NMDA-induced Hypoglossal Motoneuronal Activities in Slice Preparation from Newborn Rats

OBJECTIVE: Tongue muscles innervated by the hypoglossal nerve play an important role to ensure airway patency and nutrition intake. Although both suckling in neonates and mastication in adults involve rhythmic tongue movements, early development of the brain includes dramatical changes in morphology and function. Since these oscillations may differ in location where they are organized, and mechanisms which they are generated. Several lines of evidences indicate that masticatory rhythm is generated in the pons and medulla, meanwhile NMDA-induced sucking like activities are induced in the medulla itself. We investigated the NMDA-induced XII m activities using an in vitro preparation.

METHOD: NMDA-induced activities in hypoglossal motoneurons (XII m) were examined using whole cell patch-clamp recording in coronal brainstem slices obtained from neonatal rats ( P0-P7). We observed XII m activities induced by bath-application of NMDA with or without TTX. RESULTS: In current-clamp mode, NMDA application produced membrane depolarization followed by continuous firing. In a few XII m (3/33), rhythmic membrane potential changes superimposed of burst activity were observed after continuous firings. TTX application did not block membrane depolarization but firing. In voltage-clamp mode, NMDA application induced rhythmic membrane current changes at -65 mV of holding potential. The rhythmic current changes were enhanced by additional application of bicuculline (Bic) and strychnine (Stry). Negative slope conductance of N-shaped I-V relation obtained under NMDA application enhanced in simultaneous application of Bic and Stry. CONCLUSION: These data suggest that XII m are enables to evoke rhythmic activities during NMDA receptor activation, and that activated NMDA receptors exist in more distal dendrites than inhibitory amino acid receptor reside. During early development, motoneurons would have supplementary roles of the rhythm formation using respiration, sucking or swallowing, which roles may escape from the front stage after the maturation.