GROWTH-HORMONE RELEASING HORMONE EXPRESSION DURING POSTNATAL-DEVELOPMENT IN GROWTH HORMONE-DEFICIENT AMES DWARF MICE - MESSENGER-RNA IN-SITU HYBRIDIZATION

Several genetic mutations in mice and rats that produce lifelong growt h hormone (GH) deficiency result in overexpression of GH-releasing hor mone (GHRH) mRNA in hypothalamic arcuate nucleus neurons. In order to examine the development of this condition, GHRH mRNA expression was qu antified in Ames dwarf (df/df) and normal (DF/?) mice at 1 (day of bir th), 3, 7, 14, 21 and 60 postnatal days (d) following in situ hybridiz ation. Total mRNA was assessed using computer-assisted densitometry af ter X-ray film autoradiography, and mRNA expression per neuron was qua ntified by counts of grains per cell after emulsion autoradiography. T otal GHRH mRNA was the same in dwarf and normal mice at 1, 3 and 7d. G HRH mRNA in dwarfs increased at 14d to 240% of that in DF/? (p < 0.005 ); the percentage overexpression in dwarf mice remained greater than o r equal to 200% through 60d, although total GHRH mRNA increased in bot h dwarfs and normals during this period. GHRH mRNA per neuron was the same in normal and dwarf mice at 1 d, then increased in dwarfs to 190% of that in normals at 3d (p < 0.05), and rose to 300% of normal level s by 7d and beyond (p < 0.005). There was no sexual dimorphism in expr ession by either measure in normal or dwarf mice. These results indica te that an increase in GHRH mRNA in Ames dwarf mice is first detectabl e 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 hypophy siotropic somatostatin or dopamine in Ames dwarf mice. This difference may be due to the stimulatory action of GHRH, as opposed to the inhib itory effects of factors examined previously.