Glutamate Receptor Mechanisms Underlying Synaptic Plasticity Of Spinal Cord Motoneurons In Vitro

To study action of glutamate receptors (GluRs) on long-term potentiation (LTP) of excitatory postsynaptic potentials evoked by ipsilateral pericentral canal (iPCC), iPCC-EPSPs, in spinal cord motonuerons (MNs) and its underlying mechanisms, the intracellular recordings were carried out in the ventral horn MNs of spinal cord slices isolated from neonatal rats (7 to 14 days of age). APV (NMDA receptor antagonists) and DNQX (non-NMDA receptor antagonists) were used to analyze the maintenance mechanisms underlying LTP of iPCC-EPSP (iPCC-LTP), and bicuculline and strychnine (inhibitory amino acid receptor antagonists) were also given to observe their effects on the induction of iPCC-LTP. Electrophysiological parameters and the apparent receptor kinetics parameters were analyzed. The results are as follows:1. Apparent receptor kinetic parameters analyses in 9 MNs of iPCC-EPSPs showed that Ki value was increased with the increase of the stimulus intensity at 1T-1.2T and 1.3T-1.5T with a downward deflection between 1.2T-1.3T; K2 value was negatively correlated with the stimulus intensity (r=-0.8564, P<0.05); KT value was negatively correlated with the stimulus intensity at 1T-1.2T(r*=-0.9999, P*<0.01) and 1.3T-1.5T with a upward deflection between 1.2T-1.3T.2. In 7 MNs with iPCC-EPSPs which were applied the tetanic stimulation (100 Hz, 50 pulses/train, duration 0.4-1.0ms,6 trains, main interval 10 seconds,10-100 V),the amplitude of iPCC-EPSPs was enlarged to more than 120% of the baseline values and the enlargement lasted more than 30 min in 3 MNs (MN1-3), which could be referred to as LTP (iPCC-LTP).3. Among the 3 MNs (MN1-3) with iPCC-LTP, the amplitude of iPCC-EPSP MN3 was enlarged significantly after 30 min of tetanic stimulation. During iPCC-LTP, the area under curve and the maximum left slope were all increased to or above 120% of the baseline; the latency were all shortened; the duration were extended in MNs(MN2, MN3), another was decreased; the maximum right slope was reduced to 100% of the baseline after transient enlargement in two MNs(MN1, MN2), while another one was unchanged.4. Apparent receptor kinetics analyses of iPCC-LTP in 3 MNs showed that K1 was persistently increased after brief decrease in 10 min after tetanic stimulation and could reach 90% of the baseline in MN1, however, K1 was still below the level before tetanic stimulation, the other two MNs were unchanged; K2 and KT were decreased to 200% of the baseline after transient increase in 10 min after tetanic stimulation in MN1, while in MN2 they were unchanged, and in MN3 they were all decreased after brief increase.5. During iPCC-LTP elicited in one MN, APV(30 μ mol/L) was perfused for 15 min at least after induction of LTP lasting 30 min, the amplitude of iPCC-EPSP was increased at first then decreased, the area under curve and the duration were all reduced obviously, the latency, the maximum left slope and the maximum right slope were unchanged; after APV(50 μ mol/L) was perfused for 15 min, the changes of iPCC-EPSP parameters is not obvious.6. Bicuculline (GABA receptor antagonists,30 μ mol/L) and strychnine(glycine receptor antagonists,30 μ mol/L) were perfused in an MN to abolish an iPCC-IPSP component and unmask the iPCC-EPSP component. During superfusion of bicuculline and strychnine and after the induction of LTP lasting one hour, APV(30 μ mol/L) was perfused for 15 min, the amplitude, area under curve, duration and maximum right slope of iPCC-EPSP component were all decreased, the maximum left slope was reduced slightly, and the latency was unchanged; after DNQX(1 μ mol/L) was perfused for 5 min, the changes of iPCC-EPSP parameters is not obvious.The apparent receptor kinetics analyses of iPCC-EPSPs suggest that there are perhaps more receptors other than GluRs activated in the postsynaptic membrane and the iPCC-EPSP may be mediated by more complex transmitter-receptor mechanism with the increase of stimulus intensity. The preliminary results also indicate that the iPCC-LTP may involve more complex transmitter-receptor mechanism, GABA and glycine receptors may affect the induction of iPCC-LTP, but the GluRs may take part in the maintenance of iPCC-LTP.