GABA (γ-aminobutyric acid) an inhibitory neurotransmitter in molluscs and other animals 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 other bivalves, serotonin is an excitatory neurotransmitter that increases beating rates of gill lateral cell cilia. Previously our lab demonstrated in C. virginica serotonin’s increase of cilia beating rates is blocked by applying GABA to the visceral or cerebral ganglia. Additionally, bicuculline methchloride , a GABAa receptor antagonist, blocked the effects of GABA. By using HPLC we previously detected GABA in low ng amounts in cerebral and visceral ganglia of C. virginica . Our immunofluorescence studies showed the presence of GABA neurons in cerebral and visceral ganglia; and that some serotonin neurons had GABA receptors on their soma. Recently the genome of C. virginica and other bivalves has begun to be mapped . By conducting BLAST searches of the NCBI (National Center for Biotechnology Information) database using DNA and protein sequences of C. virginica and other invertebrate and mammalian species we found matches for GABAa and GABAb receptor genes on C. virginica chromosomes 3 and 5, respectively. Various invertebrates had Percent Identity above 60%, while humans and mice had Percent Identity of about 40% to 50% for GABAa and GABAb . We hypothesize that the ligand binding sites (LBS) for GABAa and GABAb receptors in C. virginica are evolutionarily conserved and will closely match those of other animals. To study this, we conducted searches of the NCBI database for GABAa and GABAb LBS of C. virginica and compared them to other animals. We found GABAb LBS contained 4 amino acids (N, L, A, Y) in positions 34, 122, 123, 267 that were highly conserved in LBS of other bivalves, gastropods, insects, mice, rats and humans. GABAa LBS for C. virginica has not yet been identified . We did find LBS of other animals contained 2 amino acids (L, Y) that were highly conserved among the animals in which it has been identified. The C. virginica GABAa receptor does contain L and Y amino acid s. The study complements our earlier physiology and cell biology studies demonstrating the presence and a function for GABA in C. virginica and shows the genome of C. virginica contains genes to produce GABA receptor LBS that are similar to those of other animals. This new information is valuable as it shows the simple nervous system of C. virginica can be used to conduct studies on GABA neurotransmission.
This work was supported in part by grant 2R25GM06003 of the Bridge Program of NIGMS, NIH grant K12GM09385407A1 IRACDA Program of Rutgers University and PSCCUNY grants 6234400 50 and 6343400 51.