This study was carried out to clarify the role of the periaqueductal gray matter (PAG) on the modulation of responses to noxious stimuli applied to buccal mucosa of rat. The pain modulation of PAG was related to noradrenalin, serotonin and enkephalin which were neurotransmitters of descending neurons from PAG and these neurotransmitters were influenced by reserpine treatment or adrenalectomy.
This experiment was done on normal, reserpine treated and adrenalectomized rat. The animal +;was anesthetized with urethane chloralose and anterior belly of digastric muscle was exposed. A pair of wire electordes was inserted into the muscle to record the EMG evoked by stimulation of buccal mucosa. The head of animal was fixed to stereotaxic apparatus and a bipolar concentric electrode was inserted into PAG. PAG was stimulated with constant rectangular pulse with various intensity(200 Hz, 200 usec duration) for 200 msec. The digastric EMG was elicited by stimulation of buccal mucosa with 50 msec delay after PAG conditioning stimulation. When the significant depression of the EMG of digastric muscle evoked by noxious stimuli was observed, the interval between PAGconditioning stimulation and stimulation of buccal mucosa was changed to 10, 25, ;50, 100 and 200 msec. The effect of PAGconditioning stimulation was evaluated by fractional change of digastric EMG compared with control digastric EMG.
The suppressive effect of PAG conditioning stimulation was proprotional to intensity of ;stimulation and its effect in adrenalectomized and reserpine treated rat was slightly more potent ~Athan normal rat. The suppression of digastric EMG was most at significant conditiontest interval of 25 and 50 msec, but markedly decreased at 10, 100 and 200 msec interval.
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