RHYTHMICAL ORAL-MOTOR ACTIVITY RECORDED IN AN IN-VITRO BRAIN-STEM PREPARATION

The present study employed the neonatal rat isolated brainstem prepara tion to determine whether oral-motor rhythmical activity, a substrate for the complex behaviors of suckling and chewing, could be elicited i n vitro by bath application of excitatory amino acids (EAAs). Bath app lication of EAA agonists (kainate [KA], -]-a-amino-3-hydroxy-5-methyli soxazole-4-propionic acid [AMPA], N-methyl-D L-aspartate [NMA]), in co njunction with the gamma-aminobutyric acid antagonist bicuculline, eit her failed to induce rhythmic activity (n = 17 preparations) or induce d a low-amplitude, low-frequency burst discharge (<1 Hz, n = 10 prepar ations) from the motor branches of the trigeminal nerves when the brai nstem was contiguous from the spinomedullary junction to the superior colliculus. Burst activity was in most cases bilaterally synchronous. However, when a discrete coronal transection was made at the level of the facial colliculus, between the trigeminal and facial motor nuclei, the rhythmic bursts produced by the resultant 3- to 5-mm block of tis sue following bath application of EAA agonists increased in amplitude and frequency (4-8 Hz, n = 35). Application of 6-cyano-7-nitroquinoxal ine-2,3-dione (CNQX), a non-N-methyl-D-aspartate (non-NMDA) receptor a ntagonist, blocked the rhythm induced by non-NMDA receptor agonist (n = 4) but was less effective in suppressing NMA-induced rhythmicity. In contrast, D,L-2-amino-5-phosphonovaleric acid (APV) blocked both NMA- induced (n = 5) and, in most cases, KA-induced(n = 5) rhythmicity, sug gesting an essential role for NMA receptors in production of EAA-induc ed rhythmical oral-motor activity in the neonatal rat. The present dat a demonstrate that a narrow, bilaterally distributed region of brainst em surrounding the trigeminal motor nucleus contains sufficient neuron al circuitry for the production of oral-motor rhythmogenesis.