Developmental, behavioral, and opiate receptor changes after prenatal or postnatal β-endorphin, CRF, or Tyr-MIF-1

Developmental and long-term behavioral effects of perinatal injection of beta-endorphin (BE), CRF and Tyr- Pro-Leu- Gly- NH₂ (Tyr-MIF-1) in male rats were investigated along with the possibility that opiate receptors may be altered by the injection of BE during this critical time. Daily injections of peptide were given to pregnant females (100 μg/rat) in the week before birth or to the offspring (50 μg/rat) of untreated mothers during the first week of life. Prenatal BE and CRF reduced body weight on day 1, in contrast to Tyr-MIF-1 which produced a significant increase over controls by day 7 as well as a slight but significant acceleration of eye opening. Among the postnatal treatments, CRF-treated animals showed the most dramatic changes. These included decreased body weight, accelerated eye opening, and, in adulthood, increased open field rearing behavior and a tendency for a monotonic body temperature response to low doses of morphine, in contrast to the biphasic response shown by controls. BE, when given to pregnant mothers, increased the number (B_max) of [³H]naloxone-labeled (mu) receptors in whole brains of offspring assayed on day 14, but it did not significantly alter [³H]d-Ala-d-Leu-enkephalin-labeled (delta) receptors. In contrast, a significant decrease in both mu and delta receptors was observed on day 14 in rats given BE postnatally. These differences in receptors were no longer apparent in adulthood, and no significant differences in tail-flick response were detectable at this time. Nevertheless, some of the effects of these three peptides endured well beyond their presence, and for BE included changes in the number of opiate receptors.