Behavioral effects of D-Ala2-beta-endorphin in squirrel monkeys

The effects of D-Ala2-beta-endorphin administered either intravenously (IV) or intracisternally (IC) in squirrel monkeys were tested using a number of behavioral measures: general activity, eating, social behavior, aggression/distress, analgesia, and startle/escape. There were 10 groups (N = 5) consisting of 4 dose levels administered IC (0.4, 40, 400 microgram/kg) and 6 dose levels injected IV (0, 4, 40, 80, 400, 800 microgram/kg). Every monkey was tested with all tasks on each of 5 identical repeated trials, one pre-injection baseline trial and 4 post-injection trials. After IC administration, the 2 largest doses exerted toxic effects, which were partially reversed with naloxone, producing in 2 cases muscular rigidity and profound sedation. The smaller 4 microgram/kg dose produced significant decreases in activity over trials but increased reactivity to noxious stimulation after the initial post-injection trial. With IV injection reliable changes in activity and approach to food were found. The results demonstrate significant behavioral effects of an endorphin analog in the squirrel monkeys after both central and peripheral injection.