Behavioural
and pharmacological characterization of the
mouth movements induced by muscarinic agonists
in the rat
Salamone JD, Lalies MD, Channell SL, Iversen
SD.
Abstract
Pilocarpine administered in doses of
1.25-10.0 mg/kg (IP) produced a variety of mouth
movements in the rat. The most frequent of these
movements was a chewing behaviour, which
increased up to a mean frequency of over 40 per
min at the highest doses. Tongue protrusion and
gaping also showed dose-dependent increases.
Yawning tended to increase in some doses, though
these increases were not significant, and
yawning was relatively infrequent.
Pre-treatment with scopolamine reduced these
responses, while pre-treatment with methyl
scopolamine did not. Injections of oxotremorine
or arecoline, but not carbachol, produced
dose-related increases in mouth movements
similar to those produced by pilocarpine. These
results suggest that mouth movements in the rat
are caused by stimulation of central muscarinic
receptors. This may prove to be an important
behavioural sign of central cholinomimetic
activity.
Easily detectable behavioural signs of drug
actions, such as stereotypy or catalepsy, are
widely used in psychopharmacology. Such
behaviours can help researchers identify drug
effects consistent with an action upon a
particular CNS neurotransmitter system. The
current resurgence of interest in cholinergic
psychopharmacology (Bartus et al. 1982; Rossor
1980) suggests that a simple behavioural test
for CNS cholinomimetic actions would be
valuable. The induction of various oral
activities by muscarinic agonists could prove to
be a useful cholinergic-related behaviour of
this type.
It has been reported that cholinomimetics
cause yawning in rats (Ushijima et al.
1984; Yamada and Furukawa 1980, 1981). This
yawning was shown to be more frequent in
young male rats (Urba-Holmgren et al. 1977).
Ushijima et al. (1984) observed that yawning was
frequently accompanied by tongue protrusion .
Rupniak et al. (1983) also found that
physostigmine and pilocarpine induced tongue
protrusion. This observation was made in the
context of investigating cholinergic modulation
of perioral movements following chronic
neuroleptic treatment. The anticholinergics
scopolamine and atropine attenuated the
frequency of these movements (Rupniak et al.
1983). The neuroleptic-induced oral movements
were increased by physostigmine and pilocarpine.
Acute treatment with the cholinomimetic drugs
alone produced "chewing", gaping and tongue
protrusion (Rupniak et al. 1983). Chronic
administration of pilocarpine and physostigmine
also produced mouth movements (Rupniak et al.
1985)
The present study was conducted to obtain a
more detailed behavioural and pharmacological
description of the oral activities induced by
muscarinic agonists. The frequencies of several
individual oral responses following injection of
different doses of pilocarpine were recorded. In
addition, a simpler behavioural procedure was
developed, in which the durations of periods of
mouth movements were used to assess the actions
of pilocarpine, oxotremorine, arecoline, and
carbachol, and the antagonism of
pilocarpine-induced mouth movements with
scopolamine.
4. Yawning. A gradual opening of the
mouth, followed by a retention of the open
position, frequently accompanied by a lifting
back of the head, and usually finished with a
closure of the mouth more rapid than the
original opening
Discussion
The muscarinic agonist pilocarpine produces
dose-related increases in a number of mouth
movements in the rat. Chewing and tongue
protrusion, both of which are seen in normal
control animals, are increased in frequency in a
dose-dependent manner. Gaping, which rarely
occurs in untreated animals, is also induced
reliably by pilocarpine.
These results are consistent with the
initial work of Rupniak et al. (1983), who
demonstrated that 4.0 mg/kg pilocarpine
increased mouth movements. In our study, a
dose-related graduation of effects was observed,
such that gaping was observed only at relatively
high doses, while chewing and tongue protrusion
were increased even at lower doses.
No consistent increase in yawning was
observed in this study. Ushijima et al. (1984)
previously reported that pilocarpine increased
yawning in rats. Their peak dose for obtaining
this response was 4.0 mg/kg, so our data showing
increases in yawning in some rats at 2.5 and 5.0
mg/kg were consistent with the results of their
study. However, even though the increase in
yawning reported by these authors was
statistically significant, the mean frequency of
yawning (16 yawns per 90 mm) was rather low. The
use of a different strain of rats, or other
methodological differences, could account for
the greater reliability of yawning in the
Ushijima et al. (1984) study. One should note
that in the study of Ushijima et al. (1984),
yawns were counted as "total number of mouth
openings" (p 297). It is possible that some of
the "gaping" responses that we observed were
scored as "yawning" in the Ushijima study. The
lack of consistency of yawning as a response,
and its relatively low frequency, suggest that
yawning alone might not be useful as a
cholinergic-related behaviour across a wide
variety of conditions.
The antagonism of pilocarpine effects by
scopolamine is consistent with these behavioural
effects being attributable to cholinergic
stimulation. The lack of effect of the
quaternary compound methyl scopolamine as an
antagonist demonstrates that the oral activities
probably stem from central rather than
peripheral effects, as this compound is a potent
antagonist of peripheral muscarinic receptors
but does not cross the blood-brain barrier. This
conclusion is supported by the absence of
significant mouth movements following carbachol
administration. Rupniak et al. (1983) found that
neostigmine, which does not cross the blood
brain barrier, did not produce oral activities,
while physostigmine did. In the present work,
doses of scopolamine as low as 0.125 mg/kg
resulted in an almost total blockade of mouth
movements induced by 10.0 mg/kg pilocarpine.
However doses of methyl scopolamine as high as
4.0 mg/kg yielded no reduction. The lack of
methyl scopolamine effect dissociates mouth
movements from peripheral effects such as
salivation, which methyl scopolamine blocked in
doses as low as 0.0625 mg/kg. Thus, the mouth
movements are not elicited reflexly as a
by-product of excessive salivation.
Oral activities such as chewing, tongue
protrusion and gaping could be useful
behavioural signs of central cholinomimetic
actions of drugs. These activities could be used
to augment information gained from observation
of other cholinergic responses such as
salivation. Oxotremorine and arecoline produce
movements similar to those seen with
pilocarpine. The observation of mouth movements
can yield information on the in vivo potency,
efficacy and time course of cholinomimetics. In
addition, it introduces a new behavioural model
which may enable researchers to further explore
brain cholinergic mechanisms. It is interesting
to speculate on the relationship between
cholinergic-related oral activities and those
seen during stimulant-induced stereotypes.
Though both types of movements involve the mouth
and tongue, unpublished observations from our
laboratory indicated that there are some
topographical differences between the two types
of behaviours which would allow observers to
discriminate between them. The rapid chewing
movements seen with cholinominetics do not seem
to predominate with apomorphine or amphetamine.
Stereotypy is usually directed at objects, and
can be quantified by measuring gnawing (Redgrave
et al. 1982). By definition, the oral activities
observed in the present study were not directed
at any objects. In addition, these responses
appear to be pharmacologically distinct, since
anticholinergic drugs antagonize the
cholinergic-related mouth movements, but enhance
apomorphine stereotypy (Scheel-Krflger 1970). It
is possible that both types of movements reflect
a cholinergic-dopaminergic imbalance, and that
the specific pattern of activities depends upon
which way the balance is shifted. Further
studies comparing the response topographics and
pharmacological manipulation of responses to
cholinergic and dopaminergic agonists would be
useful to clarify these points.
The finding that cholinergic agonists
enhance the frequency of some movements and
induce abnormal movements poses interesting
questions for those who advocate the use of
cholinomimetics for the treatment of Alzheimer's
Disease. Rupniak et al. (1983) suggested that
the oral activity induced by neuroleptics
reflects an acute dystonic reaction. It is
possible that the movements induced by
pilocarpine are a type of dystonia, perhaps
related to the induction of Parkinsonian
symptoms. Researchers investigating the effects
of cholinomimetics on memory in aged subjects
should also carefully examine their patients for
dystonic reactions
