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Max-Planck-Institut für Physiologische und Klinische Forschung, W. G. Kerckhoff-Institut, 61231 Bad Nauheim, Germany
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Using
extracellular electrophysiological recording in an in vitro slice
preparation, we investigated whether ANG I can be locally converted to
the functionally active ANG II within the rat subfornical organ (SFO).
ANG I and ANG II
(10
8-107
M) excited ~75% of all neurons tested with both peptides
(n = 25); the remainder were
insensitive. The increase in firing rate and the duration and the
latency of the responses of identical neurons, superfused with
equimolar concentrations of ANG I and ANG II, were not different. The
threshold concentrations of the ANG I- and ANG II-induced excitations
were both 109 M. Inhibition
of the angiotensin-converting enzyme by captopril (104 M;
n = 8) completely blocked the ANG
I-induced excitation, a 10-fold lower dose was only effective in two of
four neurons. The AT1-receptor
antagonist losartan (105 M;
n = 6) abolished the excitation caused
by ANG I and ANG II. Subcutaneous injections of equimolar doses of ANG
I and ANG II (200 µl; 2 × 104 M) in water-sated rats
similarly increased water intake by 2.4 ± 0.5 (n = 16) and 2.7 ± 0.4 ml
(n = 20) after 1 h, respectively. Control rats receiving saline drank 0.07 ± 0.06 ml under these conditions. Pretreatment with a low dose of captopril (2.3 × 103 M) 10 min
before the injection of ANG I caused a water intake of 2.8 ± 0.5 ml
(n = 10), whereas a high dose of
captopril (4.6 × 101
M) suppressed the dipsogenic response of ANG I entirely
(n = 11). These data provide direct
functional evidence for an SFO-intrinsic renin-angiotensin system (RAS)
and underline the importance of the SFO as a central nervous interface
connecting the peripheral with the central RAS.
captopril; losartan; thirst; drinking; osmoregulation; electrophysiology; renin-angiotensin system
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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THE EXISTENCE OF A BRAIN renin-angiotensin system (RAS) in addition to the peripheral or hormonal RAS has been firmly established in recent years (22, 45). All components of the peripheral RAS, which regulates plasma concentration of ANG II and thus affects blood pressure and fluid and electrolyte balance, have also been found in the brain, most notably in the subfornical organ (SFO), a brain region that regulates cardiovascular and body fluid homeostasis. The SFO is a sensory circumventricular organ (CVO) of the lamina terminalis and as such is devoid of a functional blood-brain barrier (BBB). Stimulation of the SFO either electrically or by circulating ANG II causes an increase in water (38) and salt intake (42), an elevated blood pressure (15), and a release of vasopressin (11). In the SFO of rats, some of the highest concentrations of angiotensinogen (21), reninlike activity (26), angiotensin-converting enzyme (ACE) (3, 30, 34), the peptide ANG II, (19) and ANG II receptors of the AT1-type have been found (2, 39, 45). This suggests that the SFO is a central nervous target for circulating ANG II but also the site of local ANG II production by the action of SFO-intrinsic renin and ACE, which cleave angiotensinogen and ANG I to the biologically active ANG II.
A substantial literature exists suggesting that high concentration of ACE in the SFO might be responsible for the "phenomenon of paradoxical enhancement of drinking" (17) by peripherally applied ACE inhibitors (16). While peripheral injection of a low dose of ACE inhibitors prevents formation of biologically active ANG II in the blood and thus the SFO-mediated dipsogenic effect of circulating ANG II acting on the SFO, it was suggested that the increased ANG I under these conditions is cleaved to the active ANG II within the SFO by unblocked ACE (17, 25, 33, 44). In contrast, application of high doses of ACE inhibitors, which are able to block peripheral as well as brain intrinsic ACE, abolishes the captopril-induced water intake (6, 16). Although these in vivo studies point to the SFO as a likely central nervous target in which the conversion of circulating ANG I to the biologically active ANG II might take place, functional evidence for such a mechanism is still lacking.
The aim of this study was to investigate, in an in vitro slice preparation, whether ANG I is able to alter the electrical activity of rat SFO neurons and compare its effect with the frequency, time course, and dose dependence of the excitatory effect of ANG II on the same neurons. The pharmacology of the ANG I- and ANG II-induced excitations was investigated by coapplication of the ACE inhibitor captopril and the AT1-receptor antagonist losartan. In vivo we investigated the time course of the ANG I- and ANG II-induced drinking, as an example of an SFO-mediated effect of ANG II, and compared it with the water intake induced by low and high doses of captopril. A detailed knowledge of ANG I-induced effects on SFO neurons is not just of relevance for understanding the mechanisms of action of ACE inhibitors and for certain forms of hypertension, which are dependent on the brain RAS (45, 46), but also for understanding the relative contribution of ANG I in SFO-mediated effects under physiological conditions, because plasma concentrations of ANG I are reported to be three- to fivefold higher than physiological ANG II levels (14).
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MATERIALS AND METHODS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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The materials and methods were, with minor modifications, the same as previously described (31, 37). Briefly, adult male Wistar rats (180-270 g) were decapitated, and their brains were quickly removed and superfused with ice-cold artificial cerebrospinal fluid (aCSF) of the following composition (in mM): 124 NaCl, 5 KCl, 1.2 NaH2PO4, 1.3 MgSO4, 1.2 CaCl2, 26 NaHCO3, 10 glucose, pH 7.4, equilibrated with 95% O2 and 5% CO2, 290 mosmol/kg. The brain was trimmed to a square block containing the entire hypothalamus, from which a coronal section was cut by hand at the level of the anterior commissure. A slice of the body of the fornix, containing the entire SFO, was cut by hand and was preincubated in aCSF at 35°C for 1 h. Slices were transferred to the recording chamber and fixed to the bottom of the chamber with a small metal weight. The gold-plated recording chamber was made from solid brass and, when perfused with aCSF, contained a fluid volume of ~0.7 ml. The chamber was constantly perfused with aCSF at a rate of 1.6 ml/min. aCSF entering the recording chamber was prewarmed to the same temperature as the solution already present in the chamber. The temperature was kept constant at 37°C by means of a Peltier element. Extracellular recordings were made from SFO neurons using glass-coated platinum-iridium electrodes. The SFO could easily be identified by its protrusion from the fornix and by the lateral blood vessels lining the organ on both sides. ANG I and ANG II (Sigma, Deisenhofen, Germany) were added to the aCSF shortly before the application. After a stable recording from a single neuron had been established, its responsiveness was tested in random order by switching to a perfusion solution containing the drug under consideration. The recorded action potentials were amplified and displayed on a storage oscilloscope (Gould, Germany) and were, after passing through a window discriminator (World Precision Instruments), analyzed with custom-made software (Spike2 from Cambridge Electronic Design) on a personal computer.
Normally 10 ml of aCSF containing the drug were superfused per
stimulus, except for experiments on the inhibition of converting enzymes, in which 20 ml were used. The concentrations of ANG II (108-107
M) were chosen in accordance with previous experiments (37) that showed
that this concentration induced clearly visible responses with minimum
desensitization. Captopril (Sigma) was used at concentrations between
105 and
104 M, and losartan (gift
from Merck Sharp and Dohme) was applied at a concentration of
105 M. From the
continuously recorded ratemeter counts, the average discharge rate of
each neuron was evaluated for 60 s before the stimulus. This value
(referred to as "control") was subtracted from all subsequent
changes in firing rate, and the results were expressed as the
percentage change of control. The latency of a response comprised the
time between the occurrence of the drug in the recording chamber and
the start of the excitatory or inhibitory responses. If the averaged
change of discharge rate during the entire response time was reversibly
larger than ±20%, the neuron was considered sensitive to the
applied substance; if no obvious change in the spontaneous activity was
observed, the firing rate was averaged over a 500-s period after the
drug entered the recording chamber and compared with the control
values. Mean values in the text are ±SE.
Water intake was investigated in male Wistar rats (180-200 g)
using an automated system (Omnitech, Columbus, OH) that monitored locomotor activity, food, and water intake once every minute and stored
data directly on a computer using Integra software. Animals were
adapted to the cages (40 × 40 cm) for at least 12 h before the
experiment started, and their food and water intake as well as their
locomotor activity were continuously recorded. The food consisted of
ground rat chow, which allowed precise measurement of the food consumed
by preventing the animals from removing pellets from the food container
that rested on an electric balance. Water bottles were fixed upside
down on a holder that was mounted on an electric balance (sensitivity
0.1 g) and were connected via tubing to a drinking spout. Access to
food was blocked 1 h before the experiment started to avoid
interference with prandial drinking. All drugs were dissolved in
sterile saline and were injected subcutaneously (200 µl/rat) with
small syringes (Omnican, 29 gauge, 0.5 ml, Braun, Melsungen, Germany)
at the end of the activity phase (9-10 AM). ANG II was injected in
a known effective concentration [0.20 mg/kg; i.e., 2 × 104 M per rat (4)],
and the same concentration was used for ANG I (0.26 mg/kg i.e., 2 × 104 M per rat). The
ACE inhibitor captopril was subcutaneously applied in a low dose (0.5 mg/kg; i.e., 2.3 × 103 M per rat) and a high
dose (100 mg/kg; i.e., 4.6 × 101 M per rat) 10 min
before the injection of ANG I or saline. In experiments investigating
the effect of AT1-receptor
blockade, losartan (30 mg/kg; i.e., 6.5 × 102 M per rat) was
subcutaneously injected 30 min before the injection of ANG II, ANG I,
or saline.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Electrophysiological study. Comparing
the effects of ANG II and ANG I on neurons of the SFO, only those
neurons that showed a stable spontaneous activity and could be tested
for their responsiveness to ANG II as well as ANG I were included in
these results. Furthermore, the effects of both agents had to be
reversible to be considered for quantitative evaluation. The mean
spontaneous frequency of all neurons tested was 5.5 ± 0.7 spikes/s;
the mean signal-to-noise ratio was 14:1 with an averaged spike
amplitude of 278 ± 38 µV (n = 25). ANG II excited 76% of all neurons, and the remaining were
unresponsive, i.e., not a single neuron was inhibited. All but one of
the ANG II-sensitive neurons could also be activated by ANG I (Table
1). One neuron was excited by ANG II but
missed the 20% mean response increase criteria for an excitatory
effect, with only 18% increase in spontaneous activity after
application of ANG I. The excitatory effects of equimolar ANG I and ANG
II applications on SFO neurons (n = 15) revealed no statistical difference (paired
t-test) in the mean response amplitude
(2.6 ± 0.3 Hz for ANG II and 2.3 ± 0.3 Hz for ANG I), the
response duration (551 ± 63 s for ANG II and 634 ± 104 s for ANG I), or the latency of the onset of the response after the
superfused peptides reached the recording chamber (56 ± 6 s for ANG II and 64 ± 9 s for ANG I).
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Figure 1 shows the continuous activity of a
representative rat SFO neuron that was superfused consecutively with
ANG II and ANG I. The spontaneous activity increased rapidly in
response to 107 M ANG II
and decreased asymptotically to the baseline frequency after the
application. Superfusion of the same neurons with ANG I
(107 M) caused a very
similar increase in activity, with a rapid onset and a slightly slower
reversibility of the response. Original spike recordings of the
investigated neuron during superfusion with ANG II, after recovery and
during superfusion with ANG I, are shown in Fig. 1,
top. The dose dependence of the
excitatory effects of ANG I and ANG II were investigated in a range of
1011-107
M on eight neurons each by applying increasing doses of the peptides as
shown in Fig 2. A continuous ratemeter
recording of a rat SFO neuron that was dose dependently excited by ANG
II is shown in Fig. 2A. The effective
threshold concentration was
109 M. Higher
concentrations of ANG II caused stronger and longer-lasting increases
in firing rate. Figure 2B shows a
representative rat SFO neuron that was activated by ANG I with a
threshold concentration of
109 M. A similar dose
dependence of the excitatory effect caused by increasing concentrations
of ANG I was found in each of the eight neurons investigated. The
insets in Fig. 2,
A and
B, show the mean excitatory responses
after ANG II and ANG I applications in increasing doses and underline
the similarities of the effects caused by the two peptides.
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The question whether the excitatory response observed after superfusion
with ANG I was due to a direct effect of ANG I or to a local conversion
of ANG I to ANG II within the SFO was addressed by investigating the
responsiveness of neurons before and after inhibition of endogenous ACE
by captopril. The rationale for these experiments was the assumption
that endogenous ACE was present and still active under our in vitro
slice conditions and that these enzymes were able to convert rapidly
the exogenously applied ANG I to ANG II, which is finally responsible
for the excitatory effect. Superfusion of captopril in a concentration
of 104 M was able to block
the excitatory effect of coapplied ANG I in all cases
(n = 8), whereas a 10-fold lower
concentration of captopril was only effective in two of four ANG
I-responsive neurons. Captopril alone caused either no or a small
inhibitory effect on the spontaneous activity of the tested neurons
(average firing rate during captopril was 
6 ± 2% of
baseline activity; range 0 to 19%,
n = 12). The neuron in Fig.
3 responded to ANG II
(107 M) and ANG I
(107 M) with a
quantitatively similar excitation. Superfusion with captopril
(104 M) completely
suppressed the effect of ANG I
(107 M), despite a
prolonged application time, but had no effect on ANG II-induced
excitation. The captopril-induced inhibition of the ACE was only slowly
reversible. Full reversibility of the ANG I-induced excitation after
captopril application (104
M) was observed only after 30-60 min
(n = 5). The question of whether the
ANG I-induced and captopril-sensitive excitations were due to the
excitatory effect of locally produced ANG II on specific receptors in
the SFO was investigated by using the selective AT1-receptor antagonist losartan.
Coapplication of losartan
(105 M) totally abolished
the ANG I-induced excitatory effect of all ANG I-responsive neurons
tested (n = 6). Application of
losartan (105 M) alone
reduced the spontaneous activity in 6 of 13 neurons. The average
inhibitory effect was 26 ± 2% in the responsive neurons. The continuous registration of the discharge rate of an SFO neuron that
was excited by 107 M ANG II
and 107 M ANG I is
illustrated in Fig. 4. In this recording,
superfusion of 105 M
losartan reduced the spontaneous activity from 11 to 9 impulses/s and blocked the excitatory effect of coapplied
ANG I as well as the excitatory effect of an equimolar concentration of
ANG II superfused after the losartan application.
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Drinking experiments. Subcutaneous
injection of ANG I (2 × 104 M; 200 µl) caused 13 of 16 rats to drink water within 1 h after the application. Similarly,
16 of 20 rats drank in response to an equimolar dose of ANG
II, whereas only 2 of 21 rats receiving saline consumed water
within this time period. The average amount of water consumed by all
rats, including those that did not drink at all, was 2.4 ± 0.5 ml 1 h after ANG I injection and thus significantly different from
the controls, which drank 0.07 ± 0.06 ml, but did not differ from
the effect of an equimolar dose of ANG II (2.7 ± 0.4 ml). Figure 5 shows the
average water intake of rats after subcutaneous application of ANG I
and ANG II in the absence and in the presence of losartan and different
doses of captopril. Injection of losartan (6.5 × 103 M), which did
not affect water intake itself, totally blocked water intake after
application of ANG II and ANG I. Injection of the low dose of captopril
(2.3 × 103
M) caused an average water intake of 1.0 ± 0.2 ml, whereas one of the treated rats consumed water within the first hour after application of the high dose of captopril (4.6 × 101 M). In the presence of
the high dose of captopril, the dipsogenic dose of ANG I had no effect
on water intake, whereas the same dose of ANG I combined with the low
dose of captopril resulted in drinking responses that were not
significantly different from the effect of ANG I alone.
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Because of the computer-aided registration of water intake once every
minute, precise information about drinking pattern and response times
after drug treatments could be obtained (Fig.
6). The individual water intakes after the
low dose of captopril showed a wide range in onset and magnitude of the
drinking response. Of the 31 investigated rats, 23 (74%) started
drinking within the recorded time period of 2 h. On average, the
response started 15 min after the injection and reached its maximum
within 60 min. In contrast, rats treated with the high dose of
captopril started to drink water with a 1 h delayed onset compared with
those injected with the low dose. Only 6 (43%) of 14 rats consumed
water within 120 min. Application of ANG I 10 min after the low dose of
captopril evoked a strong drinking response with a very rapid onset (10 min). As shown in Fig. 6, 9 of 10 rats consumed fluid volumes between
1.7 and 4.8 ml within the 20 min time period after subcutaneous injection of ANG I. Application of ANG I after a high dose of captopril
evoked only little water intake within 1 h, and the generally late
onset and the small volume consumed was quite similar to the effect of
the high dose of captopril alone.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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This study shows that ANG I, similar to ANG II, caused exclusively excitatory effects on neurons recorded from an in vitro slice preparation of the rat SFO. The time course and dose dependence of activation was very similar for both peptides. The ACE inhibitor captopril blocked the excitatory effect of ANG I but not ANG II, whereas the AT1-receptor antagonist losartan blocked responses to both peptides, suggesting SFO-intrinsic conversion of ANG I to ANG II to be the reason for the ANG I-induced neuronal activation. Equimolar doses of peripherally applied ANG I and ANG II caused similar dipsogenic responses in vivo, which could both be blocked by losartan. Application of a low dose of captopril, either alone or in combination with ANG I, stimulated water intake, whereas a high dose of captopril delayed the onset and reduced the amount of fluid intake significantly.
Our drinking experiments confirm previous data showing that subcutaneous application of equimolar doses of ANG I and ANG II resulted in similar water intake in rats (33). Slightly higher drinking responses after intravenous ANG I were observed in goats (5). Intracarotid in contrast to intravenous injections of ANG I in dogs caused a lower drinking response than ANG II (12). Fitzsimons (12) suggested that the lower drinking response to intracarotic in contrast to intravenously applied ANG I was a result of the reduced access of ANG I to peripheral ACE, meaning less ANG I is converted to ANG II, which acts on brain structures to stimulate water intake. Although our data, showing that coapplication of ANG I with losartan completely inhibits ANG I-induced drinking, are principally in line with such an interpretation, peripheral conversion of ANG I to ANG II cannot be the only reason for ANG I-induced water intake, because inhibition of the peripheral RAS by various ACE inhibitors did not abolish, but rather stimulated, water intake (6, 16, 33).
A dipsogenic effect of low doses of peripherally applied ACE inhibitors
is now a well-established phenomenon, although the underlying mechanism
is still not sufficiently known (6, 16, 17). Low doses of
subcutaneously applied captopril or other ACE inhibitors effectively
blocked the formation of ANG II in plasma (7, 14) and reduced ANG
II-dependent elevations in blood pressure. It was suggested early that
ANG I, which accumulated in the plasma after peripheral application of
ACE inhibitors, acts somewhere in the brain, where it is converted to
the biologically active ANG II (16, 32, 45). Although most of the ANG
I-induced drinking is presumably due to its rapid conversion to ANG II
in the periphery, the evidence remained circumstantial as to which structures in the brain are responsible for the conversion of ANG I to
the active ANG II in captopril-treated animals. Structures within the BBB are presumably not accessible to ANG I and ANG II
(35). Thunhorst et al. (43) showed that lesion of the SFO abolished captopril-induced water intake and thus provided convincing evidence that this paradoxical drinking is mediated via the SFO. Furthermore, it has been shown that direct injection of a high dose of
captopril into the SFO abolished drinking induced by peripheral injection of captopril, which further points to the SFO as the site of
conversion of ANG I to ANG II by an ACE-dependent mechanism (44). These
data may also serve as evidence for the central site of action of high
doses of peripherally applied captopril, which produced antidipsogenic
effects in the current and previous studies (6, 16, 33). Similar
conclusions were recently drawn by McKinley et al. (25), showing that a
low dose of subcutaneously applied captopril increased
c-fos expression in the SFO and
organum vasculosum of the lamina terminalis (OVLT), and this expression could be blocked by a high dose of captopril and by an
AT1-receptor blocker. On the other
hand, captopril and other ACE inhibitors can easily reach and inhibit
ACE in the SFO and other CVOs and it has been shown that an acute oral
administration of perindopril blocked binding of radiolabeled ACE
inhibitors in the SFO and OVLT but not in brain regions inside the BBB
(3). These data raise the question why peripherally applied ACE
inhibitors on the one hand can easily inhibit binding of ACE inhibitors
in these CVOs, whereas on the other hand this inhibition does not seem to be sufficient to block the conversion of ANG I to ANG II as effectively as in the periphery. To explain the concentration-dependent effectiveness and ineffectiveness of subcutaneously applied ACE inhibitors on SFO-mediated water intake, it was proposed that the
concentration or the activity of ACE in the SFO is too high to be
blocked completely by the low dose of captopril and that unblocked,
residual ACE is responsible for the local conversion of blood-borne ANG
I to the locally active ANG II (16). This model implies that, within
the SFO itself, a very effective captopril-sensitive conversion from
ANG I to ANG II must occur, and ANG II then activates SFO neurons via
AT1 receptors. The presence of
extremely high concentrations of ACE in the SFO (3) and our data
showing that ANG I and ANG II activate the same SFO neurons with very
similar potency and time course strongly support this hypothesis. Our data, showing that a high dose of captopril completely abolished water
intake for >1 h, are in line with the observation that much more ACE
inhibitor is necessary to block the central than the peripheral RAS
(45). The fact that some drinking recovered under these conditions 90 min later is presumably due to progressive accumulation of ANG I in the
plasma combined with slowly declining captopril effects. To unravel the
apparent paradox that peripherally applied ACE inhibitors effectively
block binding of ACE inhibitors in CVOs (3) but fail to inhibit ACE
activity, direct quantitative receptor-binding studies with
radiolabeled ANG I and concomitant application of low and high doses of
captopril and other ACE inhibitors are needed to evaluate the
effectiveness of ACE inhibition in the SFO and compare it with the time
courses of their dipsogenic and antidipsogenic effects under these
conditions. The questions of whether the SFO is necessary for ANG
I-induced drinking triggered by low doses of captopril and whether high
doses of captopril block drinking by acting primarily on the same
central structure are difficult to answer in in vivo studies because of
interference of the peripheral RAS and other factors, such as blood
pressure, with the drinking response. The use of an in vitro slice
preparation of the SFO eliminated such problems and allowed direct
quantitative comparison of the effects evoked by ANG I and ANG II. The
fact that the ANG I-induced neuronal excitations are blocked by
captopril as well as losartan in vitro provides the first direct
functional evidence for an effective and captopril-sensitive conversion
of ANG I to the biologically active ANG II within the SFO. The
possibility that some of the electrical responses observed after ANG I
might be due to a local conversion of ANG I to ANG-(1
7) by
ACE-independent mechanisms (9) is less likely to occur in the SFO,
because all ANG I-induced neuronal responses could be blocked by
captopril and losartan.
Electrical activation of the majority of SFO neurons by ANG II has been
shown in vivo and in vitro and is regarded as the cellular basis for
the SFO-mediated dipsogenic effect of circulating ANG II (8, 13, 18,
29, 36). Doubts were raised by some authors about the physiological
relevance of SFO-mediated ANG II-induced drinking (16, 28, 40), because
plasma concentrations of ANG II in vivo are between
1010 and
1011 M (23, 27)
and thus ~10 times lower than the threshold concentration for
increasing water intake and neuronal activity in vitro (this study and
Ref. 24). Our data, showing that ANG I is as effective as ANG II in
activating SFO neurons, and the fact that ANG I-levels in plasma are
three- to fivefold higher than ANG II levels (14, 27) and increase in
parallel under physiological conditions, suggest that ANG I and
ANG II concentrations should be summed up to evaluate
effective plasma levels of angiotensins acting on the SFO in vivo. A
single oral application of ramipril was reported to increase plasma
levels of ANG I from 0.8 to 2.6 × 1010 M, thus almost
reaching the threshold for ANG II-induced activation of SFO
neurons in vitro, and this underlines the relative importance of plasma
ANG I for SFO-mediated functions (14). Furthermore, our recent data
suggest that other blood-borne hormones, such as calcitonin and amylin,
that are released after food intake may also contribute to SFO-mediated
drinking because they activate largely the same SFO neurons as ANG I
and ANG II (36).
The fact that losartan reduced the spontaneous activity in 46% of all neurons suggests that a strong local angiotensinergic network is still active under our in vitro conditions. Histological evidence for ANG II as a neurotransmitter substance in the SFO has been presented by Lind et al. (19, 20). These authors located angiotensinergic fibers that originate mainly from hypothalamic regions and terminate in the center of the SFO and angiotensinergic somata, which are located in the rim of the SFO and give rise to angiotensinergic fibers terminating on neurons in the median preoptic nucleus (MnPO) and other hypothalamic regions (10, 41). Our electrophysiological data are compatible with the idea that ANG II-immunoreactive neurons tonically activate SFO neurons by releasing ANG II from local synapses. Together with histological evidence showing the presence of all components of the RAS in the SFO (21, 30, 34, 45), our data suggest that locally produced and released ANG II plays an important role in water intake and other SFO-mediated functions.
Perspectives
Although the function of a local angiotensinergic network in the SFO is so far unknown, it is most likely that in vivo blood-borne ANG II activates angiotensinergic neurons in the SFO, either directly or indirectly, which then could activate other neurons within the SFO by releasing ANG II from their synapses. This hypothesis is supported by data showing that the SFO-mediated water intake and vasopressin release is due to the activation of angiotensinergic neurons within the SFO that activate neurons in the MnPO or paraventricular nucleus by releasing ANG II (1, 18, 41).On the basis of these data, it is tempting to speculate that the SFO is not only an important interface connecting the peripheral with the central RAS, but the SFO-intrinsic RAS may further function as an amplifier for blood-borne ANG I and ANG II by spreading the effect of blood-borne angiotensins synaptically to nearby ANG II-sensitive neurons.
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ACKNOWLEDGEMENTS |
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Present address of H. A. Schmid: Novartis Pharma AG, Metabolic and Cardiovascular Diseases, WKL-125.08.02, CH-4002 Basel, Switzerland.
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FOOTNOTES |
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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 and other correspondence: M. Rauch, Max-Planck-Institut für physiol. und klin. Forschung, W. G. Kerckhoff-Institut, Parkstrasse 1, 61231 Bad Nauheim, Germany (E-mail: m.rauch{at}kerckhoff.mpg.de).
Received 23 October 1998; accepted in final form 15 February 1999.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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