus effects of modafinil-required very high doses: modafinil was over 200 times less potent than d-amphetamine and was also less potent than l-ephedrine. These results show that modafinil has some cocaine-like discriminative stimulus effects and, like other abused stimulants, can serve as a reinforcer at high doses.

Effects of inhaled 1,1,1-trichloroethane on locomotor activity in mice

To quantify the motoric effects of an abused solvent, photocells were added to two exposure systems. The first system utilized a static exposure chamber that recirculated vapor-laden air. A second dynamic system allowed for removal of waste gases with replenishment of fresh air combined with test vapors. In the present studies, male mice were examined for effects on locomotor activity following 30-min inhalation exposures to several concentrations of 1,1,1-trichloroethane (TCE), a widely used and abused solvent. TCE produced significant increases in locomotor activity at intermediate concentrations. Minimally effective concentrations for activity-increasing effects in the dynamic and static systems were 1,250 ppm and 2,500 ppm, respectively. At higher concentrations, motor activity was decreased with the highest dynamic system concentration (10,000 ppm), resulting in 26% of baseline control values. Biphasic, motor activity increasing and decreasing effects of TCE as a function of exposure concentration may reflect the CNS-depressant drug-like effects of abused solvents.

Evaluation of agonist-antagonist properties of nitrogen mustard and cyano derivatives of delta 8-tetrahydrocannabinol

delta 8-Tetrahydrocannabinol (delta 8-THC) is a naturally occurring cannabinoid with a characteristic pharmacological profile of in vivo effects. Previous studies have shown that modification of the structure of delta 8-THC by inclusion of a nitrogen-containing functional group alters this profile and may alkylate the cannabinoid receptor, similar to the manner in which beta-funaltrexamine (beta-FNA) alkylates the micro-opioid receptor. Two novel analogs of delta 8-THC were synthesized: a nitrogen mustard analog with a dimethylheptyl side chain (NM-delta 8-THC) and a cyano analog with a dimethylpentyl side chain (CY-delta 8-THC). Both analogs showed high affinity for brain cannabinoid receptors and when administered acutely, produced characteristic delta 9-THC-like effects in mice, including locomotor suppression, hypothermia, antinociception and catalepsy. CY-delta 8-THC shared discriminative stimulus effects with CP 55,940; for NM-delta 8-THC, these effects also occurred, but were delayed. Although both compounds attenuated the effects of delta 9-THC in the mouse behavioral tests, evaluation of potential antagonist effects of these compounds was complicated by the fact that two injections of delta 9-THC produced similar results, suggesting that acute tolerance or desensitization might account for the observations. NM-delta 8-THC, but not CY-delta 8-THC, attenuated the discriminative stimulus effects of CP 55,940 in rats several days following injection. Hence, addition of a nitrogen-containing functional group to a traditional cannabinoid structure does not eliminate agonist effects and may produce delayed attenuation of cannabinoid-induced pharmacological effects.

Effects of site-selective NMDA receptor antagonists in an elevated plus-maze model of anxiety in mice

NMDA receptor antagonists have been shown to be anxiolytic in animal models of anxiety, although they have not been tested extensively. These compounds bind to several specific sites within the NMDA-receptor complex, including the NMDA site itself, the phencyclidine site, and the strychnine-insensitive glycine site. The purpose of the present study was to examine potential anxiolytic effects of site-selective NMDA receptor antagonists in the elevated plus-maze. Drug-naive albino mice were placed in the center of an elevated maze shaped like a plus sign. Two opposing arms were enclosed by high walls; the crossing arms were open. Following injection with drug or vehicle, the number of entries and time spent in each type of arm were measured during 5-min tests. Analysis of results showed that the benzodiazepine, diazepam, and the competitive NMDA receptor antagonist, NPC 17742 (2R,4R,5S 2-amino-4,5-(1,2-cyclohexyl)-7-phosphono-heptanoic acid), increased number of open arm entries and open arm time. N-Nitro-L-arginine methyl ester, a nitric oxide synthase inhibitor which may interfere with the transduction of NMDA receptor activation, also increased open arm entries and time; however, the magnitude of these increases was small. The phencyclidine-site NMDA receptor antagonist, phencyclidine, increased open arm entries, but failed to significantly increase open arm time. ACEA 1021 (5-nitro-6,7-dichloro-1,4-dihydro-2,3-quinoxalinedione), a putative glycine-site antagonist, had significant effects only on open arm entries at the highest dose tested. These results suggest that NMDA receptor antagonists show promise as potential anxiolytic agents, but that differences among antagonists acting at different cellular sites may be expected.

Pharmacological specificity of the discriminative stimulus effects of delta 9-tetrahydrocannabinol in rhesus monkeys

delta 9-Tetrahydrocannabinol (delta 9-THC) discrimination has been used as an animal model of cannabis intoxication in humans. While numerous studies have examined the discriminative stimulus effects of cannabinoids in rats and pigeons, studies with monkeys have been rare. In the present study, rhesus monkeys, trained to discriminate delta 9-THC from vehicle in a two-lever drug discrimination procedure, were tested with a variety of psychoactive drugs, including cannabinoids as well as drugs from other classes. Results showed that delta 9-THC discrimination showed pharmacological specificity, in that none of the non-cannabinoid drugs fully substituted for delta 9-THC. In contrast, the classical cannabinoids, delta 9-THC and delta 8-THC, and the novel cannabinoids, WIN 55212-2 and 1-butyl-2-methyl-3-(1-naphthoyl)indole, produced full dose-dependent substitution for delta 9-THC in all monkeys. A heptyl indole derivative failed to substitute for delta 9-THC, but it also did not displace [3H] CP 55940 from its binding site. These findings are consistent with those of previous cannabinoid discrimination studies with rats and suggest that results of delta 9-THC discrimination studies in rhesus monkeys may be predictive of the subjective effects of cannabinoid drugs in humans.

Antagonism of the discriminative stimulus effects of delta 9-tetrahydrocannabinol in rats and rhesus monkeys

A newly developed cannabinoid antagonist, SR141716A [N-(piperidin-1-yl)- 5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxa mide hydrochloride], binds to brain cannabinoid receptors and has been shown to block characteristic pharmacological effects of the aminoalkylindole cannabinoid agonist, WIN 55,212-2 (R-(+)-(2,3-dihydro-5-methyl-3-[(4-morpho- linyl)methyl]pyrol-(1,2,3-de]-1,4-benzoxazin-6-yl)(1-n aphthalenyl)methanone monomethanesulfonate). In the present study, the effects of this compound in an animal model of cannabis intoxication were investigated. Rats were trained to press one lever after being injected with 3 mg/kg of delta 9-tetrahydrocannabinol (delta 9-THC) and to press a second lever after injection with vehicle. Rhesus monkeys also were trained to discriminate between delta 9-THC and vehicle. Results of tests with various doses of SR141716A in combination with 3 mg/kg of delta 9-THC showed that SR141716A produced reversible, dose-dependent antagonism of the discriminative stimulus properties of delta 9-THC in rats, with recovery within 24 hr. SR141716A also blocked the discriminative stimulus effects of delta 9-THC in monkeys. Furthermore, in rats, 1 mg/kg of SR141716A produced a 12-fold rightward shift in the delta 9-THC dose-effect curve and a 43-fold rightward shift in the WIN 55,212-2 dose-effect curve. When SR141716A was administered alone, it did not substitute for delta 9-THC in rats. The present results suggest that SR141716A blocks the discriminative stimulus effects of delta 9-THC via a receptor-mediated mechanism. This drug is the first reliable antagonist of cannabinoid discrimination and would be predicted to block or reverse cannabis intoxication in humans.

Discriminative stimulus effects of CP 55,940 and structurally dissimilar cannabinoids in rats

CP 55,940 is a potent synthetic bicyclic cannabinoid analog that has been used in a number of studies as a radioligand for the cannabinoid receptor. This compound shares behavioral and biochemical properties with naturally occurring cannabinoids such as delta 9-THC. The purpose of the present study was 3-fold: to establish the ability of CP 55,940 to serve as a discriminative stimulus, to determine whether this discriminative stimulus is identical to that of delta 9-THC, and to examine whether a newly developed cannabinoid antagonist, SR141716A, would antagonize the discriminative stimulus effects of CP 55,940. Rats were trained to discriminate 0.1 mg/kg CP 55,940 from vehicle in standard 2-lever operant conditioning chambers. CP 55,940 produced dose-dependent generalization from the training dose in dose-effect determinations conducted before and after testing with other drugs. The effects of the training dose of CP 55,940 were dose-dependently antagonized by co-administration of SR141716A. Results of substitution tests showed that delta 9-THC, WIN 55,212-2, and cannabinol substituted completely for CP 55,940 in a dose-dependent manner; however, CP 55,940 was approx 10-fold more potent than any of the other drugs in producing CP 55,940-like discriminative stimulus effects. Several drugs with CNS depressant properties (phencyclidine, haloperidol and diazepam) failed to produce reliable substitution for CP 55,940. These results demonstrate that CP 55,940 has discriminative stimulus effects and that it shares these effects with structurally dissimilar compounds that, like CP 55,940, bind to the cannabinoid receptor. Further, these effects are blocked by SR141716A, a cannabinoid receptor antagonist.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro and in vivo effects of cocaine and selected local anesthetics on the dopamine transporter

The effects of selected local anesthetics on in vitro and in vivo measurements of dopamine transporter activity were determined to investigate the role of local anesthetic activity in the neuronal actions of cocaine. Cocaine inhibited [3H]2-beta-carbomethoxy-3-beta-(4-fluorophenyl)tropane 1.5-naphthalenedisulfonate (CFT) binding and [3H]dopamine uptake with estimated Ki and IC50 values of 0.6 microM and 0.7 micorM, respectively. Of the local anesthetics tested, only dimethocaine showed full displacement of CFT binding (0-30 microM tested) and full inhibition of dopamine uptake (0-100 microM tested). Dimethocaine was only slightly less potent than cocaine with an estimated Ki of 1.4 micorM and an IC50 value of 1.2 microM for [3H]CFT binding and dopamine uptake. At a maximum concentration of 100 microM, the ester containing local anesthetics procaine, tetracaine, piperocaine and the amide containing local anesthetic dibucaine and bupivacaine partially inhibited dopamine uptake by 47-70%. The ester containing local anesthetic propoxycaine and the amide containing local anesthetics prilocaine, etidocaine, procainamide, and lidocaine inhibited dopamine uptake by 8-30% at 100 microM. A 10 min administration of cocaine, dimethocaine, or procaine in the dialysis solution produced dose-dependent, reversible increases in endogenous dopamine efflux from the striata of awake rats. Cocaine and dimethocaine produced similar 12-fold increases in dialysate dopamine at concentrations of 0.1 mM and 1 mM respectively. Procaine (10 mM) produced a 6-fold increase in dialysate dopamine while lidocaine (1 mM) produced a reproducible and reversible decrease (30%). These results show that the cocaine-like actions of certain local anesthetics such as dimethocaine and procaine result from their direct actions of dopamine uptake inhibitors.

Pharmacological specificity of delta 9-tetrahydrocannabinol discrimination in rats

While many previous studies have shown that a variety of cannabinoids substitute and cross-substitute for delta 9-tetrahydrocannabinol (THC) in drug discrimination procedures, few have systematically examined potential THC-like effects of non-cannabinoid compounds. The purpose of the present study was to delineate further the pharmacological specificity of THC discrimination. Rats were trained to discriminate THC (3.0 mg/kg) from vehicle. Following determination of a dose-effect curve with THC, substitution tests with selected compounds from a variety of pharmacological classes, including l-phenylisopropyl adenosine, dizocilpine, dextromethorphan, clozapine, buspirone, MDL 72222, muscimol, midazolam and chlordiazepoxide, were performed. Whereas THC produced full dose-dependent substitution, substitution tests with non-cannabinoid drugs resulted in less than chance (50%) levels of responding on the THC-appropriate lever, with the exception of (+)-MDMA (2.5 mg/kg, 50%) and diazepam (3.0 mg/kg, 67%). These results are consistent with those of previous studies and suggest that the discriminative stimulus effects of THC exhibit pharmacological specificity.

Discriminative stimulus effects of esteratic local anesthetics in squirrel monkeys

A number of esteratic local anesthetics serve as positive reinforcers and produce cocaine-like discriminative stimulus effects in animals. It has been suggested that the affinity of these compounds for a site on the dopamine transporter, and not their local anesthetic actions, is responsible for these abuse-related behavioral effects. In the present study, three local anesthetics previously shown to be self-administered in animals were examined in squirrel monkeys trained to discriminate cocaine (0.3 mg/kg) from saline in a two-lever, food-reinforced procedure. Dimethocaine (0.1-3.0 mg/kg) fully and dose-dependently substituted for cocaine. Doses of dimethocaine (1.7 mg/kg) and cocaine (0.3 mg/kg) which produced full (> 80%) substitution for cocaine were administered in combination with the dopamine D1 receptor antagonist SCH 39166 ((-)-trans-6,7,7a,8,9,13b-hexahydro-3-chloro-2-hydroxy-N-methyl-5H -benzo [d]naphtho-(2,1-b)azepine) and the dopamine D2 receptor antagonist raclopride (both at 0.003-0.03 mg/kg). SCH 39166 fully blocked the cocaine-like discriminative stimulus effects of dimethocaine and cocaine, but raclopride produced only partial antagonism of cocaine-lever selection. In addition, there was some evidence that raclopride blocked cocaine-lever responding produced by a lower dose of dimethocaine. In substitution studies, neither procaine (1-10 mg/kg) nor chloroprocaine (1-30 mg/kg) produced cocaine-like effects. These results support a role for dopamine in the behavioral effects of some local anesthetics.

Discriminative stimulus effects of excitatory amino acid agonists in rats

Sixteen male Sprague-Dawley rats were trained to discriminate 30 mg/kg i.p. NMDA from saline using a 2-lever operant procedure. Responding was maintained under a FR 32 schedule of food reinforcement. Substitution tests were completed with NMDA (3-56 mg/kg) and other putative excitatory amino acids, L-glutamate (30-560 mg/kg), L-aspartate (30-300 mg/kg), L-homocysteic acid (100-1500 mg/kg), L-cysteine (30-1000 mg/kg), monosodium glutamate (100-3000 mg/kg), kainic acid (0.1-3 mg/kg) and the selective NMDA receptor agonist, D,L-(tetrazol-5-yl)glycine (LY 285265) (0.01-1.0 mg/kg). LY 285265 fully substituted for NMDA and was approx 100-fold more potent than NMDA for producing NMDA-like discriminative stimulus effects. Partial substitution occurred with monosodium glutamate, L-glutamate and L-homocysteic acid, resulting in mean maximum levels of 49-59% NMDA-lever responding, however response rate decreases were only obtained with 3000 mg/kg monosodium glutamate, suggesting that behaviorally active doses of the other compounds may not have been fully studied. L-Cysteine, kainic acid and L-aspartate failed to substitute for NMDA or produce decreases in response rates. Unlike with other excitatory agonists tested, full substitution occurred only with LY 285265, providing evidence that selective NMDA receptor activation is the basis for the NMDA discriminative stimulus. These results also suggest that LY 285265 is a potent, systemically active, selective agonist for the NMDA receptor.

Effects of competitive and non-competitive N-methyl-D-aspartate (NMDA) antagonists in squirrel monkeys trained to discriminate D-CPPene (SDZ EAA 494) from vehicle

Drug discrimination studies have proven useful for comparing and contrasting the behavioral effects of site-selective N-methyl-D-aspartate (NMDA) antagonists. This study examined the effects of competitive and non-competitive NMDA antagonists in squirrel monkeys trained to discriminate 1 mg/kg D-CPPene [D-3-(2-carboxypiperazine-4-yl)-1- propenyl-1-phosphonic acid; SDZ EAA 494] from vehicle in a two-lever drug discrimination procedure. Results show that D-CPPene and several other competitive NMDA antagonists (NPC 17742, CGS 19755, and CGP 37849) completely substituted for D-CPPene in a dose-dependent manner. In contrast, phencyclidine (PCP) and ketamine produced only partial substitution at doses that severely suppressed response rates. These results are consistent with results of earlier studies with rats and monkeys showing differences in the discriminative stimulus effects of competitive and PCP-like non-competitive NMDA antagonists. The data support the predictions (1) that D-CPPene and the other competitive NMDA antagonists tested would have similar subjective effects in humans and (2) that some differences would be found in the subjective effects of competitive NMDA antagonists and PCP-like non-competitive antagonists.

Evaluation of the reinforcing effects of eliprodil in rhesus monkeys and its discriminative stimulus effects in rats

Two studies examined the abuse potential of the N-methyl-D-aspartate (NMDA) non-competitive antagonist eliprodil [(+/-)-alpha-(4-chlorophenyl)-4-[(fluorophenyl)methyl]-1- piperidineethanol] by evaluating its reinforcing effects in rhesus monkeys and its phencyclidine (PCP)-like discriminative stimulus effects in rats. The monkeys were trained to self-administer PCP i.v. under a fixed ratio 10 schedule of reinforcement. After the monkeys were trained, saline, vehicle and various doses of eliprodil were substituted for PCP. The rats were trained to discriminate 3 micrograms/kg PCP from saline using a standard two-lever discrimination procedure with correct-lever responding reinforced under a fixed ratio 10 schedule of food reinforcement. After acquiring the discrimination, the rats were tested with various doses of PCP, dizocilpine and eliprodil. The self-administration study showed that eliprodil did not have reinforcing effects, since it maintained injection rates comparable to the negative controls, saline and vehicle. In the discrimination study it was found that the higher doses of PCP and dizocilpine resulted in 100% PCP-associated lever responding, whereas eliprodil occasioned no responding on the PCP-associated lever. The results from these studies suggest that eliprodil has a low potential for abuse in humans as well as providing further evidence that eliprodil produces a profile of behavioral effects unlike the PCP-site selective NMDA antagonists.

A comparison of the acute behavioral effects of alkylbenzenes using a functional observational battery in mice

The acute neurobehavioral effects of six alkylbenzenes (benzene, toluene, ethylbenzene, propylbenzene, m-xylene, and cumene) were evaluated after 20-min inhalation exposures using a functional observational battery (FOB) in mice. In order to do this, an explicit protocol for the FOB developed for rats by Moser was adapted for use in mice with inhalation exposures. All six alkylbenzenes, in the concentration range of 2000 to 8000 ppm, produced a nearly identical profile of effects, a profile that was also produced by ip administration of the central nervous system depressant drug pentobarbital. These effects included changes in posture, decreased arousal and rearing, increased ease of handling, disturbances of gait, mobility, and righting reflex, decreased forelimb grip strength, increased landing foot splay, and impaired psychomotor coordination. The response to various sensory stimuli was also decreased by the alkylbenzenes and pentobarbital. These acute effects of alkylbenzenes were short-lived, with recovery beginning within minutes of removal from the exposure chamber. The finding that the alkylbenzenes produce a profile of neurobehavioral effects similar to that of pentobarbital is consistent with a growing body of other evidence that many solvents produce a profile of acute effects similar to that of central nervous system depressant drugs and ethanol.

Discriminative stimulus effects of presynaptic GABA agonists in pentobarbital-trained rats

The discriminative stimulus effects of indirect-acting GABAergic drugs were compared to those of pentobarbital (PB) and midazolam in rats trained to discriminate 5 mg/kg PB from saline under a two-lever fixed-ratio 32 schedule of food reinforcement. PB and midazolam produced dose-dependent substitution for the training dose of PB with response rate reduction only at doses above those producing full substitution. Valproic acid, an antiepileptic drug and GABA transaminase inhibitor, substituted for PB but only at a dose that produced response rate suppression. Vigabatrin, an irreversible GABA transaminase inhibitor, failed to substitute for PB, but did produce a dose-dependent decrease in response rates. The GABA uptake inhibitors, 1-[2-[bis[4-(trifluoromethyl)phenyl]-methoxy]ethyl]-1,2,5,6- tetrahydro-3-pyridinecarboxylic acid (CI-966) and (R(-)-N-[4,4-bis(3-methylthien-2-yl)but-3-enyl] nipecotic acid HCl (tiagabine), produced no greater than 40% PB-lever responding. Aminooxyacetic acid (AOAA), which is described as a nonselective presynaptic GABA agonist, yielded a maximum of 43% PB-lever responding. These results indicate that the discriminative stimulus effects of the indirect GABAA agonists, PB and midazolam, although similar to one another, differ from those of presynaptic GABAergic drugs. Differences in the discriminative stimulus properties of GABA transaminase inhibitors and uptake inhibitors also exist, indicating that not all presynaptic GABA agonists have similar behavioral profiles. These results contribute to a further understanding of the similarities and differences in the behavioral effects of drugs that enhance GABAergic neurotransmission.

The effects of delay of reinforcement and dose on the self-administration of cocaine and procaine in rhesus monkeys

Delays of reinforcement were imposed while three rhesus monkeys intravenously self-administered either cocaine or procaine to determine whether the magnitude of delay-induced reductions in infusion frequency were dose- or drug-dependent. Drug-maintained baselines demonstrating the greatest resistance to change under delay conditions might be conceptualized as involving the greatest strength of responding and include drug reinforcers with the greatest efficacy. Under baseline conditions, cocaine or procaine was infused contingent upon lever pressing according to fixed ratio 10 schedules during daily, 1-h experimental sessions. Under delay conditions, a 120-s delay of reinforcement was inserted between the completion of each fixed ratio contingency and infusion delivery. Tests under baseline and delay conditions alternated until several doses of cocaine (3, 10, 33, 100, and 300 micrograms/kg), and subsequently, of procaine (33, 100, 300, 1000, and 3000 micrograms/kg) were tested. The results indicated that during both baseline and delay conditions increases in dose of either drug resulted in increases followed by decreases in the number of infusions obtained. Introducing delays of reinforcement resulted in decreases in the number of infusions obtained at a given dose. Generally, however, when reinforcement delays were imposed the decreases in infusion number, as a percentage of baseline number, were less at the higher doses of both drugs. The doses of procaine which were maximally effective as reinforcers (i.e. during which infusion number was minimally reduced by delay) were as effective as the maximally effective doses of cocaine.(ABSTRACT TRUNCATED AT 250 WORDS)

Discriminative stimulus effects of delta 9-tetrahydrocannabinol and delta 9-11-tetrahydrocannabinol in rats and rhesus monkeys

Previous reports have suggested that delta 9-11-tetrahydrocannabinol (delta 9-11-THC), an exocyclic analog of delta 9-tetrahydrocannabinol (delta 9-THC), may have weak agonist effects as well as antagonistic properties. The purpose of the present study was to examine the effects of delta 9-11-THC in substitution and antagonism tests in rats and in rhesus monkeys trained to discriminate delta 9-THC from vehicle in two-lever drug-discrimination procedures. The substitution studies showed that delta 9-11-THC generalizes from the training dose of delta 9-THC in rats and in monkeys, although it was less potent in both species. The magnitude of the potency difference was greater in monkeys than in rats. When administered immediately following injection with the training dose of delta 9-THC, delta 9-11-THC failed to block the delta 9-THC cue in rats and showed a lack of dose-responsive inhibition in monkeys. These results suggest that delta 9-11-THC is devoid of antagonistic properties in the drug discrimination paradigm.

Pharmacological specificity of N-methyl-D-aspartate discrimination in rats

The purpose of this study was to provide further information on the usefulness of N-methyl-D-aspartate (NMDA) discrimination in rats as a behavioral model for NMDA receptor activation. The pharmacological specificity of the NMDA discriminative stimulus was examined in rats trained to discriminate 30 mg/kg, i.p. NMDA from saline using a 2-lever fixed-ratio (FR) 32 food reinforcement schedule. Pharmacologically diverse centrally-acting agents were examined for their ability to substitute for NMDA. Morphine did not substitute for NMDA; neither did the central stimulants, caffeine and (+)-amphetamine, which produced a maximum mean of only 16 and 35% NMDA-lever responding, respectively. Pentylenetetrazol and picrotoxin also did not substitute for NMDA. Compounds interacting with cholinergic neurotransmission including nicotine, physostigmine, arecoline and mecamylamine, produced at best, only intermediate levels of NMDA-lever responding (32-61%), with the highest levels of NMDA-lever responding generally occurring at doses that also reduced rates of responding. These results suggest that the discriminative stimulus properties of NMDA are dissimilar from those of a number of centrally-acting drugs. Combined with the results of studies indicating that the NMDA discriminative stimulus can be antagonized by competitive NMDA antagonists, these results provide further evidence that NMDA receptor activation is the basis of NMDA discrimination and that this model may be useful for studying site-selective NMDA agonists and antagonists.

Pharmacological specificity of the discriminative stimulus properties of 2-amino-4,5-(1,2-cyclohexyl)-7-phosphono-heptanoic acid (NPC 12626), a competitive N-methyl-D-aspartate receptor antagonist

A drug discrimination based upon the competitive N-methyl-D-aspartate (NMDA) antagonist 2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoic acid (NPC 12626) was assessed for pharmacological specificity. Adult male Sprague-Dawley rats were trained to discriminate 20 mg/kg i.p. of NPC 12626 from saline under a standard two-lever fixed ratio 32 schedule of food reinforcement. Stimulus generalization tests were conducted to examine the similarities and differences between NPC 12626, its active (2R,4R,5S) enantiomer NPC 17742, other competitive and noncompetitive NMDA antagonists and a number of drugs representative of other classes. During test sessions, the competitive NMDA antagonists NPC 12626, CGS 19755, [1-(cis-2-carboxypiperidine-4-yl)- methyl-1-phosphonic acid], NPC 17742, CSP 37849 [DL-(E)-2-amino-4-methyl-5-phosphono-3-pen-tenoic acid] and CPPene [D-3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphonic acid] all completely substituted for the training dose of NPC 12626 with ED50 values of 18.1, 2.3, 2.1, 0.8 and 0.8 mg/kg, respectively. In contrast, drugs that failed to substitute for NPC 12626 included (+)-amphetamine, baclofen, chlorpromazine, dextromethorphan, diazepam, dizocilpine (MK-801), imipramine, (-)-ketocyclazocine, L-N6-phenylisopropyladenosine, methocarbamol, morphine, muscimol, phenytoin, physostigmine and valproate. These results provide evidence that the NPC 12626 discriminative stimulus is unique and specific, shared fully only by its active enantiomer NPC 17742 and other competitive NMDA antagonists. This specificity provides further support for the hypothesis of NMDA receptor mediation of NPC 12626 discrimination, and suggests that this is a useful model to evaluate behavioral effects of competitive NMDA antagonists.

Inhaled 1,1,1-trichloroethane-produced physical dependence in mice: effects of drugs and vapors on withdrawal

1,1,1-Trichloroethane (TCE), a widely used and abused solvent, was investigated for its ability to produce physical dependence in mice. Cessation of 4 days of continuous inhalation of TCE (500-4000 ppm) resulted in a withdrawal syndrome characterized by handling-induced convulsions and increased susceptibility to pentylenetetrazol-induced convulsions. The severity of withdrawal convulsions was diminished by 30 to 60 min of reexposure to 2000 to 4000 ppm TCE or to the vapor of another widely used and abused solvent, toluene (1000-2000 ppm). Ethanol (1-2 g/kg), midazolam (0.3-1 mg/kg) and pentobarbital (30 mg/kg) were also effective in decreasing the withdrawal severity; however, chlorpromazine (3 mg/kg) and phenytoin (30 mg/kg) were without effects. These data suggest that TCE has the ability to produce physical dependence of the central nervous system depressant drug type. Taken together with other evidence for similarities in the pharmacological and behavioral effects of TCE and depressant drugs of abuse, these data support the hypothesis that the basis for TCE abuse may be its ability to produce ethanol- and depressant drug-like effects.

Effects of NMDA receptor antagonists in squirrel monkeys trained to discriminate the competitive NMDA receptor antagonist NPC 12626 from saline

Because excitatory amino acids have been implicated in several physiological phenomena, antagonists of excitatory amino acid function may have significant therapeutic potential as anticonvulsants, neuroprotectants and anxiolytics. Drug discrimination procedures in animals have proven useful to compare and contrast the behavioral effects of site-selective NMDA antagonists. In the only previous study using a competitive NMDA antagonist as a training drug, rats were trained to discriminate NPC 12626 (2-amino-4,5-(1,2-cyclohexyl)-7-phosphonohepatanoic acid) from nondrug. The major goal of the present study was to establish and characterize a nonhuman primate model of NPC 12626 discrimination. Adult male squirrel monkeys were trained to discriminate NPC 12626 from saline under a two-lever fixed ratio-30 schedule of food reinforcement. The monkeys required between 80 and 120 training sessions to acquire this discrimination after the training dose had been raised from 3 to 20 mg/kg i.m. The competitive NMDA antagonists CGP 37849 (D,L-(E)-2-amino-4-methyl-5- phosphono-3-pentanoic acid) and CPPene (D-3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphonic acid) substituted completely for NPC 12626, while the potent noncompetitive NMDA antagonist, dizocilpine (MK-801), did not. These results reflect a profile of discriminative stimulus effects which support that observed in rats and establish a primate model for use in further study of the behavioral effects of the competitive NMDA antagonists.