Regulation Of The Solitary Tract Nucleus GABAB Receptor On Rat Blood Pressure

The nucleus tractus solitarius (NTS) is a termination site forprimary afferent fibers from baroreceptors,and other peripheralcardiovascular receptors,that contain blood pressure (BP)-sensitiveneurons.As such,it mediates the inhibitory actions of baroreceptors onsympathetic discharge.The intermediate portion of the NTS is richlyinnervated by fibers from various brain nuclei that are known to have animportant role in cardiovascular control,including the area postrema andhypothalamic nuclei.Thus,it is believed that the NTS plays an importantrole in BP regulation,as well as contributing to the development andmaintenance of hypertension.Several studies have demonstrated that thecommissural NTS may be altered in the spontaneous hypertensive rat(SHR). These cardiovascular regulatory actions are carriedout by several neuronal transmitters and modulators [eg, such as γ-aminobutyric acid (GABA)].GABA is a well-known neurotransmitter that exerts inhibitoryactions in the brain,mediated through its receptors,which include theionotropic GABA A receptor (GAR) and metabotropic GABA B receptor (GBR), that are defined on the basis of pharmacological andphysiological studies.The ionotropic GAR has an intrinsic Cl-channel,which is responsible for inducing fast inhibitory postsynapticpotentials.The GBR is a G-protein-coupled receptor,and regulatesneuronal activity via activation of K+channels, which in turn induceshyperpolarization of the neuronal membrane and chronic inhibition ofneuronal activity. A high density of both GAR and GBR and a highdensity of GABA-containing nerve terminals have been found within theNTS. The GBR is a heterodimer comprised of GBR1and GBR2proteins. GBR1is critical for ligand activation of the heterodimer,whereas GBR2is merely a shuttle protein to transfer GBR1ontothe surface of the cell membrane. A large number of studies havedemonstrated that both GAR and GBR play an important role in theintegration of baroreceptor afferent inputs and baroreflexfunction.Microinjection of the GAR agonist,muscimol,or the GBRagonist, baclofen,into the NTS produces a marked pressor response viainhibition of baroreflexes, elevation of sympathetic tone,and vasopressinrelease.Conversely, microinjection of GBR antagonists removes tonicGABAergic inhibition of NTS neurons and results in a fall in arterialpressure. More interestingly, the pressor response to microinjection ofbaclofen into the NTS is enhanced in several hypertensive animalmodels, such as DOCA-salt hypertensive rats, rats with AngII-infusion induced hypertension,and1-kidney renal wrap hypertensive rats.Thosestudies demonstrated that enhanced GBR function could be associatedwith the high BP observed in those animals.However,whether GBRexpression is enhanced in the NTS during hypertension,and whether theenhanced expression of GBR in the NTS would lead to elevatedBP,remains unanswered.In the present study, we examined the GBR and GAR expression inthe NTS of SHR and WKY rats.We also investigated the functionalconsequences of enhanced GBR expression in the NTS duringhypertension by microinjection of a viral vector containing the GBRgene into the NTS.Methods:1.Real-time PCR and Western Blots were used to detect changes in theexpression of GBR in the NTS of SHR rats and WKY rats.2.Rats whose microinjection sites were within the boundaries of the NTSwere used for data analysis,and took advantage of the ways of genetransfer into the NTS. By bilateral microinjection of the AAV2-GBR1viral vector into the NTS of WKY rats.Immunofluorescence staining andwestern blots detect changes in the expression of GBR in the NTS ofWKY rats.3.A radiotelemetry system records Eeffect of GBR1gene transfer onarterial pressure,HR,and norepinephrine plasma levels. Taking use of a pressure transducer which was connectedto a Bridge Amplifier recordseffect of agonists and antagonists of GBR and GAR on BP, HR, andRSNA in SHR and WKY rats.Results:1.GBR1expression is enhanced in the NTS of SHR vs. WKY rats2.GBR was overexpressed in the NTS by bilateral microinjection of theAAV2-GBR1viral vector into the NTS of WKY rats.Effect of GBR1gene transfer on arterial pressure, HR, and norepinephrine plasma levelswere increased steadily.3.Effect of agonists and antagonists of GBR on BP, HR, and RSNA inSHR was significantly enhanced compared with WKY rats.Conclusions:GBR1expression is enhanced in the NTS of SHR vs. WKY ratsand overexpression of this gene in the NTS results in chronic elevationof blood pressure and heart rate in normotensive rats. In an acutestudy,the pressor response to baclofen microinjected into the NTS wasenhanced in SHR as compared with WKY rats.