OVEREXPRESSION OF GROWTH-HORMONE GENES IN TRANSGENIC MICE SHORTENS FREE-RUNNING PERIODS IN CONSTANT LIGHT

Transgenic mice were produced by microinjection of male pronuclei with approximately 2.7Kb DNA fragment, containing a metallothionein-I prom oter (MT) or a phosphoenolpyruvate carboxykinase (PEPCK) promoter link ed to human growth hormone (hGH) or bovine growth hormone (bGH) struct ural genes. Transgenic mice from resulting lines have substantial leve ls of circulating heterologous GH and are much larger than normal mice . Since these animals have reproductive abnormalities, and since repro ductive hormones have significant effects on the circadian timing syst em, experiments were designed to determine whether these animals had a ltered free-running periods. Transgenic female mice and their normal f emale siblings were individually housed in cages with activity wheels and exposed to constant dark (DD) or constant light (LL) for durations exceeding two weeks. Locomotor activity was continuously monitored by computer. The period of the circadian rhythm of locomotor activity an d the duration of activity were determined for each group in both ligh ting conditions. All mice free-ran in LL and DD, and as expected, the period of the free-running rhythm was related to light intensity. The period of the locomotor rhythm was longer in LL than in DD. The free-r unning period in LL, however, was significantly shorter in transgenic mice when compared to their normal siblings. In DD, free-running perio ds in transgenic and non-transgenic mice were not significantly differ ent. Thus it appears that the continuous presence of high levels of GH shortens the free-running period in LL similar to the affect of estro gen. This effect is not due to GH effects during early development, si nce only MT/bGH and MT/hGH are expressed during fetal life, yet PEPCK/ bGH animals demonstrate a similar response on period. The effect does not appear to be a characteristic of the lactogenic effect of GH, an i ndirect or permissive influence of the well known actions of estrogen on the oscillator, an indirect action of premature aging of the oscill ator, or a developmental alteration of the circadian timing system. A direct effect of GH itself remains as the prime candidate for the shor tened period in constant light.