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First published online August 30, 2006
Journal of Experimental Biology 209, 3685-3694 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02418
Noradrenergic modulation of respiratory motor output during tadpole development: role of 
-adrenoceptors
Department of Pediatrics, Université Laval, Centre de Recherche du Centre Hospitalier Universitaire de Québec, 10 rue de l'Espinay, Québec City, QC G1L 3L5, Canada
* Author for correspondence (e-mail: Richard.Kinkead{at}crsfa.ulaval.ca)
Accepted 28 June 2006
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Summary |
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-adrenoceptor antagonists. Conversely, NA application elicited dose-
and stage-dependent increases in fictive buccal ventilation frequency. We then
assessed the contribution of -adrenoceptors towards these responses by
applying specific agonists (1: phenylephrine;
2: clonidine; concentration range from 10 to 200 µmol
l-1 for both). Of the two agonists used, only phenylephrine
application consistently mimicked the lung burst frequency response observed
during NA application in each stage group. However, both agonists decreased
buccal burst frequency, thus suggesting that other (ß) adrenoceptor types
mediate this response. We conclude that modulation of both buccal and
lung-related motor outputs change during development. NA modulation affects
both types of respiratory activities in a distinct fashion, owing to the
different adrenoceptor type involved.
Key words: control of breathing, development, amphibian, Rana catesbeiana
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Introduction |
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2
adrenoceptor activation (Al-Zubaidy et al.,
1996
; Arata et al.,
1998; Errchidi et al.,
1991; Hilaire et al.,
1989) (for a review, see
Hilaire et al., 2004).
However, the functional organization of noradrenergic modulation is not
homogeneous amongst species since in in vitro preparations from
neonatal mice, exogenous NA (either bath-applied or locally injected near
medullary respiratory neurons) elicits the opposite effect; that is, an
increase in fictive breathing frequency mediated by
1-adrenoceptor activation
(Viemari and Hilaire,
2002).
During development, noradrenergic neurons and their projections, along with
the postsynaptic receptors undergo substantial reorganization
(Winzer-Serhan et al., 1996).
Thus, it is not surprising that the effects of NA on fictive breathing
frequency observed in preparations from neonatal rodents are age-dependent.
For instance, NA bath application to in vitro preparations from mice
foetuses (embryonic day 16) does not reduce but increases phrenic burst
frequency (Viemari et al.,
2003). In preparations from newborn rats and mice,
2-mediated inhibition of respiratory activity is strongest
at birth, and the strength of this inhibition subsides progressively during
the first postnatal days (Errchidi et al.,
1991; Viemari et al.,
2003) (for a review, see
Hilaire et al., 2004).
2-related modulation of respiratory activity probably
persists until adulthood since in anesthetized rats, these adrenoceptors
contribute to depression of breathing frequency following hypoxia
(Bach et al., 1999). However,
proper comparison of 2-mediated modulation of breathing
frequency across developmental stages is difficult since these data were
obtained with different experimental approaches (in vivo versus in
vitro).
Developmental change in 2-related modulation is
especially remarkable in newborn mice in which the balance between
1- and 2-mediated influences is ultimately
dominated by 1-related facilitation of breathing frequency
(Hilaire et al., 2004).
Relatively little is known about the potential role of
1-related modulation of respiratory activity in more mature
developmental stages. However, data obtained from brainstem preparations from
adult turtles suggest that this aspect of noradrenergic modulation persists
until adulthood (Johnson et al.,
1998).
Although we currently have a good understanding of how noradrenergic
modulation affects breathing frequency during early life, especially in
rodents (Hilaire et al.,
2004), our current knowledge remains limited by the absence of a
study addressing noradrenergic modulation at distinct developmental stages
(including adults) using a single model system. To address this shortcoming,
the main objective of the present study was to take advantage of the in
vitro brainstem preparation from Rana catesbeiana tadpoles and
adult bullfrogs to test the hypothesis that noradrenergic modulation of
fictive breathing frequency (both buccal and lung) changes during development.
In a second series of experiments, the use of 1- and
2-adrenoceptor agonists allowed us to address how these
receptor types contribute to noradrenergic modulation over the course of
development and the transition from aquatic to aerial breathing in this
species. Parts of this work have been reported in abstract form
(Fournier and Kinkead,
2005).
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Materials and methods |
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In vitro brainstem preparations
Animals were anesthetized by immersion in a solution of tricaine methane
sulfonate (0.06 g l-1) buffered to pH 7 with NaHCO3. For
adult frogs, the beaker was placed on ice for 40-60 min to slow metabolism and
ensure adequate anaesthesia throughout the dissection
(Winmill and Hedrick, 2003).
Once unresponsive to body pinch, tadpoles and frogs were decerebrated by a
transection just rostral to the eyes. In frogs (N=39), a hole was
drilled in the cranium to allow decerebration. Tadpoles were then placed under
the dissection microscope for determination of the developmental stage based
on the criteria of Taylor and Kollros
(Taylor and Kollros, 1946),
and assigned to one of two groups: pre- (stages VII-XI; N=39) or
metamorphic tadpoles (stages XVIII-XXIII; N=30). The cranium was
opened to expose the brainstem and rostral spinal cord and allow dissection of
the cranial nerves. The brain was irrigated with ice-cold (0-5°C)
artificial cerebrospinal fluid (aCSF) to reduce axonal conductance throughout
the dissection procedure. The composition of the aCSF was identical to the one
developed for tadpoles (Liao et al.,
1996) and adult frogs (Kinkead
et al., 1994) and consisted of (in mmol l-1): 104 NaCl;
4 KCl; 1.4 MgCl2; 10 D-glucose; 25 NaHCO3;
2.4 CaCl2 for tadpoles and 75 NaCl; 4.5 KCl; 1 MgCl2;
7.5 D-glucose; 40 NaHCO3; 2.5 CaCl2; 1
NaH2PO4 for adult bullfrogs. The superfusate was
equilibrated with a 98% O2/2% CO2 gas mixture and had a
pH of 7.9±0.1 for tadpoles and 7.8±0.1 for adult bullfrogs. The
brainstem was transected between the optic tectum and the forebrain and then
caudal to the hypoglossal nerve before being transferred to a small Petri dish
coated with Sylgard (Dow Corning, Midland, MI, USA) where it was immobilized
with insect pins. The arachnoid and pia membranes were carefully removed, and
the brain was moved to the recording chamber where it was placed ventral side
up.
Electrophysiological recordings
Bursts of respiratory-related motor activity were recorded simultaneously
from the rootlets of cranial nerves V and X using suction electrodes. The
pipettes were constructed from borosilicate glass (0.84 mm internal diameter)
pulled to a fine tip with a vertical microelectrode puller (Stoelting
Instrument, Wood Dale, IL, USA). The tip was broken and bevelled to achieve
appropriate tip diameter. Neural activity signals recorded from the suction
electrodes were amplified (gain=10 000) and filtered (low cut-off: 10 Hz; high
cut-off: 1 kHz) using a differential AC amplifier (model 1700; A-M systems,
Everett, WA, USA). Vagal and trigeminal signals were then full-wave rectified
and integrated (time constant: 100 ms) using a moving averager (model MA-821;
CEW, Ardmore, PA, USA). The raw and integrated nerve signals were viewed on an
oscilloscope and digitized for recording with a data acquisition system (model
DI-720; Dataq Instruments, Akron, OH, USA). The sampling rate of the analogue
to digital conversion for the raw signal was 2500 Hz.
Experimental protocol
Once the recording electrodes were in place, the brainstem preparation was
superfused with control (drug-free) aCSF at room temperature (20-22°C)
delivered at a rate ranging between 4 and 6 ml min-1. The
preparation was allowed to return to ambient temperature and stabilize for
30-60 min, until stable rhythmic neural activity was recorded from both
nerves.
The first series of experiments compared the effects of noradrenaline (NA)
bath application on fictive breathing frequencies (both buccal and lung)
between developmental stages. For this series, the protocol began by recording
respiratory-related motor output for 10 min before NA was added in increasing
concentrations to a second aCSF reservoir (pre-metamorphic stages:
N=12; metamorphic stages: N=10; adult=9). The brainstem was
exposed to the first NA concentration for 10 min before a higher NA
concentration was delivered to the preparation. Other studies have shown that
an equilibration period of at least 5 min is necessary to obtain measurements
that do not reflect a transient effect of the drug
(Belzile et al., 2002;
Kinkead et al., 2002;
Onimaru et al., 1998).
Brainstem preparations were exposed, in succession, to seven increasing NA
concentrations: 0.02, 0.05, 0.1, 0.5, 1, 5 and 10 µmol l-1. This
concentration range was based on values reported in the literature
(Arata et al., 1998) and
preliminary experiments, which suggested that tadpole preparations were
slightly more sensitive to NA than mammals. The final application was followed
by a `wash-out' period during which the preparation was superfused with
drug-free aCSF for a period ranging from 30 to 90 min before a final recording
of respiratory-related motor output was made. Preliminary experiments were
performed to confirm that, as in mammals, changes in respiratory activity
(especially lung-related motor output) observed during NA application are
mainly associated with -adrenoceptor activation. In these distinct
experiments, only one NA concentration (5 µmol l-1) was
bath-applied in the presence of prazosine (0.5 µmol l-1;
1-adrenoceptor antagonist) or RX821002 (25 µmol
l-1; 2-adrenoceptor antagonist). Being concerned
about potential carry over effects, each preparation was exposed to one
antagonist only. These preliminary experiments were performed on preparations
from pre-metamorphic tadpoles (N=5 for each drug) and adult bullfrogs
(N=4 for each drug) only. Since NA active drugs are light sensitive,
drug preparation and experiments were conducted with dim lights. Drug
reservoirs were covered to minimize light exposure. All drugs were obtained
from Sigma/RBI Aldrich (St Louis, MO, USA).
The second series of experiments addressed the contribution of
1- and 2-adrenoceptors in the modulation
of fictive breathing frequencies across developmental stages by comparing the
effects of 1 and 2 agonist application
between stage groups. In these experiments, brainstem preparations were
superfused for 10 min with aCSF containing increasing concentrations of the
1 receptor agonist phenylephrine (10, 25, 100 and 200
µmol l-1; pre-metamorphic: N=8; metamorphic:
N=13; adult: N=10) or the 2 receptor
agonist clonidine (10, 25, 100 and 200 µmol l-1;
pre-metamorphic: N=9; metamorphic: N=7; adult:
N=12) according to the protocol described previously.
Data analysis
Fictive breathing frequency values for respiratory burst activity were
obtained by analysing the last 3 min of application of each NA concentration
(including baseline). In vitro tadpole and frog brainstem
preparations typically produce two patterns of respiratory-related neural
activity: (1) high frequency, low amplitude, and (2) low frequency, high
amplitude, reflecting fictive buccal and lung ventilation, respectively
(Liao et al., 1996;
Torgerson et al., 1998).
Cranial nerve burst amplitude from a single electroneurogram is not always
sufficient to adequately identify fictive lung and buccal bursts
(Sanders and Milsom, 2001;
Torgerson and al., 1998).
Thus, two nerve signals were analysed simultaneously, here vagal nerve
activity was used as a sensitive marker of fictive lung activity to
distinguish between lung- and buccal-related signals
(Kogo et al., 1994;
Kogo and Remmers, 1994).
Lung and buccal burst frequencies were obtained by counting the number of lung- and buccal-related bursting events for the 3-min segment analysed, and averaged for a 1-min period.
Under baseline conditions, fictive breathing frequency differed between stage groups (stage effect: P<0.0001; Fig. 1 and Fig. 2A). Fictive breathing frequency data were therefore expressed as a percentage change from baseline to allow between-group comparisons of the fictive lung ventilation responses to drug application.
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1-adrenoceptor antagonist prazosine had no effect on fictive
lung ventilation (drug effect: P=0.4;
Fig. 3A); however, a decrease
in lung burst frequency was observed when NA was applied with RX821002
(selective 2-adrenoceptor antagonist) simultaneously (drug
effect: P=0.05; Fig.
3C).
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Stage-dependent effects of noradrenaline bath application on fictive buccal ventilation frequency
Under baseline conditions, the buccal burst frequency was similar in all
three groups (stage effect: P=0.14;
Fig. 4A). Unlike preparations
from pre-metamorphic tadpoles, fictive buccal ventilation was not always
observed in brainstems from more mature animals. For instance, only two
preparations from adult frogs produced a signal in which fictive buccal
ventilation frequency could be quantified reliably at more than two NA
concentrations. Although the mean buccal frequency values obtained for this
group are reported in Fig. 4,
these data could not be included in the ANOVA for repeated measures. Data
suggest, however, that in the adult group, NA application had no effect on
fictive buccal ventilation frequency. In preparations from both tadpole
groups, NA bath application enhanced fictive buccal ventilation frequency
(drug effect: P<0.0001; Fig.
4A). Both tadpole groups began to respond to NA at the same dose
(5 µmol l-1), but the fictive buccal ventilation frequency
increase observed in brainstems from more mature tadpoles was greater than the
one reported for the pre-metamorphic group (stagexdrug:
P=0.029; Fig. 4A).
Expressing data as a percentage change from baseline produced results similar
to those reported with absolute values (stagexdrug: P=0.013;
Fig. 4B). In both tadpole
groups, the wash-out period restored the fictive buccal ventilation frequency
values back to their initial (baseline) values. Note that neither adrenoceptor
antagonist could prevent the increase in fictive buccal ventilation observed
when 5 µmol l-1 NA was applied to preparations from either
pre-metamorphic tadpoles or adult bullfrogs (drug effect: P<0.05
for all; Fig. 3B,D).
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1-adrenoceptor activation on fictive lung ventilation frequency1-adrenoceptor agonist
phenylephrine to the aCSF altered fictive lung ventilation frequency in all
stage groups, and these effects were stage-dependent (stagexdrug:
P<0.0001; Fig. 5A).
In the pre-metamorphic group, application of low phenylephrine concentrations
(10 and 25 µmol l-1) increased fictive lung burst frequency;
however, this response was not maintained when higher concentrations of
phenylephrine were applied (Fig.
5A). These results contrast with those obtained in the metamorphic
group in which phenylephrine application decreased fictive lung burst
frequency, and preparations from adult frogs in which phenylephrine had no
significant effect (P=0.29; Fig.
5A). Fictive lung ventilation frequency returned to baseline
values during the wash-out period.
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1-adrenoceptor activation on fictive buccal ventilation frequency
Effects of 2-adrenoceptor activation on fictive lung ventilation frequency
2-adrenoceptor activation with clonidine had
stage-dependent effects on fictive lung burst frequency (stagexdrug:
P<0.0001; Fig. 6A).
In the pre-metamorphic group, clonidine bath application increased fictive
lung ventilation frequency in a dose-dependent manner; however, the response
was statistically significant at the highest dose only. In the adult group,
the increase in fictive lung ventilation frequency was significant at a lower
dose (25 µmol l-1) but then remained relatively stable when
higher clonidine concentrations were applied
(Fig. 6A). These results differ
from the response observed in the metamorphic group in which application of
low clonidine concentrations (10 and 25 µmol l-1) decreased
fictive lung ventilation frequency. These effects were reversed during the
wash-out period in both tadpole groups but persisted beyond the recovery
period in the adult group (Fig.
6A).
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2-adrenoceptor activation on fictive buccal ventilation frequency |
Discussion |
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; Arata et al.,
1998; Errchidi et al.,
1991; Viemari and Hilaire,
2002) (for a review, see
Hilaire et al., 2004) we
focused our work on the role of -adrenoceptors in these developmental
changes, and data indicate that the changes in fictive lung ventilation
frequency that follow NA bath application are probably related to these
receptor types because these responses were mimicked or prevented by selective
agonists or antagonists, respectively. Conversely, the increase in fictive
buccal ventilation frequency observed during NA application could not be
reproduced or blocked by -agonists or antagonists, thus suggesting that
these effects involve ß-adrenoceptors. Although both types of respiratory
activities require activation of essentially the same motoneurone pools, these
differences further substantiate the functional distinctions between the
neural circuits that drive lung and buccal ventilation
(Wilson et al., 2002), as
different adrenoceptor types are involved in the modulation of fictive lung
and buccal ventilation.
Critique of method
Although the value of the amphibian model for electrophysiological
investigations of respiratory control development is acknowledged
(Belzile et al., 2004;
Hedrick, 2005;
Kinkead, 1997;
Milsom et al., 1999;
Vasilakos et al., 2005), we
are well aware of the limitations inherent to the use of bath application of
pharmacological agents onto brainstem preparations
(Kinkead et al., 2002). The
results obtained from such approach must be interpreted cautiously since the
sites of action of the drugs are unknown, and the pharmacological
specificities of the agents used have been characterized in mammals and may
not apply equally in amphibians. Furthermore, clonidine, in addition to being
a selective 2-adrenoceptor agonist, also has affinity for
imidazole binding sites. Despite these limitations, this approach nonetheless
allows an initial assessment of the neural mechanisms contributing to the
maturation of the respiratory control system. However, the fact that the
changes in lung ventilation observed following in vivo clonidine
application in Bufo marinus and systemic injection of adrenaline and
noradrenaline in Rana temporaria were similar to those found in the
present study help validate our approach and data interpretation
(Niechaj, 1971;
Rives and Bernard, 2001).
The experimental design used to assess the effects of increasing NA concentrations could be a confounding factor since the time between application of the first and last dose is relatively long (over 1 h) and may be sufficient to allow dynamic changes in the response owing to adrenoceptor internalization. However, comparison of the results obtained in the dose-response experiments (Fig. 2) in which 5 µmol l-1 of NA was the sixth concentration applied (i.e. 60 min later) with those obtained in the preliminary experiments in which 5 µmol l-1 of NA was the first dose applied show that the system's responsiveness is similar and thus maintained over time.
Neuroanatomical considerations
In neonatal rodents, different groups of noradrenergic neurons are
responsible for the heterogeneous modulation of fictive breathing
(Hilaire et al., 2004). For
instance, activation of the pontine A5 noradrenergic neurons exerts an
inhibitory modulation of respiratory rhythm (via
2-adrenoceptors), whereas A6 neurons (locus coeruleus)
facilitate fictive breathing frequency
(Hilaire et al., 2004). Our
understanding of amphibian noradrenergic system in relation to respiratory
neurons is sketchy in comparison with that of mammals, and this shortcoming
makes it difficult to ascribe the effects reported in the present study to
specific groups of neurons. We know that the organization of noradrenergic
neurons of amphibians shares many features with that of mammals, but no
studies have reported the existence of A5-like neurons in amphibians.
Conversely, neuroanatomical studies have shown that, on the basis of location,
neurotransmitter content and efferent projections, the isthmic noradrenergic
cell group of amphibians is homologous to the locus coeruleus of amniotes
(Marin et al., 1996). These
neurons are present in our preparation, and probably contribute to the changes
in the respiratory motor output discussed below.
Developmental changes in noradrenergic modulation of fictive lung ventilation frequency
As we mentioned previously, these results are consistent with our
hypothesis since each of the agonists applied elicited stage-dependent changes
in fictive lung ventilation frequency. Unlike the other stage groups, the lung
burst frequency response to NA application measured from pre-metamorphic
brainstems was not homogeneous, as two distinct responses were observed. Most
preparations increased lung burst frequency in response to moderate NA
concentrations (between 0.5 and 5 µmol l-1), but we have no data
that allows us to explain why other preparations responded differently. For
instance, the nature of the response (increase versus decrease) did
not correlate with the developmental stage. Sex could affect this response,
however, male and female tadpoles were not distinguished prior to brainstem
dissection. Given that a low phenylephrine concentration was sufficient to
elicit a comparable (but significant) increase in fictive lung ventilation
frequency, we propose that, in this stage group, the response observed during
NA bath application is mainly mediated by 1-adrenoceptor
activation. The fact that the response to low clonidine concentration was
small in the pre-metamorphic group suggests that the contribution of
2-adrenoceptors towards this response is relatively
modest.
These results differ from those observed in preparations from more mature
animals in which NA application caused a net decrease in fictive lung
ventilation frequency. In the metamorphic group, this response probably
involves both -adrenoceptor subtypes since both phenylephrine and
clonidine elicited similar decreases in fictive lung ventilation frequency.
However, none of the doses used were sufficient to elicit a response of the
same magnitude, thereby suggesting that the response observed with NA reflects
simultaneous activation of more than one receptor type. In the adult group,
however, the decrease in fictive lung ventilation frequency observed during NA
application was blocked by prazosine but not by RX821002. Furthermore, the
response observed during phenylephrine application was similar to the one
reported with NA, thus indicating that 1-adrenoceptors
contribute to this response.
Most electrophysiological studies addressing noradrenergic modulation of
fictive lung ventilation have been performed on immature rodents. However,
brainstem preparations from turtles are an excellent model system for in
vitro investigation of respiratory control in a mature animal. In their
experiments, Johnson et al. (Johnson et
al., 1998) reported that both NA and phenylephrine increase the
lung burst frequency generated by turtle brainstems. These results differ
substantially from those obtained in adult frogs in which opposite effects
were observed, but are similar to those obtained in the pre-metamorphic group.
In the absence of data from immature turtles, it is difficult to determine
whether such interspecies differences are related to different developmental
trajectories or whether turtles simply do not show much change in the
organization of the neural pathways involved in noradrenergic modulation
during development.
It is noteworthy that 1-adrenoceptor activation increases
fictive lung ventilation frequency in neonatal rodents also. Although more
species need to be investigated, 1-mediated stimulation of
lung ventilation during early life could be a common configuration amongst
vertebrates. Although fewer studies have addressed the role of
ß-adrenoceptors in the modulation of respiratory rhythm in
vitro, data indicate that activation of these receptors has little effect
on the fictive breathing frequency in mammalian preparations
(Arata et al., 1998).
To the best of our knowledge, no other preparations besides amphibians and
neonatal rodents have been used to investigate 2-mediated
modulation of respiratory activity in vitro. In mammals,
2-adrenoceptor activation typically decreases fictive
breathing frequency (Al-Zubaidy et al.,
1996; Arata et al.,
1998; Errchidi et al.,
1991; Viemari and Hilaire,
2002) (for a review, see
Hilaire et al., 2004). Whereas
clonidine application elicited a modest lung burst frequency depression in the
amphibian metamorphic group, more convincing responses were observed in the
other two groups in which 2-adrenoceptor activation augments
fictive lung ventilation in this species. Again, the lack of mammalian data
from other developmental stages makes is difficult to explain these
interspecies differences.
Using low Ca2+, high Mg2+ solutions to block chemical
synaptic transmission, Arata et al. (Arata
et al., 1998) showed that, in neonatal rats, the direct effect of
adrenaline on premotor respiratory neurons is mainly excitatory, whereas the
inhibitory actions arise from indirect (GABAergic) pathways. A similar
configuration in Rana catesbeiana could contribute to the
developmental changes in lung burst frequency response to NA and phenylephrine
application because during early development, GABAA receptor
activation depolarises neurons and increases their excitability. With
maturation, Cl- gradients are better established and the effects of
GABA become mainly inhibitory (Ben-Ari,
2002). The role of indirect GABAergic pathways in noradrenergic
modulation of respiratory activity during development is currently being
addressed in our laboratory.
Developmental changes in noradrenergic modulation of fictive buccal ventilation frequency
In tadpoles, NA bath application elicited stage-dependent increases in
buccal burst frequency. Although the minimum NA concentration necessary to
elicit a response was the same in both groups (5 µmol l-1), the
response was stage-dependent, and the increase observed in metamorphic
brainstems tended to be larger than the one observed in the pre-metamorphic
group. Application of -adrenoceptor agonists clearly showed that
neither receptor type is involved in this response since both drugs decreased
buccal burst frequency. Consequently, it would appear that
ß-adrenoceptors mediate the increase in fictive buccal ventilation
frequency observed during NA application. This evidence suggests (albeit
indirectly) that the noradrenergic circuitry modulating lung versus
buccal ventilation is distinct, or that different receptor types are involved
( versus ß).
Several factors can contribute to changes in buccal ventilation frequency.
Since production of both lung- and buccal-related respiratory activity
requires activation of the same motoneurone pools, it is clear the expression
of one type of activity will reduce the time available for the expression of
the other. Given that fictive lung ventilation frequency produced by pre- and
metamorphic tadpole brainstems changed in opposite ways during NA application,
it is unlikely that this activity interfered with the production of
buccal-related activity. Thus, the increase in buccal burst frequency observed
during NA application could reflect a change in the endogenous rhythm and/or
facilitation of its expression by NA. These differences in the magnitude of
the responses observed during NA versus selective agonist application
likely reflect the opposite effects of - and ß-adrenoceptors on
buccal burst frequency. Although these results provide no clue concerning the
potential mechanisms underlying developmental changes in the magnitude of this
response, the fact that its nature does not change (increase versus
decrease) suggests that changes in adrenoceptor expression and/or the capacity
for NA release in the vicinity of respiratory neurons are probably
involved.
Perspectives
Noradrenergic modulation of respiratory motor output changes substantially
over the course of development. In mammals, many noradrenergic neurons are
hypoxia sensitive and have been implicated in the ventilatory response to
hypoxia (Neubauer and Sunderram,
2004; Roux et al.,
2000; Soulier et al.,
1997). In pre-metamorphic tadpoles, activation of noradrenergic
neurons by hypoxia would tend to facilitate both lung and buccal ventilation.
These responses could help alleviate the reduction in O2
availability and contribute to both air breathing behaviour and lung
development. However, developmental change in fictive buccal ventilation
frequency response to NA application is contrary to the changes described in
intact bullfrog tadpoles in which the buccal hyperventilatory response to
aquatic hypoxia decreases during development
(Jia and Burggren, 1997). This
change has been ascribed to progressive degeneration of the gills arches
containing O2 chemoreceptor
(Jia and Burggren, 1997), and
changes in noradrenergic modulation may occur as a reaction to the decrease in
chemosensory afferent signal to help maintain adequate buccal ventilation
during hypoxia.
The attenuation of fictive lung ventilation frequency during NA application
observed in more mature animals is more difficult to explain because
contribution of the lungs in gas exchange become increasingly important during
development (Burggren and West,
1982), and decreasing lung ventilation frequency during hypoxia
seems paradoxical, especially when, in intact frogs, enhancing pulmonary
O2 uptake is the primary adjustment to increased O2
demand (Pinder and Burggren,
1986). However, this response may be part of a larger strategy
aimed at minimizing energy expenditure when more mature (and less tolerant)
tadpoles and frogs face hypoxia.
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Acknowledgments |
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References |
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Values
statistically different from corresponding pre-metamorphic value at
P<0.05.
