Basic Hypothalamic-Pituitary Coordination
- The hypothalamus and the pituitary gland work cooperatively to coordinate the regulation of a wide variety of hormones. The relationship between these two organs is fundamentally asymmetric, with the synthesis and secretion of all pituitary hormones being under the control of the hypothalamus. Given the different anatomic relationships between the hypothalamus and the anterior and posterior pituitary, different regulatory mechanisms are responsible for their hypothalamus-controlled hormonal secretion.
|Hypothalamic-Anterior Pituitary Coordination|
- Basic Architecture
- Secretion of hormones by cells of the anterior pituitary is controlled by stimulatory or inhibitory factors synthesized by cells residing within the hypothalamus. These factors are known as hypothalamic releasing or inhibitory hormones and are secreted into the hypothalamic-pituitary portal vessels which vascularly connect the hypothalamus to the pituitary gland. These portal vessels perfuse the capillaries of the anterior pituitary thus allowing hypothalamic releasing and inhibitory hormones to immediately reach the anterior pituiatry cells which they control. In some cases, the peptide hormone secreted by the anterior pituitary is the final, physiologically important hormone that modulates target tissues (e.g. prolactin). However, in many cases the peptide hormone secreted by the anterior pituitary simply acts as an intermediate regulatory factor which induces a distant organ to release the physiologically relevant hormone (e.g. TSH).
- Basic Regulation
- Although diurnal variations may exist, most hormones released by the anterior pituitary display relatively stable concentrations within the plasma. An important question in endocrinology has been how these relatively stable concentration levels are achieved. For nearly all hormones the answer appears to be powerful negative feedback control loops in which the final physiologically relevant hormone, be it secreted by the anterior pituitary itself or a distant organ, feeds back and inhibits secretion of hypothalamic releasing hormones or the relevant anterior pituitary hormones. We will discuss the specifics of these negative feedback control mechanisms for each hormone system.
- Specific Regulation
- As mentioned, once secreted into the hypothalamic-pituitary portal vessels, releasing and inhibitory hormones act on specific anterior pituitary cells to control synthesis and secretion of particular peptide hormones. Below we provide an outline of these specific regulatory relationships.
- Hypothalamic GHRH stimulates pituitary Somatotrophs to produce Growth Hormone
- Hypothalamic GnRH stimulates pituitary Gonadotrophs to produce LH and FSH
- Hypothalamic CRH stimulates pituitary Corticotrophs to produce ACTH
- Hypothalamic TRH stimulates pituitary Thyrotrophs to produce TSH while hypothalamic Dopamine is inhibitory
- Hypothalamic PRL stimulates pituitary Lactotrophs to produce Prolactin while hypothalamic Dopamine is inhibitory
|Hypothalamic-Posterior Pituitary Coordination|
- The posterior pituitary is in reality an outgrowth of the hypothalamus and represents the hormone-secreting nerve endings of cell bodies residing within the hypothalamus. The cell bodies reside within the supraoptic and paraventricular nuclei of the hypothalamus and extend axons through the hypophysial stalk, ultimately ending in the posterior pituitary. The peptide hormones of ADH and oxytocin are released from these posterior pituitary nerve endings following stimulation of the cell bodies residing with the hypothalamic nuclei.