Table 1.

Tolerance induced by morphine but not naloxone after MORS196A-CSTA gene transferred locally into spinal cord or vlPAG.

Gene transfer site	Spinal cord		vlPAG
	morphine	naloxone	morphine	naloxone
ED50, 2 weeks after gene transfection	1.55 ± 0.45	8.42 ± 1.23	1.30 ± 0.24	7.49 ± 0.84
ED50, after chronic drug (10 mg/kg, s.c., b.i.d. for 6 days)	7.49 ± 1.16 (4.8 fold)	8.42 ± 1.23 (1.0 fold)	5.96 ± 0.87 (4.6 fold)	8.42 ± 1.23 (1.1 fold)

The ED₅₀ (mg/kg) of drugs to inhibit the tail-flick responses were determined 2 weeks after gene transfer into spinal cord or vlPAG by the up-down method. After chronically treated with drugs (10 mg/kg, s.c., b.i.d. for 6 days), animals were tested for the ED₅₀ of drugs again. The values represent the mean ± S.E.M. (n= 6–8)