Physiological evidence that d-aspartate activates a current distinct from ionotropic glutamate receptor currents in Aplysia californica neurons

Abstract

d-Aspartate (d-Asp) activates an excitatory current in neurons of Aplysia californica. Although d-Asp is presumed to activate a subset of l-glutamate (l-Glu) channels, the identities of putative d-Asp receptors and channels are unclear. Whole cell voltage- and current-clamp studies using primary cultures of Aplysia buccal S cluster (BSC) neurons were executed to characterize d-Asp-activated ion channels. Both d-Asp and l-Glu evoked currents with similar current-voltage relationships, amplitudes, and relatively slow time courses of activation and inactivation when agonists were pressure applied. d-Asp-induced currents, however, were faster and desensitized longer, requiring 40 s to return to full amplitude. Of cells exposed to both agonists, 25% had d-Asp- but not l-Glu-induced currents, suggesting a receptor for d-Asp that was independent of l-Glu receptors. d-Asp channels were permeable to Na+ and K+, but not Ca2+, and were vulnerable to voltage-dependent Mg2+ block similarly to vertebrate NMDA receptor (NMDAR) channels. d-Asp may activate both NMDARs and non-l-Glu receptors in the nervous system of Aplysia.