Genomic analyses of gas (nitric oxide and carbon monoxide) and small molecule transmitter (acetylcholine, glutamate and GABA) signaling systems in Daphnia pulex

Diffusible gasses and small molecule transmitters are classes of compounds used by neurons and other cell types for local and hormonal signaling. In crustaceans, there is evidence for the neuronal production of the gasses nitric oxide (NO) and carbon monoxide (CO), as well as the small molecule transmitters acetylcholine, glutamate and GABA. While much is known about the physiological roles played by these molecules in crustaceans, little is known about them at the molecular level. Here, we have mined the genome of Daphnia pulex for genes encoding the biosynthetic enzymes, receptors and transporters necessary for establishing each of these transmitter systems. The biosynthetic enzyme genes identified included nitric oxide synthase, heme oxygenase, choline acetyltransferase, glutaminase and glutamic acid decarboxylase. Genes encoding several transporters (e.g. vesicular acetylcholine transporter) were also characterized, as were ones involved in transmitter degradation/recycling (e.g. acetylcholine esterase); genes encoding receptors for NO and CO (i.e. soluble guanylyl cyclase), and for each small molecule transmitter (both ionotropic and metabotropic receptors for each compound) were identified. These data provide the first molecular descriptions of gas and small molecule transmitter signaling systems in D. pulex, and provide frameworks for future molecular, anatomical and physiological investigations of them in Daphnia.