A vapourized Δ⁹-tetrahydrocannabinol (Δ⁹-THC) delivery system part II: Comparison of behavioural effects of pulmonary versus parenteral cannabinoid exposure in rodents

Abstract

Introduction

Studies of the rewarding and addictive properties of cannabinoids using rodents as animal models of human behaviour often fail to replicate findings from human studies. Animal studies typically employ parenteral routes of administration, whereas humans typically smoke cannabis, thus discrepancies may be related to different pharmacokinetics of parenteral and pulmonary routes of administration. Accordingly, a novel delivery system of vapourized Δ⁹-tetrahydrocannabinol (Δ⁹-THC) was developed and assessed for its pharmacokinetic, pharmacodynamic, and behavioural effects in rodents. A commercially available vapourizer was used to assess the effects of pulmonary (vapourized) administration of Δ⁹-THC and directly compared to parenteral (intraperitoneal, IP) administration of Δ⁹-THC.

Methods:

Sprague–Dawley rats were exposed to pure Δ⁹-THC vapour (1, 2, 5, 10, and 20 mg/pad), using a Volcano® vapourizing device (Storz and Bickel, Germany) or IP-administered Δ⁹-THC (0.1, 0.3, 0.5, 1.0 mg/kg), and drug effects on locomotor activity, food and water consumption, and cross-sensitization to morphine (5 mg/kg) were measured.

Results:

Vapourized Δ⁹-THC significantly increased feeding during the first hour following exposure, whereas IP-administered Δ⁹-THC failed to produce a reliable increase in feeding at all doses tested. Acute administration of 10 mg of vapourized Δ⁹-THC induced a short-lasting stimulation in locomotor activity compared to control in the first of four hours of testing over 7 days of repeated exposure; this chronic exposure to 10 mg of vapourized Δ⁹-THC did not induce behavioural sensitization to morphine.

Discussion:

These results suggest vapourized Δ⁹-THC administration produces behavioural effects qualitatively different from those induced by IP administration in rodents. Furthermore, vapourized Δ⁹-THC delivery in rodents may produce behavioural effects more comparable to those observed in humans. We conclude that some of the conflicting findings in animal and human cannabinoid studies may be related to pharmacokinetic differences associated with route of administration.

Introduction

Studies in both humans and rodents report significant variability in the pharmacokinetic, pharmacodynamic and behavioural effects of cannabinoids, particularly for Δ⁹-tetrahydrocannabinol (Δ⁹-THC), across different routes of administration, and only a very few number of studies have directly compared the effects of pulmonary and parenteral administration of Δ⁹-THC in humans (e.g., Naef, Russman, Petersen-Felix, & Brenneisen, 2004) or rodents (e.g., Fried, 1976, Fried and Neiman, 1973, Niyuhire et al., 2007, Wilson et al., 2006, Wilson et al., 2002). In human trials, Naef et al. (2004) reported that pulmonary administration of a Δ⁹-THC liquid aerosol (0.053 mg/kg) produced peak plasma levels of Δ⁹-THC in the range of 18.7 ± 7.4 ng/ml for approximately 10 to 20 min after inhalation, and then rapidly decreased; however, plasma levels after intravenous (IV) administration of Δ⁹-THC (0.053 mg/kg) were more variable, ranging from 81.6 to 640.6 ng/ml (mean of 271.5 ± 61.1 ng/ml) within 5 min after injection (Naef et al., 2004). More importantly, Naef et al. (2004) reported that the psychological and somatic side effects of Δ⁹-THC differed according to the route of administration: although parenteral administration of Δ⁹-THC produced greater euphoria, it also produced greater anxiety, irritation, confusion and disorientation, hallucination, nausea, and headache compared to pulmonary administration, which only resulted in transient respiratory irritation.

Early studies on rodents, by Fried and Neiman (1973), reported that rats exposed to inhaled cannabis smoke showed electrical brain activity (via electroencephalographic recordings) in both cortical and hippocampal regions which was similar, but less pronounced, than that observed in rats administered intraperitoneal (IP) injections of Δ⁹-THC. Fried and Neiman (1973) also compared the effects of oral and IP administrated Δ⁹-THC, versus exposure to cannabis smoke, on rats in the open-field test; all conditions significantly reduced exploratory behaviour, with injected Δ⁹-THC having the greatest impact. However, rats administered IP or oral Δ⁹-THC, but not cannabis smoke, were reported to be hypersensitive, exhibiting greater vocalizations when handled after testing. In studies of chronic exposure to inhaled cannabis, Fried (1976) showed that male, but not female, rats developed cross-tolerance to acute exposure to IP administered Δ⁹-THC. Male and female rats exposed to cannabis smoke every other day for 32 days showed reduced locomotor activity on the initial trials (1 to 10) which then returned to baseline levels near the last trials (13 to 16); however, repeated exposure to cannabis smoke significantly increased locomotor activity after IP injection of Δ⁹-THC, indicating tolerance in male but not female rats (Fried, 1976).

More recent studies in rodents, by Wilson et al. (2002), used a metered dose inhaler (MDI) that aerosolized Δ⁹-THC to provide a systematic route of exposure in mice. They found that in male mice, intravenous (IV) administered and inhaled Δ⁹-THC produced similar levels of Δ⁹-THC in blood and brain tissue at the antinociceptive ED₅₀ dose (median effective dose of 30 mg) and that the behavioural effects of inhaled Δ⁹-THC — antinociception, hypomotility, catalepsy, and hypothermia — were all significantly antagonized by co-administration of the CB₁ antagonist/inverse agonist SR141716. Subsequently, Wilson et al. (2006) reported that although SR141716 induces withdrawal signs (e.g., paw tremors) in mice chronically exposed to either inhaled marijuana smoke or IV administered Δ⁹-THC, only co-administration of IV administered Δ⁹-THC reversed these effects; inhaled marijuana smoke did not prevent precipitated withdrawal induced by SR141716. To account for this unexpected finding, Wilson et al. (2006) suggested that levels of Δ⁹-THC from the marijuana smoke in the brain after inhalation were insufficient to reverse the effects of SR141716 and thus the mechanism of action was still likely CB₁-mediated. This conclusion was supported by additional findings of a dissociation between Δ⁹-THC levels found in blood and brain that were dependent upon the route of administration; although blood and brain levels were roughly equivalent following inhalation, brain levels were 200–300% greater in brain than in blood after IV administration (Wilson et al., 2006). In mice, Niyuhire et al. (2007) showed that the effects of Δ⁹-THC on learning and memory also differed according to the route of administration: IP administration of Δ⁹-THC (1, 3, 10 mg/kg) dose-dependently disrupted both acquisition and recall of platform location in the Morris water maze task, whereas inhaled smoke from marijuana (50, 100, and 200 mg) only impaired performance at the highest dose (estimated to have 4.2 mg Δ⁹-THC before burning). Co-administration with SR141716 reversed these effects, suggesting that the mechanism of action was also CB₁ mediated (Niyuhire et al., 2007).

All of these studies, which directly compared the effects of parenteral versus pulmonary Δ⁹-THC demonstrate that the route of administration produces qualitatively different results, which may account for some of the many conflicting findings in cannabinoid research in animal models of human behaviour. Since the combustion of cannabis releases many compounds in addition to Δ⁹-THC, including other cannabinoids (e.g., cannabidiol) and many toxins and carcinogens (e.g., anthrocyclines, nitrosamines, polycyclic aromatic hydrocarbons, terpenes, and vinyl chloride) (Turner, Bouwsma, Billets and Elsohly, 1980, Turner, Elsohly and Boeren, 1980, Sarafian et al., 1999, Zhang et al., 1999, Roth et al., 2001, Hashibe et al., 2005, Voirin et al., 2006, Aldington et al., 2008, Berthiller et al., 2008; also reviewed in Reece, 2009) which may exert their own effects, vapourization of pure Δ⁹-THC provides a more accurate assessment of the direct effects of Δ⁹-THC and its primary psychoactive metabolite, 11-hydroxy-Δ⁹-THC (11-OH-Δ⁹-THC). In the current study, a novel delivery system of vapourized Δ⁹-THC was developed and assessed for its pharmacokinetic, pharmacodynamic, and behavioural effects in rodents. In addition, studies in animals need to account for actual bioavailable levels of Δ⁹-THC in blood after injection and inhalation of pure Δ⁹-THC, not just marijuana smoke containing unknown quantities of Δ⁹-THC and numerous other toxicants. Previously, we have directly compared cannabinoid recovery levels of Δ⁹-THC and 11-OH-Δ⁹-THC in whole blood after IP and vapourization exposure to Δ⁹-THC in rodents (Manwell et al., 2014-in this issue); for the applications to animal behaviour studies, we were only interested in the psychoactive cannabinoids Δ⁹-THC and its metabolite 11-OH-Δ⁹-THC. A commercially available vapourizer, commonly used by cannabis smokers, was used to assess the effects of pulmonary administration of Δ⁹-THC and directly compared to parenteral administration of Δ⁹-THC; drug effects on locomotor activity, food and water consumption, and cross-sensitization to morphine were measured.