Chloride-dependent potentials and their effects on firing pattern in guinea-pig cingulate cortical neurons

Intracellular recordings were made from pyramidal neurons in the anterior cingulate cortex layer V of the guinea-pig slice preparation. An oscillation in membrane potential occurred when the membrane was depolarized to a subthreshold level (e.g. -60 mV), and the oscillation triggered repetitive firing. The frequency of the spontaneous firing varied with the membrane potential: frequencies were 2.9 +/- 1.4, 6 +/- 1, 13 +/- 3, and 17 +/- 3 Hz at the membrane potentials of -60, -58, -55, and -40 mV, respectively. Bicuculline (500 nM to 10 microM) increased the firing rate when the membrane potential was kept around threshold level by DC current injection. 2-Hydroxysaclofen and low Cl- (43 mM) also increased the firing rate, but 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonate (SITS, 250 microM) did not change the firing rate. Strychnine (1 to 3 microM) had no effect on the spontaneous firing. These results suggest that the firing frequency of cingulate pyramidal neurons is modulated by GABA tonically released by interneurons.