IDENTIFICATION OF GROWTH HORMONE-RELEASING HORMONE AND SOMATOSTATIN NEURONS PROJECTING TO THE MEDIAN-EMINENCE IN NORMAL AND GROWTH HORMONE-DEFICIENT AMES DWARF MICE

In the spontaneous mutant Ames dwarf mouse, GH deficiency coincides wi th a dramatic increase in the expression of both mRNA and peptide for stimulatory GHRH and reduced expression of GH-inhibitory somatostatin (SRIH) mRNA and peptide. However, both GHRH and SRIH are markedly redu ced in the dwarf median eminence (ME), suggesting that ME innervation by GHRH and SRIH neurons may be aberrant in the absence of GH. In orde r to test this hypothesis, the number of GHRH and SRIH ME-projecting n eurons was evaluated in normal and dwarf mice using a combination of r etrograde tract-tracing and neuron phenotype identification by immunoc ytochemistry (ICC). Adult animals were injected intraperitoneally with the fluorescent tract-tracer fluorogold (FG), which, in the brain, is taken up only by axons terminating in areas deprived of the blood-bra in barrier, such as the ME. Visualization of FG was achieved by either UV illumination or ICC, and was combined as appropriate with fluoresc ence or bright-field ICC for GHRH or SRIH. Cells immunoreactive for GH RH or SRIH and labeled with FG were quantified at each 180-mu m rostra l-to-caudal level through the hypothalamus. As reported previously, th e total number of hypophysiotropic GHRH neurons was markedly increased in dwarf compared with that in normal mice. However, a similar percen tage of ME-innervating GHRH cells was estimated in dwarf (73 +/- 4%) a nd normal (76 +/- 3%) animals. Such a percentage in dwarfs thus repres ents a larger population of ME-projecting GHRH cells (749 +/- 53) than in normal mice (128 +/- 15). Increased numbers of FG-labeled GHRH neu rons in dwarfs were located at the middle and posterior levels of the arcuate nucleus (2.08, 2.26 and 2.44 mm posterior to bregma). The perc entage of FG-labeled SRIH neurons was also similar for dwarf (83 +/- 2 %) and normal (87 +/- 2%) mice. Because the total SRIH-immunoreactive neuronal population in dwarfs is significantly reduced compared to tha t in normal animals, the similar FG-labeled percentage reflects a redu ced number of SRIH cells projecting to ME in dwarf (1,376 +/- 104) com pared with normal (3,192 +/- 267) mice. Fewer FG-labeled SRIH cells we re found in dwarfs at every anterior-to-posterior level of the periven tricular nucleus (p < 0.01 for comparisons at 0.28, 0.46, 0.64, and 1. 0, and p < 0.05 for comparison at 1.18 mm posterior to the bregma). Th e present study indicates that the reduction in GHRH and SRIH immunore activity in the dwarf ME may result from different phenomena for each neuronal population. The reduction in GHRH immunostaining in the ME, d espite a marked increase in the total ME-projecting GHRH neurons, may be interpreted as increased GHRH release, with consequent depletion of the ME stores. In contrast, the deficit in ME SRIH may be proportiona l to the deficit in the number of detectable SRIH periventricular nucl eus neurons.