Localization of oral-motor rhythmogenic circuits in the isolated rat brainstem preparation

Using an in vitro isolated brainstem preparation from neonatal rat (0-2 day s), the minimal circuitry for production of rhythmical oral-motor activity was determined. In the presence of the excitatory amino acid agonist, N-met hyl-D,L-aspartate (NMA), and the GABA(A) antagonist, bicuculline (BIC), rhy thmical oral-motor activity was recorded from the motor branch of the trige minal nerve. In preparations where the brainstem was isolated in continuity between the rostral inferior colliculus and the obex, oral-motor activity was not observed. However, when the brainstem was serially transected in th e coronal plane starting at the obex and proceeding rostrally, rhythmogenic activity emerged and became more stable until the level of the rostral fac ial nucleus (facial colliculus, FC) was approached. Transactions more rostr al than the FC produced rhythms that progressively deteriorated until the t rigeminal motor nucleus (MoV) was reached, at which point all activities ce ased. Surgical isolation of an ipsilateral quadrant of the brainstem encomp assing the tissue between the FC and inferior colliculus, rostro-caudally, and the midline to lateral brainstem, medio-laterally, exhibited oral-motor activity as well. The remaining contralateral side of brainstem was devoid of rhythmical trigeminal activity. However, further coronal transection of the remaining brainstem at the level of the FC induced rhythmical oral-mot or activity in the trigeminal nerve. The data suggest the existence of bila terally coordinated rhythmogenic circuits in each half of brainstem between the rostral trigeminal nucleus and the rostral facial nucleus, which are t onically inhibited by brainstem circuits caudal to the facial nucleus. (C) 1999 Elsevier Science B.V. All rights reserved.