BINDING AND UPTAKE OF THE GABA ANALOG, H-3-MUSCIMOL, IN THE RETINAS OF GOLDFISH AND CHICKEN

  • 1 January 1980
    • journal article
    • research article
    • Vol. 19  (12) , 1415-1426
Abstract
GABA is a candidate as a neurotransmitter in the vertebrate retina. The GABA analog muscimol used to probe the properties of GABA receptors in other parts of the vertebrate CNS was used to investigate potential GABA receptors in the retinas of goldfish and chick with biochemical assay techniques and light microscopic autoradiography. In both animals 3H-muscimol showed specific and saturable binding with a Kd of about 10 nM. GABA effectively inhibited 3H-muscimol binding at 50% inhibitory concentration (IC50) of 10-6 M. The labeling pattern of 7 .times. 10-7 M 3M-muscimol showed common features for both species, i.e., amacrine cell bodies were intensely labeled, horizontal cells were less intensely labeled and a laminar pattern existed throughout the inner plexiform layer (IPL). A 1 mM concentration of GABA abolished 3H-muscimol labeling in the chick retina and throughout much of the goldfish retina except for some label over amacrine cells and the distal 2/3 of the IPL. The intense somatic labeling suggests neuronal uptake of 3H-muscimol. Virtually all 3H-muscimol labeling was abolished with the addition of 0.4 mM ouabain. The uptake pattern of 3H-GABA differed from 3H-muscimol and was largely unaffected by the addition of 1 mM muscimol. 3H-Muscimol binding in retinas was adequately demonstrated biochemically but only 3H-muscimol uptake was observed with autoradiography from tissue conventionally processed through Epon. Since GABA inhibits 3H-muscimol uptake and the reverse is not true, the transport carriers for muscimol and GABA apparently are different. The strong degree of 3H-muscimol uptake by retinal neurons raises serious questions about the use of 3H-muscimol as a probe for GABA synaptic receptors in the retina with autoradiography.