Glutamate transporters in the central nervous system of a pond snail

Abstract

Previous studies on glutamate (GLU) and its receptors in the pond snail Lymnaea stagnalis have suggested that GLU functions as a neurotransmitter in various behaviors, particularly for generation of feeding rhythm. The uptake mechanism of GLU is not yet known in Lymnaea. In the present study, we characterized the GLU transporters and examined their functions in the feeding circuits of the central nervous system (CNS) in Lymnaea. First, measurement of the accumulation of (3)H-labeled GLU revealed the presence of GLU transport systems in the Lymnaea CNS. The highest accumulation rate was observed in the buccal ganglia, supporting the involvement of GLU transport systems in feeding behavior. Second, we cloned two types of GLU transporters from the Lymnaea CNS, the excitatory amino acid transporter (LymEAAT) and the vesicular GLU transporter (LymVGLUT). When we compared their amino acid sequences with those of mammalian EAATs and VGLUTs, we found that the functional domains of both types are well conserved. Third, in situ hybridization revealed that the mRNAs of LymEAAT and LymVGLUT are localized in large populations of nerve cells, including the major feeding motoneurons in the buccal ganglia. Finally, we inhibited LymEAAT and found that changes in the firing patterns of the feeding motoneurons that have GLUergic input were similar to those obtained following stimulation with GLU. Our results confirmed the presence of GLU uptake systems in the Lymnaea CNS and showed that LymEAAT is required for proper rhythm generation, particularly for generation of the feeding rhythm.