Prenatal beta-endorphin can modulate some aspects of sexual differentiation in rats.
Sexually dimorphic traits were studied in offspring of rats injected with 33 micrograms rat beta-endorphin (beta-END) three times daily from Day 14 to Day 21 of pregnancy. beta-END males had shorter neonatal anogenital distances than did controls and were more likely to show the female lordosis pattern as adults, but they did not differ in male copulatory behavior. When given a choice between spending time with an estrous female or a male, beta-END males showed a lower preference for the female than did control males. The number and somal size of neurons in the bulbocavernosus and dorsolateral nucleus of the lumbar spinal cord were unaffected by drug exposure. Elevated beta-END during fetal ontogeny apparently alters the differentiation of some, but not all, sexually dimorphic traits. The data suggest that endogenous opioids may contribute to the etiology of the prenatal stress syndrome.