Yawning is a motor pattern widely
represented in the behavioraI repertoire among
Vertebrates but its physioiogical significance
remains obscure. Apart, from its postulated role
in social communication in
primates and in man , common-place knowledge
associates yawning with the transitional phases
between sleep and the waking state, or with
conditions of physical and mental tiredness. But
very few solid facts, if any, stand out in
relation to its underlying physiological
mechanisms. Behavioral pharmacology has
nevertheless opened some leads in that
direction. As yawning behavior is elicitable in
rats both by cholinomimetic drugs (physostigmine
and pilocarpine) or by low doses of apomorphine
or other dopamine (DA) agonists, a tentative
model of organization of the central mechanisms
controlling yawning has been proposed. In this
hypothesis two sets of DA and cholinergic (ACh)
neurons, localized somewhere in the brain, are
supposed to be organized "in series", the former
tonically inhibiting the latter, which exert a
direct excitatory influence on the central
pattern generator (CPG) of yawning. Apomorphine
in low doses, by activation of DA presynaptic
autoreceptors, induces yawning by disinhibition
of the cholinergic excitatory neurons. Higher
doses of DA agonists directly inhibit the ACh
neurons and thus suppress yawning behavior. As
negative and positive modulating influences by
noradrenergic (NE) and serotonergic (5-HT)
mechanisms on yawning have also been
demonstrated, this apparently simple innate
behavior seems to be under the control of a
complex constellation of neurotransmitter
influences.
Being associated to manifestations with so
clear circadian rhythmicity as sleep and
activity, it is to be expected that yawning
should also appear with a circadian rhythm,
and justified to try to correlate it with
circadian variations in activity, reported in
the literature, of the neurotransmitter systems
postulated as subserving the central control and
regulation of yawning. Diurnal variations in
brain levels of ACh, DA, NE and 5-HT, of their
biosynthetic or metabolizing enzymes and of
their specific receptors have been vastly
explored in the last twenty years. The same is
true for different behaviors supposed to be
under the influence of these particular
neurotransmitters, because those behaviors may
be induced or modified by proper
pharmacolocgical agonists or antagonists.
Nevertheless, until 1980, spontaneous and
pharmacologically-induced yawning behavior had
escaped this detailed scrutiny for diurnal
variations. By that time ~Anias~ described that
both apomorphine- and physostigmine-induced
yawning in Wistar rats presented a clear
circadian rhythm. In these rats, kept under a
controlled 12 h-12 h light-dark (LD) cycle
(lights on at 7 a.m.),
pharmacologically--elicited yawning frequency
was lowest between 3 and 8 a.m. and showed a
high irregular plateau from 10 a.m. to 23 p.m.
Due to the very low spontaneous yawning
frequency in Wistar rats (below 1 yawn/hour), no
circadian variation in this behavior was noticed
in absence of pharmacological manipulations.
As some progress has been made in our Animal
House in Puebla in the selective breeding of a
"high yawning frequency" line of SpragueDawley
rats, it is now possible for us to communicate
observations on the circadian rhythm of
spontaneous yawning.
Our results are based on three groups of
young adult male rate, from the F4, F5 and F6
generations, observed during December-January,
March-April, and September-October. The first
two groups, formed by eight F4 and six F5
animals, were kept and observed in the same
laboratory room, under natural illumination (a
big 6 ml glass window facing East). They were
housed in groups of two or three animals, in
transparent acrylic boxes (47 X 43 X 20 cm)
containing a layer of wood shavings, covered by
a galvanized iron wire top, with a depression to
serve as food container and to hold a water
bottle. For the observation of yawning during
dark hours, the animals were illuminated from
the side or from above with a 25 W red lamp
placed at approximately 40 cm distance. The
animals were manipulated only three times a
week, when changed to clean cages. They had ad
libitum access to food (standard laboratory
rodent pellets) and drinkinc, water. The third
group, of six F6 rats, was observed in the
Animal House of the C.R.I.R.A. (Centro Regional
de Investigaciones en Reproducciôn Animal,
Panotla, Tlaxcala, México) where rats
were kept under a 14-10 LD schedule, with lights
automatically turned on from midnight to 1400,
and at an ambient temperature within 19-24'C.
This group of rats, when two months old, was
accustomed to these new conditions for 25 days
before the observation of yawning began.
Yawning occurrence was monitored through the
24 h of the day in twelve to fifteen sessions,
irregularly distributed for each group of
anirnals over 1 1/2 months. Observation sessions
did not last more than two hours, with two
observers sitting on opposite sides of the table
on which the cages were placed. Observation
implied no direct manipulation of the rats,
apart from a discrete movement of the boxes on
the table, in the first two groups, and carrying
them 5 m from a shelf to the table in the third
group. These movements of the cages were
performed at least 15 min before beginning the
clocking of each yawn during the observation
period.
The diurnal distribution of yawning frequency
(expressed as average yawns/hour) in the two
groups-of rats maintained under natural
illumination is illustrated in Fig. 2A. Apart
from the clear peak shown in the afternoon (late
light period), a tendency to exhibit additional
lower peaks also seems apparent. When the rats
were kept and observed under a 14-10 LD
schedule, with artificial illumination and
sudden transitions from light to dark (Fig. 2B),
the circadian yawning acrophase was displaced in
the time of the day, but continued to coincide
with the last hour of the light period. This
result suggests that the light-to-dark
transition might be the "primary synchronizer"
of the circadian rhythm of yawning, because the
higher frequency of yawns around this time
results highly significantly different from a
uniform distribution (P < 0.001), using a
directional test based on the coefficient of
synchronization.
If yawning is a motor pattern under
cholinergic activation and subject to DA
inhibitory regulation, one might expect that its
peak frequency should coincide with hours in the
day when cholinergic activity is highest, and
dopaminergic activity lowest. It is interesting
to note that in two studies in which clear
diurnal oscillations of ACh concentrations in
the brain have been demonstrated, low levels of
the neurotransmitter were measured at the latest
part of the light period, coïncident with
our observations of maximal yawning frequency.
The generaly accepted opinion is that high
concentrations of ACh in the brain coincide with
low firing rate of the cholinergie neurons,
the-1intracellular neurotransmitter being
protected from degradation by ACh-esterase.
Perhaps even more suggestive are Cahill and
Ehret's results on the circadian variation in DA
levels, tyrosine hydroxylase activity and
turnover rate of dopamine in the rat brain. The
lowest turnover rates of DA were calculated for
the late light hours. A decrease in dopaminergic
activity would, in our hypothesis, liberate the
cholinergic neurons exciting yawning from
inhibitory control and thus facilitate the
expression of this behavioral pattern. Turnover
rates of norepinephrine in the brainstem of the
rat, also determined by the same authors, show
two peaks, at early and late dark period hours,
when rats are more active, and according to, our
results, yawn more unfrequently than in the last
hour before the LD transition.
As some evidence exists in favour of a
serotonergic facilitation of
pharmacologically-induced yawning, it is
important to consider il serotonergic activity
may also to the circadian rhythm of spontaneous
yawmng. Quay had observed peak 5-HT
concentrations in hypothalamus, frontal cortex
and lateral portions of the lower brainstem
during light hours preceding the LD transition.
More recent studies in rats kept in either
12L-12D, or 14L-10D illumination schedules,
showed higher 5-HT turnover rates (estimated by
5-HIAA/ 5-HT ratios) during the dark period,
when yawning activity is in our experience
lower. But as 5-HT rhythms of different shapes
take place in different portions of the brain
(21, 31), it may not be altogether surprising to
encounter difficulties in correlation with a
behavioral pattern of which the neuroanatomical
structures involved in its control, and
regulation are still ignored. On the other hand
some doubts exist that total turnover of brain
5-HT may not always reflect the functional
serotonergic activity in the brain.
A final quantitative comment. Selective
breeding of "high yawning frequency"
Sprague-Dawley- rats, in four to six
generations, has brought forth an increase in
spontaneous yawning frequency to an average
(mesor) above 5 yawn/hour, ie, an order of
magnitude higher than that observed by us and
other authors in Wistar rats. Further analysis
of the factors involved in such an important
increase in yawning frequency may help in the
disclosure of the physiological mechanisms
underlying this particular behavioral
pattern
