Vaporized Cannabis Extracts Have Reinforcing Properties and Support Conditioned Drug-Seeking Behavior in Rats

Abstract

Recent trends in cannabis legalization have increased the necessity to better understand the effects of cannabis use. Animal models involving traditional cannabinoid self-administration approaches have been notoriously difficult to establish and differences in the drug used and its route of administration have limited the translational value of preclinical studies. To address this challenge in the field, we have developed a novel method of cannabis self-administration using response-contingent delivery of vaporized Δ⁹-tetrahydrocannabinol-rich (CAN_THC) or cannabidiol-rich (CAN_CBD) whole-plant cannabis extracts. Male Sprague-Dawley rats were trained to nose-poke for discrete puffs of CAN_THC, CAN_CBD, or vehicle (VEH) in daily 1 h sessions. Cannabis vapor reinforcement resulted in strong discrimination between active and inactive operanda. CAN_THC maintained higher response rates under fixed ratio schedules and higher break points under progressive ratio schedules compared with CAN_CBD or VEH, and the number of vapor deliveries positively correlated with plasma THC concentrations. Moreover, metabolic phenotyping studies revealed alterations in locomotor activity, energy expenditure, and daily food intake that are consistent with effects in human cannabis users. Furthermore, both cannabis regimens produced ecologically relevant brain concentrations of THC and CBD and CAN_THC administration decreased hippocampal CB1 receptor binding. Removal of CAN_THC reinforcement (but not CAN_CBD) resulted in a robust extinction burst and an increase in cue-induced cannabis-seeking behavior relative to VEH. These data indicate that volitional exposure to THC-rich cannabis vapor has bona fide reinforcing properties and collectively support the utility of the vapor self-administration model for the preclinical assessment of volitional cannabis intake and cannabis-seeking behaviors.SIGNIFICANCE STATEMENT The evolving legal landscape concerning recreational cannabis use has increased urgency to better understand its effects on the brain and behavior. Animal models are advantageous in this respect; however, current approaches typically used forced injections of synthetic cannabinoids or isolated cannabis constituents that may not capture the complex effects of volitional cannabis consumption. We have developed a novel model of cannabis self-administration using response-contingent delivery of vaporized cannabis extracts containing high concentrations of Δ⁹ tetrahydrocannabinol (THC) or cannabidiol. Our data indicate that THC-rich cannabis vapor has reinforcing properties that support stable rates of responding and conditioned drug-seeking behavior. This approach will be valuable for interrogating effects of cannabis and delineating neural mechanisms that give rise to aberrant cannabis-seeking behavior.

Keywords: cannabinoid; cannabis; rat; self-administration; translational; vapor.

Figure 1.

Cannabis vapor supports stable rates of active responding in male rats. A, Schematic illustration of the vapor self-administration apparatus (adapted from Fuchs et al., 2018), and (B) real-life depiction of a Long–Evans rat responding for cannabis vapor (not from the current experiments). C, Mean active (colored symbols) and inactive (open symbols) nose-poke responding for vapor containing high concentrations of CAN_THC, CAN_CBD, or VEH across increasing fixed ratio schedules of reinforcement. D, Mean number of CAN_THC, CAN_CBD, or VEH vapor deliveries earned across increasing fixed ratio schedules of reinforcement. E–G, Mean number of vapor deliveries earned organized by 15 min bins within (E) FR-1, (F) FR-2, and (G) FR-4 schedules of reinforcement. H, Nose-poke operanda discrimination index for CAN_THC, CAN_CBD, and VEH vapor across increasing fixed schedules of reinforcement. The dotted line represents a discrimination index of 0.33, which indicates a 2:1 rate of active–inactive nose-poke responding. n = 7–12/group, p ≤ 0.05. *Significant differences between CAN_THC and VEH groups. #Significant differences between CAN_THC and CAN_CBD groups. †Denotes significant differences between CAN_CBD and VEH groups.

Figure 2.

Vaporized delivery of THC-dominant cannabis extracts exhibits motivational properties. A, Mean cumulative number of active responses for CAN_THC, CAN_CBD, and VEH vapor during a 180 min progressive ratio challenge. Data are tallied and organized into 15 min bins. B, Mean break points for CAN_THC, CAN_CBD, and VEH vapor during the progressive ratio challenge (defined as an absence of active nose-poke responding for period of 15 min. C, Mean latency to initiate active nose-poke responding for CAN_THC, CAN_CBD, or VEH vapor relative to the immediately preceding vapor delivery. n = 7–12/group. p ≤ 0.05. *Significant differences between CAN_THC and VEH groups. #Significant differences between CAN_THC and CAN_CBD groups.

Figure 3.

Cannabis vapor self-administration produces physiologically relevant cannabinoid concentrations and alterations in CB1R binding. Correlations between the number of cannabis vapor deliveries (200 or 400 mg/ml) earned and plasma concentrations of (A) THC (CAN_THC-200: r = 0.51, p = 0.03; CAN_THC-400: r = 0.86, p < 0.001) and (B) CBD (CAN_CBD-200: r = 0.58, p = 0.01; CAN_CBD-400: r = 0.51, p = 0.18) at the end of the 1 h self-administration session (n = 8–17/group). Brain tissue concentration of (C) THC and (D) CBD measured 24 h after the final self-administration session in rats trained to self-administer CAN_THC or CAN_CBD vapor (n = 11/group). E, Hippocampal CB1R binding site density (pmol/mg protein) and (F) CB1R binding affinity (nm) in rats trained to self-administer CAN_THC, CAN_CBD, or VEH. Tissue was analyzed 24 h after the final self-administration session. n = 4/group. p ≤ 0.05. *Significant differences between CAN_THC and VEH groups. #Significant differences between CAN_THC and CAN_CBD groups.

Figure 4.

Self-administration of THC-rich cannabis vapor produces locomotor and metabolic alterations. A, Radio telemetry recordings of within-session locomotor activity (counts/min) over the final 10 d of self-administration in a subset of CAN_THC, CAN_CBD, and VEH self-administering rats (n = 2–3/group). B, Home cage activity measured as total time spent inactive during the 3 h immediately following CAN_THC, CAN_CBD, or VEH vapor self-administration. C, Total daily inactivity time in CAN_THC, CAN_CBD, and VEH vapor self-administering rats. D, Mean daily distance traveled in the home cage during the active and inactive phases in CAN_THC, CAN_CBD, or VEH vapor self-administering rats. E, Mean daily food consumption (grams) in CAN_THC, CAN_CBD, or VEH vapor self-administering rats. F, Mean oxygen consumption (VO₂) and (G) mean carbon dioxide consumption (VCO₂) during the active and inactive phase in CAN_THC, CAN_CBD, and VEH self-administering rats. H, Mean energy expenditure (kcal/h) during active and inactive phases of rats trained to self-administer CAN_THC, CAN_CBD, or VEH vapor. All values are presented as averages over the final 10 d of self-administration training. n = 5–6/group. p ≤ 0.05. *Significant differences between CAN_THC and VEH groups. #Significant differences between CAN_THC and CAN_CBD groups.

Figure 5.

The reinforcing effects of vaporized CAN_THC require CB1 receptor stimulation. Mean (A) active nose-poke responses for CAN_THC and (B) CAN_THC vapor deliveries following systemic administration of the CB1R antagonist AM251 (0, 1, or 3 mg/kg, i.p.). Mean (C) active nose-poke responses for CAN_CBD and (D) CAN_CBD vapor deliveries following systemic administration of the CB1R antagonist AM251 (0, 1, or 3 mg/kg, i.p.). Data are depicted as a percentage of baseline from the preceding mock injection day. p ≤ 0.05. *Significant differences between CAN_THC and VEH groups.

Figure 6.

Cannabis vapor supports conditioned drug seeking in the absence of drug availability or in the presence of drug-related cues. A, Active (colored symbols) and inactive (open symbols) responding for CAN_THC, CAN_CBD, or VEH vapor on the final day of self-administration training (left) and during the first 7 d of extinction training (right). B, Number of trials required to meet extinction criterion (i.e., ≥50% decrease in active nose-poke responses relative to the final self-administration day during the final two extinction sessions). C, Number of nose-poke responses made on the active operanda for CAN_THC, CAN_CBD, or VEH vapor on the final day of extinction (left) and during a cue-induced reinstatement test (left). n = 11–13/group. p ≤ 0.05. ‡Significant difference in responding relative to the final day of (A) self-administration or (C) extinction training. *Significant differences between CAN_THC and VEH groups. ^†Significant differences between CAN_CBD and VEH groups.