Postnatal morphine administration alters hippocampal development in rats

Morphine is frequently used as an analgesic and sedative in preterm infants. Adult rats exposed to morphine have an altered hippocampal neurochemical profile and decreased neurogenesis in the dentate gyrus of the hippocampus. To evaluate whether neonatal rats are similarly affected, rat pups were injected twice daily with 2 mg/kg morphine or normal saline from postnatal days 3 to 7. On postnatal day 8, the hippocampal neurochemical profile was determined using in vivo 1H NMR spectroscopy. The mRNA and protein concentrations of specific analytes were measured in hippocampus, and cell division in dentate gyrus was assessed using bromodeoxyuridine. The concentrations of γ-aminobutyric acid (GABA), taurine, and myo-insotol were decreased, whereas concentrations of glutathione, phosphoethanolamine, and choline-containing compounds were increased in morphine-exposed rats relative to control rats. Morphine decreased glutamic acid decarboxylase enzyme levels and myelin basic protein mRNA expression in the hippocampus. Bromodeoxyuridine labeling in the dentate gyrus was decreased by 60–70% in morphine-exposed rats. These results suggest that recurrent morphine administration during brain development alters hippocampal structure. © 2011 Wiley Periodicals, Inc. Critically ill preterm infants are frequently treated with prolonged courses of opiates to decrease pain and stress (Anand et al.,2004). The efficacy of such treatment is debated, but the clinical practice persists (Franck et al.,2000; Simons et al.,2003; Carbajal et al.,2005; Cignacco et al.,2008), because the effects of untreated pain are well established (Anand,2000; Duric and McCarson,2006). There are increasing concerns that opiates may have detrimental effects on neurodevelopmental outcomes. Neonatal morphine treatment with and without stress is associated with short-term changes in gene expression and cellular composition in the hippocampus (Vien et al.,2009; Juul et al.,2011) and long-term neurobehavioral deficits in rodents (McPherson et al.,2007; Boasen et al.,2009). In adult rats, the neurochemical profile of the hippocampus is altered during morphine administration (Corrigall,1983; Simonato,1996; Gao et al.,2007), and hippocampus-mediated learning is impaired (Spain and Newsom,1991; Bhutta et al.,2001), possibly because of decreased neurogenesis in the dentate gyrus of the hippocampus (Eisch et al.,2000; Lledo et al.,2006). To evaluate the safety of morphine for sedation in the absence of pain, we used a neonatal rat model of morphine administration (McPherson et al.,2007). We hypothesized that neonatal morphine administration would alter the neurochemical profile of the developing hippocampus and decrease neurogenesis in the dentate gyrus. The metabolites indexing neuronal and glial integrity, energy substrates and energy sufficiency, phospholipid biosynthesis, and amino acids and neurotransmitters in the developing hippocampus were assessed using high-field in vivo 1H NMR spectroscopy, followed by evaluation of mRNA and protein expression of relevant analytes in the hippocampus. We assessed cell proliferation in the dentate gyrus using bromodeoxyuridine (BrdU) histochemistry and found that rat pups exposed to recurrent morphine administration had an altered neurochemical profile, decreased glutamic acid decarboxylase (GAD) and myelin basic protein (MBP) expressions in the hippocampus, and decreased incorporation of BrdU in the dentate gyrus.