Possible involvement of a hypothalamic dopaminergic receptor in development of genetic obesity in mice

We have studied the binding of the dopaminergic agonist 2-[5,8-3H]amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene [( 3H]ADTN) to hypothalamic membranes from the genetically diabetic-obese (db/db) mice and their lean littermates. The specific binding of [3H]ADTN was defined as the difference between the radioligand binding to the membranes in the absence or presence of 1 microM (+)-butaclamol. In order to control nonspecific binding, all binding assays were performed in the presence of 0.1 mM catechol and 0.3 mM ascorbic acid. Binding of [3H]ADTN was rapid, dissociable, saturable and stereoselective. (+)-Butaclamol was very potent whereas (-)-butaclamol was ineffective in inhibiting the binding of this radioligand. Concentration-dependent binding experiments and Scatchard analysis of the data yielded dissociation constant (Kd) of 3.5-4.2 nM and number of binding sites (n) equivalent to 170-200 fmol/mg protein for lean mice. For db/db mice, the data yielded a Kd of 4.0-4.7 nM and an n of 400-500 fmol/mg protein. It was also shown that the anorexic drugs, amphetamine and fenfluramine, inhibited [3H]ADTN binding in a dose-dependent manner. Binding parameters, obtained using membranes from mice made obese by parenteral administration of gold thioglucose, were not significantly different from those obtained for the lean mice. It is concluded that the regulation of the hypothalamic dopaminergic receptors may be related to the lesion in the genetically obese mice.