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  • Title: Development of methionine-enkephalin and naloxone binding sites in regions of rat brain.
    Author: Tsang D, Ng SC, Ho KP.
    Journal: Brain Res; 1982 Apr; 255(4):637-44. PubMed ID: 6280806.
    Abstract:
    The development of opiate receptors in whole forebrain, brain stem and cerebellum of developing rats was measured by specific [3H]methionine-enkephalin ([3H]Met-Enk) and [3H]naloxone binding. The level of opiate receptor as determined by the binding of these two opiates varied with brain region as well as with age; however, the amount of [3H]naloxone bound in the same brain region obtained from animals of the same age was greater than that of [3H]Met-Enk. Kinetic analyses of these two opiate binding sites in newborns of the same age revealed that they differed in their apparent affinities and receptor numbers. Moreover, both morphine and naloxone were equally active in competing for [3H]Met-Enk binding in all 3 brain regions during the first month after birth, whereas methionine-enkephalin was only about 30-50% as active as morphine n competing for [3H]naloxone binding in that period. These results suggest that in the rat brain there are two types of opiate receptors and that the heterogeneity of opiate receptors is already apparent during early postnatal life.
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