Testing the hypothesis of neurodegeneracy in respiratory network function with a priori transected arterially perfused brainstem preparation of rat.

Degeneracy of respiratory network function would imply that anatomically discrete aspects of the brainstem are capable of producing respiratory rhythm. To test this theory we a priori transected brainstem preparations before re-perfusion and re-oxygenation at 4 rostro-caudal levels: 1.5 mm caudal to obex (n=5), at obex (n=5), 1.5 mm (n=7) and 3mm (n=6) rostral to obex. The respiratory activity of these preparations was assessed via recordings of phrenic (PNA), vagal (VNA) nerves and lumbar spinal expiratory motor output (L1). Preparations with a priori transection at level of the caudal brainstem did not produce stable rhythmic respiratory bursting, even when stimulating the arterial chemoreceptors with NaCN. Re-perfusion of brainstems that preserved the pre-Bötzinger complex (pre-BötC), showed spontaneous and sustained rhythmic respiratory bursting at low PNA amplitude that occurred simultaneously in all respiratory motor outputs. We refer to this rhythm as the pre-BötC burstlet-type rhythm. Conserving circuitry up to the pontine-medullary junction consistently produced robust high amplitude PNA at lower burst rates, while sequential motor patterning across the respiratory motor outputs remained absent. Some of the rostrally transected preparations expressed both burstlet-type and regular PNA amplitude rhythms. Further analysis showed that the burstlet-type rhythm and high amplitude PNA had 1:2 quantal relation with burstlets appearing to trigger high amplitude bursts. We conclude that no degenerate rhythmogenic circuits are located in the caudal medulla oblongata, and confirm the pre-BötC as the primary rhythmogenic kernel. The absence of sequential motor patterning in a priori transected preparations suggests that pontine circuits govern respiratory pattern formation.

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