Flowchart of four study groups. Forty-six mice were studied per group. The animals were prepared surgically under general anesthesia 3 days before anticipated delivery. S-L (saline-lidocaine) is the control group. Group S-L had a subcutaneous pump that extruded saline and 100 μl 4% lidocaine placed in the vagina as a control for vaginal manipulation that was required for group S-C (salinecapsaicin). Group M1-L (morphine 0.1 mg/kg/h -lidocaine) had a subcutaneous pump that extruded low dose morphine (0.1 mg/kg/h) before and during delivery and 100 μl 4% lidocaine was placed in the vagina. M5-L (morphine 0.5 mg/ kg/h-lidocaine) had 0.5 mg/kg/h morphine extruded from a subcutaneous pump before and during delivery and 100 μl 4% lidocaine 3 days before delivery in the vagina. Group S-C had saline extruded from the subcutaneous pump and intravaginal 100 μl 4% lidocaine and 100 μl capsaicin 0.05%. Schematic of four labor pain behaviors: A: Squashing; B: Elongation; C: Arching; D: Lateral contraction