Intrinsic responses to hypoxia and hypercapnia of neurons in the cardiorespiratory center of the ventral medulla of newborn rats.

The rostral ventrolateral medulla (RVLM) includes a variety of neurons essential for cardiorespiratory control. Although some of these neurons are thought to be intrinsically sensitive to hypercapnia and/or hypoxia, relationships between types of neurons and responses to hypoxia and/or hypercapnia are not well understood. Tyrosine hydroxylase (TH) is one of the cell-type markers of the RVLM neurons. Here, we report effects of hypoxia and hypercapnia on TH-positive or -negative neurons in the RVLM of newborn rats. Brainstem-spinal cord preparations were isolated from 0-3-day-old Wistar rats and superfused with artificial cerebrospinal fluid equilibrated with 95% O₂ and 5% CO₂, pH 7.4 at 25-26 °C. Membrane potential responses to hypoxia (95% → 0% O₂) and/or hypercapnia (2% → 8% CO₂) were examined in the presence of tetrodotoxin (TTX) after identification of the firing pattern. We found that TH-positive C1 neurons in the RVLM were sensitive to hypoxia with membrane depolarization but less sensitive to hypercapnia. TH-negative neurons in the C1 area showed responses similar to those of C1 neurons. Moreover, C1 area neurons remained depolarized by hypoxia in the presence of TTX plus gliotransmitter blockers. In contrast, Phox2b-positive and TH-negative neurons in the parafacial respiratory group were intrinsically sensitive to CO₂ but not sensitive to hypoxia. Respiratory-related neurons (Phox2b and TH negative) showed a variable response to hypoxia: unchanging, depolarizing, or hyperpolarizing. Our findings suggest that C1 area neurons in the RVLM are intrinsically sensitive to hypoxia and belong to one of the elements constituting central hypoxic sensors.