Growth Hormone Releasing Hormone Expression During Postnatal Development in Growth Hormone-Deficient Ames Dwarf Mice:mRNA in situ Hybridization

Abstract

Several genetic mutations in mice and rats that produce lifelong growth hormone (GH) deficiency result in overexpression of GH-releasing hormone (GHRH) mRNA in hypothalamic arcuate nucleus neurons. In order to examine the development of this condition, GHRH mRNA expression was quantified in Ames dwarf (df/df) and normal (DF/?) mice at 1 (day of birth), 3, 7, 14, 21 and 60 postnatal days (d) following in situ hybridization. Total mRNA was assessed using computer-assisted densitometry after X-ray film autoradiography, and mRNA expression per neuron was quantified by counts of grains per cell after emulsion autoradiography. Total GHRH mRNA was the same in dwarf and normal mice at 1, 3 and 7d. GHRH mRNA in dwarfs increased at 14d to 240% of that in DF/? (p < 0.005); the percentage overexpression in dwarf mice remained ≧200% through 60d, although total GHRH mRNA increased in both dwarfs and normals during this period. GHRH mRNA per neuron was the same in normal and dwarf mice at 1d, then increased in dwarfs to 190% of that in normals at 3d (p < 0.05), and rose to 300% of normal levels by 7d and beyond (p < 0.005). There was no sexual dimorphism in expression by either measure in normal or dwarf mice. These results indicate that an increase in GHRH mRNA in Ames dwarf mice is first detectable at 3d, a period of approximately 7d after the failure to initiate GH production, which occurs normally at embryonic day 17.5. The onset of GHRH overexpression occurs earlier than the decline of either hypophysiotropic somatostatin or dopamine in Ames dwarf mice. This difference may be due to the stimulatory action of GHRH, as opposed to the inhibitory effects of factors examined previously.

Hypothalamic Somatostatin and Growth Hormone-Releasing Hormone mRNA Expression Depend upon GABA_A Receptor Expression in the Developing Mouse

“…1C), as shown previously in the mouse [25]. A quantitative analysis of the numbers of silver grains overlying individual cells within the ARN revealed a significantly (p < 0.05) reduced signal in γ 2 –/– mice (46 ± 4 silver grains/cell; n = 7) compared with wild-type litter mates (66 ± 6 silver grains/cell; n = 7; fig.…”

Growth Hormone‐Releasing Hormone‐Producing and Dopaminergic Neurones in the Mouse Arcuate Nucleus Are Independently Regulated Populations

“…Because some arcuate nucleus neurones in rats produce both TH and GHRH, factors that support production of both the enzyme and the hypophysiotropic peptide must be present in a subset of that population. Previous studies on Ames dwarf mice have shown that the decrease in numbers of TH‐producing neurones (5) and the increase in GHRH‐immunoreactive neurones (6) are quantitatively similar and simultaneous in development (7, 8). Both TH neurone loss and GHRH neurone increase occur in the ventral arcuate nucleus (6, 9, 14).…”

“…Those observations suggest that, in GH-and prolactin-deficient animals, neurones in the arcuate nucleus change during postnatal development from dopamine-to GHRH-producing. This theory is supported by further observations that (i) the increase in identifiable GHRH-producing neurones (7) occurs developmentally at the same time as the decrease in neurones that produce tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine synthesis (8) and (ii) the summed populations of arcuate nucleus GHRH and dopamine neurones in Ames dwarf mice were nearly comparable to the same collective population in the brains of normal siblings.…”