Mepirapim, a Novel Synthetic Cannabinoid, Induces Addiction-Related Behaviors through Neurochemical Maladaptation in the Brain of Rodents

Abstract

Mepirapim is a synthetic cannabinoid that has recently been abused for recreational purposes. Although serious side effects have been reported from users, the dangerous pharmacological effects of Mepirapim have not been scientifically demonstrated. In this study, we investigated the addictive potential of Mepirapim through an intravenous self-administration test and a conditioned place preference test in rodents. Moreover, to determine whether the pharmacological effects of Mepirapim are mediated by cannabinoid receptors, we investigated whether Mepirapim treatment induces cannabinoid tetrad symptoms in mice. Lastly, to identify Mepirapim induced neurochemical maladaptation in the brains of mice, we performed microdialysis, western blots and neurotransmitter enzyme-linked immunosorbent assays. In the results, Mepirapim supported the maintenance of intravenous self-administration and the development of conditioned place preference. As a molecular mechanism of Mepirapim addiction, we identified a decrease in GABAeric signalling and an increase in dopaminergic signalling in the brain reward circuit. Finally, by confirming the Mepirapim-induced expression of cannabinoid tetrad symptoms, we confirmed that Mepirapim acts pharmacologically through cannabinoid receptor one. Taken together, we found that Mepirapim induces addiction-related behaviours through neurochemical maladaptation in the brain. On the basis of these findings, we propose the strict regulation of recreational abuse of Mepirapim.

Keywords: Mepirapim; addiction; dopamine; synthetic cannabinoid; γ-aminobutyric acid.

Figure 1

Chemical structures of Mepirapim and JWH-018.

Figure 2

Mepirapim treatment supported the maintenance of IVSA in rats. (A) Experimental schedule. Male Sprague Dawley rats (7 weeks old, 250–270 g, n = 5 per group) were used. Each group of rats self-administered either vehicle or Mepirapim (0.003, 0.01 and 0.03 mg·kg⁻¹·infusion⁻¹, i.v.) under a fixed ratio 1 schedule for seven consecutive days. All drugs were injected intravenously at a volume of 0.1 mL·infusion⁻¹. (B) Number of infusions during each session and average for 7 days. (C) Number of active lever presses during each session and average for 7 days. (D) Number of inactive lever presses during each session and average for 7 days. Data are presented as means ± SEMs. Significant differences between the vehicle group and other groups are indicated by * p < 0.05.

Figure 3

Mepirapim treatment induced the development of CPP and tetrad symptoms in mice. (A) Experimental schedule. Each arrow represents a drug treatment. Male C57BL/6J mice (7 weeks old, 20–22 g, n = 9 per group) were used. (B) CPP test. Mice were conditioned with either vehicle or Mepirapim (0.3, 1 and 3 mg·kg⁻¹, i.p.) during the conditioning period. (C,D) Open field test (OFT). Immediately before the test, mice were treated with vehicle or Mepirapim. (E) Body temperature measurement. After basal body temperature was measured, mice were treated with vehicle or Mepirapim. Data are presented as means ± SEMs. Significant differences between the vehicle group and other groups are indicated by * p < 0.05.

Figure 4

Mepirapim treatment-induced changes in levels of neurotransmitters in rat NAc. (A) Experimental schedule for microdialysis in the rat NAc. After baseline samples were collected, rats were treated with progressively increasing doses of Mepirapim (0.3, 1 and 3 mg·kg⁻¹, i.p.) every hour (n = 5 per group). Each arrow represents a drug treatment. Concentration–time profiles and areas under the concentration–time curves (AUC) for (B) dopamine (DA), (C) 3,4-dihydroxyphenylacetic acid (DOPAC) and (D) homovanillic acid (HVA). Data are presented as means ± SEMs. Significant differences between the vehicle group and other groups are indicated by * p < 0.05.

Figure 5

Mepirapim treatment-induced changes in the expression of proteins related to DA and GABA in mouse VTA and NAc. Mice in each group were treated with either vehicle or Mepirapim (1 and 3 mg·kg⁻¹, i.p.) during the CPP test and tetrad test, respectively (n = 5 per group). Western blot analysis of protein expression in VTA (A–C) and NAc (D–F). Data are presented as means ± SEMs. Significant differences between the vehicle group and other groups are indicated by * p < 0.05. CB1R: cannabinoid receptor 1, D1DR: dopamine receptor D1, GABA_A: γ-aminobutyric acid receptor A, GAD: glutamate decarboxylase, TH: tyrosine hydroxylase.

Figure 6

Mepirapim treatment-induced changes in DA and GABA levels in mouse VTA and NAc. Mice in each group were treated with either vehicle or Mepirapim (1 and 3 mg·kg⁻¹, i.p.) during the CPP test and tetrad test, respectively (n = 5 per group). Measurement of neurotransmitter levels in VTA (A,B) and NAc (C,D). Data are presented as means ± SEMs. Significant differences between the vehicle group and other groups are indicated by * p < 0.05.