Age-Related Changes in Opiate Receptor Densities in Discrete Hypothalamic Nuclei of Ovariectomized (OVX) and Estradiol (E2)-Treated Rats.

Hypothalamic opioid peptides influence a variety of diverse functions. During the aging process many aspects of hypothalamic opioid activity appear to change. Previously, we reported in young rats that steroid-induced changes in the density of naloxone binding sites parallel altered responsiveness to opiates. To determine whether age-related changes in opiate function are associated with alterations in the density of opiate receptors, we measured the density of naloxone binding sites at various times of day in young, middle-aged, and old ovariectomized or ovariectomized, estradiol-treated rats. In the medial preoptic nucleus, there is no age-related decline in the density of naloxone binding sites in ovariectomized animals. In the arcuate and ventromedial nuclei, the density of naloxone binding sites declines by middle-age and is suppressed further in the old animals. In the median eminence, there is a similar progressive age-related decline in naloxone receptor densities. In young animals estradiol treatment suppresses naloxone binding sites in all four nuclei. The ability to respond to steroid treatment persists in middle-aged and old animals except in the median eminence. These data demonstrate that the density of naloxone binding sites and the ability of estradiol to induce changes in these binding sites changes with age. The rate of these progressive changes is different in each brain region and correlates with age-related changes in opiate-mediated functions.