Brainstem and spinal projections of augmenting expiratory neurons in the rat

There are two types of expiratory neurons with augmenting firing patterns (E-AUG neurons), those in the Bötzinger complex (BOT) and those in the caudal ventral respiratory group (cVRG). We studied their axonal projections morphologically using intracellular labeling of single E-AUG neurons with Neurobiotin, in anesthetized, paralyzed and artificially-ventilated rats. BOT E-AUG neurons (n = 11) had extensive axonal projections to the brainstem, but E-AUG neurons (n = 5) of the cVRG sent axons that descended the contralateral spinal cord without medullary collaterals. In addition to these somewhat expected characteristics, the present study revealed a number of new projection patterns of the BOT E-AUG neurons. First, as compared with the dense projections to the ipsilateral brainstem, those to the contralateral side were sparse. Second, several BOT E-AUG neurons sent long ascending collaterals to the pons, which included an axon that reached the ipsilateral parabrachial and Kölliker-Fuse nuclei and distributed boutons. Third, conspicuous projections from branches of these ascending collaterals to the area dorsolateral to the facial nucleus were found. Thus, the present study has shown an anatomical substrate for the extensive inhibitory projections of single BOT E-AUG neurons to the areas spanning the bilateral medulla and the pons.