Temporal Summation of C-Fibre Afferent Inputs: Competition between Facilitatory and Inhibitory Effects on a C-Fibre Reflex in the Rat. MANUELA GOZARIU, DOMINIQUE BRAGARD, JEAN-CLAUDE WILLER AND DANIEL LE BARS. Laboratoire de Neurophysiologie, H“pital Piti‚-Salp‚triˆre, 91 Bd de l'H“pital, 75013 Paris, France, and INSERM U-161, 2, rue d'Al‚sia 75014 Paris, France.
APStracts 4:149N, 1997.
ABSTRACT
1. Long-lasting facilitations of spinal nociceptive reflexes resulting from temporal summation of nociceptive inputs have been described on many occasions in spinal non-anaesthetised rats. Since noxious inputs also trigger powerful descending inhibitory controls, we investigated this phenomenon in intact, halothane-anaesthetised rats and compared our results with those obtained in other preparations. The effects of temporal summation of nociceptive inputs were found to be very much dependent upon the type of preparation. 2. Electromyographic responses elicited by single square-wave electrical shocks (2 ms, 0.16 Hz) applied within the territory of the sural nerve, were recorded in the rat from the ipsilateral biceps femoris. The excitability of the C-fibre reflex recorded at 1.5 times the threshold (T) was tested following 20 s of electrical conditioning stimuli (2 ms, 1 Hz) within the sural nerve territory. 3. During the conditioning procedure, the C-fibre reflex was facilitated (wind-up) in a stimulus-dependent fashion in intact anaesthetised animals during the application of the first 7 conditioning stimuli; thereafter, the magnitude of the responses reached a plateau and then decreased. Such a wind- up phenomenon was seen only when the frequency of stimulation was 0.5 Hz or higher. In spinal unanaesthetised rats, the wind-up phenomenon occurred as a monotonic accelerating function which was obvious during the whole conditioning period. An intermediate picture was observed in the non- anaesthetised rat whose brain was transected at the level of the obex, but the effects of conditioning were profoundly attenuated when such a preparation was anaesthetised. 4. In intact anaesthetised animals, the reflex was inhibited in a stimulus- dependent manner during the post-conditioning period. These effects were not dependent upon the frequency of the conditioning stimulus. 5. Such inhibitions were blocked completely by transection at the level of the obex and in non-anaesthetised rats, were then replaced by a facilitation. A similar long-lasting facilitation was seen in non-anaesthetised spinal rats. 6. It is concluded that, in intact rats, an inhibitory mechanism counteracts the long-lasting increase of excitability of the flexor reflex seen in spinal animals, following high intensity, repetitive stimulation of C-fibres. It is suggested that supraspinally-mediated inhibitions also participate in long term changes in spinal cord excitability following noxious stimulation. 

Received 8 November 1996; accepted in final form 24 June 1997.
APS Manuscript Number J909-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 27 August 1997