Naltrexone does not increase brain and body development in rats

Two strains of preweanling rats (88 Long-Evans; 120 Sprague-Dawley) were given daily SC injections of either 50 mg/kg naltrexone or the equivalent volume of saline, from birth to postnatal day 21. Brain and body weights were assessed at the end of the injections, and in Long-Evans animals, significant differences were noted on both measures with the outcome favoring the control animals. No significant differences were observed between the naltrexone and control Sprague-Dawley rats. These results disconfirm the hypothesis that increased postnatal growth is accomplished by prolonged early postnatal opioid receptor blockade.