22.1 BASIC CONSIDERATIONS

TABLE 22.1: Neurohormones of hypothalamus

Hormones are products of various endocrine (ductless) glands, secreted directly into circulation for transport to target organs. Biochemically, there are three major classes of hormones: (a) steroids, derived from cholesterol, e.g.

Although present in miniscule amounts, hormones are prime regulator of various metabolic and homeostatic activities. All hormones exert their biochemical action by binding with either extracellular receptors (peptide hormones) or intracellular receptors (steroid or thyroid hormones) on target organ. Absence or non­responsiveness of these receptors is an important cause of clinical hormonal deficiency, despite normal or elevated blood levels.

Endocrine system includes three separate but inter­dependent components in order of supremacy, i.e. hypothalamus, pituitary and target endocrinal gland; each regulating the function of descendent gland in most (but not all) cases. At the same time, circulating levels of hormones from target gland also provides a positive or negative feedback to hypothalamus and regulate its endocrinal activity.

Hypothalamus-hypophyseal Axis: Hypothalamus is the supreme regulator for most of the endocrine func­tions and controls functions of anterior pituitary by secreting various neuropeptides, i.e. releasing/inhibiting hormones or factors (Table 22.1). It also synthesizes two direct-acting hormones-oxytocin and vasopressin. While hormonal Releasing/inhibitory factors are transported to anterior pituitary via minute capillary network (hypothalamic hypophyseal portal circulation), oxytocin and vasopressin are transported via axons to posterior pituitary for storage and release.

All neurohormones of hypothalamus are rapid but short-acting and release of them is controlled by many neurosensory inputs, higher function commands and

Releasing factors or Hormones

• Thyrotropin releasing hormone (TRH)

• Corticotropin releasing hormone (CRH)

• Growth hormone releasing hormone (GHRH, Ghrelin)

• Gonadotropin releasing hormone (GnRH)

• MSH-releasing hormone (MSH-RH)

Inhibiting factors or Hormones

• Growth hormone inhibiting Hormone (GHIH or Somatostatin)

• Prolactin inhibiting factor (Dopamine)

• MSH-inhibiting factor (MSH-IF)

Hormones synthesized for posterior pituitary

• Vasopressin or anti-diuretic hormone (ADH)

• Oxytocin

MSH: Melanocyte stimulating hormone

circulating levels of target-endocrine hormone. Since hypothalamus also controls many other body functions apart from endocrinal activity, i.e. sleep, hunger, thirst, sexual behavior and emotional response; hypothalamic causes of endocrinal disturbances are frequently asso­ciated with simultaneous disturbances in these functions. Common causes of hypothalamic dysfunction include developmental malformations or acquired lesions due to tumors, trauma, irradiation and infections.

22.2