The Neural Mechanism Underlying The Modulation Of DRN^5-HT-ACC Pathway Engaged In The Chronic Low Back Pain And Anxiety Comorbidities

Chronic low back pain（CLBP）is one of the most common medical conditions in the world,patients are often accompanied by comorbid emotional cognitive disorders,such as anxiety disorders,depression and other neuropsychiatric diseases.CLBP is not only lim-ited to spinal cord injuries,but also manifests as structural and functional changes in the brain.5-hydroxtryptamine（5-HT）,a monoamine transmitter that is considered to be in-volved in relieving chronic pain and anxiety.However,the distribution of 5-HT system in the brain and its mechanisms involved in CLBP is presently unclear.Object:1.To investigate the distribution of 5-HT nerve fibers throughout the brain and the projection of dorsal raphe nucleus（DRN）5-HT neurons in the brain;2.To explore the role of DRN 5-HT system in chronic pain and anxiety comorbidities;3.To elucidate structural and functional changes of DRN-ACC 5-HTergic neural circuit in chronic low back pain and anxiety comorbidities.Methods:Sert-cre and Ai34 transgenic mice were used to investigate the distribution of 5-HT nerve fibers in the whole brain by immunofluorescent methods;the projection of 5-HTergic neurons of DRN in the brain and the DRN-ACC 5-HTergic neural circuit were examined by viral tracing combined with immunofluorescent methods;the effects of DRN5-HT system and DRN-ACC 5-HTergic circuit in chronic low back pain and anxiety comorbidities were further explored by chemical genetics combined with behavioral ex-periments.Results:In the present study,the whole brain distribution of 5-HT nerve fiber endings was mapped in the brains of Sert-cre&Ai34 transgenic mice to study the structural and func-tional changes of the 5-HT system in the modified chronic compression of multiple unilat-eral DRGs（mCCD）mice.The results showed that there was a large distribution in the cortical areas such as the anterior cingulate cortex（ACC）,the primary motor cortex（M1）,the secondary motor cortex（M2）,the primary somatosensory cortex（S1）,the sec-ondary somatosensory cortex（S2）and the insular cortex（IC）,in non-cortical forebrain nuclei such as the medial septal nucleus（MS）,the lateral septal nucleus（LS）,the bed nu-cleus of the stria terminals,the lateral division,the dorsal part（BNSTLD）,the basolateral amygdaloid nucleus（BLA）,the central amygdaloid nucleus（CeA）,the caudate putamen（Cpu）,the ventral pallidum（VP）,the nucleus accumbens（NAc）,the fimbria of the hippo-campus（fi）,the reticular thalamic（Rt）and in nuclei in the brain stem such as the dorsal nucleus of the lateral lemniscus（DLL）,the median raphe nucleus（MnR）,the dorsal raphe nucleus（DRN）,and the anterior tegmental nucleus（ATg）.The primary source of innerva-tion for forebrain 5-HT neurons is the DRN.The 5-HT neurons of the DRN may also project to the forebrain nuclei,including the lateral hypothalamic region（LH）,the VP,the MS,the CeA,and the Cpu.We found that the DRN 5-HT neuronal activity was increased in mCCD mice.Activating the 5-HTergic neurons of the DRN resulted in significant de-creases in both ipsilateral and contralateral hind paw withdrawal thresholds and latencies in the sham group,but not in mCCD mice.Therefore,using the nerve bundle and AAV virus reverse tracing methods,it was observed that there was 5-HT neural circuits from the DRN to the ACC.Then the effect of DRN^5-HT-ACC neural circuit in chronic pain and anxiety-like behaviors in mCCD mice was examined by chemogenetic intervention.The results showed that specific activation of this circuit alleviated pathological hyperalgesia and anxiety-like behaviors in mCCD mice,while the inhibition of this circuit exacerbated nociceptive pain sensitization in sham mice,but had no effect on anxiety-like behaviors.Conclusions:5-HT nerve fibers are terminally distributed in many regions of the whole mouse brain and DRN 5-HT neurons project to the forebrain.The activation of DRN 5-HTergic neuron may be involved in neuropathic pain.DRN^5-HT-ACC neural circuits are involved in chronic pain and anxiety-like behaviors.