Lack of tolerance to anxiolysis and withdrawal symptoms in mice repeatedly treated with AC-5216, a selective TSPO ligand

AC-5216, a ligand for the translocator protein (18 kDa) (TSPO), produces anxiolytic-like effects in animal models of anxiety without causing the side effects normally associated with conventional benzodiazepines. This study aimed to investigate whether repeated administration of AC-5216 induces tolerance to anxiolytic-like effects of AC-5216 and produces withdrawal on abrupt cessation, and compare the results with those of diazepam. In the tolerance experiment, AC-5216 (0.1 mg/kg, p.o.) produced significant anxiolytic-like effects in both groups of mice pretreated with the vehicle and AC-5216 twice daily for 14 days. Diazepam (0.1 mg/kg, p.o.) also retained its anxiolytic effects in mice repeatedly treated with diazepam. In the withdrawal experiment, mice were orally treated with either AC-5216 (0.1, 1 or 10 mg/kg; twice daily) or diazepam (0.1, 1 or 10 mg/kg; twice daily) for 14 days, and examined, during a treatment withdrawal period, for anxiogenic-like effects in the social interaction test, and for body weight loss as indices of emotional and somatic withdrawal symptoms, respectively. In AC-5216-treated groups, neither anxiogenic-like effects nor body weight loss was observed upon treatment withdrawal at any of the doses tested. In contrast, in diazepam 1 mg/kg- and 10 mg/kg-treated groups, treatment withdrawal not only induced anxiogenic-like effects on the second day of the withdrawal period, but also decreased body weight gain and brought about body weight loss in mice. These findings indicate that AC-5216 when repeatedly administered does not induce tolerance to its anxiolytic-like effects or withdrawal symptoms.