mise à jour du
30 octobre 2003
 Physiology & Behavior
1995; 57; 5; 887-892
PVH lesions do not inhibit
stressor-induced grooming in rat
AM. Von Erp, MR. Kruk, et al
Medical pharmacology, Sylvius laboratory, U of Leiden, Netherlands


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.



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.