of
yawning behavior due to emotional stress in
rats
Kubota N, Amemiya S, Yanagita S, Kita
I.
Abstract
As yawning is often observed in stressful or
emotional situations such as tension and
anxiety, this suggests that yawning can be
considered to be an emotional behavior. However,
the neural mechanisms underlying emotion-induced
yawning remain unclear. It is well known that
the hypothalamic paraventricular nucleus (PVN)
is the most important brain structure for
induction of yawning behavior. The authors
previously showed that induction of yawning
involves the central nucleus of the amygdala
(CeA), as well as the PVN. Therefore,
emotion-induced yawning could potentially be
induced through activation of the
direct/indirect neural pathways from the CeA to
the PVN. The present study used a combination of
retrograde tracing (injection of Fluoro-Gold
(FG) into the PVN) and c-Fos
immunohistochemistry to examine the neural
pathways that evoke emotion-induced yawning.
They additionally performed lesion experiments
on the CeA using ibotenic acid, a neurotoxin, to
determine whether the CeA is involved in the
induction of emotion-induced yawning. Emotional
stress by fear conditioning induced yawning
behavior, and induced expression of
double-labeled cells for c-Fos and FG in the bed
nucleus of the stria terminalis (BNST), but not
in the CeA. Furthermore, the CeA lesions caused
by ibotenic acid abolished the induction of
emotion-induced yawning. These results suggest
that a neural pathway from the CeA to the PVN
via the BNST may be primarily involved in the
induction of emotion-induced yawning
behavior.
Résumé
Le bâillement est souvent
observé dans des situations stressantes
ou émotionnelles. Cela suggère que
le bâillement peut être
considéré comme un comportement
émotionnel. Cependant, les
mécanismes neuronaux qui sous-tendent les
bâillements induits par les
émotions ne sont toujours pas clairs. Il
est connu que le noyau paraventriculaire de
l'hypothalamus (PVN) est la structure
cérébrale la plus importante pour
l'induction de ce comportement de
bâillement. Les auteurs ont
précédemment montré que
l'induction du bâillement implique le
noyau central de l'amygdale (CeA), ainsi que le
PVN. Par conséquent, le bâillement
induit par l'émotion pourrait
potentiellement être induit par
l'activation des voies neuronales
directes/indirectes de la CeA au PVN. Cette
étude a utilisé une combinaison de
traçage rétrograde (injection de
Fluoro-Gold (FG) dans le PVN) et
d'immunohistochimie c-Fos pour examiner les
voies neuronales qui déclenchent le
bâillement induit par l'émotion.
Ils ont également réalisé
des expériences de lésion de la
CeA en utilisant de l'acide iboténique,
une neurotoxine, afin de déterminer si la
CeA est impliquée dans l'induction des
bâillements induits par les
émotions. Le stress émotionnel par
conditionnement à la peur a induit un
comportement de bâillement, et a induit
l'expression de cellules doublement
marquées pour c-Fos et FG dans le noyau
de la stria terminalis (BNST), mais pas dans la
CeA. En outre, les lésions de la CeA
causées par l'acide iboténique ont
aboli l'induction de bâillements induits
par l'émotion. Ces résultats
suggèrent qu'une voie neuronale allant de
la CeA au PVN via le BNST pourrait être
principalement impliquée dans l'induction
du comportement de bâillement induit par
l'émotion.
-Kubota
N, Amemiya S, Motoki C, Otsuka T, Nishijima T,
Kita I. Corticotropin-releasing factor
antagonist reduces activation of noradrenalin
and serotonin neurons in the locus coeruleus and
dorsal raphe in the arousal response accompanied
by yawning behavior in rats. Neurosci Res.
2012;72(4):316-323
-Kubota
N, Amemiya S, Yanagita S, Kita I. Neural
pathways from the central nucleus of the
amygdala to the paraventricular nucleus of the
hypothalamus are involved in induction of
yawning behavior due to emotional stress in
rats. Behav Brain Res. 2022 Sep 1:114091.
Yawning is a common behavior in animals and
humans, but the biological significance of
yawning remains poorly understood. In fact,
yawning is often observed in not only in the
state of boredom or drowsiness, but also
stressful emotional situations such as tension
and anxiety. These phenomena suggest that
yawning can be considered to be an emotional
behavior, which could work as a non-verbal
communica- tion tool indicating emotional
responses to stressors from inside/outside the
body. It has been reported that the emotional
arousal evoked by neuropharmacological
stimulations, including anxiogenic or anxiolytic
drugs, can potentially affect the frequency of
yawning in animals [1&endash;4]. In
addition, several clinical studies have reported
finding that patients with anxiety disorder
frequently yawn [5,6]. If yawning is an
emotional behavior, the neural circuit
generating emotional stress may be involved in
yawning occurrence. However, at the present
time, the neural mechanisms underlying the
emotion-induced yawning are poorly
undrstood.
The paraventricular nucleus (PVN) of the
hypothalamus, which plays a critical role in
integrated stress responses [7,8], is
the most important of the brain structures for
the induction of yawning behavior
[9,10]. Furthermore, it has been
reported that both parvocellular oxytocin (OT)
neurons and corticotropin-releasing factor (CRF)
neurons in the PVN are responsible for induction
of the yawning accompanied by arousal re-
sponses [11&endash;14]. Recently, we
additionally reported that emotional stress
evoked by classical fear conditioning induces
yawning behavior accompanied by neuronal
activation of the central nucleus of the
amygdala (CeA), as well as activation of both OT
and CRF neurons in the PVN in freely moving rats
[15]. The CeA, which is part of the
amygda- loid complex, is known to be essential
for the expression of emotional responses,
including behavioral, autonomic and hormonal
responses during stressful situations
[16&endash;19]. We also recently showed
that microinjection of L-glutamate into the CeA
induced arousal/yawning response accompanied by
the activation of OT and C PVN [20].
Neuroanatomical studies have shown that the CeA
not only directly innervates the PVN
[21&endash;23] but also indirectly
innervates the PVN through the bed nucleus of
the stria terminals (BNST), which is one of the
brain regions involved in emotional responses
[23,24]. Therefore, it is possible that
emotion-induced yawning could be induced by the
activation of the direct/indirect neural
pathways from the CeA to PVN.RF neurons in the
In the present study, we used a combination of
retrograde tracing and c-Fos
immunohistochemistry to examine the neural
pathways evoking emotion-induced yawning. We
also performed lesion experi- ments on the CeA
using ibotenic acid, a neurotoxin, to determine
whether the CeA is involved in the pathway that
can induce the emotion- induced yawning. Our
results suggest that emotion-induced yawning is
induced by activation of the CeA-PVN
pathway.
Discussion
The present study showed that emotional
stress by fear conditioning could induce yawning
behavior and induce expression of double-labeled
cells for c-Fos and FG in the BNST, but not in
the CeA. In addition, the CeA lesions caused by
ibotenic acid abolished the induction of
emotion- induced yawning. This is the first
demonstration that an indirect neural pathway
from the CeA to the PVN could potentially be
involved in the induction of yawning
behavior.
We have previously reported that classical
fear conditioning induced yawning behavior in
rats, i.e., emotion-induced yawning, which is
consistent with the results of our present
study, in addition to showing that the
emotion-induced yawning is accompanied by
increased anxiety- related behavior and neuronal
activation in the CeA, as well as both the OT
and CRF neurons in the PVN [15].
Furthermore, we have also recently shown that
the microinjection of L-glutamate into the CeA
was able to induce the stereotyped yawning
responses, which are charac- terized by an
initial depressor in the blood pressure and an
arousal shift on the electrocorticogram followed
by a single deep inspiration [10,20].
These findings suggest that yawning can be an
emotional behavior, and that emotion-related
neural structures and/or pathways, including the
CeA or the direct/indirect pathways from the CeA
to the PVN, may be involved in the induction of
emotion-induced yawning. However, one limitation
of this study is that it cannot be ruled out the
possibility that the total number of yawning in
emotional stress group may include not only
emotion-induced yawning following fear
conditioning but also non-emotion-induced
yawning. The data on neuronal activity could
partly support our hypothesis.
In the present study, we found that the CeA
lesions caused by ibo- tenic acid were able to
abolish the induction of emotion-induced
yawning, accompanied by impairment of the
expression of grooming. These results suggest a
crucial role of the CeA in the induction of
emotion-induced yawning. Fear conditioning is a
commonly used model to evoke a stressful
emotional state such as anxiety, which is mainly
since it cannot be ruled out that the
exitotoxicity and electrical lesion techniques
can damage not only neurons but also the fibers
passed by.
We also found that double-labeled neurons
for c-Fos and FG in the BNST, but not in the
CeA, were increased during emotion-induced
yawning. Several neuroanatomical studies have
indicated that the PVN directly receives neural
projections from the CeA and BNST
[21&endash;24, 39&endash;42]. It has
been shown that the BNST can directly receive
projections from the CeA [24,43], which
suggests that there is an indirect pathway from
the CeA to the PVN. The BNST is also involved in
the emotional responses such as fear and anxiety
[18,44&endash;48]. In addition, recent
studies have shown that neuronal activation of
the BNST induced yawning behavior
[49,50]. Although increases in c-Fos
expression in the CeA and BNST are considered to
reflect the emotional stress situation including
yawning and grooming, our data provided by a
combination of retrograde tracing and c-Fos
immunohistochemistry suggests that the indirect
neural pathway from the CeA to the PVN via the
BNST may be primarily involved in the induction
of emotion-induced yawning.
In summary, the present study investigated
the role of the CeA - PVN pathway in
emotion-induced yawning using a combination of
retrograde tracing, c-fos immunohistochemistry,
and ibotenic acid lesions. The findings of the
present study indicated that the CeA is crucial
for in- duction of emotion-induced yawning, and
that the neural CeA-PVN circuits generating
emotional stress may be primarily involved in
yawning occurrence, via the BNST, suggesting
that yawning might be an emotional behavior.
Further studies will need to be conducted to
assess the contribution of the direct/indirect
neural pathways in the induction of
emotion-induced yawning.
