Figure 5. The insertion of longer stretches of ρ subunit results in a corresponding loss in the ability of pentobarbital to potentiate GABA-evoked responses.

The upper panel shows the ability of pentobarbital (PB) to potentiate responses to 1 μM GABA. Pentobarbital has two effects. At lower pentobarbital concentrations the peak response to GABA is increased. However, at higher pentobarbital concentrations the peak may be reduced, and a large response is seen when GABA plus pentobarbital are rapidly removed (the ‘tail’). The tail results from rapid recovery from block of response by pentobarbital, so that the active and unblocked receptors cause a large conductance increase. Accordingly, the response is estimated from the larger of the peak current or the tail current. Receptors composed of α1β3 subunits are potentiated at low concentrations of pentobarbital (10 μM), while other constructs require higher concentrations. All constructs, however, are both potentiated and blocked by higher pentobarbital concentrations, as shown by the tail currents. Even the responses from homomultimeric ρ1 receptors are potentiated and blocked, although only at millimolar concentrations of pentobarbital. The lower panel shows concentration-effect plots for the relative currents elicited by co-applications of pentobarbital with 1 μM GABA, compared with the response to GABA alone. Data are means ±s.e.m. of observations obtained by recording from 5, 11, 3, 4 and 9 cells transfected with α1β3, α1c7, α1c1, α1c2 and ρ1, respectively. Note that in α1β3, α1c7 and α1c1, pentobarbital can gate GABA_A channels directly (see Fig. 6). Therefore the current increase observed with co-application of GABA and pentobarbital is the result of both gating and potentiation by pentobarbital. However, gating by pentobarbital is minimal at a pentobarbital concentration of 0.1 mM. At this concentration of pentobarbital, receptors containing α1and β3 subunits show a higher potentiation than the others tested. α1c2 receptors are very similar to homomultimeric ρ1 receptors, showing both a reduced amount of potentiation under these conditions, and an apparent increase in the EC₅₀ for potentiation.