Agonist Trapping by GABAA Receptor Channels.

Area of Science:

• Neuroscience
• Molecular Biology
• Pharmacology

Background:

• GABAergic inhibitory postsynaptic currents (IPSCs) exhibit slow deactivation.
• This slow decay suggests GABA (gamma-aminobutyric acid) may remain bound to GABA(A) receptors beyond the initial synaptic concentration.
• Previous studies hypothesized GABA "trapping" in open or desensitized states, but direct simultaneous assessment of binding and gating was lacking.

Purpose of the Study:

• To investigate the "GABA trapping" hypothesis by developing a functional assay to assess GABA binding site occupancy during channel gating.
• To determine if GABA remains bound to GABA(A) receptors during spontaneous, neurosteroid-activated, and GABA-activated currents.

Main Methods:

• Developed a novel functional assay to assess GABA binding site occupancy.
• Compared deactivation currents of GABA(A) receptors in the presence and absence of bicuculline, a competitive antagonist.
• Utilized bicuculline's allosteric inhibition of open, unbound channels as a probe.

Main Results:

• Bicuculline inhibited spontaneous and neurosteroid-activated currents, indicating unbound receptors.
• Crucially, bicuculline failed to alter the deactivation time course of GABA-activated currents.
• This protection from block implies GABA remained bound to the receptor during GABA-activated channel opening and subsequent states.

Conclusions:

• GABA is "trapped" on GABA(A) receptors during channel opening, explaining the slow deactivation of GABAergic IPSCs.
• All functional states (open, closed, desensitized) from which opening can occur appear to be GABA-liganded.
• This trapping mechanism seems specific to agonists, as diazepam unbound independently of GABA binding and channel activity.