[Mechanisms of morphine-induced rewarding effect: involvement of NMDA receptor subunits]

The glutamate receptor contributes to excitatory synaptic transmission in the central nervous system and plays an important role in memory acquisition, learning and neurological disorders. Molecular cloning studies have revealed that NMDA receptors consist of two families, the NR1 and NR2A-NR2D subunits, and NMDA receptors are thought to be pentameric or tetrameric complexes of the NR1 subunit with one or more of the NR2 subunits. It has been proposed that NMDA receptors are implicated in the development of opioid dependence. The non-selective NMDA receptor antagonist dizocilpine has been shown to suppress not only physical but also psychological dependence produced by morphine. An intracerebroventricular (i.c.v.) treatment with a specific antibody against the carboxyl-terminal region of the NR2B subunit abolishes the morphine-induced place preference, whereas antibodies against the NR1 and NR2A subunits do not affect the rewarding effect of morphine, indicating that the blockade of the NR2B subunit suppresses the development of the morphine-induced rewarding effect. Under these conditions, the NR2B subunit protein is up-regulated in the limbic forebrain of morphine-conditioned mice. These findings suggest that the NMDA receptor, especially NR2B subunit, is an important modulator of the development and/or expression of psychological dependence on morphine.