Presynaptic action of the neurosteroid pregnenolone sulfate on inhibitory transmitter release in cultured hippocampal neurons.
The effects of the neurosteroid pregnenolone sulfate (PS) were studied in 3- to 9-week-old hippocampal cultures from neonatal rats. Cells were voltage clamped using CsCl filled electrodes, while action potentials and excitatory glutamatergic currents were abolished by superfusing with a combination of tetrodotoxin, 6-cyano-7-nitroquinoxaline (CNQX) and 2-amino-5-phosphonopentanoic acid (AP-5). Under these conditions spontaneous GABAergic inhibitory postsynaptic currents (sIPSCs) were seen as inward currents at a holding potential of -70 mV. Their amplitude distributions were skewed without clearly detectable peaks. PS at 1-50 microM concentrations decreased the frequency of sIPSCs, with 1 microM being the most effective concentration. The effect appeared after 10-15 min of steroid application and the magnitude of the reduction increased during the early wash period. No recovery of sIPSC frequency was found after 30 min of washing with steroid-free medium. sIPSC amplitudes were not significantly changed at the time the effect of PS on sIPSC frequency was observed. The slow onset of this effect and its duration suggest a novel presynaptic action of the neurosteroid PS on GABAergic inhibition in the mammalian brain.