Systemic administration of D2-like
dopaminergic-receptor agonists increases yawning
behavior. However, only a few studies have been
done in animals with pathological conditions.
The taiep rat is a myelin mutant with an initial
hypomyelination followed by progressive
demyelination, being the brainstem one of the
most affected areas. In our experiments, we
analyzed the effects of systemic administration
of the D2-family agonists and antagonists on
yawning behavior, and correlated them with the
lipid myelin content in the brainstem and other
areas in the central nervous system (CNS) in
8month old male taiep and Sprague-Dawley rats.
Subjects were maintained under standard
conditions in Plexiglas cages with a 12:12
light-dark cycle, lights on at 0700 and free
access to rodent pellets and tap water. Drugs
were freshly prepared injected ip at 0800 and
subjects were observed for 60min. When
antagonists were used it was administered 15min
before the agonist.
Sprague-Dawley and taiep rats significantly
increased their yawning frequency after systemic
injection of (-)-quinpirole hydrochloride,
R(+)-7-Hydroxy-2-(dipropylamino)tetralin
hydrobromide (7-OH-DPAT) or
trans-(±)-3,4,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano
[4,3-b]-1,4-oxazin-9-ol hydrochloride
((±)-PD 128,907). Among D2-like agonists
used higher effects are obtained with
(-)-quinpirole. The effects caused by
(-)-quinpirole can be reduced by (-)-sulpiride;
and yawning caused by 7-OH-DPAT was decreased by
tiapride only in taiep rats. In Sprague-Dawley
only (-)-sulpiride is able to decrease
(-)-quinpirole-caused yawning.
In conclusion, dopaminergic D2-like agonists
are still able to cause yawning despite the
severe myelin loss in taiep rats. Similarly,
patients with various CNS illnesses that affect
myelin, such as stroke or multiple sclerosis,
are able to yawn suggesting that trigger neurons
are still able to command this innate
behavior.
1. Introduction
At the Institute of Physiology of the
Benemérita Autonomous University of
Puebla, México during a strict inbreeding
process to obtain a high-yawning (HY) subline
from Sprague-Dawley rats, we obtained as a
spontaneous mutation that affects myelin and it
is called taiep. The name is the acronym of a
neurological motor syndrome characterized by
tremor, ataxia, immobility episodes, epilepsy
and paralysis during the first year of life
(Holmgren et al., 1989). The illness is
transmitted as an autosomic recessive trait
(Duncan et al., 1992; Holmgren et al., 1989).
These mutants show an initial hypomyelination
followed by a progressive demyelination in the
central nervous system (CNS), but not in the
peripheral one (Duncan et al., 1992; Lunn et
al., 1997). At the ultrastructural level, the
taiep rat shows an abnormal accumulation of
microtubules in the cytoplasm and its processes
in the oligodendrocytes (Couve et al., 1997;
Duncan et al., 1992).
When the oligodendrocytes were cultured the
polymerization of microtubules was greater than
depolymerization, which produced an accumulation
of these cytoskeleton proteins. This alteration
was reversed by the use of nocodazole and
colchicine that suppress the microtubule
assembly and partially depolymerize the existing
microtubules (Song et al., 2003). It has been
proposed that the accumulation of microtubules
disrupts the translocation of newly synthesized
proteins from the endoplasmic reticulum to the
cis-portion of the Golgi apparatus (Couve et
al., 1997; O´Connor et al., 2000). The
mutant shows a decrease of all major myelin
proteins such as myelin basic protein (MBP),
proteolipid protein (PLP), 2',3'-cyclic
nucleotide 3'-phosphodiesterase (CNP), and the
myelin-associated glycoprotein (MAG) levels in
different portions of the CNS (Möller et
al., 1997). All the pathways that myelinate
after birth, such as corticospinal,dorsal
columns, and optic nerves are more affected than
the pathways that were already myelinated at
birth, such as ventral columns in the spinal
cord, suggesting a nonhomogeneous alteration in
the myelin pathways (Lunn et al., 1997). The
frequency of yawning is strongly dependent on
dopaminergic neurotransmission (Holmgren and
Urbá-Holmgren, 1980; Holmgren et al.,
1982; Mogilnicka and Klimek, 1977; Yamada and
Furukawa, 1981). Because the D2-like dopamine
receptors are more involved in the regulation of
this behavior, the D2 and D3 agonists have
stronger effects on yawning in different strains
of rats (Argiolas and Melis, 1998; Collins et
al., 2005; Holmgren and Urbá-Holmgren,
1980; Holmgren et al., 1982); but until now
there were no studies reporting yawning studies
in animals with brain alterations and the taiep
rats allow us to do this.
Taiep rats came from subjects with a higher
incidence of spontaneous yawning with more than
20 yawns per hour (Urbá-Holmgren et al.,
1990). Importantly, our research group showed a
strong correlation among spontaneous- and
dopaminergiccaused yawns and penile erections
(Eguibar et al., 2003; Holmgren et al., 1985).
The taiep rats are directly derived from a
high-yawning subline which have different
sensitivity to the D2-like dopaminergic agonists
producing yawning (Urbá-Holmgren et al.,
1993; Eguibar et al., 2003).
Later studies demonstrated that D2 agonists
produced an increase in yawning and penile
erection frequency in an inverted U-shape with
the ascending limb caused by a D3 effect and the
descending limb of the curve caused by an
activation of D2 receptors (Baladi et al., 2010;
Collins et al., 2005; Collins et al., 2009). In
our present experiments, we analyzed the
participation of the dopaminergic D2-like
agonists and antagonists on yawning after
systemic intraperitoneal administration of
specific agonists and antagonists in normal
outbred Sprague-Dawley and myelin mutant taiep 8
months old. We also analyzed the total protein
and lipid content in the brainstem and other
structures in the CNS of both groups of
rats.
4. Discussion
The 8 month old male taiep rats showed a
severe demyelination with just 18% of lipids in
the brainstem compared to the control group.
Similar decreases were obtained in the spinal
cord (21%), but the cortex and cerebellum were
less affected with just a 50% decrease on the
lipid content compared to the Sprague-Dawley
animals at 8 months. Detailed microscopic
studies showed a general decrease of the myelin
thickness with age in taiep rats, particularly
in all myelin tracts that mature after birth
(Lunn et al., 1997).
Because lipids are the major solid component
in the CNS, about 70%, measurements of the total
myelin content has been believed to be an
adequate way to evaluate the degree of
demyelination in myelin mutants (Norton, 1981).
Similar decreases in the lipid content were
reported in several myelin-mutant mice, such as
jimpy, shivering, and quaking (Ganser et al.,
1988). Instead of a myelin decrease, the D2-like
dopaminergic receptors are still functional to
produce a significant increase of yawning
frequency. In this study we showed that
(-)-quinpirole increases yawning frequency in
both strains of rats, being more potent than
(±)-PD 128,907 and 7-(OH)-DPAT. Similar
results were obtained by Collins and his group
in outbred Sprague-Dawley rats (Collins et al.,
2005, 2007, 2009), suggesting that myelin is not
the fundamental component to obtain maximum
effects caused by dopaminergicgenerated yawning
with these higher doses.
Binding experiments clearly demonstrated
that (±)-PD 128,907 has a similar affinity
for D3 receptors as 7-(OH)-DPAT. These D3
receptors are more abundant in the islands of
Calleja followed by the nucleus accumbens,
nucleus of the horizontal limb of the diagonal
band, and the ventral caudate-putamen (Bancroft
et al., 1998). Taiep rats show higher levels of
D1 receptors in the ventral subregions of the
basal ganglia compared to normal Sprague-Dawley
rats, with these differences greater in rats
older than 9 months. However D2-like dopamine
receptor levels are similar in both groups of
rats (Flores et al., 2002). For yawning, it has
been shown that dopaminergic pathways from
mesocortical, mesostriatal, and local
dopaminergic pathways in the paraventricular
nucleus of the hypothalamus (PVN) are involved
in the regulation of yawning (Argiolas and
Melis, 1998; Collins and Eguibar, 2010; Dourish
and Cooper, 1990). Dopaminergic drugs acting on
the PVN and through magnocellular nucleus axons
project to the brainstem in which the central
ensemble of neurons that generated and
coordinated the musculature in the pharynx,
larynx, and facial musculature are involved in
the yawn reflex pattern. The dopamine exerts a
tonic inhibitory control over cholinergic
neurons that produced yawning (Holmgren and
Urbá-Holmgren, 1980; Holmgren et al.,
1982; Yamada and Furukawa, 1981). It is clearly
stablished that an inverted U-shape doseresponse
curve was obtained with the ascending limb of
the curve mediated by D3 receptors and the
descending limb mediated by the D2 receptors
(Collins et al.,2005, 2007; Baladi et al., 2010,
2011). Because of the higher doses used in this
study, only the descending limb of the curve was
explored because at these doses there was a
significant increase in the gripping-produced
tonic-immobility episodes, a cardinal sign of
this myelin-mutant rat (Eguibar et al., 2010),
suggesting that taiep rats are less sensitive to
D3 and D2 effects on yawning. For the D2-family
antagonist used, the lack of effect is probably
caused by the low spontaneous yawning frequency.
However, tiapride is able to antagonize the
increase of yawning frequency produced by
7-OH-DPAT in taiep rats because the myelin
mutants had different sensitivity in D3
receptors and so this dose in Sprague-Dawley
rats is on the descending limb of the
dose-response curve, as previously suggested
(Baladi et al., 2010, 2011; Collins et al.,
2007, 2009). Under free access to chow taiep and
Sprague-Dawley rats showed different
sensitivities to the action of the D2-like
dopaminergic agonists, which could explain the
differences obtained when antagonists were
administered before the agonist, because at that
dose the 7-OHDPAT falls on the ascending limb of
the dose-effect curve on taiep rats, but it
could be on the descending limb in the
Sprague-Dawley rats. The agonist is acting at D3
receptors in the former and quite probably in
the D2 receptors in the latter (see Table 1 and
Figure 2). This differences in the response to
dopaminergic drugs can be because of age,
gender, genetics, and nutritional status, as
previously suggested (Baladi et al., 2011). In
our results it is quite clear that the
differences in the response to tiapride and
7-OH-DPAT are caused by the genetic background
of taiep rats and why yawning frequency differs
in both groups of rats.
In stroke patients the voluntary
contractions of paralyzed arms is impossible,
however during yawning this paralyzed arm is
stretched. In the case of taiep rats the
progressive demyelination produced, at 8-9
months old, a large reduction in the myelin
sheath reaching 100%, 70%, and 70% in the
corticospinal tract, ventral column, and the
optic nerve (Lunn et al., 1997), but they are
still able to respond to the D2-like
dopaminergic agonists as also for hemiplegic
patients in which apomorphine in low doses
increased yawning and concomitantly the
stretching of the paralyzed arm (Blin et al.,
1994). It is quite remarkable that in humans
there are only a few clinical reports in which
neurological stroke patients are able to move
their paralyzed arm during yawning (Wimalaratna,
1988). This movement is strictly concomitant
with the yawn and the arm persists in being
inert once the yawn ends (Blin et al., 1994;
Walusinski et al., 2005, 2010). For most of
these patients the lesions, which are located on
the internal capsule affecting cortical and
subcortical areas, are with the loss of myelin
one cardinal sign after a stroke (Walusinski et
al., 2005, 2009).
It is quite probable that a set of
motoneurons that produces yawning are located in
the brainstem, near the nerve centers that
control the respiratory, laryngeal, and
pharyngeal musculature (Barbizet, 1958; Heusner,
1946). These sets of motoneurons respond
reflexively to the changes of dopaminergic
transmission, as demonstrated in hemiplegic
patients and now in the myelin mutant taiep
rats. The upper parts of the CNS are able to
modulate the threshold of motoneurons that are
responsible for the control of facial
musculature involved in yawning and also the
musculature that stretches the paralyzed arm in
stroke patients, the so called Parakinesia
brachialis oscitans (Walusinski et al., 2005).
There is a strong correlation of yawning and
stretching to increase awakening when light-dark
changes happen (Provine et al., 1987), another
circumstance in which both the motor acts are
expressed together probably to increase the
awakening state.
5.- Conclusion
In conclusion, it is possible to cause
yawning in the myelin mutant taiep rat through
systemic injection of the D2-like dopaminergic
agonists. The taiep is a myelin mutant with a
progressive demyelination, and this response
correlates with clinical observations of the
presence of yawning behavior in hemiplegic
patients in which the myelin is altered, but
this reflex response commanded by dopamine
neurons happens suggesting that it is wired in a
coordinated way to produce that motor
sequence.
