Effects of inhibiting glutamine synthetase and blocking glutamate uptake on b-wave generation in the isolated rat retina

Vis Neurosci. Mar-Apr 1999;16(2):345-53. doi: 10.1017/s095252389916214x.

Abstract

The purpose of the present experiments was to evaluate the contribution of the glutamate-glutamine cycle in retinal glial (Müller) cells to photoreceptor cell synaptic transmission. Dark-adapted isolated rat retinas were superfused with oxygenated bicarbonate-buffered media. Recordings were made of the b-wave of the electroretinogram as a measure of light-induced photoreceptor to ON-bipolar neuron transmission. L-methionine sulfoximine (1-10 mM) was added to superfusion media to inhibit glutamine synthetase, a Müller cell specific enzyme, by more than 99% within 5-10 min, thereby disrupting the conversion of glutamate to glutamine in the Müller cells. Threo-hydroxyaspartic acid and D-aspartate were used to block glutamate transporters. The amplitude of the b-wave was well maintained for 1-2 h provided 0.25 mM glutamate or 0.25 mM glutamine was included in the media. Without exogenous glutamate or glutamine the amplitude of the b-wave declined by about 70% within 1 h. Inhibition of glutamate transporters led to a rapid (2-5 min) reversible loss of the b-wave in the presence and absence of the amino acids. In contrast, inhibition of glutamine synthetase did not alter significantly either the amplitude of the b-wave in the presence of glutamate or glutamine or the rate of decline of the b-wave found in the absence of these amino acids. Excellent recovery of the b-wave was found when 0.25 mM glutamate was resupplied to L-methionine sulfoximine-treated retinas. The results suggest that in the isolated rat retina uptake of released glutamate into photoreceptors plays a more important role in transmitter recycling than does uptake of glutamate into Müller cells and its subsequent conversion to glutamine.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • ATP-Binding Cassette Transporters / antagonists & inhibitors*
  • Amino Acid Transport System X-AG
  • Animals
  • Aspartic Acid / analogs & derivatives
  • Aspartic Acid / pharmacology
  • Dark Adaptation
  • Electroretinography
  • Glutamate-Ammonia Ligase / antagonists & inhibitors*
  • Glutamic Acid / metabolism
  • Glutamine / metabolism
  • Interneurons / drug effects
  • Interneurons / physiology*
  • Methionine Sulfoximine / pharmacology
  • Neuroglia / drug effects
  • Neuroglia / physiology
  • Rats
  • Rats, Sprague-Dawley
  • Retinal Ganglion Cells / drug effects
  • Retinal Ganglion Cells / physiology*
  • Retinal Rod Photoreceptor Cells / drug effects
  • Retinal Rod Photoreceptor Cells / physiology*
  • Synaptic Transmission / physiology*

Substances

  • ATP-Binding Cassette Transporters
  • Amino Acid Transport System X-AG
  • Glutamine
  • 3-hydroxyaspartic acid
  • Methionine Sulfoximine
  • Aspartic Acid
  • Glutamic Acid
  • Glutamate-Ammonia Ligase