Naltrexone administration during the preweanling period affects striatal and hypothalamic serotonergic systems, but not midbrain serotonergic or striatal dopaminergic systems in the adult rat

We have recently reported that daily administration from birth of the opioid antagonist naltrexone (1 mg/kg, s.c.) affected dopaminergic and serotonergic systems in the striatum, hypothalamus and midbrain in rats of 7, 14 and 22 days of age. Previously, we have also reported that the same dose of naltrexone administered from birth until day 21 caused diverse behavioural alterations in adulthood. In the present work, using the same naltrexone administration schedule, we demonstrate that the intermittent blockade of opioid receptors during preweanling period induces significant decreases in striatal 5-hydroxytryptamine (5-HT) and 5-hydroxy-3-indoleacetic acid (5-HIAA) concentrations, and a significant reduction of hypothalamic 5-HT levels in the adult rat. However, no effects were found on midbrain serotonergic or striatal dopaminergic systems. These results are discussed in terms of possible different sensitivities of the diverse monoaminergic systems to the naltrexone treatment. Possible correlations with the behavioural data reported previously are also suggested.