This study investigated expression of prolactin receptor (PRL-R) mRNA in selected hypothalamic nuclei of lactating rats (days 7-10 post partum) compared with dioestrous rats. Rat brains were frozen with liquid nitrogen and cut into coronal sections of 300 microm. From these sections, tissues were micropunched from the parietal cortex (CTX), choroid plexus (ChP), and five hypothalamic regions: supraoptic (SO), paraventricular (Pa), arcuate (Arc) and ventromedial hypothalamic (VMH) nuclei, and median eminence (ME). Expression of both short and long forms of PRL-R mRNA were evaluated by reverse transcription-PCR and Southern hybridisation. The results showed that the relative amount of short form mRNA in the ChP of lactating rats was significantly higher than in dioestrous rats. The short form of PRL-R mRNA was undetectable in the SO, Pa, VMH of dioestrous rats but was expressed at a significant level in lactating rats. Levels of long form mRNA in the ChP, SO, Pa and VMH in lactating rats were significantly increased compared with dioestrous rats. Moreover, the long form mRNA was induced in the CTX of lactating rats. In the Arc, levels of both forms of PRL-R mRNA tended to increase in lactating rats compared with dioestrous rats but changes were not statistically significant. Neither form of PRL-R mRNA was detectable in the ME in the two animal models. Increased expression of PRL-R mRNA in specific brain regions during lactation is consistent with the variety of PRL effects on the brain, and may help to explain profound physiological changes in the lactating mother.

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