Abstracts

GABAA receptors are the principal mediators of synaptic inhibition, and defects in GABA neurotransmission may play a critical role in the etiology of epilepsy. Many anticonvulsants used to treat epilepsy act by enhancing GABA neurotransmission, but often have undesirable side effects. A goal of modern pharmacology is to develop drugs that ameliorate seizures with minimal side effects. In order to further this goal, a better characterization of GABA receptors and how pharmacological agents act on these receptors is needed. We propose to advance this goal by elucidating the function of the exp-1 gene in the nematode Caenorhabditis elegans. Our previous data suggested that exp-1 encodes a subunit of an excitatory GABA receptor expressed in the enteric muscles of C. elegans. We mapped exp-1 to a small interval on Chromosome II and identified a gene related to GABA receptors in this interval from the C. elegans genomic sequence. Transgenes bearing this single open reading frame (ORF) completely rescued exp-1(sa6) mutants. In addition, we identified point mutations in this ORF for three alleles, which demonstrates that exp-1 encodes this GABA related protein. We isolated exp-1 cDNAs which show that EXP-1 has a divergent transmembrane 2 (TM2) from all known GABA receptor subunits. TM2 has been shown to confer ion selectivity to the ligand-gated ion channel superfamily. Consistent with our hypothesis that EXP-1 is an excitatory GABA receptor subunit, TM2 has numerous negative charges that line the pore of the receptor, suggesting this subunit might form a receptor that passes cations rather than the anion chloride. We will inject exp-1 cRNA into Xenopus laevis oocytes and determine GABA gating and ion permeability of the putative channel. Identification and characterization of the exp-1 gene may result in the identification of homologous proteins in higher vertebrates.