Departments of 1 Pediatrics (Section of Respiratory Medicine) and 2 Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06510
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
To examine the effects of chronic cyclic
hypoxia on neuronal excitability and function in mice, we exposed mice
to cyclic hypoxia for 8 h daily (9 cycles/h) for ~2 wk (starting
at 2-3 days of age) and examined the properties of freshly
dissociated hippocampal neurons obtained from slices. Compared with
control (Con) hippocampal CA1 neurons, exposed neurons (CYC) had
similar resting membrane potentials (Vm) and
action potentials (AP). CYC neurons, however, had a lower rheobase than
Con neurons. There was also an upregulation of the Na+
current density (333 ± 84 pA/pF, n = 18) in CYC
compared with that of Con neurons (193 ± 20 pA/pF,
n = 27, P < 0.03). Na+
channel characteristics were significantly altered by hypoxia. For
example, the steady-state inactivation curve was significantly more
positive in CYC than in Con (60 ± 6 mV, n = 8, for CYC and
71 ± 3 mV, n = 14, for Con,
P < 0.04). The time constant for deactivation
(
d) was much shorter in CYC than in Con (at
100 mV,
d=0.83 ± 0.23 ms in CYC neurons and 2.29 ± 0.38 ms in Con neurons, P = 0.004). We conclude that
the increased neuronal excitability in mice neurons treated with cyclic
hypoxia is due to alterations in Na+ channel
characteristics and/or Na+ channel expression. We
hypothesize from these and previous data from our laboratory (Gu XQ and
Haddad GG. J Appl Physiol 91: 1245-1250, 2001) that this
increased excitability is a reflection of an enhanced central nervous
system maturation when exposed to low O2 conditions in
early postnatal life.
Na+ channels; excitability; O2 deprivation
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
A NUMBER OF CONDITIONS and disease states are associated with tissue hypoxia that is intermittent in nature. For example, one of the important aspects of obstructive sleep apnea/hypoventilation syndrome (OSAHS) in both children and adults is a cyclic hypoxia that results from upper airway obstruction. This intermittent hypoxia occurs in OSAHS throughout the night and can repeat itself numerous times in a single night (4). In sickle cell disease, hypoxia and ischemia are also intermittent, and these may lead to major pathophysiological conditions. In cardiac ischemia, intermittent hypoxia and hypoperfusion may occur before total occlusion takes place.
The effects of cyclic hypoxia on neural properties are not clear. Although there has been a substantial literature on cellular and molecular mechanisms of adaptation to low O2 conditions (see reviews, Refs. 3, 10, 14, 24) and the mechanisms of injury (see reviews, Refs. 2, 6, 8, 9, 13, 20, 22), there are many unanswered questions, especially in relation to the role of hypoxia. For example, it is controversial as to whether a previous exposure to hypoxia renders neurons more or less susceptible to subsequent hypoxic stress (17). Because repetitive hypoxia can be severe and can occur over prolonged periods of time, it may produce central neuronal sublethal damage. It is possible, however, that central neurons may adapt during exposure, suffer less injury, and survive. Because the impact of cyclic hypoxia may depend on neuronal development and the magnitude and extent of exposure to hypoxia, we have studied, in this work, mice in early life and exposed them to a low O2 paradigm for the first 2 wk of life. We used electrophysiological methods to determine the cellular mechanisms that were altered in response to cyclic hypoxia in young mice. We focused our work on the hippocampus because 1) this has been a well-studied region and 2) we have done some of our previous work on hippocampal neurons (7).
![]() |
METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Cyclic Hypoxia
A computer-controlled chamber was developed by us for the induction and maintenance of cyclic hypoxia. This chamber (volume
|
Preparation of CA1 Cells
Mice at 12-16 days of age were used and killed after inhalation of halothane, and their hippocampi were removed and sliced into 7-10 transverse sections of 400 µm in thickness. The slices were immediately transferred to a container with 2.5 ml of fresh, oxygenated, and slightly stirred HEPES buffer at room temperature. After 30 min of exposure to trypsin (0.08%) and 20 min of protease (0.05%) digestion, the slices were washed several times with HEPES buffer and left in oxygenated solution. The CA1 region was then dissected out and triturated in a small volume (0.25 ml) of HEPES buffer. The Yale Animal Care and Use Committee has approved these studies.Electrophysiological Recording and Solutions
Electrodes for whole cell recording were pulled on a Flaming/Brown micropipette puller (model P-87, Sutter Instrument) from filamented borosilicate capillary glass (1.2-mm OD, 0.69-mm ID; World Precision Instruments or Warner Instrument). The electrodes were fire-polished, and resistances were 2-5 MFor the current-clamp experiments, the external HEPES solution bathing neurons contained (in mM) 130 NaCl, 3 KCl, 1 CaCl2, 1 MgCl2, 10 HEPES, and 10 glucose, and pH was adjusted to 7.4 with NaOH. The pipette solution contained (in mM) 138 KCl, 0.2 CaCl2, 1 MgCl2, 10 HEPES (Na+ salt), and 10 EGTA, and pH was adjusted to 7.4 with Tris. The external solutions for the voltage-clamp experiments contained similar reagents as in the current-clamp experiments, except for adding 10 mM TEA chloride, 5 mM 4-aminopyridine (4-AP), and 0.1 mM CdCl2 and reducing NaCl from 130 to about 117 mM. The internal pipette solution for the voltage-clamp experiments was also similar to the internal solution for the current-clamp experiments, except for the use of either CsF or CsCl instead of KCl [we had previously shown that there was no difference in whole cell Na+ current recorded when CsF or CsCl was used (16)]. The HEPES-buffered solutions for the enzymatic preparation and trituration of the CA1 cells contained (in mM) 125 NaCl, 3 KCl, 1.2 MgSO4, 1.25 NaH2PO4, 30 HEPES, and 10 glucose. Osmolarity of all solutions was adjusted to 290 mosM. All recordings were performed at room temperature (22-24°C). All experiments were performed on 8-27 cells for either the control or cyclic hypoxic group. One-tailed Student's t-test was performed for comparisons. Values in the text are given as means ± SE. All chemicals were purchased from Sigma.
Recording Criteria
These criteria have been previously used, as detailed in our previous publications.Morphological criteria. CA1 cells were used if they had a smooth surface, a three-dimensional contour, and were pyramidal in shape. Similar criteria have been used by us (5, 7) and others (11) on freshly triturated neurons. The CA1 cells studied were obtained from 12- to 14-day-old mice.
Electrophysiological criteria.
1) Neurons were considered for recording if the seal
resistance was >5 G. 2) Only neurons with a holding
current of <0.1 nA (command potential
100 mV) were used in the
study. 3) Series resistance was <10 M
in neurons
studied. The series resistances were compensated at 90% level with the
Axopatch 1C amplifier (Axon Instruments). Under these conditions, the
error caused by uncompensated series resistances was <1.7 mV. To
obtain adequate voltage clamp and minimize the space-clamp problem,
only small neurons with short processes were used in
Na+ current measurements. In addition, we only used cells
with current-voltage (I-V) curves that were
smoothly graded over the voltage range of activation (approximately
50 to
10 mV), as we have done in the past (5, 7,
16).
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Body Weight, Brain Weight, and Hippocampal Weight
We measured body, brain, and hippocampus weight (Table 1) to follow the effect of cyclic hypoxia on development. Although body weight was significantly lower in mice exposed to cyclic hypoxia, neither brain weight, hippocampal weight, nor the ratios of brain/body weight, hippocampus/body weight, and hippocampus/brain weight were significantly different.
|
Neuronal properties and membrane excitability.
All control (Con) (n = 7) and cyclic hypoxia-exposed
(CYC) (n = 9) CA1 neurons fired APs when they were held
at 75 mV and given depolarizing currents in the current-clamp mode.
No Con and CYC neurons, however, fired spontaneous APs. CYC CA1 neurons had a similar Vm to those of Con neurons
(
28 ± 3, n = 8 for CYC vs.
29 ± 3, n = 12 for Con), but they had a significantly lower Rm (568 ± 86 M
, n = 16 for CYC vs. 875 ± 150 M
, n = 15 for Con neurons, P = 0.04). To determine the excitability of
Con and CYC neurons, we systematically determined the rheobase.
Stepping all neurons from the same Vm (
75 mV)
in the current-clamp mode, the amount of current required to generate
one AP was 54 ± 24 pA, n = 8, in CYC but more
than double that in Con neurons (117 ± 18 pA, n = 18, P = 0.03; Fig. 2)
(Table 2).
|
|
Na+ Current Magnitude
Because 1) neuronal excitability is largely dependent on the characteristics and magnitude of Na+ channels, and 2) CYC and Con neurons showed differences in excitability even when neurons were stepped from the same Vm, we examined the Na+ channel properties in both groups of neurons. Under voltage clamp, steps from a holding potential of
|
Na+ current characteristics. We next examined the characteristics of the Na+ current to determine whether there are other differences between CYC and Con neurons that could be important in determining excitability.
Activation.
With CA1 neurons held at 130 mV, depolarizing voltages were given
from
70 to +80 mV with 10-mV increments. For both groups of neurons,
the threshold for Na+ channel activation was
60 mV, and
the Na+ current reached a peak at
20 mV. The midpoint for
the voltage-conductance relation (g/gmax) was
about the same in both groups (
35 ± 3 mV, n = 10, for Con neurons, and
41 ± 3 mV, n = 9, for
CYC neurons). Similarly, the activation slope factors, 3.1 ± 0.6 for Con and 3.3 ± 0.5 for CYC, were not different in both groups
of neurons (Fig. 4). Although there was
no difference between the two groups either in the midpoint of
voltage-conductance curves or slope factors, at some particular
voltages,
30 mV, for example,
g/gmax was statistically different
(P = 0.02) between the Con (0.56 ± 0.10, n = 10) and CYC (0.86 ± 0.06, n = 9) groups.
|
Steady-state inactivation.
Steady-state inactivation of the Na+ current was studied
using a prepulse potential from 130 to
20 mV and then stepping
Vm to
20 mV (Fig. 5, A and
B). Compared with Con, the
steady-state inactivation curve of CYC was significantly shifted in a
depolarizing direction by more than 10 mV (Fig. 5C). The
h
1/2 was
60 ± 6 mV (slope factor = 5.4 ± 0.3, n = 8) and
71 ± 3 mV (slope factor = 6.6 ± 0.5, n = 14) for CYC and Con neurons,
respectively (P < 0.04).
|
Recovery from inactivation.
To examine the recovery from inactivation, we used a two-pulse protocol
with increased interval durations between the two pulses. Although the
time constant for recovery (h) was not much different
for both groups (2.3 ± 0.6 ms, n = 9, and
2.8 ± 0.7 ms, n = 11, for CYC and Con neurons,
P = 0.29), there were significant differences if two
pulses were below about 5 ms apart. For example, when the time
(t) between the two pulses was 2.6 ms, the ratio of the peak
of the second current to that of the first
(Ipeak2/Ipeak1) for Con
was 0.16 ± 0.008 and 0.53 ± 0.09 for CYC (P = 0.002). At t = 5.12 ms, the ratio
Ipeak2/Ipeak1 for Con was
0.52 ± 0.08 and 0.74 ± 0.07 for CYC (P = 0.02) (Fig. 6C).
|
Deactivation characteristics.
We also examined the deactivation properties, i.e., the transition from
the open to the resting closed state for both groups of neurons. We
held CA1 neurons at 100 mV, depolarized them for 1 ms to
10 mV, and
repolarized to
100 mV (Fig. 7, A and
B). Current traces in response
to this protocol are shown separately in Fig. 7, A (Con) and
B (CYC), and superimposed in Fig. 7C. The normalized current traces for both Con and CYC are superimposed in Fig.
7D. Notice that Fig. 7C represents the actual
recordings overlaid, whereas Fig. 7D represents scaled
recordings. The averaged time constant for deactivation,
d at
100 mV, was significantly smaller for CYC neurons
(0.8 ± 0.2 ms, n = 9) than that for Con neurons
(2.3 ± 0.4 ms, n = 14, P = 0.004)
(Fig. 7E). At
70 mV, the results showed a similar
difference (1.9 ± 0.6 ms, n = 9, for CYC and
3.1 ± 0.5 ms, n = 14, for Con) (Table 2).
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
In this study, we have found that CA1 neurons are more excitable in mice exposed to cyclic hypoxia. Our results also suggest mechanisms that can explain the differences in excitability between CYC and Con CA1 neurons.
In spite of the fact that CYC neurons have a lower input resistance,
they still exhibited a lower rheobase for excitability. There are
several reasons for the increase in excitability of CYC neurons. First,
the Na+ current density was significantly (~73%) higher
in the exposed neurons than in control counterparts. Second, the
steady-state inactivation curve was significantly shifted in the
depolarizing direction, i.e., there were more channels available for
recruitment in CYC than that in Con neurons, especially at
physiological voltages, such as between 40 and
80 mV. Third, the
probability of available channels after depolarizations was higher in
CYC than Con neurons, especially in the immediate period after
stimulation. Fourth, it is also possible that the faster deactivation
time constant of the Na+ channels in CYC neurons is a
factor that allows these neurons to be more readily recruitable than
control neurons.
Because our data were collected in the whole cell configuration and these results demonstrated that the whole cell Na+ current was larger in CYC than that in Con neurons, we cannot presently differentiate between the various reasons that led to the increased whole cell current. For example, the Na+ channel expression in the plasma membrane may be higher upon exposure to cyclic hypoxia. Alternative possibilities are that the single channel ionic conductance and the open probability of the channel may have been increased.
One additional interesting finding in this work was that the capacitance in CYC neurons was significantly larger than that in Con neurons. Although there could have been a bias in the selection of these cells, it is possible that cells in CYC mice were larger than their counterparts in Con mice. This may not be surprising because other investigators have shown that repeated hypoxia induces structural changes in hippocampal CA1 neurons, such as a reduction in the number of dendritic branching (21). Hence, it would be useful at a later stage to characterize the morphological changes in these neurons quantitatively because these alterations, such as reduction in the number of dendritic branching and increase in cell size, may have profound effects on cell-cell communications.
We have previously published data on the effect of cyclic hypoxia in
mice after about 4 wk of exposure. Our previous data showed that the
animals exposed to cyclic hypoxia had a much lower excitability than
those that were not exposed; their Na+ currents were also
downregulated compared with control animals (7). Our data
in this work would strongly suggest that the maturation of the in vivo
exposed neurons over the first month of life in mice is quite different
from that of the control, nonexposed neurons. Indeed, there are two
main ideas that we highlight in this work. First, the paradigm for the
hypoxia exposure in vivo is very important. Here, we observed that 2 wk
of the cyclic hypoxic stress induced a very different profile from the
same stress for 4 wk. Second, the maturation in early life of certain
membrane proteins, such as the Na+ channels, in terms of
properties and regulation/expression, depends on environmental
oxygenation and on the length of hypoxic exposure. Figure
8A shows that Na+
current, for example, increased by 3.5-fold from 2 to 4 wk of age in
Con, but by only <10% in CYC. The same maturational pattern was also
true for Na+ current density (Fig. 8A).
Furthermore, in almost every characteristic of Na+ channel
tested, CYC neurons had much smaller changes compared with normal
controls (Fig. 8, B-D). Hence, the changes
in Na+ channel characteristics were much less dramatic for
CYC treated neurons in the period between 2 and 4 wk, whereas the
maturational changes in Na+ channel characteristics of Con
neurons were substantial during the same period. In contrast, compare
this maturational change to that in the first 2 wk in cyclic hypoxia.
CYC neurons, as we observe in this work, were much more excitable than
Con neurons and had different properties of Na+ channels.
We hypothesize, therefore, from our previous data, as well as from the
data in this work, that cyclic hypoxia, in early life, induces two
stages of development. The first 2 wk seem to have an accelerated
maturation that enhances Na+ channel expression or
regulation and enhances excitability of neurons with cyclic hypoxia
(with Na+ current of 634 and 1,749 pA and current density
of 192 and 333 pA/pF for Con and CYC at 14 days). The second period (2 to 4 wk) would seem to induce a stabilization of these Na+
channel properties and neuronal excitability at a period when maturation of naive neurons is accelerated with cyclic hypoxia (with
Na+ current increasing by 2,633 pA for Con and decreasing
by 127 pA for CYC; the current density increased by 299 pA/pF for Con and only 23 pA/pF for CYC from 14 to 28 days)(7). The end
result would manifest itself in an enhanced excitability after 2 wk of cyclic hypoxia and a decreased excitability after 4 wk.
|
The changes in Con neurons reflect developmental changes of Na+ channels characteristics during normal maturation, as we and others have demonstrated in rodents (1, 12, 18, 23). In summary, neurons obtained from exposed mice seem to "mature" in vivo at a faster rate when exposed to cyclic hypoxia early in life in the first 2 wk. However, this maturation seems to halt and, by about 28 days, the phenotype of the exposed mice had lagged behind (Fig. 7).
One question that can be raised from our current work is whether the changes observed in mice after the first 2 wk of exposure reflect an adaptive strategy that could benefit the overall survival of these neurons and possibly the animal itself. Although it is difficult to be certain, it would be intriguing to speculate that the increase in Na+ channel and increase in excitability in the first few weeks of life in hypoxia is adaptive and beneficial to the organism. It has been shown that the enhancement of Na+ channels in early life (5) is critical in the process of synaptogenesis and the refinement of synaptic connections (19). Hence, it is possible that hypoxia in early life enhances synaptic connectivity. This, in turn, will enhance brain development and can counteract the potential hypoxia-induced metabolic depression and its negative impact on brain development.
Although hypoxia could have directly affected neuronal excitability (16), it could also have induced the neuronal changes indirectly via other means. For example, maternal stress could have affected growth and development of the litter pups. We do not believe, however, that this is the case from previous experiments done by Mortola et al. (15). In addition, similar experiments that we had done in our laboratory on rat pups showed that foster mothers raised in normoxia (the majority of the day) showed that the effect on body size was related to hypoxia rather than maternal effects (unpublished observations). Other factors that could have played a role include alterations in the hormonal milieu, extracellular pH, neurotransmitter release, or growth factors in the microenvironment of neurons.
In summary, we have shown that the younger mice exposed to cyclic hypoxia have an overall higher hippocampal neuronal excitability. This increased excitability can be explained by 1) an upregulation of the Na+ current and 2) alterations in the channel characteristics, including activation, steady-state inactivation, recovery from inactivation, and deactivation. This investigation documents the existence of an important functional link between cyclic hypoxia and the voltage-sensitive Na+ channel. The importance of this link is related to the idea that the changes in the Na+ channel documented in this work may be critical for the increased excitability of neurons and their metabolic demands in their early life.
![]() |
ACKNOWLEDGEMENTS |
---|
We thank Ralph Garcia, Aaron Hochberg, Hillary Sunamoto, and Mary Catherine Muenker for invaluable technical assistance.
![]() |
FOOTNOTES |
---|
This work was supported by National Institutes of Health Grants P01 HD-32573, R01 NS-35918, and R01 HL-66327.
Address for reprint requests and other correspondence: G. G. Haddad, Dept. of Pediatrics, Office of the Chairman, Albert Einstein College of Medicine, Kennedy Neuroscience Center, 1410 Pelham Parkway South, Bronx, NY 10461 (E-mail: ghaddad{at}aecom.yu.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
10.1152/ajpcell.00432.2002
Received 19 September 2002; accepted in final form 30 December 2002.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Arroyo, EJ,
Xu T,
Grinspan J,
Lambert S,
Levinson SR,
Brophy PJ,
Peles E,
and
Scherer SS.
Genetic dysmyelination alters the molecular architecture of the nodal region.
J Neurosci
22:
1726-1737,
2002
2.
Banasiak, KJ,
Xia Y,
and
Haddad GG.
Mechanisms underlying hypoxia-induced neuronal apoptosis.
Prog Neurobiol
62:
215-249,
2000[ISI][Medline].
3.
Barouki, R,
and
Morel Y.
Oxidative stress and gene expression.
Journal de la Société de Biologie
195:
377-382,
2001[Medline].
4.
Block, AJ,
Boysen PG,
Wynne JW,
and
Hunt LA.
Sleep apnea, hypopnea and oxygen desaturation in normal subjects. A strong male predominance.
N Engl J Med
300:
513-517,
1979[Abstract].
5.
Cummins, TR,
Xia Y,
and
Haddad GG.
Functional properties of rat and human neocortical voltage-sensitive sodium currents.
J Neurophysiol
71:
1052-1064,
1994[Abstract].
6.
Fung, ML.
Role of voltage-gated Na+ channels in hypoxia-induced neuronal injuries.
Clin Exp Pharmacol Physiol
27:
569-574,
2000[ISI][Medline].
7.
Gu, XQ,
and
Haddad GG.
Decreased neuronal excitability in hippocampal neurons of mice exposed to cyclic hypoxia.
J Appl Physiol
91:
1245-1250,
2001
8.
Haddad, GG,
and
Jiang C.
O2 deprivation in the central nervous system: on mechanisms of neuronal response, differential sensitivity and injury.
Prog Neurobiol
40:
277-318,
1993[ISI][Medline].
9.
Hagberg, H,
Peebles D,
and
Mallard C.
Models of white matter injury: comparison of infectious, hypoxic-ischemic, and excitotoxic insults.
Ment Retard
8:
30-38,
2002.
10.
Hahn, AG,
and
Gore CJ.
The effect of altitude on cycling performance: a challenge to traditional concepts.
Sports Med
31:
533-557,
2001[ISI][Medline].
11.
Hamill, O,
Marty A,
Neher E,
Sakmann B,
and
Sigworth FJ.
Improved patch-clamp techniques for high-resolution current recording from cells and cell-free membrane patches.
Pflügers Arch
391:
85-100,
1981[ISI][Medline].
12.
Hendricks, SJ,
Stewart RE,
Heck GL,
DeSimone JA,
and
Hill DL.
Development of rat chorda tympani sodium responses: evidence for age-dependent changes in global amiloride-sensitive Na+ channel kinetics.
J Neurophysiol
84:
1531-1544,
2000
13.
Li, C,
and
Jackson RM.
Reactive species mechanisms of cellular hypoxia-reoxygenation injury.
Am J Physiol Cell Physiol
282:
C227-C241,
2002
14.
Lopez-Barneo, J,
Pardal R,
and
Ortega-Saenz P.
Cellular mechanism of oxygen sensing.
Annu Rev Physiol
63:
259-287,
2001[ISI][Medline].
15.
Mortola, JP,
Xu LJ,
and
Lauzon AM.
Body growth, lung and heart weight, and DNA content in newborn rats exposed to different levels of chronic hypoxia.
Can J Physiol Pharmacol
68:
1590-1594,
1990[ISI][Medline].
16.
O'Reilly, JP,
Cummins TR,
and
Haddad GG.
Oxygen deprivation inhibits Na+ current in rat hippocampal neurones via protein kinase C.
J Physiol
503:
479-488,
1997[Abstract].
17.
O'Reilly, JP,
and
Haddad GG.
Chronic hypoxia in vivo renders neocortical neurons more vulnerable to subsequent acute hypoxic stress.
Brain Res
711:
203-210,
1996[ISI][Medline].
18.
Shah, BS,
Stevens EB,
Pinnock RD,
Dixon AK,
and
Lee K.
Developmental expression of the novel voltage-gated sodium channel auxiliary subunit 3, in rat CNS.
J Physiol
534:
763-776,
2001
19.
Shrager, P,
and
Novakovic SD.
Control of myelination, axonal growth, and synapse formation in spinal cord explants by ion channels and electrical activity.
Brain Res Dev Brain Res
88:
68-78,
1995[ISI][Medline].
20.
Snoeckx, LH,
Cornelussen RN,
Van Nieuwenhoven FA,
Reneman RS,
and
Van Der Vusse GJ.
Heat chock proteins and cardiovascular pathophysiology.
Physiol Rev
81:
1461-1497,
2001
21.
Trojan, S,
and
Pokorny J.
The development of the brain and perinatal hypoxia.
Ceskoslovenska Neurologie a Neurochirurgie
52:
364-371,
1989[Medline].
22.
Wong, TM,
and
Shan J.
Modulation of sympathetic actions on the heart by opioid receptor stimulation.
J Biomed Sci
8:
299-306,
2001[ISI][Medline].
23.
Xia, Y,
Fung ML,
O'Reilly JP,
and
Haddad GG.
Increased neuronal excitability after long-term O2 deprivation is mediated mainly by sodium channels.
Brain Res Mol Brain Res
76:
211-219,
2000[ISI][Medline].
24.
Zhuang, J,
and
Zhou Z.
Protective effects of intermittent hypoxic adaptation on myocardium and its mechanism.
Biol Signals
8:
316-322,
1999[ISI].