Effect of naloxone on the long-term body weight gain induced by repeated postnatal stress in male mice.

Stress during the early postnatal life of laboratory animals can induce body weight gain in adulthood. Exposure of mice to repeated postnatal stress has been shown to increase body weight some weeks after the recording of a hyposensitivity to thermal nociceptive stimulation. Since the increase in nociceptive threshold after repeated environmental manipulations can be interpreted as a behavioral index of stress-induced endogenous opioid release, the present experiment was designed to study whether the stress-induced body weight gain could be prevented by a chronic treatment with the opioid antagonist naloxone. Sixteen litters of CD-1 male mice were used. Treatments took place from day 2 to day 19 of postnatal life. As well as body weight we measured spontaneous motor activity and food and water consumption. Body length and epididymal fat pad weight were also measured, at the end of the experiment. Stress (i.e. a daily saline injection schedule) increased the rate of body weight gain and the epididymal fat pad weight; a daily injection schedule of naloxone did not induce these effects. No significant differences were evident among groups regarding body length, motor activity, or food and water intake. The data suggest that repeated postnatal saline injections may induce long-term modifications in body and epididymal fat weights and that these effects may be mediated by endogenous opioids.