Effects of continuous opioid receptor blockade on alcohol intake and up-regulation of opioid receptor subtype signalling in a genetic model of high alcohol drinking

Abstract

Effects of a continuous naloxone infusion via osmotic pumps on alcohol drinking and opioid receptor density and function in the high-drinking AA (Alko, Alcohol) rats were examined. AA rats were trained to drink 10% (v/v) ethanol in a 1-h limited access procedure and implanted with subcutaneous osmotic pumps delivering either saline, a low dose (0.3 mg/kg per hour), or a high dose (3.0 mg/kg per hour) of naloxone for 7 days. The pumps were then removed and alcohol, food and water intakes were measured for another 4 days. Compared with saline, both naloxone doses significantly suppressed 1-h alcohol intake during the 7-day infusion. The suppression was smaller than that by a bolus injection of the same daily dose 15 min before the session, although a complete blockade of morphine-induced antinociception was achieved even with the smaller naloxone infusion. Significant decreases were also seen in daily food and water intake during the first days, but they quickly returned to their previous baselines. After pump removal, rats of both naloxone-treated groups rapidly increased their alcohol drinking and reached the pretreatment baseline, while their food and water intakes significantly surpassed their baselines. Naloxone infusion at 3.0 mg/kg per hour for 7 days significantly decreased 24-h alcohol drinking without affecting alcohol preference. Twenty-four hours after pump removal, autoradiography with [3H]DAMGO, [3H]DPDPE and [3H]U-69,543 revealed an up-regulation of mu-, delta- and kappa-opioid receptor binding sites in many brain areas of these animals. This receptor up-regulation was functional, because receptor coupling to G-protein activation was enhanced by agonist ligands, as revealed by [35S]GTPgammaS autoradiography. A good correlation existed between ligand binding densities and G-protein activation for mu- and kappa-receptors in control and naloxone-treated brain sections. Furthermore, morphine-induced analgesia in a hot-plate test showed a leftward shift in the morphine dose-response curve after naloxone treatment. These results suggest that the usefulness of a chronic opioid antagonist dosing regime could be limited by nonspecific effects of the antagonist on ingestive behaviour, an up-regulation of opioid receptors with high antagonist doses, and the resulting supersensitivity to opioid agonists after the discontinuation of the treatment.

Comparison between δ‐opioid receptor functional response and autoradiographic labeling in rat brain and spinal cord

“…In this respect, a particularly strong correlation has been reported for both (r ϭ 0.90) and (r ϭ 0.98) opioid receptors in rat brain (Hyytia et al, 1999). However, in an analogous study of DOR using DPDPE as both radioligand and agonist, only a tenuous correspondence was seen (r ϭ 0.61, nonsignificant; Hyytia et al, 1999). In the present study, radioligand binding revealed a highly significant Third, efficiency of coupling between DOR and G-proteins may be regionally heterogeneous, as has been proposed for mouse DOR (Oakley et al, 2003) and for rat MOR (Sim-Selley et al, 2000).…”

“…In the main experiment, the sections were then incubated in assay buffer in the presence of a nonsaturating concentration of 0. Hyytia et al (1999). Sections were thawed at room temperature and rehydrated for 20 minutes in assay buffer containing 50 mM Tris-HCl, 5 mM MgCl 2 , 100 mM NaCl, and 1 mM EDTA (pH 7.4).…”

Morphine-Induced Changes in δ Opioid Receptor Trafficking Are Linked to Somatosensory Processing in the Rat Spinal Cord

“…Slides were stored at Ϫ80°C until additional use. Agoniststimulated GTP␥S (a nonhydrolysable analog of GTP) binding was assessed using a procedure adapted from Hyytia et al (1999). Briefly, sections were thawed to room temperature and rehydrated for 20 min in assay buffer containing 50 mM Tris HCl, 5 mM MgCl 2 , 100 mM NaCl, and 1 mM EDTA, pH 7.4.…”

δ-Opioid Agonists: Differential Efficacy and Potency of SNC80, Its 3-OH (SNC86) and 3-Desoxy (SNC162) Derivatives in Sprague-Dawley Rats

“…Some previous work has demonstrated that ␦-opioid agonist-stimulated GTP␥S binding is greatest in the striatum (Hyytia et al, 1999). SNC86, SNC80, and SNC162 dose dependently stimulated […”