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.
Pharmacological
and environmental regulation on yawning in the
high-yawning (HY) subline of
Sprague&endash;Dawley rats.
Jose R.
Eguibar,
Cortés Ma del Carmen and Uribe
Carlos
Instituto de
Fisiología, Benemérita Universidad
Autónoma de Puebla, Puebla,
México
The high-yawning (HY)
subline was obtained at the Institute of
Physiology in the Benemérita Autonomous
University of Puebla through a strict inbreeding
process along 75 generations. This group of rats
showed a spontaneous yawning frequency more than
20 times/h. On the other hand, low-yawning (LY)
rats have a mean yawning frequency around 2
times/h, and were selected from another stock of
Sprague-Dawley by inbreeding process through 60
generations.
The availability of HY
rats allows us to analyze the environmental
influences that regulated yawning expression.
So, we demonstrated that yawning showed a
circadian oscillation with a clear peak before
darkness. This circadian rhythm did not show a
free running under constant light conditions,
but it is possible to synchronize it whit a
restricted availability of food or water, which
produced a clear peak of yawning that predicts
their presentation.
On the other hand, HY
rats are more sensible to presynaptic
dopaminergic D2/D3 agonist such as: apomorphine,
3-PPP, quinpirole or 7-OH-DPAT. Central
administration of adrenocorticotropin hormone
(ACTH1-24) no clear effect were obtained on
yawning frequency in the HY subline, probably
due to displacement produced by the increase in
frequency and duration of grooming bouts. On the
other hand, central administration of bombesin,
a gastrointestinal peptide, inhibits yawning in
HY rats. Also is the case with GABAa and GABAb
agonists which inhibits spontaneous and
cholinergic-induced yawning. In the case of
cholinergic agonists such as physostigmine or
pilocarpine produced larger increase on yawning
frequency in HY respect to LY rats.
HY rats showed different
responses in the elevated plus maze and burying
test. In fact, HY subline showed lower
corticosterone level after immobilization in a
cylinder respect to LY ones. These results
clearly indicate that stress induced different
responses in hypothalamus-pituitary-adrenal
glands in both sublines.
In conclusion, HY subline
allows us to analyze with detail pharmacological
and environmental factors that modulate yawning
frequency. It is clear that the behavioural and
environmental differences in HY rats are due to
differences in the biochemical pathways involved
in synthesis, release or metabolism of
neurotransmitters and modulators involved in the
regulation of yawning.
This work is
partly supported by VIEP-BUAP SAL/G/2009 and
CONACYT 106,694 grants to JRE and by Dr. E.
Agüera-Ibañez,
Rector-BUAP.
Dopamine Agonist-Induced Yawning in Rats: A
Dopamine D3 Receptor Mediated Behavior
A specific role for the dopamine D3 receptor
in behavior has yet to be elucidated. We now
report that dopamine D2/D3 agonists elicit
dose-dependent yawning behavior in rats,
resulting in an inverted U-shaped dose-response
curve. A series of experiments was directed
toward the hypothesis that the induction of
yawning is a D3 receptor mediated effect, while
the inhibition of the yawning observed at higher
doses is due to competing D2 receptor
activity.
Although its physiological function is still
poorly understood, yawning is under the control
of several neurotransmitters and neuropetides at
the central level as this overview of
litterature on neurochemistry of yawning show.
Among these substances, the best known are
dopamine, excitatory aminoacids, acetylcholine,
serotonin, nitric oxyde, adrenocorticotropic
hormone-related petides and oxytocin, that
falicitate yawning and opiod peptides that
inhibit this behavioral reponse. Some of the
above compounds interact in the paraventricular
nucleus of the hypothalamus to control yawning.
This hypothalamic nucleus contains the cell
bodies of oxytocinergic neurons projecting to
extra-hyopthalamic brain areas that play a key
role in the expression of this behavioral event.
