WIDESPREAD SITES OF BRAIN-STEM VENTILATORY CHEMORECEPTORS

We produced local tissue acidosis in various brain stem regions with 1-nl injections of acetazolamide (AZ) to locate the sites of central chemoreception. To determine whether the local acidosis resulted in a stimulation of breathing, we performed the experiment in chloralose-urethan anesthetized vagotomized carotid-denervated (cats) paralyzed servo-ventilated cats and rats and measured phrenic nerve activity (PNA) as the response index. Measurements of extracellular brain tissue pH by glass microelectrodes showed that AZ injections induced a change in pH at the injection center-equivalent to that produced by an increase in end-tidal Pco2 of almost-equal-to 36 Torr and that the change in brain pH was limited to a tissue volume with a radius of <350 mum. We found AZ injections sites that caused a significant increase in PNA to be located 1) within 800 mum of the ventrolateral medullary surface at locations within traditional rostral and caudal chemosensitive areas and the intermediate area, 2) within the vicinity of the nucleus tractus solitarii, and 3) within the vicinity of the locus coeruleus. Single AZ injections produced increases in PNA that were less-than-or-equal-to 69% of the maximum value observed with an increase in end-tidal Pco2. We conclude that central chemoreceptors are distributed at many locations within the brain stem, all within 1.5 mm of the surface, and that stimulation of a small fraction of all central chemoreceptors can result in a large ventilatory response.