Modulation of adrenal catecholamine release by DA2 dopamine receptors in the anaesthetized dog

Roles of dopaminergic d(1) and d(2) receptors in catecholamine release from the rat adrenal medulla

The aim of the present study was designed to establish comparatively the inhibitory effects of D(1)-like and D(2)-like dopaminergic receptor agonists, SKF81297 and R(-)-TNPA on the release of catecholamines (CA) evoked by cholinergic stimulation and membrane depolarization from the isolated perfused model of the rat adrenal medulla. SKF81297 (30microM) and R-(-)-TNPA (30microM) perfused into an adrenal vein for 60 min, produced great inhibition in the CA secretory responses evoked by ACh (5.32x10(- 3) M), DMPP (10(-4) M), McN-A-343 (10(-4) M), high K(+) (5.6x10(-2) M), Bay-K-8644 (10microM), and cyclopiazonic acid (10microM), respectively. For the release of CA evoked by ACh, high K(+), DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid, the following rank order of inhibitory potency was obtained: SKF81297>R-(-)-TNPA. However, R(+)-SCH23390, a selectve D(1)-like dopaminergic receptor antagonist, and S(-)-raclopride, a selectve D(2)-like dopaminergic receptor antagonist, enhanced the CA secretory responses evoked by ACh, high K(+), DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid only for 0~4 min. The rank order for the enhancement of CA release evoked by high K(+), McN-A-343 and cyclopiazonic acid was R(+)-SCH23390>S(-)-raclopride. Also, the rank order for ACh, DMPP and Bay-K-8644 was S(-)-raclopride > R(+)-SCH23390. Taken together, these results demonstrate that both SKF81297 and R-(-)-TNPA inhibit the CA release evoked by stimulation of cholinergic (both nicotinic and muscarinic) receptors and the membrane depolarization from the isolated perfused rat adrenal gland without affecting the basal release, respectively, but both R(+)-SCH23390 and S(-)-raclopride facilitate the CA release evoked by them. It seems likely that the inhibitory effects of SKF81297 and R-(-)-TNPA are mediated by the activation of D(1)-like and D(2)-like dopaminergic receptors located on the rat adrenomedullary chromaffin cells, respectively, whereas the facilitatory effects of R(+)-SCH23390 and S(-)-raclopride are mediated by the blockade of D(1)-like and D(2)-like dopaminergic receptors, respectively: this action is possibly associated with extra- and intracellular calcium mobilization. Based on these results, it is thought that the presence of dopaminergic D(1) receptors may play an important role in regulation of the rat adrenomedullary CA secretion, in addition to well-known dopaminergic D(2) receptors.

Functional and autoradiographic characterization of dopamine D2-like receptors in the guinea pig heart

Dopamine receptors include the D1- (D1 and D5 subtypes) and D2-like (D2, D3, and D4 subtypes) families. D1-like receptors are positively and D2-like receptors negatively coupled to the adenylyl cyclase. Dopamine D2-like (D4 subtype) receptors have been identified in human and rat hearts. However the presence of D2 and D3 receptor subtypes is unclear. Furthermore, their role in cardiac functions is unknown. By autoradiographic studies of guinea pig hearts, we identified D3 and D4 receptors, using the selective radioligands [3H]-7-OH-DPAT and [3H]emonapride (YM-09151-2 plus raclopride). Western blot analysis confirmed D3 and D4 receptors in the right and left ventricle of the same species. Selective agonists of D3 and D4 receptors (+/-)-7-OH-DPAT and PD 168 077 (10(-9) to 10(-5) M, respectively) induced a significant negative chronotropic and inotropic effect in the isolated guinea pig heart preparation. Negative inotropic effect induced by PD 168 077 was associated with an inhibition in cyclase activity. No changes in cyclase activity were found with (+/-)-7-OH-DPAT. The aim of this study is to support the presence of D3 and D4 receptors in the heart. Although our results suggest that D3 and D4 receptors are functionally active in the heart, we need additional information with an antagonist and an agonist of improved potency and selectivity to understand the respective roles of D3 and D4 receptors in the cardiac functions.

Polymorphism in exon 6 of the dopamine D(2) receptor gene (DRD2) is associated with elevated blood pressure and personality disorders in men

A deficient dopamine D(2) receptor (DA2) formation or action may contribute to hypertension via an increase of catecholamine release. In addition, Axis II personality disorders that appears odd or eccentric (cluster A) is associated with a low density of DA2. This study sought to examine if a NcoI restriction fragment length polymorphism (C to T transition) in exon 6 of the dopamine D(2) receptor gene (DRD2) was associated with these characteristics. The genotypes (CC, CT and TT) were compared in anthropometric, endocrine, metabolic and haemodynamic variables as well as estimates of personality disorders in 284 randomly selected 51-year-old men. Homozygotes for the C allele constituted 49% of the men and homozygotes for the T allele 9%, while heterozygotes were 41%. The TT genotype was associated with elevated systolic and diastolic blood pressure, independent of obesity and endocrine abnormalities, including the hypothalamic-pituitary-adrenal axis regulation. Moreover, the TT genotype was significantly more frequent among subjects with grade 1 (mild) hypertension (>140/90 mm Hg) compared to normotensive subjects (<130/85 mm Hg). The polymorphism in exon 6 of the DRD2 was also significantly associated with cluster A personality disorders. These results suggest that a polymorphism in exon 6 of the DRD2examined with the restriction enzyme NcoI is associated with an elevated blood pressure, independent of obesity. Paranoid or schizoid personality disorders is also associated with a polymorphism of the DRD2, which might be associated with a previously demonstrated low density of this receptor.

Dopamine receptors: from structure to function

The diverse physiological actions of dopamine are mediated by at least five distinct G protein-coupled receptor subtypes. Two D1-like receptor subtypes (D1 and D5) couple to the G protein Gs and activate adenylyl cyclase. The other receptor subtypes belong to the D2-like subfamily (D2, D3, and D4) and are prototypic of G protein-coupled receptors that inhibit adenylyl cyclase and activate K+ channels. The genes for the D1 and D5 receptors are intronless, but pseudogenes of the D5 exist. The D2 and D3 receptors vary in certain tissues and species as a result of alternative splicing, and the human D4 receptor gene exhibits extensive polymorphic variation. In the central nervous system, dopamine receptors are widely expressed because they are involved in the control of locomotion, cognition, emotion, and affect as well as neuroendocrine secretion. In the periphery, dopamine receptors are present more prominently in kidney, vasculature, and pituitary, where they affect mainly sodium homeostasis, vascular tone, and hormone secretion. Numerous genetic linkage analysis studies have failed so far to reveal unequivocal evidence for the involvement of one of these receptors in the etiology of various central nervous system disorders. However, targeted deletion of several of these dopamine receptor genes in mice should provide valuable information about their physiological functions.

Autoradiographic localization of dopamine D2-like receptors in the rat adrenal gland

The pharmacological profile and the anatomical localization of dopamine D2-like receptors were studied in sections of the rat adrenal gland using combined radioligand binding and autoradiographic techniques with [3H]-spiroperidol as a ligand. [3H]-Spiroperidol was bound to sections of the rat adrenal gland in a manner consistent with the labelling of dopamine D2-like receptor sites. The binding was time-, temperature- and concentration-dependent and of high affinity with a dissociation constant (Kd) value of 1.6 +/- 0.04 nM and a maximum density of binding sites (Bmax) of 60 +/- 3.6 fmol/mg tissue. Experiments on the pharmacological specificity of [3H]-spiroperidol binding to sections of the rat adrenal gland suggest the labelling of dopamine D3 and/or D4 receptors. The presence of dopamine D3 and D4 receptors in the rat adrenal gland was confirmed by the demonstration of a specific binding for the D3 radioligand [3H]-7-hydroxy-N,N-di-n-propyl-2-aminotetralin (DPAT) and for the D4 radioligand [3H]-clozapine. Light microscope autoradiography showed the highest accumulation of silver grains which correspond to [3H]-spiroperidol binding sites in the rat adrenal medulla. In the adrenal cortex, where density of silver grains is about 40% lower than in the medulla, the radioligand is accumulated primarily in the zona glomerulosa and to a lesser extent in the zona reticularis. These findings suggest that dopamine D2-like receptor sites in the rat adrenal gland cortex are primarily involved in the modulation of catecholamine secretion from the medulla and of aldosterone secretion from the cortex. The possible relevance of the occurrence of dopamine D3 and D4 receptor subtypes in the adrenal gland is discussed.

Acute dopaminergic influence on plasma adrenaline levels in the rat

This study was aimed at in vivo characterisation of the possible role of dopamine receptors in the modulation of adrenaline release from the adrenal medulla in rats. Quinpirole (0.3, 1 and 3 mg/kg s.c., 30 min), an agonist at dopamine D2-like receptors induced a statistically significant increase not only in adrenal dopamine but also in plasma and heart adrenaline levels. The effects of the lowest dose of quinpirole were blocked by domperidone (5 mg/kg s.c., 150 min). Implantation of catheters followed by blood sampling appeared to be a stressful procedure, inducing itself an elevation of adrenal dopamine and of heart adrenaline by 100 and 250%, respectively. To explore the possibility of determining the plasma levels of adrenaline without blood sampling, regression modelling was performed by means of partial least squares regression (PLS) using treatment and levels of heart adrenaline and adrenal dopamine as predictor variables. The selected variables were found to be good predictors of plasma adrenaline levels. Accordingly, the increase in adrenal dopamine and heart adrenaline levels following administration of the dopamine autoreceptor agonist, talipexole, and the classical non-selective dopamine receptor agonist, apomorphine, were interpreted as indicators of the increased adrenomedullary adrenaline release. Neither of the dopamine D2 receptor antagonists used, i.e. domperidone, supposed to have only peripheral effects, nor raclopride, had significant effects on adrenal dopamine and heart adrenaline. Our results support the presence of peripherally located dopamine D2-like receptors, capable of acutely stimulating not only the synthesis of catecholamines, but also the release of adrenaline from adrenals in the conscious rat.