Influences of entopeduncular nucleus stimulation upon electromyogram activity of masticatory muscles

Influences of stimulation of the entopeduncular nucleus (Ep) upon electromyogram (EMG) activity of masticatory muscles were examined. In the rat lightly anesthetized with halothane, high frequency (HF) microstimulation (trains of 20, 333-Hz cathodal pulses at 30–60 μA) and GABA microinjection (0.2–0.6 μl of 10 mM GABA dissolved in physiological saline) were performed in the Ep by using a three-barreled microelectrode. EMG activity was recorded from the anterior digastrics and the anterior superficial masseter muscles by using two fine enamel-insulated copper wires. The EMG activity was also evoked by the GABA microinjection. The effect of the GABA microinjection was negated by the microinjection of bicuculline prior to the GABA microinjection. The EMG activity was classified into the tonic spike-type, burst-type, or mixed type on the basis of the waveform. In each rat, the location of the microelectrode tip was estimated by observing a series of serial frontal sections through the whole rostrocaudal extent of the Ep. The present data suggested that Ep neurons involved in elicitation of tonic spike-type activity in the jaw muscles might be located mainly in the rostral third of the Ep, and that Ep neurons implicated in provocation of burst-type activity in jaw muscles might be located in the caudal third of the Ep. Possible neuronal pathways from the Ep to motoneurons innervating the masticatory muscles were discussed. The present data shed new light on the control mechanisms of the basal ganglia upon jaw movements.