Effects of gamma-aminobutyric acid and (-)-baclofen on calcium and potassium currents in cat dorsal root ganglion neurones in vitro.

Calcium and potassium currents were examined in dialysed, isolated dorsal root ganglion neurones of the cat by use of a single electrode voltage clamp. Two calcium currents were identified, a low threshold inactivating (transient) current and a higher threshold slowly inactivating (sustained) current. Both currents were blocked by 1 mM cadmium and replacement of calcium with manganese, which revealed an underlying nonspecific outward current. The sustained current disappeared with internal dialysis over a period of 20 to 30 min. (-)-Baclofen (100 microM) and gamma-aminobutyric acid (GABA, 100 microM) were found to reduce the peak amplitude of the sustained calcium current, an effect which became more pronounced with increasing concentrations of external magnesium (1-5 mM). In 5 mM external magnesium, 100 microM baclofen reduced the calcium current by 28%. The voltage-activated delayed rectifier appeared to be the most prominent potassium current in these cells. We were unable to find any evidence for a significant contribution from calcium-activated potassium conductance or a transient potassium conductance under our recording conditions. Baclofen and GABA at 100 microM had no consistent effect on the voltage-activated potassium current. Baclofen did not change the resting potassium conductance.