Comparison of locomotor activity between offspring groups in response to a novel environment.

<p>Locomotor activity is shown on days 1–3, after saline or morphine administration on days 4–8 (10 mg/kg, s.c.), and after a final morphine challenge on day 16 (5 mg/kg, s.c.). A: Total activity scores in the vertical plane and B: in the horizontal plane, reveal that habituation was greater among naltrexone-exposed offspring. C: Total horizontal locomotor activity did not differ between groups treated with saline and challenged with morphine (saline-morphine). D: Total horizontal locomotor activity did not differ between groups treated with morphine, although sensitization was expressed in both groups after challenge with morphine (morphine-morphine). E: Analysis of data expressing total morphine-induced locomotor activity as a function of basal locomotor activity on day 3 revealed significantly greater activity values for naltrexone-exposed offspring, indicating an increased development of sensitization. Data are expressed as vertical plane entries (A) or horizontal distance moved in centimetres (B, C, D and E) and represent the mean ± SEM (one-way ANOVA: * <i>p</i><0.05, ** <i>p</i><0.01, *** <i>p</i><0.001; days 1–3 (A and B): placebo: n = 37, naltrexone: n = 53; days 4–16 (C): placebo: n = 12, naltrexone: n = 19; days 4–16 (D and E): placebo: n = 20, naltrexone: n = 26).