S-adenosyl-L-methionine restores prolactin receptors in the aged rabbit brain.

The number of prolactin (PRL) receptors in the hypothalamus and substantia nigra of aged rabbits is significantly lower than the number measured in young animals. The treatment of aged rabbits for 30 days with S-adenosyl-L-methionine (SAM) restored the number of PRL binding sites to levels found in the hypothalamus and substantia nigra from young animals. We did not observe a clear-cut difference between the dissociation constants of untreated young, and untreated or SAM-treated aged rabbits, whereas the binding capacity varied. Moreover, in vitro addition of SAM to hypothalamic membranes from aged rabbits resulted in a significant increase in the number of binding sites. This in vitro effect was antagonized by S-adenosyl-L-homocysteine, a specific in vitro inhibitor of methyltransferase activity. The reduction in the number of PRL receptors is assumed to be related to the decrease in membrane fluidity induced by aging, while its increase after SAM treatment might be ascribed to the ability of this methyl donor to increase the fluidity of cell membranes by stimulating phospholipid synthesis.