CHARACTERIZATION OF COCAINE BINDING-SITES IN THE RAT TESTES

Purpose: An estimated 29 million individuals use cocaine in the United States. Studies have shown a high affinity for dose dependent binding of cocaine in the testes. Recent work done in our laboratory has show n that chronic administration of cocaine to male rats has an adverse e ffect on fertility and spermatogenesis by producing extensive morpholo gical changes in the testes, leading to reduction in sperm production. As a first step toward understanding this process, we characterized a nd identified the pharmacological properties of [H-3]cocaine binding s ites in the testes. Materials and Methods: Crude membranes from the te stes were prepared from 35 days old male Sprague-Dawley rats. [H-3]coc aine binding was measured by using the method of Madras et al. (1989) with modifications. The data from saturation binding assays were analy zed by Inplot (GraphPad Software, San Diego, CA) to determine the Kd a nd Bmax. Results: Specific binding of [H-3]cocaine was linearly depend ent on membrane protein concentrations ranging from 0.2 to 8 mg./ml. T he pooled data from three independent experiments revealed a mean affi nity of 36 +/- 2.0 nM and Bmax of 1.84 +/- 0.13 pmol/mg. The present s tudy demonstrates that testicular tissue has receptor protein that bin ds [H-3]cocaine saturably and specifically. Competition displacement e xperiments revealed a shallow displacement curve for (-)cocaine and Wi n 35,428 with r(2) = 0.96, indicative of multiple binding components. Computer analysis confirmed that a two component binding model was pre ferred statistically over a one component model in all three experimen ts (p < 0.001). Conclusion: The results from these studies suggest tha t the testicular tissue contains a protein that binds [H-3]cocaine in a saturable and specific manner. It has a different sensitivity from t he [H-3]cocaine binding protein in the brain and placenta. Further cla rification of the relationship between cocaine and its recognition sit e is necessary to understand the mechanism of testicular damage after cocaine exposure.