Pharmacological characterization of the excitatory 'Cys-loop' GABA receptor family in Caenorhabditis elegans

Ionotropic GABA receptors are evolutionarily conserved proteins that mediate cellular and network inhibition in both vertebrates and invertebrates. A unique class of excitatory GABA receptors has been identified in several nematode species. Despite well-characterized functions in Caenorhabditis elegans, little is known about the pharmacology of the excitatory GABA receptors EXP-1 and LGC-35. Using a panel of compounds that differentially activate and modulate ionotropic GABA receptors, we investigated the agonist binding site and allosteric modulation of EXP-1 and LGC-35.

We used two-electrode voltage clamp recordings to characterize the pharmacological profile of EXP-1 and LGC-35 receptors expressed in Xenopus laevis oocytes.

The pharmacology of EXP-1 and LGC-35 is different from that of GABA_A and GABA_A -ρ receptors. Both nematode receptors are resistant to the competitive orthosteric antagonist bicuculline and to classical ionotropic receptor pore blockers. The GABA_A -ρ specific antagonist, TPMPA, was the only compound tested that potently inhibited EXP-1 and LGC-35. Neurosteroids have minimal effects on GABA-induced currents, but ethanol selectively potentiates LGC-35.

The pharmacological properties of EXP-1 and LGC-35 more closely resemble the ionotropic GABA_A -ρ family. However, EXP-1 and LGC-35 exhibit a unique profile that differs from vertebrate GABA_A and GABA_A -ρ receptors, insect GABA receptors and nematode GABA receptors. As a pair, EXP-1 and LGC-35 may be utilized to further understand the differential molecular mechanisms of agonist, antagonist and allosteric modulation at ionotropic GABA receptors and may aid in the design of new and more specific anthelmintics that target GABA neurotransmission.