- Grooming occurrence in several
species of animals, as a result of environmental
manipulations, has generated an increasing
interest in the study of this behavior, and many
authors have suggested that grooming serves a
variety of adaplive functions. In rodents,
grooming can be elicited by either exposure to a
novel environment, including handling and
transportation of the animal to an observation
room, or immersion in water, which allows the
recording of repetitive and natural sequences of
grooming for long periods. Grooming and other
behaviors, particularly yawning, which is
less susceptible to modification by
environmental manipulations, are increased by
the i.c.v. administration of ACTH, a-MSH and
related neuropeptides suggesting that these
behaviors might share some of their
underlying neurochemical mechanisms.
The fact that the prior administration of
naloxone, a nonselective opioid antagonist, and
other neurotransmitter substances prevent
environmental or pharmacological induced
grooming, suggests a complex interaction among
several neural systems. For instance, the
muscarinic receptor antagonists atropine and
scopolamine can specifically antagonized, in a
dose-dependent manner, ACTH-induced grooming.
Similarly, a previous injection of atropine into
the ventral tegmental area inhibits grooining
induced by the i.c.v. adininistration of a-MSH.
A prior injection of muscarinic antagonists also
suppressed the effect of bombesin-stimulated
grooming. Spontaneous and drug-induced yawning
have also been related to a cholinergic
influence, especially to an excitatory action by
the administration of cholinomimetic
substances.
These findings indicate that grooming and
yawning are under the influence of the
muscarinic cholinergic system. However,
there are no studies that reveal a direct
relation between grooming frequency and
cholinergic drugs, as there are for yawning.
This is because most pharmacological studies
have focused ou the ability of cholinergic
antagonists to reverse grooming induced by
neuropeptides. In addition, most studies have
been carried out either on grooming or yawning,
but rarely considering the relationship
between both behaviors, probably because in
contrast to grooming, yawning occurs witlh a low
spontaneous frequency which restricts its
analysis to pharmacological manipulations.
ln our laboratory we have developed two
sublines of Sprague-Dawley
rats, selectively bred for high-(HY) and
lowyawning (LY) frequency, and it bas been shown
that they differ in their responses to
cholinergic and dopaminergic drugs as well as in
emotional reactivity and hierarchical
composition of grooming elements. In this study
we tested whether novelty and water
immersion-induced grooming frequency differ
between HY and LY rats in a way that it
parallels the difference in yawning between both
groups of rats. We also examined whether
pilocarpine increases grooming as it docs with
yawning. The results indicate a genotypic
variation between HY and LY rats that
supports the first hypothesis of the study, but
not the latter. [...]
-
- Discussion : The present results show
that HY rats groom more than LY when they are
exposed to a novel environment or after
immersion in water. Thee latter produces a
higher amount of grooming than the novel testing
condition, which is in accordance with other
experiments. The difference in grooming between
HY and LY rats, which parallels that of
spontaneous yawning frequency shown by both
sublines, indicates a positive correlation
between yawning and grooming and suggests
that the selection for yawning frequency also
affected grooming.
- Previous results showed that HY and LY rats
also differ in open field activity and the
hierarchical organization of grooming elements,
which supports the findings of this report and
demonstrates that the differences between both
strains of rats go beyond yawning
frequency.
- Whether these behavioral differences are a
direct or secondary consequence of the inbred
selection is still not clear, although it is
consistent with other inbred selection studies
that have revealed that genetic influences are
ubiquitous for animal behavior. It is possible
that yawning and grooming have a motivational or
functional relationship, other than that
concerning their pharmacological induction. It
has previously been suggested that these
behaviors are after responses to stressful
stimuli or circumstances. If this were the case,
il would follow that HY rats are more sensitive
to stressful circumstances, a conclusion
partially supported by preliminary results, and
to the pituitary-adrenal system, which is
activated as an animal's reaction to exposure to
novel stimuli.
- Indeed, it has been reported that the
central release of ACTH can be involved in the
increased grooming observed in a novel
environment, and probably in that after
immersion in water. The finding that HY animals
maintain a high level of grooming in a novel
condition and after immersion in water, supports
the idea that both kinds of grooming response
insight be similar with respect to the
neurochemical mechanisms of their generation.
Stressors such as fur moistening may differ from
exposure to a novel condition and from immersion
in water, which not only disturbs the fur of the
animal but also makes it too dry. Therefore,
drying after water immersion seems to overcorne
grooming caused by solely moistening the fur. It
is interesting that the difference in
novelty-induced grooming between HY and LY rats
is restricted to face washing and scratching,
which have been considered as components of two
sub-branches in the grooming system, suggesting
that HY and LY rats differ in both of them.
Although pilocarpine diminished novelty- and
water immersion-induced grooming of both strains
of rats, with a higher sensitivity to this drug
shown by HY rats, the inhibition was more marked
in the novel than in the water immersion
condition. This indicates that the inhibitory
influence of this drug depends on the
manipulation to which the animals are previously
subjected. In addition, pilocarpine seems to
inhibit grooming by disrupting mainly body
washing, which is a transitional element between
rostro and caudal components and also a crucial
element in HY grooming structure.
-
- Conversely, pilocarpine appears not to
affect indivichial LY grooming components, but
all of them to a similar degree. This is
probably because LY grooming structure as a
whole is more resistant to modification even
after immersion in water, in which we observed
greater number of grooms, this leaves ont any
possibility of statistical bias due to the
occurrence of few events. The pharmacological
effect of pilocarpine was due to a direct
influence on grooming response rather than to a
generalized inhibition of locomotor activiiy, as
reflected by the finding that activity in open
field was not affected by this drug suggesting a
specific effect on grooming. The fact that
pilocarpine at closes of 3.75 mg/kg induced
chewing response, which has been proposed as a
reliable index of central muscarinic agonist
activity in rats, indicates that it affected
mostly central cholinergic activity. In
addition, the doses of pilocarpine used in this
study are within those (0.5-10 mg/kg) frequently
found in other reports on yawning and are
below those currently used to produce massive
effects.
The genotypic
variation in grooming and yawning
between HY and LY rats may be the result of
differences in the expression of cholinergic
neurotransmission and ils interaction with other
systems, as several neurotransmitters have been
evidenced in other, comparative studies in
inbred strains of rodents (7). Resulis,,from our
laboratory raise the possibility that HY may
have a high~r cholinergic tone than LY rats
(34). However, the differences in grooming and
yawning bctween HY and LY rats cannot be
dependent only on the effect of pilocarpine,
because in contrast with the inhibitory effect
on grooming, that on yawning is a dose-dependent
increase in both sublines of animals. The
cholinergic system affects these behaviors in
opposite directions and probably along distinct
pathways. In fact, the cholinergie neurons
involved in yawning are thought to be influenced
by dopaminergic rienrons (38). The results
presented here are not conclusive on the type of
muscarinic receptors, which pilocarpine is
mostly affecting, and il is likely that other
neurotransmitter systems are involved in the
production of grooming.
There is evidence that ACTH, in vitro,
inhibits the quinuclidinylbenzilate (muscarinic
antagonist) receptor-binding. This might account
for the increase of the acetylcholine turnover
rate after central administration of ACTH. This
is consistent with the finding that inhibition
of muscarinic receptor binding in the brain may
lead to an increased release of acetylcholine.
Therefore, we expected pilocarpine to increase
grooming, which contrasts with the results
reported here. On the basis of these
experimental findings, it appears that the
effects of pilocarpine and ACTH on grooming
differ, a hypothesis that has been suggested for
yawning too. The situation is even more
intricate than il may appear since the sole
i.c.v. administration of pirenzepine or
AFDX-116, selective antagonists of M1 and M2
muscarinic receptors respectively, does not
modify grooming behavior in rat, but decreases
yawning. Furthermore, the administration of
scopolamine in hamsters receiving artificial
cerebrospinal fluid did not change grooming.
However, some authors have reported that not
only muscarinic but also nicotinic receptors are
involved in ACTH-induced grooming. We are
currently attempting to test whether the
differences in grooming between HY and LY rats
also involve peptidergic influences. Initial
results indicate that HY and LY rats differ in
their grooming response to the administration of
ACTH.
In conclusion, these results have revealed
evidence for a significant genotypic variation
in grooming response between IIY and LY rats as
well as differences in sensitivity to the
behavioral effects of pilocarpine. This
difference parallels that of yawning which makes
HY rats to be an excellent tool for studying
neurochemical mechanisms involved in the
generation of both behaviors, and their
implications in stressful
circumstances.
-
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