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NEUROPHARMACOLOGY
-Aminobutyric Acid (GABA)A and GABAB Receptors in Paraventricular Nucleus in Control of Sympathetic Vasomotor Tone in Hypertension
Department of Anesthesiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania; and Division of Anesthesiology and Critical Care, University of Texas M.D. Anderson Cancer Center, Houston, Texas
Received June 16, 2006; accepted October 26, 2006.
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; Anderson et al., 1989; Allen, 2002). In this regard, sympathetic nerve activity is elevated in animal models of hypertension including spontaneously hypertensive rats (SHRs), renin transgenic rats, Dahl salt-sensitive rats, and deoxycorticosterone acetate-salt rats as well as in hypertensive patients (Judy et al., 1976; Takeda and Bunag, 1980; Greenwood et al., 1999; Mancia et al., 1999; Cabassi et al., 2002). However, the mechanisms underlying heightened sympathetic outflow in hypertension remain poorly understood. Most studies on the central alterations in hypertensive animals have focused on the hypothalamus and brainstem (Ciriello et al., 1984; Esler and Kaye, 2000; de Wardener, 2001; Allen, 2002; Ito et al., 2002). The hypothalamic paraventricular nucleus (PVN) is an important brain region controlling sympathetic outflow and arterial blood pressure (ABP) through projections to the intermediolateral cell column of the spinal cord and the rostral ventrolateral medulla (RVLM) (Swanson and Sawchenko, 1983; Dampney, 1994; Pyner and Coote, 2000; Hardy, 2001; Allen, 2002). Because lesion of the PVN reduces ABP and attenuates the development of hypertension in SHRs and deoxycorticosterone acetate-salt rats (Ciriello et al., 1984; Nakata et al., 1989; Takeda et al., 1991), the PVN presympathetic neurons probably play an important role in the development of hypertension.
GABA is a predominant inhibitory neurotransmitter, and GABAergic synaptic inputs in the PVN tonically inhibit sympathetic outflow and ABP (Zhang and Patel, 1998; Zhang et al., 2002). It has been shown that the excitatory effect of bicuculline on PVN presympathetic neurons and the postsynaptic GABAA current in PVN presympathetic PVN neurons are reduced in SHRs compared with those in normotensive rats (Li and Pan, 2006). However, other studies suggest that the influence of the PVN on sympathetic activity in SHRs is increased. For instance, activation of GABAA receptors in the PVN with muscimol produces more profound sympathoinhibitory and depressor responses in SHRs than in normotensive controls (Allen, 2002). Although GABAergic inhibition in the PVN is diminished initially in rats with renal hypertension (Martin and Haywood, 1998), disinhibition of the PVN with bicuculline causes an augmented pressor response in rats with chronic renal hypertension (Haywood et al., 2001). Hence, alterations of GABAergic function in the PVN during hypertension are not fully delineated.
In addition to the change of GABAA receptors, GABAB receptors appear to play a greater role in regulation of the excitability of PVN presympathetic neurons in hypertension. In this regard, blockade of GABAB receptors increases the activity of the PVN-RVLM output neurons in hypertensive SHRs but not in normotensive rats (Li and Pan, 2006). In this study, using specific GABAA and GABAB receptor agonists and antagonists, we systematically determined the function and sensitivity of GABAA and GABAB receptors in the PVN in control of sympathetic vasomotor tone in normotensive and hypertensive rats.
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Materials and Methods |
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). The procedures and protocols were approved by the Animal Care and Use committee of the Pennsylvania State University College of Medicine and conformed to the National Institutes of Health Guide for the Care and Use for Laboratory Animals. Blood Pressure and Sympathetic Nerve Recordings. To monitor the ABP of WKY and SD rats and SHRs, we measured ABP in all the rats using a noninvasive tail-cuff system (model 29-SSP; IITC Life Science, Woodland Hills, CA). Blood pressure was measured every day for at least 1 week before the final experiment. The SHRs had an increase in blood pressure starting at 8 weeks of age and reached a stable state of hypertension approximately 13 weeks after birth.
Rats were initially anesthetized using 2% halothane in O2, and halothane was discontinued after a mixture of 
-chloralose (60–70 mg/kg) and urethane (800 mg/kg) was given i.p. Adequate depth of anesthesia was assessed before surgery by the absence of corneal reflexes and paw withdrawal response to a noxious pinch. The trachea was cannulated for mechanical ventilation using a rodent ventilator (CWE, Ardmore, PA) with 100% oxygen. Expired CO2 concentration was monitored with a CO2 analyzer (Capstar 100; CWE) and maintained at 4 to 5% by adjusting the ventilation rate (50–70 breaths/min) or tidal volume (2–3 ml) throughout the experiment. ABP was measured with a pressure transducer (PT30; Grass Instruments, Quincy, MA) through a catheter placed into the left carotid artery. Heart rate (HR) was counted by triggering from the blood pressure pulse. The right femoral vein was cannulated for i.v. administration of drugs. Supplemental doses of -chloralose and urethane were administered as necessary to maintain an adequate depth of anesthesia.
For lumbar sympathetic nerve activity (LSNA) recordings, a small branch of left lumbar postganglionic sympathetic nerve was isolated through a retroperitoneal incision with the aid of an operating microscope. The lumbar sympathetic nerve was then cut distally to ensure that afferent activity was not recorded (Pan et al., 2001). The nerve was then immersed in mineral oil and placed on a stainless steel recording electrode. The nerve signal was amplified (20,000–30,000) and band-pass filtered (100–3000 Hz) by an alternating current amplifier (model P511; Grass Instruments). LSNA was monitored through an audio amplifier (Grass Instruments). We chose to record the lumbar sympathetic nerve activity because it has been shown that the change is greater in the lumbar than renal sympathetic nerve in response to inhibition of the PVN (Stocker et al., 2005). LSNA and ABP were recorded using a 1401-plus analog-to-digital converter and Spike2 system (Cambridge Electronic Design, Cambridge, UK), displayed, and stored on the hard disk of a computer. The background noise was determined after the rats were killed by an overdose of sodium pentobarbital at the end of each experiment. Respective noise levels were subtracted from the nerve activity, and the percentage change in LSNA from the baseline value was calculated.
PVN Microinjections. Rats were placed in a stereotaxic frame (Kopf Instruments, Tujunga, CA). The dorsal surface of the skull was exposed and a small hole drilled to expose the brain. A glass microinjection pipette (tip diameter, 20–30 µm) was advanced to the PVN. The stereotaxic coordinates used were 1.6 to 2.0 mm caudal from bregma, 0.5 mm lateral to the midline, and 7.0 to 7.5 mm ventral to the dura (Zahner and Pan, 2005). The injection sites of the PVN were first verified by the depressor responses to microinjection of 5.0 nmol GABA (20 nl, 250 mM). This approach was chosen to identify the pressor regions of the PVN as described previously (Zahner and Pan, 2005). Drugs were pressure ejected using a calibrated microinjection system (Nanoject II; Drumond Scientific Co, Broomall, PA) and monitored using an operating microscope. After microinjection of the drugs, the glass pipette was left in place for 1 to 2 min to ensure adequate delivery of the drug to the injection site. The micropipette was then withdrawn and placed at a respective stereotaxic coordinates for injection into the contralateral PVN. GABA microinjections were separated by a 10- to 15-min interval to allow recovery of the possible depressor response. A total of six GABA injections were performed in each rat. The PVN vasomotor site was considered when GABA injection decreased the ABP by at least 10 mm Hg and the stereotaxic coordinates from the same rat in which the prior GABA microinjection elicited the greatest depressor responses were used for the subsequent microinjection of GABAA and GABAB receptor agonists and antagonists.
Different doses of GABA receptor agonists and antagonists were microinjected bilaterally into the PVN of each rat. The volume of the injectant (50 nl) was consistent for each injection. After each injection, the pipette was retracted to load different concentrations of drugs and positioned again to the PVN using the identical coordinates for microinjection. In the preliminary experiment, bilateral microinjection of vehicle (5% microspheres in saline, 50 nl) into the PVN up to six times over a period of 3.5 h had no significant effect on the LSNA, ABP, and HR in two SHRs and four WKY rats. To determine the doses-response relationship of bicuculline, baclofen, and CGP52432, individual doses of the drug effects were tested after the LSNA, ABP, and HR returned to the control level from the previous drug injection. However, because of the long-lasting effect of muscimol, cumulative doses of muscimol were tested in our study.
GABA, muscimol, baclofen, and bicuculline were obtained from Sigma-Aldrich (St. Louis, MO). CGP52432 was purchased from Tocris Cookson Inc. (Ellisville, MO). All the drugs were dissolved in saline (pH adjusted to 7.3). The microinjection doses for muscimol (Allen, 2002), bicuculline (Zahner and Pan, 2005), baclofen (Takenaka et al., 1996), and CGP52432 (Urban et al., 2005) were derived from previous studies and determined in the pilot experiments.
Histology. The location of the pipette tip and diffusion of the injectant in the PVN were examined and confirmed histologically in all rats. The drug solution contained 5% rhodamine-labeled fluorescent microspheres (0.04 µm; Molecular Probes, Eugene, OR) to identify the dispersion of the drug throughout the PVN and its surrounding area. The fluorescence dye was not included in the GABA solution used initially to map the PVN vasomotor site. At the completion of the experiment, the rat brain was removed rapidly and fixed in 10% buffered formalin solution overnight. Frozen coronal sections (40 µm) were cut on a freezing microtome and mounted on slides. Rhodamine-labeled regions were identified under an epifluorescence microscope and plotted on standardized sections from the Paxinos and Watson atlas (Paxinos and Watson, 1999). Rats with micropipette misplacement or injectants spreading outside of the PVN were excluded from data analysis.
Data Analysis. Values are presented as means ± S.E.M. Data were analyzed using Spike2 software (Cambridge Electronic Design). Mean arterial pressure was derived from ABP and calculated as diastolic pressure plus one third of the pulse pressure. The lumbar sympathetic nerve signals were rectified, and background noise was subtracted using the level obtained after the rats were sacrificed by an overdose of sodium pentobarbital. The nerve signal was then integrated with 1-s time constant. Control values were obtained from an average of the 60-s period immediately before each injection. The LSNA is presented as the percentage change from baseline activity due to the variability in baseline LSNA in each animal. The ABP, HR, and LSNA values following each intervention were calculated by averaging the parameters over 30 s when the maximal responses occurred. To compare the difference in responses of ABP, LSNA, and HR with different doses of agonists or antagonist microinjection within the experimental groups, repeated-measures analysis of variance (ANOVA) with Dunnett's post hoc test was performed. A two-way ANOVA with Bonferroni's post hoc test was used to compare both the raw data and relative changes of the parameters among experimental groups. P < 0.05 was considered statistically significant.
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There was no difference in the distribution of microinjection sites within the PVN among WKY and SD rats and SHRs (Fig. 1). We included fluorescence microspheres in the microinjection pipette to mark the possible drug injection site and spreading. However, it should be recognized that the diffusion area of microspheres does not accurately reflect the region where the drug acts. In all rats included for analysis, the size of the fluorescence microsphere spread (50 nl) was typically 0.25 to 0.40 mm around the injection site (Fig. 1). The spread of the dye involved the dorsal cap region, magnocellular subnucleus, and ventrolateral subnucleus throughout the rostral-caudal extent of the PVN without penetrating the third ventricular ependymal lining. Spread of the dye, however, was not consistently observed in a nucleus outside of the PVN. Three SHRs, two WKY rats, and three SD rats were excluded from the study because of micropipette misplacement and injectant spreading outside of the PVN. In the preliminary experiment, bilateral microinjection of vehicle (5% microspheres in saline, 50 nl) into the PVN up to six times over a period of 3.5 h had no significant effect on the LSNA, ABP, and HR in two SHRs and four WKY rats.
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; Zahner and Pan, 2005) into the PVN dose-dependently decreased LSNA, ABP, and HR in six WKY rats and six SD rats tested (Figs. 2 and 3). The latency of the response of LSNA, ABP, and HR to muscimol injection was 0.55 ± 0.06 min. The peak responses of the LSNA, ABP, and HR appeared 3 to 4 min after muscimol microinjection. Because of the long-lasting effect of muscimol, microinjection of muscimol into the PVN caused a prolonged decrease in LSNA, ABP, and HR. As a result, LSNA, ABP, and HR did not return to the baseline level 3 to 4 h after muscimol microinjection. Compared with that in WKY and SD rats, the reduction in LSNA, ABP, and HR was significantly attenuated in six SHRs at higher doses (0.45 and 1.5 nmol) of muscimol tested (Fig. 3). The latency of the response of LSNA, ABP, and HR to muscimol injection in SHRs (3.5 ± 0.5 min) was not significantly different from that in WKY (3.4 ± 0.4 min) and SD (3.4 ± 0.8 min) rats. The inhibitory effect of muscimol on LSNA, ABP, and HR was reversed by microinjection of bicuculline (0.3 nmol, 50 nl) into the PVN after the last microinjection of muscimol (Fig. 3). The reversal effect of bicuculline lasted approximately 30 to 45 min, and the parameters then fell to the level seen after muscimol alone.
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; Chen and Toney, 2003) into the PVN dose-dependently increased LSNA and ABP in normotensive WKY (n = 6) and SD (n = 6, Fig. 4) rats. However, bicuculline microinjection did not significantly alter HR at all doses tested (Fig. 4C). The onset of the response of LSNA and ABP to bicuculline injection was 0.76 ± 0.15 min, and the peak response of the LSNA and ABP appeared 2.5 ± 0.6 min after bicuculline administration. The increases in LSNA and ABP elicited by bicuculline microinjections were significantly attenuated in SHRs (n = 7) in all doses tested, compared with those in WKY and SD rats (Fig. 4). The LSNA and ABP returned to baseline level 30 to 45 min after each dose of bicuculline injection. The recovery time for the LSNA and ABP was not significantly different among WKY (38.3 ± 2.5 min) and SD (39.4 ± 3.1 min) rats and SHRs (38.9 ± 3.6 min). In three WKY rats and two SHRs, microinjection of bicuculline into the site lateral to the PVN had no effect on LSNA and ABP.
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). To determine the effect of activation of GABAB receptors in the PVN on LSNA, ABP, and HR, a GABAB agonist, baclofen, was used in WKY (n = 6) and SD (n = 6) rats and SHRs (n = 7). Bilateral microinjection of baclofen (0.3–4.5 nmol, 50 nl) (Takenaka et al., 1996) into the PVN only significantly decreased LSNA, ABP, and HR in WKY and SD rats at the highest dose (4.5 nmol) tested (Figs. 5 and 6). However, baclofen microinjection produced a profound decrease in LSNA, ABP, and HR in hypertensive SHRs (Figs. 5 and 6). The maximal decrease in LSNA, ABP, and HR by baclofen injection was 32.8 ± 2.1%, 37.5 ± 2.4 mm Hg, and 30 ± 8 bpm in SHRs. The onset of the response of LSNA and ABP to baclofen microinjection was between 2 and 3 min, and the response of the LSNA and ABP reached maximum 4 to 5 min after baclofen microinjection. The recovery time for the LSNA and ABP after baclofen injection was not significantly different among WKY (38.3 ± 2.5 min) and SD (39.4 ± 3.1 min) rats and SHRs (38.9 ± 3.6 min). The effect of microinjection of the highest dose of baclofen (4.5 nmol) into the PVN on LSNA, ABP, and HR was blocked by microinjection of a GABAB receptor antagonist, CGP52432 (3.0 nmol) (Fig. 6).
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Effect of Microinjection of CGP52432 into the PVN on the LSNA, ABP, and HR in WKY and SD Rats and SHRs. We then determined the effect of blocking GABAB receptors in the PVN on LSNA, ABP, and HR in WKY (n = 6) and SD (n = 6) rats and SHRs (n = 6). CGP52432 is a water-soluble GABAB receptor antagonist (Lanza et al., 1993; Urban et al., 2005). Bilateral microinjection of CGP52432 (0.15–3.0 nmol, 50 nl) into the PVN had no significant effect on LSNA, ABP, and HR in WKY and SD rats in all doses tested (Fig. 7). However, CGP52432 dose-dependently increased LSNA and ABP in SHRs with a maximal increase in LSNA (17.5 ± 1.3%) and ABP (23.6 ± 3.3 mm Hg, Fig. 7). Microinjection of CGP52432 had no significant effect on HR in all three groups of rats. The onset of the response of LSNA and ABP to CGP52432 microinjection was between 3 and 4 min, and the peak response of the LSNA and ABP appeared 6 to 7 min after CGP52432 microinjection into the PVN in SHRs. The LSNA, ABP, and HR returned to the baseline level 25 to 35 min after each dose of CGP52432 microinjection in SHRs.
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The PVN is an important brain region regulating sympathetic vasomotor tone and may play a role in the development or maintenance of hypertension (Swanson and Sawchenko, 1983; Ciriello et al., 1984; Takeda et al., 1991; Allen, 2002). We have shown that the postsynaptic GABAA receptor function of the PVN-RVLM neurons is profoundly decreased in SHRs (Li and Pan, 2006). In this regard, disinhibition of the PVN with bicuculline in brain slices produces a less excitatory or even an inhibitory effect on the firing activity of PVN-RVLM neurons in hypertensive SHRs (Li and Pan, 2006). In the present study, we found that microinjection of bicuculline dose-dependently increased sympathetic activity and ABP in both normotensive WKY and SD rats. However, these responses to bicuculline were significantly attenuated in SHRs. Furthermore, we observed that activation of GABAA receptors with muscimol in the PVN produced a smaller decrease in LSNA and ABP in SHRs than in WKY and SD rats. These data suggest that in the PVN, GABAA receptor-mediated inhibition of sympathetic vasomotor tone is attenuated in SHRs. Microinjection of muscimol into the PVN caused a similar decrease in HR in normotensive rats and SHRs, probably due to decreased sympathetic outflow. Unlike bicuculline-induced increase in LSNA and ABP, we found that microinjection of bicuculline into the PVN did not significantly increase HR in both normotensive and hypertensive rats. HR can be accelerated by an increased sympathetic outflow. On the other hand, increased ABP initiates the baroreflex to reduce HR. Consequently, these factors may interact and result in no net changes in the HR in response to bicuculline microinjection. In the present study, the magnitude of the pressor responses to bicuculline administered into the PVN of normotensive rats appears larger than that reported in previous studies. It should be recognized that bicuculline was delivered into the PVN unilaterally in most previous studies in anesthetized rats. For example, unilateral microinjection of bicuculline into the PVN increases blood pressure of 19 (Ito et al., 2002) or 30 to 34 mm Hg (Tagawa and Dampney, 1999; Zhang et al., 2002; Chen and Toney, 2003) in normotensive rats. In conscious normotensive rats, slow infusion of bicuculline through a chronic cannula into the PVN increases blood pressure 15 to 19 mm Hg (Martin and Haywood, 1993; De Novellis et al., 1995; Schlenker et al., 2001). In the present study, we first mapped the vasomotor site in the PVN using GABA before subsequent microinjection of bicuculline. It is possible that a larger increase in ABP by a higher concentration of bicuculline in our study was due to a bilateral blockade of GABAA receptors in the PVN.
A reduction in presynaptic GABA release or postsynaptic GABAA receptor function in the PVN may lead to attenuated responses of LSNA and ABP to bicuculline and muscimol in SHRs. We have shown that both the frequency and amplitude of GABAergic inhibitory postsynaptic currents of PVN-RVLM neurons are largely diminished in 13-week-old SHRs (Li and Pan, 2006). In addition, the GABAA current induced by puff application of GABA is significantly reduced in the PVN-RVLM neurons in 13-week-old SHRs compared with that in normotensive controls (Li and Pan, 2006). Furthermore, the binding sites for GABAA receptors are reduced in the PVN in 12-week-old SHRs compared with age-matched WKY rats (Kunkler and Hwang, 1995). Therefore, it is possible that a decrease in postsynaptic GABAA receptor function may be responsible for attenuated vasomotor responses to bicuculline and muscimol in the PVN in SHRs. However, some studies have shown that the GABAA receptor function in the PVN is increased. In this regard, microinjection of muscimol into the PVN produces a more profound decrease in the sympathetic nerve discharges in 14- to 17-week-old SHRs compared with age-matched WKY rats (Allen, 2002; Akine et al., 2003). In addition, microinjection of bicuculline into the PVN causes an enhanced pressor response in 14- to 16-week-old SHRs (Ito et al., 2002). The reasons for the discrepancy regarding the function of GABAA receptors in the PVN between the present and previous studies are not clear. It is possible that the SHRs of different lineages (i.e., substrains) were used in different labs. The different magnitudes of the response of sympathetic vasomotor tone caused by microinjection of GABA agents in the PVN also could be due to differences in the types of anesthetics used, methods for identification of the PVN vasomotor site, and the injection volume/dose and protocols (i.e., unilateral versus bilateral injection). For instance, unilateral microinjection of bicuculline into the PVN was performed in the study by Ito et al. (2002). Furthermore, previous studies used a single dose of bicuculline and muscimol for microinjection. In the present study, the GABAA receptor sensitivity in the PVN of SHRs was assessed using the dose-response effect of bicuculline and muscimol. Nevertheless, we cannot exclude the possibility that GABAA function in the PVN may be reduced during the initial stage (8–13 weeks old) of hypertension but increased as a compensatory change in a later stage of the sustained hypertension (14–17 weeks old). In support of this possibility, microinjection of bicuculline into the PVN produces an attenuated pressor response at the onset of renal hypertension (4 days after establishment of hypertension) (Martin and Haywood, 1998) but an augmented pressor response in chronic renal hypertension (3 weeks after hypertension) in rats (Haywood et al., 2001).
A novel finding from the present study is that in contrast to the reduced GABAA receptor function in the PVN, the GABAB receptor function in the PVN in the control of sympathetic vasomotor tone is augmented in SHRs. We found that activation of GABAB receptors with baclofen in the PVN only decreased LSNA and ABP at the highest dose in normotensive WKY and SD rats. However, microinjection of baclofen into the PVN significantly decreased LSNA and ABP in all of the doses tested in the SHRs. Furthermore, blockade of GABAB receptors with CGP52432 significantly increased LSNA and ABP in SHRs but not in normotensive WKY and SD rats. These data strongly suggest that GABAB receptors in the PVN are not normally involved in tonic GABAergic inhibition of sympathetic outflow in normotensive rats. On the other hand, in SHRs, tonic GABAergic inhibition of sympathetic vasomotor tone is mediated by both GABAA and GABAB receptors. This change in GABAergic function in the PVN is consistent with our recent finding that blockade of GABAB receptors increases the excitability of PVN-RVLM neurons in hypertensive SHRs but not in normotensive controls in a brain slice preparation (Li and Pan, 2006). Therefore, the current in vivo physiological study provides further evidence that GABAB receptors in the PVN play a greater role in tonic GABAergic control of sympathetic vasomotor tone in SHRs. The increase in the GABAB receptor function in the PVN may dampen the hypertension in SHRs.
The underlying mechanisms of the functional plasticity of GABAA and GABAB receptors in the PVN of hypertensive rats are little known. Alteration of the receptor number and/or its affinity, phosphorylation state of the receptor, and receptor subunit composition may be involved in impaired GABAA receptor function in the PVN in hypertension (Rabow et al., 1995). In addition, other neurotransmitters may modulate the function of GABAA receptors in the brain (Chen and Wong, 1995; McCarson and Enna, 1999). For example, NMDA suppresses the GABAA current through dephosphorylation of GABAA receptors, an effect mediated by the Ca2+-dependent phosphatase calcineurin (Chen and Wong, 1995). However, virtually nothing is known how GABAB receptor function is up-regulated in the PVN in SHRs. In contrast to down-regulation of GABAA receptors by the glutamatergic input, the GABAB receptor is up-regulated by potentiated glutamatergic synaptic input. For instance, expression of GABAB receptor mRNA is increased in the dorsal root ganglia and dorsal horn of the lumbar spinal cord in response to increased nociceptive inputs from the hindlimb (McCarson and Enna, 1999). In the present study, we did not compare the effect of microinjection of GABA into the PVN on sympathetic vasomotor tone. The GABA action is limited by the local reuptake system (GABA transporters). Hence, it cannot access equally to GABAA and GABAB receptors in the PVN upon microinjection. Furthermore, because the effect of microinjection of GABA on the LSNA and ABP only lasts 2 to 3 min, it is technically impossible to do bilateral injection due to the time limitation. Nevertheless, it would be interesting to determine whether the function of the GABA reuptake system in the PVN is altered in SHRs. In addition, it should be acknowledged that high circulating oxygen tensions, resulting from supplemental oxygen used in our preparation, may have some influence on the results. It has been reported that the chemoreflex is augmented, attenuated, or not changed in SHRs (Przybylski and Brozyna, 1981; Fukuda et al., 1987; Hayward et al., 1999). It remains unclear the extent to which supplemental oxygen used in this study can affect the results.
Together with our recent in vitro findings (Li and Pan, 2006), the present study provides new functional evidence that GABAA-mediated inhibition is reduced, but GABAB-mediated inhibition is enhanced, in the control of sympathetic vasomotor tone in the PVN of SHRs. Because of the distinct effect of microinjection of GABAA and GABAB agents into the PVN on the sympathetic outflow, the altered GABAA and GABAB receptor function in the hypothalamus may be important for the development of certain hypertension such as stress-induced hypertension. Our data suggest that in SHRs, there may be differential GABAergic inputs to different receptor types in the PVN or an alteration of receptor responsiveness such as receptor number and intracellular signaling. Further studies are warranted to determine the molecular mechanisms underlying altered GABAA and GABAB receptor function in the PVN in hypertension.
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
ABBREVIATIONS: SHR, spontaneously hypertensive rat; PVN, paraventricular nucleus; SD, Sprague-Dawley; ABP, arterial blood pressure; RVLM, rostral ventrolateral medulla; WKY, Wistar-Kyoto; HR, heart rate; LSNA, lumbar sympathetic nerve activity; CGP52432, 3-[[[[(3,4-dichlorophenyl)methyl]amino]propyl]diethoxymethyl]phosphinic acid; bpm, beats per minute.
Address correspondence to: Dr. Hui-Lin Pan, Division of Anesthesiology and Critical Care, Unit 409, University of Texas M.D. Anderson Cancer Center, 1400 Holcombe Boulevard, Houston, TX 77030-4009. E-mail: huilinpan{at}mdanderson.org
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