The effects of β-FNA, a highly selective and irreversible μ opioid receptor antagonist, in altering body and brain development in preweaning rats were determined. Animals given β-FNA did not differ from controls in body weights, brain and cerebellar weights, macroscopic dimensions of the brain, the area of the cerebellum, or in organ weight. The dosage of β-FNA utilized (5 mg/kg) blocked morphine-induced analgesia (2 mg/kg morphine sulfate, SC) for each injection period (i.e., 48 hr). In contrast to β-FNA treatment, rats given naltrexone (50 mg/kg SC) in a regimen which completely blocked the opioid receptor throughout ontogeny exhibited marked increases in somatic and neurobiological growth. These results suggest that, in and by themselves, μ receptors selectively antagonized by β-FNA do not play an important role in regulating development.
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