Slow changes in the flexion reflex of the rat following arthritis or tenotomy

(1) The flexor reflex was measured in control decerebrate spinal rats by recording the motor axon activity in the nerve to biceps femoris evoked by a pressure stimulus to the hindpaw. A brief (1 Hz, 20 s) conditioning stimulus to peripheral nerves with sufficient strength to activate C fibres results in a prolonged increase in this flexor reflex. If the conditioning stimulus is applied to a cutaneous nerve, the sural, the facilitation lasts 5 (5.0 +/- 1.6, n = 28) min. However, if the same conditioning stimulus is applied to the muscle nerve to gastrocnemius, the facilitation lasts up to an hour (54.0 +/- 8.3 min, n = 16). (2) Ankle joint urate arthritis was induced by the injection of 1.25 mg sodium urate crystals into one ankle joint. Two hours after the injection, conditioning of the flexor reflex by brief stimulation of the sural or gastrocnemius nerves produced the same effect as in control animals. However, 24 h after the injection, sural nerve conditioning produced the same effect on the flexor reflex as in controls but gastrocnemius nerve conditioning produced only 8 min (8.3 +/- 1.6, n = 8) of facilitation instead of the expected 54 min. (3) The decreased ability of the muscle nerve to produce prolonged facilitation was not dependent on a saturation of the flexor reflex since the reflex could still be enhanced briefly by the conditioning stimuli. It was also not dependent on a continuous input from the arthritic ankle since the decrease is still apparent when the ankle has been denervated 24 h after the beginning of the arthritis.(ABSTRACT TRUNCATED AT 250 WORDS)