Yawning responses induced by local hypoxia in the paraventricular nucleus of the rat

Yawing was induced by microinjections of L-glutamate, cyanide and a nitric oxide-releasing compound (NOC12) into the paraventricular nucleus of the hy pothalamus (PVN) in anesthetized, spontaneously breathing rats. To evaluate physiological aspects of yawning, we monitored intercostal electromyogram (EMG) as an index of inspiratory activity, digastric EMG, blood pressure an d electrocorticogram (ECoG). Microinjection of L-glutamate in the medial pa rvocellular subdivision (mp) elicited a stereotyped yawning response, i.e. an initial depressor response and an arousal shift in ECoG followed by a si ngle large inspiration with mouth opening. The same sequential events were observed during spontaneous yawning, indicating that the rnp is responsible for triggering yawning. Microinjection of cyanide into the mp caused the s ame yawning responses as the ones elicited by microinjection of L-glutamate , suggesting that the mp is sensitive to chemical hypoxia or ischemia withi n the PVN. Microinjection of NOC12 into the mp elicited a single large insp iration with a variable onset delay, suggesting that diffusible nitric oxid e (NO) within the mp may act as a paracrine agent to cause a yawning respon se. We hypothesize that the mp of the PVN contains an oxygen sensor that ca uses a yawning response. (C) 2000 Elsevier Science B.V. All rights reserved .