Repeated Exposure to the "Spice" Cannabinoid JWH-018 Induces Tolerance and Enhances Responsiveness to 5-HT1A Receptor Stimulation in Male Rats

Abstract

Naphthalen-1-yl-(1-pentylindol-3-yl)methanone (JWH-018) is a synthetic compound found in psychoactive "spice" products that activates cannabinoid receptors. Preclinical evidence suggests that exposure to synthetic cannabinoids increases 5-HT_2A/2C receptor function in the brain, an effect which might contribute to psychotic symptoms. Here, we hypothesized that repeated exposures to JWH-018 would enhance behavioral responsiveness to the 5-HT_2A/2C receptor agonist DOI. Male Sprague-Dawley rats fitted with subcutaneously (sc) temperature transponders received daily injections of JWH-018 (1.0 mg/kg, sc) or its vehicle for seven consecutive days. Body temperature and catalepsy scores were determined at 1, 2, and 4 h post-injection each day. At 1 and 7 days after the final repeated treatment, rats received a challenge injection of either DOI (0.1 mg/kg, sc) or the 5-HT_1A receptor agonist 8-OH-DPAT (0.3 mg/kg, sc), then temperature and behavioral responses were assessed. Behaviors induced by DOI included wet dog shakes and back muscle contractions (i.e., skin jerks), while behaviors induced by 8-OH-DPAT included ambulation, forepaw treading, and flat body posture. On the first day of repeated treatment, JWH-018 produced robust hypothermia and catalepsy which lasted up to 4 h, and these effects were significantly blunted by day 7 of treatment. Repeated exposure to JWH-018 did not affect behaviors induced by DOI, but behavioral and hypothermic responses induced by 8-OH-DPAT were significantly augmented 1 day after cessation of JWH-018 treatment. Collectively, our findings show that repeated treatment with JWH-018 produces tolerance to its hypothermic and cataleptic effects, which is accompanied by transient enhancement of 5-HT_1A receptor sensitivity in vivo.

Keywords: JWH-018; receptor; serotonin; spice; synthetic cannabinoid.

Figure 1

Core temperature measures and summed catalepsy scores for rats receiving acute subcutaneous injections of 0.1, 0.3, and 1.0 mg/kg JWH-018 or its vehicle. Core temperature and behavioral score were recorded at 0, 0.25, 0.5, 0.75, 1, 1.5, 2, and 4 h post-injection, as described in the Section “Acute JWH-018 Administration and Rimonabant Antagonism.” Data are mean ± SEM for N = 9 rats per group. *Represents significant effects when compared to the corresponding vehicle-treated group for temperature (Bonferroni, p < 0.05) and catalepsy (Dunn’s, p < 0.05).

Figure 2

Core temperature measures and summed catalepsy scores for rats receiving either subcutaneous (sc) vehicle (VEH) or 1.0 mg/kg JWH-018 (JWH), 30 min after pretreatment with either sc vehicle (VEH) or 1.0 mg/kg rimonabant (RIM). Core temperature and behavioral score were recorded at 0, 0.25, 0.5, 0.75, 1, 1.5, 2, and 4 h post-injection, as described in the Section “Acute JWH-018 Administration and Rimonabant Antagonism.” Data are mean ± SEM for N = 9 rats per group. *Represents significant effects when compared to the corresponding vehicle/vehicle-treated group for temperature (Bonferroni, p < 0.05) and catalepsy (Dunn’s, p < 0.05).

Figure 3

Core temperature measures and summed catalepsy scores for rats receiving either subcutaneous vehicle (VEH) or 1.0 mg/kg JWH-018 (JWH) once daily for seven consecutive days. Core temperature and behavioral score were recorded at 0, 1, 2, and 4 h post-injection each day for 7 days, as described in the Section “Repeated dosing with JWH-018.” Data are mean ± SEM for N = 9 rats per group. *Represents significant effects when compared to the vehicle group from day 1 of treatment for temperature (Bonferroni, p < 0.05) and catalepsy (Dunn’s, p < 0.05).

Figure 4

Summed scores for wet dog shakes and back muscle crawls (skin jerks) induced by a subcutaneous challenge injection of 0.1 mg/kg DOI at 1 and 7 days after cessation of repeated JWH-018 treatment. Behavioral scores were recorded at 0, 0.25, 0.5, 0.75, 1, 1.25, 1.5, and 2 h post-injection, as described in the Section “Challenge injection with serotonergic agonists.” Data are mean ± SEM for N = 9 rats per group.

Figure 5

Summed scores for serotonin syndrome behaviors and mean temperature recordings induced by a subcutaneous challenge injection of 0.3 mg/kg 8-OH-DPAT at 1 and 7 days after cessation of repeated JWH-018 treatment. Behavioral scores and core temperatures were recorded at 0, 0.25, 0.5, 0.75, 1, 1.25, 1.5, and 2 h post-injection, as described in the Section “Challenge injection with serotonergic agonists.” Data are mean ± SEM for N = 9 rats per group. *Represents significant effects when compared to the group that received repeated vehicle treatment (Mann–Whitney, p < 0.05).

Figure 6

Time-course of core body temperature changes induced by a subcutaneous challenge injection of 0.3 mg/kg 8-OH-DPAT at 1 and 7 days after cessation of repeated JWH-018 treatment. Temperatures were recorded at 0, 0.25, 0.5, 0.75, 1, 1.25, 1.5, and 2 h post-injection, as described in the Section “Challenge injection with serotonergic agonists.” Data are mean ± SEM for N = 9 rats per group. *Represents significant effects when compared to vehicle-pretreatment group at specific time points (Bonferroni, p < 0.05).