Table 1.

Tolerance induced by morphine but not naloxone in MOR-KO mice that had been transfected with the dsAAV2-MORS196A-EGFP gene

	ED50 of morphine (mg/kg)	ED50 of naloxone (mg/kg)
Acute effect	4.4 ± 1.0	8.4 ± 1.2
Sub-chronic effect	7.2 ± 1.1 (×1.63)	8.4 ± 1.2

The ED₅₀ values of morphine and naloxone that inhibit the tail-flick responses were determined using the up-and-down method 3 weeks after gene transfer. The mice were then sub-chronically treated with morphine (10 mg/kg) or naloxone (10 mg/kg) subcutaneously twice a day for 4 days. ED₅₀ values were then redetermined. Data are means ± the standard error of the mean (SEM) (n = 6).