Increased rewarding properties of morphine in perinatally protein-malnourished rats.

In the current research, we assessed the influence of a protein malnutrition schedule from the 14th day of gestation up to 40 days of age (D-rats) on the rewarding properties of morphine in adult rats by means of the conditioned place preference paradigm. Well-nourished animals (C-rats) administered with different doses of morphine (0.75, 1.5, 3, 6, 12 or 24 mg/kg i.p.) exhibited a conditioning place preference with doses of 3 and 6 mg/kg, whereas in D-rats such a conditioning effect was observed with doses of 1.5 and 3 mg/kg. No adverse effects were observed in either C- or D-rats for the higher doses of morphine. In addition, when animals of both groups were pretreated twice a day for 3 days with increasing doses of morphine (5, 10 and 20 mg/kg s.c.), only D-rats elicited sensitization to the conditioning effect with the lowest dose of morphine (0.75 mg/kg i.p.). Furthermore, sensitized D-rats showed a selective and significant increase in FosB expression in the nucleus accumbens (core and shell), basolateral amygdala and medial prefrontal cortex, brain areas that are functionally related to the rewarding neural circuit. These results demonstrate that a deficient nutritional status during the perinatal period results in adult subjects having neural alterations, leading to an increased responsiveness to morphine and/or enhanced reinforcement effects, which correlates with an overexpression of FosB in selective brain areas related to the rewarding network.