Abstract
D-Aspartate (D-Asp) is an endogenous compound found in the central nervous system (CNS) of a variety of organisms. Despite its prevalence, however, relatively little understood of its physiological role. The prevailing theory is that D-Asp is an alternate agonist of N-methyl-D-aspartate receptor (NMDAR) channels. The goal of this work was to characterize the currents activated by D-Asp in neurons Aplysia californica, focusing on cells of the buccal S cluster (BSC). First, a general electrophysiological characterization was carried out, examining ion permeability, agonist dose-response, and the kinetics of activation, inactivation, and desensitization. D-Asp activated non-specific cation currents characterized by permeability to Na+ and K+. D-Asp-induced currents shared similar current-voltage relationships and time courses of activation and inactivation with L-glutamate (L-Glu)-induced currents. D-Asp currents, however, were subject to prolonged desensitization. Additionally, D-Asp activated currents independently of L-Glu, the known agonist of NMDAR channels, suggesting a non-NMDAR-dependent role of D-Asp. Next, select antagonists were used in an effort to pharmacologically characterize D-Asp receptor channels. These experiments suggested that D-Asp whole cell currents may be characterized by activation of multiple receptor sites, including NMDARS, excitatory amino acid transporters (EAATs), and a putative non-L-Glu D-Asp receptor. Furthermore, bath-applied D-Asp attenuated L-Glu-activated currents. Finally, D-Asp currents were compared to those evoked by acetylcholine (ACh) and serotonin (5-HT) in BSC cells. Results suggested that D-Asp activated receptor channels independently of ACh and 5-HT. Ten minute bath application of 5-HT was found to potentiate D-Asp current responses, likely through activation of a protein kinase C (PKC)-dependent mechanism, suggesting that D-Asp induced currents may be subject to synaptic plasticity associated with learning. While the identity of the putative D-Asp receptor remains elusive, the current work has advanced our understanding of the role D-Asp may play in the nervous system. These results should provide the groundwork for future studies aimed at identifying this unknown receptor channel, as well as investigation of the potential relationship of D-Asp receptor modulation to learning and memory in Aplysia, which may have relevance in higher organisms.