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Neurophysiology of yawning
O. Walusinski & B. Deputte
see a complete new chapter in
The Mystery of Yawning in Physiology and Disease
Frontiers of Neurology and Neuroscience tome 28
Karger, Basel 2010
Neurophamacology of yawning
Collins GT, Eguibar JR.
Front Neurol Neurosci
The Neurophamacology of Yawning
Gregory T. Collins
Department of Pharmacology University of Michigan Medical School Ann Arbor USA
Jose R. Eguibar
Instituto de Fisiología, Benemérita Universidad Autónoma de Puebla, Puebla, México
Yawning is a common behavioral event that is observed in humans, as well as other mammals, birds, and reptiles. In humans, yawning often occurs just before bed and upon waking up, and is also associated with tedious or boring situations. Although the physiologic roles of yawning have yet to be fully elucidated, the past 50 years of research has led to a much greater understanding of the neuropharmacologic regulation of yawning. While many of the early studies concluded that yawning was primarily driven by changes in cholinergic neurotransmission, we now know that numerous neurotransmitters and neurohormones are involved in the mediation of yawning, including acetylcholine, dopamine, glutamate, serotonin, oxytocin, GABA, opioids, adrenergics, nitric oxide, as well as the POMC-derived peptides ACTH and a-MSH. Furthermore, antagonist interaction studies have clearly defined at least three distinct neural pathways involved in the induction of yawning, as well as the hierarchical order through which these different neurotransmitter systems interact to regulate yawning. The following sections will discuss the state of knowledge for each of the major neurotransmitters and neurohormones involved in the regulation of yawning, their interactions with one another, and their place in the hierarchical organization of yawning.
The involvement of the dopaminergic system can be evidenced through the administration of small doses of apomorphine, a mixted agonist of the D1-D2 receptors of dopaminergic synapses which induces yawning. Strong doses make them disappear and cause motor stereotypies in animals or dyskenesias of the face in human. Such yawns induced by apomorphine are antagonised by typical and atypical neuroleptics, but not by domperidone, a blocking agent of peripheral dopaminergic receptors because it cannot cross the blood-brain barrier. The disappearance of yawning in extrapyramical syndromes confirms the importance of the dopaminergic pathway. The correlate is that giving an injection of apomorphine to a parkinsonian triggers, during the next ten minutes, a yawn a minute, with a repeat cycle at the end of the test one hour later. Similarly, a parkinsonian who loosens up under the effet of L-DOPA yawns when this occurs.
Endogenous peptide system : Hypophysectomy prevents yawning from happening. ACTH, MSH (melanocyte-stimulating hormone), LH-RH, all hypophysial peptides administered to rats by intrathecal injection, induce yawns and an erection. Ocytocine injected in the paraventricular nucleus of the hypothalamus (its destruction prevents any yawning) triggers yawns, while an ocytocine inhibitor prevents them. The role of the paraventricular nucleus of the hypothalamus and of the pituitary gland is controlled by an ocytocinergic network which receives dopaminergic activating influences and opioid inhibiting influences. This network reaches the hippocampus, on one hand, and the bulbopontine region (the one that probably commands yawning), on the other hand. At the level of the hypothalamic nucleus one finds a parallel between the activation or not of yawning and the presence or not of nitric oxide synthetase (NO).
The serotoninergic influence has been demonstrated by various pharmacological experiments. For instance, mCPP (1-3-chlorophenyl-piperazine) which is selective for 5-HT2c receptors is a powerful inducer of yawns in Man as well as animals. This probably explains the iatrogenic effect of yawn salvos in patients receiving serotoninergic antagonists such as antidepressants (serotonine reuptake inhibiting action). On the other hand, the stimulation of 5-HT1a and 5-HT2 receptors prevents the development of yawns induced by apomorphine, as well as by mCPP or nitric-acid inhibitors. The paraventricular nucleus of the hypothalamus, which receives major serotoninergic projections originating in the nucleus of the dorsal raphe, would be the seat of a modulation of dopaminergic and ocytocinergic neurons by the serotocinergic receptors of these neurons.
There is probably a final cholinergic pathway because pilocarpine and physostigmine, muscarinic agonists (acethylcholinesterase inhibiting) are powerful yawn triggers that are inhibited by atropine or scopolamine, acting as antagonists. These experiments show that the cholinergic pathways are the terminal, or executive, link common to all mechanisms triggering pharmacologically induced yawns.
Role of sexual hormones : in castrated rats, the response to apomorphine or ocytocine disappears. An injection of testosterone restores erection, but it must be accompanied by injections of estradiol for the yawn-erection diptych to occur. Tamoxifen, an anti-estrogen, prevents the reoccurrence of yawns induced by apomorphine after a testosterone-estradiol treatment. In non-castrated rats, progesterone increases and estradiol inhibits yawns triggered by apomorphine, without impact on erection. However, tamoxifen prevents inhibition of this type of yawn by estradiol. Because of their structure, steroids offer an activation mechanism that differs from that of the other major hormones. Some hormones, such as ocytocine, are proteins that act on membrane receptors located outside the cell and cannot penetrate the double lipid layer of which cell membranes are made. But, steroidian hormones, given their lipid nature, cross the cell wall and act on the intracytoplasmic receptors, which gives them direct access to the nucleus and the regulation of genic expression. Differences in the concentration of these receptors in various cerebral regions are responsible for the differential action of these hormones. It also appears that these hormones act by modifying the microsomial metabolism of neurotransmitters. The facilitating effect of dihydro-testosterone on yawning would intervene at the cholinergic and serotoninergic level. There is as yet no proposed explanation regarding the reason for the disappearance of this phenomenon in human primates.
-Sanna F, Succu S, Melis MR, Argiolas A. Dopamine agonist-induced penile erection and yawning: Differential role of D(2)-like receptor subtypes and correlation with nitric oxide production in the paraventricular nucleus of the hypothalamus of male rats. Behav Brain Res. 2012
Neural basis of drug induced yawning
Cooper SJ, Dourish CT in Neurobiology of Stereotyped Behaviour
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