Functional and topographical properties of field potentials evoked in rat dorsal horn by cutaneous C‐fibre stimulation.

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Abstract
Extracellular field potentials in the lumbosacral dorsal horn evoked by stimulation of cutaneous C fibers in the sural nerve were explored in the halothane-anesthetized rat. C-fiber-evoked field potentials were prominent in lamina II and lamina V of the dorsal horn. These potentials had a latency of 80-130 ms and a duration of > 200 ms. A peak in the C-fiber-evoked field potential, termed the CI potential, with a median latency of 120 ms, range 105-150 ms, was distinguished in lamina II. The time from onset to peak of the CI potential was, on average, 13 ms, when all C fibers were activated. The amplitude of the CI potential in lamina II was directly proportional to the amplitude of the C-fiber-evoked nerve volley; the relation between the C-fiber nerve volley and the C-fiber-evoked field potential in lamina V was nonlinear. A selective block of A fibers did not influence the amplitude of these field potentials. Following stimulation of C afferent fibers in the medial sural nerve, at frequencies higher than 0.1 Hz, the CI potential in lamina II, but not the C-fiber-evoked field potential in lamina V, was increased. There was no concomitant change of the A-fiber-evoked field potentials. The magnitude of the potentiation of the CI potential was dependent on the frequency and the number of stimuli. Mean percentage potentiation was 200%, range 150-300%, after 70 stimulations at a frequency of 1.0 Hz. During the stimulation the CI potential increased monotonically. The decay of the potentiation of the CI potential was well described by 2 exponentially declining phases. There was a positive correlation between the size of the time constants of the decay and the number of stimuli during conditioning. Following noxious radiant heat (temperature 50-55.degree. C) applied to a restricted part of the skin (area 20-30 mm2) within the receptive field of the medial sural nerve for 10-20 s, the CI potential was potentiated by 50-130%. The duration of this potentiation was 3-15 min. This potentiation was somatotopically organized. There was no effect on the amplitude of the CI potential following innocuous skin stimulation (slowly moving contact, brushing the skin, warmth 40.degree. C). The potentiation of the CI potential generated during electrical stimulation is probably mainly due to activity in nociceptive afferent fibers.