Introduction motoneurons’’ [3], their role in CNS control of breathing is
Brain Research 880 2000 118–130 www.elsevier.com locate bres
Research report
Specific subnuclei of the nucleus tractus solitarius play a role in determining the duration of inspiration in the rat
a b
a a ,
Adam M. Wasserman , Niaz Sahibzada , Yvonne M. Hernandez , Richard A. Gillis
a
Department of Pharmacology Georgetown University Medical Center Washington , DC 20007 USA
b
Department of Psychology University of the District of Columbia Washington , DC 20008 USA
Accepted 2 July 2000
Abstract
Our previous data obtained in the cat suggest that the neurons of the ventrolateral subnucleus of the tractus solitarius vlNTS act as an inspiratory off-switch and terminate the inspiratory phase of the respiratory cycle Berger et al., Eur. J. Pharmacol. 277 1995 195–208;
Gillis et al., Neurosci. Abstr. 23 1997 725. The purpose of the present study was to determine whether inhibition of the region of the vlNTS of the rat using drugs that hyperpolarize, disfacilitate or block both axonal conduction and action potential generation would alter
the inspiratory phase of the respiratory cycle. Experiments were conducted in anesthetized, vagotomized and spontaneously breathing rats while monitoring diaphragmatic electromyogram activity. Vagus nerves were sectioned in order to rule out prolongation of inspiration
evoked by microinjection of agents into the vlNTS which block excitatory drive from lung afferent inputs. Bilateral microinjection of the inhibitory amino acid g-aminobutyric acid GABA 25 nmol 45 nl produced an immediate prolongation of inspiratory duration 484618
to 1291684 ms and an apneustic pattern of breathing. Other effects observed were a significant shortening of expiratory duration 778636 to 432638 ms, rise in blood pressure 8364 to 10866 mmHg and a small but significant increase in heart rate 439617 to
1
452618 beats min. Bilateral microinjection of the ionotropic glutamate receptor antagonist kynurenic acid 1 nmol and the Na channel
blocker tetrodotoxin 10 pmol into the region of the vlNTS consistently produced a similar prolongation of inspiratory duration and an apneustic pattern of breathing. These results support the hypothesis that neurons in the region of the vlNTS promote the transition from
inspiration to expiration and function as part of the ‘Inspiratory Off Switch’.
2000 Elsevier Science B.V. All rights reserved.
Theme : Endocrine and autonomic regulation
Topic : Respiratory regulation
Keywords : Apneusis; Respiration; Nucleus tractus solitarius; Inspiratory Off Switch
1. Introduction motoneurons’’ [3], their role in CNS control of breathing is
now considered to be relatively minor, especially in Respiratory rhythm and pattern formation are thought to
rodents [2,22,35,39]. Indeed, removal of the DRG appears be products of respiratory neurons concentrated into two
to have no effect on rhythm generator neurons in a slice or discrete areas of the medulla oblongata [14,23]. One of
brainstem–spinal cord preparation [34,39]. These findings these areas of respiratory neurons is located in the dor-
raise the question as to what role is played by the DRG in somedial medulla around the obex and has been designated
the control of breathing and addressing this question is the the dorsal respiratory group DRG. In the cat, the DRG
focus of the present study. corresponds most closely to the ventrolateral nucleus of the
Currently, the consensus of opinion is that the DRG can solitary tract. While these neurons were at one time
convey respiratory rhythmic drive to phrenic and intercost- suggested as the ‘‘principal respiratory drive to phrenic
al motoneurons [5]. This opinion is based on studies carried out in cats where neurons of the DRG, specifically
those neurons located in the vlNTS monosynaptically excite phrenic and intercostal motoneurons [12,22]. In the
Corresponding author: Tel.: 11-202-687-1607; fax: 11-202-687-
rat, however, very few neurons of the vlNTS monosynapti-
6437. E-mail address
: gillisrgunet.georgetown.edu R.A. Gillis.
cally excite motoneurons in the spinal cord that control
0006-8993 00 – see front matter
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A .M. Wasserman et al. Brain Research 880 2000 118 –130
119
muscles for breathing [9,31,43]. The vlNTS respiratory incision below the chest cavity and inserting a hooked
neurons can also transfer afferent information from the bipolar platinum–iridium electrode to the right half of the
lungs to VRG bulbospinal inspiratory neurons [5]. How- costal diaphragm. The electrode was coupled to a Tek-
ever, data obtained for both the cat and rat indicate that tronix AM 502 differential AC preamplifier Tektronix,
this is not the role of the vlNTS neurons but is the function Wilsonville, OR. The output signal of the amplifier was
of neurons located in the medial subnucleus of the tractus fed into an audiometer and displayed on a storage oscillos-
solitarius mNTS [6,26]. In a recent study of the role of cope and computer monitor. Data were stored on computer
DRG neurons in control of respiration in the rat it was Apple Macintosh PowerPC connected to MacLab for
concluded based on cross-correlation analysis that DRG later viewing and analysis. The dEMG signal and the
neurons did not appear to convey respiratory rhythmic software-generated integration iEMG of the raw signal
drive to phrenic and intercostal motoneurons [42]. It was using a 100-ms time window were continuously recorded
also concluded that DRG neurons had little role in and stored for off-line analysis. Heart rate was obtained
transmitting afferent information from the lungs to VRG from the blood pressure signal and was averaged over
bulbospinal inspiratory neurons. eight beat bins to minimize beat-to-beat fluctuations.
Our previous data obtained in the cat suggest another possible role for the vlNTS, namely to act as an inspiratory
off switch and terminate the inspiratory phase of the 2.2. Stereotaxic surgery and microinjection procedure
respiratory cycle [4,15]. In these studies we found that microinjection of NMDA receptor antagonists into the
Animals were oriented ventral-side down and placed in vlNTS produced an increase in inspiratory duration and an
a Kopf small-animal stereotaxic frame Tujunga, CA. The apneustic breathing pattern. Since numerous investigators
dorsal medulla was exposed via a limited occipital have reported the existence of respiratory-related neurons
craniotomy with reflection of the dura and retraction of the in the vlNTS and adjacent interstitial subnucleus of the
cerebellum in an analogous fashion as described in the cat tractus solitarius iNTS of the rat [6,9,26,28,29,38,41,42],
[33]. Preliminary studies not shown using the calamus we set out to test the hypothesis that these neurons in the
scriptorius i.e., caudal tip of the area postrema as our rat can function as inspiratory off-switch neurons.
reference point were used to determine microinjection coordinates for the specific subnuclei of the NTS. Briefly,
Fast Green FCF dye 5 was microinjected into a series of coordinates and the resulting dye locations compared