Functional anatomical evidence for respiratory rhythmogenic function of endogenous bursters in rat medulla

Endogenous burster neurons (EBs) have been found at the level of the facial nucleus (VIIn), and 500 mum caudally, within the pre-Bötzinger complex (preBötC). They have been proposed as either causal to or playing no role in respiratory rhythmogenesis. Little is known about their broader distribution in ventrolateral medulla. Here, a Ca(2+) indicator was used to record respiratory network activity in ventrolateral medulla, and, following synaptic blockade, to identify EBs active at perfusate K(+) concentrations ([K(+)](o)) of 3, 6, and 9 mm. Recordings were made along the respiratory column, extending 300 mum rostrally, and 1100 mum caudally from the caudal pole of VIIn (VIIc), in the in vitro tilted sagittal slab preparation, isolated from neonate male and female Sprague Dawley rats. Activity under matching [K(+)](o) in the intact respiratory network was subsequently investigated. Respiratory neurons (n = 401) formed statistically significant clusters at the VIIc, within the preBötC, and 100 mum caudal to the preBötC. EBs (n = 693) formed statistically significant clusters that overlapped with respiratory clusters at the VIIc and preBötC. EB activity increased significantly as [K(+)](o) was increased, as did neurons that remained coupled following synaptic blockade. The overlap between respiratory and EB clusters in regions of ventrolateral medulla identified as rhythmogenic supports the hypothesis that EBs are constituents of rhythmogenic networks. In addition, the observation of truncated inspiratory bursts and ectopic bursting in respiratory neurons when [K(+)](o) was elevated in the intact network is consistent with a causal role for EBs in respiratory rhythmogenesis.