Chemosensory and cholinergic stimulation of fictive respiration in isolated cns of neonatal opossum

Authors


  • Author's email address
    J. G. Nicholls: nicholls@ubaclu.unibas.ch

  • Author's present address
    J. Eugenín: Department of Biology, Faculty of Chemistry and Biology, Universidad de Santiago de Chile, Casilla 40-Correo 33, Santiago, Chile.

Abstract

  • 1The aim of the present experiments was to characterize the central chemical drive of fictive respiration in the isolated CNS of the newborn opossum, Monodelphis domestica. This opossum preparation, in contrast to those of neonatal rats and mice, produces respiratory rhythm of high frequency in vitro.
  • 2Fictive respiration was recorded from C3–C5 ventral roots of the isolated CNS of 4- to 14-day-old opossums using suction electrodes. At room temperature (21–23 °C) the frequency of respiration was 43 ± 5.3 min−1 (mean ±s.e.m., n= 50) in basal medium Eagle's medium (BMEM) equilibrated with 5% CO2–95% O2, pH 7.37-7.40. Respiratory discharges remained regular throughout 8 h experiments and continued for more than 20 h in culture.
  • 3Superfusion of the brainstem confirmed that solutions of pH 6.3–7.2 increased both the amplitude and frequency of respiration. High pH solutions (7.5–7.7) had the opposite effect and abolished the rhythm at pH7.7. Addition of ACh (50–100 μm) or carbachol (0.01–10 μm) to the brainstem superfusion also increased the amplitude and frequency of respiratory activity, as did physostigmine (50–100 μm) or neostigmine (20–50 μm). Conversely, scopolamine (50–100 μm) reduced the amplitude and frequency of the basal respiratory rhythm by about 30%.
  • 4H+ - and cholinergic-sensitive areas on the surface of the isolated CNS were explored with a small micropipette (outer tip diameter, 100 μm) filled with BMEM (pH 6.5) or 1 μm carbachol. Carabachol applied to H+ - and cholinergic-sensitive areas in the ventral medulla mimicked the changes of respiratory pattern produced by low pH application. Responses to altered pH and carbachol were abolished by scopolamine (50 μm). Histochemistry demonstrated several medullary groups of neurons stained for acetylcholinesterase. The superficial location of one of these groups coincided with a functional and anatomically well-defined pH- and carbachol-sensitive area placed medial to the hypoglossal roots.
  • 5Exploration of chemosensitive areas revealed that application of drugs or solutions of different pH to a single well-defined spot could have selective and distinctive effects upon amplitude and frequency of respiratory activity.
  • 6These results show that fictive respiration in the isolated CNS of the newborn opossum is tonically driven by chemical- and cholinergic-sensitive areas located on the ventral medulla, the activity of which regulates frequency and amplitude of respiration. They suggest that a cholinergic relay, although not essential for rhythm generation, is involved in the central pH chemosensory mechanism, or that cholinergic and chemical inputs converge upon the same input pathway to the respiratory pattern generator.

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