Regulation of seasonal reproduction by hypothalamic activation of thyroid hormone

Abstract

Organisms living outside the tropics measure the changes in the length of the day to adapt to seasonal changes in the environment. Animals that breed during spring and summer are called long-day breeders, while those that breed during fall are called short-day breeders. Although the influence of thyroid hormone in the regulation of seasonal reproduction has been known for several decades, its precise mechanism remained unknown. Recent studies revealed that the activation of thyroid hormone within the mediobasal hypothalamus plays a key role in this phenomenon. This localized activation of the thyroid hormone is controlled by thyrotropin (thyroid-stimulating hormone) secreted from the pars tuberalis of the pituitary gland. Although seasonal reproduction is a rate-limiting factor in animal production, genes involved in photoperiodic signal transduction pathway could emerge as potential targets to facilitate domestication.

Keywords: ependymal cell; iodothyronine deiodinase; mediobasal hypothalamus; pars tuberalis; seasonal reproduction; thyroid hormone; thyrotropin.

Figure 1

Calendar of seasonal breeding animals. Most animals mate in a specific time of a year. Small animals with short gestation or incubation period mate in spring and summer, while large animals that have a 6-month gestation period mate in fall to give birth in spring.

Figure 2

Photoperiodic signal transduction pathway in mammals and birds. In mammals, light information is received by the eye and transmitted to the pineal gland via the circadian pacemaker, the suprachiasmatic nucleus (SCN). The duration of the pineal melatonin signal encodes the length of night and regulates TSH secretion in the pars tuberalis. The pars tuberalis TSH acts on TSH receptor expressed in the ependymal cells lining ventrolateral walls of the third ventricle (VIII) to induce DIO2 and reduce DIO3. Local thyroid hormone activation within the mediobasal hypothalamus (MBH) by DIO2/DIO3 switching plays a key role in the regulation of seasonal reproduction. In contrast, light information received by deep brain photoreceptors induces TSH secretion from the pars tuberalis in birds. Nevertheless, melatonin is not involved in the seasonal reproduction of birds. The schematic is a modified version of illustration published by Ikegami and Yoshimura (40).

Figure 3

Neuro–glial interaction between GnRH nerve terminals and glial endfeet. Locally activated thyroid hormone within the MBH regulates neuro–glial interaction in the median eminence and these morphological changes appear to regulate or modulate seasonal GnRH secretion from the hypothalamus to portal capillary. The illustration has been modified from that published by Yoshimura (89).