Oxytocin-Induced Paw Sucking in Infant Rats

In addition to the role of oxytocin in affiliative behaviors of adult mammals, some evidence also indicates that oxytocin may modulate the affiliative behavior of mammalian infants. Pharmacological and comparative studies have indicated that central oxytocin may attenuate the production of ultrasonic vocalizations in isolated rodents.'.2 Recently it was reported that in infant rats, central oxytocin antagonism blocked the acquisition of a preference for a maternally associated odor.3 Understanding the behavioral significance of oxytocin in infant rats may provide insights into the developmental establishment of affiliative behaviors. We report here a behavioral phenomenon, paw sucking, that we have observed in infant rats following central oxytocin administration. Ten-day-old pups (n = 27, from 6 litters) were removed from the nest and injected intracranially with saline solution, oxytocin (50 ng), or the oxytocin antagonist OTA (500 ng). Injections in a volume of 5 pl were made directly through the uncalcified skull. Pups were then placed in a glass dish for 10 minutes. For the first 5 minutes, pups were not disturbed. For the second half of the test session, pups were intermittently stroked with a cotton swab directed at the side of the face and vibrissae. Pups were videotaped from a camera placed underneath the dish during the test period. These tapes were later analyzed for oral behaviors. A two-variable ANOVA for drug treatment and stimulation revealed a significant drug effect on paw sucking (F (2,24) = 5.5, p < 0.05) and a significant stimulation effect on mouthing (F (1,24) = 16.4, p c 0.001). A significant drug effect was also evident on grooming, but only during the stimulation phase (F (2,24) = 7.0, p c 0.01). In the adult rat, many affiliative behaviors such as licking, grooming, nesting, and retrieving young engage the perioral region, and many of these behaviors are thought to be mediated by central oxytocinergic projections. In the rat pup, affiliative behavior likewise involves a great deal of perioral activity. Suckling, probing, and even huddling contain strong perioral components. The central administration of oxytocin induced an array of oral behaviors in 10-day-old rats, and the central administration of an oxytocin antagonist reduced oral behaviors. These drug effects were most pronounced during somatosensory stimulation. These results indicate that the oxytocinergic substrates for oral behaviors and perhaps the oxytocinergic substrates for certain affiliative behaviors develop at an early age in rats. Furthermore