Two Distinct Oscillatory States Determined by the NMDA Receptor in Rat Inferior Olive

1. The effects of N-methyl-D-aspartate (NMDA) receptor activation and blockade on subthreshold membrane potential oscillations of inferior olivary neurones were studied in brainstem slices from 12- to 21-day-old rats. 2. Dizocilpine (MK-801), a non-competitive NMDA antagonist, at 1-45 microM abolished spontaneous subthreshold oscillations, without affecting membrane potential, input resistance, or the low-threshold calcium current, I(T). Ketamine (100 microM), a non-competitive NMDA antagonist, and L-689,560 (20 microM), an antagonist at the glycine site of the NMDA receptor, also abolished the oscillations, while the competitive non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 20-50 microM) had no effect. 3. NMDA (100 microM) induced 4.1 Hz subthreshold oscillations and reversibly depolarized olivary neurones by 13.7 mV. In contrast, 10 microM alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and 20 microM kainic acid depolarized the membrane equivalently but did not induce oscillations. 4. Both NMDA-induced and spontaneous subthreshold oscillations were unaffected by 1 microM tetrodotoxin and were prevented by substituting extracellular calcium with cobalt. 5. Removing magnesium from the perfusate did not affect spontaneous subthreshold oscillations but did prevent NMDA-induced oscillations. 6. NMDA-induced oscillations were resistant to 50 microM mibefradil, an I(T) blocker, in contrast to spontaneous oscillations. Both oscillations were inhibited by 20 microM nifedipine, an L-type calcium channel antagonist, and 200 nM omega-agatoxin IVA, a P-type calcium channel blocker. Bay K 8644 (10 microM), an L-type Ca(2+) agonist, significantly enhanced the amplitude of both spontaneous and NMDA-induced oscillations. 7. The data indicate that NMDA receptor activation induces olivary neurones to manifest high amplitude membrane potential oscillations in part mediated by L- and P- but not T-type calcium currents. Moreover, the data demonstrate that NMDA receptor currents are necessary for generation of spontaneous subthreshold oscillations in the inferior olive.