CHEMOSENSORY AND CHOLINERGIC STIMULATION OF FICTIVE RESPIRATION IN ISOLATED CNS OF NEONATAL OPOSSUM

1. The aim of the present experiments was to characterize the central chemical drive of fictive respiration in the isolated CNS of the newbo rn opossum, Monodelphis domestica. This opossum preparation, in contra st to those of neonatal rats and mice, produces respiratory rhythm of high frequency in vitro. 2. Fictive respiration was recorded from C3-C 5 ventral roots of the isolated CNS of 4- to 14-day-old opossums using suction electrodes. At room temperature (21-23 degrees C) the frequen cy 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% O-2, p H 7.37-7.40. Respiratory discharges remained regular throughout 8 h ex periments and continued for more than 20 h in culture. 3. Superfusion 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 pH 7.7. Additio n of ACh (50-100 mu M) or carbachol (0.01-10 mu M) to the brainstem su perfusion also increased the amplitude and frequency of respiratory ac tivity, as did physostigmine (50-100 mu M) or neostigmine (20-50 mu M) . Conversely, scopolamine (50-100 mu M) reduced the amplitude and freq uency of the basal respiratory rhythm by about 30%. 4. H+- and choline rgic-sensitive areas on the surface of the isolated CNS were explored with a small micropipette (outer tip diameter, 100 mu m) filled with B MEM (pH 6.5) or 1 mu M carbachol. Carabachol applied to H+- and cholin ergic-sensitive areas in the ventral medulla mimicked the changes of r espiratory pattern produced by low pH application. Responses to altere d pH and carbachol were abolished by scopolamine (50 mu M). Histochemi stry demonstrated several medullary groups of neurons stained for acet ylcholinesterase. The superficial location of one of these groups coin cided with a functional and anatomically well-defined pH- and carbacho l-sensitive area placed medial to the hypoglossal roots. 5. Exploratio n of chemosensitive areas revealed that application of drugs or soluti ons of different pH to a single well-defined spot could have selective and distinctive effects upon amplitude and frequency of respiratory a ctivity. 6. These results show that fictive respiration in the isolate d CNS of the newborn opossum is tonically driven by chemical- and chol inergic-sensitive areas located on the ventral medulla, the activity o f which regulates frequency and amplitude of respiration. They suggest that a cholinergic relay, although not essential for rhythm generatio n, is involved in the central pH chemosensory mechanism, or that choli nergic and chemical inputs converge upon the same input pathway to the respiratory pattern generator.