Adult behavior and adrenocortical function following neonatal morphine treatment in rats |
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Authors: | Theo Sonderegger Emery Zimmermann |
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Affiliation: | (1) Department of Anatomy and Brain Research Institute, School of Medicine, UCLA, 90024 Los Angeles, California, USA;(2) Present address: Department of Psychology, University of Nebraska, Lincoln, Nebraska, USA |
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Abstract: | Adult behavioral and endocrine effects of neonatal administration of morphine (M) were studied in female rats injected s.c. with M on either days 3–12 (M1) or days 12–21 (M2). The dose given twice daily was increased to 8 mg/kg in group M1 and to 16 mg/kg in group M2. Compared to saline-treated controls, growth rates were temporarily suppressed (P<0.05) and body weights were reduced (P<0.05) util day 20 in M1 and until day 64 in M2 rats. The open-field test performed on days 29–31 failed to differentiate between neonatal treatment groups. On days 90–95 M1 but not M2 animals showed impaired learning of a conditioned emotional responses (CER). On day 40 all groups showed similar increased levels of plasma corticosterone 30 min following injection of naloxone (5 mg/kg). Compared to controls on day 156, both M1 and M2 rats showed diminished (P<0.05) analgesic responses (hot-plate test) to M (10 mg/kg). In response to M challenge (40 mg/kg) on day 170, all groups showed comparable acute increases in plasma corticosterone levels. These findings provide further evidence that early exposure to M results in growth retardation and protracted tolerance and that, depending on dose and time of exposure, neonatal M may result in impaired learning of CER in adulthood. These effects suggest that early morphine can produce long-lasting alterations of learned behavior without lasting impairment of pituitary-adrenal function. |
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Keywords: | Morphine Neonatal addiction Protracted tolerance Growth Analgesia Conditional emotional response Corticosterone Open field |
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