The early ontogeny of D-amphetamine-induced one-trial behavioral sensitization was characterized using male and female preweanling and preadolescent rats. In Experiment 1, rats were injected with saline or D-amphetamine (1, 4, or 8mg/kg) in activity chambers or the home cage on postnatal day (PD) 12, PD 16, PD 20, or PD 24. One day later, rats were challenged with either 0.5 or 2mg/kg D-amphetamine and distance traveled was measured in activity chambers for 120min. In Experiment 2, saline or D-amphetamine was administered in activity chambers on PD 24, while a challenge injection of D-amphetamine (0.25-4mg/kg) was given on PD 25. At younger ages (PD 13 and PD 17), a strong sensitized response was evident on the test day regardless of whether rats were pretreated with D-amphetamine (4 or 8mg/kg) before being placed in the activity chamber or 30min after being returned to the home cage. Rats did not display D-amphetamine-induced behavioral sensitization on PD 21, nor was context-dependent sensitization apparent on PD 25 even when a broad dose range of D-amphetamine was used. When low doses of D-amphetamine were administered on the pretreatment and test days (1 and 0.5mg/kg, respectively), sensitized responding was not evident at any age. In summary, D-amphetamine-induced one-trial behavioral sensitization was only apparent within a narrow developmental window during early ontogeny. This ontogenetic pattern of sensitized responding is similar to the one produced by methamphetamine and distinct from the pattern produced by cocaine. The unique sensitization profiles resulting from repeated D-amphetamine and cocaine treatment may be a consequence of their different mechanisms of action.
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