Glutamate (Glu) neurons are major excitatory neurons in mammalian brains and various invertebrate ganglia . In humans, disfunction of Glu neurons are associated with a number of disorders including Parkinson’s disease, Alzheimer’s disease, Huntington’s disease, autism, depression and schizophrenia. In higher animals Glu receptors are classified as ionotropic NMDA, AMPA and kainate types; or metabotropic group I, II, III and GluR7 types. NMDA type Glu receptors recently have been reported to be involved in regulating bivalve metamorphosis in C. gigas , Mercenaria mercenaria and Mya arenaria. Other studies also are showing that genes for i onotropic Glu NMDA (iGluRs) type are present in C. gigas. I n C. virginica and other studied bivalves, gills are innervated by serotonin and dopamine nerves from their visceral ganglia (VG) . Serotonin is cilio-excitatory , while dopamine is cilio-inhibitory to gill lateral cell (GLC) . Recently our lab detected Glu neurons in ganglia of C. virginica , and showed they excite serotonin neurons to increase GLC cilia beating rates. Using immunohistochemistry and PAGE and Western Blotting we also showed the presence of the ionotropic GluR1 receptor in the VG. Previously, the presence of Glu neurons had not been reported in the adult C. virginica or to have a physiological function. In this project we sought to further study the physiology of ionotropic Glu receptors in VG of C. virginica. We hypothesize that applying a GluR1 agonist to the VG of C. virginica would stimulate the receptor, whereas a GluR1 antagonist would block the receptor, preventing the cilio -excitatory actions of Glu on the GLC. To test this, we used VG preparations in which the VG innervation to the gill is kept intact. We examined effects of applying a GluR1 ionotropic agonists and antagonists to the VG while we observed GLC cilia beating rates. Shells were removed and preparations placed into chambers with a barrier so drugs could be discretely applied to the VG without coming in contact with the gill. Beating rates of GLC cilia were measured by stroboscopic microscopy. Our results show that a pplying the ionotropic agonist homocysteic acid (10-5 – 10-3M) to the VG caused a dose dependent increase in cilia beating from 12 to 18 beats/second. Applying 10-5 M of the ionotropic antagonist DL-2 amino-5-phosphonopentanoic acid (DL2) to the VG prevented Glu (10-5 – 10-3 M) from increasing cilia beating rates. The study thus far confirms a physiological role for Glu as an excitatory neurotransmitter in the VG , most likely exciting serotonin neurons to cause an increase in GLC cilia beating rates. The results of the ionotropic agonist homocysteic and antagonist DL2 indicate that the Glu receptor type is ionotropic. The bivalve mollusc gill is a useful model to study regulatory mechanisms of ciliary activity as well as the pharmacology of drugs affecting biogenic amines and other neurotransmitters.
This work was supported in part by grants 2R25GM06003 of the Bridge Program of NIGMS, 0537231071 of the CSTEP Program of NYSED, P120A210054 of the MSEIP Program of the DoEd, and NIH grant K12GM093854-07A1 IRACDA Program of Rutgers University.