Introduction : Apomorphine (Apo), a
dopamine (DA) receptor agonist in animals and
man, induces yawning in both rats and humans. In
rodenu the response is blocked by both typical
and atypical neuroleptics which points to the
involvement of DA receptors. Dose-responst
curves vith Apo show a bell shaped form with
induction of yawning with low doses and
absence vith large doses. These findings,
together with other evidence, are compatible
with the view that Apo-induced yawning is
mediated by stimulating DA autoreceptors and
that yawning behaviour mediated by DA agouists
may provide an index of central DA autoreceptor
function. Whereas a large body of literature has
accumulated on Apo-induced yawning in animals.
Such information that is available consists of
incidental observations and subjective
impressions. The lack of systematic studies on
Apo-induced yawing in humans is probably the
consequence of the lack of a simple technique
for the objective measurement of this
phenomenon.
In the present paper we describe a
simple, reliable and accurate method for the
recording and measurement of Apo-induced yawning
behaviour in man.
Methods : A pair of linearlized
magnetometers was used to measure displacement
of the lower jaw by attaching one sensor to the
forehead below the hairline and a second sensor
under the chin. The magnetometers consist of two
separate wire coils about 1 cm long and 0.5 cm
in diameter mounted on a flat hard plastic base.
A double sticky tape vas used to fix the base to
the skin. One of the coils generates a magnetic
field. The second coil by moving in and out of
the field (follwing the movements of the jaw)
generates a current which in proportional to the
distance between both coils. The output of the
magnetometers was fed into a DC amplifier and
displayed on a strip chart recorder. Upward
deflections indicate jaw opening. Jaw opening
associated with yawning could be readily
identified. Other jaw movements caused by
swalloving or coughing are associated with
smaller, short-lived deflections. Breathing
movements do not interfere vith. jaw opening.
Apo HCl (0.5 mg,-sc) was administered to five
healthy normal subjects aged 18-47 (3 men; 2
women) on no medication. Yawning was monitored
for 15 minutes before and 60 minutes after Apo
administration. Wo of the subjects also received
saline placebo. The volunteers remained
recumbent in the supine position throughout the
recording and were asked to refrain from
talking. Subjects were observed throughout the
sessions and yawning and other movements of the
mouth (swalloving, coughing) noted and
correlated with deflections produced on the
strip charts.
A complete yawn, which might consist of one
or several sequential inspiratory movements, was
defined as the interval between mouth opening
and closure of the mouth. The tracings vere
evalusted by two independent raters. The number
of yawns and total yawning time was calculated.
Concordance between observed yawns and recorded
yawns vas assessed.
Results : Apo induced yawning in each
of the five subjects. Reither subject recelving
placebo yawned. Each yawn corresponded vith a
major deflection on the tracing at the
commencement of yawning and a return to,
baseline et the end. Each subject showed 'a
characteristic tracing in which individuel yawns
could be easily identified by independent
evaluators. Artifacte due to swallowing,
coughing or talking vere readily distinguished.
The number of yawn and total yawning time could
easily be calculated. There was a complets
concordance between the raters for the number of
yawns documented on the tracing. These numbers
showed complete concordance vith the number of
directly observed yawns. Preliminary
observations showed thât yawning ceased if
sleep vas induced. The two women showed a
considerably grester number of yawns and total
yawning time than the three men.
Discussion : There is little
information on spontaneous yawing in man.
Whether a DA mechanism subserves spontaneous
yawning is unknown. It is of interest that
spontaneous yawning in believed to be rare in
schizophrenia and Parkinson'a disease whereas
there is a propensity for yawning in
Huntington's chorea. In all three conditions
there in evidence for a disturbance of DA
function. During the development of the prenant
method va observed that subjects who yawned
spontaneously had an identical yawing pattern to
yawning induced in the same subjects by Apo..
The present investigation shows that yawing can
be objectively recorded by measuring
displacement of the lower jaw in a simple,
reliable manner and the data can be easily
analyzed. In the rat Apo is believed to induce
yawning by stimulating central DA autoreceptors.
Incidental observations with domperidone suggest
that au in the rat Apo-induced yawning is also
mediated by an effect on DA receptors within the
blood brain barrier but whether autoreceptors
are involved is unknown. Apo-induced yawning may
provide a relatively noninvasive approach to
evaluating DA autoreceptor function in
psychiatric and neurological disorders.
The anatomical site for spontaneous yawning
in man in unknown but the medulla oblongata,
hypothalamus and striatum have all been
suggested. In animals electrical stimulation
within the hypothalamus induces yawning.
Apo-induced yawning in the rat may be a
striatal-media ed phenomenon. The method
described for measuring yawning lands itself to
the investigation of the phytiological and
pharmacological regulation of the dopamine
system mediating Apo-induced yavning. The rapid
onset of the yawning after sc administration of
Apo has obvious advantages for clinical
investigation. The observation that sleep
inhibits yawing suggests that subjects should
remain awake during the period of monitoring.
The preliminary finding that yawning was more
marked in the two women compared with the three
men is of interest as in animals yawning is
essentially a male phenomenon. Thus, the female
monkey and baboon rarely yawn spontaneously.
Castration of the male rhesus monkey causes a
precipitous decline in yavning and testosterone
administration to ovariectomized females
increases yawning. Physostigmine and Apo-induced
yawning are significantly grester in the male
than female rat. Testosterone restores
physostigmine induced yawning in castrated male
rats. Our preliminary observations in humans
points to a species difference.
Conclusions : Apo-induced yawning can
be objectively recorded by measuring the
displacement of the lower jaw. Measuring this
behavioural response may provide a simple way of
assessing DA autoreceptor function in
psychiatric and neurological disorders.
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