The minimal inhibitory synaptic currents evoked in neonatal rat motoneurones.

1. Tight-seal whole-cell recordings were made from lumbar motoneurones visually identified in thin slices of neonatal rat spinal cord. The inhibitory postsynaptic currents (IPSCs) were evoked by extracellular stimulation of a neighbouring internuncial neurone in the presence of glutamate receptor antagonists. 2. Glycinergic IPSCs were recorded in the presence of bicuculline. The IPSCs appeared in an all-or-none manner as the graded stimulus intensity exceeded a certain threshold. Their latencies showed a unimodal distribution with a mean of 0.81 ms at 37 degrees C. Thus, the observed IPSCs are suggested to be monosynaptically evoked unitary IPSCs. The mean conductance of unitary IPSCs was 2.9 +/- 1.2 nS (+/- S.D.). 3. When the external Ca2+ concentration ([Ca2+]o) was reduced, the number of failures in response to stimulation increased, thereby reducing the mean amplitude of IPSCs. The mean amplitude of IPSCs was linearly related to the [Ca2+]o (0.35-1.4 mM) with a mean slope of 3.1 +/- 0.67 on double logarithmic co-ordinates. 4. The amplitude of individual IPSCs decreased with decrease in [Ca2+]o. However, below 0.7 mM [Ca2+]o, the mean amplitude of IPSCs (excluding failures) reached a steady minimum level. The mean conductance of these IPSCs measured in 0.5 mM [Ca2+]o was 657 +/- 281 pS. 5. The minimal IPSCs had a coefficient of variation of 0.50 +/- 0.13. No clear correlation was observed between the rise time and the amplitude of minimal IPSCs evoked in individual motoneurones, indicating that the amplitude variability is not due to the different synaptic locations. 6. Spontaneous miniature IPSCs were recorded from motoneurones in the presence of tetrodotoxin. The miniature IPSCs had a mean conductance of 739 +/- 278 pS, being comparable to the minimal evoked IPSCs. 7. Under various internal and external Cl- concentration, the reversal potential of the IPSCs (EIPSC) approximately coincided with the Cl- equilibrium potential. A 730-fold change in the potassium concentration gradient across the membrane did not affect the EIPSC. The permeability ratio of K+ to Cl- (Pk/PCl) was less than 0.05. 8. It is concluded that the IPSCs are carried almost exclusively by Cl- and that the minimal evoked IPSCs represent the quantal response of the transmitter.