D - Aspartate activates a current distinct from ionotropic 1 glutamate receptor currents in Aplysia californica neurons 2

18 D-Aspartate (D-Asp) activates an excitatory current in neurons of Aplysia californica. 19 While D-Asp is presumed to activate a subset of L-glutamate (L-Glu) channels, the 20 identities of putative D-Asp receptors and channels are unclear. Whole cell voltage and 21 current clamp studies using primary cultures of Aplysia buccal S cluster (BSC) neurons 22 were executed to characterize D-Asp-activated ion channels. Both D-Asp and L-Glu 23 evoked currents with similar current-voltage relationships, amplitudes, and relatively 24 slow time courses of activation and inactivation when agonists were pressure applied. D25 Asp-induced currents, however, were faster and desensitized longer, requiring 40 s to 26 return to full amplitude. Of cells exposed to both agonists, 25% had D-Aspbut not L27 Glu-induced currents, suggesting a receptor for D-Asp that was independent of L-Glu 28 receptors. D-Asp channels were permeable to Na and K, but not Ca, and were 29 vulnerable to voltage-dependent Mg block similarly to vertebrate NMDAR channels. 30 D-Asp may activate both NMDARs and non-L-Glu receptors in the nervous system of 31 Aplysia. 32 33