Mini review: Bidirectional Regulation of Circadian Rhythm by Suprachiasmatic Nucleus and Nuclear Receptors in Female Mammals

The anterior region of the hypothalamus accommodates a bilateral structure called the suprachiasmatic nucleus (SCN), which controls, modulates, and perpetuates the homeostasis of circadian rhythm and sleep hormone release. These SCN have a predominance over multitudinous peripheral tissues like the uterus, liver, intestine, pancreas, endocrine system, immune system, reproductive system, and cardiovascular system. This peripheral clock acts as a pacemaker for circadian rhythm timing, which regulates crucial metabolic pathways and organizes numerous activities in the female reproductive network of mammals. The circadian CLOCK genes are expressed in various reproductive organs. The CLOCK, BMAL1, CRY, and PER genes harmonize the balance and manifestation of nuclear receptors (NRs) expression, and the other way round, NRs regulate these circadian genes. Several NRs, in particular estrogen, progesterone, androgen, and PPARs, nurture the ovary and uterus. Bidirectional coordination between SCN and NRs maintains the circadian rhythm of the hypothalamic-pituitary-gonadal (HPG) axis of the female reproductive organs.