Immunoreactive beta-endorphin in the hypothalamus of female rats: changes in content and release during prepubertal...

Female Wistar rats of different ages (1-45 days) were used. Extracts were made of the mediobasal hypothalamus (MBH) and beta-endorphin immunoreactivity (beta-ENDi) was quantitated by radioimmunoassay. Low but significant amounts of beta-ENDi (6.5 ng/MBH) were present on the first postnatal day. Hypothalamic beta-ENDi content did not change during the first week but decreased during the second week to a minimum (4.5 ng/MBH) on day 14. Thereafter, beta-ENDi increased rapidly to 13 ng/MBH on day 28 and remained at this level. Gel filtration showed that beta-ENDi substances with chromatographic characteristics identical to those of beta-END and beta-LPH were present in MBHs of 14-, 20- and 45-day-old rats. A beta-ENDi substance, possibly representing beta-END1-27, was nearly absent on day 14, but represented a major component of the MBH of the 45-day-old rat. In vitro incubation of MBH resulted in spontaneous release of beta-ENDi. Depolarization of neuronal membranes by incubation in medium containing 45 mMK+ stimulated beta-ENDi release. Both the spontaneous and K+-stimulated release of beta-ENDi were low on day 10 but reached postpubertal levels on day 20. These observations lead us to propose that the beta-ENDi-containing neurons in the hypothalamus of developing female rats rapidly mature between 14 and 20 days after birth. This may be causally related to the rapid decrease in circulating FSH levels that occurs during this period.