Endogenous Glutamatergic Control of Rhythmically Active Mammalian Respiratory Motoneurons In Vivo

The transmission of rhythmic drive to respiratory motoneurons in vitro is critically dependent on glutamate acting primarily on non-NMDA receptors. We determined whether both non-NMDA and NMDA receptors contribute to respiratory drive transmission at respiratory motoneurons in the intact organism, both in the state of anesthesia and in the same animals during natural behaviors. Twenty-seven rats were implanted with electroencephalogram and neck electrodes to record sleep-wake states and genioglossus and diaphragm electrodes for respiratory muscle recordings. Microdialysis probes were inserted into the hypoglossal motor nucleus (HMN). Under anesthesia, non-NMDA or NMDA receptor antagonism significantly decreased respiratory-related genioglossus activity, indicating a contribution of each receptor to respiratory drive transmission at the HMN. However, despite the presence of respiratory-related genioglossus activity in the same rats across sleep-wake states, neither non-NMDA receptor antagonism at the HMN nor glutamate uptake inhibition had any effect on respiratory-related genioglossus activity. These results showed that, compared with anesthesia, respiratory drive transmission through the non-NMDA receptor is low in the behaving organism. In contrast, glutamate uptake inhibition increased tonic genioglossus activity in wakefulness and non-rapid-eye-movement sleep, indicating a functional endogenous glutamatergic modulation of tonic, but not respiratory, motor tone. Such an effect on tonic drive may contribute to the suppression of both tonic and respiratory-related genioglossus activity in wakefulness and sleep with NMDA receptor antagonism at the HMN. These data do not refute previous identification of a glutamatergic (mostly non-NMDA receptor activating) respiratory drive to hypoglossal motoneurons, but this mechanism is more prominent in anesthetized or in vitro preparations.