Abstract
The susceptibility of synaptic transmission to the stimulation frequency is a characteristic feature of immature animals, and has been attributed to the incompleteness of transmitter release mechanisms. In an isolated spinal cord preparation of newborn rat, monosynaptic reflexes (MSRs) evoked by dorsal root stimulation, were mediated by both NMDA and non-NMDA glutamate receptors. In normal conditions, MSRs were constant in amplitude at 1/15sec, and were completely eliminated by CNQX, which suggested normal MSRs were apparently dependent on non-NMDA glutamate receptor. When stimulus rate was increased to 1/sec, MSR amplitudes were greatly reduced initially, and recovered later. This recovery of MSR was eliminated by APV. In the presence of APV, (non-NMDA component of) MSRs were depressed and not recovered at 1/sec. This non-NMDA component of MSRs appeared in constant amplitudes in the presence of cyclothiazide at 1/sec. In the presence of CNQX and 0-Mg, (NMDA component of) MSRs were depressed initially and recovered at 1/sec. From these results, non-NMDA glutamate receptor mediated almost all of MSRs at 1/15sec in normal conditions, but not at 1/sec due to desensitization. In contrast, NMDA glutamate receptor does not mediate normal MSRs at 1/15sec, but activated at 1/sec in place of non-NMDA receptor. In conclusion, incompleteness of transmitter release mechanisms could not be solely attributed to the immature feature of synaptic transmission, that is, the susceptibility to the stimulation frequency. [J Physiol Sci. 2007;57 Suppl:S155]
