Quisqualate and ACPD are agonists for a glutamate-activated current in identified Aplysia neurons

Abstract

1. Glutamate is an important neurotransmitter in both vertebrates and invertebrates, yet the characterization of molluscan glutamate receptors and their relationships to vertebrate receptors is incomplete. This study uses two-electrode voltage clamp to characterize glutamate-evoked currents in cultured neurons from the marine gastropod mollusk Aplysia californica. 2. The identified buccal ganglion neurons, B1 and B2, display two pharmacologically distinct current responses to pressure-applied glutamate. One response is a desensitizing chloride current similar to that found in other large buccal ganglion neurons. The other is a nondesensitizing potassium current that is strongly outwardly rectifying. 3. The potassium current response has a higher sensitivity to glutamate than the chloride response. 4. Individual neurons, isolated in primary cell culture, exhibit different relative proportions of the two currents. 5. The glutamate agonists quisqualate and (1S,3R)-aminocyclopentane-1,3-dicarboxylic acid specifically evoke the potassium response but not the chloride response. The glutamate agonist (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid does not evoke any appreciable response. Kainate and N-methyl-D-aspartate also have no effect on these cells. Another glutamate analogue, ibotenate, evokes the transient chloride current but not the potassium current. 6. These results indicate that the glutamate receptor mediating the outward potassium response does not conform in its pharmacological profile to any of the known vertebrate glutamate receptor types.