Buccal rhythm generation in metamorphic bullfrogs is dependent on glycine

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FASEB Journal






Fish and early developmental amphibians use gill structures to exchange gases in water. In jawless fish, the neural circuit responsible for generating gill ventilation (buccal) rhythm is adjacent to the trigeminal nerve (CN V). In amphibians, a homologous rhythm generating site has been identified just caudal to the vagus nerve (CN X). The amphibian buccal oscillator is reported to be driven by Cl−mediated mechanisms, and subsequent inhibition of either GABA or glycinergic receptors abolishes this rhythm. This study investigates the effects of Cl− mediated antagonism of buccal rhythm generation and the distribution of buccal rhythm generating circuits.

We isolated brainstems from bullfrog tadpoles, placed them in a recording chamber, and allowed them to recover for 1 hour perfused with artificial cerebral spinal fluid (aCSF) equilibrated with 98.5% O2 and 1.5% CO2 (pH 7.8). During this time, neural activity from the facial (CN VII) and hypoglossal (CN XII) cranial nerves was recorded. Following recovery, brainstems were either transected at the level of the glossopharyngeal nerve (CN IX), removing the caudal medulla (rostral to the amphibian “buccal oscillator”) or left intact. Activity from the trigeminal (CN V), facial (CN VII), or hypoglossal (CN XII) nerves were recorded. A cocktail containing bicuculline methochloride (5 μM) and strychnine (5 μM) or bicuculline or strychnine alone was subsequently bath applied for 30 minutes.

Small amplitude rhythms were observed following transection, similar to buccal rhythms of intact isolated brainstems. These were accompanied by large amplitude rhythms, which we consider putative lung rhythms. Upon application of bicuculline, putative buccal rhythms in transected and intact brainstems remained and lung activity increased. Application of strychnine alone or in the presence of bicuculline abolished the small amplitude rhythm, and still enhanced lung activity. These results suggest that buccal-like rhythms are glycine dependent, and may be produced by a diffuse medullary network.