Molecular evidence for the involvement of NR2B subunit containing N-methyl-D-aspartate receptors in the development of morphine-induced place preference.

The present study was designed to clarify the role of the NR1, NR2A and NR2B subunits of N-methyl-D-aspartate receptors in the development of morphine-induced place preference using specific antibodies to N-methyl-D-aspartate receptor subunits in the mouse. An i.c.v. injection with specific antibodies against the carboxyl-terminal region of either NR1, NR2A or NR2B subunits caused the attenuation of seizures induced by i.v. infusion of N-methyl-D-aspartate in the mouse. Using western blotting, NR1, NR2A and NR2B subunits were found to be highly expressed in the cerebral cortex and hippocampus of the mouse brain, which are key areas in producing seizures regulated by N-methyl-D-aspartate receptors. These findings indicate that all three subunits of the N-methyl-D-aspartate receptor in these areas are likely to be critical for the seizure induced by i.v. infusion of N-methyl-D-aspartate. Furthermore, present data provide evidence that these antibodies when given into the brain specifically act on the target sites, resulting in the blockade of the respective N-methyl-D-aspartate receptor subunit. Under these conditions, i.c.v. treatment with the antibody against NR2B subunits abolished the morphine-induced place preference, whereas antibodies against NR1 and NR2A subunits did not affect the rewarding effect of morphine. Furthermore, the NR2B subunit protein was specifically up-regulated in the limbic forebrain of morphine-conditioned mice, whereas the protein levels of either NR1 or NR2A subunits were not altered.From these results, we suggest that the stimulation of the carboxyl-terminal region of NR2B subunits in the limbic forebrain may contribute to the development of morphine-induced rewarding effect.