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Le bâillement, du réflexe à la pathologie
Le bâillement : de l'éthologie à la médecine clinique
Le bâillement : phylogenèse, éthologie, nosogénie
 Le bâillement : un comportement universel
La parakinésie brachiale oscitante
Yawning: its cycle, its role
Warum gähnen wir ?
 
Fetal yawning assessed by 3D and 4D sonography
Le bâillement foetal
Le bâillement, du réflexe à la pathologie
Le bâillement : de l'éthologie à la médecine clinique
Le bâillement : phylogenèse, éthologie, nosogénie
 Le bâillement : un comportement universel
La parakinésie brachiale oscitante
Yawning: its cycle, its role
Warum gähnen wir ?
 
Fetal yawning assessed by 3D and 4D sonography
Le bâillement foetal
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Brain Res
1991;538(2):203-210
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.

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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.