NMDA Actions on Rat Abducens Motoneurons

Intracellular recordings were made from rat abducens motoneurons in vivo during local extracellular micro-ionophoretic application of N-methyl-d-aspartate (NMDA) and NMDA receptor antagonists. Typical NMDA responses, at a resting potential of -60 mV, consisted of a slow depolarization with an apparent increase in membrane resistance, bursts of action potentials followed by stable repetitive firing. Ionophoretic applications of aminophosphonovalerate (APV), kynurenate or MK801 reduced or blocked the NMDA-induced responses. The NMDA responses were voltage-dependent. NMDA responses induced by short (< 30 s) NMDA application pulses were blocked by hyperpolarizing the neuron. Long duration (> 30 s) NMDA applications induced rhythmic plateau potentials in hyperpolarized abducens motoneurons. The rhythmic depolarizations (15 - 30 mV) were modulated in both frequency and duration by current injection. They were abolished by further hyperpolarization or replaced by stable repetitive firing when hyperpolarization was removed. Our data show that NMDA receptors are present in rat abducens motoneurons and may be involved in the induction of rhythmic activities. The voltage-dependent blockade of somatic NMDA receptor-associated ion channels by cell hyperpolarization may be important for these oscillations. It is suggested that the rhythmic behaviour is due to the activation of dendritic NMDA receptors.