(Hypertension. 1996;27:1291-1298.)
© 1996 American Heart Association, Inc.
Articles |
Role of 
-Aminobutyric Acid B Receptors in Baroreceptor Reflexes in Hypertensive Rats
From the Department of Neuroscience, University of Pittsburgh (Pa).
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Abstract Previous studies demonstrated that stimulation of
-aminobutyric acid B (GABAB) receptors in the
nucleus tractus solitarius of spontaneously hypertensive rats (SHR)
elicited a larger increase in arterial pressure compared
with control Wistar-Kyoto rats. Since stimulation of GABAB
receptors in the nucleus tractus solitarius attenuates
cardiovascular responses evoked by electrical
stimulation of the aortic depressor nerve in normotensive rats and
there is evidence of a central neural attenuation of aortic depressor
nerve–evoked responses in SHR, we conducted studies to test the
hypothesis that enhanced stimulation of GABAB receptors in
the nucleus tractus solitarius in SHR is responsible for the
attenuation of the aortic depressor nerve–evoked responses.
Electrical stimulation of the left aortic depressor nerve resulted in
frequency-dependent decreases in arterial pressure,
heart rate, and splanchnic sympathetic nerve activity in
urethane-anesthetized control rats. These responses were
not significantly altered by injection of the GABAB
receptor antagonist CGP 35348 into the ipsilateral nucleus
tractus solitarius. The responses evoked by aortic depressor nerve
stimulation were attenuated in SHR. This attenuation was particularly
apparent with more prolonged periods (>15 seconds) of
high-frequency (25-Hz) stimulation, with the depressor and
sympathetic nerve responses diminishing during the course of
stimulation. This time- and frequency-dependent attenuation of
baroreceptor-evoked depressor responses was reversed by injection
of CGP 35348 into the ipsilateral nucleus tractus solitarius. Rats made
hypertensive by treatment with deoxycorticosterone plus salt did not
have attenuated aortic depressor nerve–evoked responses. These
results suggest that alterations in GABAB-mediated neural
transmission in the nucleus tractus solitarius contribute to the
attenuation of the baroreceptor reflex observed in SHR.
Key Words: nucleus tractus solitarius • baroreflex • desoxycorticosterone • pressoreceptors • rats, inbred SHR • receptors, GABA
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Introduction |
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Previous studies have suggested that alterations in CNS control of the circulation may contribute to the pathogenesis of hypertension in certain animal models of hypertension. Using various models of experimental hypertension, many different laboratories have shown changes in the CNS associated with hypertension. For example, in SHR, possibly the most studied of the experimental models of hypertension, numerous reports cite differences in catecholamine-mediated neural transmission at specific loci within the brain.1 2 3 4 Specific alterations in catecholamine-mediated neural transmission have also been reported in other genetic models of hypertension, for example, the Lyon strain of hypertensive rat,5 as well as nongenetic models, such as the DOCA-salt–treated rat.6 Although catecholamine-mediated neural transmission has been the focus of much of this type of work, largely because of centrally acting antihypertensive drugs that act on catecholaminergic systems, changes in other neurotransmitters have been demonstrated as well. For example, altered levels of amino acid neurotransmitters have been noted.7
Another approach to the study of differences in the CNS in hypertensive animals has been to electrically or chemically manipulate specific loci in the CNS and determine whether the evoked cardiovascular response is altered in some way. For example, many laboratories have shown differences between hypertensive and normotensive rats in the cardiovascular responses produced by drugs that affect GABA-mediated neural transmission.8 9 Included among such studies are reports from this laboratory10 11 that the cardiovascular effects of stimulation or blockade of GABAB receptors in the NTS are exaggerated in SHR. Specifically, the increase in AP elicited by bilateral injection into the NTS of the direct-acting GABAB receptor agonist baclofen, as well as indirect-acting GABA agonists, is exaggerated in SHR compared with WKY. Conversely, the decrease in AP elicited by injection of a GABAB receptor antagonist into the NTS is enhanced in SHR. Similar observations have been noted in DOCA-salt hypertensive rats.
Studies conducted in normotensive rats have demonstrated that stimulation of GABAB receptors in the NTS results in an attenuation of baroreceptor reflex responses.12 13 Florentino et al13 reported that bilateral injection of baclofen into the NTS eliminated reflex bradycardia elicited by increasing AP with intravenously administered phenylephrine. In addition, unilateral injection of baclofen into the NTS attenuated depressor and bradycardic responses elicited by electrical stimulation of the ipsilateral ADN.12 Consistent with these observations is the finding that in an in vitro brain stem slice preparation, responses of NTS neurons evoked by tractus solitarius stimulation can be antagonized by the application of baclofen to the slice.14 Furthermore, this inhibitory effect of baclofen on evoked postsynaptic currents in NTS neurons is largely due to an effect of the drug on presynaptic GABAB receptors.14
The observations that (1) stimulation of GABAB receptors in the NTS attenuates the baroreceptor reflex and (2) the cardiovascular effects of endogenous GABA acting on GABAB receptors in the NTS seem to be enhanced in hypertensive rats lead to the prediction that baroreceptor reflex processing should be altered at the level of the NTS in hypertensive rats. Indeed, several studies have demonstrated an attenuation of baroreceptor-evoked cardiovascular responses at the level of the CNS in hypertensive animals. For example, Gonzales and colleagues15 have demonstrated that cardiovascular and sympathetic nerve responses elicited by direct stimulation of the ADN are attenuated in adult SHR compared with WKY. In addition, Nakamura et al16 reported that DOCA-salt hypertensive rats also had attenuated cardiovascular responses to ADN stimulation. Furthermore, they noted that ADN-evoked responses were attenuated within 5 days of treatment with DOCA and salt, before the development of hypertension. The purpose of the present studies was to test directly the hypothesis that the central attenuation of ADN-evoked responses in hypertensive rats results from enhanced action of GABA on GABAB receptors in the NTS.
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Methods |
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Animals
These experiments were conducted on adult male SHR (n=17) and WKY (n=19) (16 to 20 weeks of age, Taconic Farms, Germantown, NY) and DOCA-salt–treated rats (n=22) and normotensive control rats (n=15) of a Sprague-Dawley strain (Zivic-Miller, Allison Park, Pa). Rats were housed singly in wire mesh cages in a temperature-controlled room on a 12-hour light/dark cycle with food (Purina 5001) and tap water available ad libitum for at least 1 week before use in experiments. All animal protocols were in strict accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals and were approved by the Institutional Animal Care and Use Committee of the University of Pittsburgh.
For production of DOCA-salt hypertension, rats weighing approximately 200 g were subjected to right unilateral nephrectomy while anesthetized with pentobarbital sodium (50 mg/kg IP). Immediately after nephrectomy, a pellet containing 50 mg DOCA (Innovative Research Products) was placed subcutaneously in the back of the neck. The drinking water was replaced with 0.9% saline for the first 2 weeks after nephrectomy and was then switched to 0.45% saline. Rats were used either 3 weeks after nephrectomy (DOCA-HT rats) or 5 to 7 days after nephrectomy (DOCA-preHT rats). Control rats consisted of heminephrectomized rats not treated with either DOCA or salt.
Protocol
Rats were initially anesthetized with halothane (2% in
100% O2 administered through a cone placed over the nose).
Catheters (PE-50 tubing filled with saline containing 40 U/mL heparin)
were then inserted into the right femoral artery and vein as previously
described. Urethane (1.5 g/5 mL water) was then infused
intravenously at a rate of 4 mL/h until the rat had
received a dose of 1.5 g/kg; the halothane was terminated approximately
10 minutes after initiation of the urethane infusion. A tracheal
catheter was then inserted and the rat ventilated (2.5 mL stroke
volume, 70 breaths per minute) with 100% O2 and treated
with a muscle relaxant (d-tubocurarine, 0.5 mg/kg IV,
supplemented with 0.2 mg/kg at hourly intervals). The rat's body
temperature was maintained at 37°C with a thermostatically controlled
heating pad. Stimulating electrodes were placed on the left ADN as
previously described12 and anchored in place with
polyvinylsiloxane dental impression material (Impra-Mix Light, Kent
Dental).
Some rats were then prepared for recording of sSNA.17 The greater splanchnic nerve was isolated by a retroperitoneal approach and placed on a pair of polytetrafluoroethylene-insulated stainless steel wire electrodes (A-M Systems). The signal was amplified 10 000-fold, band-pass filtered between 30 and 10 000 Hz (BMA-931 amplifier, CWE Inc), and monitored with a computer-based data-acquisition system (MacLab 2e). Once a suitable signal was obtained (maximal signal exceeding 20 µV, robust inhibition to 25-Hz stimulation of ADN, initial assessment of signal-to-noise based on the signal remaining during 25-Hz ADN stimulation >2:1), the electrode was isolated from surrounding tissue and anchored in place with polyvinylsiloxane dental impression material.
Rats were placed in a stereotaxic instrument with the incisor bar positioned 11 mm below the interaural line. The dorsal surface of the medulla was exposed by limited craniotomy, and with the aid of a surgical microscope, the area postrema was visualized. Experiments were begun at least 20 minutes after the completion of all surgery.
Throughout each experiment, pulsatile AP and sSNA (when monitored) were recorded on computer disk with a MacLab 2e data-acquisition system at a sampling rate of 1000 Hz. In addition, MAP was monitored on a chart recorder (model 7 polygraph, Grass Instrument Co). MAP and HR values were determined off-line from the data acquired on computer. The sSNA signal was full-wave rectified and integrated in discrete 2-second bins.18 The noise signal (signal remaining after ganglionic blockade with 5 mg/kg trimethaphan camsylate) measured at the conclusion of the experiment was subtracted from this value to provide a measure of sSNA in microvolts per second.
In the first experiment, frequency-response curves for ADN-evoked changes in MAP, HR, and sSNA were generated in SHR and WKY. The ADN was electrically stimulated (model S88 equipped with a PSIU6 stimulus isolation unit, Grass Instrument Co) with 30-second trains of 0.5-millisecond pulses of 150 µA at frequencies of 2.5, 5, 10, and 25 Hz. This stimulus intensity is sufficient to maximally excite both A- and C-fiber afferents. Frequencies were tested in ascending order with 5 to 10 minutes between stimulations. After the completion of the ADN frequency-response curve, the effect of unilateral injection of CGP 35348 (5 nmol) into the NTS was examined. This was followed approximately 1 hour later by bilateral injection of baclofen (40 pmol) into the NTS. Injections of drugs were made into the NTS with single-barreled glass micropipettes and a PicoPump (WPI). Drugs were delivered over several seconds in a volume of 100 nL artificial cerebrospinal fluid (mmol/L: NaCl 144, CaCl2 1.2, KCl 2.8, MgCl2 0.9). The volume of drug injected was carefully monitored by watching the movement of the fluid meniscus in the calibrated micropipette. For bilateral injections, the drug was initially injected on one side, the pipette withdrawn and repositioned on the contralateral side, and the second injection made; thus, injections were made approximately 1 minute apart. Doses refer to the amount injected into each NTS.
In a separate group of SHR and WKY, the response to approximately 1 minute of ADN stimulation at 25 Hz was measured. This was repeated approximately 15 minutes later, except that CGP 35348 (5 nmol) was injected into the ipsilateral NTS approximately 30 seconds into the period of ADN stimulation. Approximately 30 minutes later, the effect of ADN stimulation was retested; this was followed by the injection of CGP 35348 without ADN stimulation.
Additional studies examined the effect of ADN stimulation on MAP, HR, and sSNA in DOCA-HT, DOCA-preHT, and control rats. ADN frequency-response curves were generated as described above for SHR and WKY.
At the conclusion of the study, the rat was injected intravenously with a drug that blocks ganglionic transmission (trimethaphan camsylate, 5 mg/kg) to obtain a background noise level for sympathetic nerve activity. In preliminary experiments, the signal obtained after trimethaphan injection was not significantly different from that recorded 10 minutes after death caused by an overdose of anesthetic. In SHR and WKY before ganglionic blockade, sSNA was recorded for 30 seconds, during which the respirator was turned off, to elicit a robust increase in sSNA. This evoked increase in sSNA was used to normalize sSNA measures within a range of activity between zero (noise) and high activity (apnea) as one way of evaluating differences in baseline sSNA between SHR and WKY.
In rats that had received microinjections into the NTS, the injection site was marked by injection of 100 nL of 1% fast green into the NTS with the same micropipette previously used for drug injections. The rat was then decapitated and the brain stem rapidly removed and frozen in isopentane on dry ice. Brain stems were subsequently cut into 40-µm sections with a cryostat, and sections were mounted on glass microscope slides. Sections were stained with cresyl violet or neutral red and examined with low-power light microscopy. All microinjection sites were centered in the medial subnucleus of the NTS at the level of the area postrema.
CGP 35348 and trimethaphan camsylate were generously donated by CIBA-Geigy (Basel, Switzerland) and Hoffmann–La Roche (Nutley, NJ), respectively. Baclofen was purchased from RBI. All other drugs were purchased from Sigma Chemical Co.
Data are expressed as mean±SE. Frequency-response data for ADN stimulation in SHR and WKY were analyzed by two separate two-way ANOVAs. First, the initial decrease in MAP, HR, or sSNA was compared with a two-way ANOVA (rat strainxstimulus frequency, run as a repeated measure). This was followed by a two-way ANOVA for each strain (stimulus frequencyxduration, run as a repeated measure). Post hoc analysis of significant (P<.05) effects was done with the Tukey-Kramer test. A similar approach was used for analysis of data from the DOCA-salt–treated rats. Other comparisons between SHR and WKY were performed with either a t test or ANOVA, as specified in individual experiments. All statistical analyses were done with SYSTAT (Systat Inc).
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Results |
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Spontaneously Hypertensive Rats
Electrical stimulation of the ADN elicited frequency-dependent decreases in AP, HR, and sSNA in both SHR and WKY (Figs 1
and 2, Table 1). The
initial decrease in MAP, expressed as millimeters of mercury change
from baseline, was the same in both rat groups (P>.1 for
rat strain, two-way ANOVA with stimulus frequency as a repeated
measure). During 30 seconds of ADN stimulation, the initial depressor
response was maintained in WKY. In contrast, at the higher stimulation
frequencies in SHR, the response diminished over time (Figs 1 and 2).
For example, the depressor response evoked by 30 seconds of ADN
stimulation at 25 Hz was only 58±5% of the maximal response compared
with 99±3% in WKY (P<.01 between strains, t
test). This response pattern was also apparent in sSNA (Table 1, Fig 1). The attenuation of ADN-evoked sympathoinhibition over time observed
in WKY was markedly exaggerated in SHR. ADN-evoked changes in HR were
different between the two strains, with the response being smaller in
SHR compared with WKY at all stimulation frequencies (Fig 2). In both
strains, the initial bradycardic response was maintained throughout the
entire 30 seconds of ADN stimulation.
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significant difference
(P<.05) in SHR compared with WKY based on a two-way
repeated measures ANOVA (strainxfrequency) for either the 10- or
30-second time point. Baseline MAP values for these rats were 119±2
and 165±5 mm Hg in WKY and SHR, respectively. Baseline HR values were
359±15 and 368±13 beats per minute in WKY and SHR, respectively.
(Examples of AP responses to ADN stimulation in SHR and WKY are
provided in Fig 1
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In these same rats, the effects of unilateral injections of CGP 35348
(5 nmol) into the NTS and the effects of bilateral injections of
baclofen (40 pmol) were also tested. As previously noted in rats
anesthetized with chloralose,11 CGP 35348 elicited
a depressor response that was larger in SHR than in WKY, and baclofen
elicited an increase in AP that was larger in SHR than in WKY (Table 2). Changes in sSNA followed the pattern for changes in
AP (Table 2). The depressor response evoked by CGP 35348 injection was
accompanied by a significant decrease in HR in SHR but not in WKY. In
contrast, baclofen injections resulted in a small increase in HR that
was similar in the two strains.
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Because stimulation of GABAB receptors in the NTS has been
shown to attenuate ADN-evoked responses in Sprague-Dawley
rats12 and the cardiovascular responses
evoked by actions on GABAB receptors in the NTS are
enhanced in SHR, we examined whether the increased responses evoked by
GABAB receptors in the NTS might contribute to the
attenuated ADN-evoked responses observed in SHR.
Urethane-anesthetized rats were prepared for
recording of AP and HR, stimulation of the left ADN, and
microinjection into the NTS. The ADN was then stimulated for
approximately 60 seconds. This stimulus was repeated with CGP 35348
injected into the ipsilateral NTS approximately 30 seconds into the
stimulation period (Fig 3). Stimulation of the ADN at 25
Hz produced responses similar to those described above. ADN stimulation
in WKY resulted in decreases in MAP and HR that were maintained
throughout the stimulation period. In contrast, in SHR, the initial
depressor response was not maintained but stabilized at approximately
50% of the initial response (Fig 3). At this point, injection of CGP
35348 reduced MAP to a level that was not significantly different from
the initial response to ADN stimulation (Fig 3). Furthermore, the
depressor response evoked by CGP 35348 in SHR was greater during
prolonged ADN stimulation than without ADN stimulation (Fig 4). In contrast, in WKY, CGP 35348 had no effect on AP
during ADN stimulation (Figs 3 and 4).
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DOCA-Salt–Treated Rats
As with SHR and WKY, electrical stimulation of the ADN evoked
frequency-dependent decreases in AP, HR, and sSNA in DOCA-HT and
control rats (Fig 5, Table 3). These
responses did not differ between DOCA-HT and control rats. A group of
DOCA-preHT rats (n=11), rats that had been treated with DOCA and salt
for 5 days, was also studied because a previous report had described an
attenuation of ADN-evoked responses in rats treated in this
manner.16 However, as with DOCA-HT rats, the DOCA-preHT
rats had responses to ADN stimulation that did not differ from those in
control rats (n=7) (data not shown).
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Discussion |
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ADN-Evoked Responses Are Altered in SHR Compared With WKY
The present results demonstrate that ADN-evoked cardiovascular responses are attenuated in SHR compared with WKY. In agreement with the earlier report of Gonzales et al,15 electrical stimulation of the ADN resulted in frequency-dependent decreases in MAP that differed between SHR and WKY. The most apparent difference is that the response to sustained ADN stimulation (what Gonzales et al referred to as the "static phase" of the ADN-evoked response) was diminished in SHR compared with WKY and compared with the initial response (the "dynamic phase" of Gonzales et al). Thus, the pattern of the ADN-evoked change in MAP is different in the two strains. In WKY, the initial depressor response is maintained throughout the period of ADN stimulation, whereas in SHR, the response diminishes over time, especially at higher stimulation frequencies. However, the initial decrease in MAP is similar between the two strains. Although this conclusion is at odds with the report by Gonzales at al, this contrast is based totally on the manner in which the data are analyzed rather than on the actual results. In both studies, the ADN-evoked depressor responses are the same in the two strains when expressed as absolute change in MAP. However, if the data are expressed as percent change in MAP, the response in SHR is significantly smaller than in WKY, owing to the higher baseline MAP in SHR. Nonetheless, independent of how the data are calculated, the ADN-evoked decreases in MAP are sustained in WKY and not in SHR.
Similar conclusions can be drawn from the sSNA results; ADN-evoked inhibition of sSNA is attenuated in SHR. ADN stimulation results in inhibition of sSNA that attenuates over time, even in WKY, in which the decrease in MAP is maintained. Thus, in WKY, stimulation of the ADN at 25 Hz initially decreases sSNA by 90%, but 10 seconds into the stimulation period, the sSNA response is inhibited by only 60% compared with baseline. A similar rapid attenuation of ADN-evoked inhibition of sSNA in WKY was reported by Gonzales et al15 and also occurs in other rat strains.19 Compared with WKY, in SHR this time-dependent attenuation of ADN-evoked sympathoinhibition is exaggerated. Although comparisons of evoked changes in sSNA between SHR and WKY are complicated by differences in baseline levels of nerve activity, certain differences are apparent. First, as noted above, during 30 seconds of ADN stimulation, the evoked changes in sSNA attenuate to a greater extent in SHR. Second, the extent to which sSNA can be inhibited by ADN stimulation is less in SHR; high-frequency ADN stimulation initially elicits a 90% inhibition of sSNA in WKY compared with 75% in SHR. Indeed, ADN-evoked inhibition of sSNA is less in SHR than WKY at all stimulus frequencies.
ADN-evoked bradycardia is also different between SHR and WKY. At all stimulation frequencies, the evoked bradycardia is attenuated in SHR compared with WKY. However, unlike the MAP and sSNA responses, which diminish during continuing ADN stimulation in SHR, the bradycardic responses are maintained throughout the stimulation period. Comparisons with the study by Gonzales et al15 are not possible because HR data were not included in that study.
ADN fibers provide excitatory input to the NTS, and the effects of direct excitation of the NTS by local microinjection of glutamate11 provide an interesting comparison. Indeed, the comparisons between strains are similar for ADN stimulation and microinjection of glutamate into the NTS; the evoked change in MAP is similar in the two strains when expressed as absolute decrease in MAP, whereas the decrease in HR in response to each treatment is reduced in SHR. When expressed as percent change from baseline, MAP and HR responses to either ADN stimulation or glutamate injection into the NTS are reduced by approximately 40% in SHR compared with WKY. Stimulation of cardiac afferents, which also elicits decreases in MAP and HR via an excitatory input to the NTS, results in similarly attenuated responses in SHR compared with WKY.20 The similarity of responses to afferent stimulation and glutamate injection into the NTS suggests that the differences in the magnitude of the initial cardiovascular response between SHR and WKY occur distal to the NTS in the baroreceptor reflex circuitry. Since the maximal decrease in MAP obtainable by sympathoinhibition is larger in SHR than in WKY, but similar when expressed as percent change in MAP,21 22 23 it appears that there is a general attenuation of baroreceptor reflex (both sympathetic vasomotor and cardiac components) that occurs centrally, distal to the NTS. Even so, there is an additional impairment of ADN-evoked inhibition of sSNA and MAP in SHR that is apparent with prolonged ADN stimulation at high frequencies; this time-dependent attenuation of ADN-evoked depressor responses in SHR is discussed more fully below.
The significance of this central attenuation of the initial phase of ADN-evoked responses in SHR to the pathogenesis of hypertension in SHR is unclear at present. It is unknown whether this trait would cosegregate in crossbreeding experiments. Even if it did cosegregate with hypertension in SHR, it would be uncertain whether the attenuation of ADN-evoked responses occurred in response to the hypertension or independent of it. Nonetheless, as discussed below, it is not a universal characteristic of hypertension, because with a protocol similar to that used in SHR, ADN-evoked responses were normal in DOCA-salt hypertensive rats.
The finding of central attenuation of ADN-evoked responses in SHR, which is in full agreement with the report of Gonzales et al,15 is in contrast with the study of Sun and Guyenet24 showing that the central neural processing of baroreceptor afferent signals is the same in SHR and WKY. Although differences in substrain of SHR, type or level of anesthesia, or sympathetic nerve that was monitored may account for these differences, it is most likely that differences in the experimental protocol (eg, electrical stimulation of the ADN versus stimulation of the ADN by increasing AP) account for the different conclusions.
GABAB Receptors in the NTS of SHR Are Involved in the
Attenuation of the ADN-Evoked Depressor Response
We have previously demonstrated that stimulation of
GABAB receptors in the NTS attenuates ADN-evoked
responses,12 and we have presented the hypothesis
that GABA acting on GABAB receptors in the NTS may
contribute to frequency-dependent attenuation of baroreceptor
afferent processing in the NTS.25 This hypothesis leads to
the prediction that high-frequency stimulation of the ADN should
result in a response that attenuates over time, which is what we (see
above) and others15 have observed in SHR. Furthermore,
this hypothesis predicts that the attenuation of depressor responses to
high-frequency ADN stimulation in SHR should be reversed by
administration of a GABAB receptor antagonist;
this was observed in the present studies. The depressor response
elicited by injection of a GABAB receptor
antagonist into the NTS was potentiated during prolonged
ADN stimulation at 25 Hz in SHR. Thus, the present data in SHR are
consistent with the hypothesis that frequency- and
time-dependent attenuation of ADN-evoked responses occurs in vivo
and is at least partly due to GABAB receptor–mediated
attenuation of the baroreceptor reflex. Furthermore, this observation
supports the notion that the diminution of ADN-evoked depressor
responses during prolonged ADN-stimulation is due to processes
occurring in the NTS.
Given this explanation, it is surprising that a similar effect was not observed for ADN-evoked decreases in HR in SHR. This would imply differential processing in the NTS of baroreceptor control of sympathetic vasomotor outflow and vagal cardiomotor outflow. Furthermore, this would suggest either that the modulation of the depressor response does not occur at the primary afferent synapse or that different baroreceptor afferent fibers are involved in the two different baroreceptor reflex responses.
Since frequency-dependent attenuation of visceral afferent transmission in the NTS has been demonstrated with a variety of experimental protocols in normotensive animals,26 27 28 it is surprising that time- and frequency-dependent diminution of ADN-evoked depressor responses was not observed in WKY (or normotensive Sprague-Dawley rats, see below) as it was in SHR. However, the NTS in SHR appears to be more sensitive to drugs acting on GABAB receptors, possibly the result of an increased number of GABAB receptors.29 Thus, because of the enhanced sensitivity to manipulation of the GABAB receptor in the NTS of SHR, it may simply be that it is easier to demonstrate frequency-dependent inhibition of ADN-evoked depressor responses in vivo in SHR. Nevertheless, time-dependent attenuation of ADN-evoked sympathoinhibition does occur in normotensive animals. However, the role of the NTS, or specifically GABAB receptors in the NTS, in mediating this response has not been examined.
ADN-Evoked Responses Are Not Altered in DOCA-HT Rats
The comparison of ADN-evoked responses and effects of drugs that
act on GABAB receptors in the NTS in SHR and WKY is
complicated by the differences in baseline AP between the two strains
as well as by the genetic complexity of this hypertensive model. Thus,
a similar alteration in ADN-evoked responses in DOCA-salt–treated
rats that has been reported to develop before the development of
elevated AP16 seemed to provide an ideal model for study
of the relevance of GABAB receptor mechanisms in the
central attenuation of ADN-evoked responses and the significance
of this to the pathogenesis of hypertension. However, we have been
unable to replicate the findings of Nakamura et al.16 The
prominent attenuation of ADN-evoked decreases in MAP, HR, and sSNA in
DOCA-HT and DOCA-preHT rats reported by Nakamura et al is in marked
contrast to the lack of an effect of DOCA-salt treatment on ADN-evoked
responses in the present study. Although a variety of differences
between the two studies might account for these different results (eg,
Nakamura et al used Wistar rats, whereas we used
Sprague-Dawley rats in the present study), the
present results clearly demonstrate that attenuation of ADN-evoked
responses is not a characteristic of DOCA-HT rats.
Summary and Conclusions
In summary, the present results confirm previous studies that
there is a central attenuation of baroreceptor reflex responses in SHR.
Furthermore, enhanced frequency- and time-dependent inhibition of
ADN-evoked depressor responses is likely related to enhanced action of
GABA on GABAB receptors in the NTS in attenuating the
baroreceptor reflex. The significance of this finding to the
pathogenesis of hypertension in SHR is unclear at present, but
results in DOCA-HT rats suggest that it is not simply a result of
hypertension.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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These studies were supported by grants from the National Institutes of Health (HL-38786) and the American Heart Association, Pennsylvania Affiliate.
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Footnotes |
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Reprint requests to Dr Alan F. Sved, Department of Neuroscience, 446 Crawford Hall, University of Pittsburgh, Pittsburgh, PA 15260. E-mail sved@bns.pitt.edu.
Received November 22, 1995; first decision January 8, 1996; accepted February 9, 1996.
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