Vol. 87, Issue 3, 1066-1074, September 1999
Rhythmic sympathetic nerve discharges in an in vitro neonatal
rat brain stem-spinal cord preparation
Chun-Kuei
Su
Institute of Biomedical Sciences, Academia Sinica, Taipei, Taiwan
11529, Republic of China
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ABSTRACT |
To understand the origination of sympathetic
nerve discharge (SND), I developed an in vitro brain stem-spinal
cord preparation from neonatal rats. Ascorbic acid (3 mM)
was added into the bath solution to increase the viability of
preparations. At 24°C, rhythmic SND (recorded from the splanchnic
nerve) was consistently observed, but it became quiescent at
<16°C. Respiratory-related SND (rSND) was discernible and was
well correlated with C4 root
activity. Power spectral analysis of SND revealed a dominant 2-Hz
oscillation. In most preparations (86%), such oscillation was
persistent, whereas it only slightly reduced its magnitude after
isolation from the brain stem. The removal of neural structures rostral
to the superior cerebellar artery (equivalent to the level of facial
nuclei) reduced rSND, increased tonic SND, but did not affect the
temporal coupling between SND and
C4 root activity. Our data suggest
a prominent contribution of SND from the neural mechanisms confined
within the neonatal rat spinal cord. This ascorbic acid-enhanced in
vitro preparation is a very useful model to study neural mechanisms underlying sympathorespiratory integration.
ascorbic acid; autonomic control; sympathetic development; sympathorespiratory integration
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INTRODUCTION |
SYMPATHETIC NERVE DISCHARGE (SND) maintains the
vasomotor tone for an appropriate blood perfusion to different organs.
The origination of SND has been attributed mainly to the supraspinal neural structures, especially to those neurons located in the rostral
ventrolateral medulla (5, 13, 31). Some rostral ventrolateral medulla
neurons have pacemaker-like activity or receive tonic excitatory drives
from other neurons and subsequently deliver these excitatory signals
via their axonal projection to the sympathetic preganglionic neurons,
intermediate lateral (IML) column cells, located in the thoracolumbar
spinal cord (2, 14, 18, 21, 22, 35, 38). This laboratory has previously demonstrated that the neurons in dorsomedial medulla are also involved
in the maintenance of vasomotor tone and have differential control over
different sympathetic outflows (35, 37).
Spontaneous activities of IML cells can also contribute to the
generation of SND. Substantial background SND is found in some cervical
sympathetic preganglionic fibers or splanchnic nerves in a spinal
preparation of cats (4, 23). Some intraspinal synaptic inputs to IML
cells can maintain a low level of ongoing synaptic activities on a
sympathetic preganglionic neuron (6, 10). In animals with a spinal
preparation, rhythmic SND could still be elicited by asphyxia or
through direct application of strychnine or kainic acid to the spinal
cord (1, 12). These observations imply that the neural mechanisms,
including IML cells in the spinal cord, may also contribute
significantly to SND.
An in vitro brain stem-spinal cord preparation from neonates was
recently developed to study the neural inputs from the brain stem to
IML cells (7, 24). However, it is unclear whether such an in vitro
preparation can indeed generate rhythmic SND endogenously. Comparable
with the above-mentioned experimental model, there is also another in
vitro brain stem-spinal cord preparation (extending from the lower
brain stem to the cervical spinal cord) that can generate rhythmic
respiratory activities (33, 40). Because the anatomic locations of
cardiovascular and central respiratory neurons in the brain stem are
within the proximity of each other (8, 17), it is very
likely that this in vitro preparation could spontaneously generate SND
as well.
In this study, we describe an in vitro brain stem-spinal cord
preparation that can endogenously generate respiratory outflow at the
C4 cervical spinal cord ventral
root and SND at the splanchnic nerves. Sections were made at different
levels of the brain stem and spinal cord to evaluate the contribution
of different neural structures to SND. Our observations suggest that,
in neonatal rats, a significant portion of SND is derived from the
neural mechanisms confined in the spinal cord.
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METHODS |
Neonatal Sprague-Dawley rats (postnatal days
0-2) were used in this study.
General procedures were modified from the methods previously described
(36). The neural tissue that extends from the brain stem to the lumbar
spinal cord, encased by the skull and vertebrae, was immersed in a
10°C bath solution for further dissection under microscope (Wild
M32). Temperature of the bath solution was controlled by a circulation
pump (B401-D, Firstek Scientific) and was monitored by a temperature
amplifier (13-4615-474029 Gould) by using a thermoprobe (YSI
probe 401, Yellow Springs Instruments). The pH of bath solution was
also monitored (PHM83, Radiometer, Copenhagen, Denmark). Bath solution
(in mM: 128 NaCl, 3 KCl, 1.5 CaCl2, 1.0 MgSO4, 24 NaHCO3, 0.5 NaH2PO4,
and 30 D-glucose) was equilibrated with 95% O2-5%
CO2. In some experiments,
CO2 concentration ([CO2]) was adjusted
by a gas proportioner (03218-50, Cole-Parmer) and monitored by a
CO2 analyzer (Normocap
CD-102-28-02, Datex). Complete exposure of the brain stem and
cervical spinal cord was achieved by removing the surrounding bones.
The cervical ventral roots were carefully preserved and were used later
for monitoring the respiratory activities of the preparation. Dorsal
parts of the vertebrae encasing the thoracic-lumbar spinal cord were
also removed to allow for better perfusion, whereas the ventral parts of the vertebrae were left intact to preserve the sympathetic efferent
pathways. Excess tissues attached to the ventral parts of the vertebrae
were removed, retaining only minimal tissues in the bath. By tracing
those nerves that exit from the sympathetic chain and innervate the
celiac ganglion, located adjacent to the adrenal gland, we can easily
identify the splanchnic nerves. The distal ends of the nerves were cut
at the level before innervation of the celiac ganglion. The whole
bundle of splanchnic nerves, containing both major and minor branches
encapsulated by a nerve sheath, was used to record the preganglionic
SND. This dissected preparation was fixed to the Sylgard-gel floor of
the recording chamber (Fig. 1).
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Fig. 1.
Ventral view of in vitro brain stem-spinal cord preparation. Double-end
arrow indicates brain stem was trimmed rostrally at a level between the
trigeminal nerves and superior cerebellar artery. Splanchnic nerves
were identified by tracing the nerves originated from the lower
sympathetic chain and innervated celiac ganglion (not shown).
Bottom: endogenous rhythmic neural
discharges. 
C4, leaky integration of
C4 ventral root activity; SND,
sympathetic nerve discharge.
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C4 ventral root and splanchnic
nerve activities were recorded by suction electrodes. Neural signals
were amplified, filtered (band-pass filter: 0.1-1 kHz; DAM50,
WPI), and stored in a PCM-tape recorder (DR-890, Neuro-Corder). To
acquire the envelope of C4 root
activity
(C4) or
SND (SND), the signals were rectified and
integrated by using a leaky integrator. The total SND was also measured
by a time-based integrator (13-4615-70, Gould) with a
resetting time of 5 s. Background noise level of SND recording was
determined by integration of the signals at a bath temperature
<15°C (when nerve activities were virtually quiescent) or after a
transection at the level of T8
spinal cord. C4 root
activity-triggered average of SND and power spectral analysis of the
neurogram were conducted by using a Pclamp system (Axon Instruments)
and were further analyzed by Axograph (version 3.0). In constructing
the C4 root activity-triggered
average of SND, two analog-delay modules (NL740 Neurolog system,
Digitimer) were used to acquire the pretriggering signals. Rhythmic
oscillation of SND was examined by power spectral analysis of SND
(low-pass filtered at 200 Hz), with each episode of the signal
registering 8.192 s and sampling at 1 kHz. The averaged power spectrum
was acquired from 32 episodes.
Student's t-test was used to test
whether neural signals were significantly altered after a treatment.
The 
2 test was used to evaluate
the incidence ratio of certain observations. A
P value of <0.05 was considered
significant. All the values are presented as means ± SE.
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RESULTS |
Optimal conditions for recording endogenous SND in the in vitro
preparation.
After dissection was conducted in a 10°C bath solution, the
preparation was promptly thawed back to room temperature while SND was
monitored. As shown in Fig. 2, the recovery
of rhythmic nerve activities is temperature dependent. At a bath
temperature of <16°C, there was an absence of significant SND
generated by the splanchnic nerves. SND became apparent as the bath
temperature was raised to >20°C, reached a plateau at
~24°C, and decayed progressively when bath temperature was
increased (data not shown). When bath temperature was maintained at
27°C, supposedly an optimal temperature for recording respiratory
activities in similar studies (3, 33, 40), SND decayed promptly.
Therefore, the optimal temperature in this preparation to record SND
was ~24°C.
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Fig. 2.
Temperature-dependent recovery of brain stem-spinal cord preparation to
generate SND and C4 root
respiratory activities.
0-5 SND
(resetting time, 5 s), time-based integration of SND; T°C, bath
temperature. Periods labeled at top of slower traces
(A, B, C, and
D) are stretched and shown
respectively on bottom (A-D) and at right
(a-d). SND was integrated with a
resetting time of 5 s, as shown in
0-5
SND. When the bath temperature was <16 °C
(A), height of
0-5 SND
indicates level of background noise (dashed line). Arrow head
(top), acute section at
C8 spinal cord.
C4 respiratory activities were not
affected by sectioning. Comparing C
with D, note significant diminution of
SND, but some activities still remained after sectioning was
performed.
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To acquire a sustainable preparation that can generate stable and
long-lasting SND, the effects of ascorbic acid in preservation of the
viability of brain slices were evaluated. The viability of the brain
stem-spinal cord preparation in generating SND was compared between the
experiments with or without incubation of 3 mM ascorbic acid. In the
preparations incubated with 3 mM ascorbic acid, the rundown of SND was
observed only in 2 of 17 (12%) experiments. (Rundown of SND: when bath
temperature was maintained at 24 ± 1°C, the neural signals
disappeared gradually within 2 h, leaving an integrated signal not
different from the level when bath temperature was
<15°C.) In contrast, in the absence of ascorbic
acid, four of seven (57%) experiments showed an apparent rundown of
SND within 2 h. The 2 test
revealed that a significantly higher incidence of viable preparation
occurred with incubation of 3 mM ascorbic acid
(P < 0.05). These findings are
consistent with the previous reports that ascorbic acid improves the
conditions of neuronal growth and viability of brain slices (16, 19).
In the presence of 3 mM ascorbic acid and with maintenance of the bath
temperature at 24°C, SND could be sustained for more than 5 h
without apparent deterioration. Figure 3
shows the acute effects of ascorbic acid. By adding 3 mM ascorbic acid
to the bath solution, both SND and respiratory rhythmic discharge of
C4 root activity were increased significantly (SND: 25 ± 4%, P < 0.01; respiratory frequency: 38 ± 2%,
P < 0.001;
n = 4).
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Fig. 3.
Ascorbic acid enhances SND only in intact brain stem-spinal cord but
not in isolated spinal cord preparations. Addition of 3 mM ascorbic
acid (arrowhead) into bath solution increased both SND and burst
frequency of C4 ventral root
activities in an intact brain stem-spinal cord preparation
(A and
B,
top). In contrast, addition of 3 mM
ascorbic acid to another preparation
(C and
D; a section made at the level of
C8 to remove synaptic inputs from
brain stem to thoracic levels of spinal cord) only increased
respiratory frequency but did not affect SND
(bottom).
Middle panels: periods labeled at the
top of slower traces
(A-D) are stretched and shown.
Lower right: background noise of SND
recording, which is determined 10 min after sectioning performed at
T8.
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The effects on neural activities of changing
[CO2] in the bubbling
air were also examined. The
[CO2] of the bubbling
air was changed from 5 to 2 or 8%; this resulted in a change of the pH value in the bath solution from 7.39 to 7.14 or 7.64, respectively. As
shown in Fig. 4, the change of SND only
parallels the change of
[CO2] in an isolated
spinal cord preparation, but not in an intact brain stem-spinal cord
preparation. In isolated spinal cord preparations, SND consistently
decreased when milieu
[CO2] was decreased
from 5 to 2% (
31 ± 4%,
n = 4;
P < 0.01), but was only
insignificantly increased when milieu
[CO2] was increased from 5 to 8% (13 ± 7%, n = 4).
In intact preparations with the brain stem, the change of
[CO2] did not result
in a consistent SND response. An increase of
[CO2], from 5 to 8%,
did not change SND significantly (n = 3) or cause a biphasic SND response with either an initial decrement or
increment followed by a delayed enhancement or depression
(n = 3). A decrease of
[CO2], from 8 to 2%,
might reduce (n = 3), enhance SND
(n = 2), or cause an initial decrement
followed by a delayed increment (n = 2). At equilibrium states (10 min after changing milieu
[CO2]), the overall effects induced by the change of
[CO2] on SND were
pooled. In comparison with the level of SND at 5%
[CO2], SND was not
significantly altered either by elevation (3 ± 15%,
n = 6) or reduction (13 ± 16%,
n = 6) of the milieu
[CO2]. However, the
burst frequency of C4 root
activity consistently changed in parallel with
[CO2], i.e., the
frequency of respiratory bursts was higher as
[CO2] increased and
lower as [CO2]
decreased (n = 6). In comparison with
the respiratory frequency at 5%
[CO2], an increase of
[CO2] to 8%
accelerated the respiratory frequency significantly (34 ± 8%,
P < 0.05), whereas a decrease of
[CO2] to 2%
diminished the rate (68 ± 17%,
P < 0.05).
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Fig. 4.
Effects of changing CO2
concentration ([CO2])
on neural activities. A: SND and
C4 root activity recorded from
brain stem-spinal cord preparation. Note that
C4 root activity disappeared after
[CO2] was lowered from
8 to 2% and gradually recovered after
[CO2] was raised back
to 5%. Fluctuation of SND was not in parallel with change of
[CO2].
B: SND recorded from an isolated
spinal cord preparation. Recordings started 1 h after a sectioning was
performed at C1. Note: change of
SND parallels change of
[CO2].
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Brain stem- and spinal cord-derived components of SND.
Surprisingly, not many preparations showed a significant amount of SND
that depended on the synaptic inputs from the brain stem.
Only in 2 of 14 (14%) preparations, was brain stem-derived SND
component manifested by a significant diminution of SND after an acute
section at the C1 or
C8 level (Fig. 2). In most
experiments (86%) after the section, SND was reduced only transiently
and within 10 min could recover up to 90% of its activity before the section (Fig. 5). However, a further
section at the
T7-T9
spinal cord consistently abolished SND (Fig.
5D). Because ascorbic acid may
stimulate SND (Fig. 3, A and
B), we further evaluated whether such prominent SND generated spontaneously from the isolated spinal cord is caused by a direct action of ascorbic acid to enhance SND at
the level of spinal cord. In contrast to the observation in Fig. 3,
A and
B, the addition of 3 mM ascorbic acid
to an isolated spinal cord did not alter SND (Fig. 3,
C and
D). This indicates that ascorbic
acid mainly acts at the level of the brain stem, which also results in
an increase of respiratory frequency (36 ± 3%,
n = 4;
P < 0.001). On the other hand, the
transient and slight reduction of SND immediately after the brain stem
section was also observed in experiments with no ascorbic acid added to the bath solution (n = 2). These
results exclude the possibility of direct stimulant effects of ascorbic
acid on SND preganglionic neurons. The prominent recovery of SND after
isolation of the synaptic inputs from the brain stem suggests that a
significant component of SND originates endogenously from the spinal
cord.
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Fig. 5.
Spinal cord-derived SND component. Arrow heads,
left to
right, represent sectioning performed
at level of superior cerebellar artery,
C1, and
T8 spinal cord, respectively.
After sectioning was performed at level of superior cerebellar artery,
SND and C4 burst frequency
increased. A prompt section at C1
spinal cord caused only transient inhibition and slight reduction of
SND. SND disappeared after section at level of
T8 spinal cord. Periods labeled
A-D at
top of slower traces are stretched and
shown in bottom panels. Dashed line,
background noise level, acquired after sectioning was performed at
T8.
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In taking advantage of the landmark lying on the
rostroventral surface of the medulla, we further deciphered whether the
neural structure rostral to the superior cerebellar arteries
(equivalent to the level at facial nuclei) is essential for the
generation of SND. Figures 5 and 6 show a
slight but persistent increase of SND after the removal of the neural
structure rostral to the level of superior cerebellar arteries.
Although the neural structure in the brain stem rostral to the facial
nuclei is not essential for the generation of SND, we did notice that,
in its absence, a dominant component of SND related to the respiratory
activities (correlated with C4
ventral root activities) was diminished (Fig. 6). To manifest the
respiratory-related component of SND (rSND), a leaky integrator was
used to reveal the envelope of SND fluctuation. After sectioning of the
brain stem was performed at the level of superior cerebellar artery,
respiratory frequency and amplitude increased, but rSND diminished
(Fig. 6). Figure 7 shows the power spectral
analysis of SND. The rhythmic oscillation of SND was dominant at a
frequency of ~1-2 Hz. This basic rhythm was not altered after
removal of the neural structure higher than the facial nuclei, despite
the fact that a lower frequency component of SND (<1 Hz) was reduced
after the section (Fig. 7B,
n = 7). Also, the dominant rhythm of
SND at 1-2 Hz persisted in isolated spinal cord preparations (Fig.
7). Figure 8 shows the average of neural
activities triggered by C4 root
activity. The peak activity of SND appeared consistently
after the onset of inspiratory C4 activities with a delay of 103 ± 11 ms (range: 65-208 ms,
n = 13). After sectioning was
performed at the level of superior cerebellar arteries, there was a
tonic elevation of SND, whereas the temporal relationship between peak
C4 activities and SND remained
unchanged (n = 7).
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Fig. 6.
Removal of neural structures rostral to superior cerebellar artery
reduced respiratory-related SND (rSND). Brain stem was sectioned at
level of superior cerebellar artery, equivalent to level of facial
nuclei. After the sectioning was performed, burst frequency of
C4 ventral root activities and
amplitude of SND increased. Periods labeled at
top of slower traces
(A and
B) are stretched and shown in
bottom panels. Note: slow fluctuation
of rSND (arrows, A) diminished after
the section was performed (B).
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Fig. 7.
Power spectral analysis of SND envelope.
A: power spectrum was acquired by an
average of 32 episodes (8.192 s/episode, sampling at 1 kHz) collected
under different bath temperatures (solid lines), revealing a rhythmic
SND at ~2 Hz. Dashed lines, spectrum of SND envelope after serial
sections made at level of superior cerebellar artery,
C8, and
T8, respectively. Arrow, reduction
of lower frequency component (<1 Hz) after removal of neural
structures rostral to superior cerebellar artery. Rhythmic SND at
1-2 Hz was sustained after section was performed at
C8, but power was virtually
abolished after section was performed at
T8. Top
right: each episode is 1 s, showing original envelopes
of SND under different conditions. B:
normalized change of power spectrum after section was performed at
level of superior cerebellar artery. Data from 7 experiments were
pooled. Gray lines, normalized change of individual experiments,
acquired through dividing power spectrum after section was performed by
that before the sectioning occurred. Black line, average of normalized
change of power spectrum. Arrowhead (compare with arrow in
A) indicates an averaged decrease of
the power (<1 Hz) after section perfoermed at level of superior
cerebellar artery.
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Fig. 8.
Average of SND triggered by C4
root activity. Each trace is average of 32 episodes. Leaky integration
of neural activities, before and after section performed at superior
cerebellar artery, is superimposed to reveal temporal coupling of
SND and
C4. Temporal
coupling between activities was not altered, as shown by persistence of
rSND after the section was performed. Compared with apparent rSND
acquired from C4-triggered
average, random pulse-triggered average from same stretch of neural
signals only revealed a general elevation of tonic SND after section
was performed.
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DISCUSSION |
We have developed an in vitro neonatal rat brain stem-spinal cord
preparation that can endogenously generate respiratory activities and
SND. Because rSND is discernible and well correlated with C4 respiratory-burst activities,
this preparation could be a useful model to study the neural mechanisms
that underlie the sympathorespiratory integration. Our results also
clearly demonstrate that a significant portion of SND in neonatal rats
is derived from the neural mechanisms of the spinal cord.
Optimal conditions for recording SND from the in vitro preparation.
On the basis of our empirical observation, the optimal ambient bath
temperature for this in vitro preparation is 24°C, which is a
temperature lower than that for the preparation routinely used to study
respiratory rhythmogenesis (27-28°C) (3, 29, 33). Oxygen
consumption for neurons may be higher in a warmer environment due to
the higher cellular metabolism. Thus, when less free oxygen is
available for the neurons closer to the core of preparation, a
microenvironment of hypoxic hypercapnia would result. In
sinoaortic-denervated cats, hypoxia depresses inspiration-synchronous sympathetic activity while it increases the tonic component of the
activity (32). Intravertebral injection of NaCN, to cause a local
hypoxia in either the brain stem or an isolated spinal cord, stimulates
sympathetic excitation (27). Asphyxia of sufficient duration in cats
with a spinal preparation increased sympathetic nerve activities,
characterized by a 2- to 3-Hz oscillation (12). Thus hypoxia or
hypercapnia could have a direct stimulant effect on SND at the level of
the spinal cord and might produce the spinal cord-derived component of
SND. However, the lower optimal ambient temperature in our experimental
conditions did not support such a view (that higher
[CO2] in the core of
preparation resulting from the higher ambient temperature is required
to produce the endogenous SND). In fact, we observed that respiratory
frequency is positively correlated with the change of
[CO2] (Fig. 4). This observation indicates that the accumulated
CO2 in the core of the preparation
could be higher in a warmer environment to stimulate respiratory
rhythmogenesis but not SND. Furthermore, there is no consistent effect
of changing the [CO2]
on SND in our intact in vitro experimental model. Only in the in vitro
isolated spinal cord, but not in the intact brain stem-spinal cord
preparation, did we observe a consistent SND response in parallel with
change of [CO2] (Fig.
4). Such an observation indicates a complex action of
CO2 on different orders of neurons
responsible for eliciting SND, and, possibly, an inhibitory and
excitatory action at the level of the brain stem and the spinal cord, respectively.
The mammalian brain contains a high level of ascorbic acid. The level
is even higher in fetal and newborn rats than in adult rats (20).
Ascorbic acid reduces lipid peroxidation and enhances viability of
neurons (19, 26). All these findings are consistent with our
observation that the viability of our in vitro preparation is
significantly better in the presence of 3 mM ascorbic acid. In our
study, ascorbic acid exerts no direct effect at the level of spinal
cord, yet it enhances SND in intact brain stem-spinal cord
preparations. This suggests a direct action of ascorbic acid on the
brain stem neurons that generate SND (Fig. 3). Thus, in the presence of
ascorbic acid, the overall SND could be biased to a stronger synaptic
input from the brain stem. However, an acute isolation of the spinal
cord from the influence of the brain stem only slightly reduced SND in
most preparations. This observation also suggests that a
major portion of SND is derived from the spontaneous
neural activities confined within the neonatal rat spinal cord.
Characteristics of endogenous rhythmic SND in vitro.
The most dominant rhythm of SND in our in vitro preparation is
~1-2 Hz. The power of rhythmic SND decayed promptly at the frequency >5 Hz. Such a pattern of activities was not altered after
the section was made at the junction between the brain stem and the
spinal cord. Intriguingly, other studies also suggest that the basic
rhythmic generator of SND is located in the spinal cord. In spinal
preparations of cats, splanchnic activities during asphyxia have a
2-3 Hz oscillations (12). In the isolated spinal cord of adult
rats, a nonpatternized stimulation of neural circuits at the spinal
cord (by exogenous application of kainic acid) is sufficient to elicit
a patternized rhythmic 2-6 Hz SND (1). Furthermore, disruption of
neural mechanisms in the brain stem by intracisternal injection of
kynurenate to block glutamate synaptic transmission does not
desynchronize the sympathetic activity (9). Thus the neural circuits in
the spinal cord in either neonatal or adult rats can be independent
from the brain stem and can be sufficient to elicit a basic rhythmic SND.
By power spectral analysis of SND, we observed subtle changes of SND
after sectioning was performed at different brain stem levels. The
lower frequency component of SND (<1 Hz, Fig. 7), equivalent to rSND,
was diminished yet persisted after sectioning of the brain stem at the
level of superior cerebellar artery (Figs. 6-8). In contrast, the
same sectioning increased tonic SND (Figs. 5-8). The appearance of
rSND indicates a functioning neural circuit of our in vitro preparation
that conveys the respiratory drives to the brain stem sympathetic
premotoneurons and subsequently to the splanchnic nerves. After
sectioning was performed at the level of superior cerebellar artery,
the latency between the peak activity of SND and the onset of
C4 root inspiratory activity was
not altered (Fig. 8). Studies of central circuits underlying the
sympathorespiratory coupling have recently elucidated some likely
pathways that propagate respiratory messages toward sympathetic premotoneurons (13, 30, 39). To date, the neurons that can carry
respiratory messages to sympathetic premotoneurons have been found in
the Bötzinger complex and the caudal ventrolateral medulla (28,
39). Thus our findings further demonstrate that the neural structure
rostral to the superior cerebellar artery (at the level of facial
nuclei) is not essential to bridge these two systems.
Physiological significance of rhythmic SND in neonatal rats.
At birth, the development of the rat sympathetic nervous system is
incomplete (11, 25). Sympathetic ganglionic neurotransmission only
functions after postnatal days
5-10 (15, 34). In the present study, we used rats
with an age of 0-2 postnatal days and observed a significant
spinal cord-derived component of SND. This observation may simply
reflect a maturation process of the sympathetic nervous system and may
not directly link to the vasomotor regulation at this developmental
stage. The discrepancy between rats, in terms of maturation at birth,
may in turn give rise to a variable dependency on the synaptic inputs
from the brain stem. This possibility could be reflected by our
observation that 14% of the preparations revealed a greater proportion
of SND derived from the brain stem (Fig. 2). Undoubtedly, it would be
of interest to examine the postnatal changes of SND genesis and to
determine whether such a spinal cord-derived rhythmic SND is involved
in the activity-dependent fine tuning of the spinal neural circuits in
the neonates.
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ACKNOWLEDGEMENTS |
The author is grateful to Drs. C.-Y. Chai for support, T.-N. Lin
for technical assistance, and M. Seah for editing this manuscript.
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FOOTNOTES |
This work was supported by grants from National Science Council of the
Republic of China (NSC 88-2314-B-001-002) and Shih-Chun Wang
Research Memorial Fund.
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: C.-K. Su,
Institute of Biomedical Sciences, Academia Sinica, Taipei, Taiwan
11529, Republic of China (E-mail: csu{at}ibms.sinica.edu.tw).
Received 25 January 1999; accepted in final form 21 May 1999.
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