Morphine-induced opioid receptor down-regulation detected in intact adult rat brain cells

Intact brain cells dissociated from the brains of adult rats were used to study the regulation of opioid receptors by in vivo chronic morphine treatment. When the specific binding of [³H]naloxone in a physiological iso-osmotic buffer was compared in cells obtained from sham-operated rats and in those treated for 6 days with four (2 × 2) 75 mg morphine pellets, it was found that morphine treatment resulted in a significant reduction in the density of [³H]naloxone binding sites. This receptor down-regulation was not accompanied by a change in the receptor affinity for the ligand. This effect of morphine was reversed upon the removal of morphine pellets for 18 h after tolerance induction. When similar experiments were performed using brain homogenates prepared and assayed in the same physiological buffer, there was an increase in the number of [³H]naloxone binding sites in morphine-treated animals compared to controls. On the other hand, when the binding experiments were conducted in 50 mM Tris-HCl buffer, no difference in ligand binding was apparent between control and morphine-treated groups. The present results demonstrate opioid receptor down-regulation by chronic morphine treatment measured in intact brain cells, and suggest that different conclusions may be reached when other tissue preparations are used to assess the receptor density.