Patch-Clamp Analysis on the Mechanism for Inhibitory Synaptic Transmission in Rat Spinal Motoneurone.

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Tight-seal whole-cell recordings of Inhibitory Postsynaptic Currents (IPSCs) were made from neonatal rat spinal motoneurones. The IPSCs were evoked by stimulating a single internuncial neurone extracellularly. The latency measured from the stimulus artifact to the onset of IPSCs was stable when stimulated at various frequencies (0.2-50 Hz) with the mean latency of 0.6-0.8 ms at 37 C, suggesting the monosynaptic nature of the IPSCs. When the external Ca^<2+> concentration ([Ca]_o) was reduced, the number of failures of IPSCs increased, and the mean amplitude of IPSCs decreased. The relation between [Ca]_o (0.35-1.4mM) and the mean amplitude of IPSCs was linear with a mean slope of 3.1 on double logarithmic co-ordinates. The amplitude of IPSCs decreased with a decrease in [Ca]_o. However, below 0.7 mM [Ca]_o, the amplitude of observed IPSCs (excluding failures) reached a steady 'minimum' level. The meah conductance of the 'minimum' IPSCs measured in 0.5 mM [Ca]_o was 650 pS, which was comparable to the conductance of spontaneous miniature IPSCs recorded from motoneurones under tetrodotoxin. It is suggested that the 'minimum' IPSCs observed at low [Ca]_o may represent the quantal IPSCs. The quantal IPSCs had a coefficient of variation of 0.50 on average. The mean amplitude of quantal IPSCs varied among different motoneuromes by a factor of 3. No correlation was observed between the rise time and amplitude of quantal IPSCs, indicating that the dispersion in quantal size is not due to different locations of release sites. The statistical nature of the IPSCs indicated that the amplitude of IPSCs cannot be fitted with a binomial distribution. It is concluded that the quantal size constututing IPSCs may not be uniform at individual inhibitory synapses of neonatal rat spinal cord.