Quantitative autoradiography of adenosine receptors in brains of chronic naltrexone-treated mice

1. Manipulation of micro opioid receptor expression either by chronic morphine treatment or by deletion of the gene encoding micro opioid receptors leads to changes in adenosine receptor expression. Chronic administration of the opioid receptor antagonist naltrexone leads to upregulation of micro receptor binding in the brain. 2. To investigate if there are any compensatory alterations in adenosine systems in the brains of chronic naltrexone-treated mice, we carried out quantitative autoradiographic mapping of A(1) and A(2A) adenosine receptors in the brains of mice treated for 1 week with naltrexone (8 mg(-1) kg(-1) day(-1)), administered subcutaneously via osmotic minipump. 3. Adjacent coronal brain sections were cut from chronic saline- and naltrexone-treated mice for the determination of binding of [(3)H] D-Ala(2)-MePhe(4)-Gly-ol(5) enkephalin ([(3)H] DAMGO), [(3)H]1,3-dipropyl-8-cyclopentylxanthine ([(3)H] DPCPX) or [(3)H] 2-[p-(2-carbonylethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine ([(3)H] CGS21680) to micro, A(1) and A(2A) receptors, respectively. 4. A significant increase in micro and A(1) receptor binding was detected in chronic naltrexone-treated brains. The changes in micro receptors were significant in several regions, but changes in A(1) were relatively smaller but showed significant upregulation collectively. No significant change in A(2A) receptor binding was detected in chronic naltrexone-treated brains. 5. The results show that blockade of opioid receptors causes upregulation of A(1) receptors, but not A(2A) receptors, by as yet undefined mechanisms.