A posterior hypothalamic to midbrain circuit orchestrating nociceptive behaviours in male mice

Background and Purpose: Chronic pain is a widespread and debilitating condition with limited treatment options. While the ventral tegmental area (VTA) and posterior hypothalamic nucleus (PH) have been independently implicated in pain modulation, the specific neural circuitry linking them remains unclear. This study aimed to investigate the role of the PH → VTA glutamatergic projection in chronic pain regulation. Experimental Approach: Using a mouse model of neuropathic pain induced by chronic constriction injury (CCI), we combined viral tracing, immunofluorescence, in vitro electrophysiology, optogenetics, chemogenetics and fibre photometry. Behavioural assays were used to assess mechanical allodynia and thermal hyperalgesia following circuit manipulation. Key Results: We identified a glutamatergic projection from PH to VTA dopamine (DA) neurons that modulates pain behaviour. Optogenetic activation of the PH → VTA circuit in naïve mice induced pain-like hypersensitivity. Conversely, inhibition of this pathway reduced pain-related behaviours in CCI mice. Mechanistically, PH activation increased glutamatergic input onto VTA DA neurons, enhancing dopamine release in the nucleus accumbens (NAc). Fibre photometry confirmed increased activity in this circuit during nociceptive states. Conclusions and Implications: Our results define a functional PH^Glu → VTA^DA → NAc circuit that contributes to chronic pain processing. Hyperactivity in this pathway facilitates nociceptive behaviours, while its inhibition exerts analgesic effects. Modulating this circuit may offer new therapeutic strategies for treating chronic pain.