Transmission of the respiratory rhythm to trigeminal and hypoglossal motor neurons in the American Bullfrog (Lithobates catesbeiana)

Abstract

Spatially distinct, interacting oscillators in the bullfrog medulla generate and coordinated buccal and lung ventilatory rhythms, but how these rhythms are transmitted onto trigeminal and hypoglossal motor neurons is unknown. Using a vertically-mounted isolated brainstem preparation, the Sheep Dip, we identified the regions of the brainstem containing motor nuclei using a solution capable of blocking synaptic release and, following washout, locally exposed these regions to 5 μM NBQX and/or 50 μM AP5. Local application of NBQX significantly reduced the amplitude of buccal and lung bursts on the trigeminal nerve, and lung bursts on the hypoglossal nerve. Local AP5 caused a significant reduction in lung burst amplitude on both nerves, but for buccal bursts, hypoglossal amplitude increased and trigeminal amplitude was unchanged. Local co-application of NBQX and AP5 eliminated fictive respiratory motor output completely in both nerves. These results are consistent with mammalian data, suggesting a critical role for glutamate in transmission of respiratory activity from oscillators to motor neurons.

Keywords: Cranial motor neurons; Lithobates catesbeiana; Rana catesbeiana; Respiratory motor neurons; Upper airway motor neurons.