The new muscarinic
M1-receptor agonist YM796 evokes
yawning
and increases
oxytocin secretion
from the posterior
pituitary gland in rats
Fujikawa M, Yamada K, Nagashima M, Domae M,
Furukawa T
Department of Pharmacology,
Fukuoka University, Japan
The present experiments were performed to
examine the effects of a new muscarinic
M1-receptor agonist, (-)-YM796
((-)-S-2,8-dimethyl-3-methylene-1-oxa-8-azaspiro[4.5]decane
L-tartrate monohydrate), on yawning and oxytocin
secretion from the posterior pituitary gland in
rats YM796, at doses of 2.5-50 mg/kg (SC),
elicited yawning.
The yawning response was markedly increased
by pretreatment with a beta-adrenoceptor
antagonist, pindolol (20 mg/kg, IP), which per
se did not elicit yawning. The yawning induced
by YM796 (10 mg/kg, SC) in combination with
pindolol (20 mg/kg, IP) was inhibited by
scopolamine (0.5 mg/kg, SC), a muscarinic
receptor antagonist, and pirenzepine (300
micrograms/ rat, ICV) and EEDQ
(N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline)
(5 mg/kg, IP), muscarinic M1-receptor
antagonists, but not by spiperone (0.5 mg/kg,
SC), a dopamine D2-receptor antagonist, 4-DAMP
(4-diphenylacetoxy-N-methylpiperidine
methiodide) (100 micrograms/rat, ICV), a
muscarinic M3-receptor antagonist, and
[d(CH2)5, Tyr(Mc)2, Orn8]-vasotocin (100
ng/rat, ICV), an oxytocin receptor antagonist.
YM796 at 2.5-50 mg/kg (SC) did not exert an
action on prolactin levels but increased
oxytocin secretion from the posterior pituitary
gland in rats.
This augmentation of oxytocin secretion by
YM796 was inhibited by scopolamine (0.5 mg/kg,
SC) and pirenzepine (3 mg/kg, SC), but not by
mecamylamine (1 mg/kg, IP), a nicotinic receptor
antagonist. The present findings obtained with
YM796 suggest that the muscarinic M2-receptor
stimulation participates in causing yawning
behavior and oxytocin secretion in rat.
PREVIOUS behavioral studies, including our
experimental findings, have shown that the
yawning behavior was induced by dopamine
D2-receptor agonists and was blocked by both
dopamine D,-receptor and muscarinic receptor
antagonists, thereby indicating that the
behavior involves dopaminergic and cholinergic
activation. Cholinesterase inhibitors and
muscarinic receptor agonists also induce yawning
behavior, which is blocked by muscarinic
receptor antagonists. but not by dopamine
receptor antagonists, suggesting that the
dopaminergic activation precedes cholinergic
one. Thus, the dopaminergic-cholinergic
activation seems to participate essentially in
eliciting yawning behavior. Besides, since
administration of oxytocin has been reported to
cause yawning, oxytocinergic neuronal activities
seems to be participated in producing the
yawning behavior. On the other hand,
intraventricular administration of bethanechol
or carbachol produces the sustained release of
oxytocin, which is abolished by atropine, but
not by mecamyl-amine or hexamethonium. In
cultured bovine granulosa, continuous exposure
to acetylcholine resulted in a dosedependent
increase in oxytocin secretion, which was
completely abolished by the specific muscarinic
receptor antagonists, atropine and scopolamine,
but not by nicotinic receptor antagonists,
implying that cholinergic neurotransmitters play
a direct role in the regulation of oxytocin
release through muscarinic receptors.
The muscarinic receptors have been
designated as M,- or M,-receptors, depending on
whether they have high or low affinity for
pirenzepine. Recently. molecular cloning studies
have revealed the existence of five distinct
muscarinic receptor proteins (M,-M,). but their
functions are as yet tentative.
(-t)-YM796 was reported to exert muscarinic
Ml-receptor agonistic activity in the central
nervous system, with relatively weak muscarinic
M,-receptor and/or muscarinic M,-receptor
agonistic activity for behavioral changes at a
high dose. Moreover, the (-)-isomer of YM796
having higher affinity for muscarinic
M,-receptors than (-t)-YM796 was identified from
the ratio of the IC,,, value.
To determine the possible involvement of the
M, subtype of muscarinic receptor in yawning and
oxytocin release, we examined whether a new
muscarinic M,-receptor agonist, (-)- YM796,
elicits yawning responses and affects
oxytocin
DISCUSSION
(-)-YM796, a novel muscarinic M,-receptor
agonist, was capable of inducing yawning in this
study. Thus, the present result confirms the
previous reports describing that RS-86 and
AFlO2B, putative muscarinic M,-receptor
agonists. induce yawning. As for the potency of
producing yawning, the frequencies were 6.8 -t
2.2 yawns in 60 min by YM796, a muscarinic
M,-receptor agonist, alone at 10 mgikg SC and
20.4 -C 3.0 yawns by YM796 in combination with
pindolol in this study. The frequency produced
by pilocarpine, a muscarinic M,- and M,-receptor
agonist. was 3.1 + 0.6 yawns in 60 min at 4
mg/kg, IP alone and 20.9 + 3. I yawns in
combination with pindolol under similar
experimental conditions in our previous reports
(28). Studies on the agents that act selectively
on muscarinic M,-receptors have not been
reported. Accordingly, the muscarinic
M,-receptors are definitely involved in causing
yawning, but the possible involvement of
muscarinic MZ- and M,-receptors may not be
dismissed at present.
In our previous reports blockade of
B-adrenoceptors caused by central B-adrenoceptor
antagonists, pindolol. and others, which reach
the brain through the blood-brain barrier, but
not by peripheral B-adrenoceptor antagonists,
facilitates the occurrence of yawning induced by
dopaminergic agonists. such as apomorphine and
talipexole (B-HT 920). Administration of
LY-78335 and IJK-I 187A,
phenylethanolamine-Nmethyltransferase
inhibitors, which decrease adrenaline formation
without affecting dopamine and noradrenaline
levels in the brain, similarly potentiates the
incidence of yawning as does B-adrenoceptor
blockade. Therefore. the central adrenergic
neuronal system was proposed to participate in
the regulation of yawning for suppressive
direction via padrenoceptors.
The yawning induced by YM796 was also
potentiated by treatment with the P-adrenoceptor
antagonist, pindolol, in the present study. The
yawning induced by muscarinic receptor agonists,
AF102B and RS-86, has been reported to be
blocked by muscarinic receptor antagonists but
to be unaffected by dopamine receptor
antagonists. In the present study, the yawning
behavior elicited by YM796 administered after
pindolol was inhibited by scopolamine. a
muscarinic receptor antagonist, and was also
antagonized by pirenzepine and EEDQ, muscarinic
M,-receptor antagonists, without being affected
by 4-DAMP, a muscarinic M?-receptor antagonist.
and spiperone. a dopamine D,-receptor
antagonist. Thus, muscarinic M,-receptors are
involved in the induction of the yawning
behavior. Nicotinic receptor stimulation causes
an increase of oxytotin release, and the
muscarinic pathway also seems to be involved in
regulating oxytocin release. In the present
study. serum oxytocin levels were increased in a
dose-dependent manner not only by nicotine but
also by YM796. indicating the participation of
both nicotinic and muscarinic receptor
mechanisms.
The elevation of oxytocin levels produced by
YM796 was inhibited by scopolamine, a muscarinic
receptor antagonist. but not by mecamylamine. a
nicotinic receptor antagonist. thus implying
that the muscarinic receptor stimulation
participates in the acceleration of oxytocin
release from the posterior pituitary gland. In
further experiments. pirenzepine, a muscarinic
M,-receptor antagonist. did not affect the
static oxytocin levels but antagonized the
elevation of the oxytocin level by YM796. These
findings indicate that muscarinic M,-receptors
play a role in increasing oxytocin release. As
for possible involvement of oxytocinergic
neuronal activities in the yawning, central
administration of oxytocin has been reported to
elicit yawning behavior. Therefore, it might be
a problem whether or not the release of oxytocin
found in the plasma is related to the yawning
response. In this study, the yawning induced by
YM796 after pindolol was not prevented by the
oxytocin receptor blockade. Accordingly, the
release of oxytocin from the neurohypophysis
after YM796 may not be related to the yawning
response.
This would also suggest that central and
peripheral oxytocin release is differentially
regulated and YM796 acts at another level in the
central nervous structures involved in the
regulation and/or expression of this behavior.
Moreover, as described in the introduction, the
dopaminergic-cholinergic neuronal linkage seems
to be essentially involved in eliciting yawning
behavior. In fact, YM796-induced yawning was
inhibited by muscarinic antagonists, but not by
a dopamine D,-receptor antagonist in the present
study. There has been also the proposal that the
expression of yawning induced by dopaminergic
agonists involves dopamine-oxytocin, but not
oxytocin-dopamine, neuronal linkage. Besides,
the oxytocin-elicited yawning behavior is
inhibited by oxytocin receptor antagonists and
muscarinic receptor antagonists, but not by
dopamine receptor antagonists. In this study,
the YM796-induced yawning was antagonized by
muscarinic M,-receptor antagonists, but not by
the oxytocin receptor antagonist.
Therefore, the findings suggest that the
oxytocinergic mechanism precedes the muscarinic
one and is in position between dopaminergic and
muscarinic mechanism in producing yawning
behavior. The present findings suggest that the
muscarinic M,-receptor stimulation is involved
in the occurrence of yawning behavior and
secretion of oxytocin from the posterior
pituitary gland in rats.
