If the modulation and direct activation by DZP were the same,
as opposed to different mechanisms acting via different
binding sites, then the EC50’s for activation and modulation
should be similar. To test this possibility, we compared the
sensitivities of activation and modulation by DZP. The top
row of current traces in Figure 4a are direct activation by
various concentrations of DZP in wild-type abg receptors
(ultrahigh expression). The bottom row of traces in Figure 4a
are currents from DZP-mediated activation of abY205Sg. This
mutation is in the GABA binding site and results in a reduced
sensitivity such that no GABA-mediated current can be
detected at GABA concentrations as high as 20 mM (Amin &
Weiss, 1993). In this case, however, we can still detect DZPmediated
currents with high expression. The resulting dose–
response relationships for abg (filled circles) and abY205Sg
(open circles) are plotted in Figure 4b. The EC50’s for DZPmediated
activation were 72.072.0 and 115.076.2 nM for abg
and abY205Sg, respectively (Table 1). The shaded line in
Figure 4b is the dose–response relationship for wild-type abg
in the presence of 3 mM GABA and increasing concentrations
of DZP taken from a previous study (Amin et al., 1997). In this
case, the EC50 for modulation was 64.673.7 nM, very close to
that for direct activation in abg and abY205Sg. These data
support the hypothesis that DZP-mediated activation and
modulation are through the same DZP binding site.