Role And Mechanism Of Paraventricular Nucleus In Regulating Cardiac Sympathetic Afferent Reflex In Rats

BackgroundThe cardiac sympathetic afferent reflex(CSAR)is a sympatho-excitatory reflex.The sympathetic afferent endings innervating the heart are stimulated by epicardial application of bradykinin(BK), capsaicin,or hydrogen peroxide,the afferent signals from cardiac receptors arrive the central,or then reflexly increases sympathetic tone and arterial pressure.During myocardial ischemia and infarction,CSAR is induced.Chronic heart failure(CHF)is a common disease threatening human's health with its high mortality.It is well accepted that CHF is characterized by enhanced sympathetic tone which plays a critical role in the pathogenesis of CHF.The degree of sympathoexcitation is positively correlated with mortality in the CHF state.The CSAR is enhanced in CHF and partially contributes to the over-excitation of sympathetic nervous system.The paraventricular nucleus(PVN)is an important integrative region in the control of sympathetic outflow and arterial pressure.Our previous studies have shown that angiotensinâ…¡(Angâ…¡)and AT₁ receptors in the PVN modulate the CSAR in normal rats and dogs and partially contribute to the enhanced CSAR in CHF.Recently,we have found that NAD(P)H oxidase-derived reactive oxygen species especially superoxide anions and hydrogen peroxide in the PVN contribute to the effect of Angâ…¡in the PVN on the CSAR in rats.These results suggest the importance of PVN in the central control of the CSAR.However the central pathway of the CSAR is not clear.The present study was designed to more conclusively demonstrate that the PVN is an important component of the central neurocircuitry of the CSAR and further elucidate the central mechanism responsible for modulation of the CSAR.It will lay a foundation in finding new drugs for management of CHF.Objective1.The effects of inhibiting and exciting the PVN,or bilateral electrolytic and chemical lesion of the PVN on the CSAR were respectively determined.2.The roles of GABA_A and GABA_B receptors as well as GABA in the PVN in the control of the CSAR and sympathetic outflow were investigated.MethodsIn anesthetized rats with sinoaortic denervation and cervical vagotomy,renal sympathetic nerve activity(RSNA),mean arterial pressure(MAP)and heart rate(HR)were recorded in vivo on a PowerLab data acquisition system.The CSAR was evaluated by the RSNA response to epicardial application of bradykinin(BK)or capsaicin.The coordinates for PVN were determined in a stereotaxic instrument according to the Paxinos and Watson rat atlas.All the drugs are bilaterally microinjected into PVN.1.The effects of lidocaine and glutamate,which respectively inhibit and activate the neurons in the PVN on the CSAR,were investigated. Lidocaine(8.5 nmol),L-glutamate(1 nmol)and saline were respectively microinjected into the bilateral PVN in three groups of rats(n=7 for each group).The CSAR was determined before microinjection,5 and 30 minutes after microinjection.2.Electrolytic lesion and sham lesion of the PVN were carried out in two groups of rats(n=6 for each group).The CSAR was respectively determined before lesion and 10 minutes after the lesion.3.Kainic acid(KA)was used to creat a chemical lesion to destroy neuronal perikarya in the PVN but spare axons of passage and terminals in the vicinity of the injection site.The effects of microinjection of KA(2 nmol)and saline into the PVN on the CSAR were determined in two groups of rats(n=6 for each group).The CSAR was evaluated before microinjection and 90 minutes after microinjection.To exclude the possibility that the effect of KA on the CSAR was caused by diffusion to other brain area,the effect of microinjection of same dose of KA into the anterior hypothalamic area,which is adjacent to the PVN,on the CSAR was also determined(n=3).Furthermore,the long-term effect of KA on the CSAR was observed at the eighth day after microinjection(n=3).4.The rats were randomly divided into six groups(n=6 for each group), which were subjected to the bilateral PVN microinjection of saline,three doses of GABA_A receptor agonist isoguvacine(0.1,1 or 10 nmol), GABA_A receptor antagonist gabazine(0.1 nmol),or gabazine plus the high dose of isoguvacine(pretreatment with gabazine 10 min before isoguvacine).The CSAR was evaluated 10 minutes after the microinjection.5.The rats were randomly divided into six groups(n=6 for each group), which were subjected to the bilateral PVN microinjection of saline,three doses of GABA_B receptor agonist baclofen(0.01,0.1 or 1 nmol),GABA_B receptor antagonist CGP-35348(10 nmol),or CGP-35348 plus the high dose of baclofen(pretreatment with CGP-35348 10 min before baclofen). The CSAR was evaluated 10 minutes after the microinjection.6.The rats were randomly divided into five groups(n=6 for each group), which were subjected to the bilateral PVN microinjection of saline, GABA-transminase inhibitor vigabatrin(10 nmol),vigabatrin plus gabazine,vigabatrin plus CGP-35348,and vigabatrin plus combined gabazine and CGP-35348.The gabazine and/or CGP-35348 were microinjected into the PVN 30 minutes after microinjection of vigabatrin. The CSAR was evaluated 70 minutes after microinjection of saline or vigabatrin. Result1.The effects of inhibiting and exciting the neurous in PVN on the CSAR and sympathetic outflow.Microinjection of lidocaine into the PVN abolished the CSAR without significant effects on the baseline RSNA and MAP,while glutamate which excites the neurons in the PVN enhanced the CSAR and increased the baseline RSNA and MAP.2.The effects of electrolytic lesion of the PVN on the CSAR and sympathetic outflow.Electrolytic lesions of the PVN irreversibly abolished the CSAR without significant effects on the baseline RSNA and MAP.3.The effects of selective lesion of the neuronal perikarya in PVN on the CSAR and sympathetic outflow.Selective lesion of the neuronal perikarya in the PVN with kainic acid induced rapid and great increases in both RSNA and MAP,which lasted about 60 minutes and then remained at the baseline levels.At the 90th minute after kainic acid,epicardial application of bradykinin or capsaicin failed to induce the CSAR.4.The effects of GABA_A receptor agonist and antagonist in the PVN on the CSAR and sympathetic outflow.The PVN microinjection of isoguvacine,a GABA_A receptor agonist, induced dose-dependently decreases in RSNA and MAP,and inhibited the CSAR.The high dose of isoguvacine caused great decreases in the baseline RSNA and MAP,but only showed a weakly inhibitory effect on the CSAR.These effects of isoguvacine were completely abolished by gabazine,a GABA_A receptor antagonist.However,gabazine alone did not cause any significant effect on the CSAR,but increased the baseline RSNA and MAP.5.The effects of GABA_B receptor agonist and antagonist in the PVN on the CSAR and sympathetic outflow.The PVN microinjection of baclofen,a GABA_B receptor agonist, dose-dependently caused decreases in RSNA and HR,and inhibited the CSAR evoked by either capsaicin or BK.The high dose of baclofen caused great decreases in the baseline RSNA and MAP,and almost completely abolished the CSAR.These effects of baclofen were completely antagonized by CGP-35348,a GABA_B receptor antagonist. Moreover,CGP-35348 alone enhanced the CSAR,but only slightly increased the baseline RSNA and MAP.Intravenous administration of same dose of baclofen had no significant effect on the CSAR,baseline RSNA and MAP.6.The effects of GABA-transaminase inhibitor in the PVN on the CSAR and sympathetic outflow.The PVN microinjection of vigabatrin,a selective GABA-transaminase inhibitor which increases endogenous GABA level, caused great decreases in baseline RSNA,MAP and HR,and almost completely abolished the CSAR.These effects of vigabatrin were partially inhibited by either GABA_A receptor antagonist gabazine or GABA_B receptor antagonist CGP-35348,and were completely blocked by gabazine plus CGP-35348.Conclusions1.PVN microinjection of lidocaine to inhibit the neural activity of the PVN abolishes the CSAR,but excitatory amino acid L-glutamate enhances the CSAR.Either nonselective electrolytic lesion of the PVN or selective lesion of the neuronal perikarya in the PVN with KA abolishes the CSAR.The results indicate that the PVN is a pivotal component of the central neurocircuitry of the CSAR.2.Activating the GABA_A receptors in the PVN attenuates the CSAR, while activating the GABA_B receptors abolishes the CSAR.The blockade of the GABA_A receptors in the PVN has no significant effect on the CSAR,while blockade of the GABA_B receptors enhances the CSAR.The increased endogenous GABA level in the PVN caused by the GABA-transaminase inhibitor inhibits the CSAR,which is mediated by both the GABA_A and GABA_B receptors.These results indicate that the GABA in the PVN plays an important role in inhibiting the CSAR,which is involved in both the GABA_A and GABA_B receptors in the PVN.