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DEVELOPMENT AND TISSUE PLASTICITY

Ontogeny of central CO₂ chemoreception: chemosensitivity in the ventral medulla of developing bullfrogs

¹Department of Physiology, Dartmouth Medical School, Lebanon, New Hampshire 03756-0001; ²Biology Department, University of Texas-San Antonio, San Antonio, Texas 78249

Submitted 9 May 2003 ; accepted in final form 4 August 2003

Sites of central CO₂ chemosensitivity were investigated in isolated brain stems from Rana catesbeiana tadpoles and frogs. Respiratory neurograms were made from cranial nerve (CN) 7 and spinal nerve 2. Superfusion of the brain stem with hypercapnic artificial cerebrospinal fluid elicited increased fictive lung ventilation. The effect of focal perfusion of hypercapnic artificial cerebrospinal fluid on discrete areas of the ventral medulla was assessed. Sites of chemosensitivity, which are active continuously throughout development, were identified adjacent to CN 5 and CN 10 on the ventral surface of the medulla. In early- and middle-stage tadpoles and frogs, unilateral stimulation within either site was sufficient to elicit the hypercapnic response, but simultaneous stimulation within both sites was required in late-stage tadpoles. The chemosensitive sites were individually disrupted by unilateral application of 1 mg/ml protease, and the sensitivity to bath application or focal perfusion of hypercapnia was reassessed. Protease lesions at CN 10 abolished the entire hypercapnic response, but lesions at CN 5 affected only the hypercapnic response originating from the CN 5 site. Neurons within the chemosensitive sites were also destroyed by unilateral application of 1 mM kainic acid, and the sensitivity to bath or focal application of hypercapnia was reassessed. Kainic acid lesions within either site abolished the hypercapnic response. Using a vital dye, we determined that kainic acid destroyed neurons by only within 100 µm of the ventral medullary surface. Thus, regardless of developmental stage, neurons necessary for CO₂ sensitivity are located in the ventral medulla adjacent to CN 5 and 10.

isolated brain stem preparation; central carbon dioxide chemosensitivity; kainic acid; metamorphosis; amphibian; hypercapnia