Sensitization and habituation of dorsal horn cells in cats.

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Abstract
Extracellular recordings were obtained from spinal horn cells in acutely spinalized cats anaesthetized with sodium pentobarbitone. The dorsal horn cells responded to ipsilateral tactile stimulation of the central pad of the hind foot. Eleven short latency dorsal horn cells driven by electrical stimulation of the foot pad were studied intensively; these short latency dorsal horn cells all discharged within 1.5 ms of the arrival of an afferent volley at the dorsal root entry zone. Electrode tip sites were histologically verified to lie near the medial border of the dorsal horn in the 7th lumbar segment, in Rexed''s laminae III and IV. Electrical stimulation of the foot pad not only activated the dorsal horn cells studied, but also produced a reflex discharge which was monitored by recording from the ipsilateral first sacral ventral root, which had been sectioned intradurally and mounted on bipolar recording electrodes. Repeated stimulation of the foot pad at moderate intensities and frequencies (e.g., 3 times threshold for the ventral root response at 5 Hz) typically produced a transitory increase in the magnitude of the reflex discharge (sensitization) followed by a marked waning of the reflex magnitude (habituation). Changes in response patterns of the dorsal horn cells were compared to those of the reflex discharges. The short latency dorsal horn cells fell into 2 distinct patterns of response and were characterized by intermediate thresholds to tactile stimulation, comparable to that of the reflex discharge itself; relatively low numbers of responses per stimulus (mean: 1.3/stimulus); and low spontaneous activity rates (once per 10 s or less). The firing patterns of the other class of short latency dorsal horn cells did not parallel the response pattern of the reflex discharge. These 5 dorsal horn cells were characterized by thresholds of tactile stimulation considerably below that of the reflex discharge itself; bursts of responses following each stimulation (mean: 7.1/stimulus); high spontaneous activity rates (mean: 2.4/s). The intermediate threshold short latency dorsal horn cells are apparently capable of mediating the reflex discharge elicited by central pad stimulation; the changes in firing patterns of these dorsal horn cells with repeated stimulation may contribute appreciably to both the sensitization and the habituation of the reflex discharge.