The influence of altered endogenous GH status on somatostatin (somatot
ropin release-inhibiting hormone; SRIF) gene expression was studied in
two transgenic mouse models. Transgenic dwarf mice carried the rat GH
gene promoter fused to the diphtheria toxin A-chain gene, placing tox
in expression under GH promoter control. As a result, the toxic produc
t of the transgene ablated all GH-expressing cells, resulting in undet
ectable circulating GH, reduced weight (10.6 +/- 1.0 g for transgenic
dwarfs vs. 29.5 +/- 1.7 g for controls; P < 0.001), and no detectable
somatotrophs. Transgenic giant mice contained a construction combining
a widely expressed metallothionein promoter and the human GH-releasin
g hormone (hGHRF) structural gene. Transgene expression of hGHRF resul
ted in over-production of endogenous mouse GH in the anterior pituitar
y and weight increases (42.7 +/- 2.7 g for giants vs. 29.5 +/- 1.7 g f
or controls; P < 0.005). Using in situ hybridization, control mice, tr
ansgenic dwarfs, and transgenic giants were compared for levels of pre
pro-SRIF mRNA. Hybridization signal intensities for prepro-SRIF mRNA w
ere similar in transgenic dwarfs to those in littermate nontransgenic
mice in non-GH-regulating regions of the brain, such as cortex (contro
l, 31 +/- 2 U; dwarf, 27 +/- 2) and reticulothalamic nucleus (control,
41 +/- 2 U; dwarfs, 39 +/- 3). Transgenic giant mice had hybridizatio
n intensity of SRIF mRNA similar to that of normals in cortex (control
s, 31 +/- 2 U; giant, 27 +/- 1) and reticulothalamic nucleus (controls
, 41 +/- 2 U; giant, 40 +/- 4). In the GH-regulating neurons of the an
terior periventricular hypothalamus (PeN), prepro-SRIF mRNA signal in
transgenic dwarf mice decreased to 60% of that in controls (88 +/- 13
U for dwarfs vs. 147 +/- 17 U for controls; P < 0.01), although the nu
mbers of mRNA-expressing cells in the PeN were not different between t
he transgenic dwarfs and controls (dwarfs, 69 +/- 6 cells; controls, 7
2 +/- 4 cells). The transgenic giant mice had 230% higher prepro-SRIF
mRNA signal than control mice in the PeN (343 +/- 30 U in giants vs. 1
47 +/- 17 U in controls; P < 0.001). Again, the numbers of mRNA-expres
sing cells were not different in giants (57 +/- 9) and normals (72 +/-
4). These results suggest that while the lack of endogenous GH is acc
ompanied by a slight decrease in transcriptional expression of SRIF in
the PeN, the overproduction of endogenous GH greatly stimulates hypot
halamic SRIF steady state mRNA levels.