Sexually dimorphic effects of aging on tuberoinfundibular dopaminergic neurons and lactotropes of rats

The effect of aging on plasma prolactin (PRL) levels, hypothalamic tuberoinfundibular dopaminergic (TIDA) neurons and pituitary lactotropes was evaluated in prolactinoma-free young (5-month-old) and old (23- to 24-month-old) Long-Evans rats of either sex. The young female rats were in diestrus, the old ones in recurrent pseudo-pregnancy. The tyrosine hydroxylase (TH)-labelled neurons in the arcuate nucleus (AN) and axons in the median eminence (ME) as well as the PRL-labelled lactotropes in the pituitary gland were studied by morphometry and densitometric immunohistochemistry. Further, we investigated the secretory function of isolated lactotropes by reverse hemolytic plaque assay (RHPA) and by cell culture in comparable animal groups. Compared with young animals, the plasma PRL levels of old rats of both sexes were similar or reduced. All morphometric and densitometric measurements of the AN neurons, ME axons (except number) and pituitary lactotropes were comparable in young and old female rats. In old male rats the AN and ME measurements were mostly decreased, while the lactotropes remained almost unchanged. The RHPA generally showed a reduced PRL release from lactotropes of old animals of both sexes. The PRL release from the cultured lactotropes, on the contrary, was greatly increased in old female rats and unchanged in old male rats. Our functional and morphological data suggest that the in vivo function of lactotropes in old prolactinoma-free female and male rats does not seem to be strongly influenced by the mildly reduced TIDA neuron activity, yet emphasize the differences of the aging process in the two sexes