Effect of conditioning stimulation of the limbic system on tooth pulp-driven neuron in cat's somatosensory cortex (SI).

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In order to detemine if the limbic system has any role in pain processing, we investigated the effect of conditioning stimulation at hippocampus, amygdala and the surrpunding cortex on the response of tooth pulp-driven (TPD) neurons in the first somatosensory cortex (SI).Cats (n=26) were anesthetized with N_2O-O_2 (2:1) and 1%-halothane and immobilized by curare. The tooth pulp test stimulus was a single 5-15v rectangular pulse of 0.5msec duration and the conditioning stimuli of limbic system were trains of 33 pulses (50-500 a) having duration of 0.5msec delivered at 330Hz.TPD neurons have been classified according to their discharge patterns and latencies at fast (F-), slow (S-), and fast- accompanied by after-discharges (Fa). The conditioning stimuli of hippocampus or amygdaloid nucleus did not alter the activities of TPD neurons and their spontaneous discharges. On the other hand, the conditioning stimulation of the periamygdaloid area (PAA) that located in the ventral side to the amygdaloid nucleus markedly suppressed the response of the S-type neurons and the after-discharges of the Fa-types, although the conditioning stimulation had almost no effect on: 1) The discharge of F-type TPD neurons, 2) the fast responses of the Fa-type, and 3) the spontaneous discharge of all three TPD types. The inhibitory period was 300-800msec. This inhibition persited even after the destruction of stria terminalis that was a major efferent tract from the amygdaloid nucleus, and conditioning stimulation of the stria terminalis did not mimic the inhibitory effect. In a related study, digastric EMG activity (jaw opening reflex) induced by tooth pulp stimulation was suppressed 60-100% by the PAA conditioing stimulation for a period of 100-300msec in cats anesthetized with Nembutal. These findings indicate that the PAA modulated central mechanisms of nociception at the bulbar level via the pathway other than the stria terminalis, presumably the ventral amygdalofugal fiber.