CB1 antinociceptive tolerance is delayed in S426A/S430A mice and accelerated in older mice in the formalin test of inflammatory pain.

Inflammatory pain represents a significant detriment to patient quality of life and currently lacks adequate treatment options. Conditions associated with inflammatory pain can occur across the lifespan but are most common in aged individuals. There has been increasing public interest in the analgesic potential of cannabinoids, and CB1 agonists have been preclinically tested for several conditions. However, their use is potentially limited by the development of tolerance. The S426A/S430A transgenic mouse line was previously generated to explore this tolerance, with two point mutations preventing the phosphorylation that represents the first step in CB1 desensitization. Therefore, we investigated the antinociceptive efficacy of CB1-selective agonism (ACEA) in the formalin model of inflammatory pain, using mice that varied in sex (female vs male), age (3- vs 12-month-old [MO], and genotype (C57BL/6J vs S426A/S430A). ACEA provided significant antinociception that was equivalent across all tested group. The S426A/S430A mutation significantly delayed tolerance at both ages tested. Additionally, the S426A/S430A mutation was associated with increased body weight in 3MO, 12MO female, and 12MO male mice. Mice that were 12MO had accelerated tolerance relative to 3MO mice. No significant differences in rotarod (motor coordination), open field (locomotion and center duration), or rectal temperature were noted. These results demonstrate the antinociceptive benefit of CB1 agonists and the significant limitation in their use (tolerance). Further studies are needed to investigate the mechanisms of CB1 tolerance development, the increase in body weight with the S426A/S430A mutation, and the potential clinical significance of these findings.