Effect of Barbiturates on 'quantal' Synaptic Transmission in Spinal Motoneurones

1. Monosynaptic excitatory post-synaptic potentials (EPSPs) produced in triceps surae motoneurones of the cat by stimulation of single afferent fibres in the muscle nerve were recorded with intracellular electrodes before and after the administration of thiopentone or pentobarbitone.2. The average amplitude of the ;unit' EPSP was 0.11-0.21 mV and remained unchanged after the administration of the barbiturates (10 mg/kg, I.V.).3. Mean quantum content (m) ranged from 1.9 to more than 5 before drug administration. The m was reduced by thiopentone (10 mg/kg, I.V.) and pentobarbitone (10 mg/kg, I.V.) by 23.1 and 24.7% respectively.4. The barbiturates, in the doses employed, produced no alterations in the input resistance of the motoneurone membrane or in the strength-duration relation obtained by passing depolarizing current pulses through the micro-electrode.5. It is concluded that the action of thiopentone and pentobarbitone, in the doses used, is confined to the presynaptic nerve terminals and results in a reduction in the amount of transmitter released by afferent impulses.