Effects of a distant noxious stimulation on A and C fibre-evoked flexion reflexes and neuronal activity in the dorsal horn of the rat.

In the halothane-anaesthetized rat, the responses of 49 neurons in the lumbo-sacral cord and the reflex discharge in the common peroneal nerve following electrical stimulation of the sural nerve were recorded in order to study possible relations between neuronal events and reflex nerve discharges. A distant noxious stimulus (to activate Diffuse Noxious Inhibitory Controls (DNIC) of Le Bars et al.) was used as a conditioning stimulus. Only the responses of neurons receiving an input from both A and C fibres were studied. The neurons were classified as class 1 (low threshold mechanoreceptive input only, n = 2), class 2 (nonnoxious and noxious inputs, n = 34) or class 3 (responding to noxious stimuli only, n = 13). During conditioning stimulation the C fibre evoked discharge was inhibited in 32 out of 34 class 2 neurons. The A fibre-evoked discharge was simultaneously inhibited in 29 of these neurons. The main effect of the distant noxious stimulation on the C fibre evoked neuronal discharge was to decrease the discharge by a constant number of spikes, independent of the level of evoked activity. Only one class 3 neuron was inhibited during conditioning stimulation and none of the class 1 cells were influenced by DNIC. During conditioning stimulation the late and prolonged C fibre evoked reflex nerve discharge (latency 160-200 ms, duration up to several hundred ms) was strongly depressed. Concomitantly, a short-lasting reflex nerve discharge appeared over the interval 115-160 ms. This released reflex nerve discharge (RR) had a constant latency. There was no simultaneous change of the A beta evoked reflex nerve discharge. After the end of the distant noxious stimulation the late C fibre evoked reflex nerve discharge (latency 160-200 ms) recovered. Concomitantly, the RR disappeared. The possibility that the class 2 neurons and the class 3 neurons are intercalated in different reflex pathways is discussed.