BRAIN
RESEARCH
ELSEVIER
Brain Research 671 (1995) 314-316
Short communication
Role of the parabrachial nuclei in the airway dilation evoked
by the Hering-Breuer reflex
Ann M. Motekaitis *, Irene C. Solomon, Marc P. Kaufman
Division of Cardiovascular Medicine, Departments of Internal Medicine and Human Physiology, University of California, Davis, TB 172,
Davis, CA 95616, USA
Accepted 22 November 1994
Abstract
We tested the hypothesis that blockade of glutamatergic receptors in the parabrachial nucleus (PBN) of chloralose-anesthetized cats attenuated the reflex airway dilation evoked by activation of pulmonary stretch receptors. Unilateral microinjection of
kynurenic acid (50 nl, 100 mM) into the PBN reversibly attenuated the reflex relaxation of the trachealis muscle in 7 cats. These
findings suggest that the PBN is part of the central pathway mediating the airway dilation component of the Hering-Breuer
reflex.
Keywords: Airway smooth muscle; Autonomic nervous system; Pneumotaxic center
The Hering-Breuer reflex is well known to terminate inspiration. The parabrachial nuclei (PBN), and
the Kolliker-Fuse nucleus, which comprise the pontine
respiratory group, appear to be part of the central
pathway evoking this ventilatory effect. For example,
activation of pulmonary stretch receptors by inflation
of the lungs, inhibits the discharge of cells in the
medial PBN [7]. Further, the medial PBN and the
Kolliker-Fuse nucleus have been shown to control the
sensitivity of the Hering-Breuer reflex. Electrolytic lesions in these areas increase the lung volume threshold
necessary to terminate inspiration [6,8,12].
Another component of the Hering-Breuer reflex is
dilation of the airways [13]. Activation of pulmonary
stretch receptors, the afferent arm of the HeringBreuer reflex, dilates the airways by withdrawing
cholinergic tone to airway smooth muscle [2]. Although
the medial PBN has been implicated as essential for
full expression of the ventilatory component of the
Hering-Breuer reflex, its role in causing the airway
dilation component is unknown. Previous work from
our laboratory demonstrated that stimulation of the
medial and lateral PBN with a glutamate receptor
agonist decreased total lung resistance by withdrawing
* Corresponding author. Fax: (1) (916) 752-3264.
0006-8993/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSDI 0 0 0 6 - 8 9 9 3 ( 9 4 ) 0 1 3 7 1 - 3
cholinergic tone to the airways [11]. We therefore
hypothesized that the PBN may be part of the reflex
arc dilating the airways when the Hering-Breuer reflex
is evoked by activation of pulmonary stretch receptors.
A detailed description of the methods has been
published elsewhere [11]. In nine cats, anesthesia was
induced by vaporized halothane (5%) and maintained
with a-chloralose (initial 35 mg/kg; supplement 5
mg/kg, i.v.). The cats were paralyzed (vecuronium
bromide; 0.1 mg/kg, i.v.) and their lungs were mechanically ventilated. The tracheal smooth muscle response
to stimulation of pulmonary stretch receptors was
recorded before and after unilateral blockade of glutamatergic receptors with kynurenic acid (Kyn; 50 nl, 100
mM) in the PBN. Pulmonary stretch receptors, whose
afferent fibers travel in the vagus nerves, were activated with positive end-expiratory pressure (PEEP).
This was accomplished by placing the expiratory outlet
of the ventilator under an additional 2-6 cm H20.
Transverse tension of the trachealis muscle was
recorded isometrically [3]. The site of Kyn microinjection into the PBN was selected on the basis that
chemical stimulation (DL-homocysteic acid; 25 nl, 100
mM) of this nucleus, decreased tracheal smooth muscle
tension and total lung resistance. In two cats, control
microinjections of xanthurenic acid (Xan; 50 nl, 100
mM), an inert analog of Kyn, were made prior to Kyn.
A.M. Motekaitis et al. / Brain Research 671 (1995) 314-316
315
K
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12
20
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t
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it
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rr, o~,l
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Fig. 1. Schematic drawings of coronal sections of the rostral pons
showing sites of kynurenic acid microinjection. Numbers on the left
side of the figure represent mm posterior (P) to interaural zero [1].
Filled circles represent sites where kynurenic acid was microinjected.
Open circles represent sites where xanthurenic acid followed by
kynurenic acid was microiniected. Abbreviations: BC, brachium conjuctivum; IC, inferior colliculus; P, pyramidal tract; PBI, lateral
parabrachial nucleus; PBm, medial parabrachial nucleus; SO, superior olive; TB, trapezoid body.
I n five cats, p h r e n i c n e r v e activity was r e c o r d e d a n d
i n t e g r a t e d by the m e t h o d described by E l d r i d g e [5].
Baseline was c a l c u l a t e d by averaging p h r e n i c n e r v e
discharge for the o n e m i n u t e p e r i o d p r e c e e d i n g the
a p p l i c a t i o n of P E E P .
P E E P was i n c r e a s e d for o n e m i n u t e a n d the maximal relaxation of the trachealis muscle was recorded.
P h r e n i c nerve activity was i n t e g r a t e d , averaged over
this o n e m i n u t e period, a n d t h e n c o m p a r e d with its
b a s e l i n e value. All data are r e p o r t e d as m e a n + S.E.M.
Statistical significance ( P < 0.05) was d e t e r m i n e d with
a repeated measures one-way A N O V A .
U n i l a t e r a l m i c r o i n j e c t i o n of Kyn into the m e d i a l
( n = 6) a n d the lateral ( n = 1) P B N (Fig. 1) reversibly
a t t e n u a t e d the trachealis muscle r e s p o n s e to a n increase in P E E P (Figs. 2 a n d 3). T h e increase in P E E P
also evoked a small (i.e. 8 + 1 m m H g ) , n o n - s i g n i f i c a n t
decrease in m e a n arterial p r e s s u r e which did n o t
c h a n g e p o s t - K y n microinjection. I n two of these cats,
m i c r o i n j e c t i o n of X a n into the P B N did n o t a t t e n u a t e
the tracheal relaxation b u t s u b s e q u e n t m i c r o i n j e c t i o n
of Kyn into the same site did. T r a c h e a l relaxation was
n o t a t t e n u a t e d in two a d d i t i o n a l cats by m i c r o i n j e c t i o n
of Kyn into t h r e e different sites o u t s i d e of the P B N (2
sites in o n e cat, 1 site in the other). Histology revealed
that two of these sites were c a u d a l a n d o n e was m e d i a l
to the p a r a b r a c h i a l n u c l e u s (not shown). I n o n e cat
d u r i n g the time of a t t e n u a t i o n of the reflex relaxation
of the trachealis muscle, we i n c r e a s e d the stimulus
from 2 to 7 cm H 2 0 to test w h e t h e r the airway smooth
muscle was still reactive. T h e g r e a t e r stimulus r e s u l t e d
(o)
\
/--~
/
16
1
+
+
lSl
- - .
Fig. 2. Computer-aided reconstruction of tile reflex response of the
trachealis muscle to activation of the pulmonary stretch receptors.
Arrows indicate an increase in PEEP from 1 to 5 cm H20. Note that
with the increase in PEEP, the trachealis muscle relaxed and after
the increase in PEEP was removed, returned to baseline tension
levels (panel A). Sixty minutes after unilateral microiniection of
l~nurenic acid into the parabrachial nucleus, the trachealis did not
respond to an increase in PEEP (panel B) but by 90 minutes,
recovery occurred (panel C).
J
~
•
I-I Baseline
i~
m
mPEEP
*
e
Tracheal
Tension
(g)
tS
10
Control AttenuationRecov~
Atrop~
Fig. 3. Summary data from 7 cats showing the effects of unilateral
microinjection of kynurenic acid (KYN; 50 nl, 100 mM) into the
parabrachial nucleus on the tracheal smooth muscle response to an
increase in PEEP. Open bars represent baseline means and filled
bars represent mean responses to an increase in PEEP. Peak attenuation occurred at 33 + 10 min (shown as attenuation). Full recovery
of the response occurred at 69 + 9 min (shown as recovery). Asterisks
represent statistically significant difference (P < 0.05) between baseline and its corresponding response to PEEP; brackets across the top
represent statistically significant difference (P < 0.05) between the
mean responses to PEEP. Note that the decrease in tracheal tension
evoked by PEEP during lqmurenic injection was significantly less
(P < 0.05) than the decrease in tracheal tension evoked by PEEP
during the control or recovery periods. Also note that even though
baseline muscle tension decreased during the course of the experiment, the trachealis muscle was still capable of further relaxation as
indicated by its response to atropine (1 mg/kg, i.v; hatched bar).
316
A.M. Motekaitis et al. /Brain Research 671 (1995) 314-316
in a reflex relaxation of 3.0 g, whereas the smaller
stimulus resulted in a reflex relaxation of only 0.4 g.
The latency between the increase in PEEP and the
start of tracheal muscle relaxation was 7 + 1 s before
microinjection of Kyn into the PBN (n = 7). In three
cats in which the response was attenuated but not
abolished by microinjection of Kyn, the latency increased, non-significantly, to 12 + 4 s. The latency of
the response after recovery (11 + 3 s) still tended to be
non-significantly longer than the latency of the control
response.
In five cats, an increase in PEEP evoked a decrease
in phrenic nerve activity to 57 + 19% of baseline of
nerve activity. Microinjection of Kyn into the PBN did
not change the magnitude of this response. For example, at the time of peak attenuation (i.e. 33 min, Fig. 3)
of the trachealis muscle response to PEEP, phrenic
nerve activity decreased to 58 + 16% of baseline. Likewise, at the time of recovery (i.e. 120 min, Fig. 3),
phrenic nerve activity decreased to 60 + 17% of baseline.
We have shown that unilateral blockade of glutamatergic receptors in the PBN reversibly attenuated
the reflex relaxation of tracheal smooth muscle evoked
by P E E P but had no effect on phrenic response to this
maneuver. Previous studies have demonstrated that the
medial PBN and the Kolliker-Fuse nucleus are essential for the full expression of the ventilatory component
of the Hering-Breuer reflex [6,8,12]. The reason that
our microinjections of Kyn did not have an effect on
the ventilatory component of the Hering-Breuer reflex
may have been due to the criterion used in selecting
the site in the PBN to be blocked. This criterion was
that stimulation evoked a decrease in tracheal tension
and a parallel decrease in total lung resistance. The
concomitant phrenic response was not a criterion and
consisted of neural apnea (n = 2), a decrease (n = 2),
or a small increase (n = 1) in phrenic discharge. Our
findings agree with previous studies which reported
stimulation of the PBN evoked various effects on
phrenic nerve discharge [4,9,11].
Another reason that we did not effect the ventilatory component of the Hering-Breuer reflex may be
the type and extent of blockade used. Previous studies
examining the effect of blockade in the parabrachial
nucleus on ventilatory control used large bilateral electrolytic lesions that irreversibly destroyed both cells
and fibers of passage [6,8,12]. A recent study examining
ventilatory control used large volumes (80-3,000 nl) of
a N M D A receptor antagonist, MK-801, which were
bilaterally microinjected into the PBN region [10]. This
study found a large variation in the magnitude of the
prolongation of phrenic inspiratory time evoked by
withholding lung inflation. Moreover, the larger the
volume and the closer the microinjection was to the
Kolliker-Fuse nucleus, the larger the effect on prolonging inspiratory time [10].
In contrast to the above studies, we unilaterally
blocked glutamatergic receptors in a small area of the
PBN that was shown to relax airway smooth muscle
when stimulated with oL-homocysteic acid. Therefore,
the possibility exists that a larger a n d / o r different area
needs to be blocked to have an effect on the ventilatory
component of the Hering-Breuer reflex.
The PBN has been demonstrated to be essential for
the full expression of the ventilatory component of the
Hering-Breuer reflex [6,8,12] but it was not known if
this region was also essential for the airway dilation
component of this reflex. Previously, we had demonstrated that stimulation of the PBN evoked dilation of
the airway smooth muscle by withdrawing cholinergic
tone [11]. This study has extended those findings by
demonstrating that the PBN plays a role in causing the
airway dilation evoked by the Hering-Breuer reflex.
The PBN might be an essential relay in the reflex arc
causing this dilation or, alternatively, it might function
to lower the volume 'threshold' needed to evoke the
airway component of this reflex.
We thank Mr. Michael Wong for his technical assistance. This work was supported by NIH Grant
HL40910.
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