Hypothalamo-pituitary Axis

1. Hypothalamo-pituitary Axis And Pineal Gland

2. The Endocrine Hypothalamus The hypothalamus is a small area of the brain lying between midbrain and forebrain, it is now considered as an �endocrine gland� The hypothalamus receives nerve fibers directly or indirectly from all areas of the brain. Thus, its activity is in part regulated by higher centers. Extremely significant in controlling autonomic function Has a number of neurons which synthesize and secrete neurohormones The neurohormones secreted by the hypothalamus are either releasing or inhibiting hormones that control secretion of adenohypophyseal (anterior pituitary) hormones The hypothalamus plays a central role in the endocrine system.

4. The hypothalamic - pituitary axis The pituitary is connected to the hypothalamus by a stalk (the infundibulum) which carries axons to the neurohypophysis (posterior pituitary) as well as blood vessels to the adenohypophysis (anterior pituitary).

5. The Hypophyseal Portal System

6. Characteristics of Hypothalamic releasing and inhibiting hormones Hypothalamic releasing and inhibiting hormones are carried directly ONLY to the anterior pituitary gland via hypothalamic hypophyseal portal veins. Secretion is pulsatile They have short half lives so their actions on the pituitary are of short duration. Their actions are limited by negative feedback mechanisms. They stimulate release of anterior pituitary hormones. They stimulate synthesis of anterior pituitary hormones. They stimulate hyperplasia and hypertrophy of target cells.

7. Hypothalamic Hormones Secretion: Is influenced by emotions Can be influenced by the metabolic state of the individual Delivered to the anterior pituitary via the hypothalamic-hypophyseal portal system Usually initiates a three-hormone sequence

8. Hypothalamic Neurohormones Seven neurohormones are made in the hypothalamus Thyrotropin-releasing hormone (TRH) Corticotropin-releasing hormone (CRH) Gonadotropin-releasing hormone (GnRH) Growth hormone-releasing hormone (GHRH) Growth hormone-release inhibiting hormone (GHIH) or somatostatin Prolactin-releasing factor (PRF) Prolactin-inhibiting hormone (PIH)

9. Pituitary Gland It lies in a depression of the sphenoid bone called hypophyseal fossa It consists of two parts: neurohypophysis of neural origin developed as a downgrowth from the floor of the third ventricle (posterior pituitary) Adenohypophysis of ectodermal origin formed as an up growth from the buccal cavity (anterior pituitary) Neurohypophysis has three parts: the median eminence which is connected to the neural lobe by the infundibular stem. Adenohypophysis is also divided into three parts: the pars distalis, the pars intermedia and pars tuberalis

10. Hypothalamic releasing & inhibiting hormones and the Pituitary hormones they control

11. Control of Anterior Pituitary It has no neural connection with the hypothalamus There is a capillary plexus in the hypothalamus It gives rise to the Hypothalamo-hypophyseal portal system Neurohormones from hypothalamus pass through this portal system to stimulate or inhibit the anterior pituitary

13. Control of Posterior Pituitary Neurons arising in : Paraventricular nucleus Supraoptic nucleus Sends neurons down pituitary stalk (infundibulum) They synapase with capillaries in the posterior pituitary Secrete the neurohormones Oxytocin and vasopressin ADH Directly into the blood stream

16. Hypothalamus-pineal axis The Pineal gland is a small gland attached to the posterior wall of the 3rd ventricle by a thin stalk Consists of pinealocytes Secretes melatonin in a circadian pattern of release Light inhibits release of melatonin

17. Pineal Gland It is an unpaired, cone shaped gland. Pineal gland is innervated by postganglionic fibers originating from the superior cervical ganglia. Pineal gland is therefore an effector organ. Pineal Gland secretes Melatonin, an indoleamine hormone.

21. Melatonin Biosynthesis Melatonin is a lipid soluble hormone. Melatonin is not stored, once synthesized it is secreted by passive diffusion. The precursor of melatonin is tryptophan. Melatonin is also synthesized in the retina, blood leukocytes and gastro-intestinal tract. In these tissues melatonin acts as paracrine or autocrine hormone.

23. Melatonin Metabolism Melatonin is primarily metabolized in the liver. It is secreted in the urine as sulphate or glucuronide. Other metabolic pathways are catalyzed in the brain.

25. Control of Melatonin Synthesis Major control by the post ganglionic sympathetic nerve fibers (adrenergic control). By releasing noradrenaline Other regulatory mechanisms are present.

26. Endocrine Role of Melatonin Many process regulated by the endocrine system is affected by melatonin. Examples: coat growth, body weight, lactation, hibernation, insulin release, gastro-intestinal physiology etc� No single organ can escape the influence of melatonin.

27. Neuroendocrine Role of Melatonin Melatonin causes increased sensitivity of the hypothalamopituitary axis to the negative feedback effect of steroid hormones. It is known as an antigonadotropic hormone because it inhibits the secretion of LH and FSH, testosterone and prolactin. It increases basal GH release and GH responsiveness to GHRH. Melatonin plays a facilitatory role at the hypothalamic level by inhibiting endogenous somatostatin release . It also inhibits the release of CRH.