In vivo effects of synthetic cannabinoids JWH-018 and JWH-073 and phytocannabinoid Δ⁹-THC in mice: Inhalation versus intraperitoneal injection

Highlights

• Injected or inhaled synthetic cannabinoids (SCBs) caused CB1-mediated effects.

• Potency (JWH-018>JWH-073>Δ⁹-THC) was consistent across routes of administration.

• SCBs were Δ⁹-THC-like in the tetrad and in drug discrimination by both routes.

• Cataleptic and convulsant effects of SCBs were only noted after injection.

Abstract

Human users of synthetic cannabinoids (SCBs) JWH-018 and JWH-073 typically smoke these drugs, but preclinical studies usually rely on injection for drug delivery. We used the cannabinoid tetrad and drug discrimination to compare in vivo effects of inhaled drugs with injected doses of these two SCBs, as well as with the phytocannabinoid Δ⁹-tetrahydrocannabinol (Δ⁹-THC). Mice inhaled various doses of Δ⁹-THC, JWH-018 or JWH-073, or were injected intraperitoneally (IP) with these same compounds. Rectal temperature, tail flick latency in response to radiant heat, horizontal bar catalepsy, and suppression of locomotor activity were assessed in each animal. In separate studies, mice were trained to discriminate Δ⁹-THC (IP) from saline, and tests were performed with inhaled or injected doses of the SCBs. Both SCBs elicited Δ⁹-THC-like effects across both routes of administration, and effects following inhalation were attenuated by pretreatment with the CB1 antagonist/inverse agonist rimonabant. No cataleptic effects were observed following inhalation, but all compounds induced catalepsy following injection. Injected JWH-018 and JWH-073 fully substituted for Δ⁹-THC, but substitution was partial (JWH-073) or required relatively higher doses (JWH-018) when drugs were inhaled. These studies demonstrate that the SCBs JWH-018 and JWH-073 elicit dose-dependent, CB1 receptor-mediated Δ⁹-THC-like effects in mice when delivered via inhalation or via injection. Across these routes of administration, differences in cataleptic effects and, perhaps, discriminative stimulus effects, may implicate the involvement of active metabolites of these compounds.

Introduction

Over the past 5 years, synthetic cannabinoids (SCBs) rapidly emerged as popular drugs of abuse in Europe and the US. Commercial preparations (typically branded as “K2” in the US or as “Spice” in Europe) are readily available online and in business establishments such as convenience stores and truck stops (Vardakou et al., 2010). Most of these preparations consist of inert plant materials laced with SCBs, typically from the aminoalkylindole (AAI) family (Fattore and Fratta, 2011), and are presumed to possess pharmacological properties similar to Δ⁹-tetrahydrocannabinol (Δ⁹-THC), the primary psychoactive constituent of marijuana (Gaoni and Mechoulam, 1964). The widespread over-the-counter availability of these products has led to the perception that they are safe to use, and this, combined with the fact that their active constituents are not detected in standard drug screens, has spurred use of SCBs to epidemic levels on many college campuses (Vandrey et al., 2012). Similarly, one in nine high school seniors admitted using SCBs over the past year, making these compounds the 2nd most frequently used recreational drug, after marijuana, in this population (Johnston et al., 2011). State and federal scheduling of some of the more common SCBs under the Controlled Substances Act has largely failed to curtail drug availability, and commercial preparations containing these drugs remain quasi-legal and easily obtainable (Seely et al., 2012).

Although structurally distinct from Δ⁹-THC, the synthetic AAI cannabinoid compounds also bind and activate cannabinoid CB1 receptors (CB1Rs) (Estep et al., 1990, Eissenstat et al., 1990). The abuse liability of AAI SCBs therefore most likely results from their capability to potently and efficaciously activate these CB1Rs. While a plethora of different SCBs are reported to be present in various commercial preparations, two of the most commonly observed are JWH-018 [1-pentyl-3-(1-naphthoyl)indole] and JWH-073 [1-butyl-3-(1-naphthoyl)indole] (Logan et al., 2012, Seely et al., 2013). Previous studies revealed that these SCBs have high affinity for CB1Rs, and possess much higher efficacy at these receptors than Δ⁹-THC (Lindigkeit et al., 2009, Atwood et al., 2010).

In this regard, although humans typically smoke commercial preparations of SCBs (Vandrey et al., 2012), almost all preclinical studies with these compounds have involved systemic injection. Drugs administered via inhalation largely bypass first-pass metabolism, whereas systemic injection allows for significant first-pass effects (Pond and Tozer, 1984). Importantly, we have recently reported that several phase I hydroxylated metabolites of JWH-018 and JWH-073 retain biological activity (Brents et al., 2011, Brents et al., 2012), which could have implications for human use. As such, it may be the case that laboratory animal models employing systemic injection of SCBs maximize formation of active phase I metabolites, whereas the human condition, i.e. smoking, would be expected to minimize metabolite formation. At the time of this writing, only a single study has evaluated the effects of a single inhaled SCB, JWH-018, in mice (Wiebelhaus et al., 2012), demonstrating dose-dependent effects on all measures of the cannabinoid tetrad, and reversal of these drug effects by prior administration of the CB1R antagonist/inverse agonist rimonabant. In the present studies, utilizing a whole-body exposure system, we extend these previous observations by directly comparing the effects of multiple doses of JWH-018 to its structural analogue JWH-073 and to Δ⁹-THC using the cannabinoid tetrad (Martin et al., 1991) following inhaled exposure or intraperitoneal injection. The CB1R receptor antagonist/inverse agonist rimonabant was used to determine whether any observed drug effects in the tetrad assay following inhalation of cannabinoids were mediated via CB1R actions. Additional studies compared the interoceptive effects of inhaled or injected SCBs to those of intraperitoneal Δ⁹-THC using drug discrimination. This further evaluation of the effects of SCBs via inhalation in mice may increase our understanding of their biological effects and may provide a more translational approach to the study of these compounds, as compared to systemic injection.