- Abstract
- It has long been known
that the intracerebroventricular administration
of ACTH1-24 increases the duration of grooming
episodes and the frequency of yawning in rats.
The objective of this study was to investigate
in what way these episodes are prolonged
andwhether and to what extent genotype
influences such effects. We compared the effect
of increasing doses of
intracerebroventricularinjections of ACTH1-24 on
grooming and yawning in males of two inbred
strains of SpragueÐDawley rats with distinct
yawningfrequency, high-yawning (HY) and
low-yawning (LY). In LY rats the duration of
grooming episodes increased, while in HY
ratsgrooming episodes augmented both in number
and duration. In LY rats the duration of
grooming components increased likewise, inHY
rats however, neither the number nor the
duration of the components changed. The grooming
rate in both strains of rats wasslowed, though
more so in LY than in HY rats. Yawning increased
in LY rats but not in HY rats. We conclude
therefore thatACTH1-24 increases the duration of
grooming episodes by slowing the grooming rate
according to genotype, and may or may notalter
the frequency of yawning.
-
- Introduction
- The
intracerebroventricular (ICV) administration of
ACTH1-24 prolongs grooming in rodents without
affecting the number of episodes of this
behaviour (Gispenand Isaacson, 1981). The exact
nature of the way grooming episodes become
longer has not yet been identified (but see
Bressers et al., 1995; Gispen and Isaacson,
1981; Spruijt et al., 1992; van Wimersma
Greidanus et al., 1985).
-
- ACTH1-24 could either
increase the number of grooming components per
episode, make grooming components longer or
affect both number and duration of grooming
components. A study to distinguish between these
alternatives would help us to understand better
the role of ACTH1-24 in grooming behaviour.
Since it is unlikely that following the
administration of ACTH1-24, rats can
simultaneously groom faster and longer, we
hypothesize that ACTH1-24 slows grooming rate,
i.e., rats spend more time grooming each part of
their body. Besides the effect on grooming, the
ICV administration of ACTH1-24 produces yawning
(Ferrari, 1958;Ferrari et al., 1963).
-
- Both grooming and yawning
are stereotyped behaviours (Fentress, 1988;
Dourish andCooper, 1990) and hence they might
have a strong genetic component. A comparative
study using inbred strains of rats, which differ
in yawning and grooming may indicate the extent
to which the effect of ACTH1-24 on these
behaviours is genetically dependent. In this
study, we have attempted to determine in what
way grooming episodes are prolonged after the
ICV administration of ACTH1-24 and evaluate the
extent to which the response to ACTH1-24 is
affected by genotype. To this purpose, we used
two strains of SpragueÐDawley rats,
previously selected for high- (HY) and
low-yawning (LY) frequencies (Urba-Holmgren et
al.1990). They have been shown to differ in
novelty-induced grooming as well (Eguibar and
Moyaho1997).
-
- Discussion
- The purpose of the
present study was an in-depthanalysis of the
prolonging effect of ACTH1-24 ongrooming
behaviour. In LY rats results corroborate
theoutcome of former studies (Gispen and
Isaacson, 1981) that ACTH1-24 increases the
duration but not the number of grooming
episodes, and show that this iscaused by a
longer duration of the components of grooming
behaviour. In HY rats, in comparison,
bothduration and number were increased. This is
an unusual effect of ACTH1-24, which can be
attributed to genetic differences between HY and
LY rats, but whose specificmechanisms are beyond
this study. Similarly, the durationof grooming
components after the administrationof ACTH1-24
differed between LY and HY rats: HY groomed each
part of their body for a shorter time.
-
- Previous studies suggest
that such differential response might be due to
the fact that HY rats interrupt
systematicallytheir ongoing activities (e.g.,
grooming,resting, mating), and that they can
hardly complete anorderly sequence of behaviour
(Moyaho et al., 1995). Accordingly, these rats
might be classified as rats with an attention
deficit, but additional research is required to
support this hypothesis. Administration of
ACTH1-24 augmented yawning frequency in LY rats,
which is consistent with previous reports on
other animals (Argiolas et al., 2000;
Ferrari,1958; Poggioli et al., 1995). However,
ACTH1-24 did notaffect yawning in HY rats. This
difference cannot be attributed to a ceiling
effect since there are other drugs (e.g.,
colinomimetics and dopamine-receptor stimulants)
that greatly increase yawning in HY rats
(Eguibar and Moyaho, 1997; Eguibar et al., 2003;
Urba-Holmgren et al., 1993). It seems instead
that there is a maximum level above which
ACTH1-24 does not increase yawning in HY rats.
This finding is very interesting because no drug
with inductive effects on yawning has failed to
increase the frequency in HY rats. That yawning
could have appeared as a rebound effect after
the 120 min of observation is improbable,
judging the trend of response to increasing
doses of ACTH1-24, and data from previous
reports (Argiolas et al., 2000).
-
- Another explanation of
the insensitivity of yawning to the
administration of the drug could be a lack of
receptors, but since ACTH1-24 did indeed affect
grooming, distinct types of ACTH1-24 receptors
might be involved in yawning and grooming. There
are several subtypes of melanocortin receptors
in the brain area (De Wied, 1999; Wikberg et
al., 2000), of which the MC4 type is involved in
both behaviours (Adan et al., 1994; Adan and
Gispen, 2000; Argiolas et al., 2000; Vergoni et
al., 1998). Yet, the fact that yawning occurs
after a delay while grooming responds promptly
(results not presented), suggests that different
melanocortin receptors might be implicated.
Since ACTH1-24 requires the activation of
dopamine receptors (Guild and Dunn, 1982), an
alternative explanation for the differential
effect on HY and LY rats could be that they
differ in dopamine neurotransmission. In
addition to dopamine, other neurotransmitters
are involved in ACTH-induced yawning, for
example acetylcholine,opioïd peptides, Ca2+
ions and nitric oxide (for a review see Argiolas
and Melis, 1998).
-
- Accordingly, ACTH-induced
yawning is prevented by muscarinic receptor
antagonists (Yamada and Furukawa, 1980),
bymorphine (Bertolini and Gessa, 1981), by
x-conotoxin or organic Ca2+ channel blockers
(Argiolas et al., 1990; Poggioli et al., 1993)
and by nitric oxide synthase inhibitors(Melis
and Argiolas, 1997). Therefore, differences in
yawning frequency between HY and LY rats can
also result from an interaction between ACTH1-24
and other neurotransmitter systems. Preliminary
results from our laboratory indicate that indeed
dopamine levels in the nucleus accumbens are
higher in LY than inHY rats.
-
- The diminution of the
grooming rate suggests that ACTH1-24 affects or
interacts with neural circuits involvedin
''recurrent routines'' as it produces repetitive
sequences of behaviour. This effect resembles
behaviour related to obsessiveÐcompulsive
disorder in humans in which patients are unable
to stop doing certain behaviours (e.g., washing,
dressing or other toileting behaviours (Swedo,
1989). Patients lack a sense of confidence that
leads them to checking until ''reaching
satisfaction''(Insel and Pickar, 1983).
-
- Neuropeptides have indeed
been involved in obsessiveÐcompulsive
behaviours (Sandyk,1987; but see Insel and
Pickar, 1983). In addition grooming is
displacement behaviour (Cohen and Price,1979)
and displacement behaviours have been proposed
for modelling obsessiveÐcompulsive disorder.
Therefore, the effect of ACTH1-24 on grooming
may be useful to understanding recurrent
pathological behaviours.
-
- In conclusion, these
findings show that ACTH1-24 prolongs duration of
grooming episodes by slowing the grooming rate.
Such effect depends on genotype and may
originate strain differences.
-
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