The effects of morphine tolerance and abstinence on the characteristics of N-methyl-D-aspartate (NMDA) receptors, labeled with [3H]MK-801, were determined in the brain regions and spinal cord of the rat. 2. Male Sprague-Dawley rats were rendered tolerant to and physically dependent on morphine by subcutaneous implantation of six morphine pellets during a 7-day period. In tolerant (non-abstinent) rats, the pellets were left intact at the time of sacrificing, whereas in the abstinent rats the pellets were removed 16 hr prior to sacrificing. 3. The binding of [3H]MK-801, an NMDA receptor antagonist, to membranes prepared from spinal cord and brain regions (cortex, striatum, amygdala, hippocampus, hypothalamus, midbrain and pons-medulla) was determined using 5 nM concentration of the ligand in the presence of 30 microM glycine and 50 microM of glutamate. 4. In non-abstinent morphine tolerant rats, the binding of [3H]MK-801 was decreased by 40 and 33% in the midbrain and spinal cord, respectively, in comparison with their placebo controls. In morphine abstinent rats, the binding of [3H]MK-801 was decreased by 42, 29 and 50% in hypothalamus, midbrain and spinal cord, respectively, in comparison with their placebo controls. The binding of [3H]MK-801 to other brain regions and spinal cord of morphine tolerant and abstinent rats did not differ from their respective placebo controls. 5. Thus, these studies demonstrate, for the first time, that in the presence of glutamate and glycine, NMDA receptors of selected brain regions and spinal cord are down-regulated in rats treated chronically with morphine.
