Differential down-regulation of delta opioid binding sites during physical dependence on methionine enkephalin in the rat.

Post-synaptic receptor modulation is thought to be one important mechanism involved in the adaptation of a neuronal system during chronic exposure to a drug. However, initial studies of opioid receptor regulation following chronic in vivo administration of narcotic agonists, such as morphine, reported no down-regulation in the number of opioid receptors in the brain. Subsequent studies, employing in vitro preparations, have reported evidence of opioid receptor down-regulation under specific conditions. It remains to be determined whether the in vitro phenomena of opioid receptor plasticity is relevant to the intact mammalian central nervous system. The data in this report shows that chronic in vivo administration the opioid peptide methionine enkephalin, results in a significant, regionally specific down-regulation of delta opioid receptors in rat brain: 30% decrease in receptor density in the striatum; no change in hypothalamus.

The effect of chronic met-enk on various dopamine systems in the rat.

Morphine and enkephalins act preferentially via different receptors, mu and delta, respectively. Chronic administration of morphine or methionine enkephalin results in development of physical dependence. Data from our laboratory suggests that there are some differences in the effect of these two opioids with regards to certain neurochemical systems.