Behavioural Brain Research, 58 (1993) 141-154 © 1993 Elsevier Science Publishers B.V. All rights reserved. 0166-4328/93/$06.00 141 BBR 1508 Thresholds of electrically induced defence reaction of the rat: short- and long-term adaptation mechanisms Eliana Cristina Murari Sudrf, Maria Rufina de Barros, Gilberto Neves Sudrf, Luiz Carlos Schenberg* Department of Physiological Sciences, Biomedical Centre, Federal University of Espirito Santo, Vitfria, ES (Brazil) (Received 5 December 1992) (Revised version received 26 July 1993) (Accepted 27 July 1993) Key words: Defence reaction; Fear; Flight behavior; Freezing behavior; Periaqueductal gray; Superior colliculus The thresholds of electrically induced defence reaction of the rat were studied through the logistic fitting of the response output. When stepwise increasing stimuli were applied at the dorsal midbrain, hierarchically organized mean thresholds, spaced 10/~A apart, were observed for im- mobility, running and jumping defensive behaviours. The parallel threshold functions of these responses, ranked in the above order, denote that they have distinct output probabilities when induced with sequential stepwise increasing stimuli. In contrast, when single dally stimuli were given in a random order, virtually superimposed threshold functions were obtained for these defensive behaviours. In this case, since the same out- put probabilities would be expected for immobility, running and jumping behaviours, the defence system seems to operate in a state of maxi- mum entropy. The above data suggest that the dorsal midbrain, including the deep collicular layers and the periaqueductal gray, may encode hierarchical or non-hierarchical defensive patterns which, respectively, mimic either the attentive behaviour of the prey watching the approaching predator or its chaotic behaviour when cornered by a sudden attack. On the other hand, whereas quite stable thresholds were observed for the somatic defensive responses when 5 stimulation sessions were repeated over 15 days, the defecation and micturition output underwent a marked and progressive lessening. Since these autonomic responses have long been considered as reliable indexes of fear, their attenuation throughout the repeated sessions could express the rat adaptation to fear by the recurrence of the aversive experience. Taken together, these data suggest that while short-term neuronal adaptation could be responsible for the hierarchical threshold structure of the short interval stepwise stimula- tion, long-term neuronal adaptation could underlie the selective decrease of defecation and micturition responses over repeated sessions of in- tracranial stimulation. INTRODUCTION The defence reaction is a highly organized pattern of somatic and autonomic responses. Hess coined the term "affective defence reaction" (affektiven Abwehr- reaktion) to describe the behaviours produced by hy- pothalamic stimulation in cats, particularly, to describe the immobile aggressive display with slightly hunched back, flattening of the ears, teeth baring, hissing, growl- ing, unsheeted claws, piloerection and marked mydria- sis. This peculiar pattern, however, could culminate in either attack or sudden flight with stronger stimula- tion 41. These responses were identical to the cat innate behaviours and were accompanied by a widespread *Corresponding author. Department of Physiology, The Medical School, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. Fax: (44) (21) 414-6924. sympathetic activation that, according to Hess 4°, was in agreement with Cannon's concept of "emergency reaction ''17. Hess also suggested that flight and defeca- tion were typical manifestations of "fear", while the defence reaction itself, i.e., the immobile aggressive dis- play, was the expression of "rage ''4°. Thus, pari passu with the former ethologists TM Hess work out the notion of the ambivalent nature of agonistic behaviours which would result from the simultaneous activation of attack and flight systems. A plain neural topography representing the cat de- fensive behaviours was proposed some years later by Hunsperger and collaborators 34. According to these authors, the cat defence system would be comprised of two core zones controlling the immobile aggressive dis- play, namely, the perifornical hypothalamus and the periaqueductal gray, surrounded by a more diffuse flight zone extending from the preoptic area to the caudal