d‐Aspartate as a putative cell–cell signaling molecule in the Aplysia californica central nervous system

Abstract

The content, synthesis and transport of D-aspartate (D-Asp) in the CNS of Aplysia californica is investigated using capillary electrophoresis (CE) with both laser-induced fluorescence and radionuclide detection. Millimolar concentrations of D-Asp are found in various regions of the CNS. In the cerebral ganglion, three adjacent neuronal clusters have reproducibly different D-Asp levels; for example, in the F-and C-clusters, up to 85% of the free Asp is present in the D-form. Heterogeneous distribution of D-Asp is also found in the individual identified neurons tested, including the optical ganglion toplayer neurons, metacerebral cells, R2 neurons, and F-, C-and G-cluster neurons. The F-cluster neurons have the highest percentage of D-Asp ( 58% of the total Asp), whereas the lowest value of 8% is found in R2 neurons. In pulse-chase experiments with radiolabeled D-Asp, followed by CE with radionuclide detection, the synthesis of D-Asp from L-aspartate (L-Asp) is confirmed. Is D-Asp in the soma, or is it transported to distantly located release sites? D-Asp is clearly detected in the major nerves of A. californica, including the pleuroabdominal and cerebrobuccal connectives and the anterior tentacular nerves, suggesting it is transported long distances. In addition, both D-Asp and L-Asp are transported in the pleuroabdominal connectives in a colchicine-dependent manner, whereas several other amino acids are not. Finally, D-Asp produces electrophysiological effects similar to those induced by L-Asp. These data are consistent with an active role for D-Asp in cell-to-cell communication.

A Novel Pyridoxal 5′-Phosphate-dependent Amino Acid Racemase in the Aplysia californica Central Nervous System

“…Using CE-LIF we measured the presence of D-Ser and D-Asp in the cells containing the enzyme to determine if they co-localize. It is particularly telling that the enzyme was present in the F-, C-, and G-clusters where high levels of D-Asp content were previously reported (21,24); we now confirm the co-localization of D-Ser and D-Asp in these structures. The correlation of DAR1 and its products supports our determination that the enzyme is involved in the actual biosynthesis of D-Asp and D-Ser.…”

“…The correlation of DAR1 and its products supports our determination that the enzyme is involved in the actual biosynthesis of D-Asp and D-Ser. Several years ago, we hypothesized that D-Asp was a neurotransmitter in the A. californica CNS (24). Since then, we have accumulated evidence of D-Asp release from and uptake by the cerebral ganglia.…”

“…But it is unknown if the enzyme is also present in bivalve nervous and endocrine systems. In a prior study (24), we detected high levels of D-Asp (%d/l ϩ d) in the F-, C-, and G-clusters of the cerebral ganglion of the A. californica CNS, with the highest D-Asp content (85%) found in the insulin-producing F-cluster cells. We also observed enzyme activity in the cerebral ganglion that could transfer [ 14 C] from L-Asp to D-Asp in radioisotope pulse-and-chase experiments.…”

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

“…Numerous studies have confirmed the widespread presence of D-Asp in each of the ganglia of the Aplysia CNS in multiple species such as A. fasciata (D'Aniello et al 1993), A. limacina (Spinelli et al 2006), and A. californica (Liu et al 1998;Miao et al 2005Miao et al , 2006. L-Glu receptors are found throughout the Aplysia nervous system, including the well-studied NMDA- (Ha et al 2006) and AMPA-type receptors that activate channels (Antzoulatos and Byrne 2004), as well as the invertebrate-specific L-Glu-activated Cl Ϫ channels (King and Carpenter 1989).…”

“…Meanwhile, Huang et al (2005) found that D-Asp did not activate AMPA/ kainate or metabotropic glutamate receptors. Miao et al (2006) investigated the electrophysiological responses to D-Asp in Aplysia neurons of the abdominal and cerebral ganglia. They found that D-Asp induced depolarizations similarly to L-Glu; however, in some neurons D-Asp responses had opposite polarity from L-Glu responses, similarly suggesting that D-Asp may have actions independent of L-Glu and its target receptors.…”