Cannabinoid agonists (e.g., THC) dose-dependently decrease locomotor activity and body temperature and produce antinociception and catalepsy. Drugs that produce this tetrad of effects within a limited dose range are likely to function as CB1 receptor agonists. A structure activity relationship study from our laboratory investigating analogs of the CB1 antagonist rimonabant revealed that certain alterations in the 3-substituent of rimonabant’s pyrazole core conferred agonist-like properties in the tetrad. Interestingly, these effects were present in CB1 -/- mice, and were not reversed by rimonabant in wild-type mice. The present study evaluated two novel 3-substituent rimonabant analogs, O-6629 and O-6658 in the tetrad and drug discrimination, a preclinical model of drug subjective effects that possesses a high degree of pharmacological specificity. Drugs that elicit cannabinergic psychoactive effects in humans are likely to produce THC-like operant responding in animals trained to discriminate between the interoceptive stimuli produced by THC relative to vehicle. O-6629 and O-6658 decreased locomotor activity and body temperature and produced catalepsy. O-6629, but not O-6658 produced significant antinociception. However, these drugs differed from THC in regard to the magnitude of tetrad effects observed. These analogs also failed to elicit THC-like discriminative stimulus effects, nor did they antagonize THC’s discriminative stimulus in mice discriminating 5.6 mg/kg THC from vehicle. Finally, mice were trained to discriminate 5.6 mg/kg O-6629 from vehicle. O-6658 produced full substitution for O-6629, whereas the cannabinoid agonists THC and anandamide did not. O-6629’s discriminative stimulus failed to generalize to rimonabant, cocaine or morphine, whereas WIN 55,212-2 and nicotine evoked partial substitution. These results suggest that these analogs might exert their pharmacological properties through a novel cannabinoid receptor, as has been proposed for WIN 55,212-2 and anandamide. Additionally, O-6629’s discriminative stimulus may involve nicotinic acetylcholine or dopaminergic components. Future directions include determining whether the partial substitution observed with nicotine was mediated through a nicotinic mechanism. Tests with chlorpromazine, an antipsychotic that is a false positive in the tetrad, and diazepam, which produces partial substitution for THC’s discriminative stimulus through a GABAergic mechanism are also planned.

There is accumulating evidence from many directions indicating that gender plays a critical role in drug abuse. Biological factors, including gonadal sex hormones, contribute in a significant although incompletely understood manner, to gender differences in drug abuse. Female sex hormones have been shown to affect central nervous system function and modulate the effects of drugs of abuse. For example, GABAA receptor function is positively modulated by progesterone. There is evidence from preclinical in vitro and in vivo studies as well as some clinical research suggesting that progesterone and its metabolites may enhance the behavioral effects of benzodiazepines, which also serve as positive modulators of GABAA receptors. The three studies presented here utilize within subject designs to assess the role of progesterone on the discriminative stimulus, subjective, performance and cardiovascular effects of triazolam, a short-acting benzodiazepine, in healthy, premenopausal women. The first study examined the effect of menstrual cycle phase on the discriminative stimulus effects of triazolam (0.00, 0.06, 0.12 and 0.25 mg/70 kg). The results of this study indicated that when progesterone levels peak (mid luteal phase), the discriminative stimulus effects of triazolam (0.12 mg/70 kg) are enhanced. The second study examined the separate and combined effects of a range of acute doses of oral micronized progesterone (0, 100 and 200 mg) and oral triazolam (0.00, 0.12 and 0.25 mg/70 kg) on the subjective, psychomotor and physiological effects of these medications, tested under conditions of low circulating sex hormones. The results of this study indicated that progesterone alone has some short-acting, sedative-like effects and enhances the subjective and performance effects of triazolam. The final study examined the effects of progesterone (0 and 100 mg) on the discriminative stimulus effects of triazolam (0.00, 0.06, 0.12 and 0.25 mg/70 kg), also under conditions of low circulating sex hormones. The results of this study indicated that the parent hormone progesterone does not appear to alter sensitivity to the discriminative stimulus effects of triazolam. Increases in sensitivity to triazolam in studies 1 and 2 may have been the result of neuroactive progesterone metabolites (e.g., allopregnanolone, TH-DOC), although future studies will be required to further examine this possibility. Taken together, these studies help clarify the manner in which the ovarian hormone progesterone and its metabolites modulate the behavioral effects of the benzodiazepines.

Ziprasidone (ZPD) is an atypical antipsychotic drug (APD) that has been shown to fully substitute in C57BL/6 mice for the discriminative stimulus properties of the atypical APD clozapine (CLZ). In rats, however, it has failed to substitute for either 1.25 mg/kg or 5.0 mg/kg training doses of CLZ. Here the discriminative stimulus properties of ZPD were examined by training 19 adult male Sprague-Dawley rats to discriminate 2.0 mg/kg ZPD from vehicle in a two-lever drug discrimination procedure (ED50 = 0.07 mg/kg). The atypical APD CLZ produced full substitution (ED50 =0.76 mg/kg), as did the atypical APDs zotepine (ED50 = 0.63 mg/kg), olanzapine (ED50 = 0.25 mg/kg), quetiapine (ED50 = 0.93 mg/kg), and risperidone (ED50 = 0.09 mg/kg). The 5-HT2A antagonist ritanserin also fully substituted for ZPD (ED50 = 1.27 mg/kg). Partial substitution (2A/B/C receptors play an important role in the discriminative stimulus properties of ZPD and perhaps the ratio of binding to 5-HT2A/B/C and D2 receptors. While it will be necessary to test additional APDs, these initial findings suggest that ZPD drug discrimination may be a useful model to differentiate atypical from typical APDs.

Abstract ESTABLISHING THE DISCRIMINATIVE STIMULUS PROPERTIES OF THE ATYPICAL ANTIPSYCHOTIC AMISULPRIDE IN C57BL/6 MICE Antipsychotic medications are used to treat schizophrenia. The present study used the drug discrimination paradigm to measure the subjective effects of the atypical antipsychotic amisulpride and to examine the underlying neuropharmacological mechanisms responsible for the discriminative stimulus property of the drug. Male C57BL/6 mice were trained to discriminate 10 mg/kg (-)S amisulpride from vehicle in a two-lever drug discrimination task. A dose effect curve for (-)S amisulpride yielded an ED50 = 1.77 mg/kg 95% CI [1.28, 2.45 mg/kg]. Substitution testing was conducted for the isomer (+)R amisulpride, racemic (±)SR amisulpride, the atypical antipsychotics clozapine, aripiprazole and the typical antipsychotic haloperidol. There was partial substitution for (+)R amisulpride, and full substitution for (±)SR amisulpride with a significant rightward shift in the dose effect curves. Clozapine, aripiprazole, and haloperidol failed to fully substitute with significant rate suppression at the higher doses. These results demonstrated that (-)S amisulpride has a unique discriminative stimulus that differs from other antipsychotic drugs.

The increase in heroin use is one factor contributing to the current opioid epidemic in the United States. There are three Food and Drug Administration (FDA) approved medications for the treatment of opioid use disorder (OUD), and these include agonist (i.e. methadone and buprenorphine) and antagonist (i.e. naltrexone) therapies. Although these medications are effective for some patients, regulatory constraints for agonist therapies limit access and patient compliance for naltrexone is poor. The development of new therapies, such as immunopharmacotherapies, for the treatment of OUD is a priority for the National Institute of Drug Abuse. A heroin immunopharmacotherapy, or vaccine, produces heroin selective antibodies that bind to and sequester heroin in the periphery. One formulation of a heroin-tetanus toxoid (TT) conjugate vaccine has shown promise in preclinical studies in mice in monkeys but has not been fully assessed to determine independent variables that might impact vaccine effectiveness such as heroin route of administration, species of animal or abuse-related behavioral endpoints. Chapter II of this dissertation aimed to determine effectiveness and selectivity of the heroin-TT conjugate vaccine to alter the antinociceptive effects of subcutaneous and intravenous heroin in male and female rats. In addition, maximal vaccine effects were compared to effects of a positive control naltrexone. Vaccine effectiveness to reduce heroin antinociception was selective, but effectiveness did not depend on route of administration. Furthermore, maximum effects were less than those seen with a clinically meaningful dose of naltrexone. Combining the vaccine with naltrexone enhanced the effectiveness of naltrexone to block the antinociceptive effects of heroin. Chapter III determined vaccine effectiveness and selectivity to block heroin’s discriminative-stimulus effects in nonhuman primates and compared maximal effects produced by vaccine and naltrexone. The heroin vaccine weakly but selectively reduced the abuse-related subjective-like effects of heroin in one of two monkeys. However, chronic naltrexone treatment nonselectively antagonized the abuse-related effects of both heroin and fentanyl, and naltrexone effects were more robust than those of the vaccine. Chapter IV established a translational procedure to assess candidate medication effects on the reinforcing effectiveness of heroin in a heroin versus food choice procedure in rats. In the procedure, rats choose between a liquid food reinforcer and ascending doses of heroin in a 5-component choice procedure. Heroin versus food choice was found to be sensitive to an environmental manipulation of altering response requirement for both reinforcers. Chronic buprenorphine decreased heroin choice, consistent with its FDA-approved indication for treating OUD. Collectively, these data suggest that the current formulation of the heroin-TT conjugate vaccine may not be as effective as naltrexone at decreasing heroin use. However, one potential indication the vaccine may be useful for is as an adjunctive therapy to clinically available agonist or antagonist medications and a heroin versus food choice procedure in rodents would be one way to full assess this preclinically.

The 129S2 inbred mouse strain is often used as a background strain in the production of genetically altered mice (i.e. knockout and transgenic mice). It is important to establish the behavioral phenotype of wild-type mice before making comparisons to genetically altered mice. Also, those comparisons can assist in the evaluation and interpretation of the in vivo effects of drugs. The drug discrimination assay measures the subjective effects of drugs and provides a measure of underlying neuropharmacological mechanisms responsible for the discriminative stimulus properties of drugs. The present study established the atypical antipsychotic drug clozapine as a discriminative stimulus in male 129S2 inbred mice and compared clozapine’s discriminative stimulus properties in 129S2 mice to C57BL/6 and DBA/2 inbred mice. By comparing the discriminative stimulus properties between inbred strains of mice we hope to obtain a fuller picture of the underlying neuropharmacological mechanisms of antipsychotic drugs.

Nitrous oxide (N2O) is a widely used anesthetic adjunct in dentistry and medicine that is also commonly abused. N2O alters the function of several receptors in vitro and ex vivo, however, the receptors systems underlying its abuse-related intoxicating effects are poorly understood. The goals of this dissertation were to (1) establish N2O as a discriminative stimulus, (2) characterize the temporal properties of the discriminative stimulus, (3) determine the degree of similarity between N2O and other inhalants and (4) explore the neurochemical effects responsible for the stimulus properties of N2O. Twenty-four mice were trained to discriminate 10 minutes exposure to 60% N2O+40% O2 from 100% O2 in daily 5 minute food-reinforced operant sessions. Mice acquired the discrimination in a mean of 38 sessions. N2O produced concentration-dependent full substitution for itself. Full substitution required 7 minutes of N2O exposure but the offset of stimulus effects following cessation of N2O exposure were more rapid. Varying degrees of partial substitution for N2O were engendered by abused vapors and vapor anesthetics. The aromatic hydrocarbon toluene produced the most robust substitution for N2O. One or more toluene concentrations produced full substitution for N2O in 7 of 8 subjects, suggesting that these two abused inhalants share common neurochemical mechanisms. The NMDA receptor open channel blockers (+)-MK-801, ketamine and memantine produced dose-dependent partial substitution for N2O. A competitive NMDA antagonist and NMDA glycine site antagonist did not substitute for N2O. Pretreatment with (+)-MK-801 as well as ethanol produced dose-dependent leftward shifts in the N2O concentration effect curve further suggesting some overlap in their mechanisms of action. GABAA agonists and positive allosteric modulators, opioid agonists, serotonergic agonists, nicotine, a nNOS inhibitor and the psychomotor stimulant amphetamine all failed to appreciably substitute for N2O and/or failed to alter the N2O concentration effect curve when administered prior to N2O exposure. No drug tested produced greater than 80% mean N2O-lever selection leaving open the possibility of other neurochemical contributors to the stimulus effects of N2O.

Amisulpride, a benzamide derivative, is an atypical antipsychotic drug used to treat both schizophrenia and depression. Amisulpride is a selective antagonist at dopamine D2 and D3 receptors and at serotonin 5-HT2B and 5-HT7 receptors and displays an atypical clinical profile with reduced extrapyramidal motor effects. The drug has a chiral center and is a mixture of two optical isomers: (S)-amisulpride and (R)-amisulpride. The present study used a two-lever drug discrimination assay to allow a direct comparison between amisulpride and its two isomers. Additionally, substitution testing was conducted with the typical antipsychotics, atypical antipsychotics, antidepressants, the anxiolytic chlordiazepoxide, several benzamide derivatives, and selective ligands with receptor mechanisms relevant to amisulpride. C57BL/6 mice were trained to discriminate 10 mg/kg rac-amisulpride from vehicle in a two-lever drug discrimination task for food reinforcement in an average of 35.7 sessions (range 6-89). The amisulpride dose-response curve (0.078 – 10.0 mg/kg) yielded an ED50 = 0.64 mg/kg, 95% CI [.47, 0.84 mg/kg]. The isomers fully substituted for amisulpride with a significant left-ward shift in the dose-response curve for (S)-amisulpride as compared to rac-amisulpride and (R)-amisulpride. The benzamide derivatives sulpiride and the (S)-sulpiride isomer fully substituted for amisulpride; tiapride produced partial substitution (76.4% DLR); none of the other tested drugs substituted for rac-amisulpride’s discriminative stimulus. These results showed that the rac-amisulpride stimulus was readily acquired in C57BL/6 mice, and that it has a unique and robust discriminative stimulus that is dose-dependent, time-dependent and stereoselective and is not shared with other antipsychotic or antidepressant drugs.

The glutamate system dysfunctions present in schizophrenia raise new questions about possible glutamatergic actions of the atypical antipsychotic clozapine. While clozapine has been shown to partially substitute for the discriminative stimulus of the glutamate agonist N-methyl D-aspartate (NMDA) in rats, NMDA discrimination has not previously been established in mice. The present study was designed to explore the possible role of NMDA activity in clozapine’s discriminative stimulus. Two groups of C57BL/6 mice were trained to discriminate either 2.5 mg/kg CLZ from vehicle or 30 mg/kg NMDA from vehicle in a standard two-lever drug discrimination task. NMDA drug discrimination was successfully established in C57BL/6 mice. While NMDA did not substitute for clozapine, clozapine partially substituted for NMDA at the 0.625 mg/kg dose, demonstrating an asymmetrical relationship between clozapine’s and NMDA’s discriminative stimuli. Dose combination tests further investigated this relationship. It was found that 0.625 mg/kg CLZ + 30 mg/kg NMDA produced partial substitution (61.82% DLR), while 0.625 mg/kg CLZ + 56 mg/kg NDMA full substitution (92.82% DLR) in CLZ-trained mice. In addition, combination testing with 10 mg/kg NMDA + 2.5 mg/kg CLZ and 10 mg/kg NMDA + 5.0 mg/kg CLZ produced full substitution in NMDA-trained mice ((80.04% DLR and 100% DLR, respectively). Finally, it was found that the α1- adrenoreceptor antagonist prazosin fully substituted for both CLZ (3.0 mg/kg = 92.20% DLR) and NMDA (1.0 mg/kg = 98.77% DLR and 3.0 mg/kg = 99.62% DLR). These findings suggest that interactions between clozapine’s and NMDA’s discriminative stimuli may involve antagonism of α1- adrenoreceptors, but further research of other mechanisms including serotonergic, histaminergic, and cholinergic receptor activity or metabolic interactions is needed. Finally, these initial finding suggest that that drugs active at glutamatergic receptors may have potential as therapeutic drugs for treatment of schizophrenia.

Due to its unique receptor binding profile and its relationship to clozapine, N-desmethylclozapine (NDMC) has been examined as a possible antipsychotic drug (APD). Clozapine has been trained as discriminative stimulus in our lab, but NDMC has not yet been established as a discriminative stimulus. In experiment 1, 12 C57BL/6 mice were trained to discriminate 10.0 mg/kg NDMC from VEH using a standard-two lever operant procedure to assess antipsychotic substitution. The typical APD clozapine fully substituted for NDMC at 2 doses tested (2.5 and 5.0 mg/kg), while typical APD haloperidol failed to substitute for NDMC. In Experiment 2, 11 mice were given repeated administration of NDMC to assess cross-tolerance development to the discriminative stimulus of clozapine. NDMC was successfully trained as a discriminative stimulus and was also shown to induce cross-tolerance to clozapine’s discriminative stimulus, indicating similar underlying pharmacological mechanisms of action between NDMC and clozapine.