Stereospecific blockade of alpha 2-adrenoceptors by (+)-butaclamol: implications for the characterization of dopamine receptors

(+)-sulpiride antagonises the renal effects of gamma-L-glutamyl-L-dopa in man

1. gamma-L-glutamyl-L-dopa (gludopa) was given by intravenous infusion to six healthy salt-replete men on two occasions, with and without pretreatment with (+)-sulpiride. 2. Gludopa increased sodium excretion, glomerular filtration rate and effective renal plasma flow whilst decreasing plasma renin activity. 3. (+)-sulpiride had no significant effect on baseline natriuresis, renal haemodynamics or plasma renin activity, but significantly attenuated the rise in sodium excretion, glomerular filtration rate and effective renal plasma flow produced by gludopa. 4. (+)-sulpiride abolished the acute fall in plasma renin activity seen with gludopa. 5. (+)-sulpiride raised serum prolactin concentration but did not affect the ris in urine dopamine excretion rate caused by gludopa. 6. Gludopa exerts its renal effects by stimulating specific dopamine receptors which are principally of the DA1 subtype.

Pharmacological characterization of binding sites identified in rat brain following in vivo administration of [3H]-spiperone

[3H]-spiperone is commonly used to label dopamine receptors in vitro in brain tissue. However, spiperone also interacts with brain 5-hydroxytryptamine and noradrenaline receptors. In vivo, [3H]-spiperone has been used for identifying dopamine receptors in both animals and man but the nature of the sites identified is unknown. The in vivo administration of [3H]-spiperone to rats leads to a selective accumulation of radioactivity in the olfactory lobes, tuberculum olfactorium, nucleus accumbens, striatum, substantia nigra, hippocampus, frontal cortex and hypothalamus, when compared to the cerebellum. In vivo drug displacement studies suggest that the binding of [3H]-spiperone in these areas may be to dopamine, 5-HT or noradrenaline receptors. [3H]-spiperone in vivo mainly labels dopamine receptors in striatum, tuberculum olfactorium, hypothalamus, substantia nigra and olfactory lobes. However, in the frontal cortex and nucleus accumbens specific binding involves not only dopamine receptors but also 5-HT and/or noradrenaline receptors. Interpretation of in vivo studies in man using radioactive spiperone and its derivatives must take into account the fact that this ligand only labels dopamine receptors in some brain areas.

The DA1 receptor agonist fenoldopam (SK & F 82526) is also an alpha 2-adrenoceptor antagonist

The alpha-adrenoceptor activity of fenoldopam was evaluated in three different in vitro tissue preparations. In rabbit isolated aortic rings fenoldopam displayed weak alpha 1-adrenoceptor antagonist activity (-log KB = 5.41) and displayed no alpha 1-adrenoceptor agonist activity. In contrast, fenoldopam demonstrated moderately potent antagonist properties at the alpha 2-adrenoceptor in two other tissue preparations. In in vitro model systems used for the characterization of alpha 2-adrenoceptors, fenoldopam competitively antagonized the effects of the alpha 2-adrenoceptor agonist B-HT 920. In the dog isolated saphenous vein and in the isolated field-stimulated guinea-pig ileum, fenoldopam antagonized the effects of B-HT 920 with -log KB values of 7.78 and 7.60, respectively. These data indicate that in addition to being an agonist at DA1 receptors, fenoldopam is also a relatively selective antagonist at alpha 2-adrenoceptors.

Interactions of dopamine and the release of [3H]-taurine and [3H]-glycine from the isolated retina of the rat

1 The dose-related, calcium-dependent, potassium-stimulated release of preloaded [(3)H]-dopamine from the superfused rat retina has been demonstrated.2 A high-affinity uptake system for dopamine exists in rat retina in vitro; K(m) value was calculated as 1.89 muM, V(max) value as 1.4 nmol g(-1) tissue h(-1).3 Dopamine (0.8 and 4 mM) inhibited the spontaneous release of [(3)H]-glycine from retina, and in the case of 0.8 mM dopamine this inhibitory effect was antagonized by 10 muM (+)-butaclamol but not by 10 muM (-)-butaclamol.4 The potassium-evoked (25 mM) release of [(3)H]-glycine from rat retina was similarly inhibited by dopamine (0.4-4 mM) in a dose-related manner when added to the superfusate with the potassium. The effect of 0.8 mM dopamine was antagonized by 10 muM (+)-butaclamol but not by 10 muM (-)-butaclamol.5 Dopamine (4 mM) significantly reduced the spontaneous release of [(3)H]-taurine from rat retina.6 The potassium-stimulated (25 mM) release of [(3)H]-taurine occurred after the cessation of the depolarizing stimulus. This delayed release of [(3)H]-taurine was unaffected if dopamine was applied to the superfusate at the same time as the potassium, but it was significantly reduced if dopamine (0.8 and 4 mM) was applied after the depolarizing stimulus had been removed and during the actual amino acid release phase.7 The inhibition of K(+)-stimulated (25 mM) delayed release of [(3)H]-taurine by applying dopamine (0.8 mM) after the depolarizing stimulus was blocked by 10 muM (+)-butaclamol but not by 10 muM (-)-butaclamol.8 The results are discussed with respect to the possible neurotransmitter role for dopamine within the rat retina, and its possible interaction with glycine and taurine.