Visual Neuroscience



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


BARRY S.  WINKLER a1c1, NATALIA  KAPOUSTA-BRUNEAU a2, MATTHEW J.  ARNOLD a1 and DANIEL G.  GREEN a2
a1 Eye Research Institute of Oakland University, Rochester
a2 Department of Ophthalmology, University of Michigan, Ann Arbor

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.

(Received April 28 1998)
(Accepted September 1 1998)


Key Words: Electroretinogram; b-wave; Glutamate-glutamine cycle; Müller cells; Glutamine synthetase; Glutamate transporters.

Correspondence:
c1 Correspondence and reprint requests to: Barry S. Winkler, Eye Research Institute, 416 Dodge Hall, Suite 416, Oakland University, Rochester, MI 48309, USA.