Involvement of N-methyl- D-aspartate receptors for the Ptychodiscus brevis toxin-induced depression of monosynaptic and polysynaptic reflexes in neonatal rat spinal cord in vitro

Abstract

The effects of Ptychodiscus brevis toxin (PbTx) on the Ia-α motoneuron synaptic transmission in neonatal rat spinal cord in vitro was examined. The stimulation of a dorsal root evoked monosynaptic (MSR) and polysynaptic reflex (PSR) potentials in the segmental ventral root in Mg 2+-free medium. Superfusion with PbTx (2.8–84 μM) depressed the MSR and the PSR in a concentration-dependent manner. At 2.8 μM of PbTx, the depression of MSR and PSR was 24±8.3% and 37±9.7%, respectively. The maximal depression was seen at 84 μM of the toxin (78% for MSR and 96% for PSR). The concentration of toxin required to produce 50% depression was 28.3±6.4 μM for MSR and 5.5±1.1 μM for PSR. The PbTx (28 μM) did not alter the magnitude of the dorsal root or the ventral root potentials. Addition of MgSO 4 (1.3 mM) or DL-2-amino-5-phosphonovaleric acid (APV; 10 μM) to the physiological solution abolished the PSR totally and decreased the MSR by about 30%. In both the conditions, the PbTx-induced depression of the MSR was attenuated significantly. The PbTx-induced depression was blocked completely in the presence of APV+6-cyano-7-nitroquinoxaline-2,3-dione (0.1 μM). NMDA (1 μM) by itself did not alter the magnitude of MSR or PSR but enhanced the PbTx-induced depression (28 μM) of PSR significantly. 7-Chlorokynurenic acid (3 μM; glycine B antagonist) did not block the PbTx-induced depression of MSR. D-serine (glycine B agonist) did not reverse the PbTx-induced depression of reflexes although it reversed the 7-chlorokynurenic acid-induced depression of PSR. The results indicate that the PbTx depressed the spinal reflexes without altering the magnitude of dorsal root or ventral root activity. The depression of the PSR involved NMDA receptors while that of the MSR involved NMDA and non-NMDA receptors. The PbTx actions did not involve the glycine B site of the NMDA receptor.