Developmental assessment of hypothalamic tuberoinfundibular dopamine in prolactin-deficient dwarf mice

Development of the hypophysiotropic hypothalamus in PRL-deficient Ames dwarf (df/df) mice was examined for steady state dopamine (DA) by visualization using formaldehyde-induced catecholamine histofluorescence and by quantification using catecholamine HPLC at selected postnatal ages (7, 14, 21, 30, and 90 days). Phenotypically normal (DF/?) littermate mice were compared with dwarfs by both methods at each age. The studies were designed to investigate whether the known deficiency in hypothalamic tuberoinfundibular DA in adult dwarfs is present neonatally or develops over the postnatal period. The anterior pituitary of each mouse was processed for GH and PRL immunocytochemistry. At 7 days of age, GH immunostaining was robust, and scattered PRL-positive cells were noted in DF/? pituitary. Homogeneously distributed PRL cells increased in number through 30 days of age in normal mice. Neither GH nor PRL immunoreactivity was present in df/df mice at any age. At 7, 14, and 21 days of age, hypothalamic DA tuberoinfundibular histofluorescence was comparable in df/df and DF/? mice. At 90 days of age, tuberoinfundibular histofluorescence in normal mice remained intense, but was virtually undetectable in dwarfs. The developmental change affected only tuberoinfundibular neurons, since DA histofluorescence in nonhypophysiotropic areas, such as substantia nigra (SN), was qualitatively comparable for df/df and DF/? for all ages examined. Norepinephrine (NE) fluorescence in hypothalamus was also comparable for df/df and DF/?. Catecholamine HPLC provided quantitative confirmation of histofluorescence observations. DA and NE levels in both hypothalamus and ventral midbrain, including SN, increased during development in both df/df and DF/? brains. NE levels were not different between dwarf and normal animals at any age in either medial basal hypothalamus (MBH) or SN. The DA concentration in SN was not different between df/df and DF/? at any age examined. MBH DA was comparable in df/df and DF/? mice at 7, 14, and 21 days of age; at 30 and 90 days, MBH DA was markedly lower (P < 0.001) in dwarf than in normal mice. Although MBH DA in dwarfs was comparable to that in normal mice at 21 days, the increase in dwarfs between 14 and 21 days was not statistically significant. Thus, the hypothalamic DA deficit that exists in adult dwarf mice is not present neonatally and represents a failure to increase DA compared with normal mice after 14 days of age. The failure of continued development of hypophysiotropic tuberoinfundibular DA neurons in dwarf mice is correlated chronologically with absent pituitary PRL production.