This study examined the allosteric modulation of the NMDA receptor by nanomolar concentrations of neurosteroids in rats treated long term with morphine The neurosteroids dehydroepiandrosterone sulfate (DHEAS) pregnenolone sulfate (PS) and pregnanolone sulfate (3 alpha 5 beta S) are important mediators in the central nervous system They induce rapid responses by non-classical steroidal mechanisms e g via interaction with the N-methyl-D-aspartate (NMDA) receptor and are known to modify the binding of ifenprodil to the NMDA receptor subunit NR2B The NMDA receptor is involved in several processes including memory learning synaptic plasticity and neuronal development Morphine a mu-opioid receptor agonist has an important role in the clinical treatment of pain The main drawback of morphine treatment is the associated development of dependence and tolerance The mechanisms behind these phenomena are still to be elucidated but several reports suggest the involvement of the NMDA receptor The results of the present study indicate that the allosteric modulation induced by the neurosteroids DHEAS PS and 3 alpha 5 beta s was similar in all tested brain regions This suggests that the NR2B receptor subunit behaves independently of its site of expression Moreover the NR2B subunit was up-regulated in the frontal cortex but not in the hippocampus or hypothalamus It is concluded that morphine does not affect the neurosteroid modulatory effect on ifenprodil binding in the rat hippocampus or hypothalamus but does significantly affect both the expression of the NR2B subunit and the 3 alpha 3 beta S modulatory effect on ifenprodil binding in the frontal cortex It is suggested that the observed effect of long term morphine on the properties of NR2B in the frontal cortex may be associated with the mechanism underlying the development of opiate dependence (C) 2010 Elsevier Inc All rights reserved