The possibility of change in opioid receptors after pretreatment of opioids and their antagonists in vivo and in vitro was examined. Acute administration of opioid agonists and antagonists induced increase in opioid receptor binding to mouse brain membranes. Naloxone was the most potent in enhancing receptor bindings. At 10 mg/Kg, naloxone induced increase in 3H-dihydromorphine (3H-DHM), 3H-D-Ala2,D-Leu5-enkephalin and 3H-ethylketocyclazocine bindings 90, 30 and 40 percent, respectively, while morphine was less potent in eliciting these increases than naloxone. Scatchard analysis revealed that number of binding site increased without altering affinity in naloxone injected mice. On the other hand, there was no change in 3H-DHM binding between saline and naloxone injected mice after membrane fractions were dialysed with 100 mM NaCl-con-taining buffer solution. Furthermore, treatment in vitro of membrane with 10−7M naloxone or morphine also induced increase in 3H-DHM binding 38 and 20 percent, respectively, whereas D-Ala2,D-Leu5-enkephalin (DADLE) treatment enhanced 3H-DADLE binding 20 percent.
