This study material has been compiled from a lecture audio transcript and copy-pasted text, originally presented by Prof. Nedret Altıok on Medical Pharmacology – Hypothalamic and Pituitary Hormones.

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📚 Hypothalamic and Pituitary Hormones: A Pharmacological Overview

Introduction

The hypothalamic and pituitary hormones form the central links of the endocrine system, regulating numerous fundamental physiological processes in the body. Anterior pituitary hormones are primarily controlled by hypothalamic hormones, and with the exception of prolactin, they mediate their effects by regulating the production of other hormones in peripheral tissues. This complex regulatory system offers multiple avenues for pharmacological intervention.

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I. Anterior Pituitary Hormones

A. General Overview

✅ Hormones of the anterior pituitary are central to the hypothalamic-pituitary endocrine axis. ✅ All anterior pituitary hormones are under the control of a hypothalamic hormone. ✅ With the exception of prolactin, all mediate their effects by regulating the production of other hormones in peripheral tissues. ✅ Four anterior pituitary hormones (TSH, LH, FSH, and ACTH) and their hypothalamic regulators are subject to feedback regulation by the hormones they control.

B. Growth Hormone (GH) and Mecasermin

1. Physiology of Growth Hormone (GH)

📚 Growth Hormone (GH): Essential for normal growth during childhood and adolescence. It is a crucial regulator of lipid and carbohydrate metabolism and lean body mass throughout life. 💡 Its effects are primarily mediated by regulating the production of Insulin-like Growth Factor 1 (IGF-1 or Somatomedin) in peripheral tissues.

2. Therapeutic Uses of GH (Somatropin)

✅ Somatropin (recombinant human GH) is used for: * GH deficiency in children and adults. * Children with genetic diseases associated with short stature (e.g., Turner syndrome, Noonan syndrome, Prader-Willi syndrome). * Children with failure to thrive due to chronic renal failure or small-for-gestational-age condition. * Adults with AIDS-associated wasting and GH deficiency. * Improving gastrointestinal function in patients who have undergone intestine resection and developed malabsorption syndrome.

3. Adverse Effects of GH

⚠️ In Children: * Pseudotumor cerebri * Slipped capital femoral epiphysis * Progression of scoliosis * Edema * Hyperglycemia ⚠️ In Adults: Generally tolerate GH less well than children. * Peripheral edema * Myalgia * Arthralgia

4. Mecasermin (Synthetic IGF-1)

📚 Mecasermin: Recombinant human IGF-1. ✅ Used for children with IGF-1 deficiency who are unresponsive to GH therapy. ✅ Administered parenterally. ⚠️ Most important toxicity: Hypoglycemia. This can be prevented by consuming a snack or meal shortly before administration.

C. Growth Hormone Antagonists

GH-secreting pituitary adenomas cause:

• Acromegaly in adults.
• Gigantism in children and adolescents who have not completed their growth phase (rarely). Pharmacologic treatment aims to:

1. Inhibit GH secretion.
2. Interfere with GH effects.

1. Somatostatin Analogs

📚 Somatostatin: A 14-amino-acid peptide that inhibits the release of GH, glucagon, insulin, TSH, prolactin, and gastrin. Produced by neuroendocrine neurons of the hypothalamus, delta cells in the pyloric antrum, duodenum, and pancreatic islets. ✅ Octreotide and Lanreotide: Long-acting synthetic analogs of somatostatin. * Uses: Acromegaly, carcinoid, gastrinoma, glucagonoma, and other endocrine tumors. * Administration: * Regular octreotide: Subcutaneously 2-4 times daily. * Slow-release intramuscular formulation (octreotide or lanreotide): Every 4 weeks for long-term therapy. * Adverse Effects: Significant GI disturbances, gallstones, and cardiac conduction abnormalities.

2. Dopamine D2 Receptor Agonists

✅ Bromocriptine: More effective at inhibiting prolactin release than GH release from the anterior pituitary, but can also affect GH. ✅ High doses can be used in the treatment of acromegaly.

3. Pegvisomant

📚 Pegvisomant: A GH receptor antagonist. ✅ Approved for the treatment of acromegaly. 💡 It is a long-acting derivative of a mutant GH that can cross-link GH receptors but cannot induce the conformational changes needed for receptor activation. ✅ Administered subcutaneously.

D. Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH) (Gonadotropins) and their Analogs

1. Physiology of Gonadotropins

✅ In Women: FSH directs follicle development; FSH and LH collaborate in ovarian steroidogenesis. ✅ In Men: FSH is the primary regulator of spermatogenesis; LH is the main stimulus for testicular androgen production.

2. Therapeutic Uses

✅ Used in combination to: * Stimulate spermatogenesis in infertile men. * Induce ovulation in women with anovulation unresponsive to less complicated treatments. * Controlled ovarian hyperstimulation for in vitro fertilization. ✅ All gonadotropin preparations are administered parenterally.

3. Gonadotropin Preparations

• Menotropins: Mixture of FSH and LH purified from the urine of postmenopausal women.
• FSH and its analogs:
  • Urofollitropin: Purified preparation extracted from the urine of postmenopausal women.
  • Recombinant forms of human FSH:
    • Follitropin alpha
    • Follitropin beta (identical to natural human FSH)
• LH and its analogs:
  • Human Chorionic Gonadotropin (hCG): Placental protein supporting the corpus luteum during early pregnancy. Structure nearly identical to LH, mediates effects via LH receptors. Purified from pregnant woman's urine or recombinant hCG is used for LH activity.
  • Lutropin: Recombinant form of human LH.

E. Gonadotropin-Releasing Hormone (GnRH) and its Analogs

1. Physiology of GnRH

📚 GnRH: A decapeptide that stimulates gonadotropin release when secreted in a pulsatile pattern by the hypothalamus.

2. GnRH Agonists

✅ Examples: Leuprolide, goserelin, histrelin, nafarelin, triptorelin. 💡 Mechanism: Continuous dosing inhibits gonadotropin release by downregulating GnRH receptors in pituitary cells. ✅ Therapeutic Uses (to suppress endogenous gonadotropin secretion): * Induction of spermatogenesis in infertility (men). * In women undergoing ovulation induction with gonadotropins. * In women with gynecologic disorders benefiting from ovarian suppression (e.g., endometriosis, polycystic ovary). * In men with advanced prostate cancer. * In children with central precocious puberty. ⚠️ Adverse Effects: * In Women: Typical menopause symptoms (hot flushes, sweats, headache). Long-term treatment avoided due to risk of bone loss and osteoporosis. * In Men: Hot flushes, sweats, gynecomastia, reduced libido, decreased hematocrit, reduced bone density.

3. GnRH Antagonists

✅ Examples: Ganirelix, cetrorelix, degarelix. ✅ Therapeutic Uses: * Ganirelix & Cetrorelix: Used during ovulation induction to suppress endogenous gonadotropin production (alternative to GnRH agonists). * Degarelix: Approved for the treatment of advanced prostate cancer. ⚠️ Adverse Effects: Similar to continuous GnRH agonist treatment, except GnRH antagonists do not cause a tumor flare when used for advanced prostate cancer.

F. Prolactin Antagonists (Dopamine D2 Receptor Agonists)

1. Physiology of Prolactin

✅ The anterior pituitary hormone prolactin regulates lactation. ✅ Dopamine is the physiological inhibitor of prolactin release.

2. Hyperprolactinemia

✅ Can result from prolactin-secreting adenomas, leading to infertility and galactorrhea in women and men.

3. Therapeutic Uses

✅ Dopamine D2 receptor agonists (e.g., bromocriptine, cabergoline, pergolide) are effective in: * Reducing serum prolactin concentrations. * Restoring fertility in hyperprolactinemia. * High doses can also be used in the treatment of acromegaly.

G. Adrenocorticotrophic Hormone (ACTH, Corticotropin)

1. Physiology of ACTH

📚 ACTH (Corticotropin): Anterior pituitary secretion that controls the synthesis and release of glucocorticoids and the mineralocorticoid aldosterone from the adrenal cortex. ✅ Corticotropin-Releasing Factor (CRF): A peptide that releases ACTH and β-endorphin from corticotrophs. ✅ Blood ACTH concentration is reduced by glucocorticoids, forming the basis of the dexamethasone suppression test for Cushing's syndrome diagnosis.

2. Therapeutic and Diagnostic Uses

• Therapy: ACTH itself is not often used in therapy due to less predictable action than corticosteroids and potential for antibody formation.
• Diagnosis:
  • Tetracosactide: A synthetic polypeptide (first 24 N-terminal residues of human ACTH).
    • Actions on adrenal cortex: 1️⃣ Stimulation of glucocorticoid synthesis and release (within minutes). 2️⃣ Trophic action on adrenal cortical cells, regulating key mitochondrial steroidogenic enzymes.
    • Main use: Diagnosis of adrenal cortical insufficiency. Administered IM or IV, followed by plasma hydrocortisone measurement.

3. Inhibitors of ACTH Secretion and Function

• Pasireotide: A somatostatin analog (agonist at four of five somatostatin receptor subtypes, high affinity for type 5).
  • Mechanism: Inhibits GH secretion (used in acromegaly) and ACTH secretion.
  • Uses: Reduces circulating cortisol levels in patients with ACTH-producing pituitary tumors; used in Cushing's disease patients not candidates for surgery or with recurrent disease.
  • Administration: Subcutaneous or IM (long-acting preparation).
• Cabergoline: Potent, long-acting dopamine (D2) receptor agonist.
  • Uses: Primarily for hyperprolactinemia. Also inhibits ACTH secretion from corticotroph tumors (often D2 receptor positive) and is occasionally used off-label for this purpose.

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II. Posterior Pituitary Hormones

A. Oxytocin

1. Physiology

📚 Oxytocin: A nonapeptide synthesized in the paraventricular nuclei of the hypothalamus and transported to the posterior pituitary.

2. Therapeutic Uses

✅ Effective stimulant of uterine contraction. ✅ Used intravenously to induce or reinforce labor.

3. Antagonists

✅ Atosiban: An oxytocin receptor antagonist. ✅ Used in some countries as a tocolytic (drug to treat preterm labor).

B. Vasopressin (Antidiuretic Hormone [ADH])

1. Physiology

📚 Vasopressin (ADH): Synthesized in neuronal cell bodies in the hypothalamus and released from nerve terminals in the posterior pituitary. ✅ V2 Receptors: Located in the distal tubule and collecting ducts of the kidney. Activation increases insertion of water channels, increasing membrane permeability to water, thus providing an antidiuretic effect. ✅ Extrarenal V2-like Receptors: Regulate release of coagulation factor VIII and von Willebrand factor from endothelial cells. ✅ V1 Receptors: Located on vascular smooth muscle. Activation causes vasoconstriction.

2. Therapeutic Uses

• Vasopressin:
  • Due to vasoconstrictor effect, sometimes used to treat bleeding from esophageal varices or colon diverticula.
  • Intravenous vasopressin may cause coronary artery spasm, leading to angina.
• Desmopressin: Selective agonist of V2 receptors.
  • Administration: Oral, nasal, or parenteral.
  • Uses: Pituitary diabetes insipidus, mild hemophilia A, or von Willebrand disease.

3. Vasopressin Receptor Antagonists

✅ Examples: Conivaptan, tolvaptan. ✅ Uses: Treatment of hyponatremia, especially in patients with congestive heart failure, liver cirrhosis, or Syndrome of Inappropriate ADH Secretion (SIADH). 📚 SIADH: Water retention increases blood volume (hypervolemia) combined with urinary salt loss, often resulting in dilutional hyponatremia. ✅ Demeclocycline: Counteracts ADH action on renal tubules. Can be used to treat SIADH caused by excessive ADH secretion.

4. Other Vasopressin Analogs

• Terlipressin: Increased duration of action, low but protracted vasopressor action, minimal antidiuretic properties.
• Felypressin: Short-acting vasoconstrictor. Injected with local anesthetics (e.g., prilocaine) to prolong their action.