Gentle stroking stimuli induce affiliative responsiveness to humans in male rats

Gentle tactile stimuli have been shown to play an important role in the establishment and maintenance of affiliative social interactions. Oxytocin has also been shown to have similar actions. We investigated the effects of gentle stroking on affiliative relationships between humans and rats and the effects of gentle stroking on activation of oxytocin neurons. Male rats received 5-min stroking stimuli from an experimenter every other day for 4 weeks between 3 and 6 weeks of age (S3-6 group), for 4 weeks between 7 and 10 weeks of age (S7-10 group), or for 8 weeks between 3 and 10 weeks of age (S3-10 group). Control rats did not receive stroking stimuli. Rats in the S7-10 and S3-10 groups emitted 50-kHz calls, an index of positive emotion, more frequently during stroking stimuli. Rats in the S3-6, S7-10, and S3-10 groups showed affiliative behaviors toward the experimenter. Oxytocin neurons in the hypothalamic paraventricular nucleus of rats in the S3-6, S7-10, and S3-10 groups were activated following stroking stimuli. These findings revealed that post-weaning repeated stroking stimuli induce an affiliative relationship between rats and humans and activation of oxytocin neurons.