The Circadian Transcription Factor NPAS2 Modulates Fentanyl-mediated Withdrawal and Hyperalgesia but Not Tolerance, in a Sex-dependent Manner.

Abstract

Prescription opioids are the mainstay treatment for severe pain. Unfortunately, their chronic use can cause misuse, addiction, and overdoses death. Risks for these life-threatening outcomes are increased by side-effects which develop upon long-term opioid use, including analgesic tolerance, opioid-induced hypersensitivity, and physical dependence. In addition, altered circadian rhythmicity and disruption of sleep patterns are also a common trait in patients on opioid therapy. Prior research suggested intricate bidirectional interactions between circadian rhythmicity, the pathophysiology of pain, and opioid-mediated analgesia and side-effects. Circadian rhythms are generated by the rhythmic expression of clock genes such as the circadian transcription factor Neuronal PAS domain protein 2 (Npas2) that is expressed in DRG, spinal cord and brain structures involved in pain modulation and opioid analgesia. However, whether it is involved in opioid side-effects was never investigated. Thus, the present study was designed to explore the possible role of NPAS2 signaling in fentanyl-induced side-effects. Using male and female mutant mice with non-functional NPAS2 protein (NPAS2-/-), we performed behavioral studies to evaluate the impact of the absence of NPAS2 signaling on behavioral models of fentanyl-induced analgesia, tolerance, hyperalgesia, and dependence. Our findings show that thermal pain thresholds, acute analgesia, and tolerance development to a fixed dose of fentanyl were unchanged in male and female NPAS2-deficient mice. However, we found that female but not male NPAS2-defficient mice displayed a decreased potency of high doses of fentanyl after tolerance development. In addition, female NPAS2-defficient mice showed markedly more symptoms of physical dependence when withdrawal was precipitated by naloxone, which was not observed in male mice. Conversely, while female NPAS2-defficient mice developed similar fentanyl-induced hypersensitivity, male NPAS2-defficient mice had higher fentanyl-induced hypersensitivity, when compared to matched-sex WT littermate controls. Together, our findings suggest sex-specific effects of NPAS2 signaling in the regulation of fentanyl-induced tolerance, hyperalgesia, and dependence. Grant support from K01DA038654; R21DA041872; R01HL150432. Prescription opioids are the mainstay treatment for severe pain. Unfortunately, their chronic use can cause misuse, addiction, and overdoses death. Risks for these life-threatening outcomes are increased by side-effects which develop upon long-term opioid use, including analgesic tolerance, opioid-induced hypersensitivity, and physical dependence. In addition, altered circadian rhythmicity and disruption of sleep patterns are also a common trait in patients on opioid therapy. Prior research suggested intricate bidirectional interactions between circadian rhythmicity, the pathophysiology of pain, and opioid-mediated analgesia and side-effects. Circadian rhythms are generated by the rhythmic expression of clock genes such as the circadian transcription factor Neuronal PAS domain protein 2 (Npas2) that is expressed in DRG, spinal cord and brain structures involved in pain modulation and opioid analgesia. However, whether it is involved in opioid side-effects was never investigated. Thus, the present study was designed to explore the possible role of NPAS2 signaling in fentanyl-induced side-effects. Using male and female mutant mice with non-functional NPAS2 protein (NPAS2-/-), we performed behavioral studies to evaluate the impact of the absence of NPAS2 signaling on behavioral models of fentanyl-induced analgesia, tolerance, hyperalgesia, and dependence. Our findings show that thermal pain thresholds, acute analgesia, and tolerance development to a fixed dose of fentanyl were unchanged in male and female NPAS2-deficient mice. However, we found that female but not male NPAS2-defficient mice displayed a decreased potency of high doses of fentanyl after tolerance development. In addition, female NPAS2-defficient mice showed markedly more symptoms of physical dependence when withdrawal was precipitated by naloxone, which was not observed in male mice. Conversely, while female NPAS2-defficient mice developed similar fentanyl-induced hypersensitivity, male NPAS2-defficient mice had higher fentanyl-induced hypersensitivity, when compared to matched-sex WT littermate controls. Together, our findings suggest sex-specific effects of NPAS2 signaling in the regulation of fentanyl-induced tolerance, hyperalgesia, and dependence. Grant support from K01DA038654; R21DA041872; R01HL150432.