N-Methyl-D-Aspartate (NMDA) and Non-Nmda Type Glutamate Receptors are Present on Squid Giant Axon Schwann Cells

Abstract

ABSTRACT The role of the amino acid glutamate in neuronal-glial signalling is becoming increasingly recognised in both vertebrate and invertebrate nervous systems (Barres, 1989). Although the ability of glutamate to depolarize cultured astrocytes has been known for some time (Bowman and Kimelberg, 1984; Kettenmann and Schachner, 1985), there has been a debate over whether this was due to a direct activation of glutamate receptors or resulted from an electrogenic glutamate uptake mechanism (Brew and Attwell, 1987; Cull-Candy et al. 1988). However, recent evidence using patch-clamping techniques suggests the presence of ionic currents activated by glutamate receptors in cultured astrocytes (Sontheimer et al. 1988; Usowicz et al. 1989). Responses to glutamate have also been obtained from glial cells in a number of intact preparations, including the adaxonal glia of the squid giant axon (Villegas, 1978) and the glia of the optic nerve of Necturus maculosa (Tang and Orkand, 1986). The responses of the former preparation (Lieberman et al. 1989) and of the neuropile glia of the leech segmental ganglion (Ballanyi et al. 1989) have been demonstrated to be due to glutamate receptor activation. In all the above studies the glutamate receptors present on glial cells have been demonstrated pharmacologically to belong to the non-NMDA subtype. Here we present evidence, for the first time in any glial preparation, that NMDA type glutamate receptors are present in the adaxonal glial or Schwann cells of the giant axon of the tropical squid Sepioteuthis sepioidea and that they mediate a slow depolarization of these cells. In addition, these Schwann cells also possess a second distinct population of glutamate receptors that mediate rapid signalling responses and are pharmacologically similar to the non-NMDA glutamate receptors described in other glial preparations (Ballanyi et al. 1989; Lieberman et al. 1989; Sontheimer et al. 1988; Usowicz et al. 1989).