Male and female rat pups were injected with β-endorphin, 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 β-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 β-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. © 1979.

beta-endorphin-like immunoreactivity was measured by radioimmunoassay in the brains of adult rats treated neonatally with beta-endorphin, naloxone, or vehicle. After treatment with beta-endorphin, the decreases observed in beta-endorphin-like immunoreactivity in the hypothalamus, pineal, midbrain, pons-medulla, hippocampus, striatum, frontal cortex, occipital cortex, and posterior cortex were highly significant but the 23% decrease in the thalamus was not significantly different from that of control rats. Neonatal administration of naloxone only resulted in a significant decrease in beta-endorphin-like immunoreactivity in the hypothalamus. In contrast, no differences were discernible in content of either beta-endorphin-like immunoreactivity or ACTH-like immunoreactivity in the pituitary of rats treated with beta-endorphin, naloxone, or vehicle in the neonatal period. These same rats had shown an increased threshold to painful thermal stimulation by the tail-flick test after administration of either beta-endorphin or naloxone at birth. The results suggest that neonatally injected beta-endorphin may alter the levels of beta-endorphin-like immunoreactivity in rat brain as well as the response to pain.