Signalling is one of the key processes in biological systems. It enables communication within a single cell as well as among the many cells in higher organisms. Specialized neurotransmitter and hormone systems have evolved in order to coordinate the many cellular and organ functions in the body. These systems have long been known to contain feedback loops that control and regulate their activity. For example, in the hypothalamic-pituitary-peripheral gland axes, the hormones released from the peripheral glands modulate release of the regulatory hormones both in the hypo-