The Effect And Mechanistic Study Of PVN^oxytocin-CeA Circuit On Comorbidity Of Pain And Anxiety

BackgroundPain often comorbid with some psychiatric disorders including anxiety and depression.In China,up to 54.16% pain patients present with anxiety disorders.The combination of pain and anxiety leads to poorer treatments outcomes and overall functioning than either condition alone.However,the underlying mechanisms of this comorbidity and their dynamic interactions remain largely unknown.It is of great importance to investigate the pathophysiology of comorbid pain and anxiety in order to improve the efficacy of diagnosis and treatment response.Oxytocin,a neuropeptide,is secreted from the hypothalamic supraoptic(SON)and paraventricular(PVN)nuclei of the hypothalamus,and processed along the axonal projections to the posterior lobe of the pituitary,where they are stored in secretory vesicles and released into peripheral circulation.In addition to this release from axonal terminals,there is also evidence that some oxytocin-containing neurons project to other areas of the central nervous system.Oxytocin and vasopressin are closely related peptides,so they can both bind to the oxytocin receptor and vasopressin receptor 1A,fostering the secretion of hormones.Some researchers have found that oxytocin is associated with modulation of pain experience and regulation of mental disorders,suggesting that it might have a key role throughout the pathophysiology of comorbid pain and anxiety and is emerging as target for novel treatment approach.However,the effects of this centrally-released oxytocin are not as clearly as understood.The aim of this study is to investigate the effect and mechanism of oxytocin on comorbidity of pain and anxiety.MethodsCFA was injected into the right hind paws of male adult C57/BL6 mice to induce the inflammatory pain and its associated anxiety-liked disorders.Anxiety-like behaviours were tested by the elevated plus maze(EPM),the marble burying test(MB),the light-dark transition test,while the mechanical thresholds were determined by Von Frey tests.After intra-cerebronventricular injection of oxytocin,we evaluated the effect of oxytocin on anxiety-like behaviors and mechanical hyperalgesia and the brain region involved.The activity of oxytocin neurons in PVN and neurons in central amygdala(Ce A)were assessed through immunofluorescence technique.The effect of oxytocin neuron activation/inhibition in PVN on comorbid pain and anxiety were investigated by optogenetic and chemogenetic approaches.Results1.CFA-induced inflammation pain produced a long-lasting decrease in paw-withdrawal mechanical threshold and anxiety-like disorders.Intra-cerebronventricular injection of oxytocin blocked hyperalgesia and anxiety-like behaviors induced by inflammatory pain.2.Pretreatment with oxytocin and vasopressin V1 a receptor antagonist reversed oxytocin-induced antinociception,indicating that oxytocin and vasopressin V1 a receptors mediated the pain modulation of oxytocin.However,the oxytocin-induced anxiolytic effect was blocked by vasopressin V1 a receptor but not oxytocin receptor,suggesting that vasopressin V1 a receptor participated in the regulation of anti-anxiety effect of oxytocin.3.Pain-anxiety model was accompanied with decreased excitability of oxytocin neurons in PVN,activation of these neurons reversed pain associated anxiety-like behaviors in animal models.Inhibition of PVN oxytocin neurons induced anxiety-like behaviors in naive animals.4.Pain–anxiety model was accompanied with decreased oxytocin m RNA level in Ce A.Intra-Ce A but not basolateral amygdala(BLA)injection of oxytocin relieved anxiety-like behaviors induced by pain.Pretreatment with oxytocin and vasopressin V1 a receptor antagonist in Ce A prevented analgesia effect of oxytocin,while only vasopressin V1 a receptors antagonist blocked anxiolytic effect of oxytocin.5.Photogenetic activation of Ce A projecting PVN oxytocin neurons prevented pain-induced anxiety-like behaviors.ConclusionIn summary,we successfully developed animal model of inflammatory pain-induced anxiety and identified the analgesic and anxiolytic effect of oxytocin.Oxytocin and vasopressin V1 a receptors mediated the pain modulation of oxytocin,vasopressin V1 a receptors participated in the regulation of anxiolytic-like behaviors of oxytocin.Ce A is a key brain region for regulation of comorbidity pain and anxiety.Comorbidity pain and anxiety was companied by decreased excitability of oxytocin neurons in PVN and neurons in Ce A.Activation/ Inhibition of these neurons can reverse/induce pain associated anxiety-like behaviors in animal models.Activation of Ce A projecting PVN oxytocin neurons decreased pain-induced anxiety-like behaviors,indicating PVN-Ce A oxytocinergic projections are crucial for comorbidity pain and anxiety.Our results indicated that PVN-Ce A oxytocinergic projections play a key role in pain comorbid psychiatric disorders.