Control of Circadian and Reproductive Behavior and Physiology

Abstract

Mammalian reproduction is mediated by the hypothalamic-pituitary-gonadal axis. The gonadotropin-releasing hormone (GnRH) neuron is the final central output of the hypothalamic portion of this axis, and the proper development and function of this neuronal type is required for normal fertility. Several genes have been identified as necessary for GnRH neuron development and function, including the homeodomain protein Six6, a homologue of Drosophila optix.

Mice lacking the Six6 gene have several previously identified phenotypes—predominantly phenotypes of the eye and retina—and additional phenotypes are described here. Male and female Six6-/- mice have impaired fertility due, in part, to significantly reduced numbers of GnRH neurons relative to wild-type littermate control animals.

Additional phenotypes identified in the Six6-/- mice were circadian- and suprachiasmatic nucleus- (SCN-) related. Six6 -/- mice displayed a variety of circadian behavioral activity patterns, with some mice showing periodicity during constant darkness, and other mice completely lacking circadian periodicity even in entrainment photoperiods. However, regardless of behavioral phenotype or the previously identified optic nerve phenotype, Six6-/- mice lacked suprachiasmatic nuclei as indicated by immunostaining for prototypical SCN markers and observation of hypothalamic morphology.

The circadian clock controls many aspects of mammalian physiology, including reproduction in many species. Inactivation of the circadian clock in mice, via knock-out of Bmal leads to infertility, though the mechanisms underlying this infertility remained incompletely understood. Using transgenic, physiological, and behavioral techniques, the reproductive phenotype of Bmal-/- mice was investigated. Tissue-specific inactivation of Bmal in all neurons or in GnRH neurons did not appreciably affect fertility, though neuronal Bmal inactivation resulted in a shortening of the free-running circadian period. Female Bmal-/- mice were unable to respond to estrogen stimulation with an appropriate LH surge. Male Bmal-/- mice showed severely impaired mating behavior, and even Bmal-/- mice given testosterone replacement exogenously did not show normal mating behavior.