SELF-GROOMING can be evoked by electrical
stimulation of the paraventricular and dorsal
hypothalamic area, depending on the exact
localization of the electrode tip. Chemical
stimulation of this area with neuropeptides,
such as ACTH, aMSH, and oxytocin, or low doses
of the excitatory amino acid agonists NMDA and
kainic acid also induces grooming responses.
Even mild mechanical stimulation of the PVH
induces grooming, although to a lesser extent
than after local injection of ACTH or a-MSH. The
precise spatial distribution of grooming sites
in the hypothalamus suggests the involvement of
a local neurotransmitter or neuropeptide, such
as oxytocin. However, grooming patterns are
still present after decerebration, suggesting
that the HGA itself is not required for the
central organization of grooming per se.
Therefore, il is not clear what the specific
role of the PVH in the regulation of grooming
is. One possibility is that the hypothalamic
grooming area is involved in the effects of
stressors on grooming.
Self-grooming behaviour often occurs as an
aftereffect of mild stressors, such novelty or
handling of the animal; such grooming is
supposed to play a role in dearousal of the
animal after exposure to a stressor. Because the
PVH is an important part of the hypothalamus
-pituitary - adrenal axis, which is activated by
stressors, it may have a dual role in both the
physiological and behavioural responses to
stressors. To investigate what role the PVH
plays in grooming behaviour, we studied the
effect of PVH lesions on grooming induced by
external events.
Grooming behaviour is controlled by both
central and peripheral mechanisms. The influence
of peripheral sensory mechanisms appears to be
smaller in stressor-induced grooming than in fur
cleaning. Therefore, we studied the effects of
lesions on grooming induced by mild, manual
restraint and grooming induced by moistening of
the fur. To exclude the well-known effects of
novelty stressors on grooming, the animals were
observed in their home cages. The time spent on
grooming, the time course of grooming, and the
time spent on specific grooming elements were
recorded.
[...]
DISCUSSION
The main finding from this experiment is that
large lesions, which completely destroy the area
of the hypothalamus where one can easily induce
grooming, do not inhibit grooming induced by
mild, manual restraint or moistening of the fur.
The changes observed in total time spent on
grooming are small, not significant, and in the
opposite direction. This suggests that the
PVH is not a prerequisite for the execution of
grooming behaviour per se. These results are
in agreement with the hypothesis that grooming
movements are generated by a diffusely organized
network in the hindbrain and, therefore,
resistant to CNS damage. This idea is also
supported by our recent finding that complete
lesions of the periaquaductal gray, a major
projection area of the PVH/HGA, do not affect
grooming induced by electrical stimulation of
the PVH, despite the fact that such lesions also
cause severe behavioural and motor deficits.
The question on the role of the PVH in
stressor-related grooming remains open. It is
possible that the mild stressors used here do
not activate the stress response sufficently to
induce stressorrelated grooming, possibly
because the animals have been handled before the
experiment. However, stronger stressors do not
necessarily induce more grooming. It is also
possible that the central mechanisms involved in
grooming have reorganized during postsurgical
recovery.
Previous studies showed that different
manipulations of the PVH evoked different
grooming responses. There is evidence that
different mechanisms in the PVH have a role in
the timing of different constituent elements of
grooming. Injection of ACTH induces yawning
responses in addition to grooming;
electrical stimulation of the PVH evokes
grooming, but inhibits scratching behaviour.
Injection into the PVH of related substances
such as ACTH 1-24 and a-MSH induces different
amounts of scratching behaviour. Low doses of
different excitatory amino acids, such as NMDA
and kainic acid, induced slightly different
grooming responses with respect to the amount of
genital grooming and scratching. Others
reported an increase in penile erection, genital
grooming, and yawning after injection of
oxytocin into the PVH. The distribution of
the area where electrical stimulation evokes
grooming coincides to a remarkable degree with
oxytocin-immunoreactive fibres and cell bodies.
The fact that electrical and mechanical
stimulation evokes grooming responses within
seconds after onset of stimulation suggests a
role of this area in the initiation of grooming.
We recently showed that oxytocin infusions into
the PVH in resting rats do indeed initiate
grooming, whereas infusions of a-MSH are not
effective. ACTH1-24 and a-MSH prolong grooming
initiated by novelty or mechanical stimulation
of the PVH, suggesting different mechanisms for
initiation and continuation of grooming. One
would expect then that lesions of local oxytocin
mechanisms would affect the ordered grooming
pattern.
The fact that PVH lesions do change the
grooming pattern seems to confirm that the PVH
has such a modulatory role in grooming
behaviour. Also, direct observation of the
lesioned rats suggests that these animals switch
between grooming elements; more or less
randomly, by passing the usual cephalo-caudal
pattem that is present in normal grooming
episodes. However, a detailed sequential
analysis comparing differences in grooming
pattern, following much more selective lesions
of the hypothalamic grooming area, would be
required to confirm the hypothesis that the
PVH regulates the ordering of the different
elements in the grooming pattern. The large
lesions in this study also affected areas
outside the hypothalamic grooming area, and it
is very well possible that the lesioning of
these areas is responsible for the effects on
the grooming pattern and its constituent
elements. The data presented here can only show
that, despite serious behavioural deficits
caused by the extensive lesions, complete
destruction of the hypothalamic grooming area
does not abolish grooming induced by a mild
stressor and by an external stimulus.
