Effect Of Tachykinin NK-1Receptor On Somatic Pain Perception In Anterior Cingulate Gyrus

Pain is a warning to tell us the body being hurted，which can protect the bodyto avoid further damage. But severe pain is unbearable and can cause physiologicalfunction disorder of the body，also. An ideal effective method to treat pain inclinic has not been found. Hence development of pain study has importanttheoretical and practical significance.Anterior cingulate gyrus (ACG) is located in the medial surface of cerebralhemispheres，and has extensive fiber connections with the nuclei involved in painperception and modulation，and an important center of algesia. Our previous studyfound ACG has somatic nociceptive neurons and the saphenous nerve (SN)representative area. Many studies show that dopamine system concerns with pain.The pathway of dopaminergic neurons projects to ACG. The expression of c-fos genehas a close relationship with pain. The c-fos gene and its encoded protein areused to judge and track the conduction path of nociceptive information. Ourprevious study found that SN somatic nociceptive information can activate ACGdopaminergic neural system，and increase significantly ACG neuron Fos proteinexpression. However，which kinds of receptors are involved in these processes needfurther study.Substance P is one of the earliest discovered neuropeptide，is an importantmember of the tachykinin family，is widely expressed in nervous system. SP is a neurotransmitter or neuromodulator， is involved in nociceptive informationtransmission，neurogenic inflammation，visceral smooth muscle contraction anddilation of blood vessels etc. Tachykinin NK-l receptor is generally accepted asSP receptor. Behavior experimental evidence show that activation of NK-l receptorcan cause pain reaction. Peripheral SP/NK-1receptor system is involved in allergyto heat pain or nerve injury associated with inflammation and orofacial persistentpain induced by formalin. Application of NK-l receptor antagonists on neuropathicpain and inflammatory pain in rats can block or attenuate hyperalgesia. But whethertachykinin NK-l receptor takes part in the processes of the c-fos gene expressionof ACG neurons and activation of ACG dopaminergic neural system induced by SNafferent nociceptive information have not been reported now. Therefore，in thepresent study，noxious stimulation of SN by the strong current was used as a modelof somatic pain，and HPLC-ECD，and immunohistochemistry methods were used to studyeffects of NK-l receptor antagonist GR82334on changes of both c-fos geneexpression and DA content in rat ACG induced by the strong current noxiousstimulation of SN. The experiment could investigate the role of tachykinins inthe transfer of somatic pain information，enable mankind to get beneficialinspiration to research the analgesic mechanism and apply analgesia methods，to provide a theoretical basis for the clinical analgesic therapy. The experimentcould evaluate the value of application of NK-1receptor antagonists in the newanalgesic drug research and development，and provide the experimental basis forthe development of new effective and non-addictive pain killers.84male Wistar rats weighting from200g to270g were used in the present study. 42rats were used to study effects of NK-l receptor antagonist GR82334on changesof c-fos gene expression，and DA content in the rats ACG induced by the strongcurrent noxious stimulation of SN，respectively. The rats were randomly dividedinto blank control group, sham stimulation control group (That is the stimulatingelectrode was placed in the central end of the exposed left SN without electricalstimulation.)，electrically stimulating SN group(That is30min after electricstimulating SN with1.0mA.)，GR82334（ith）group(That is10min after GR82334intrathecal injection，then electric stimulating SN)，NS（ith）group(That is10minafter NS intrathecal injection，then electric stimulating SN)，GR82334（iv）group(That is10min after GR82334injection by caudal veins followed by30min after1.0mA electric stimulating SN)，and NS（iv）group(That is10min after NS injectionby caudal veins followed by30min after1.0mA electric stimulating SN). These ratswere decapitated and their brains were taken，respectively. After weighed，homogenated，and centrifugated the samples of ACG with high speed，the DA contentof20μl supernatant liquid of the ACG samples was determined with HPLC-ECD system.In addition，the coronary frozen section of ACG brain tissue was made，and thenstained by the conventional SABC method. Fos protein expression in ACG neuronswas analyzed by using microscopic image analyzer.The experimental data showed compared with blank control group and shamstimulation control group，Fos protein expression and DA content in ACG wereincreased significantly (P<0.01) at30min after strong current noxious stimulationSN(That is electrically stimulating SN group). Fos protein expression and DAcontent in ACG were also increased significantly (P<0.05，P<0.01) at10min after GR82334intrathecal injection followed by30min after strong current noxiousstimulation SN(That is GR82334（ith）group). But the magnitudes of increases inFos protein expression and DA content in ACG in GR82334（ith）group were smallerthan those in both electrically stimulating SN group and SN（ith）group（P<0.05，P<0.01）. The data suggested that the nociceptive afferent information of SN canarrive ACG and activate ACG neurons，and increase Fos protein expression of ACGneurons；And there is some kind of structural and functional connection betweenSN and ACG dopaminergic nervous system，and SN afferent somatic nociceptiveinformation can activate ACG dopaminergic nervous system to increase DA contentin ACG，which is involved in pain；And central NK-1receptors are involved in theprocesses of augment in Fos protein expression and DA content of ACG caused bySN afferent nociceptive information. But there are other central pathways of SNnociceptive input to ACG to cause significant increases in Fos protein expressionand DA content of ACG，in which other neurotransmitters and receptors are involved.The experimental results showed compared with blank control group and shamstimulation control group，ACG Fos protein expression and DA content had notsignificant variation at10min after caudal vena injection NK-1receptorantagonist GR82334followed by30min after strong current noxious stimulation SN(P>0.05).But compared with blank control group and sham stimulation control group，ACG Fos protein expression and DA content augmented obviously at10min after caudalvena injection normal saline followed by30min after strong current noxiousstimulation SN (P<0.01). The data indicated that the significant increases in bothFos protein expression and DA content in ACG caused by strong current noxious stimulation SN were antagonized by GR82334（iv），suggested that peripheral NK-1receptors partake the processes of increases in Fos protein expression and DAcontent of ACG induced by strong current noxious stimulation SN.