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 ra ts 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 caus ed 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 peri od induces significant decreases in striatal 5-hydroxytryptamine (5-HT ) and 5-hydroxy-3-indoleacetic acid (5-HIAA) concentrations, and a sig nificant reduction of hypothalamic 5-HT levels in the adult rat. Howev er, no effects were found on midbrain serotonergic or striatal dopamin ergic systems. These results are discussed in terms of possible differ ent sensitivities of the diverse monoaminergic systems to the naltrexo ne treatment. Possible correlations with the behavioural data reported previously are also suggested.