Effects of inhibiting glutamine synthetase and blocking glutamate uptake on b-wave generation in the isolated rat retina.
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.
Winkler
B S
BS
Eye Research Institute of Oakland University, Rochester, MI 48309, USA.
Kapousta-Bruneau
N
N
Arnold
M J
MJ
Green
D G
DG
D018528:ATP-Binding Cassette Transporters / Q000037:antagonists & inhibitors*; D027322:Amino Acid Transport System X-AG; D000818:Animals; D001224:Aspartic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D003623:Dark Adaptation; D004596:Electroretinography; D005974:Glutamate-Ammonia Ligase / Q000037:antagonists & inhibitors*; D018698:Glutamic Acid / Q000378:metabolism; D005973:Glutamine / Q000378:metabolism; D007395:Interneurons / Q000187:drug effects / Q000502:physiology*; D008717:Methionine Sulfoximine / Q000494:pharmacology; D009457:Neuroglia / Q000187:drug effects / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012165:Retinal Ganglion Cells / Q000187:drug effects / Q000502:physiology*; D017948:Retinal Rod Photoreceptor Cells / Q000187:drug effects / Q000502:physiology*; D009435:Synaptic Transmission / Q000502:physiology*
D018528:ATP-Binding Cassette Transporters; D027322:Amino Acid Transport System X-AG; D005973:Glutamine; C011274:3-hydroxyaspartic acid; D008717:Methionine Sulfoximine; D001224:Aspartic Acid; D018698:Glutamic Acid; D005974:Glutamate-Ammonia Ligase