STEREOTYPED YAWNING RESPONSES INDUCED BY ELECTRICAL AND CHEMICAL-STIMULATION OF PARAVENTRICULAR NUCLEUS OF THE RAT

Yawning was evoked by electrical or chemical stimulation in the parave ntricular nucleus (PVN) of anesthetized, spontaneously breathing rats. To evaluate physiological aspects of yawning, we monitored polygraphi c measures as follows; a coordinated motor pattern of yawning was asse ssed by monitoring breathing [intercostal electromyogram (EMG)], mouth opening (digastric EMG), and stretching of the trunk (back EMG). We a lso recorded blood pressure (BP), heart rate, and the electrocorticogr am (ECoG) to evaluate autonomic function and arousal responses during yawning. A stereotyped yawning response was reproducibly evoked by ele ctrical stimulation or microinjection of L-glutamate or NOC-7, a nitri c oxide (NO)-releasing compound, into the PVN. The stereotyped yawning response consisted of two sequential events, an initial response repr esented a depressor response and an arousal shift in the ECoG to lower voltage and faster rhythms. These initial changes were followed by a yawning behavior characterized by a single large inspiration with mout h opening and stretching of the trunk. A similar sequence of events oc curred during spontaneous yawning; a fall in BP and ECoG arousal prece ded a yawning behavior. An increase in the frequency of spontaneous ya wns was also observed after microinjection of L-glutamate or NOC-7 int o the PVN. Intravenous administration of N-G-monomethyl-L-arginine, an inhibitor of nitric oxide synthase (NOS), prevented the stereotyped y awning response evoked by chemical stimulation of the PVN. Histologica l examination revealed that effective sites for the yawning responses were located in the medial part of the rostral PVN, the site of parvoc ellular and magnocellular neurons. NADPH-diaphorase histochemistry sho wed the existence of NOS-containing cells in yawning-evoked sites of t he PVN. In summary, the sequential events of yawning may be generated by NOS-containing parvocellular neurons in the medial part of the rost ral PVN projecting to the lower brain stem.