Evaluating acute pain in experimental animals. First part.

Objective: To describe tests of nociception which appear in the "pre-clinic al" literature. Data sources: References obtained by computerized bibliographic research (M edline (R)) and the authors' personal data. Data synthesis: Ethical problems arising from the study of the pain in awak e animals, problems arising from the choice of stimulus and stimulus parame ters and the quantification of responses are presented. Pain in animals can be estimated only by examining their reactions, but at the same time, the existence of a reaction does not necessarily mean that there is a concomita nt sensation. A description of the signs of pain in mammals is proposed. A noxious stimulus can be defined by its physical nature, its site of applica tion and what has previously happened to the tissues at this site. Electric al stimulation short-circuits the process of transduction at free nerve end ings and is not specific; however it has the advantage that it can be appli ed suddenly and briefly and thus results in synchronised signals in the rel evant primary afferent fibres which can be differentiated into A delta and C fibres. Heat selectively stimulates thermoreceptors and nociceptors, but the low calorific power of conventional stimulators restricts their usefuln ess. Radiant sources have the disadvantage of emitting waves in the visible and the adjacent infrared spectra, for which the skin is a poor absorber a nd good reflector. Thermodes have the disadvantage of activating mechanorec eptors and thermoreceptors simultaneously; furthermore, their capacity for transferring heat depends on the quality of contact with skin and thus on t he pressure with which they are applied. These problems can be overcome by using CO, lasers but even today, the cost of these is a major disadvantage. Chemical stimuli differ from those mentioned above by the progressive onse t of their effectiveness, their duration of action and the fact that they a re of an inescapable nature. Experimental models employing chemical stimuli are undoubtedly the most similar to acute clinical pain. A wide spectrum o f reactions are observed in nociceptive tests, but in almost every case the y involve motor responses. After defining the ideal characteristics of a no ciceptive test, tests based on the use of short duration and longer duratio n stimuli are presented. In tests of phasic pain, reactions are evoked by t hermal (tail-flick test, hot-plate test), mechanical or electrical (flinch- jump test, vocalisation test) stimuli. Tests of tonic pain employ injection s of algogenic agents intradermally (formalin test) or intraperitoneally (w rithing test) or even the dilation of hollow organs. All these tests will b e critically appraised in a subsequent paper [1]. Conclusion: The tail-flick and hot-plate tests are the most used, but there is an increasing recourse to the formalin test and tests involving foot wi thdrawal after mechanical stimulation. (C) 2001 Editions scientifiques et m edicales Elsevier SAS.