Nucleus tractus solitarius mediates hyperalgesia induced by chronic pancreatitis in rats

BACKGROUND Central sensitization plays a pivotal role in the maintenance of chronic pain induced by chronic pancreatitis(CP).We hypothesized that the nucleus tractus solitarius(NTS),a primary central site that integrates pancreatic afferents apart from the thoracic spinal dorsal horn,plays a key role in the pathogenesis of visceral hypersensitivity in a rat model of CP.AIM To investigate the role of the NTS in the visceral hypersensitivity induced by chronic pancreatitis.METHODS CP was induced by the intraductal injection of trinitrobenzene sulfonic acid(TNBS)in rats.Pancreatic hyperalgesia was assessed by referred somatic pain via von Frey filament assay.Neural activation of the NTS was indicated by immunohistochemical staining for Fos.Basic synaptic transmission within the NTS was assessed by electrophysiological recordings.Expression of vesicular glutamate transporters(VGluTs),N-methyl-D-aspartate receptor subtype 2B(NR2B),andα-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subtype 1(GluR1)was analyzed by immunoblotting.Membrane insertion of NR2B and GluR1 was evaluated by electron microscopy.The regulatory role of the NTS in visceral hypersensitivity was detected via pharmacological approach and chemogenetics in CP rats.RESULTS TNBS treatment significantly increased the number of Fos-expressing neurons within the caudal NTS.The excitatory synaptic transmission was substantially potentiated within the caudal NTS in CP rats(frequency:5.87±1.12 Hz in CP rats vs 2.55±0.44 Hz in sham rats,P<0.01;amplitude:19.60±1.39 pA in CP rats vs 14.71±1.07 pA in sham rats;P<0.01).CP rats showed upregulated expression of VGluT2,and increased phosphorylation and postsynaptic trafficking of NR2B and GluR1 within the caudal NTS.Blocking excitatory synaptic transmission via the AMPAR antagonist CNQX and the NMDAR antagonist AP-5 microinjection reversed visceral hypersensitivity in CP rats(abdominal withdraw threshold:7.00±1.02 g in CNQX group,8.00±0.81 g in AP-5 group and 1.10±0.27 g in saline group,P<0.001).Inhibiting the excitability of NTS neurons via chemogenetics also significantly attenuated pancreatic hyperalgesia(abdominal withdraw threshold:13.67±2.55 g in Gi group,2.00±1.37 g in Gq group,and 2.36±0.67 g in mCherry group,P<0.01).CONCLUSION Our findings suggest that enhanced excitatory transmission within the caudal NTS contributes to pancreatic pain and emphasize the NTS as a pivotal hub for the processing of pancreatic afferents,which provide novel insights into the central sensitization of painful CP.

Anterior Cingulate Cortex Mediates Hyperalgesia and Anxiety Induced by Chronic Pancreatitis in Rats

Central sensitization is essential in maintaining chronic pain induced by chronic pancreatitis(CP),but cortical modulation of painful CP remains elusive.Here,we examined the role of the anterior cingulate cortex(ACC)in the pathogenesis of abdominal hyperalgesia in a rat model of CP induced by intraductal administration of trinitrobenzene sulfonic acid(TNBS).TNBS treatment resulted in long-term abdominal hyperalgesia and anxiety in rats.Morphological data indicated that painful CP induced a significant increase in FOS-expressing neurons in the nucleus tractus solitarii(NTS)and ACC,and some FOS-expressing neurons in the NTS projected to the ACC.In addition,a larger portion of ascending fibers from the NTS innervated pyramidal neurons,the neural subpopulation primarily expressing FOS under the condition of painful CP,rather than GABAergic neurons within the ACC.CP rats showed increased expression of vesicular glutamate transporter 1,and increased membrane trafficking and phosphorylation of the N-methyl-D-aspartate receptor(NMDAR)subunit NR2B and theα-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor(AMPAR)subunit GluR1 within the ACC.Microinjection of NMDAR and AMPAR antagonists into the ACC to block excitatory synaptic transmission significantly attenuated abdominal hyperalgesia in CP rats,which was similar to the analgesic effect of endomorphins injected into the ACC.Specifically inhibiting the excitability of ACC pyramidal cells via chemogenetics reduced both hyperalgesia and comorbid anxiety,whereas activating these neurons via optogenetics failed to aggravate hyperalgesia and anxiety in CP rats.Taken together,these findings provide neurocircuit,biochemical,and behavioral evidence for involvement of the ACC in hyperalgesia and anxiety in CP rats,as well as novel insights into the cortical modulation of painful CP,and highlights the ACC as a potential target for neuromodulatory interventions in the treatment of painful CP.