Yawning/cortical activation induced by microinjection of histamine into the paraventricular nucleus of the rat.

The effects of microinjection of histamine into the paraventricular nucleus (PVN) of the hypothalamus on yawning responses were investigated in anesthetized, spontaneously breathing rats. Yawning responses were evaluated by monitoring the intercostal electromyogram (EMG) as an index of inspiratory activity and digastric EMG as an indicator of mouth opening. We also recorded the electrocorticogram (ECoG) to determine the arousal response during yawning. Autonomic function was evaluated by measuring blood pressure and heart rate. Microinjection of histamine into the medial parvocellular subdivision (mp) of the PVN elicited a yawning response, i.e. a single large inspiration with mouth opening, and an arousal shift in ECoG to lower voltage and faster rhythms. Microinjection of HTMT dimaleate, an H1 receptor agonist, into the PVN also caused the yawning/arousal response. Pretreatment with pyrilamine, an H1 receptor antagonist, inhibited the histamine induced yawning behavior. These data demonstrate that a histamine receptive site for triggering yawning/arousal responses exists in the PVN, and suggest that these responses are mediated by activation of H1 receptor within the PVN. Copyright 2002 Elsevier Science B.V.