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The Journal of Neurophysiology Vol. 83 No. 3 March 2000, pp. 1243-1252
Copyright ©2000 by the American Physiological Society
Department of Neurobiology, University of California, Los Angeles, California 90095-1763
Shao, X. M. and
J. L. Feldman.
Acetylcholine Modulates Respiratory Pattern: Effects Mediated by
M3-Like Receptors in PreBötzinger Complex Inspiratory
Neurons. J. Neurophysiol. 83: 1243-1252, 2000. Perturbations of cholinergic neurotransmission in the
brain stem affect respiratory motor pattern both in vivo and in vitro; the underlying cellular mechanisms are unclear. Using a medullary slice
preparation from neonatal rat that spontaneously generates respiratory
rhythm, we patch-clamped inspiratory neurons in the preBötzinger
complex (preBötC), the hypothesized site for respiratory rhythm
generation, and simultaneously recorded respiratory-related motor
output from the hypoglossal nerve (XIIn). Most (88%) of the
inspiratory neurons tested responded to local application of
acetylcholine (ACh) or carbachol (CCh) or bath application of
muscarine. Bath application of 50 µM muscarine increased the frequency, amplitude, and duration of XIIn inspiratory bursts. At the
cellular level, muscarine induced a tonic inward current, increased the
duration, and decreased the amplitude of the phasic inspiratory inward
currents in preBötC inspiratory neurons recorded under voltage
clamp at
60 mV. Muscarine also induced seizure-like activity evident
during expiratory periods in XIIn activity; these effects were blocked
by atropine. In the presence of tetrodotoxin (TTX), local ejection of 2 mM CCh or ACh onto preBötC inspiratory neurons induced an inward
current along with an increase in membrane conductance under voltage
clamp and induced a depolarization under current clamp. This response
was blocked by atropine in a concentration-dependent manner. Bath
application of 1 µM pirenzepine, 10 µM gallamine, or 10 µM
himbacine had little effect on the CCh-induced current, whereas 10 µM
4-diphenylacetoxy-N-methylpiperidine methiodide blocked
the current. The current-voltage (I-V) relationship of the CCh-induced response was linear in the range of
110 to
20 mV
and reversed at
11.4 mV. Similar responses were found in both pacemaker and nonpacemaker inspiratory neurons. The response to CCh was
unaffected when patch electrodes contained a high concentration of EGTA
(11 mM) or
bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (10 mM). The response to CCh was reduced greatly by substitution of 128 mM Tris-Cl for NaCl in the bath solution; the I-V
curve shifted to the left and the reversal potential shifted to
47 mV. Lowering extracellular Cl
concentration from 140 to
70 mM had no effect on the reversal potential. These results suggest
that in preBötC inspiratory neurons, ACh acts on M3-like ACh
receptors on the postsynaptic neurons to open a channel permeable to
Na+ and K+ that is not Ca2+
dependent. This inward cation current plays a major role in
depolarizing preBötC inspiratory neurons, including pacemakers,
that may account for the ACh-induced increase in the frequency of
respiratory motor output observed at the systems/behavioral level.
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