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Department of Physiology, Wayne State University, School of Medicine, Detroit, Michigan 48201
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ABSTRACT |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Activation of adenosine
A2a and ATP
P2x purinoceptors in the
subpostremal nucleus tractus solitarii (NTS) via microinjection of the
selective agonists CGS-21680 and 
,
-methylene ATP (,-MeATP), respectively, elicits large dose-dependent decreases in arterial pressure and heart rate, differential regional vasodilation, and differential inhibition of regional sympathetic outputs. With marked
hypotensive hemorrhage, preganglionic adrenal sympathetic nerve
activity (pre-ASNA) increases, whereas renal (RSNA) and postganglionic
adrenal sympathetic nerve activity (post-ASNA) decrease. In this
setting, adenosine levels in the brain stem increase. Therefore, we
investigated whether stimulation of specific purinoceptors in the NTS
may evoke differential sympathetic responses. RSNA was recorded
simultaneously with pre-ASNA or post-ASNA in chloralose-urethan-anesthetized male Sprague-Dawley rats.
CGS-21680 (2 and 20 pmol in 50 nl) inhibited RSNA and post-ASNA,
whereas pre-ASNA increased markedly. ,-MeATP (25 and 100 pmol in
50 nl) inhibited all sympathetic outputs. Sinoaortic denervation plus
vagotomy markedly prolonged the responses to
P2x-purinoceptor stimulation.
Glutamate (100 pmol in 50 nl) caused differential inhibition of all
sympathetic outputs similar to that evoked by ,-MeATP. We
conclude that NTS A2a-purinoceptor
activation evokes differential sympathetic responses similar to those
observed during hemorrhage, whereas
P2x-purinoceptor and
glutamate-receptor activation evokes differential inhibition of
sympathetic outputs similar to arterial baroreflex responses.
adrenal sympathetic nerve; renal sympathetic nerve; purinergic receptor subtypes; nucleus of the solitary tract; cardiovascular control
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INTRODUCTION |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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THE NUCLEUS TRACTUS SOLITARII (NTS), the primary integrative center for cardiovascular control and other autonomic functions, contains a great variety of neurotransmitters/neuromodulators and is organized in a viscerotopic manner (21, 27, 36). The combination of these two factors (neurochemical and neuroanatomic) may create specific regional patterns of cardiovascular and neuroregulatory responses to different physiological and pathological stimuli.
Among the numerous neuroactive substances operating in the NTS, the
purines ATP and adenosine recently have been shown to play an important
role in central cardiovascular control (24). Several lines of evidence
from our laboratory and from others suggest that ATP, operating via
P2x purinoceptors, may act as a
fast neurotransmitter in NTS interneurons, whereas adenosine operating
via A1 and
A2a purinoceptors plays a
neuromodulatory role in NTS mechanisms of cardiovascular control (1, 2, 5, 6, 10, 12-14, 17, 22-24, 28, 31, 33, 34). Neuronally released ATP may subsequently become a source of adenosine when catabolized by ectonucleotidases (42). This was recently confirmed in
vivo at the level of the NTS and rostral ventrolateral medulla (33).
Microinjection of ATP and its synthetic analogs, ,-methylene ATP
(,-MeATP) and 2-methylthio-ATP, into the subpostremal NTS in vivo
produce abrupt, marked, dose-related reductions in mean arterial blood
pressure (MAP), heart rate (HR), and sympathetic activity, simulating
baroreflex responses (5, 14, 28). Similarly, microinjection of
adenosine into the subpostremal and caudal NTS decreases MAP, HR, and
efferent sympathetic nerve activity (1, 22, 23, 34); however, the
responses develop slower and last longer than those evoked by ATP. This
depressor action of adenosine is mediated via
A2a purinoceptors (2, 5, 31). Recently we have shown that selective activation of
P2x and
A2a purinoceptors in the
subpostremal NTS elicits distinct patterns of dose-dependent,
differential regional vasodilation and differential inhibition of renal
(RSNA) versus lumbar sympathetic nerve activity (LSNA) (5, 28, 31).
Specific physiological mechanisms (e.g., hemorrhage, stress, exercise,
etc.) have characteristic regional distribution of vascular and
neuroregulatory effects (15, 26, 35, 39, 41). A most impressive example
of differential sympathetic regulation is the dissociation between
adrenal sympathetic nerve activity (ASNA) versus RSNA observed in
several physiological and pathological situations. For example, early
stages of hemorrhage or hypoglycemia lead to a greater increase in ASNA
than RSNA (9, 35, 39), whereas during prolonged severe hemorrhage,
stimulation of cardiac chemoreceptors or the paraventricular nucleus of
the hypothalamus, or intravenous injections of morphine, ASNA increases
while RSNA decreases (8, 11, 16, 18, 35, 39). These differential response patterns were usually related to specific reflex mechanisms or
certain central neurotransmitters/neuromodulators. Because adenosine is
naturally released into the NTS and other central structures under
conditions of severe hemorrhage, central hypoxia, or ischemia
(25, 38, 40), it is possible that this neuromodulator may facilitate
the dissociation between ASNA and RSNA. Adenosine, acting via
A2a purinoceptors, may stimulate
neuronal release of serotonin in the NTS (4) or facilitate
cardiopulmonary reflexes via an increase in glutamate release from
primary afferents terminating on NTS neurons (10). Both these
mechanisms may elicit an increase in ASNA and a simultaneous decrease
in RSNA (16, 32). To test this hypothesis, we compared ASNA and RSNA
responses to selective stimulation of
A2a purinoceptors in the
subpostremal NTS. To evaluate whether the pattern of sympathetic
responses (ASNA vs. RSNA) is specific to stimulation of
A2a purinoceptors, we compared
these responses with those evoked by selective stimulation of
P2x purinoceptors and nonselective
stimulation of glutamate receptors in the same site of the NTS. To
distinguish between primary responses to stimulation of the
purinoceptor subtypes in the NTS and reflex compensation to these
responses via arterial and cardiopulmonary afferents, the effects
evoked by microinjections of selective
A2a- and
P2x-purinoceptor agonists
(CGS-21680 and ,-MeATP, respectively) were compared in intact
versus sinoaortic denervated and vagotomized (SAD + VX) animals.
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MATERIALS AND METHODS |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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All protocols and surgical procedures employed in this study were reviewed and approved by the Institutional Animal Care and Use Committee and were performed in accordance with the "Guiding Principles in the Care and Use of Animals" endorsed by the American Physiological Society and the Guide for the Care and Use of Laboratory Animals published by the National Institutes of Health.
Design.
The effect of activation of A2a
and P2x purinoceptors in the
subpostremal region of the NTS on simultaneously recorded sympathetic nerve activity directed to the adrenal gland and the kidney was investigated in 26 intact and 11 SAD + VX male Sprague-Dawley rats
(350-400 g) (Charles River Laboratories, Wilmington, MA). MAP and
HR responses were also recorded. Activation of purinoceptors was
accomplished via microinjections of the selective
A2a- and P2x-purinoceptor agonists
CGS-21680 and ,-MeATP, respectively. In some animals the effects
of microinjection of vehicle [artificial cerebrospinal fluid
(ACF)] (5, 28, 31) on cardiovascular and neural parameters were
observed over the average time of the response to the maximal dose of
CGS-21680 or ,-MeATP, respectively. In an additional five animals
with intact baroreceptor and vagal afferents, nonselective
glutamatergic receptors were stimulated via microinjections of sodium
glutamate into various sites of subpostremal and immediately adjacent
commissural NTS. This was performed to compare responses evoked by
selective activation of purinoceptor subtypes with a standard
"nonspecific" stimulation of NTS neurons in this area.
Instrumentation and measurements.
All the procedures were described in detail previously (2, 5, 14, 28,
29, 31). Briefly, rats were anesthetized with a mixture of
-chloralose (80 mg/kg) and urethan (500 mg/kg ip) and
tracheotomized. Intact animals breathed spontaneously, similar to our previous studies (28, 31). SAD + VX animals were
connected to a small animal respirator (SAR-830, CWE, Ardmore, PA) to
compensate for the changes in respiratory pattern. All animals breathed
oxygen-enriched air. Arterial blood gases were tested occasionally
(ABL500, OSM3; Radiometer), and ventilation was adjusted to maintain
PO2,
PCO2, and pH within normal ranges.
The right femoral artery and vein were catheterized to monitor arterial
blood pressure and infuse drugs. SAD + VX was accomplished and its
completeness tested as described previously (29).
Microinjections into the NTS.
Unilateral microinjections of CGS-21680, ,-MeATP, glutamate, or
vehicle (ACF) were made with multibarrel glass micropipettes into the
medial region of the caudal subpostremal NTS as described previously
(2, 5, 28, 31). Some microinjections of glutamate were made into the
adjacent portions of the NTS, i.e., commissural and rostral
subpostremal NTS. Nonselective activation of glutamatergic receptors
allowed evaluation of whether the relative ratio between regional
neural responses (ASNA vs. RSNA) to a nonspecific stimulus (glutamate)
depends on the precise anatomic location of the stimulus in this area
of the NTS. All microinjection sites were verified histologically as described previously (28, 30, 31) and presented
schematically in Fig. 1.
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,-MeATP were the same as those used in
our previous studies (5, 28, 31): 1)
the approximate threshold hypotensive dose (2 and 25 pmol for CGS-21680
and ,-MeATP, respectively), and
2) the maximally effective
hypotensive dose (20 and 100 pmol for CGS-21680 and ,-MeATP,
respectively) (5, 28, 31). We have previously shown that the effects
elicited with the high doses of both purinoceptor agonists were
completely and selectively blocked by microinjection of the selective
A2a-purinoceptor antagonist
CGS-15943A (2) or P2-purinoceptor
antagonist suramin, respectively (14, 28). To avoid the effect of
desensitization of purinoceptors, in all experiments only one dose of
purinoceptor agonist was microinjected into the left and/or
right side of the NTS. If purinergic agonists were injected
bilaterally, at least a 90-min interval between the injections was
allowed. Because glutamate (100 pmol in 50 nl) elicits relatively
short-lasting responses, the effects of up to three unilateral
microinjections into separate areas of the NTS (comissural, rostral
subpostremal, and caudal subpostremal) were performed. All the drugs
were dissolved in ACF and the pH adjusted to 7.2. Microinjections of
ACF (50 nl) served as a vehicle control.
Data analysis.
Hemodynamic and sympathetic nerve responses were quantified in two
ways: 1) the maximal percent
difference from a 30-s basal control period taken immediately before
microinjection, and 2) integration
of the percent changes from control over the period of the change in
MAP. The HR responses, calculated from pulse intervals, were expressed
in absolute values (beats/min). Neural recordings were additionally
filtered using a running average in 10-s intervals to minimize the
effect of random spikes on maximum response values. Because the
responses to ,-MeATP frequently exhibited a biphasic pattern,
i.e., fast recovery to ~80% of the depressor response followed by
variable residual depression, only the fast part of the response (to
80% of recovery in MAP) was analyzed, similar to our previous study
(28). However, time to recovery was measured for both fast and slow
components of the response. Maximal and integral responses to control
microinjections of ACF were measured over the time to recovery of MAP
after the high dose of CGS-21680 and/or ,-MeATP,
respectively. One-way ANOVA for independent measures was used to
compare MAP and HR responses to ACF versus different doses of CGS-21680
and ,-MeATP. A two-way ANOVA for independent measures was used to
compare neural responses versus doses of the drug and ASNA versus RSNA.
Differences observed were further evaluated by the test of simple
effect. An -level of P < 0.05 was
used to determine statistical significance.
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RESULTS |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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The resting values for MAP and HR measured in intact animals
(n = 31) before microinjection of
drugs or vehicle into the subpostremal NTS were 81.8 ± 1.3 mmHg and
367 ± 4 beats/min. The effects of microinjections of CGS-21680 and
,-MeATP at the high doses (20 and 100 pmol/rat, respectively) on
MAP, HR, and simultaneously recorded pre-ASNA and RSNA are presented in
Fig. 2. Microinjection of CGS-21680
produced gradually developing and long-lasting reductions in MAP, HR,
and RSNA; however, pre-ASNA increased markedly. In contrast to the
directionally opposite responses of RSNA and pre-ASNA to CGS-21680,
,-MeATP evoked decreases in both sympathetic outputs. The
responses to ,-MeATP started abruptly and usually showed a
biphasic pattern of recovery (fast and slow components), especially for
the high dose of the drug (Fig. 2). Fast recovery, to ~80% of the
depressor response, was followed by a weak residual depression, cardiac
slowing, and sympathoinhibition, lasting several times longer than the
fast component of the response (Fig. 2 and Table 1). Both drugs demonstrated a dose
dependency of the duration of the depressor responses (Table 1).
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A2a-purinoceptor stimulation. The average neural and hemodynamic responses to ACF (50 nl) and both doses of CGS-21680 (2 and 20 pmol in 50 nl) are shown in Fig. 3. Microinjections of CGS-21680 evoked substantial dose-dependent increases in pre-ASNA, whereas RSNA and hemodynamic parameters markedly decreased in a dose-dependent manner. The differences between neural responses were highly significant for both methods of evaluation of the effect (i.e., maximum and integral responses, P < 0.0001). HR and RSNA responses were similar in magnitude and time course to those observed in previous studies from our laboratory (5, 31). However, the decrease in MAP was ~30-50% smaller compared with that previously observed for both doses of the drug in terms of both the maximal and integral responses (5, 31; see DISCUSSION). Microinjections of the same volume of ACF did not evoke marked integral responses measured over the average time to recovery of MAP for the high dose of CGS-21680 (Fig. 3). The integral RSNA, pre-ASNA, and HR responses to ACF were not different from zero (P > 0.05), and the integral response of MAP did not decrease but rather tended to increase slightly (P = 0.034). Because hemodynamic and neural parameters fluctuate spontaneously, some decreases and equivalent increases in all parameters were observed over the relatively long time period of measurement (almost a half hour); however, these minor spontaneous fluctuations were significantly smaller than the responses to even the low dose of CGS-21680 (P < 0.05) (Fig. 3). Note that pre-ASNA responses to CGS-21680 (increases) were compared with the maximal, spontaneous increases in the activity recorded after microinjections of ACF, whereas responses in RSNA and hemodynamic parameters (decreases) were compared with maximal spontaneous decreases in these parameters. The magnitude of spontaneous increases was not different from the magnitude of spontaneous decreases for all recorded parameters (P > 0.05; the remaining data for all these comparisons are not presented).
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P2x-purinoceptor stimulation.
Averaged fast components of hemodynamic and neural responses to both
doses of ,-MeATP compared with respective vehicle control (ACF)
are presented in Fig. 4. Microinjection of
,-MeATP produced powerful, dose-related reductions in the
activity of both sympathetic outputs and decreases in MAP and HR. The
decreases in MAP, HR, and RSNA were similar in magnitude and time
course to those observed in our previous studies (5, 28). The responses
evoked by the high dose of ,-MeATP were significantly greater
than those caused by the low dose for both maximal and integral changes
(P < 0.05 for all comparisons).
However, dose-related differences were much more pronounced for
integral changes, especially for the neural activity, which was
abruptly inhibited, sometimes to virtually zero levels for a very short
time (a few seconds) just after the microinjection. Large dose-related
differences for integral responses and small dose-related differences
for maximum responses reflect the observation that higher doses of
,-MeATP evoked much longer-lasting inhibitory effects compared
with those evoked with low doses (Table 1; compare also Fig. 2,
right, and Fig. 5, left).
The fast responses to ,-MeATP were several times shorter than
those evoked by CGS-21680, especially for the small, near-threshold doses of both drugs (Table 1).
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,-MeATP and for both methods of evaluation
of the effects (P < 0.05).
Microinjection of vehicle (ACF) into the same site of the NTS had no
marked effect on any of the parameters measured over the average time
of fast depressor responses to the higher dose of ,-MeATP (Fig.
4).
Sinoaortic denervation and vagotomy. Bilateral sinoaortic denervation and subsequent vagotomy resulted in a marked, sustained increase in HR and transient elevation in MAP. HR measured in SAD + VX animals (n = 11) just before the microinjections of the drugs (3-10 h after denervation) remained elevated compared with HR measured in intact animals (441 ± 6 vs. 367 ± 4 beats/min, respectively; P < 0.0001). In contrast, MAP returned gradually toward resting values during the 30-60 min after denervation, and no differences between SAD + VX vs. intact animals were observed throughout the time course of experiments (82.9 ± 1.3 vs. 81.8 ± 1.3 mmHg, respectively; P = 0.681).
The comparison of average maximum and integral responses evoked by microinjections of the high doses of CGS-21680 and,-MeATP in
intact and SAD + VX animals is presented in Table
2. The opposite regional neural responses
to stimulation of A2a
purinoceptors, i.e., a decrease in RSNA and an increase in pre-ASNA,
were not affected by SAD + VX. Also, the magnitude of hemodynamic
responses to CGS-21680 was not markedly changed by SAD + VX, although
the maximum responses tended to decrease (Table 2). However, SAD + VX slightly increased the time to recovery of MAP (Table 1).
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,-MeATP in SAD + VX
versus intact animals (Tables 1 and 2). However, HR responses were not
significantly altered with SAD + VX. A slight tendency to decrease
maximum HR responses and to increase integral HR responses in SAD + VX
versus intact animals did not reach statistical significance
(P = 0.429 and
P = 0.0968, respectively).
Glutamate microinjections.
Similarities between neural and hemodynamic responses evoked by the low
dose of ,-MeATP (25 pmol in 50 nl) and a moderate dose of
glutamate (100 pmol in 50 nl) are shown in Fig.
5
(left). The microinjection of
glutamate elicited a short-lasting decrease in MAP, HR, RSNA, and ASNA.
The time course of these responses was very similar to that of the fast
component of the responses evoked by a low dose of ,-MeATP. Both
drugs inhibited pre-ASNA to a slightly lesser extent than RSNA. The
ratio between maximal neural responses (
%pre-ASNA/%RSNA) was
virtually the same for both agonists microinjected into the same area
of the caudal subpostremal NTS [0.668 ± 0.055 vs. 0.641 ± 0.063 for ,-MeATP (n = 9) and glutamate (n = 7), respectively;
P = 0.750].
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Post-ASNA responses. In contrast to pre-ASNA, post-ASNA responded similarly to RSNA. Stimulation of A2a purinoceptors evoked a decrease in predominantly post-ASNA in contrast to the increase observed in predominantly pre-ASNA (compare Figs. 3 and 6). The inhibition of post-ASNA was smaller than that for RSNA; however, predominantly post-ASNA exhibited almost 40% of preganglionic activity, which presumably increased in response to stimulation of A2a purinoceptors, whereas RSNA was ~96% postganglionic.
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,-MeATP evoked virtually the same inhibition of
post-ASNA and RSNA in terms of both maximal and integral responses
(P > 0.05 for both comparisons).
Also, the responses to glutamate were very similar for both post-ASNA
and RSNA. Integral neural responses to glutamate were virtually the
same between post-ASNA and RSNA, although a very small (3.2%)
"significant" (P = 0.047)
difference for the maximum responses was observed. The ratio between
post-ASNA and RSNA responses to glutamate (%post-ASNA/%RSNA) was significantly closer to 1 than that calculated for all
(n = 17) pre-ASNA versus RSNA
responses (0.953 ± 0.016 vs. 0.676 ± 0.036, respectively,
P = 0.006).
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DISCUSSION |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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This is the first study to investigate the effects of selective stimulation of A2a and P2x purinoceptors located in subpostremal NTS on pre- and postganglionic sympathetic nerve activity directed to the adrenal gland. Also, for the first time, hemodynamic and neural responses to stimulation of purinoceptor subtypes in the NTS were compared in intact versus SAD + VX animals. The major new finding of the present study is that pre-ASNA increased in response to stimulation of NTS A2a purinoceptors and decreased in response to stimulation of P2x and glutamate receptors, whereas simultaneously recorded RSNA and post-ASNA decreased in response to stimulation of all of these receptor subtypes. These differential neural responses were dose dependent and were accompanied by respective decreases in MAP and HR, as observed previously (5, 28, 31). In addition, SAD + VX markedly prolonged the responses to activation of P2x purinoceptors and slightly extended the responses to activation of A2a-purinoceptor stimulation without marked changes in their initial magnitudes. SAD + VX experiments demonstrate that the increase in pre-ASNA after stimulation of A2a purinoceptors was not a baroreflex response to the concomitant decrease in MAP but a primary response to central stimulation of these receptors.
Differential sympathetic control via NTS neurons.
Stimulation of P2x purinoceptors
in the subpostremal NTS inhibited pre-ASNA to a lesser extent than
RSNA; however, there was no difference between the inhibition of
post-ASNA and RSNA. More dramatic differences among these regional
sympathetic outputs were observed in response to
A2a-purinoceptor stimulation:
pre-ASNA increased, whereas post-ASNA decreased and RSNA decreased to
an even greater extent. In addition, our previous studies showed that
stimulation of P2x purinoceptors
inhibited RSNA to a lesser extent than LSNA (directed mostly to the
hindlimb), whereas stimulation of
A2a purinoceptors, which inhibited
RSNA to an extent similar to that in the present study, caused no
significant change in LSNA (28, 31). The ratio between regional
sympathetic responses to selective stimulation of
A2a and
P2x purinoceptors was different and characteristic for each pair of sympathetic outputs
(%pre-ASNA/%RSNA, %post-ASNA/%RSNA, and
%LSNA/%RSNA) and each purinoceptor subtype (A2a and
P2x). These differential
sympathetic responses to stimulation of different purinoceptor subtypes
were evoked from the same anatomic site of the NTS. It is worthwhile to
note that the relative ratio between inhibitory neural responses to
nonselective glutamatergic stimulation (%pre-ASNA/%RSNA)
remained virtually the same for all the microinjection sites located in
various parts of subpostremal NTS and the immediately adjacent
comissural NTS. This indicates that the differential neural response
patterns were not related to a very discrete anatomic location of
neurons targeting different sympathetic output. Instead, it is likely
that NTS neurons finally targeting sympathetic outputs directed to
different organs exhibited differential expression of
A2a versus
P2x and glutamate receptor subtypes. In support of this concept, a recent study confirmed that
relative expression of various types of
K+ channels was different for
subpostremal and comissural NTS neurons that are involved in arterial
baroreflex versus cardiopulmonary reflex control of the circulation
(7). Taken together, the results of the present and our previous
studies support the hypothesis that different
neurotransmitters/neuromodulators operating in the same site of the NTS
may be related to specific functional subsystems differentially
controlling regional sympathetic outputs.
Physiological implications. Stimulation of A2a purinoceptors in subpostremal NTS created a very similar pattern of hemodynamic and differential sympathetic neural responses to that observed during the latter hypotensive stage of severe hemorrhage, i.e., hypotension accompanied with a large and long-lasting decrease in HR and RSNA and a contrasting, marked increase in pre-ASNA (8, 35, 39). Adenosine naturally accumulates in the brain stem, including the NTS, during severe hemorrhage, hypoxia, or cerebral ischemia (38, 40). This coincidence strongly suggests that adenosine naturally released in the NTS during hemorrhage may participate via its A2a-purinoceptor action in creating specific regional responses that occur during hemorrhagic shock.
Interestingly, the decreases in MAP in response to stimulation of NTS adenosine receptors recorded in the present study were ~30-50% smaller than those observed previously, whereas the decreases in HR and RSNA were virtually the same (5, 31). One major difference among the experimental preparations was that in the present study the left adrenal nerve was cut for recording ASNA, whereas in previous studies sympathetic innervation of both adrenal glands remained intact. Considering that stimulation of A2a purinoceptors in the subpostremal NTS evoked large, long-lasting increases in pre-ASNA directed predominantly to the adrenal medulla, it is likely that release of epinephrine is increased in this setting. In the rat, stimulation of pre-ASNA results in a 4:1 increase in the release of epinephrine versus norepinephrine as measured in the adrenal vein (20). Epinephrine operating via vascular-receptors located preferentially
in the muscle vascular bed (37) may contribute to the depressor
response evoked by stimulation of
A2a purinoceptors in the NTS via
preferential vasodilation of skeletal muscle. We previously observed
that stimulation of NTS A2a
purinoceptors evoked marked, preferential vasodilation of the hindlimb
versus the renal and mesenteric vascular beds (5). In the present
study, because one adrenal gland was denervated, less epinephrine would
be released, thereby limiting peripheral vasodilation and hypotension
via this mechanism. In support of this concept, we recently reported
that the hypotensive and hindlimb vasodilator responses to stimulation
of NTS A2a purinoceptors were
abolished by systemic -adrenergic blockade (19).
Stimulation of P2x purinoceptors
exerted MAP and HR depression and sympathoinhibition of a very rapid
onset and biphasic pattern of recovery; the first ~80% of the
response recovered relatively fast (in 28 s for the low dose of
,-MeATP), whereas weak residual depression lasted several times
longer (Table 1). This biphasic time course of the responses suggests
that two different mechanisms were triggered by stimulation of
P2x purinoceptors in the NTS; presumably, neuromediator-like action was followed by
neuromodulator-like action. Interestingly, the fast
response elicited by the low, near-threshold dose of ,-MeATP was
similar to that evoked by microinjection of glutamate and mimicked
baroreflex response (Fig. 5). The ratio between these neuroinhibitory
responses of pre-ASNA and RSNA was virtually the same for ,-MeATP
and glutamate. In addition, blockade of
P2 purinoceptors with suramin
microinjected into the same site of the NTS markedly impaired HR
baroreflex responses (30). Previous studies indicate that ATP operating via P2x purinoceptors may serve as
a fast neurotransmitter between central neurons (12, 13, 17) and that
ATP is neuronally released into the NTS (33). Therefore, it is possible
that ATP may act as a fast neurotransmitter between NTS interneurons
possibly linked in chain with glutamatergic neurons operating in the
baroreflex arc. Alternatively, stimulation of
P2x purinoceptors may trigger the
release of glutamate in the baroreflex arc at the level of the NTS, as
was postulated for hippocampal structures (17). The secondary weak,
slow, and long-lasting response may be mediated via triggering of the
release of one of numerous neuromodulators operating in the NTS (21).
This interesting possibility awaits further, detailed investigations.
In summary, different neurotransmitters/neuromodulators microinjected
into the same site of the NTS evoked different patterns of regional
sympathetic responses. Differential neural response to stimulation of
A2a purinoceptors resembled
differential neural response to severe hemorrhage, whereas the rapid
onset and fast initial recovery of the responses to stimulation of
P2x purinoceptors were similar to
typical responses evoked with stimulation of glutamate receptors and
resembled rapid baroreflex responses. The ratio between neural
responses to selective stimulation of
A2a and
P2x purinoceptors was
characteristic for each pair of sympathetic outputs and each
purinoceptor subtype. We believe that a comparison of specific patterns
of regional peripheral responses to administration of specific
neurotransmitters/neuromodulators into discrete groups of central
neurons performed in vivo in a whole animal may help explain the
physiological role and significance of molecular mechanisms described
at the cellular level in vitro.
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ACKNOWLEDGEMENTS |
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The authors gratefully acknowledge the generous gift of arfonad by Hoffmann-La Roche (Nutley, NJ). We also gratefully acknowledge the technical assistance of C. Cupps.
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FOOTNOTES |
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This study was supported by National Institutes of Health Grants MH-47181, GM-08167, and HL-02844.
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. §1734 solely to indicate this fact.
Address for reprint requests: D. S. O'Leary, Dept. of Physiology, Wayne State Univ., School of Medicine, 540 E. Canfield Ave., Detroit, MI 48201.
Received 5 May 1998; accepted in final form 25 August 1998.
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REFERENCES |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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, intact animals; 
, sinoaortic denervated and
vagotomized (SAD + VX) animals; 
, intact animals; 
, SAD + VX animals; 
, glutamate].
