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1 Department of Physiology and Biophysics, State University of New York at Stony Brook, Stony Brook, NY, USA
* To whom correspondence should be addressed. E-mail: ICSolomon{at}physiology.pnb.sunysb.edu.
We have previously demonstrated that microinjection of DL-homocysteic acid (DLH), a glutamate analog, into the pre-Botzinger complex (pre-BotC) can produce tonic excitation of phrenic nerve discharge. Although this DLH-induced tonic excitation can be modified by systemic hypercapnia, the role of focal increases in pre-BotC CO2/H+ in this modulation of the DLH-induced response remains to be determined. Therefore, we examined the effects of unilateral microinjection of DLH (10 mM; 10-20 nl) into the pre-BotC before and during increased focal pre-BotC CO2/H+ (i.e., focal tissue acidosis) in chloralose-anesthetized, vagotomized, mechanically ventilated cats. Focal tissue acidosis was produced by blockade of carbonic anhydrase using either focal acetazolamide (AZ) or methazolamide (MZ) microinjection. For these experiments, we selected sites in which unilateral microinjection of DLH into the pre-BotC produced a non-phasic tonic excitation of phrenic nerve discharge (n=10). Microinjection of 10-20 nl AZ (50 µM) or MZ (50 µM) into these 10 sites in the pre-BotC increased the amplitude and/or frequency of eupneic phrenic bursts, as previously reported. Subsequent microinjection of DLH produced excitation in which phasic respiratory bursts were superimposed on tonic discharge. These DLH-induced phasic respiratory bursts had an increased frequency compared to the pre-injection baseline frequency (p < 0.05). These findings demonstrate that modulation of phrenic motor activity evoked by DLH-induced activation of the pre-BotC is influenced by focal CO2/H+ chemosensitivity in this region. Further, these findings suggest that focal increases in pre-BotC CO2/H+ may have contributed to the modulation of the DLH-induced responses previously observed during systemic hypercapnia.
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