The phrenic nucleus was identified by microinjections of N-methyl-D-aspartic acid in urethan-anesthetized adult male Wistar rats. Microinjections of GABA_A and GABA_B receptor agonists (muscimol and baclofen, respectively) at the same site decreased the phrenic nerve burst amplitude. Microinjections of GABA_A and GABA_B receptor antagonists (bicuculline and 2-hydroxysaclofen, respectively) blocked as well as reversed the effects of their respective agonists. These results were confirmed by recording extracellular action potentials from single phrenic neurons. Micropressure applications of muscimol and baclofen decreased the activity of single neurons in the phrenic nucleus; this effect was blocked as well as reversed by micropressure applications of bicuculline and 2-hydroxysaclofen, respectively. These results demonstrated the presence of GABA receptors on the neurons in the phrenic nucleus and suggested that their activation results in the decrease of the phrenic nerve burst amplitude. The importance of these results in the identification of neural circuits mediating inhibition of phrenic neurons is discussed.

baclofen; bicuculline; 2-hydroxysaclofen; muscimol; phrenic neurons; transmitters

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