In vitro responses of neurons in the periaqueductal gray to hypoxia and hypercapnia

Hypoxia-sensitive neurons in the caudal hypothalamus (CH) have been shown t o project to the periaqueductal gray (PAG) which, in rum, sends descending projections to an area of the ventrolateral medulla (VLM) containing neuron s inherently excited by hypoxia. The purpose of this study was to determine if neurons in the PAG are excited by hypoxia or hypercapnia in an in vitro environment. Extracellular responses to hypoxia and hypercapnia of neurons located throughout the PAG were recorded in a rat brain slice (400-500 mu m thick) preparation. Hypoxic (10% O-2/5% CO2/85% N-2) and hypercapnic (7% CO2/93% O-2) stimuli were delivered to the tissue through gas bubbled into the brain slice chamber. A majority (39 of 53) of the neurons tested respon ded to hypoxia. Of these neurons, 92% responded to hypoxia with an increase in firing rate. Neurons in the dorsolateral/lateral regions increased firi ng rates to a greater extent than neurons located in ventrolateral regions. All neurons tested (n = 6) also responded to hypoxia after perfusion of th e tissue with a low Ca2+/high Mg2+ solution to block classic synaptic trans mission. Only a small proportion (7/33) of neurons tested responded to hype rcapnia. These findings indicate that neurons in the periaqueductal gray re gion of the brain have an inherent responsiveness to hypoxia and, thus, may contribute to the overall coordination of cardiorespiratory responses to s ystemic hypoxia. (C) 1999 Elsevier Science B.V. All rights reserved.