Yawning.

YAWNING IS A COMMON PHYSIOLOGICAL EVENT THAT CAN BE DIVIDED INTO THREE DISTINCT PHASES: a long inspiratory phase, a brief acme and a rapid expiration. The aim of yawning is not yet well defined. However this semi-voluntary event increases vigilance and aims to alert when drowsiness occurs. Yawning probably has an important role for social communication as well. Yawning can be responsible for pain, luxation or even transient ischaemic attack. Abnormal yawning is present in various pathologies: migraine, Parkinson's disease, tumours, psychiatric diseases, infections or iatrogenic pathologies. The neuro-pharmacology of yawning is complex and knowledge of its mechanisms is incomplete. While under the control of several neurotransmitters, yawning is largely affected by dopamine. Dopamine may activate oxytocin production in the paraventricular nucleus of the hypothalamus. Oxytocin may then activate cholinergic transmission in the hippocampus and, finally, acetylcholine might induce yawning via the muscarinic receptors of the effectors. This is an over-simplification; many other molecules can modulate yawning, such as nitric oxide, glutamate, GABA, serotonin, ACTH, MSH, sexual hormones and opium derivate peptides. Dopamine involvement in yawning could have practical applications in the study of new drugs or the exploration of neurological diseases such as migraine or psychosis. 2001 Harcourt Publishers Ltd