Excitatory amino acids and synaptic transmission in embryonic rat brainstem motoneurons in organotypic culture

We used brainstem motoneurons recorded in organotypic slice co-cultures mai ntained for more than 18 days in vitro, together with multibarrel ionophore tic applications of glutamate receptor agonists and bath applications of sp ecific blocking agents, to study the responses of rat brainstem motoneurons to glutamate receptor activation, and the contribution of these receptors to synaptic transmission. Differentiated brainstem motoneurons in vitro are depolarized by glutamate, N-methyl-D-aspartate (NMDA) and DL-alpha-amino-3 -hydroxy-5-methyl-4-isoxazol-propionic acid (AMPA) iontophoresis, and expre ss NMDA, AMPA and also specific kainate receptors, as evidenced by (+/-)2-a mino-5-phosphonovaleric acid (APV)- and (-)1 -(4-aminophenyl)-3-methyl-carb amoyl-4-methyl-7,8-methylenedioxy-3,4-dihydro-5H-2,3-benzo-diazepine[GYKI 5 3784 (LY303070)]-resistant depolarizations. Electrical stimulations applied to the dorsal part of the explant trigger excitatory synaptic potentials w ith latencies distributed in three regularly spaced groups. Excitatory post synaptic potentials (EPSPs) in the earliest group have a similar latency an d time course and correspond to monosynaptic activation. EPSPs in later gro ups have more scattered latencies and time courses and correspond to polysy naptic activation. Monosynaptic EPSPs are insensitive to the specific NMDA blocker APV, and are completely and reversibly suppressed by the non-compet itive AMPA receptor antagonist GYKI 53784 (LY303070), Detailed analysis of the spontaneous excitatory synaptic activity shows that APV decreases the f requency of spontaneous EPSPs without modifying their shape or amplitude. W e conclude that excitatory synapses on brainstem motoneurons in vitro are m ainly activated through AMPA receptors (AMPA-Rs), NMDA receptors (NMDA-Rs) are present in the membrane, but are located either at extrasynaptic sites or silent synapses, and are not directly involved in synaptic transmission on motoneurons. On the contrary, NMDA receptors contribute to synaptic tran smission within the premotor interneuronal network.