Printed in the U . S . A . NEONATAL ADMINISTRATION OF $ - ENDORPHIN 1 PRODUCES " CHRONIC " INSENSITIVITY TO THERMAL STIMULI

Male and female rat pups were injected with ~-endorpbin, naloxone or a saline control solution during days 2-7 postnatally. At 90 days of age the rats were tested for analgesia with the tail flick test. Testing was conducted during the first 2 hours of the light and the dark cycle. In both sexes and during both phases of the light cycle rats treated with 8-endorphin as infants evidenced a significant elevation in threshold for painful thermal stimuli. Early treatment with naloxone also resulted in elevated threshold for thermal stimuli. Administration of naloxone to these rats as adults did not reverse the analgesic effect. It was concluded that early exposure to B-endorphin results in permanent changes in behavior perhaps by altering the interaction of endogenous opiates with their binding sites during a ciritcal period of opiate receptor development.