Effect of temperature and ionic environment on the specific binding of 3H(-)sulpiride to membranes from different rat brain regions

Sulpiride is an antipsychotic drug endowed with the properties of a dopamine antagonist. The failure of sulpiride to inhibit neostriatal dopamine stimulated adenylate cyclase activity indicated that this drug is a selective D₂ receptor antagonist. In this study we used a novel synthesized ²H(-)sulpiride with very high specific activity (72 Ci/mol) and characterized the temperature sensitivity of the binding sites labeled by this compound. Kinetic analysis of ³H(-)sulpiride binding in rat striatum showed unstable behavior when incubation was performed at 37 or 30°C. However when experiments were carried out at 15 or 10°C, binding reached a stable steady-state within 10 min. Scatchard analysis of binding isotherms obtained at 10°C showed a 5-fold increase in the maximum number of binding sites and a decrease in K_d values to one-third those obtained at 37°C. Pharmacological characterization of the binding sites labeled by ³H(-)sulpiride at 10°C showed a greater affinity for antagonists but not for agonists than 37°C. Under both experimental condition, ³H(-)sulpiride binding sites were Na⁺ and GTP-sensitive. The temperature sensitive binding phenomenon appeared to be area specific. ³H(-)sulpiride binding sites in tissues other than from striatum were influenced less or not at all by changes in incubation temperature.