Long-term effects of food deprivation: II. Impact on morphine reactivity

Exposure to a series of inescapable shocks elicits a hormonally mediated opioid hypoalgesia in rats. In addition, it has a long-term effect on the opioid system: it increases reactivity to a low dose of morphine. Here we looked at whether another manipulation that elicits a hormonally mediated opioid hypoalgesia—food deprivation—has a similar sensitizing effect. Subjects were placed on a restricted diet for 48 h. Opiate reactivity was then assessed 24 h after food was returned. In Experiment 1, previously food-deprived subjects exhibited a stronger hypoalgesia on the tailflick test 30 min after an injection of morphine. In Experiment 2, a history of food deprivation increased the duration of morphine hypoalgesia as well as its onset. In Experiment 3, food deprivation per se induced hypoalgesia, but only during the early part of the dark cycle.