Hypercapnia-induced activation of brainstem GABAergic neurons during early development

During early development, GABAergic mechanisms contribute to the regulation of respiratory timing in response to CO₂. In 5-7 day old piglets, a double labeling technique was used to determine whether GABA-containing neurons are activated by normoxic hypercapnia (10% CO₂, 21% O₂, and 69% N₂). The c-Fos gene encoded protein (c-Fos) was employed to localize CO₂ activated cells within the piglet medulla oblongata. Parvalbumin was used as a marker for GABAergic neurons. In animals breathing room air, only scant c-Fos-like immunoreactive neurons were observed. A marked increase in c-Fos positive cells was induced after a 60 min exposure to hypercapnia. Colocalization studies revealed that hypercapnia significantly increased c-Fos expression in GABA-containing neurons in the medulla oblongata, especially in the ventral aspect of the medulla, within the Bötzinger region, the gigantocellular reticular nucleus, and the caudal raphe nuclei. Only a few double-labeled cells were observed within the nucleus tractus solitarius. Therefore, brainstem GABAergic neurons are part of the neural networks that respond to CO₂ and may contribute to respiratory frequency responses to hypercapnia during early development.