Sex difference and estrous cycle: expression of prolactin receptor mRNA in rat brain.

The short and long forms of prolactin receptor (PRL-R) mRNA have been detected in the female rat brain. The present study aimed to investigate: (1) if the PRL-R mRNA is expressed in the male rat brain; (2) if expression levels in the female brain vary during the estrous cycle. All animals were sacrificed between 12:00 and 14:00 h. Radioactive RNase protection assay was used to measure mRNA levels. The results showed that both forms of PRL-R mRNA were expressed to varying degrees in the choroid plexus (ChP), preoptic area (POA), mediobasal hypothalamus (MBH), cerebral cortex (CTX) and pons-medulla PON) in both male and female rats. The average amount of both forms of PRL-R mRNA in the ChP, POA, MBH of cycling females was significantly higher than in the male rat. Among cycling female rats, the expression levels of both forms of PRL-R mRNA in the ChP, POA and MBH during proestrous were significantly greater than during diestrous or estrous. In proestrous females, the ChP expressed the highest levels of mRNA whereas the CTX contained the lowest. The ratios of short:long form mRNA were not significantly changed according to sex, estrous stage or brain regions although a slightly higher amount of the short form was observed. The detection of PRL-R mRNA in the male rat implicates that PRL may be involved in regulation of brain function in the male subject. The higher levels of PRL-R mRNA in female rats on proestrous suggest that PRL-R may be regulated by PRL or steroid hormones that show a surge on this day.