Papers of the Week

The "nociplastic pain," a recently proposed novel mechanistic pain descriptor, is defined as pain occurring through altered nociception without nociceptor activation and nerve injury. Nociplastic pain is often characterized by widespread pain sensitization (WSP) in multiple body regions (Fitzcharles et al., 2021). As many patients with primary chronic pain would have nociplastic backgrounds, developing appropriate methods to evaluate drug effects against nociplastic pain in animal model is in great demand. Using two rat models with the WSP involving central amygdala (CeA) activation by orofacial inflammation or direct chemogenetic activation (Sugimoto et al., 2021), we examined whether widely used analgesics, acetaminophen (AcAph), pregabalin (PGB), and duloxetine (DLX) could attenuate the WSP. AcAph (100 or 200 mg/kg, i.p.) significantly elevated 50%-paw withdrawal threshold (PWT), which had been lowered significantly by upper lip injection of formalin, or systemic injection of clozapine-N-oxide in the rats with excitatory designer receptors (hM3Dq) expressed in the right CeA. This effect lasted for > ∼4 h. PGB (30 mg/kg, i.p.) also significantly counteracted the lowered PWT in rats with orofacial formalin injection for >∼6 h. DLX was ineffective on the WSP. Based on these results, we propose that these preclinical models could be used to evaluate drug effects for primary chronic pain. We conclude that the widely used pain killers, AcAph and PGB, also relieve nociplastic widespread sensitization in the absence of ongoing nociceptor activation and nerve injury.