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  • Title: Properties of dopamine agonist and antagonist binding sites in mammalian retina.
    Author: Makman MH, Dvorkin B, Horowitz SG, Thal LJ.
    Journal: Brain Res; 1980 Aug 04; 194(2):403-18. PubMed ID: 6770965.
    Abstract:
    Retinal homogenates of calf, rat, rabbit and Cebus appella and Macaca mulata monkeys were found to contain stereospecific binding sites for the dopamine antagonist [3H]spiroperidol. In further studies with calf and rat retina, stereospecific binding sites were also found for the dopamine agonist [3H]ADTN (2-amino-6,7,-dihydroxy-1,2,3,4-tetrahydronapththalene). The [3H]spiroperidol binding sites in calf retina were pharmacologically similar to the dopaminergic spiroperidol binding sites previously demonstrated to be present in striatum. However, calf and rabbit retina contained less than 1/10 the concentration of [3H]spiroperidol binding sites found in striatum. Saturation studies and Scatchard analyses showed a single class [3H]spiroperidol binding sites with Kd (apparent dissociation constant) = 0.3 and 0.2 nM and Bmax (binding site number) = 38 and 24 fmol/mg protein in calf retina and rabbit retina respectively. Rates of [3H]spiroperidol association and dissociation were also evaluated in calf retina. Drug specificity for [3H]ADTN binding in calf retina resembled that previously reported for striatal [3H]ADTN binding and thus differed from retinal [3H]spiroperidol binding. Calf retinal [3H]ADTN binding sites had a Kd = 9 nM and Bmax = 113 +/- 12 fmol/mg protein. Thus, the total number of [3H]ADTN sites in retina was at least twice that of [3H]spiroperidol sites. Guanine nucleotides (GTP and Gpp (NH)p) but not ATP reduced the affinity of the dopamine agonist ADTN for [3H]spiroperidol binding, and also reduced the specific binding of [3H]ADTN itself up to a maximal value of about 50% of control binding. Saturation studies of calf retinal [3H]ADTN binding confirmed that Gpp(NH)p-displaceable sites were a discrete saturable subset of stereospecific [3H]ADTN sites with Kd = 9 nM and Bmax = 50 +/- 6 fmol/mg protein. The Gpp(NH)p insensitive sites had a Kd = 9 nM and Bmax = 63 +/- 7 fmol/mg protein. It is proposed that although [3H]ADTN sites differ pharmacologically from [3H]spiroperidol sites, since [3H]spiroperidol sites are guanine nucleotide-sensitive and similar in number to the guanine nucleotide-sensitive class of [3H]ADTN sites, they may possibly be related to these sites as well as to adenylate cyclase. In addition, retina contains guanine nucleotide-insenstive [3H]ADTN sites, possibly presynaptic and probably not coupled to adenylate cyclase.
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