MODULATION OF RESPIRATORY RHYTHM IN-VITRO - ROLE OF G(I O) PROTEIN-MEDIATED MECHANISMS/

Slice preparations from neonatal rat medulla that generate respiratory rhythm in vitro were used to test, for G(i/o) protein-mediated mechan isms affecting breathing rhythm in mammals. The frequency of inspirato ry motor discharge recorded from hypoglossal (XII) nerve roots decreas ed with bath application of gamma-aminobutyric acid (GABA) and norepin ephrine, as well as agonists specific for GABA(B), alpha(2)-adrenergic , and mu-opioid receptors; 5-hydroxytryptamine had little effect on fr equency. Microinjection of these specific agonists into the pre-Botzin ger complex, the site of respiratory rhythm generation in vitro, also decreased frequency. In contrast, substance P (SP) increased frequency when it was bath applied or microinjected into the pre-Botzinger comp lex. To test for involvement of G(i/o), proteins, pertussis toxin (PTX ) was injected into the cerebrospinal fluid of newborn rats, and slice s from these animals were tested 48 h later for block of drug effects on rhythm. In PTX-treated slices the frequency decrease due to GABA(B) , mu-opioid, and alpha(2)-adrenergic receptor activation was attenuate d (P less than or equal to 0.05), whereas the SP receptor-mediated res ponse was unaltered. To test for involvement of K+ conductances linked to G(i/o) proteins, Ba2+ (0.2 mM) was added to the bath before applic ation of drugs. Ba2+ attenuated the decrease in frequency associated w ith GABA(B) (P less than or equal to 0.05) and mu-opioid (0.10 less th an or equal to P less than or equal to 0.05) receptor activation, wher eas the alpha(2)-adrenergic and SP responses were unaltered. We conclu de that GABA(B) and mu-opioid, but not alpha(2)-adrenergic and SP, rec eptor activation modulates respiratory frequency via a G(i/o) protein- dependent Ba2+-sensitive ionic conductance mechanism on neurons within the medullary locus for rhythm generation. This mechanism may be a co nvergent pathway for control of respiratory frequency.