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

Using an in vitro isolated brainstem preparation from neonatal rat (0-2 days), the minimal circuitry for production of rhythmical oral-motor activity was determined. In the presence of the excitatory amino acid agonist, N-methyl-D,L-aspartate (NMA), and the GABAA antagonist, bicuculline (BIC), rhythmical oral-motor activity was recorded from the motor branch of the trigeminal 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 the coronal plane starting at the obex and proceeding rostrally, rhythmogenic activity emerged and became more stable until the level of the rostral facial nucleus (facial colliculus, FC) was approached. Transections more rostral than the FC produced rhythms that progressively deteriorated until the trigeminal motor nucleus (MoV) was reached, at which point all activities ceased. Surgical isolation of an ipsilateral quadrant of the brainstem encompassing 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-motor activity in the trigeminal nerve. The data suggest the existence of bilaterally coordinated rhythmogenic circuits in each half of brainstem between the rostral trigeminal nucleus and the rostral facial nucleus, which are tonically inhibited by brainstem circuits caudal to the facial nucleus.