Abstract
Most mammals use photoperiod to synchronize their breeding season, so that young are raised during the spring and summer months. The pineal hormone melatonin is the major endocrine message used to relay photoperiodic information to the reproductive axis. This chapter first discusses the ecological context for seasonal breeding and the pivotal role that nocturnal melatonin secretion plays in its regulation. Melatonin acts through high affinity G protein–coupled receptors, and we discuss the emergence of the pars tuberalis of the anterior pituitary as a principal site of melatonin action. We go on to discuss recent evidence that local metabolism of thyroid hormone by tanycyte cells within the mediobasal hypothalamus is the key seasonal gateway for activation of the hypothalamic gonadotropin-releasing hormone pulse generator. Melatonin controls tanycyte function via a pathway involving a novel paracrine action of thyrotropin released by pars tuberalis cells. Downstream targets of this control pathway include RF-amide neurons implicated in hypothalamic gonadotropic signaling, suggesting an integrated model for the control of seasonal reproduction in mammals. We end by focusing on current hypotheses for mechanisms of tanycyte action and the cellular consequences of altered hypothalamic triiodothyronine metabolism.
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