Maintenance of beta-endorphin analgesia across age cohorts

Age-related decreases occur in analgesic responses following morphine, 2-deoxy-D-glucose, inescapable foot shock and cold-water swims. Decreased affinity and concentration of opiate receptors and levels of endogenous opioids are also observed. The present study evaluated the dose-dependent (0.1, 0.5, 1.0, 5.0 micrograms, ICV) and time-dependent (15, 30, 45, 60 min) properties of beta-endorphin analgesia on the jump test across three age cohorts of rats (8, 18 and 30 months of age). The different age cohorts failed to display differences in the magnitude of beta-endorphin analgesia across doses and times, except for a transient (30 min) decrease in the 30-month group following the 0.5 microgram dose. This maintenance of beta-endorphin analgesia across age cohorts stands in marked contrast to the age-related decrements in morphine and opiate-sensitive environmental analgesia and occurs despite decreased levels of beta-endorphin. These data are discussed in terms of differential alterations in opiate receptor subpopulations, and represent the first instance of maintained opioid analgesia across cohorts.