Schematic overview of the pair feeding and time course experiments.

<p>Baseline food intake (FI) and body weight (BW) data were obtained on days −2 and −1. On day 0 rats were subcutaneously implanted with placebo or morphine pellets. Based on the food consumption data of morphine implanted rats, a group of placebo implanted, pair fed rats was added on day 1. These rats received the same amount of food daily that was consumed by their morphine-implanted pairs on the previous day. Some morphine and placebo treated rats were decapitated on day 2 along their placebo-implanted, pair fed partners on day 3. On day 7, placebo and morphine-implanted groups were further divided into two groups half of the rats from each group received subcutaneous injection of saline or naloxone and sacrificed at different time points thereafter. Animals from pair fed group were decapitated on day 8 (7^th day after pellet implantation).