Orexin Receptor‐1 Inactivation Facilitates Lung Motor Activity during In Vitro Hypercapnic Stimulation of Pre‐metamorphic Bullfrogs

The mechanisms regulating amphibian respiration change dramatically during development as the animal transitions from aquatic to aerial breathing. Gill ventilation is the primary mechanism for oxygen acquisition in early stages, but infrequent lung breaths also occur. Interestingly, activating central chemoreceptive pathways (i.e. hypercapnia, hypoxia) can stimulate lung motor activity in tadpoles. Yet, the mechanisms which modulate chemosensitivity at various stages of development remain poorly understood in amphibians. Orexin is a critical hypothalamic neurotransmitter that is highly conserved across vertebrates and regulates important functions including sleep‐wake states, feeding, and breathing. In many vertebrates, orexinergic neurons are profoundly excited by increased CO 2 (and decreased pH), suggesting that they are intrinsically chemosensitive. We hypothesized that antagonism of the orexin receptor‐1 (OX 1 R) would blunt the increased ventilatory response to hypercapnia in pre‐metamorphic tadpoles of the American bullfrog ( Lithobates catesbeianus ). Using tadpole brainstem preparations (Taylor‐Kollros stages: IV–XII), we determined the effects of hypercapnia alone (5% CO 2 , 20 mins), or in the presence of an OX 1 R antagonist (SB‐334867, 10μM) on respiratory motor activity recorded from cranial nerves V, VII and X. Isolated brainstems were transected rostral to the optic nerve to maintain an intact hypothalamus. Contrary to previous reports performed on preparations without the hypothalamus, our data showed no significant effect of hypercapnia on the frequency of lung bursting in this stage group. However, blocking OX 1 R's during hypercapnia revealed a significant ventilatory response observable as an increase in lung burst frequency (baseline respiratory motor activity was not affected by SB‐334867). Although these results were contrary to our hypothesis, they demonstrate that orexin inhibits the central CO 2 chemoreflex of pre‐metamorphic bullfrog tadpoles. Support or Funding Information This work was supported by the Emerging Leaders in the Americas Program and Fonds de Recherche de Quebec ‐ Nature et Technologies awarded to E. Fonseca. Additional support was provided by the Fonds de Recherche du Québec ‐ Santé (FRQS) for T. Janes and an NSERC Discovery Grant awarded to R. Kinkead. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .