Inhibition of the nociceptive response during the Transport Response. (A, B) The pups’ responses to the noxious tail pinch at PND 13. The tail was pinched by an artery clip with 160 g (A) or 200 g (B) of force during the undisturbed condition (UD), after a 10-s immobilization between the experimenter’s fingers on the floor (Touching) or after a 10-s immobilization by manual carrying (Carrying). (C–E) The length of immobilization time during manual carrying (C), the representative posture during manual carrying (D), and the pups’ responses to the tail pinch (E) in wild-type (+/+), heterozygous (+/–), and homozygous (–/–) Oprm knockout mice at PND 13. (F) The responses of C57BL/6 pups to the tail pinch after a 10-s immobilization by manual carrying in pups that received no injection (NI), a saline (Sal) injection, or a naloxone (Nx) injection. The number of tested pups is provided in the parentheses. **p < 0.01.