The neuropeptide melanin-concentrating hormone (MCH) is a potent orexigenic signal in rodents.  During lactation there is a marked, but transient, expression of MCH in a discrete population of neurones in the medial preoptic area (mPOA) of the hypothalamus of rats that appears to peak at around day 19 of lactation.  The aim of this study was to investigate whether prolactin, which is elevated during lactation, induces MCH expression in the mPOA. 

MCH neurones were identified by immunohistochemistry.  Dual-label inmmunohistochemistry was done to determine if phosphorylated STAT5, a marker of long-form prolactin receptor activation, co-localized with MCH neurones.  Female Sprague Dawley lactating rats were perfused with 4 % PFA on day 19 of lactation having been given either bromocriptine (500 µg, sc; n=6) to suppress endogenous production of prolactin, or vehicle (n=6) twice daily for 2 days before perfusion.  Pups were cross-fostered every 12 hours to ensure adequate suckling and milk supply.  Another group had pups removed on day 17 and the brains collected 2 days later (non-suckling; n=4).  For comparison, brains were also collected a group of rats in dioestrus.

A population of immunopositive MCH neurones was consistently detected in the mPOA of day 19 lactating rats, in addition to the well-described MCH populations in the lateral hypothalamus and zona incerta.  The majority of MCH cells present in the mPOA were also immunopositive for nuclear pSTAT5, indicative of prolactin regulation.  No prolactin-induced pSTAT5 was seen in the other MCH populations.    MCH-immunoreactive neurones were not detected in the mPOA when prolactin was suppressed either by bromocriptine treatment or removing the suckling stimulus, whereas in the vehicle-treated lactating rats MCH-immunoreactive neurones were present in similar numbers to that seen in non-treated lactating rats. These results suggest prolactin may be involved in mediating changes in the MCH neuronal system during lactation.