In the oyster Crassostrea virginica and other bivalves , gills are innervated by serotonin and dopamine nerves . The innervation originates in the cerebral ganglia (CG), which connect to the visceral ganglia (VG) and then to the gills. In gill lateral cells (GLC), s erotonin is cilio-excitatory and dopamine is cilio-inhibitory. In mammals glutamate (Glut) neurons are major excitatory neurons. In humans, disfunction of Glut neurons are associated wit h several disorders including Parkinson’s disease, Alzheimer’s disease, Huntington’s disease, autism, depression and schizophrenia. Glut receptors are generally classified as ionotropic NMDA, AMPA and kainate types or metabotropic group I, II, III and GluR7 types. Not much has been reported concerning Glut neurons in bivalves. However, ionotropic Glut receptors were found involved in regulating bivalve larvae metamorphosis in C. gigas , Mercenaria mercenaria and Mya arenaria; and genetic studies showed genes for i onotropic Glut receptors present in Pacific oysters. Glut neurons had not previously been reported in or have a physiological function in adult C . virginica. Recently our lab used immunohistofluorescence and Western Blotting to detect Glut neurons and the ionotropic Glut receptor GluR1 in VG of C. virginica. We also showed t he Glut neurons in VG excited serotonin neurons to increase GLC cilia beating rates. Based on these findings, we sought to determine if Glut had a neurophysiological role in CG of C. virginica. We h ypothesize Glut would be an excitatory neurotransmitter in CG, resulting in an increase in GLC cilia beating rates. To test this Glut was applied directly to the CG of CG preparations in which gill innervation from CG and VG were kept intact. Shells were removed and preparations placed in chambers with a barrier so drugs could be discretely applied to CG without coming in contact with VG or gill. Beating rates of GLC cilia were measured by stroboscopic microscopy. Our results showed applying Glut (10-5 - 10-3 M) to CG, of CG preparation, caused a dose dependent increase in GLC cilia beating rates from a basal of 13 to 19 beats/sec. This response was mimicked by applying the ionotropic agonist, homocysteic acid (10-5 – 10-3 M) to the CG, which also caused a similar dose dependent increase in cilia beating from 13 to 19 beats/sec. This cilio -excitatory effect of Glut on the CG was prevented by the presence of the Glut ionotropic antagonist, DL-2 amino-5-phosphonopentanoic . The study confirms a physiological role for Glut as an excitatory neurotransmitter in CG, most likely exciting CG serotonin neurons to increase GLC cilia beating rates. Our pharmacological results also show the Glut receptors in CG are ionotropic. The bivalve mollusc gill is a useful model to study regulatory mechanisms of cilia activity as well as the pharmacology of drugs affecting biogenic amines in nervous systems.
This work was supported in part by grants 0537231071 of the CSTEP Program of NYSED, P120A210054 of the MSEIP Program of the DoEd, 2R25GM06003 of the Bridge Program of NIGMS , and NIH grant K12GM093854-07A1 IRACDA Program of Rutgers University.