Sémiologie et examen clinique






haut de page

mise à jour du
8 avril 2004
Behavioural Brain Research
1993; 59; 95-101

Periaqueductal gray lesions do not affect grooming, induced electrically in the hypothalamic paraventricular area in the rat
AM Van Erp, MR Kruk, W Meelis, JG Veening
Ethopharmacology Group, Sylvius Laboratory, University of Leiden,
Department of Anatomy and Embryology, University of Nijrnegen,
The Netherlands


Introduction : Electrical stimulation of specific areas in the hypothalamus evokes complex behavioural patterns. Depending on the exact location of the electrode, different responses are evoked, such as attack behaviour or self-grooming. Self-grooming can be evoked in the paraventricular nucleus (PVH) and dorsal hypothalamic area (DHA). Several descending and ascending pathways have been described that may be involved in the execution of these behavioural responses. One particular pathway runs from the hypothalamus through the periaqueductal gray area (PAG) and the adjoining parts of the brain stem. The PAG seems to be involved in aggressive and defensive behaviour and has been shown to be crucial for ACTH-induced grooming. However, Mos et al showed that PAG lesions only slightly and transiently reduce aggression evoked by hypothalamic stimulation and aggression in a territorial setting (resident/intruder paradigm). Therefore, it seemed worthwhile to investigate the effect of PAG lesions on hypothalamically induced grooming behaviour, to study the role of the PAG in the execution of self-grooming. [...]
Discussion : Our study shows that lesions of the periaqueductal gray area have no effect on thresholds for hypothalamically evoked self-grooming behaviour. This is in line with the effect of PAG lesions on hypothalamic attack responses. Mos et al showed that PAG lesions only slightly and transiently reduce thresholds for hypothalamic attack. In general, hypothalamic responses may consist of either changes in the introductory phase of behaviour, e.g. after ventromedial hypothalamic stimulation or consist of a clearcut response, often of a compulsive nature, e.g. attack or grooming responses. Animals interrupt ongoing behaviour immediately and show the particular response elicited within seconds after onset of stimulation. Behaviourally it seems that a system involved in the execution phase of the behaviour, rather than the introductory or decision making phase, is activated.
Which pathways are involved in these hypothalamic responses? Fibres descend towards the lower parts of the brainstem either via the ventral tegmental area and the central tegmental field, or along the midline via the PAG. In addition, ascending fibres and fibres with a thalamic destination may play a role, as well as other ascending projections to the forebrain. Interestingly, hypothalamic projections to the PAG show a topographical and the fibres originating from the hypothalamic attack area (caudal, lateral PAG) show a distribution that is very different from fibres originating from the hypothalamic grooming area (dorsal and lateral PAG).
Spruijt et al showed that the PAG is a prerequisite for ACTHinduced grooming. PAG lesions diminished the response to i.c.v. injections of ACTH 1-21. However, grooming did not disappear completely, but was reduced to baseline level. There are several possible explanations for these seemingly contradictory results. Recent experiments, in which grooming was induced by direct injection of neuropeptides into the PVH suggest that peptides such as ACTH and alpha-MSH prolong a grooming response that was initiated by other factors, e.g. handling of the rat or exposure to a novel environment. In contrast, a peptide such as oxytocin seems to initiate grooming. Apparently, separate systems exist in the central nervous system that regulate the initiation of groorning, its continuation and probably the interruption of this behaviour. We hypothesise that ACTH, for which binding sites are present in the PAG serves to modulate grooming that has been initiated by other factors outside the PAG. This could explain the fact that after PAG lesions grooming is still performed at baseline level. The modulatory effect of the PAG on grooming has been confirmed recently by Van Wimersma Greidanus, who showed that PAG lesions attenuate ACTH- but not oxytocin-induced grooming. Nevertheless, our results suggest that the PAG is not the major destination station involved in hypothalamically evoked grooming responses.

A recent study by Roeling showed that one of the many efferent pathways originating from the PVH that is possibly involved in the execution of grooming, is the connection descending via lateral hypothalamus, ventral tegmental area (VTA) and central tegmental field. Interestingly, the VTA is one of the areas of the brain that has been reported to be involved in grooming behaviour: injection of neuropeptides, such as oxytocin, CCK and alpha-MSH into the VTA induces selfgrooming behaviour. The possible involvement of the VTA in the control of hypothalamic responses deserves special attention in future studies, although a similar study using VTA lesions may prove difficult to perform because of severe effects on autonomic functions.

Lesions of the periaqueductal gray resulted in severe behavioural deficits. Some of these effects inay have been caused by lesions of tissue surrounding the PAG, especially in animals with large lesions in which ascending serotonergic and/or noradrenergic pathways may have been disrupted. Animals didn't eat or drink (also reported by Mos et al): they seemed to have -forgotten- how to do it, because they would drink readily when helped and showed interest in wet mesh after they had been drinking. It is interesting to not that several animals showed moderate to strong defensive reactions upon touch, despite the intensive handling period before surgery. They didn't try to bite, but upon touch by the experimenters hand tried to escape by running and jumping and sometimes they screamed when picked up. This observation is at odds with reports that PAG lesions attenuate defensive behaviour for which we have no explanation. Others have reported that PAG lesions do not change defensive reactions induced by other means either, e.g. inferior colliculus stimulation.

In addition, some animals showed reflex-like face washing movements upon stimulation of the mouth region with a wet syringe during drinking sessions. These movements were quite strong and compulsive, performed in a position in which a rat normally would never groom (restrained, lying on its back). These movements were not just attempts to remove the syringe, but included the repetitive "front paw over car" movements that arc characteristic for face washing. This is an important observation, showing that the animals are still responding to sensory stimuli. Apparently the sensory input pathways are intact, as well as the motor components of the grooming behaviour. In spite of the fact that they have a problem in maintaining their body posture, they still groom, even while lying on their back. We could not correlate the nature of observed behavioural deficits (defensive behaviour, face washing upon stimulation of mouth region) to the size or extension of the lesions. These clear deficits, however, did not affect the hypothalamically evoked grooming response, neither inhibitory nor facilitatory.