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Department of Physiology and Biophysics, University of Nebraska Medical Center, Omaha, Nebraska 68198-4575
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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One characteristic of heart failure
(HF) is increased sympathetic activation. The paraventricular
nucleus (PVN) of the hypothalamus (involved in control of sympathetic
outflow) has been shown to have increased neuronal activation during
HF. This study examined the influence of endogenous GABA input
(inhibitory in nature) into the PVN on renal sympathetic nerve
discharge (RSND), arterial blood pressure (BP), and heart rate (HR) in
rats with HF induced by coronary artery ligation. In 
-chloralose-
and urethane-anesthetized rats, microinjection of bicuculline (a GABA
antagonist) into the PVN produced a dose-dependent increase in RSND,
BP, and HR in both sham-operated control and HF rats. Bicuculline
attenuated the increase in RSND and BP in HF rats compared with control
rats. Alternatively, microinjection of the GABA agonist muscimol
produced a dose-dependent decrease in RSND, BP, and HR in both control and HF rats. Muscimol was also less effective in decreasing RSND, BP,
and HR in HF rats than in control rats. These results suggest that
endogenous GABA-mediated input into the PVN of rats with HF is less
effective in suppressing RSND and BP compared with control rats. This
is partly due to the post-release actions of GABA, possibly caused by
altered function of post-synaptic GABA receptors in the PVN of rats
with HF. Reduced GABA-mediated inhibition in the PVN may contribute to
increased sympathetic outflow, which is commonly observed during HF.
paraventricular nucleus; renal sympathetic nerve activity; muscimol; bicuculline
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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GABA IS A WELL-KNOWN INHIBITORY neurotransmitter in the central nervous system. A large body of evidence suggests that GABA plays an important role in central cardiovascular control. Intracerebroventricular injections of GABA agonists produce decreases in arterial blood pressure, heart rate (HR), and peripheral sympathetic nerve activity (2, 4, 11, 14, 21). Conversely, intracerebroventricular administration of GABA antagonists such as bicuculline methiodide or picrotoxin results in marked increases in blood pressure and HR due to an increase in sympathetic nerve activity (3, 18, 19). In addition, GABA in the central nervous system may modulate vasopressin, a humoral substance involved in cardiovascular regulation in the body. Intracerebroventricular or intravenous application of GABA markedly decreases the plasma level of vasopressin (33, 39). Therefore, it appears that central GABA exerts its cardiovascular effect through both autonomic and humoral pathways. The cardiovascular effects of GABA in the central nervous system may occur at different levels in the brain because GABA is widely localized within discrete autonomic centers of the brain. The paraventricular nucleus (PVN) is one of the candidate sites in which these effects of GABA are mediated. This is because the PVN contains cells that are either involved in autonomic regulation or secrete vasopressin into the circulation. GABA is reported to be a dominant inhibitory neurotransmitter in the PVN. Iontophoretically applied GABA depresses the firing rate of PVN neurosecretory cells (34). Microinjection of bicuculline into the PVN elicits significant increases in arterial blood pressure and HR that are blocked by pretreatment of a ganglionic blocking agent. This suggests that GABA in the PVN is tonically inhibitory to sympathetic outflow. Taken together, these observations suggest that GABAergic mechanisms in the PVN are important in the control of the sympathetic nervous system outflow. It is conceivable that any alteration in the GABA mechanisms within the PVN may lead to alteration of sympathetic nerve activity.
One hallmark of congestive heart failure (HF) is increased sympathetic activation. Both humans and experimental animals with HF display an elevated sympathetic activation (9, 25). However, the mechanisms that engender increased sympathetic drive in HF remain poorly defined. In recent years, we have examined the involvement of central cardiovascular processing mechanisms in the elevated neurohumoral drive during HF (24-28, 41, 46). One key area within the hypothalamus that is important in dictating sympathetic outflow is the PVN (36, 37), which demonstrates increased neuronal activity during HF (27, 38). Combining these observations with the sympathoinhibitory role of GABA within the PVN mentioned above led us to hypothesize that an altered GABAergic mechanism within the PVN may be involved in the regulation of sympathetic outflow during HF and that this altered mechanism may contribute to increased sympathetic nerve activity during HF.
To test this hypothesis, this study was designed to examine whether 1) reduced endogenous GABA activity within the PVN contributes to increased sympathetic outflow in rats with HF, and 2) the response of renal sympathetic nerve activity to the microinjection of muscimol (a GABA agonist) into the PVN is altered in rats with HF.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Induction of Myocardial Infarction
All procedures utilized in this study were approved by the University of Nebraska Medical Center Institutional Animal Care and Use Committee, and the experiments were conducted according to the Guiding Principles for Research Involving Animals and Human Beings. Male Sprague-Dawley rats weighing 210-280 g were obtained from Sasco Breeding Laboratories (Omaha, NE) and were randomly assigned to one of two groups: HF and sham. HF was produced by coronary artery ligation as previously described (10, 16, 31). Briefly, each rat was anesthetized with pentobarbital sodium (50 mg/kg ip) and the trachea was cannulated to facilitate mechanical ventilation. A left thoracotomy was performed, and the heart was lifted from the thorax. The left coronary artery was ligated near its branch point from the aorta between the pulmonary artery outflow tract and the left atrium. Sham rats underwent thoracotomy and manipulation of the heart, but the coronary artery was not ligated. After these maneuvers were completed, the heart was returned to its original position and the thorax was closed. The air within the thorax was removed, which allowed the rats to resume spontaneous respiration. The trachea was closed, the neck incision was sutured, and the rats were allowed to recover from anesthesia. Analgesics (Nubain-Stadol, 1 ml/kg sc) were administered after surgery, and subsequently Tylenol with codeine elixir (codeine phosphate and acetaminophen) was given in the drinking water for 2 days after surgery. Each rat was caged individually in an environment maintained with ambient temperature at 22°C and humidity at 30-40%. Laboratory chow (Purina) and tap water were available ad libitum. The acute experiments were conducted 6-8 wk after ligation of the left coronary artery (HF group) or sham surgery in control rats. At the end of each acute experiment, a 2-Fr micromanometer-tipped catheter (Millar Instruments) was advanced through the right carotid artery into the left ventricle to determine left ventricular (LV) pressures. LV end diastolic pressure (LVEDP) was determined from the LV pressure recording. LVEDP measurement was determined to provide a measure of diastolic dysfunction.Recording of Efferent Renal Sympathetic Nerve Discharge
On the day of the experiment, the rat was anesthetized with urethane (0.75 g/kg ip) and-chloralose (70 mg/kg ip supplemented with 0.3 g/kg iv if necessary). The left femoral vein was cannulated for drug administration, and the left femoral artery was cannulated for
recording of arterial blood pressure and HR. The left kidney was
exposed through a left retroperitoneal flank incision. A branch of
renal nerve was isolated, and the distal end of the nerve was placed on
a thin bipolar platinum electrode. The nerve-electrode junction was
insulated electrically from the surrounding tissues with mineral oil.
The electrical signal from the electrode was linked via a
high-impedance probe (HIP5) to a Grass P511 band-pass amplifier (gain
of 10-50 × 1,000) with high- and low-frequency cutoffs of
1,000 and 100 Hz, respectively. The output from the Grass amplifier was
directed to a Grass integrator, which rectified the signal and
integrated the raw nerve discharge. The output of the Grass integrator
was displayed as an integrated voltage that is proportional to the
renal nerve discharge. The average rectified signal
[resistor-capacitor (RC) circuit filtered, time constant, 0.5 s] was
then recorded and stored for later analysis in a computer-based
data-acquisition system (MacLab). Efferent renal sympathetic nerve
discharge (RSND) at the beginning of the experiment was defined as
basal nerve discharge. All renal nerve activity recordings were
corrected by subtraction of background noise, which is defined as the
signal that remains post mortem. The response of renal nerve discharge
to the administration of drugs into the PVN during the experiment was
subsequently expressed as a percent change from the basal value. We
have observed that RSND, arterial blood pressure, and HR do not change
significantly over the time frame of this experiment. This was evident
in experiments where we missed the PVN in both sham-operated and HF rats.
Microinjections
The rat was placed in a stereotaxic apparatus (David Kopf Instruments, Tujanga, CA). The coordinates for the left PVN were determined from the atlas by Paxinos and Watson (30) as 1.8 mm posterior and 0.4 mm lateral to the bregma and 7.8 mm ventral to the dura. A small burr hole was placed in the skull, and a long, thin needle (0.5 mm OD and 0.1 mm ID), which was connected to a microsyringe (0.5 µl, Hamilton Microsyringe), was lowered into the PVN. For bicuculline microinjections, the microsyringe was filled with a 1 mM solution of bicuculline methiodide, and 50, 100, and 200 pmol (50, 100, and 200 nl, respectively) were injected into the PVN. Each injection was made over a period of 2 min. For muscimol microinjections, the microsyringe was filled with a 4 mM solution of muscimol, and 0.2, 0.4, and 0.8 nmol of muscimol (50, 100, and 200 nl, respectively) were injected into the PVN. The vehicle solution was artificial cerebrospinal fluid (aCSF, pH 7.4) for all compounds. As a control group, similar volumes of aCSF (50, 100, and 200 nl) were also injected into the PVN. These volumes of aCSF injections into the PVN did not produce any significant changes in RSND (2.3 ± 2.0%), blood pressure (3.1 ± 2.0 mmHg), or HR (5.6 ± 3.0 beats/min) for a 100 nl microinjection (n = 4) over the time frame of these experiments. We have previously shown (44) that microinjections of vasoactive substances (dissolved in aCSF of similar volumes) do not produce a change in RSND, blood pressure, or HR. The responses in mean arterial blood pressure, HR, and RSND over the 20-30-min period were recorded after each dose of these drugs or aCSF. At the end of the experiment, monastral blue dye (200 nl) was injected into the PVN for histological verification of the injection site.For comparison with bicuculline studies, we determined the ability of rats with HF and sham-operated control rats to increase RSND in response to another stimulus that is known to maximally activate RSND (7). The RSND response to blocking the airway for 40 s in rats with HF and in sham-operated rats was elicited. Such a stimulus is known to increase RSND by 150-200% of baseline activity in normal animals. These responses were compared with responses to the maximal dose of bicuculline (200 pmol) in rats with HF. To substantiate the concept that any responses of blood pressure and RSND to bicuculline are mediated by the blockade of GABA within the PVN and not a peripheral action, in four normal rats, the effect of an intravenous injection of 500 pmol of bicuculline (500 nl of 10 mM solution of bicuculline in saline over a period of 2 min) on these responses was examined.
Brain Histology
At the end of the experiment, the rat was killed and the brain was removed and fixed in formalin for at least 24 h. The brain was then frozen, and serial transverse sections (30 µm) were cut using a cryostat (
20°C) and thaw-mounted onto
chrome-alum-coated microscope slides. Sections were stained using a
neutral red (1% aqueous) staining procedure. The presence of blue dye
within the PVN was verified microscopically.
Cardiac Histology
At the end of each experiment, the heart was removed, weighed, and fixed in 10% buffered formalin for histological study. The hearts were coded so that determinations of infarct size were made without knowledge of the responses to central administrations. The infarct size was determined as previously described (10, 25, 27, 31). Briefly, the hearts were sectioned into four major segments located from the atrium to the apex: A, B, C, and D. Segments A (the atria and the connecting inflow and outflow tracts) and D (mainly the apex) were not analyzed for histological damage. Segments B and C (representing the bulk of the left ventricle) were subjected to graded dehydration with ethanol, embedded in paraffin, cut into sequential 10-µm sections, and stained with hematoxylin-eosin. Every tenth section was projected onto a screen, and the outline of the tissue was diagramed after microscopic examination of the infarcted areas of the ventricle to identify the edges of the infarct. The scale drawing thus obtained was utilized for measuring LV outer (epicardial) and inner (endocardial) circumferences as well as the arc length of the infarcted region, and the average value was then calculated. The infarcted fraction of the LV wall was calculated based on the above measurements (infarcted arc length/total LV circumference). Minimum LV wall thickness was also measured from the cardiac diagrams to assess transmural damage.Data Analysis
Responses of RSND to the various doses of drugs were expressed as a percent change over the basal value. Responses of arterial blood pressure and HR to drugs were expressed as the difference between the basal value and the value after each dose of a drug. The data were subjected to two-way repeated-measures ANOVA and subsequent comparison for individual differences using the Newman-Keuls test (42). Blood pressure and HR were compared between groups using the unpaired t-test. P < 0.05 was considered to indicate statistical significance. All data are presented as means ± SE.| |
RESULTS |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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General Data
Table 1 summarizes the salient characteristics of sham and HF rats used in the present study. There were no statistically significant differences in basal mean arterial blood pressure and HR measurements between the sham and HF groups. Renal nerve discharge was significantly higher in rats with HF. Upon examination of the histological data, HF rats with myocardial infarcts involving >30% of the LV wall were used in this study. The HF group displayed infarcts extending over ~42% of the endocardial surface. Sham rats had no observable damage to the myocardium. The minimum ventricular thickness was significantly less in HF rats than in the sham group, which is indicative of transmural damage. Heart weight was significantly greater in HF rats than in sham rats, which suggests compensatory hypertrophy of noninfarcted regions of the myocardium. LVEDP was significantly elevated in HF rats compared with sham rats. Similar findings have been previously observed by researchers in our laboratory (17, 25-29, 46) and others (10, 16, 31). Taken together, the observations of increased LVEDP, cardiac hypertrophy, and histological damage to the myocardium of 34-42% in our present study suggest that the rats in the HF group had decreased cardiac contractile function and were experiencing HF. Figure 1 illustrates the termination of injection tracts within the hypothalamus. Injector placements were found throughout the rostrocaudal extent of the PVN. Among the 36 injections targeting the PVN, 28 injections were in or immediately adjacent to the PVN area, whereas 8 injections were outside but adjacent to the PVN. Among the 28 injections that were in or very close to the PVN, 14 injections were in sham-operated rats and 14 injections were in rats with HF. The 50- to 200-nl injection volumes targeting the PVN would be expected to distribute the drug in or within <0.5 mm away from the rostrocaudal and mediolateral boundaries of the PVN (35).
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Microinjection of Bicuculline
Typically, microinjection of bicuculline produced increases in RSND, blood pressure, and HR that peaked between 4-6 min from the start of the injection and were followed by a gradual decrease back to the preinjection levels (within 20-30 min). An example of the peak responses in RSND, blood pressure, and HR to bicuculline in sham and HF rats is illustrated in Fig. 2. Microinjection of bicuculline (50, 100, and 200 pmol) elicited significant increases in efferent RSND, arterial blood pressure, and HR reaching 127 ± 13%, 32 ± 7 mmHg, and 63 ± 13 beats/min, respectively, at the highest dose in sham rats (Fig. 3). Although there were significant increases in RSND and arterial blood pressure to bicuculline in HF rats, the responses were significantly blunted compared with sham rats. It should be noted that although HF rats have an increased basal sympathetic nerve activity, the absolute increases in RSND to bicuculline were still significantly less in rats with HF. These data indicate that the blockade of endogenous GABA receptors in the PVN of rats with HF is less effective in raising the RSND and blood pressure compared with sham rats. The RSND response to blocking the airway of rats with HF (130 ± 27%) for 40 s was comparable to control rats (155 ± 55%). The response in HF rats was over three times higher than that elicited by the highest dose of bicuculline (40 ± 9%). These data demonstrate that RSND can increase above the level elicited by the maximal dose of bicuculline given within the PVN in this experiment.
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In normal rats, we also examined the possibility that the observed effects of bicuculline were mediated by its peripheral effect. Intravenous administration of 0.5 nmol of bicuculline (n = 4) elicited no significant change in RSND, blood pressure, or HR.
After injections of bicuculline into hypothalamic sites distant from
the PVN, we did not see any response in RSND and blood pressure (Fig.
4). There was a small but significant
response in HR. We termed this group the anatomic control group. This
group can also be used as a time control, because these experiments were carried out over the same time frame as the experiments mentioned above. There were no significant changes in RSND and arterial blood
pressure during the time frame of these experiments to microinjection of bicuculline in adjacent sites within the hypothalamus in either sham
or HF rats.
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Microinjection of Muscimol
Microinjection of muscimol (0.2, 0.4, and 0.8 nmol) caused significant decreases in efferent RSND, arterial blood pressure, and HR, reaching67 ± 5%, 28 ± 7 mmHg, and 78 ± 7 beats/min, respectively, at the highest dose in sham rats (Fig.
5). Although there were significant
decreases in RSND and HR to muscimol in HF rats, the responses
(RSND, arterial blood pressure, and HR) were significantly blunted
compared with sham rats. There were no significant effects on RSND and
arterial blood pressure when muscimol was injected into close adjacent
areas (dorsal and anterior to the PVN; Fig.
6). There was a decrease in HR with
muscimol injections into adjacent areas of the PVN.
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In normal rats, we also examined the possibility that the observed effects of muscimol were mediated by its peripheral effect. Intravenous administration of 2 nmol of muscimol (n = 3) elicited no significant changes in RSND, arterial blood pressure, or HR.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The major findings of this study are that 1) endogenous GABA inhibition in the PVN, which restrains tonic sympathetic drive in normal animals, is markedly impaired in rats with HF; and 2) reduced responses to an exogenous GABA agonist microinjected into the PVN implicate a diminished responsiveness of the GABA receptors in HF. These results may provide insight for better understanding of altered central mechanisms involved in symapthoexcitation that is commonly observed during the late phases of HF.
GABA is a dominant neurotransmitter in the PVN (32). The
blockade of the GABA system in the PVN increases sympathetic outflow (20), which indicates that it is tonically inhibitory to
sympathetic outflow. Therefore, it is conceivable that a deficiency or
hypofunction of GABAergic mechanisms in the PVN may be related to
pathogenesis of some clinical conditions characterized by altered
sympathetic nerve activity such as HF. In this study, the
sympathoexcitatory effect of bicuculline administered into the PVN was
significantly reduced in HF compared with sham rats. Commensurate with
the response of RSND, there was a reduced pressor response to the
administration of bicuculline into the PVN in HF rats. This suggests
that there is less GABA inhibitory input within the PVN in HF rats.
Therefore, significantly blunted increases in RSND and blood pressure
to the blockade of the GABA system were observed in HF rats. However, one alternative possibility is that reduced RSND and blood pressure responses are due to an increased basal sympathetic nerve activity in
HF rats, which cannot be further elevated by bicuculline. The latter
possibility is unlikely because RSND could be further elevated over
baseline by blocking the airway of the HF rat. The reduced inhibition
of renal sympathetic outflow by GABA in the PVN could occur because of
the following possibilities: 1) decreased synthesis of GABA
in the releasing neuron, 2) increased breakdown of GABA, 3) increased uptake of GABA upon release, 4)
decreased release of GABA by the neuroterminals, and/or 5)
an abnormal interaction between GABA and effector neurons in the PVN
(i.e., GABA receptors). The physiological activities of GABA are
mediated through its interaction with two types of receptors in the
central nervous system, which are referred to as GABAA and
GABAB. The cardiovascular effects of GABA are primarily
mediated by GABAA receptors, although GABAB
receptors may also be involved in this action (8). The GABAA receptor is a complex protein with multiple
recognition sites (8) and contains (minimally) a GABA
binding site coupled to a Cl
channel. The binding of GABA
or the GABA agonist muscimol to recognition sites on the
GABAA receptor opens Cl channels in the cell
membrane (13). The influx of Cl into the
cell hyperpolarizes the membrane and thus inhibits the neuron. The
possibility of an abnormal interaction between GABA and its receptors
was tested in this study by examining the responses of RSND, blood
pressure, and HR to the microinjection of muscimol into the PVN of sham
and HF rats. Compared with control rats, there were significantly
reduced responses in RSND, blood pressure, and HR to the administration
of muscimol into the PVN of rats with HF. This indicates that there is
an abnormal interaction between GABA and effector neurons in the PVN
during HF thus exhibiting reduced responsiveness of PVN neurons to GABA
or its agonists. This blunted response is probably not due to the
increased uptake of GABA, decreased synthesis of GABA, or decreased
release of GABA. The reduced responsiveness of PVN neurons to muscimol
could be the result of a reduced number of GABA receptors in the PVN, a
dysfunction of receptors, or dysfunction in message transduction downstream from the activation of GABA receptors during HF. These possibilities remain to be examined.
Some data suggest that nitric oxide (NO) may activate the endogenous
GABA system in the PVN. Because both NO and GABA are present in the PVN
(32, 40), this provides an opportunity for interaction
between NO and GABA. It has been reported that NO can cause the release
of GABA from neurons via peroxynitrite (1, 23). This
interaction may exist in the PVN because it has been reported that
perfusion of the PVN with NO that contains cerebrospinal fluid elicits
an increase in the concentration of GABA in the perfusate
(15). Furthermore, both NO and GABA (20) are
sympathoinhibitory. It is conceivable that the activation of the GABA
system may play an important role in mediating the renal
sympathoinhibitory effect of NO within the PVN. Administration of a NO
donor into the PVN causes decreases in RSND and blood pressure that can
be eliminated by blockade of the GABAA receptor in the PVN
with bicuculline (45). Thus the effect of NO is mediated by GABA in the PVN. Consistent with this concept, administration of
N
-nitro-L-arginine methyl ester
into the PVN elicited significant increases in RSND, blood pressure,
and HR that were abolished by the blockade of the GABA system with
bicuculline (45).
If the NO-GABA mechanism is a physiological regulator of sympathetic outflow within the PVN, it is reasonable to speculate that any alterations in the NO-GABA mechanism within the PVN will influence sympathetic nerve activity and the cardiovascular system. Previously we have shown (28) that the genetic message for neuronal NO synthase (nNOS) is decreased in the hypothalamus of rats with HF. We subsequently stained the PVN for nNOS-positive neurons by using the NADPH diaphorase staining method (46), which is known to correlate with the immunocytochemistry technique (5). The results demonstrate that there were decreased nNOS-positive neurons in the PVN of rats with HF compared with sham-operated control rats (46). Recently we have investigated the function of endogenous NO in the PVN to regulate sympathetic outflow during HF. The response of efferent RSND to NG-monomethyl-L-arginine (a blocker of NO synthesis) microinjected into the PVN was significantly reduced in rats with HF compared with the sham-operated control group (43). It is possible that the reduced NO synthesis-release contributes to reduced GABA release, and this may partly explain the blunted responses to bicuculline in rats with HF. However, microinjection of sodium nitroprusside (a NO donor) into the PVN resulted in significant decreases in efferent renal sympathetic nerve discharge and arterial blood pressure in the sham-operated control group but not in rats with HF. These results are consistent with diminished responsiveness of the GABA receptors. These data suggest that reduced renal sympathoinhibition mediated by NO and subsequently GABA within the PVN may contribute to elevated sympathetic nerve activity during HF.
It is conceivable that the deficiency or hypofunction of GABAergic mechanisms in the PVN may be related to pathogenesis of a variety of clinical conditions characterized by altered sympathetic nerve activity such as hypertension and HF. Dysfunction in the GABA mechanisms has been implicated in hypertension, which is characterized by increased sympathetic nerve activity. Hambley and colleagues (12) reported that in spontaneously hypertensive compared with Wistar-Kyoto rats, there were significant reductions in endogenous hypothalamic GABA concentrations and the density of muscimol binding to hypothalamic membranes, which is an indication of a reduced number of GABA receptors. Because GABA is a dominant inhibitory neurotransmitter in the hypothalamus, it was hypothesized that decreased inhibition of GABA on sympathetic outflow contributes to the pathogenesis of hypertension (22). Because HF and hypertension share at least one common feature, namely, increased sympathetic nerve activity, we hypothesize that the dysfunction of GABA mechanisms in the brain is involved in increased sympathetic nerve activity during HF.
According to histological data, we conclude that the difference in the response to microinjections of bicuculline and muscimol in the area of PVN between the two groups is not due to the difference in the termination of injection in the brain, because all data were taken from those rats with the termination of injection located in the area of the PVN. However, we did test the possibility that these responses were mediated through the diffusion of bicuculline or muscimol to areas adjacent to the PVN. Injections of bicuculline or muscimol into sites outside but adjacent to the PVN did not produce significant changes in RSND or arterial blood pressure. This suggests that the area of the PVN is a major site where the actions of bicuculline and muscimol observed in this study are mediated. In this study, any response or the absence of a response to the microinjections of drugs in the area of the PVN was not due to the mechanical damage of the PVN by the placement of the cannula, because the mere placement of a cannula into the area of the PVN did not elicit any changes in the basal RSND, blood pressure, or HR. A similar observation has also been made by other investigators (6, 20). The observed responses in RSND, arterial blood pressure, and HR to the microinjections of bicuculline and muscimol were not due to the possible leakage of these drugs into the peripheral circulation. This was confirmed by the observation that there were no significant changes in these parameters to the intravenous administration of more than twice the maximal injected dose of bicuculline or muscimol. The changes in HR to either bicuculline or muscimol must be interpreted with caution, because there were significant changes in HR with the administration of either bicuculline or muscimol into regions adjacent to the PVN. These results suggest that the changes in HR were not strictly mediated by the area of the PVN and the changes in HR were likely due to nonspecific effects of these drugs that diffused to adjacent sites away from the PVN.
In conclusion, our current study demonstrates that there is reduced endogenous GABA-mediated inhibition on renal sympathetic outflow during HF. The abnormal interaction between GABA and its receptors may also contribute to this phenomenon.
Perspectives
HF is characterized by increased sympathetic nerve activity. The mechanisms that underlie increased sympathetic nerve activity are not clear. Previously, we have reported that hexokinase activity (an index of neuronal activity) within the PVN is increased during HF. This change in neuronal activity may be related to the imbalance between excitatory and inhibitory inputs coming into the PVN during HF. This study demonstrates that decreased endogenous GABA input and abnormalities in post-GABA mechanisms within the PVN may contribute to the increased sympathetic nerve activity observed during HF. Future research is necessary to establish the origin and nature of the stimuli that activate these mechanisms to regulate sympathetic outflow. Defining the precise nature of afferent signaling, interactions with sympathoexcitatory mechanisms such as glutamate and ANG II within the PVN, and the mechanisms of activation of the hypothesized GABA-mediated sympathoinhibition require further investigation.| |
ACKNOWLEDGEMENTS |
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Technical assistance from Peggy Mazzeo is appreciated.
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FOOTNOTES |
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This work was supported by National Heart, Lung, and Blood Institute Grant HL-62222.
Address for reprint requests and other correspondence: K. P. Patel, Dept. of Physiology and Biophysics, Univ. of Nebraska Medical Center, 984575 Nebraska Medical Center, Omaha, NE 68198-4575 (E-mail: kpatel{at}unmc.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
10.1152/ajpregu.00241.2001
Received 26 April 2001; accepted in final form 15 November 2001.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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) compared with sites of termination of injections
within the region from heart failure (HF) rats (*) and those outside of
the PVN region (
). AH, anterior hypothalamic nucleus;
f, fornix; 3V, third ventricle. Horizontal bar represents 0.5 mm.
B: photomicrograph of an injection into the PVN of a rat.
Arrow indicates the site of termination of the microinjection.
