Localization and characterization of dopamine D1 receptors in sheep hypothalamus and striatum

Dopamine receptors are pharmacologically grouped as D1 and D2 receptors. Previous research in the ewe has shown that central D1 receptors may have a role in facilitating prolactin release. The aims of this study were therefore to localize and characterize D1 binding sites in the hypothalamus of sheep. For comparison, a known D1 receptor-rich tissue (striatum) was also studied. The bioactivities of several D1 analogues were also assessed for their efficacy in sheep tissue. In vitro autoradiography with [125l]-SCH23982 was used to localize D1 binding sites. The ventromedial hypothalamic nucleus (VMH) displayed moderate levels of specific binding, localized to the medial portion of the nucleus. Low levels of specific binding were seen in the preoptic area, supraoptic nucleus and anterior hypothalamic area. The suprachiasmatic nucleus, median eminence and arcuate nucleus did not show specific binding. As expected the striatum displayed high levels of specific binding. The VMH, preoptic area, median eminence, striatum and anterior pituitary were examined with radioligand binding studies to quantify and characterize D1 binding sites. Scatchard analysis gave KD 1.04 nM and Bmax 127.4 fmol/mg protein for VMH and KD 1.99 nM and Bmax 454.6 fmol/mg protein for striatum. While specific binding occurred in the preoptic area and median eminence this binding did not show saturation characteristics. Specific binding was not observed in the anterior pituitary. Affinities determined by competitive binding studies showed that the binding sites in both VMH and striatum have the characteristics of a D1 receptor, that is, high affinity for the D1 agonists and antagonists, low affinity for dopamine and the serotonergic antagonist ketanserin and extremely low affinity for the D2 agonists and noradrenaline. Adenylate cyclase studies showed that in the striatum dopamine and the D1 agonists, fenoldopam and SKF38393, were able to cause significant dose-dependent increases in adenylate cyclase activity. In contrast the D1 agonist, SKF82958, was inactive in this system. The D1 antagonists SCH23390 and SCH39166, but not SKF83566, abolished the adenylate cyclase response to 50 microM dopamine. In the VMH the D1 agonist SKF38393, but not dopamine, stimulated adenylate cyclase activity. In conclusion, these results demonstrate that D1 binding sites exist within the hypothalamus in the VMH and that these binding sites have the characteristics of D1 receptors. These receptors are a potential site of action for dopamine in facilitating prolactin release. In addition, the results show that at least for some dopamine analogues, receptor binding affinity does not always correlate with biological activity.