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1 School of Medicine, University of Louisville, Louisville, Kentucky, United States
2 Pediatrics, University of Louisville, Louisville, Kentucky, United States
3 Pediatrics, University of Louisville, Louisville, Kentucky, United States; Pediatrics , University of Louisville, 570 South Preston Street, Louisville, Kentucky, 40202, United States
* To whom correspondence should be addressed. E-mail: nicholas.mellen{at}louisville.edu.
In mammals, respiration-modulated networks are distributed rostrocaudaly in the ventrolateral quadrant of the medulla. Recent studies have established that in neonate rodents, two spatially separate networks along this column, the parafacial respiratory group (pFRG) and the pre-Boetzinger complex (preBoetC), are hypothesized to be sufficient for respiratory rhythm generation, but little is known about the connectivity within or between these networks. In order to be able to observe how these networks interact, we have developed a neonate rat medullary tilted sagittal slab, which exposes one column of respiration-modulated neurons on its surface, permitting functional imaging with cellular resolution. Here we examined how respiratory networks responded to hypoxic challenge and opioid-induced depression. At the systems level, the sagittal slab was congruent with more intact preparations: hypoxic challenge led to a significant increase in respiratory period and inspiratory burst amplitude, consistent with gasping. At opioid concentrations sufficient to slow respiration, we observed periods at integer multiples of control, matching quantal slowing. Consistent with single-unit recordings in more intact preparations, respiratory networks were distributed bimodally along the rostrocaudal axis, with respiratory neurons concentrated at the caudal pole of the facial nucleus, and 350 microns caudally, at the level of the pFRG and the preBoetC respectively. Within these regions neurons active during hypoxia- and/or opioid-induced depression were ubiquitous, and interdigitated. In particular, contrary to earlier reports, opiate-insensitive neurons were found at the level of the preBoetC.
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