INSULIN-LIKE GROWTH-FACTOR-I IN SYRIAN-HAMSTERS - INTERACTIONS OF PHOTOPERIOD, GONADAL-STEROIDS, PINEALECTOMY, AND CONTINUOUS MELATONIN TREATMENT

Four experiments in Syrian hamsters examined the role and possible int eraction of photoperiod, gonadal steroids, and the pineal on circulati ng levels of insulin-like growth factor-1 (IGF-1). In the first experi ment, female hamsters were exposed to long photoperiod (LP; 14:10 LD) or short photoperiod (SP; 8:16 LD); an additional group of SP-exposed females was pinealectomized (PX). SP induced a significant depression in IGF-1 concentrations which PX partially prevented. In Experiment 2, two groups (control and castrate [CX]) of adult male hamsters were ke pt in LP, and three groups (intact, CX, and CX + PX) of hamsters were kept in SP for five weeks. The four groups of animals that were CX and /or maintained in SP had approximately the same mean level of IGF-1, a nd all four groups were significantly (P < 0.001) higher than the LP-c ontrol hamsters. In Experiment 3, four groups (intact controls, CX, CX + melatonin pellet [MEL PEL], and MEL PEL only) were kept in LP. Mela tonin pellets (1 mg melatonin/24 mg beeswax/every two weeks) were impl anted sc twice during the experiment. Castration induced a rise (P < 0 .001) in IGF-1 levels, and this was not prevented by MEL PEL. In Exper iment 4, testosterone and dihydrotestosterone pellets implanted in LP- exposed CX males prevented the CX-induced rise in IGF-1; testosterone implants also reduced IGF-1 levels in CX males treated with progestero ne. In conclusion, SP treatment depresses IGF-1 in female hamsters and raises it in males. These results substantiate previous studies in ot her models of gonadal steroid deficient animals. They lend further cre dence to the hypothesis that there is a sexual dimorphism in circulati ng IGF-1 concentrations in the Syrian hamster that may be at least par tially related to the presence of gonadal steroids.