NMDA-INDUCED BURST DISCHARGE IN GUINEA PIG TRIGEMINAL MOTONEURONS IN VITRO. Kim, Yang In, and Scott H. Chandler. Department of Physiological Science, University of California at Los Angeles, Los Angeles, CA 90024 U. S. A. and Department of Physiology, Korea University College of Medicine, Seoul, Korea 136-705.
APStracts 2:0081N, 1995.
SUMMARY AND CONCLUSIONS
1. The responses of guinea-pig trigeminal motoneurons (TMNs) to N-methyl-D,L- aspartate (NMA) were studied using brainstem slice preparations and whole-cell patch-clamp (n=89) or conventional microelectrode (n=22) recording techniques. The primary goals of this study were to determine if N-methyl-D-aspartate (NMDA) receptor activation would produce spontaneous bursting activity in TMNs and, if so, the underlying mechanisms responsible for the generation of these bursts. 2. Bath-applied NMA (100-300 [mu]M, n=80) in standard perfusion medium elicited depolarization, increase in apparent input resistance (R inp ) and rhythmic burst discharges (1-90 sec duration) from TMNs. These effects were blocked by the NMDA receptor antagonist DL-2-amino-5-phosphonopentanoic acid (AP5, 30 [mu]M, n=6), but not by the non-NMDA receptor antagonist 6-cyano-7- nitroquinoxaline-2,3-dione (CNQX, 5-10 [mu]M, n=10). Furthermore, the burst- inducing effect of NMA was not mimicked by the non-NMDA receptor agonists, kainate (KA, 5-10 [mu]M, n=6) and ( ¯+ )-a-amino-3-hydroxy-5-methylisoxazole- 4-propionic acid (AMPA, 5-10 [mu]M, n=5). 3. In tetrodotoxin (TTX)-treated conditions (n=13), NMA elicited depolarization, an increase in apparent R inp and rhythmic membrane potential oscillations without action potential bursts (i.e. plateau potentials), suggesting that the effects of NMA observed in the TTX-free condition resulted from activation of postsynaptic NMDA receptors. 4. Graded depolarization of neurons (n=20) by intracellular DC current injection generally led to a graded increase in frequency and duration of the NMA- induced bursts and plateau potentials until these rhythmic events eventually became transformed into continuous spike discharge and maintained depolarization, respectively. Removal of Mg 2+ from the perfusion medium (n=11) also turned the bursts and plateau potentials into continuous spike discharge and maintained depolarization, respectively. 5. The effects of NMA on the current-voltage (I-V) curve following a depolarizing ramp voltage-clamp command (15-20 mV/s) were examined (n=40). Under NMA (100-300 [mu]M) conditions, the I-V relationship exhibited a region of negative slope conductance (NSC) between -60 and -35 mV, thus making the I-V relationship "N"-shaped. The NSC was abolished by AP5 (30 [mu]M, n=8), but not by CNQX (5-10 [mu]M, n=6). The I-V relationship in AMPA (3-10 [mu]M, n=5) or KA (3-10 [mu]M, n=5) was almost linear between -80 and -30 mV. In perfusion medium lacking Mg 2+ , the NMA-induced NSC was abolished leaving the I-V relationship linear in the region between -80 and -30 mV (n=6). 6. The role of Ca 2+ in the generation and termination of individual NMA-induced bursts was examined (n=13). Application of 0- or low-Ca 2+ medium either 1) blocked completely the generation of NMA-induced plateau potentials (n=6) , 2) increased the duration of plateau potentials (n=5) or 3) prevented the termination (i.e., repolarization) of plateau potentials (n=2). The I-V curves examined in low-Ca 2+ medium were almost linear in the vast majority of the cases (7 of 8) due to the reduction of NMA-induced inward current in the region of the NSC. 7. The role of Ca 2+ -dependent K + currents (I KCa ) in terminating individual NMA-induced bursts was assessed using the bee venom apamin and the scorpion venom iberiotoxin, which block the SK and BK types of K + channels, respectively. While apamin (200 nM, n=5) completely blocked the afterhyperpolarization following each action potential, it (100 nM, n=2; 200 nM, n=3) failed to prevent the termination of NMA-induced bursts; only a slight to moderate increase of the duration and the underlying depolarization of the bursts were apparent during application of apamin. Iberiotoxin (30 nM) had no obvious effects on either the bursts or the action potential waveform (n=5). 8. The data from this study demonstrate that selective activation of postsynaptic NMDA receptors induces rhythmical burst discharges in TMNs. Additionally, the data suggest that the NMA-induced bursts arise from activation of a Mg 2+ and voltage-dependent NMDA conductance which is manifest as a region of NSC in the NMA I-V relationship. Furthermore, the bursts are dependent upon the presence of Ca 2+ in the ACSF in a subpopulation of TMNs. 9. Therefore, it is concluded that any models of rhythmical oral-motor activity must include the unique properties of the NMDA channel. 

Received 31 October 1994; accepted in final form 16 March 1995.
APS Manuscript Number J686-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 19 April 1995.