Figure 5. Novel-Object Recognition Test.

Graphs illustrate interaction time (sec) during two phases of the Novel-Object Recognition test performed at a younger age (PD31; Panel A) and later in adulthood (PD55-56; Panel B). Animals were initially treated with morphine sulfate (10mg/kg sc twice-daily) or equivalent volume of saline from PD1-14. During the ‘training session,’ animals interacted with each of the two identical objects (familiar objects) for equal amount of time both at the young age (PD31; N=10 for saline group; N=7 for morphine group; Panel A) and in adulthood (PD55-56; N=17 for saline group; N=21 for morphine group; Panel B). In a second phase (‘testing session after 1hr’), one of the familiar objects was replaced by a novel object. At a young age, both saline and morphine-treated animals spent significantly more time interacting with the novel object. In adulthood, althought both saline and morphine groups interacted longer with a novel object, only morphine group reached significance. Panels A′ and B′ illustrate discrimination ratio (the time spent with the novel object divided by total interaction time). During the training session, a discrimination ratio of 0.5 indicates that animals spent equal amounts of time with both objects, whereas values greater than 0.5 during the testing session after 1 hr indicate that animals interacted more with the novel object. We report significantly increased discrimination ratio for both young (Fig. A′) and adult animals (Fig. B′). Although morphine treated animals discriminated between the familiar and the novel object at both ages, calculated discrimination ratio was significantly higher in saline treated young animals (Fig. A′). This difference disappeared in adulthood (Fig. B′). One-way ANOVA with Tukey HSD test; *, p<0.05; **, p<0.001.