Grooming induced by
intrahypothalamic injection of ACTH in the rat:
comparison with grooming induced by
intrahypothalamic electrical stimulation
and i.c.v.
injection of ACTH
Van Erp AM, Kruk MR, Willekens-Bramer DC,
Bressers WM, Roeling TA, Veening JG, Spruyt
BM.
Ethopharmacology Group,
Sylvius Laboratory, Leiden, The
Netherlands.
Intracerebroventricular (i.c.v.) injection
of adrenocorticotropic hormone (ACTH) elicits
grooming in the rat, but the neural organization
of this response is still obscure. Electrical
stimulation (EHS) in an area around the
hypothalamic paraventricular nucleus (PVH) also
elicits grooming. This hypothalamic area
contains many ACTH-immunoreactive fibres.
Injection of ACTH1-24 (0.3 microgram/0.3
microliters) in the same area elicits intense
grooming responses in the rat. Latency,
intensity and precise patterning of the grooming
response are dependent upon the exact site of
injection. Comparison of grooming responses
elicited by EHS, ACTH injected i.c.v. and ACTH
injected in the PVH reveals that these are
slightly dissimilar. This may provide clues as
to the brain mechanisms involved in the
organization of the different components of
grooming. EHS does not elicits scratching and
even reduces 'spontaneous' scratching. Also,
EHS-elicited grooming is characterized by short
pauses. The time-course of appearance of
yawning differs between ACTH-PVH and
ACTH-i.c.v. injections. Excited locomotion
elicited only by ACTH-i.c.v. is apparently
caused by ACTH-sensitive systems outside the
PVH. The results suggest that the
ACTH-containing part of the hypothalamus around
the PVH is crucially involved in the
organization of grooming behaviour. We believe
that at this level in the brain, the subroutines
of grooming, scratching and yawning are
integrated into one skin maintenance
behaviour.
INTRODUCTION
Grooming is easily induced in rodents by
manipulation of the environment or manipulation
of the animal's fur". It can also be elicited by
intracerebroventricular (i.c.v.) injection of
peptides such as adrenocorticotropic
hormone(1-24) (ACTH,-,,; for review see Gispen'2
and Spruyt34). Detailed behavioural studies of
Fentress5'6 and intracranial administration of
various grooming-inducing substances indicate a
predominant control of this behaviour by the
central nervous system. Behavioural descriptions
of grooming induced by these substances suggest
that grooming bouts consist of several different
behavioural subroutines which may have a
different causation. For instance, i.c.v.
injection of bombesin induces much more
scratching behaviour than the injection of
ACTH". Also, the time course of appearance and
disappearance of the different behavioural
components is not the same. Stretching and
yawning only appear after a prolonged
period of extensive grooming", suggesting that
these subroutines are mediated by different
central mechanisms. ACTH- and
melanocyte-stimulating hormone (MSH)-induced
grooming do resemble 'spontaneous grooming 34,
suggesting a physiological role for ACTH or MSH
in natural grooming.
The brain mechanisms involved in grooming
are only partially understood. Grooming can be
elicited by injection of ACTH1_24 into the
central gray of the midbrain33. It has also been
elicited by electrical stimulation (EHS) in a
region of the paraventricular nucleus of the
hypothalamus (PVH) of the rat25, and by
injection of kainic acid into the same area 29.
This particular region is rich in ACTH-like
immunoreactivity19. Gessa'° suggested that
the site of action for stretching and
yawning induced by ACTH in cats was
located in the hypothalamus near the third
ventricle. In addition, Dunn4 reported that the
grooming response induced by i.c.v. ACTH1_24 in
rats is located in the vicinity of the anterior
ventral third ventricle. Thus the efficacy of
electrical stimulation and the presence of ACTH
in the hypothalamus suggest that this part of
the hypothalamus is involved in the organization
of grooming behaviour. Therefore, it seems
worthwhile to investigate whether direct
injection of ACTH, into this part of the
paraventricular complex would also elicit
grooming behaviour. The aims of this study are
(1) to study the effects of hypothalamic ACTH on
grooming behaviour; (2) to delineate the area
responsible for the induction of grooming
behaviour by ACTH; (3) to compare the grooming
responses induced by different stimuli. In doing
so we hope to be able to unravel the neural
mechanisms involved in grooming behaviour in the
hypothalamus.
DISCUSSION
Injection of ACTH1_24 in an area with high
ACTH immunoreactivity and electrical stimulation
of this area both induce intense grooming
responses. Interestingly, these grooming
responses are not exactly the same and differ
from grooming behaviour induced by injection of
ACTH into the ventricle.
We have observed in this study that sites
where injections of ACTH induce grooming are
closely grouped within a specific area of the
PVH-complex. Comparison of the location of sites
that induced grooming with electrical
stimulation 25 (present report), by injection of
kainic acid 29 or ACTH (present report) would
suggest that the grooming responses originate
from the same specific area of the hypothalamus.
If that is true, the neural substrate involved
in the organization of grooming is probably a
group of cell bodies sensitive to activation by
ACTH and excitatory amino acids.
Interestingly, this area contains many ACTH-
and CRF-immunoreactive fibres (Fig. 1, bottom)
and 21. The distribution of ACTH-immunoreactive
fibres was found to be very similar to the
distribution of grooming inducing electrode
sites (Figs. 1 and 2). Many of the electrode
sites from which grooming was most easily
induced in an earlier study by Lammers25 were
found in the periventricular area, along the
entire length of the ventricle in the
hypothalamus. A few sites were even located in
the hypothalamic arcuate nucleus, an area that
also contains a high density of ACTH-sensitive
fibres. Injections of CRF into the PVH have also
been reported to induce grooming behaviour 21,
although this area seems to contain no CRF
receptors28. Oxytocin and cholecystokinin (CCK)
are also present in the PVH-complex. These
peptides can also induce grooming behaviour when
injected i.c.v. or in the ventral tegmental
area
However, our guess would be that the
distribution of ACTH immunoreactivity matches
the distribution of grooming inducing sites
best. Moreover, we have no detailed information
on the structure of CCK and oxytocin induced
grooming.
Although the grooming responses evoked by
intrahypothalamic ACTH, i.c.v. ACTH and
intrahypothalamic electrical stimulation
appeared to be very similar, we observed some
differences: Electrical stimulation of the PVH
reduces scratching. The absence of scratching
sustains the hypothesis that scratching and
grooming per se are different subroutines of the
total skin maintenance behaviour. Moreover, this
suggests that the PVH-complex is important in
the integration of scratching and grooming
subroutines. It is possible that electrical
stimulation activates a subpopulation of
neurones facilitating grooming, and at the same
time inhibits scratching.
Scratching often occurred immediately after
the cessation of stimulation, which could be
seen as a rebound of the inhibited scratching
response. Scratching and grooming are
incompatible motor responses. Scratching
requires the recruitment of the hind legs while
the front paws remain on the ground, whereas the
grooming of the body requires the hind legs
firmly on the ground while the front paws and
snout are active in body care. It is known that
many behavioural responses are recruited
rostrocaudally3'56"3'24'30. Our results suggest
that the different subroutines for control of
hindpaws and forepaws 13.24 in grooming can be
activated differentially even at the level of
the hypothalamus. It has been shown very
recently that the PVH becomes intensely labelled
after application of transneuronal retrograde
tracers in the ulnar nerve of the front paw".
This labelling may occur as a result of the
paraventriculo-spinal projections35. Injection
of the same tracers in the hindpaw, however, do
not appear to induce the same amount of PVH
labelling. The concept that scratching and
grooming are different behavioural subroutines
is also supported by the fact that other
grooming-inducing peptides, such as bombesin and
/3-endorphin, elicit more scratching behaviour
than ACTH or MSH when injected in the
ventricle".
Furthermore we observed that electrical
stimulation increased the number of pauses in
grooming bouts. We believe that electrical
stimulation activates neurones and/or fibres
less selectively than ACTH injected locally, and
may evoke competing and incompatible responses
simultaneously. Possibly, as a result of the
simultaneous activation of systems that start or
stop the grooming response, interruptions occur
in grooming bouts, leading to an increase in the
frequency of almost all grooming elements (Fig.
4, right panel). Our results suggest furthermore
that electrical stimulation does not elicit the
grooming response as easily as ACTH injection in
the PVH. Rats do not only interrupt grooming
more often with EHS, they also often make a half
turn before they start grooming (Fig. 5).
Whether the lower total time spent on grooming
with EHS is due to the stimulation technique is
not clear. However, it seems that the difference
in occurrence of scratching between local ACTH
injection and electrical stimulation is
independent from the dosage or current intensity
used. Increasing the current intensity above the
grooming threshold decreased the amount of
scratching while increasing the amount of other
grooming elements, whereas ACTH-induced grooming
increased scratching concomitantly with other
grooming elements.
The injection of ACTH in this PVFI area
elicits grooming almost exclusively. I.c.v.
injection of ACTH, however, also induces hopping
and rearing behaviour (Fig. 5). We suggest that
this is a consequence of the activity of the
peptide on other ACTH-sensitive strucures
elsewhere in the brain.
If we consider the grooming response, our
results so far suggest that the hypothalamic
paraventricular complex is crucially involved in
the organization of body care. We suggest that
at this level in the brain, the behavioural
subroutines of grooming, scratching and
yawning are integrated into one skin
maintenance behaviour, by an yet unknown
mechanism. The fact that all sites where
yawning was induced are lying directly in
the PVH, suggests that this area is also
important in the integration of grooming and
yawning behaviour. Because EHS leads to
an increase in the pauses in grooming bouts in
contrast to injection of ACTH in the PVH, we
assume that the interruptions in grooming are
also organised at this level in the
hypothalamus. These interruptions may have a
functional significance for the animal, giving
it the opportunity to investigate whether the
environment is safe enough to remain engaged in
grooming behaviour.
Our results on hypothalamic behavioural
responses suggest that at the level of the
hypothalamus, different behavioural subroutines
are integrated. The hypothalamus may have a role
in determining the priority of competing
behavioural responses. Stimulation of an attack
eliciting site completely blocks the grooming
response elicited in a grooming site, even at
intensities well below the intensity required to
produce an attack 26. The fact that electrical
stimulation in the hypothalamus increases one
aspect of a behavioural system and reduces
another has been reported earlier in
hypothalamic 'aggression'. Stimulation of the
hypothalamic attack area in the rat increases
violent attack, but reduces sideways threat
behaviour and other 'ambivalent' postures which
are usually attributed to the same
'motivational' system20' 23,24 Such results
suggest that the behavioural responses elicited
in the hypothalamus are the expression of quite
specific subroutines involved in the direct
execution of responses, below the conceptual
level of what is usually called a 'motivational'
system.
