Patch-clamp analysis of direct steroidal modulation of glutamate receptor-channels

Steroid hormones regulate the neuroendocrine and behavioral functions of the brain by using a number of diverse cellular mechanisms. Many steroids exert rapid electrophysiological effects on neurons, involving specific interactions with membrane components, such as neurotransmitter receptors. Previous studies suggest that the steroids, estrogen and pregnenolone sulfate (PS), might directly modulate glutamate receptors. The present experiments utilized patch-clamp recording of glutamate receptor-channels in excised membrane patches to test for direct modulation by these steroids. Characteristic single-channel activity from N-methyl-D-aspartate (NMDA) receptors could be elicited in both inside-out and outside-out patches excised from acutely dissociated hippocampal neurons. PS, but not 17 beta-estradiol, increased the open probability of NMDA channel activity in inside-out and outside-out patches. The PS-induced increase in open probability could be attributed to an increase in both frequency of opening and mean open time of the NMDA receptor, though the effect on frequency of opening was more prominent. The non-NMDA agonist, kainate, induced continuous shifts and increased noise in the baseline current of outside-out patches, but rarely activated clearly resolvable single-channel openings. 17 beta-estradiol and PS had no apparent effect on the kainate-induced currents. These findings suggest that some steroids can directly modulate glutamate receptors, but other steroids may utilize indirect mechanisms for regulating glutamatergic synaptic transmission.