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Pharmacological Research, Vol . 25, Supplement 1, 1992

BIOCHEMICAL AND FUNCTIONAL IDENTIFICATION OF DOPAMINE RECEPTORS IN RAT BROWN ADIPOSE TISSUE Enzo Nisoli, Cristina Tonello, and Michele 0 . Carruba. Department of Biomedical Sciences and Biotechnologies, Section of Pharmacology, Toxicology and Experimental Therapeutics, School of Medicine, University of Brescia, Brescia, Italy .

High dopamine (DA) concentration is present in the rat brown adipose tissue (BAT), however until now no definitive data have been reported about the presence, the type and the function of DA receptors in this tissue. The present work was designed to fill this gap . To this purpose, the dorsal interscapular BAT was removed, homogenized and used for either membrane preparations for binding studies, according to Levin and Sullivan (1986), or adenylate cyclase (AC) activity measurement, according to Clement-Cormier et al . (1972) . Measurements of lipid metabolites release from brown adipocytes were performed according to Bukowiecki et al . (1980), for functional studies . It was found that DA receptor agonists, bromocriptine, (-)propylnorapomorphine (NPA), lisuride, apomorphine and quinpirole, inhibited both basal (IC50 values are reported in Table 1) and (-)isoproterenol-stimulated AC activity in BAT homogenates . These inhibitory effects were antagonized by DA receptor antagonists, such as haloperidol and butaclamol, but not by the (2adrenoceptor antagonist, yohimbine, or the serotonin receptor antagonist, ketanserin . The selective D1 agonist, fenoldopam, was unable to modify the AC activity . Furthermore, DA and various DA agonists (lisuride, NPA, bromocriptine, apomorphine, quinpirole) were able to reduced the AC activity stimulated by forskolin . Lisuride, as an example, inhibited forskolinstimulated AC activity with an IC50 of 20 µ M and its action was antagonized by 100 ,a M haloperidol . It was also found that DA and various DA receptor agonists, induced a significant decrease of the release of glycerol and non-esterified fatty acids from brown adipocytes stimulated by the B -adrenoceptor agonist (-)isoproterenol (IC50 values are shown in Table 1) . This effect was selectively antagonized by haloperidol and butaclamol . [ 3H]-Spiroperidol bound in a saturable, specific, andreversible manner to BAT membranes with a KD of 26 nM and a Bmax of 312 fmol/mg prot) . This binding was demonstrated to be selectively displaced by DA agonists and antagonists with a rank order of potency pharmacologically consistent with receptors dopaminergic in nature . The above results indicate that the DA receptors present in the BAT belong to the family of D2 DA receptors because of their main pharmacological characteristics and because negatively coupled to AC, however, they differ from classical D2 in their lower affinity and lack of

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© 1992 The Italian Pharmacological Society

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Pharmacological Research, Vol . 25, Supplement 1, 1992 stereoselectivity for agonists and antagonists . DA, by activating these atypical DA receptors, is able to lower the efficacy of the brown fat cells metabolism stimulation induced by the sympAthetic system, thus partecipating in thermogenesis control .

TABLE 1

Dopamine agonists inhibit adenylate cyclase activity and (-)isoproterenol-induced glycerol release in homogenates or in dispersed cells, respectively, of rat brown adipose tissue (BAT) .

----------------------------------------------------------------------------------------------------------------IC50 (p .. M) Drugs

Adenylyl cyclase

Glycerol Release

---------------------------------------------------------------------------------------------------------------Lisuride

2 .4

3 .8

(-)NPA

2 .7

2 .4

Bromocriptine

6 .3

8 .4

Quinpirole Apomorphine

12 .3

9 .8

A14 .1

15 .3

-----------------------------------------------------------------------------------------------------------------Values are the mean ± S .E.M . of at least three experiments runned in triplicate.

Bukowiecki, L ., Follda, N ., Paradis, A ., and Collet, A . : Stereo- specific stimulation of brown adipocyte respiration by catecholamines via B 1-adrenoreceptors . Am . J . Physiol . 238 : E552-E563, 1980 . Clement-Cormier, Y .C ., Kebabian, J.W ., Petzold, G .L ., Greengard, P . : Dopamine -sensitive adenylate cyclase in mammalian brains : a possible role of action of antipsychotic drugs . Proc . Natl . Acad . Sci ., U .S .A . 71 : 155-162, 1974 . Levin, B .E . and Sullivan, A .C . : Beta-1 receptor is the predominant Beta-adrenoreceptor on rat brown adipose tissue . J . Pharmacol . Exp . Ther . 236: 681-688, 1986 .

Biochemical and functional identification of dopamine receptors in rat brown adipose tissue.

91 Pharmacological Research, Vol . 25, Supplement 1, 1992 BIOCHEMICAL AND FUNCTIONAL IDENTIFICATION OF DOPAMINE RECEPTORS IN RAT BROWN ADIPOSE TISSU...
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