CO₂/H⁺ chemoreception in the cat pre-Botzinger complex in vivo

We examined the effects of focal tissue acidosis in the pre-Botzinger complex (pre-BotC; the proposed locus of respiratory rhythm generation) on phrenic nerve discharge in chloralose-anesthetized, vagotomized, paralyzed, mechanically ventilated cats. Focal tissue acidosis was produced by unilateral microinjection of 10-20 nl of the carbonic anhydrase inhibitors acetazolamide (AZ; 50 μM) or methazolamide (MZ; 50 μM). Microinjection of AZ and MZ into 14 sites in the pre-BotC reversibly increased the peak amplitude of integrated phrenic nerve discharge and, in some sites, produced augmented bursts (i.e., eupneic breath ending with a high-amplitude, short- duration burst). Microinjection of AZ and MZ into this region also reversibly increased the frequency of eupneic phrenic bursts in seven sites and produced premature bursts (i.e., doublets) in five sites. Phrenic nerve discharge increased within 5-15 min of microinjection of either agent; however, the time to the peak increase and the time to recovery were less with AZ than with MZ, consistent with the different pharmacological properties of AZ and MZ. In contrast to other CO₂/H⁺ brain stem respiratory chemosensitive sites demonstrated in vivo, which have only shown increases in amplitude of integrated phrenic nerve activity, focal tissue acidosis in the pre-BotC increases frequency of phrenic bursts and produces premature (i.e., doublet) bursts. These data indicate that the pre-BotC has the potential to play a role in the modulation of respiratory rhythm and pattern elicited by increased CO₂/H⁺ and lend additional support to the concept that the proposed locus for respiratory rhythm generation has intrinsic chemosensitivity.