Sex-specific role of the circadian transcription factor NPAS2 in opioid tolerance, withdrawal, and analgesia

ABSTRACT Opioids like fentanyl are the mainstay treatment for chronic pain. Unfortunately, opioids high addiction liability has led to the current opioid crisis. This is in part related to long-term use side-effects, including analgesic tolerance (gradual decrease in analgesia), and physical dependence (withdrawal symptoms upon opioid interruption), causing dose-escalation and preventing usage interruption. Altered circadian rhythmicity and sleep patterns are common in patients on opioid therapy. Prior research showed intricate bidirectional interactions between circadian rhythmicity, opioid analgesic efficacy, and side-effects. However, underlying mechanisms are largely unknown. Neuronal PAS domain protein 2 (NPAS2) is a circadian transcription factor that is highly expressed in structures of the central nervous system that modulate pain and opioids. In this study, we used male and female mice expressing non-functional NPAS2 (NPAS2 deficient, NPAS2-/-) to investigate the role of NPAS2 in fentanyl analgesia, tolerance, hyperalgesia, and dependence. In NPAS2-/- mice, we found that thermal pain thresholds, acute analgesia, and tolerance to a fixed dose of fentanyl were largely like wild-type mice. However, female NPAS2-/- mice augmented behavioral state of analgesic tolerance and exhibited significantly more behavioral symptoms of physical dependence. Conversely, only male NPAS2-/- mice had increased fentanyl-induced hypersensitivity, when compared to matched-sex littermate controls. Together, our findings suggest sex-specific effects of NPAS2 signaling in the regulation of fentanyl-induced tolerance, hyperalgesia, and dependence.