Pharmacology of [ [formula omitted]] R(+)-7-OH-DPAT binding in the rat caudate-putamen

Abstract

Dopamine D 3 receptors may be involved in drug addiction and in disorders such as schizophrenia and Parkinson’s disease. To determine the pharmacological properties of dopamine D 3 receptors in the rat caudate-putamen, we have investigated R(+)-[ 3 H ]7-hydroxy- N, N-di- n-propyl-2-aminotetralin ([ 3 H ] R(+)-7-OH-DPAT) binding to membrane preparations from the rat caudate-putamen. Kinetic analyses showed that [ 3 H ] R(+)-7-OH-DPAT binding reached equilibrium in approximately 1 h and that both association and dissociation curves were composed of at least two components. Likewise, saturation curves showed at least two binding components with a combined B max value of about 600 fmol/mg protein, which is three times higher than what is present in the subcortical limbic area. Competition curves were performed with agonists such as R(−)-propylnorapomorphine, dopamine, PD 128907, quinpirole, and bromocriptine, and antagonists such as haloperidol, raclopride, clozapine, GR 218231x, remoxipride, and U99194A. These experiments revealed that [ 3 H ] R(+)-7-OH-DPAT binding could be resolved into three specific binding sites ( R 1– R 3) and one nonspecific binding site, with R 1– R 2 probably representing D 3 receptor binding and the minor R 3 representing D 2 receptor binding. The low affinities of (±)-8-OH-DPAT and 1,3-di(2-tolyl)guanidine to inhibit [ 3 H ] R(+)-7-OH-DPAT binding indicate negligible involvement of 5-HT 1A or sigma binding sites, respectively. The pharmacological profile of [ 3 H ] R(+)-7-OH-DPAT (2 nM) binding in the caudate-putamen was similar to that of dopamine on [ 125 I ]iodosulpride binding in the cerebellar lobule X, which contain D 3 but not D 2 receptors. Mg 2+ increased and GTP and Na + decreased the binding of [ 3 H ] R(+)-7-OH-DPAT, suggesting a coupling of endogenous D 3 receptors to G proteins. Taken together, these results suggest that dopamine D 3 receptors display multiple agonist binding states, and that D 3 receptors are present in high concentrations in the rat caudate-putamen. These results may have implications for the physiological and pathological roles of dopamine D 3 receptors in the brain.