Quantitative analysis of [³H]spiroperidol binding to rat forebrain sections: Plasticity of neostriatal dopamine receptors after nigrostriatal injury

The binding of [³H]spiroperidol to rat coronal sections in vitro was investigated using two procedures: swabbing studies, in which the tissue sections are wiped from the microscope slides after incubation in the presence of [³H]spiroperidol, and autoradiographic studies, in which the autoradiographic negatives are analyzed using computer-assisted densitometry. In the swabbing studies, the pharmacological and kinetic properties of butaclamol-displaceable binding were investigated, and the following results suggest that [^3H]spiroperidol binds specifically to only a single site within the basal forebrain of tissue sections and that the site is the dopamine D-2 receptor. (1) The pseudo-first order and first order plots for the rate of association to and dissociation from tissue sections appeared to be linear. (2) Dopamine antagonists, such asd haloperidol and butaclamol, were much more effective than dopamine agonists or the serotonin S-2 ligand, ketanserin, in inhibiting [³H]spiroperidol binding. (3) The ability of dopamine agonists to inhibit [³H]speroperidol binding was markedly reduced by the guanine nucleotide, Gpp(NH)p. (4) Saturation analysis of specific [³H]speroperidol binding revealed a K_d and B_max of 0.93 nM and 447 fmol/mg protein, and a Hill coefficient of 1.05. The findings are also compatible with the possibility that [³H]spiroperidol binds to several sites that have identical affinities for this ligand. Densitometric studies were used to assess the effect of lesions on [³H]spiroperidol binding in the neostriatum. Intrastriatal injection of kainic acid substantially reduced 1 μM (+)-butaclamol-displaceable binding, indicating that the receptors are in large part on intrinsic striatal neurons. Neostriatal [³H]spiroperidol binding was investigated 7 days after destruction of the mesotelencephalic dopamine system by the ventral tegmental injection of 6-hydroxydopamine. As determined by saturation analysis, the average values for K_d and B_max were 0.66 nM and 1212 fmol/mg protein in the intact striatum, and 0.82 nM and 1504 fmol/mg in the denervated striatum. The finding of a significant 23.8% increase in receptor density by the end of the first postoperative week, a period during which behavioral supersensitivity to apomorphine increases rapidly, supports the hypothesis that a proliferation of D-2 receptors underlies the behavioral manifestations of denervation supersensitivity.