The role of vagal afferents and carbon dioxide in the respiratory response to thyrotropin-releasing hormone.

Rats treated neonatally with pargyline and 5,7-dihydroxytryptamine to decrease central serotonin-containing neurons have an accentuated respiratory response to i.c.v. thyrotropin-releasing hormone (TRH). Since these treated rats also evidence an elevated PaCO2, we sought to evaluate the importance of CO2 in determining the magnitude of the respiratory response to TRH. Neonatal treatment with capsaicin or acute vagotomy also produced adult animals whose basal PaCO2 was elevated and whose respiratory response to TRH was greater than that seen in control rats with lower PaCO2 values. In normal rats, however, administration of CO2 immediately before and after TRH administration does not alter the subsequent response to TRH. Thus, it appears that TRH facilitates the processing of CO2-dependent afferent impulses, and that CO2 does not alter disposition or pharmacokinetics of TRH.