Effects of Suprachiasmatic Transplants on Circadian Rhythms of Neuroendocrine Function in Golden Hamsters**This work was supported by NIH Grants MH-44132, KO2-MH-00914, and F32-HD-07673. A preliminary report of this research was presented at the 23rd Annual Meeting of the Society for Neuroscience (Neurosci Abstr 19:236.17, 1993).

Jan 14, 1999Endocrinology

How Transplants of the Brain’s Internal Clock Affect Daily Hormone Cycles in Golden Hamsters

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Abstract

SCN grafts restored circadian locomotor rhythms in SCN-lesioned hamsters but did not reinstate endocrine rhythms.

SCN lesions disrupted estrous cycles and locomotor rhythms in hamsters under various light conditions.
Grafts that reinstated locomotor activity failed to restore estrous cycles and urinary melatonin metabolite rhythms.
LH surges were triggered by estradiol in brain-intact females but not in SCN-lesioned, grafted females.
Sham-lesioned males exhibited clear circadian rhythms of serum corticosterone and cortisol, while lesioned-grafted males did not.
Testicular regression occurred in controls but was absent in both SCN-lesioned hamsters with grafts and those that regained locomotor rhythmicity.

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Grafts of fetal tissue including the suprachiasmatic nucleus (SCN) of the hypothalamus restore locomotor rhythmicity to behaviorally arrhythmic, SCN-lesioned Syrian hamsters. We sought to determine whether such transplants also reinstate endocrine rhythms in SCN-lesioned hamsters. In Exp 1, SCN lesions interrupted estrous cycles in a 14 h light, 10 h dark photoperiod and locomotor rhythms in constant dim red light (DD). SCN grafts that reinstated behavioral circadian rhythms consistently failed to reestablish estrous cycles. After ovariectomy, estradiol implants triggered LH surges at approximately circadian time 8 in 10 of 12 brain-intact control females and 0 of 9 SCN-lesioned, grafted females. Daily rhythms of the principal urinary melatonin metabolite, 6alpha-sulfatoxymelatonin, were not reestablished by behaviorally functional grafts. In Exp 2, SCN lesions eliminated locomotor rhythmicity in adult male hamsters maintained in DD. Seven to 12 weeks after restoration of locomotor activity rhythms by fetal grafts, hosts and sham-lesioned controls were decapitated at circadian times 4, 8, 12, 16, 20, or 24. Clear circadian rhythms of both serum corticosterone and cortisol were seen in sham-lesioned males, with peaks in late subjective day. No circadian rhythms in either adrenal hormone were evident in serum from lesioned-grafted males. Testicular regression, observed in intact and sham-lesioned males maintained in DD, was absent not only in arrhythmic SCN-lesioned hamsters given grafts of cerebral cortex, but also in animals in which hypothalamic grafts had reinstated locomotor rhythmicity. The pineal melatonin concentration rose sharply during the late subjective night in control hamsters, but not in SCN-lesioned animals bearing behaviorally effective transplants. Even though circadian rhythms of locomotor activity are restored by SCN transplants, circadian endocrine rhythms are not reestablished. Endocrine rhythms may require qualitatively different or more extensive SCN outputs than those established by fetal grafts.

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