The long-term effects of repeated heroin vapor inhalation during adolescence on measures of nociception and anxiety-like behavior in adult Wistar rats.

RATIONALE: Adolescents represent a vulnerable group due to increased experimentation with illicit substances that is often associated with the adolescent period, and because adolescent drug use can result in long-term effects that differ from those caused by drug use initiated during adulthood. OBJECTIVES: The purpose of the present study was to determine the effects of repeated heroin vapor inhalation during adolescence on measures of nociception, and anxiety-like behavior during adulthood in female and male Wistar rats. METHODS: Rats were exposed twice daily to 30 min of heroin vapor from post-natal day (PND) 36 to PND 45. At 12 weeks of age, baseline thermal nociception was assessed across a range of temperatures with a warm-water tail-withdrawal assay. Anxiety-like behavior was assessed in an elevated plus-maze (EPM) and activity was measured in an open-field arena. Starting at 23 weeks of age, baseline thermal nociception was re-assessed, nociception was determined after acute heroin or naloxone injection, and anxiety-like behavior was redetermined in the EPM. RESULTS: Adolescent heroin inhalation altered baseline thermal nociception in female rats at 12 weeks of age and in both female and male rats at ~ 23 weeks. Heroin-treated animals exhibited anxiety-like behavior when tested in the elevated plus-maze, showed blunted heroin-induced analgesia, but exhibited no effect on naloxone-induced hyperalgesia. CONCLUSIONS: The present study demonstrates that heroin vapor inhalation during adolescence produces behavioral and physiological consequences in rats that persist well into adulthood.

Appetitive, antinociceptive, and hypothermic effects of vaped and injected Δ-9-tetrahydrocannabinol (THC) in rats: exposure and dose-effect comparisons by strain and sex.

Advances in drug vapor exposure systems have enabled evaluation of Δ-9-tetrahydrocannabinol (THC) vapor effects in laboratory animals. The purpose of this study was to 1) establish a range of parameters of THC vapor exposure in rats sufficient to produce a behavioral dose-effect curve in a battery of tasks sensitive to THC; and 2) to investigate sex differences in the effects of THC vapor exposure and THC injection (intraperitoneal, IP) on these behaviors in two strains of outbred rats. Male and female Sprague Dawley and Wistar rats (N = 22, 5-6/sex per group) received THC via passive vapor exposure (200 mg/mL; 5 conditions) and IP injection (1-20 mg/kg) in a within subject design. The effects of vaped and injected THC on appetite was determined using progressive ratio responding for food pellets. THC effects on nociception, measured using the tail withdrawal assay, and body temperature were also assessed during a 5-h test period for evaluation of time course of effects. Plasma THC concentrations were assessed after THC vapor and 10 mg/kg IP THC. THC vapor produced exposure-related increases and decreases in motivation to obtain food under the progressive ratio schedule. IP THC (3-20 mg/kg) reduced breakpoints. Vaped and injected THC produced exposure and dose-dependent antinociception and hypothermia. Sex and strain differences in THC effects were also observed. Plasma THC concentrations were higher after 10 mg/kg IP THC (152 ng/mL) compared to the highest vapor exposure condition tested (38 ng/mL), but magnitude of behavioral effects were comparable. THC vapor exposure produced reliable, dose orderly effects on food-maintained behavior, nociception, and body temperature that are comparable to effects of IP THC, although there were differences in the time course of behavioral outcomes.

Effects of nicotine and THC vapor inhalation administered by an electronic nicotine delivery system (ENDS) in male rats.

BACKGROUND: Electronic nicotine delivery systems (ENDS, e-cigarettes) are increasingly used for the self-administration of nicotine by various human populations, including previously nonsmoking adolescents. Studies in preclinical models are necessary to evaluate health impacts of ENDS including the development of nicotine addiction, effects of ENDS vehicles, flavorants and co-administered psychoactive substances such as Δ9-tetrahydrocannabinol (THC). This study was conducted to validate a rat model useful for the study of nicotine effects delivered by inhalation of vapor created by ENDS. METHODS: Male Sprague-Dawley rats (N = 8) were prepared with radio telemetry devices for the reporting of temperature and activity. Experiments subjected rats to inhalation of vapor generated by an electronic nicotine delivery system (ENDS) adapted for rodents. Inhalation conditions included vapor generated by the propylene glycol (PG) vehicle, Nicotine (1, 10, 30 mg/mL in the PG) and THC (12.5, 25 mg/mL). RESULTS: Nicotine inhalation increased spontaneous locomotion and decreased body temperature of rats. Pretreatment with the nicotinic cholinergic receptor antagonist mecamylamine (2 mg/kg, i.p.) prevented stimulant effects of nicotine vapor inhalation and attenuated the hypothermic response. Combined inhalation of nicotine and THC resulted in apparently independent effects which were either additive (hypothermia) or opposed (activity). CONCLUSIONS: These studies provide evidence that ENDS delivery of nicotine via inhalation results in nicotine-typical effects on spontaneous locomotion and thermoregulation in male rats. Effects were blocked by a nicotinic antagonist, demonstrating mechanistic specificity. This system will therefore support additional studies of the contribution of atomizer/wick design, vehicle constituents and/or flavorants to the effects of nicotine administered by ENDS.

Binge-like acquisition of α-pyrrolidinopentiophenone (α-PVP) self-administration in female rats.

RATIONALE: The synthetic cathinone α-pyrrolidinopentiophenone (α-PVP) has been associated with bizarre public behavior in users. Association of such behavior with extended binges of drug use motivates additional investigation, particularly since a prior study found that half of male rats experience a binge of exceptionally high intake, followed by sustained lower levels of self-administration during the acquisition of intravenous self-administration (IVSA) of a related drug, 3,4-methylenedioxypyrovalerone. OBJECTIVES: The binge-like acquisition pattern is novel for rat IVSA; thus, the present study sought to determine if this effect generalizes to IVSA of α-PVP in female rats. METHODS: Female Wistar rats were trained in IVSA of α-PVP (0.05 mg/kg/inf) in experimental chambers containing an activity wheel. Groups were trained with the wheels fixed (No-Wheel group), fixed for the initial 5 days of acquisition or free to move throughout acquisition (Wheel group). The groups were next subjected to a wheel access switch and then all animals to dose-substitution (0.0125-0.3 mg/kg/inf) with the wheels alternately fixed and free to move. RESULTS: Approximately half of the rats initiated their IVSA pattern with a binge day of exceptionally high levels of drug intake, independent of wheel access condition. Wheel activity was much lower in the No-Wheel group in the wheel switch post-acquisition. Dose-effect curves were similar for wheel access training groups, for binge/no binge phenotypic subgroups and were not altered with wheel access during the dose-substitution. CONCLUSION: This confirms the high reinforcer effectiveness of α-PVP in female rats and the accompanying devaluation of wheel activity as a naturalistic reward.

Effects of D1 and D2 receptor antagonists on the discriminative stimulus effects of methylendioxypyrovalerone and mephedrone in male Sprague-Dawley rats trained to discriminate D-amphetamine.

Psychopharmacology research has amassed substantial evidence for similarities between synthetic cathinones and other commonly abused psychostimulants. Few studies have utilized drug discrimination methods to investigate synthetic cathinones, and the precise neurochemical substrates underlying their interoceptive effects have not been examined. The present study assessed the involvement of D1 and D2 dopaminergic receptors in the stimulus effects of 3,4-methylenedioxypyrovalerone (MDPV) and mephedrone (MEPH) in rats trained to discriminate D-amphetamine. Eight male Sprague-Dawley rats were trained to discriminate 0.5 mg/kg D-amphetamine (AMPH) from saline. Dose-response curves were then generated with AMPH (0.0-1.0 mg/kg), MDPV (0.0-1.0 mg/kg), and MEPH (0.0-2.0 mg/kg). Subsequently, Sch 39166 (0.3 mg/kg) and haloperidol (0.5 mg/kg) were administered in combination with select doses of MDPV and MEPH. Both MDPV and MEPH produced full substitution for AMPH. Sch 39166 produced a downward shift in the MDPV and MEPH dose-response curves and haloperidol produced similar results with MDPV. These preliminary findings indicate that MDPV and MEPH produce interoceptive stimuli that are similar to those produced by AMPH and that D1 and D2 dopamine receptors contribute to these effects. Additional studies are warranted to investigate the contribution of other receptor mechanisms involved in the interoceptive stimuli produced by synthetic cathinones.

Differential effects of 3,4-methylenedioxypyrovalerone (MDPV) and 4-methylmethcathinone (mephedrone) in rats trained to discriminate MDMA or a d-amphetamine + MDMA mixture.

RATIONALE: Recent reports on the abuse of novel synthetic cathinone derivatives call attention to serious public health risks of these substances. In response to this concern, a growing body of preclinical research has characterized the psychopharmacology of these substances, particularly mephedrone (MEPH) or methylenedioxypyrovalerone (MDPV), noting their similarities to 3,4-methylenedioxymethamphetamine (MDMA) and cocaine. Few studies have utilized drug discrimination methodology to characterize the psychopharmacological properties of these substances. OBJECTIVES: The present study employed a rodent drug discrimination assay to further characterize the stimulus effects of MEPH and MDPV in comparison to MDMA and to a drug mixture comprised of d-amphetamine and MDMA. METHODS: Eight male Sprague-Dawley rats were trained to discriminate 1.5 mg/kg MDMA, and eight rats were trained to discriminate a mixture of 1.5 mg/kg MDMA and 0.5 mg/kg d-amphetamine (MDMA + AMPH) from vehicle. Substitution tests were conducted with MDMA, d-amphetamine, MDPV, MEPH, and cocaine. RESULTS: Dose-response curves generated with MDMA and MEPH were comparable between training groups. In contrast, AMPH, MDPV, and cocaine produced only partial substitution in animals trained to discriminate MDMA but produced full substitution in animals trained to discriminate the MDMA + AMPH mixture. CONCLUSIONS: These findings indicate that MDPV's effects may be more similar to those of traditional psychostimulants, whereas MEPH exerts stimulus effects more similar to those of MDMA. Additional experiments with selective DA and 5-hydroxytryptamine (5-HT) receptor antagonists are required to further elucidate specific receptor mechanisms mediating the discriminative stimulus effects of MDPV and mephedrone.