CHANGES IN NOCICEPTIVE REFLEX FACILITATION DURING CARRAGEENAN-INDUCEDARTHRITIS

Facilitation of neuronal responses induced by repetitive electrical st imulation of C-fibres (wind-up) is thought to be a substrate of hypera lgesia. There is little information on how these responses are in turn modified during hyperalgesia, and the extent to which hyperalgesic st ates also induce a facilitation of the neuronal responses mediated by A-fibres. The current study was undertaken in order to evaluate the ef fects of peripheral inflammation and stimulus presentation on the faci litation of nociceptive reflexes. Flexor reflexes, recorded as single motor units, were evoked in rats by cycles of low and high frequency e lectrical stimulation with pulse durations of 0.2, 0.5 and 2 ms. Respo nses were studied in control and inflammatory conditions, using the ca rrageenan-induced mono-arthritis model. The results show that the faci litation of late (C-fibre mediated) responses was proportional to the pulse duration of stimulation, as well as to the stimulation frequency . Facilitation was always higher when animals were subjected to inflam mation. In inflammatory conditions, facilitation of reflexes was obser ved not only for late (C-fibre mediated) but also for early (A-fibre m ediated) reflex responses. However, the facilitation of these early re sponses was not proportional to the intensity of stimulation. Thus, in arthritic animals, late (C-fibre mediated) flexion reflexes elicited from the skin, are facilitated and early (A-fibre mediated) reflexes a re not only facilitated but, in addition, show a novel wind-up phenome non.