Characterization of a GABAA receptor ß subunit in the abalone Haliotis asinina that is upregulated during larval development

In the tropical abalone Haliotis asinina, the neurotransmitter γ-aminobutyric acid (GABA) is a potent inducer of larval settlement, a process beginning with the onset of a behavioral search for a suitable substratum and ending with metamorphosis. In the natural environment, larvae can encounter GABA or GABA-like molecules through association with conspecific foot mucus and crustose coralline algae. To understand the role of GABA in the molecular process leading to settlement required identification and analysis of GABA's cognate receptor. We now have isolated the first abalone full-length GABAA receptor (Has-GABAAR) β subunit gene, which encodes a protein of 485 amino acids, from juvenile H. asinina neural tissue. Similar to other metazoan GABAARs, the abalone GABAAR contains four transmembrane domains, a conserved cysteine loop in the Nterminal extra-cellular domain, and highly conserved sequence motifs. The Has-GABAAR gene is expressed at extremely low levels in unfertilized eggs, but increases significantly just prior to settlement, peaking at 120 h post fertilization (hpf). We further demonstrate that during the period of larval competence (96–144 hpf), gene transcripts and the encoded Has-GABAAR were localized in a cluster of cells along the dorsal and lateral edges of the foot, as well as the posterior epithelium. In functional settlement assays using GABA and 5-AVA, we found that there was significantly lower settlement of veligers pre-treated with antibodies to an external domain of the Has-GABAAR than those treated with preimmune serum, or untreated veligers. We postulate that this receptor may act as a highly sensitive chemical sensor, whose activation is necessary to trigger chloride-mediated sensory neuron activation or inhibition, leading to the initiation of settlement and metamorphosis events.