Figure 3. Glutamate-gated Chloride (GluCl) Channel Structure, Ivermectin and Glutamate Binding Sites, and Channel Currents.

(A) Diagram of a GluCl channel subunit. It is about 500 amino acids long and begins at the N terminal on the extracellular face of the membrane. The subunits that contain vicinal cysteines are recognized as α-subunits. Each subunit has four α-helical transmembrane regions: M1, M2, M3, and M4. The M2 region lines the pore of the transmembrane ion channel. Between the M3 and M4 regions there is an extended cytoplasmic loop. The C terminal is also found at the extracellular surface of the membrane. (B) Five subunits are arranged like the staves of a barrel around the central pore of the ion channel to form a pentomer. Ivermectin binds to the channel in the transmembrane region of the ion channel in the outer bilayer of the lipid bilayer membrane and can open the channel. (C) The five subunits of the GluCl channel viewed from above. If the five subunits of the ion channel are all the same, then the ion channel receptor is referred to as homomeric. If it is made of nonidentical subunits it is referred to as heteromeric. The orthosteric ligand-binding site, where glutamate binds, is located between two adjacent subunits (green) and in the extracellular region of the ion channel. The ivermectin binding site is deep in the ion channel and is involved in contact with the M2 region near the pore, and it contacts the M3 region as well. (D) The upper trace shows a cell-attached patch recording at −50 mV with 800 μM glutamate in the patch-pipette. The rectangular current steps representing the opening and closing of GluCl channels can be seen. Each current step is 2 pA (10⁻¹² A). The probability of the channels being open (pOpen) is 0.08, that is, each channel was open for 8% of the time. The lower trace shows the effect of adding 1 μM ivermectin to the bath, and followed after a few minutes, allowing time for ivermectin to move into and along the membrane, the number of GluCl opening increased as did the probability of the channel being open. It increased to 0.14, or 14% of the time. The opening of many ion channels together allows more Cl⁻ current to flow through the membranes and to increase the conductance of the membrane.