A vapor exposure method for delivering heroin alters nociception, body temperature and spontaneous activity in female and male rats

Adult Consequences of Repeated Nicotine Vapor Inhalation in Adolescent Rats

INTRODUCTION

There has been a resurgence in nicotine inhalation in adolescents due to the popularity and availability of Electronic Nicotine Delivery Systems (ENDS). Almost five times as many US high-school seniors inhale nicotine vapor daily compared with those who smoke tobacco. This study was conducted to determine the impact of repeated adolescent vapor inhalation of nicotine on behavior in adulthood.

METHODS

Male and female Sprague-Dawley rats were exposed to 30-minute sessions of ENDS vapor inhalation, twice daily, from post-natal day (PND) 31-40. Conditions included vapor from the propylene glycol (PG) vehicle or nicotine (30 mg/mL in the PG). Animals were assessed for effects of nicotine on open field (PND 74-105) and wheel activity (PND 126-180) and for volitional exposure to nicotine vapor (PND 285-395). Plasma nicotine and cotinine were assessed in separate groups of male and female Wistar and Sprague-Dawley rats after a single nicotine inhalation session.

RESULTS

Group mean plasma nicotine ranged from 39 to 59 ng/mL post-session with minimal strain differences detected. Adolescent nicotine exposure enhanced sensitivity to the locomotor stimulating effects of nicotine (0.1-0.8 mg/kg, s.c.) in an open field in female rats, but didn't change the effects of nicotine on wheel activity. Female rats exposed to nicotine (30 mg/mL) vapor as adolescents responded more vigorously than PG-exposed females to nicotine vapor in a fixed ratio 5 challenge.

CONCLUSIONS

Repeated adolescent nicotine vapor inhalation leads to enhanced liability for volitional exposure to nicotine vapor in adulthood in female rats, but minimal change in spontaneous locomotor behavior.

IMPLICATIONS

These results show that adolescent vaping of nicotine can lead to lasting sensitization to the effects of nicotine in adulthood, including volitional responding for nicotine vapor. Demonstration of this in a controlled animal model establishes causality in a manner not possible from longitudinal evidence in human populations. These findings further highlight the importance of decreasing adolescent nicotine exposure to e-cigarettes to reduce consumption in adulthood.

Persistent effects of repeated adolescent and adult heroin vapor inhalation in female Wistar rats

Adolescent drug exposure has been associated with more severe mental health outcomes related to substance abuse and anxiety disorders. The aim of the present study was to contrast the long-term effects of repeated heroin vapor inhalation during adolescence with similar heroin exposure in adulthood. Groups of female Wistar rats underwent twice daily 30-minute sessions of heroin or propylene glycol (control) vapor inhalation from postnatal days (PND) 36-45 or PND 85-94, respectively. Nociception was assessed after vapor inhalation sessions and forty days later, for the Adolescent-Exposed and Adult-Exposed groups. Anxiety-like behavior was assessed with an elevated plus-maze (EPM) and spatial learning was assessed with a Barnes maze. Acute effects of naloxone (0.3 mg/kg, i.p.) and heroin (0.5 and 1.0 mg/kg, s.c.) on thermal nociception were determined on PND 140/189 and PND 149/198, respectively. Repeated heroin vapor inhalation produced anti-nociceptive tolerance across sessions in both adolescent and adult rats, with the adolescents exhibiting more complete tolerance. Heroin vapor inhalation produced anxiolytic effects, regardless of age of exposure. There were no effects of heroin on spatial learning. Naloxone produced acute hyperalgesia in all but the Adolescent-Exposed heroin group, and heroin anti-nociception was blunted in both heroin-exposed groups at the highest heroin dose. Repeated heroin vapor inhalation can produce lasting effects on nociception and anxiety-like behavior that persist for months after the exposure. Importantly, these findings suggest that adolescent exposure to heroin vapor produces specific effects on nociception that are not observed when exposure occurs in adulthood.

Adult consequences of repeated nicotine and Δ9-tetrahydrocannabinol (THC) vapor inhalation in adolescent rats

RATIONALE

Use of electronic drug delivery systems (EDDS, "e-cigarettes") to ingest nicotine and Δ9-tetrahydrocannabinol (THC) has surged in adolescents in the USA; five times as many high-school seniors vape nicotine daily using tobacco. At the same time, 19.5% of seniors use cannabis at least monthly, with 12% using EDDS to deliver it.

OBJECTIVES

This study was conducted to examine the impact of repeated adolescent vapor inhalation of nicotine and THC in rats.

METHODS

Female Sprague-Dawley rats were exposed to 30-min sessions of vapor inhalation, twice daily, from post-natal day (PND) 31 to PND 40. Conditions included vapor from the propylene glycol (PG) vehicle, nicotine (60 mg/mL in the PG), THC (100 mg/mL in the PG), or the combination of nicotine (60 mg/mL) and THC (100 mg/mL). Rats were assessed on wheel activity, heroin anti-nociception and nicotine and heroin vapor volitional exposure during adulthood.

RESULTS

Nicotine-exposed rats exhibited few differences as adults, but were less sensitive to anti-nociceptive effects of heroin (1 mg/kg, s.c.). THC- and THC + nicotine-exposed rats were less spontaneously active, and obtained fewer nicotine vapor deliveries as adults. In contrast, THC-exposed rats obtained volitional heroin vapor at rates indistinguishable from the non-THC-exposed groups. Repeated THC exposure also caused tolerance to temperature-disrupting effects of THC (5 mg/kg, i.p.).

CONCLUSIONS

These studies further confirm that the effects of repeated vapor exposure to THC in adolescence last into early to middle adulthood, including decreased volitional consumption of nicotine. Effects of repeated nicotine in adolescence were comparatively minor.

Hyperactivity Induced By Vapor Inhalation of Nicotine in Male and Female Rats

Rationale

Preclinical models of electronic nicotine delivery system (ENDS; "e-cigarette") use have been rare, so there is an urgent need to develop experimental approaches to evaluate their effects.

Objective

To contrast the impact of inhaled nicotine across sex.

Methods

Male and female Wistar rats were exposed to vapor from a propylene glycol vehicle (PG), nicotine (NIC; 1-30 mg/mL in PG), or were injected with NIC (0.1-0.8 mg/kg, s.c.), and then assessed for changes in temperature and activity. The antagonist mecamylamine (2 mg/kg) was administered prior to NIC to verify pharmacological specificity. Plasma levels of nicotine and cotinine were determined after inhalation and injection.

Results

Activity increased in females for ~60 minutes after nicotine inhalation, and this was blocked by mecamylamine. A similar magnitude of hyperlocomotion was observed after s.c. administration. Body temperature was reduced after nicotine inhalation by female rats but mecamylamine increased this hypothermia. Increased locomotor activity was observed in male rats if inhalation was extended to 40 minutes or when multiple inhalation epochs were used per session. The temperature of male rats was not altered by nicotine. Plasma nicotine concentrations were slightly lower in male rats than in female rats after 30-minute nicotine vapor inhalation and slightly higher after nicotine injection (1.0 mg/kg, s.c.).

Conclusions

Nicotine inhalation increases locomotor activity in male and female rats to a similar or greater extent than by subcutaneous injection. Sex differences were observed, which may be related to lower nicotine plasma levels, lower baseline activity and/or a higher vehicle response in males.

Rats chasing the dragon: A new heroin inhalation method

BACKGROUND

Despite extensive human use of inhalation for ingesting opioids, models in rodents have mostly been limited to parenteral injection and oral dosing. Methods using electronic drug delivery systems (EDDS; "e-cigarettes") have shown efficacy in rodent models but these do not faithfully mimic the most popular human inhalation method of heating heroin to the point of vaporization.

NEW METHOD

Middle aged rats were exposed to vapor created by direct heating of heroin HCl powder in a ceramic e-cigarette type atomizer. Efficacy was determined with a warm water tail withdrawal nociception assay, rectal temperature and self-administration.

RESULTS

Ten minutes of inhalation of vaporized heroin slowed response latency in a warm water tail withdrawal assay and increased rectal temperature in male rats, in a dose-dependent manner. Similar antinociceptive effects in female rats were attenuated by the opioid antagonist naloxone (1.0 mg/kg, s.c.). Female rats made operant responses for heroin vapor in 15-minute sessions, increased their response rate when the reinforcement ratio increased from FR1 to FR5, and further increased their responding when vapor delivery was omitted. Anti-nociceptive effects of self-administered volatilized heroin were of a similar magnitude as those produced by the 10-minute non-contingent exposure.

COMPARISON WITH EXISTING METHODS

Inhalation of directly volatilized heroin successfully produces heroin-typical effects, comparable to EDDS inhalation delivery.

CONCLUSIONS

This study shows that "chasing the dragon" methods of inhalation of heroin can be modeled successfully in the rat. Inhalation techniques may be particularly useful for longer term studies deep into the middle age of rats.

Nicotine, THC, and Dolutegravir Modulate E-Cigarette-Induced Changes in Addiction- and Inflammation-Associated Genes in Rat Brains and Astrocytes

E-cigarette use has been marketed as a safer alternative to traditional cigarettes, as a means of smoking cessation, and are used at a higher rate than the general population in people with HIV (PWH). Early growth receptor 2 (EGR2) and Activity-Regulated Cytoskeleton-Associated Protein (ARC) have a role in addiction, synaptic plasticity, inflammation, and neurodegeneration. This study showed that 10 days of exposure to e-cigarette vapor altered gene expression in the brains of 6-month-old, male, Sprague Dawley rats. Specifically, the e-cigarette solvent vapor propylene glycol (PG) downregulated EGR2 and ARC mRNA expression in frontal cortex, an effect which was reversed by nicotine (NIC) and THC, suggesting that PG could have a protective role against NIC and cannabis dependence. However, in vitro, PG upregulated EGR2 and ARC mRNA expression at 18 h in cultured C6 rat astrocytes suggesting that PG may have neuroinflammatory effects. PG-induced upregulation of EGR2 and ARC mRNA was reversed by NIC but not THC. The HIV antiretroviral DTG reversed the effect NIC had on decreasing PG-induced upregulation of EGR2, which is concerning because EGR2 has been implicated in HIV latency reversal, T-cell apoptosis, and neuroinflammation, a process that underlies the development of HIV-associated neurocognitive disorders.

Adult consequences of repeated nicotine and Δ 9 -tetrahydrocannabinol (THC) vapor inhalation in adolescent rats

The use of Electronic Drug Delivery Systems (EDDS, "e-cigarettes") to ingest nicotine and Δ 9 -tetrahydrocannabinol (THC) has surged in adolescent populations in the United States, as five times as many high-school seniors vape nicotine daily as use tobacco. At the same time 19.5% of seniors use cannabis at least monthly, with 12% using EDDS to deliver it. This study was conducted to examine the impact of repeated adolescent vapor inhalation of nicotine and THC in rats. Female Sprague-Dawley rats were exposed to 30-minute sessions of vapor inhalation, twice daily, from Post-Natal Day (PND) 31 to PND 40. Conditions included vapor from the propylene glycol (PG) vehicle, Nicotine (60 mg/mL in the PG), THC (100 mg/mL in the PG) or the combination of Nicotine (60 mg/mL) and THC (100 mg/mL). Rats were assessed on wheel activity, heroin anti-nociception and nicotine and heroin vapor volitional exposure during adulthood. Nicotine exposed rats exhibited few differences as adults, but were less sensitive to anti-nociceptive effects of heroin (1 mg/kg, s.c.). THC- and THC+Nicotine-exposed rats were less spontaneously active, and obtained fewer nicotine vapor deliveries as adults. In contrast, THC exposed rats obtained volitional heroin vapor at rates indistinguishable from the non-THC-exposed groups. Repeated THC exposure also caused tolerance to temperature-disrupting effects of THC (5 mg/kg, i.p.). These studies further confirm that the effects of repeated vapor exposure to THC in adolescence last into early to middle adulthood, including decreased volitional consumption of nicotine. Effects of repeated nicotine in adolescence were comparatively minor.

Rats Chasing the Dragon: A new heroin inhalation method

Rationale

Despite extensive human use of the inhalation route for ingesting opioids, models in rodents have mostly been limited to parenteral injection and oral dosing. Methods using electronic drug delivery systems (EDDS; "e-cigarettes") have shown efficacy in rodent models but these do not faithfully mimic the most popular human inhalation method of heating heroin to the point of vaporization.

Objective

This study was designed to determine if direct volatilization of heroin hydrochloride delivers effective heroin doses to rodents.

Methods

Middle aged rats were exposed to vapor created by direct heating of heroin HCl powder in a ceramic e-cigarette type atomizer. Efficacy was determined with a warm water tail withdrawal nociception assay, rectal temperature and self-administration.

Results

Ten minutes of inhalation of vaporized heroin slowed response latency in a warm water tail withdrawal assay and increased rectal temperature in male rats, in a dose-dependent manner. Similar antinociceptive effects in female rats were attenuated by the opioid antagonist naloxone (1.0 mg/kg, s.c.). Female rats made operant responses for heroin vapor in 15-minute sessions, increased their response rate when the reinforcement ratio increased from FR1 to FR5, and further increased their responding when vapor delivery was omitted. Anti-nociceptive effects of self-administered volatilized heroin were of a similar magnitude as those produced by the 10-minute non-contingent exposure.

Conclusions

This study shows that "chasing the dragon" methods of inhalation of heroin can be modeled successfully in the rat. Inhalation techniques may be particularly useful for longer term studies deep into middle age of rat species.

Extended access to fentanyl vapor self-administration leads to addiction-like behaviors in mice: Blood chemokine/cytokine levels as potential biomarkers

Rodent models are useful for understanding the mechanisms that underlie opioid addiction, but most preclinical studies have focused on rewarding and consummatory aspects of opioids without components of dependence-induced escalation of drug taking or seeking. We characterized several opioid-related behaviors in mice using a model of vaporized fentanyl self-administration. Male and female C57BL/6J mice were assigned to short-access (ShA; 1 h, nondependent) or long-access (LgA; 6 h, dependent) fentanyl vapor self-administration and subsequently tested in a battery of behavioral tests, followed by blood collection during withdrawal. Compared with mice in the ShA group, mice in the LgA group escalated their fentanyl intake, were more motivated to work to obtain the drug, exhibited greater hyperalgesia, and exhibited greater signs of naloxone-precipitated withdrawal. Principal component analysis indicated the emergence of two independent behavioral constructs: "intake/motivation" and "hyperalgesia/punished seeking." In mice in the LgA condition only, "hyperalgesia/punished seeking" was associated with plasma levels of proinflammatory interleukin-17 (IL-17), chemokine (C-C motif) ligand 4 (CCL-4), and tumor necrosis factor α (TNF-α). Overall, the results suggest that extended access to opioids leads to addiction-like behavior, and some constructs that are associated with addiction-like behavior may be associated with levels of the proinflammatory cytokines/chemokines IL-17, TNF-α, and CCL-4 in blood.

Reinforcing effects of fentanyl and sufentanil aerosol puffs in rats

RATIONALE

Rapidly evolving e-cigarette technology developed for self-administering nicotine aerosol has the potential to be utilized to self-administer other aerosolized drugs of abuse. Rodent models which mirror characteristics of human e-cigarette use are necessary to explore the degree to which this may be a public health concern.

OBJECTIVES

Our goal was to develop a highly translational model of discrete nose-only aerosol puff drug delivery to explore the reinforcing effects of fentanyl and sufentanil aerosols in rats.

METHODS

Male and female Sprague-Dawley rats were trained to perform a multiple schedule FR1 lever-press, 4-s (second) nose hold operant during which the subject's orofacial areas were exposed to drug-free glycerol/propylene glycol aerosol produced by a commercial e-cigarette at a power setting of 18 watts. Each completed 4-s drug-free vehicle aerosol exposure resulted in a 3-s presentation of a 0.1-ml dipper of sweetened milk solution. After training, rats were then allowed to self-administer 4-s nose-only puffs of fentanyl (100-6000 µg/ml) or sufentanil (30-500 µg/ml) aerosol in the absence of paired milk dipper reinforcers.

RESULTS

All 31 rats learned the lever-press/nose-poke multiple schedule for milk dippers alone and 25 accepted exposure to 4 s of 18 watts of drug-free vehicle aerosol when paired with milk dipper presentations. In the absence of paired milk dipper presentations, fentanyl aerosol puffs at concentrations of 1000 and 3000 µg/ml as well as 100 µg/ml puffs of sufentanil served as reinforcers compared to both air puffs and drug-free vehicle aerosol puffs. There were no significant differences between males and females in number of fentanyl or sufentanil puffs self-administered.

CONCLUSIONS

Discrete nose-only puffs of two potent opioids under exposure conditions comparable to puff durations in human e-cigarette users serve as reinforcers in rats. This outcome suggests that under appropriate conditions e-cigarettes might be a potential alternative delivery mechanism for illicit opioids.

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.

Effects of combined THC and heroin vapor inhalation in rats

RATIONALE

Opioids are effective medications, but they have several key limitations including the development of tolerance, establishment of dependence, diversion for non-medical use, and the development of addiction. Therefore, any drugs which act in an additive or synergistic fashion with opioids to address medical applications have the potential to reduce opioid-related harms.

OBJECTIVES

To determine if heroin and Δ⁹-tetrahydrocannabinol (THC) interact in an additive or independent manner to alter nociception, body temperature, and spontaneous locomotor activity when inhaled or injected.

METHODS

Groups of female and male rats, implanted with radiotelemetry transmitters, were exposed to vapor generated from heroin (50 mg/mL in propylene glycol vehicle; PG), THC (50 mg/mL), or the combination for assessment of effects on temperature and activity. Thermal nociception was assessed with a warm water tail-withdrawal assay.

RESULTS

Heroin inhalation increased temperature and activity whereas THC inhalation decreased temperature and activity in both female and male Sprague-Dawley rats. Effects of combined inhalation were in opposition, and additional experiments found the same outcome for the injection of heroin (0.5 mg/kg, s.c.) and THC (10 mg/kg, i.p.) alone and in combination. In contrast, the co-administration of heroin and THC by either inhalation or injection produced additive effects on thermal nociception in both male and female Sprague-Dawley and Wistar rats.

CONCLUSIONS

This study shows that additive effects of THC with an opioid on a medical endpoint such as analgesia may not generalize to other behavioral or physiological effects, which may be a positive outcome for unwanted side effects.

Female rats self-administer heroin by vapor inhalation

Over the last two decades the United States has experienced a significant increase in the medical and non-medical use of opioid drugs, resulting in record numbers of opioid-related overdoses and deaths. There was an initial increase in non-medical use of prescription opioids around 2002, followed later by increased heroin use and then most recently fentanyl. Inhalation is a common route of administration for opioids, with a documented history spanning back to Mediterranean antiquity and up through modern use with e-cigarette devices. Unfortunately, preclinical studies using inhalation as the route of administration remain relatively few. This study was conducted to determine the efficacy of e-cigarette vapor inhalation of heroin in rats. Non-contingent exposure to heroin or methadone vapor produced anti-nociceptive efficacy in male and female rats. Female rats were trained to self-administer heroin vapor; the most-preferring half of the distribution obtained more vapor reinforcers when the concentration of heroin was reduced in the vapor vehicle and when pre-treated with the opioid receptor antagonist naloxone. The anti-nociceptive effect of heroin self-administered by vapor was identical in magnitude to that produced by intravenous self-administration. Finally, anxiety-like behavior increased 24-48 h after last heroin vapor access, consistent with withdrawal signs observed after intravenous self-administration. In sum, these studies show that rewarding and anti-nociceptive effects of heroin are produced in rats by vapor inhalation using e-cigarette technology. Importantly, self-administration models by this route can be deployed to determine health effects of inhaled heroin or other opioids.