Hypothalamic clock governs circadian pain

Pain sensitivity in humans exhibits daily fluctuations. Patients with neuropathic pain or rheumatoid arthritis frequently experience an exacerbation of symptoms in the evening. However, the neural mechanisms underlying circadian pain remain poorly understood. Although rhythmic variations have been reported in the central nervous system, evidence supporting a direct correlation between circadian rhythms in neuronal activity and pain sensitivity is still limited.

Research on circadian pain has largely focused on characterizing its perceptual and clinical manifestations, but the recognition of circadian pain rhythms has not been matched by mechanistic insight. Although the functions of many pain-processing brain regions have been characterized, their diurnal activity patterns under pathological pain conditions are poorly defined. Crucially, it is unknown whether and how the central circadian clock orchestrates pain-related regional activities to drive circadian pain.

Mice with neuropathic pain exhibit differentially diurnal and nocturnal responses to pain, showing higher pain sensitivity at zeitgeber time 5 (ZT5, 5 hours after light onset) in the daytime (the resting phase of mice) and lower at ZT14 in nighttime (the active phase of mice). Fiber photometry recordings and chemogenetic modulations reveal that the rhythmic activity of vasoactive intestinal peptide neurons in the suprachiasmatic nucleus (SCN^VIP) is higher at ZT5 and lower at ZT14 in neuropathic pain mice, which corresponds to concomitant changes in nociceptive sensitivity.

Fos-TRAPed neuronal labeling and in vivo recordings demonstrate that the activity of GABAergic neurons in the ventrolateral periaqueductal gray (vlPAG^GABA) is higher at ZT5 compared to ZT14. The retrograde tracing reveals that vlPAG^GABA neurons are innervated by glutamatergic neurons in the paraventricular nucleus (PVN^Glu), which exhibit distinct daily rhythms in neuropathic pain mice, characterized by elevated activity at ZT5 and reduced activity at ZT14. Optogenetic manipulations show that the rhythmic PVN^Glu→vlPAG^GABA circuit activity entrains rhythmic pain sensitivity of mice with neuropathic pain.

Photoactivation of SCN^VIP neuronal terminals in the PVN at ZT5 and local SCN^VIP neurons at ZT14 induced opposite pain behavioral effects. Triple retrograde tracing and fiber photometry recordings indicate that the activity of PVN-projecting GABAergic neurons in the subparaventricular zone (SPZ^GABA) exhibits a synchronized, but inverse, pattern of rhythmicity relative to SCN^VIP neuronal activity. Ablation of SPZ^GABA neurons establishes the SPZ as an essential inhibitory relay in the SCN→SPZ→PVN→vlPAG circuit for maintaining daily rhythms in pain sensitivity of neuropathic pain mice.

vlPAG^GABA neuronal rhythmicity drives daily rhythms of the descending analgesia system via the vlPAG→rostral ventromedial medulla (RVM)→spinal cord circuit. At ZT5, the decreased activity of RVM-innervated GABAergic neurons in the spinal cord (SC^GABA) leads to increased activity of ascending ventral posterolateral nucleus (VPL)–projecting glutamatergic neurons in the spinal cord (SC^Glu) that they target, thereby inducing higher pain sensitivity of mice with neuropathic pain.

Mice exhibit daily rhythms in pain sensitivity, accompanied with rhythmic activity of the endogenous descending analgesia system, which is coordinated by the hypothalamic master clock. This study explains how the circadian system influences the pain modulation system to generate time-of-day–dependent pain behaviors, offering a mechanistic framework for chronotherapeutic strategies in pain management.