Table 1.

Summary of groups, subgroups, treatments and tests in our experiment

Group and subgroup	Treatment and test
Sham	1. Hot plate test: base record 15 min before injection;
	2. Morphine injection: single dose (10 mg/kg);
	3. Hot plate test: 5 times (15 min interval after injection), MPE calculation.
Sham-Tol-Sal	1. Morphine injection for inducing tolerance: 30 mg/kg, 3 times/d, for 3 d;
	2. Co-treatment with morphine: saline (10 ml/kg), 3 times/d, for 3 d;
	3. Hot plate test: base record 15 min before injection;
	4. Morphine injection: single dose (10 mg/kg);
	5. Hot plate test: 5 times (15 min interval after injection), MPE calculation.
Sham-Tol-LA	1. Morphine injection for inducing tolerance: 30 mg/kg, 3 times/d, for 3 d;
	2. Co-treatment with morphine: L-arginine (50 mg/kg), 3 times/d, for 3 d;
	3. Hot plate test: base record 15 min before injection;
	4. Morphine injection: single dose (10 mg/kg);
	5. Hot plate test: 5 times (15 min interval after injection), MPE calculation.
OVX	1. Hot plate test: base record 15 min before injection;
	2. Morphine injection: single dose (10 mg/kg);
	3. Hot plate test: 5 times (15 min interval), MPE calculation.
OVX-Tol-Sal	1. Morphine injection for inducing tolerance: 30 mg/kg, 3 times/d, for 3 d;
	2. Co-treatment with morphine: saline (10 ml/kg), 3 times/d, for 3 d;
	3. Hot plate test: base record 15 min before injection;
	4. Morphine injection: single dose (10 mg/kg);
	5. Hot plate test: 5 times (15 min interval after injection), MPE calculation.
OVX-Tol-LA	1. Morphine injection for inducing tolerance: 30 mg/kg, 3 times/d, for 3 d;
	2. Co-treatment with morphine: L-arginine (50 mg/kg), 3 times/d, for 3 d;
	3. Hot plate test: base record 15 min before injection;
	4. Morphine injection: single dose (10 mg/kg);
	5. Hot plate test: 5 times (15 min interval after injection), MPE calculation.