THE EFFECTS OF NANOLITER EJECTIONS OF LIDOCAINE INTO THE PONTOMEDULLARY RETICULAR-FORMATION ON CORTICALLY INDUCED RHYTHMICAL JAW MOVEMENTS IN THE GUINEA-PIG

In the ketamine/urethane anesthetized guinea pig, electromyographic (EMG) responses of the anterior digastric muscle were studied when loci within the lower brainstem were microejected with lidocaine (2%) during rhythmical jaw movements (RJMs) evoked by repetitive electrical stimulation of the masticatory area of the cortex. The area investigated was between the trigeminal motor nucleus (Mot V) and the rostral pole of the inferior olive. Microejections of lidocaine, contralateral to the cortical stimulus site, into the ventral-medial portion of Mot V where digastric motoneurons are known to be located, resulted in reduction or complete abolishment of the digastric EMG activity ipsilateral to the ejection with no effective change in mean cycle duration (CD) or mean percent normalized integrated amplitude of the contralateral digastric EMG. Microejections of lidocaine, contralateral to the cortical stimulus site, into the ponto-medullary reticular formation in areas that included portions of the caudal nucleus pontis caudalis (PnC), nucleus gigantocellularis (GC), medial nucleus parvocellularis (PCRt), and dorsal paragigantocellularis (dPGC), in most cases produced a bilateral reduction in the mean normalized integrated amplitude and a bilateral increase in the mean cycle duration. In these sites, the bilateral increase in mean cycle duration of digastric EMG bursts was also associated with a significant increase of coefficient of variation in CD. In many cases, microejection of lidocaine completely abolished rhythmical digastric activity, bilaterally. HRP injections into Mot V were performed to determine the locations of trigeminal premotoneurons and their relationship to effective lidocaine sites for rhythmical jaw movement suppression. Retrogradely labeled cells were found mainly in the mesencephalic nucleus of V; trigeminal principal and spinal V sensory nuclei, bilaterally; and within the intermediate and lateral regions of reticular formation, bilaterally. No labeling was found in the medial reticular formation, including the nucleus gigantocellularis and dorsal paragigantocellularis. The results of this study indicate that regions in the lower brainstem, including portions of the nuclei gigantocellularis and dorsal paragigantocellularis, may participate in masticatory timing and pattern formation, lending further support to the previous hypothesis that (1) both GC and dPGC nuclei of the medial reticular formation contain components of the oscillatory network necessary for RJM production; (2) this region does not contain trigeminal premotoneurons, suggesting a segregation between central timing neurons within these regions of brainstem and trigeminal premotoneurons; and (3) ipsilateral cortex activates neurons located contralaterally within the brainstem reticular core which are critical for bilateral digastric EMG activity during cortically induced RJMs. © 1990.