Mechanisms of the trigeminal-lateral parabrachial-rostroventral medullary circuits involved in experimental occlusal interference induced chronic orofacial pain.

BACKGROUND: Chronic orofacial pain is a prevalent and debilitating condition, with complex bidirectional interactions between sensory and emotional processing. The spinal trigeminal nucleus (Sp5), lateral parabrachial nucleus (LPBN), and rostral ventromedial medulla (RVM) form putative circuits involved in pain modulation, yet their structural and functional connectivity remains poorly understood. This study aims to elucidate the functional contribution of the Sp5-LPBN-RVM circuits to experimental occlusal interference (EOI)-induced chronic orofacial pain. METHODS: Viral tracing and immunofluorescence were employed to investigate the structural connectivity of the circuits. A chronic orofacial pain model induced by EOI was established. Following chemogenetic manipulation of the Sp5-LPBN(glutamatergic) pathway, we recorded the neuronal activities in the RVM and Sp5 through in vivo electrophysiology and fiber photometry respectively. We also evaluated the mechanical head withdrawal threshold (mHWT), conditioned place aversion (CPA) or conditioned place preference (CPP). RESULTS: Anatomically, the Sp5 directly projects to the LPBN. Among the LPBN neurons receiving Sp5 input, 85.8% are glutamatergic and 4.1% are GABAergic. The LPBN, in turn, sends projections to the RVM, with 71.4% of these projecting neurons being glutamatergic and 9.1% GABAergic. However, few of the glutamatergic LPBN neurons that receive Sp5 input further project to the RVM. Functionally, activation of the Sp5-LPBN(glutamatergic) pathway enhanced the evoked responses of RVM ON-cells and Sp5 wide-dynamic range (WDR)/non-specific (NS) neurons, increased calcium signals in RVM neurons and glutamatergic Sp5 neurons, induced orofacial hyperalgesia, and elicited pain-related aversive behavior. Crucially, inhibiting this pathway reversed the analogous parameter changes seen in EOI models. CONCLUSIONS: The Sp5-LPBN-RVM circuit functions as a trigeminal-parabrachial-medullary positive-feedback loop. This circuit amplifies nociception through descending facilitation from the RVM and central sensitization at the Sp5, concurrently exacerbating pain-related negative affect. Thus, it constitutes a critical neural substrate underlying chronic orofacial pain and represents a potential therapeutic target. GRAPHICAL ABSTRACT: [Image: see text] SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s10194-026-02292-4.