Using rapid agonist applications to transfected HEK-293 cells, we investiga
ted pregnenolone sulfate (PS) effects on deactivation and desensitization o
f recombinant NMDA receptors subtypes. PS prolonged the deactivation of res
ponses produced by brief applications of L-glutamate with all subunit combi
nations tested. The action of PS was larger on NR1a/NR2A than on NR1a/NR2B
channels. PS slowed the rate of macroscopic desensitization of the response
s with all subunit combinations tested. In contrast, PS had little effect o
n current rise time and had much reduced action on responses with L-cysteat
e, a low affinity agonist.
Our results suggest that PS decreases agonist unbinding. However, this acti
on is counteracted by decreased desensitization. Since desensitization prod
uces slow deactivating components, particularly with NR1a/NR2B receptors, t
his underlies the decreased PS effect with these subtypes. Indeed PS action
was mainly observed on the fast component of deactivation. Furthermore, pr
olongation of NR1a/NR2A responses was similar to that of responses from NR1
b/NR2B receptor, a subtype characterized by reduced desensitization.
PS prolongation of evoked NMDA receptor mediated synaptic currents from cor
tical neuronal primary culture(s) was not significantly different from that
of responses with NR1a/NR2B receptors indicating that native receptors in
these neurons comprised at least some heteromeric combinations of these two
subunits. (C) 2001 Elsevier Science Ltd. All rights reserved.