The Role Of Hypothalamic Arcuate Nucleus In The Regulation Of Inflammatory Nociception Of Rats

Part Ⅰ.The role of hypothalamic arcuate nucleus in regulations of the discharge activity of spinal dorsal horn neurons and the nociceptive response of CFA-imflamed ratsObjective The hypothalamic arcuate nucleus(ARC)is an important component of the endogenous pain modulation system.However,it remains not fully clear how the ARC is involved in the modulation of inflammatory pain.In this part of studies,we aimed to identify the role of ARC in regulations of the neuronal activity in the spinal dorsal horn and the nociceptive responses of rats with persistent inflammation in the left hind paw.Methods Inflammation was induced by sub-plantar injection of complete Freund’s adjuvant(CFA)in rats.The lesion of ARC was produced in rats by electrolytic lesion or by a neonatal intraperitonial(i.p.)injection of monosodium glutamate(MSG,2 mg/g).Extracellular single unit recordings were conducted in the dorsal horn of the spinal cord.Electrical stimulation(frequency: 0.2 Hz,intensity: 20 V two-pulses with 0.5 ms duration and 10 ms interval)of the right sciatic nerves was performed to mimic noxious stimulation.Spontaneous and electrical stimulation-evoked discharges were recorded in wide dynamic range(WDR)neurons,which responded to both innocuous and noxious stimulations,and nociceptive specific(NS)neurons,which responded only to noxious stimulations.Thermal responses of rats were measured by the radiant heat-withdrawalmethod.Mechanical responses were measured by the von Frey method.L-glutamate(0.2mM/0.5 μl)was micro-injected into the ARC to activate ARC neurons.Effects of the ARC injection of L-glutamate on nociceptive neurons in the spinal dorsal horn and on rats’ responses to the heat and mechanical stimulations were recorded respectively at 2min,8 min,14 min after the ARC injection.Results When compared with those in control animals both the spontaneous and noxcious electrical stimulation-evoked discharges of WDR and NS neurons in the spinal dorsal horn were significantly increased(P<0.001)in CFA-inflamed rats Lesions of ARC produced by electrolysis or by a neonatal intraperitonial injection of MSG significantly blocked the electrical stimulation-induced increase in neuronal discharges in both WDR(P<0.05)and NS(P<0.01)neurons of CFA-inflamed rats,while no change was noted in their spontaneous activity.Following microinjection of L-glutamate into the ARC area,noxcious electrical stimulation-induced discharges were significantly increased in the spinal dorsal horn neurons of CFA-inflamed rats(P<0.01).Furthermore,animals’ nociceptive responses to both thermal and mechanical stimulations were also significantly(P<0.01)enhanced.These effects induced by microinjection of L-glutamate lasted about12 min.Conclusions Spinal cord dorsal horn neurons were sensitized following peripheral inflammation.While destroying ARC neurons(by electrolytical lesion or by a neonatal intraperitonial injection of MSG)significantly prevented the increase in neuronal activity induced by inflammation,exciting ARC neurons by microinjection of L-glutamate into the ARC area further increased nociceptive responses in the spinal dorsal horn and the whole animal evoked by inflammation.Thus,we conclude that ARC may act as a descending facilitatory component critically involved in the central sensitization process associated with peripheral inflammation.Part Ⅱ.The mechanism of hypothalamic arcuate nucleus in regulations of the activity of spinal dorsal horn neurons of CFA-imflamed ratsObjective Previous studies have shown that endogenous pain modulation system which including the periaqueductal gray(PAG)and the rostral ventromedial nucleus(RVM)regulate the nociceptive response of spinal dorsal horn neurons by the descending pathway.In this part of studies,we investigated potential pathway(s)and mechanisms by which ARC neurons modulate inflammation-induced nociception.Methods Electrophysiological and behavioral studies were conducted to examine effects of lidocaine or methysergide(MSD)microinjected into the nucleus raphe magnus(NRM)area on the neuronal discharge activity in the spinal dorsal horn and on the behavioural response of CFA-inflamed animals to thermal and mechanical stimulation with or without microinjection of L-glutamate into the ARC area.Through ELISA assay concentrations of 5-hydroxytryptamine(5-HT),norepinephrine(NE),glutamate(Glu)andγ-aminobutyric acid(GABA)in the NRM and spinal dorsal horn of rats were examined.Results Microinjection of either lidocaine or MSD into the NRM did not significantly affect both the neuronal discharge activity in spinal dorsal horn and the animals’ responses to thermal and mechanical stimulation in CFA-inflamed rats.However,these drug injections blocked the facilitative effects induced by the microinjection of L-glutamate into the ARC area.ELISA assay showed that when compared with those in the control group of rats,in the spinal dorsal horn of CFA-inflammatory rats,the concentrations of 5-HT(P<0.01)and Glu(P<0.01)were increased but NE(P<0.01)and GABA(P<0.01)decreased.In the NRM,the concentrations of 5-HT(P<0.01)and GABA(P<0.01)were increased,but Glu(P<0.01)decreased.The ARC-lesion prevented all of these changes in both the spinal dorsal horn and NRM of CFA-inflammatory rats.Conclusions Taken all the findings together,we conclude that NRM is a criticalcomponent,through which ARC play a facilitative role resulting in the central sensitization in rats with peripheral inflammation,and that up-regulating the concentrations of 5-HT,Glu in both the NRM and spinal dorsal horn,as well as down-regulting Glu in the NRM,and GABA and NE in the spinal dorsal horn may be important mechanisms underlying the action of ARC neurons in the modulation of nociception associated with CFA inflammation.