title: Dopamine receptors in rat brain regions. Optimal conditions for 3H-agonist binding, pH dependency and lack of inhibition by ascorbic acid.
abstract: The binding of dopaminergic agonist and antagonist radioligands was investigated in rat brain regions. A 30-min preincubation of homogenates of caudate nucleus or mesolimbic brain regions at 37 degrees induced a several-fold increase in the stereospecific binding of [3H]-dopamine or [3H]apomorphine to the subsequently washed particulate fraction, whereas it induced a slight decrease in [3H]spiroperidol binding. Stereospecific 3H-agonist binding was not observed in brain regions devoid of dopaminergic innervation. Guanosyl nucleotides, EDTA or ethyleneglycol-bis-(beta-amino-ethyl ether) N,N'-tetraacetate (EGTA), included in the preincubation buffer, antagonized the stimulation of 3H-agonist binding. The stereospecific binding sites of 3H-agonists were saturable with equilibrium dissociation constants (Kd) of 1-2 nM. High-affinity binding was pH dependent, with different pH optima for each radioligand. Several dopamine receptor agonists and antagonists were potent inhibitors of stereospecific [3H]dopamine binding, whereas l-ascorbic acid was inactive at concentrations as high as 1.0 mM. The stereospecific binding of [3H]apomorphine or [3H]spiroperidol was also unaffected by ascorbic acid. The nonspecific (d-butaclamol-insensitive) portion of 3H-agonist binding was weakly inhibited by concentrations of ascorbic acid higher than 0.01 mM. This effect was also observed in the cerebellum and spinal cord, where none of the 3H-agonist binding was stereospecific. It is concluded that the portion of 3H-agonist or 3H-antagonist radioligand binding which is related to dopamine receptors is unaffected by ascorbic acid.
mesh_terms: D000818:Animals; D001205:Ascorbic Acid / Q000494:pharmacology*; D001923:Brain Chemistry; D002421:Caudate Nucleus / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D008024:Ligands; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D011954:Receptors, Dopamine / Q000187:drug effects / Q000378:metabolism*; D013116:Spinal Cord / Q000378:metabolism; D013237:Stereoisomerism
chemical_list: D008024:Ligands; D011954:Receptors, Dopamine; D001205:Ascorbic Acid