THE INFLUENCE OF TEMPORAL SUMMATION ON A C-FIBER REFLEX IN THE RAT - EFFECTS OF LESIONS IN THE ROSTRAL VENTROMEDIAL MEDULLA (RVM)

In intact rats, an inhibitory mechanism counteracts the increase in ex citability of a flexor reflex seen in spinal animals following high-in tensity, repetitive stimulation of C-fibres. We tested the hypothesis that the rostral ventromedial medulla (RVM) is involved in these proce sses. Electromyographic responses elicited by electrical stimulation o f the sural nerve, were recorded from the ipsilateral biceps femoris i n halothane-anaesthetised, sham-operated or RVM-lesioned rats. There w ere no significant differences between the C-fibre reflexes in the two groups in terms of their thresholds, latencies, durations or mean rec ruitment curves. The excitability of the C-fibre reflex was tested fol lowing 20 s of high-intensity homotopic electrical conditioning stimul i at 1 Hz. During the conditioning period, the EMG responses first inc reased in both groups (the wind-up phenomenon), but then decreased in the sham-operated rats and plateaued in the RVM-lesioned rats. These e ffects were followed by inhibitions that were very much smaller in the RVM-lesioned rats, both in terms of their magnitudes and their durati ons. It is concluded that the RVM is involved in inhibitory feedback m echanisms elicited by temporal summation of C-fibre afferents that bot h counteract the wind-up phenomenon and trigger long periods of inhibi tion. (C) 1998 Elsevier Science B.V.