Ptychodiscus brevis toxin enhances the frequency-dependent depression of the monosynaptic reflex in neonatal rat spinal cord in vitro

Abstract

The involvement of frequency-dependent depression (FDD) of synaptic transmission for the depressant action of the Ptychodiscus brevis toxin (PbTx) was investigated in neonatal rat spinal cord in vitro. The stimulation of a dorsal root by train of pulses (five stimuli) at different frequencies evoked potentials in the ventral root (monosynaptic reflex, MSR). Amplitude of the fifth response as percent of first response at 0.1, 0.2, 0.5, 1.0 and 2.0 Hz were 90, 80, 75, 70 and 50%, respectively. In Mg 2+-free medium, PbTx depressed the MSR and also enhanced the FDD in a concentration-dependent manner. Further, the PbTx-induced depression can well be correlated with the enhancement of FDD ( r=0.98). In the presence of Mg 2+ (1.3 mM), the FDD was greater than that in the absence of Mg 2+. But in the presence of Mg 2+ PbTx did not alter FDD, eventhough there was 25% depression at 28 μM (significantly lesser than in Mg 2+-free medium). The results indicate that the Mg 2+-sensitive component of PbTx-induced depression of MSR is mediated via the neuronal systems involving FDD.