Regional brain structural changes during the transition from acute to chronic neuropathic pain: a longitudinal study in a rat model of orofacial pain.

Brain imaging studies have identified structural alterations in individuals with chronic neuropathic pain, particularly in regions involved in sensory, emotional, and cognitive processing. However, most studies remain cross-sectional, leaving the temporal progression of these changes during pain chronification unclear. This gap is critical, as both neural and non-neural cells undergo dynamic changes during pain chronification. The primary aim of this study was to track longitudinally, regional brain volume and density changes before and after induction of orofacial neuropathic pain in male rats, using a chronic constriction injury of the infraorbital nerve (ION-CCI). This model enables assessment of changes in the spinal trigeminal nucleus (SpVn). T2-weighted magnetic resonance imaging scans were acquired 7-days prior to injury and then at +2, +7, +14, and +28 days, in ION-CCI (n=11), sham-injured (n=6), and naïve (n=8) groups. Voxel-based and deformation-based morphometry were used to identify early- and late-onset density and volumetric changes, respectively. Early after injury, transient, bidirectional volume and/or density changes in the ipsilateral primary somatosensory cortex (S1), contralateral SpVn, and bilateral dorsomedial periaqueductal grey, ipsilateral retrosplenial and contralateral prelimbic cortices, were observed. In contrast, late-onset changes were more widespread, with volume increases observed bilaterally in the SpVn pars interpolaris (SpVo), in the ipsilateral dorsomedial SpVn (SpVdm), bilateral lateral hypothalamus, and contralateral amygdalo-hippocampal complex, and volume decreases in contralateral retrosplenial, prelimbic, and bilateral S1 cortices. These findings reveal early dynamic reorganisation of sensory and sensory-modulatory brain regions, followed by sustained structural alterations across trigeminal brainstem nuclei, limbic-hypothalamic systems, and higher-order cortical networks.Significance Statement Chronic neuropathic pain is associated with structural brain changes, but cross-sectional human studies cannot determine when these alterations emerge, particularly at the primary afferent synapse. Using longitudinal magnetic resonance imaging in a rat model of trigeminal neuropathic pain, we mapped the temporal changes in brain anatomy from pre-injury through pain chronification. We identified an early, transient reorganisation in sensory and modulatory regions, followed by widespread, sustained structural changes in trigeminal brainstem nuclei, including the spinal trigeminal nucleus, limbic-hypothalamic systems, and higher-order cortical networks. These findings provide the first temporal blueprint of brain reorganisation during the transition to chronic orofacial neuropathic pain and establish a mechanistic foundation for identifying early biomarkers and therapeutic targets.