Glutamatergic regulation of respiratory rhythm generation during development in the bullfrog brainstem

Abstract

This study examined the role of NMDA and non-NMDA glutamate receptors for regulating respiratory rhythm generation during development in the bullfrog, Rana catesbeiana. We used an in vitro brainstem preparation from pre-metamorphic tadpoles (T-K stages 8–16) and late-stage, post-metamorphic tadpoles (T-K stages 24–25). Respiratory-related neural activity corresponding with gill/lung activity in pre-metamorphic tadpole brainstems and buccal/lung activity in post-metamorphic brainstems were examined before and after blockade of ionotropic glutamate receptors. In both pre-metamorphic and post-metamorphic tadpoles, blockade of non-NMDA receptors with CNQX (0.1, 0.5, 1, 5 μM) significantly increased gill/buccal burst frequency (P<0.01), but reversibly abolished lung burst frequency at 5 μM (P<0.01). Blockade of NMDA receptors with AP-4 (0.1, 5, 10, 50, 100 mM) had no significant effect on gill/buccal or lung burst frequency at any stage of development, but did affect gill burst amplitude of pre-metamorphic tadpoles during the washout period. These data indicate that glutamate, acting at non-NMDA receptors, is inhibitory to gill burst activity but excitatory to lung burst activity and may be critical for activation of the lung rhythm generator. Glutamate acting at NMDA receptors, however, appears to have a role burst pattern formation, but not respiratory rhythm generation. These results indicate that glutamate differentially affects gill and lung rhythm generation with fast synaptic non-NMDA receptors having a major role on respiratory rhythm generation throughout development. Supported by NIH SO6 GM48135.