Genomic Study of GABA Receptors in the Bivalve Crassostrea virginica

Abstract

GABA (γ-aminobutyric acid) is a major inhibitory neurotransmitter in molluscs and other animals, but it has not been well studied in bivalves. In humans impairment of GABA neurotransmission can cause epilepsy. In the bivalve mollusc Crassostrea virginica, as well as many other bivalves, serotonin is an excitatory neurotransmitter that increases the beating rate of gill lateral cell cilia. This serotonin innervation originates in the cerebral and visceral ganglia of the animals. Previous physiology work of our lab demonstrated that in C. virginica the action of serotonin on accelerating the beating rates of gill lateral cell cilia is blocked by applying GABA to the visceral or cerebral ganglia. Additionally, the GABAa receptor antagonist bicuculline methchloride blocked the inhibitory effect of GABA. By using HPLC we previously had detected GABA in low ng amounts in the cerebral and visceral ganglia of C. virginica. Our earlier immunofluorescence histochemistry studies also showed the presence of GABA neurons in both the cerebral and visceral ganglia; and that some serotonin neurons in the ganglia had GABA receptors on their soma. Recently the genome of C. virginica and other bivalves have begun to be mapped. We hypothesize that C. virginica contains genes for GABA receptors and that these receptors are similar to those found in other animals, including mammals and humans. To study this we conducted BLAST searches of the NCBI (National Center for Biotechnology Information) database using the DNA and protein sequences from C. virginica and other invertebrate and mammalian species GABAa and GABAb receptor genes. We found matches for GABAa and GABAb receptor genes located on C. virginica chromosomes 3 and 5, respectively. Doing BLASTS of the receptors of other invertebrates and mammals found matches with very low Expect Values (E values) and high Percent Identity, signifying high similarities of the GABAa and GABAb receptors of C. virginica to those in other bivalves, gastropods, insects mice, rats and humans. Various invertebrates had Percent Identity above 60%, while humans and mice had Percent Identity of about 40% for GABAa and 50% for GABAb. This study complements our earlier physiology and cell biology studies demonstrating the presence and a function for GABA in C. virginica, and shows that the genome of C. virginica contains genes to produce GABA receptors that are similar to those found in other animals. This new information is valuable as it shows that the simple nervous system of C. virginica can be used to conduct studies on GABA neurotransmission.