Effects of lesions on 3H-apomorphine and 3H-spiperone stereospecific binding sites in subcellular fractions of rat striatum

Abstract

3H-spiperone and 3H-apomorphine showed saturable stereospecific binding to membranes of both a microsomal and a mitochondrial fraction of rat striatum. For each ligand, the K D -values were similar in both subcellular fractions, but the microsomal fraction contained a 2 times higher binding site density than the mitochondrial fraction. In both subcellular preparations the number of 3H-spiperone stereospecific binding sites was about two times higher than the number of 3H-apomorphine stereospecific binding sites. Unilateral striatal kainic acid lesions caused a reduction of 67% of 3H-spiperone and of 71% of 3H-apomorphine stereospecific binding in the microsomal fraction. In the mitochondrial fraction the reduction in stereospecific binding sites was only 33% and 40% for the 3H-ligands respectively. Scatchard plots of 3H-spiperone stereospecific binding were linear in the two subcellular fractions of both lesioned and control striata, and binding affinities were similar throughout. Scatchard plots of 3H-apomorphine stereospecific binding appeared to be non-linear in conditions where non-specific binding exceeded stereospecific binding i.e. in the mitochondrial fractions and in the microsomal fraction of the lesioned striata. To allow clear analysis of the data, original experimental measurements of total binding and non-specific binding are reported. The impact of variations in blank values on the final outcome of the analysis of specific binding is discussed. Unilateral cortical ablation caused a 22% reduction of both 3H-spiperone and 3H-apomorphine stereospecific binding in the microsomal fraction, but did not affect the binding in the mitochondrial fraction. 3H-spiperone and 3H-apomorphine stereospecific binding sites appear to occur concomitantly and to be affected in parallel by the lesions. Binding sites recovered in the striatal microsomal and mitochondrial fraction show the same binding characteristics but are apparently of different cellular origin. Dopamine receptors on intrastriatal neurones and cortical striatal afferents are mostly recovered in the microsomal fraction, and are likely to be localized on plasma membranes or associated with small subcellular transport particles. The origin of dopamine receptors recovered in the mitochondrial fraction is unknown. The presently reported findings corroborate the hypothesis that 3H-spiperone and 3H-apomorphine label the same unitary dopamine receptor complex, but probably interact with different sub-unit sites of the receptor.