The Journal of Neuroscience, September 15, 2001, 21(18):7363-7371
Phasic Vagal Sensory Feedback Transforms Respiratory Neuron
Activity In Vitro
Nicholas M.
Mellen and
Jack L.
Feldman
Department of Neurobiology, University of California Los Angeles,
Los Angeles, California 90095-1763
The isolated neonatal rat medulla generates respiratory-related
rhythms recorded from cervical spinal cord ventral roots. When lungs
and their vagal innervation are retained, respiratory activity is
modulated by lung mechanoreceptor feedback: transient lung inflation
triggered off inspiratory onset (phasic inflation) shortens inspiration
and increases respiratory frequency. In this study, the activity of six
respiratory neuron classes before and during phasic inflation was
studied. Type 1 and 2 inspiratory neurons, identified in the transverse
slice, were distinguished by the presence of a transient outward
current or a hyperpolarization-activated inward current, respectively.
Cell types only identified in the en bloc medulla included type II and
III inspiratory neurons, distinguished by delayed onset and
peri-inspiratory inhibition, respectively, and preinspiratory neurons,
active before and after but silent during inspiration. Biphasic
neurons, identified in the preparation used here, fired briskly during
lung inflation but are otherwise quiescent. During phasic inflation,
biphasic neurons showed a decrementing expiratory pattern of activity, matched by augmented postinspiratory hyperpolarization in type 1 neurons only, suggesting that biphasic neurons inhibit type 1 neurons,
removing drive to other inspiratory neurons and terminating the
inspiratory burst. This mechanism could account for a phasic inflation-induced increase in respiratory frequency via resetting effects. Alternatively, the phasic inflation-induced respiratory frequency increase may be attributable to slow facilitation. Slow modulation consistent with facilitation was apparent in the earlier onset of pre-I firing before inspiration and loss of postinspiratory firing and in the earlier onset of depolarization in type 2 neurons. On
the basis of relative onset times and responses to phasic inflation, connectivity between these cell types is proposed.
Key words:
mammal; neonate rat; respiration; pulmonary afferents; vagus; medulla; pre-Bötzinger complex
Copyright © 2001 Society for Neuroscience 0270-6474/01/21187363-09$05.00/0
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