Prolactin replacement must be continuous and initiated prior to 21 d of age to maintain hypothalamic dopaminergic neurons in hypopituitary mice.

The prolactin (PRL) deficit in mice homozygous for the spontaneous Ames dwarf (df) mutation coincides with a marked reduction in the number of PRL-regulating tuberoinfundibular dopaminergic (TIDA) neurons. The TIDA deficit develops after 14 21 d postnatally and may be prevented by PRL replacement initiated at 12, but not at 60, d of age. The present study was designed to define further the developmental period during which PRL can prevent the deficit in the number of TIDA neurons in df/df mice, as well as to evaluate whether exposure to PRL neonatally affects the response to PRL by TIDA neurons in later development. To address the first aim, litters of df/df and normal (DF/df) mice were treated daily with ovine PRL (50 microg intraperitoneally), starting at 12, 21, or 30 d of age. To address the second aim, DF/df and df/df animals treated with PRL for 30 d starting at 12 d of age were subjected to PRL withdrawal (15 d of saline vehicle treatment), followed by PRL retreatment. All brains were evaluated using both catecholamine histofluorescence and tyrosine hydroxylase (TH) immunocytochemistry. Total numbers of TH-immunostained cells were counted in area A12 (TIDA neurons) and in A13 (medial zona incerta). Qualitatively, catecholamine fluorescence in A12 perikarya and terminals in df/df mice was enhanced by PRL treatment initiated at 12 or 21, but not at 30, d of age. TH immunostaining intensity was enhanced in all df/df PRL-treated groups, compared with saline treatment. However, total numbers of TH-positive TIDA neurons were reduced significantly in df/df mice treated with PRL beginning at 21 or 30 d, as well as with saline at 12 d, compared with similarly treated DF/df groups and with df/df animals treated with PRL beginning at 12 d (p < 0.01 for all comparisons). Among dwarf mice treated with PRL beginning at 12 d, followed by PRL withdrawal, the numbers of TH-positive TIDA neurons were greater than those of saline-treated dwarfs, but less than those in DF/df mice (p < 0.05 for both comparisons). In dwarfs retreated with PRL after withdrawal, the TIDA population was also smaller than that in normal animals (p < 0.05), although it was larger than in vehicle-treated dwarfs of the same age (p < 0.05).