Inhibitory Miniature Synaptic Potentials in Rat Motoneurons

In the newborn rat spinal cord, spontaneous potentials were recorded, with KCl electrodes, from motoneurons in the presence of tetrodotoxin (10(-6) g ml-1) to abolish nerve impulses. These potentials occurred at low frequencies (less than 2 Hz), and their mean amplitude was a fraction of 1 mV. An increase of osmolarity with sucrose or an increase of extracellular K+, increased the frequency of miniature synaptic potentials. The amplitude of the spontaneous potentials was increased by intracellular injection of Cl-. Strychnine (2-25 microM) completely abolished the spontaneous potentials. It is suggested that these potentials are produced by the spontaneous release of packages of inhibitory transmitter at synapses on motoneurons.