Endocrinology Study Guide

Neuroendocrine Anatomy

General

• Sella Turica

  • saddle-shaped depression in the Sphenoid bone

• Diencephalon

  • “Interbrain”

  • region including the: thalamus, hypothalamus, epithalamus and subthalamus

• Ventricular System

  • four connected cerebrospinal fluid-filled cavities within the brain

    • continuously circulates through ventricles and into subarachnoid spaces of CNS

  • CSF produced in choroid plexus of ventricles

  • third ventricle surrounded by hypothalamic nuclei

• Pineal Gland

  • on the dorsal surface of the midbrain

  • secretes melatonin in response to changing photoperiods

    • relevant for seasonal breeders

• Optic Chiasma

  • anterior to the hypothalamus

Hypothalamus

General:

• the hypothalamus is a specialized ventral portion of the brain consisting of groups of nerve cell bodies called hypothalamic nuclei that appear as lobules

• neural control center for reproductive hormones

• maintains homeostasis

  • influences autonomic nervous system and manages hormones

• synthesizes: GnRH, OT, SST, CRH, GHRH

Components:

• Paraventricular Nucleus (PVN)

  • produces Oxytocin

• Supraoptic Nucleus (SON)

  • produces Vasopressin

    • anti-diuretic hormone

• Principle Mamillary Fasciculus (PM)

• Posterior Hypothalamic Area (PHA)

• Dorsal Hypothalamic Area (DHA)

• Dorsomedial Nucleus (DMN)

• Mammillary Bodies

  • recollective memory/direction

• Surge Control Center (collection of nuclei)

  • female only

• Tonic Control Center (collection of nuclei)

  • male and female

GnRH Surge Control Center

• Female only

• Consists of the:

  • ventromedial nucleus (VMN)

  • arcuate nucleus (ARC)

  • median eminence (ME)

    • one of the three portal systems in the body (hepatic, renal, brain)

    • one of seven areas of the brain without the blood-brain-barrier

    • the site where hypothalamic releasing hormones are released into the portal capillary bed to be transported to the anterior pituitary

GnRH Tonic Control Center

• Female and Male

• Consists of the:

  • preoptic nucleus (PON)

  • suprachiasmatic nucleus (SCN)

  • anterior hypothalamic area (AHA)

Pituitary Gland

General:

• two lobes: Anterior and Posterior

  • both housed in sella turica

Anterior Pituitary (adenohypophysis):

• non-neural tissue

• connected to hypothalamus via blood supply

  • via hypophyseal portal vessels carrying releasing hormones

• synthesizes and stores:

  • FSH, & LH (Gonadatrops)

  • PRL, GH, TSH, ACTH, MSH

    • ACTH & MSH share same precursor molecule (pro-opiomelanocortin)

Posterior Pituitary (neurohypophysis):

• neural tissue

• structurally connected to hypothalamus

  • nerve endings coming directly from hypothalamus

• axons from neurons in the hypothalamus (PVN) directly here and release neurohormones into a simple arteriovenous capillary plexus

  • PVN produces oxytocin → transported down axons to terminals in the PL

    • if neuron is stimulated → OT is released into the blood

• does not synthesize, only stores

  • stores OT and VP

    • synthesized in hypothalamus

Transport of Hormones from Hypothalamus

Hypothalamus-Pituitary Interrelationships

Vasculature

• general capillary system

  • drains blood into heart directly from brain

  • regular vein: capillary bed → heart

• portal capillary system

  • drains blood into another capillary system through veins

    • portal vein → connects capillary beds

    • the portal system in the median eminence allows the hypothalamus to communicate with the peripheral endocrine system

      • by sending and receiving signals from portal vasculature

Hypothalamohypophyseal Portal System (Anterior Pituitary)

• carries hypothalamic hormones specifically to anterior pituitary without dilution in the system blood

  • allows rapid response

  • little dilution of peptide hormones

    • peptide hormones have a short ½ life

• specific hypothalamic nuclei secrete releasing hormones or releasing inhibiting factors that control release of anterior pituitary hormones

  • GnRH stimulates release of LH and FSH

  • Preovulatory LH surge is controlled by GnRH from the surge center

  • Tonic LH release is controlled by GnRH from the tonic center

• Parvocellular Neurosecretory Cells in PVN

  • project to the median eminence

  • integral to the hypophyseal portal system to the anterior lobe

    • corticotropic releasing hormone

• Components:

  • Medial Hypophyseal Artery (MHA)

  • Primary Portal Plexus (PPP)

  • Portal Vessel (PV)

    • carry releasing hormones to anterior pituitary

  • Superior Hypophyseal Artery (SHA)

    • a branch from the C6 segment of the internal carotid artery

  • Secondary Portal Plexus (SPP)

    
    

Hypothalamic Hormones to Posterior Pituitary

• Neurophysin-Hormone Complex

  • carrier proteins which transport the hormones oxytocin and Vasopressin to the Posterior Pituitary

• Neurophysin

  • specific binding protein

  • transported down the unmyelinated axons of the cells to posterior lobe

• Magnocellular Neurosecretory Cells

  • neuroendocrine neurons whose cell bodies are mainly in the PVN

• PVN → Oxytocin & Vasopressin

• SON → Vasopressin

Receptors and Pathways

Cells of Target Organs Have Specific Receptors for Hormones

• Endocrine Gland → Secretes Hormones

• Target Tissue → Specific Receptors

Hormones Bind to a Receptor & Stimulate a Specific Cellular Response

Protein & Peptide Hormones

• receptors in plasma membrane

Steroid Hormones

• receptors in nucleus and plasma membrane

Prostaglandin Hormones

• receptors in plasma membrane

General Concepts:

• the number of receptors on or in a cell regulates the degree of cell stimulation and cellular response to the hormone

  • more sensitive = more receptors to bind

• hormones can regulate whether there is an increase or decrease in the number of receptors

• timing of hormone release is crucial

  • receptor needs to be available

Signaling Pathway

• GTP (Guanosine Triphosphate)

Spare Receptors

• in most systems, the max biological response is achieved at concentrations of hormone lower than required to occupy all the receptors on/in a cell

• maximum response with 2-3% receptor occupancy

  • 97% of receptors are “spare”

• the greater the proportion of spare receptors…

  • the more sensitive the target cell to the hormone

    • lower concentration of hormone required to achieve half-max response

• examples:

  • insulin stimulates max glucose oxidation in adipocytes

    • with 2-3% of receptors bound

  • LH stimulates max testosterone in leydig cells

    • when 1% of receptors are bound

Concept of Receptor Up and Down Regulation

• during estrus

  • estrogen stimulates increase in progesterone receptor in endometrium

  • CL: progesterone secretion over 8-10 days

    • progesterone down-regulates its own receptor over 8-10 days

    • loss of P4 receptor in epithelium, not stroma

  • loss of progesterone receptor permits increase of estrogen receptor in endometrial epithelium

  • anytime there is increased estrogen, there is an increase in oxytocin uptake of receptors

Steroidogenic Pathway

Mechanisms of Steroid Hormone Action:

• steroid hormones are not water soluble

  • need a carrier protein → albumin

    • sex hormone binds globulin

    • corticosteroid binds globulin

    • thyroxin binds globulin

• fast response

  • steroid binding to membrane receptors

  • adenylate cyclase activation

  • protein kinase activation

• slow response

  • hours to days

  • SRE: Serum Response Element

    • short sequences of DNA within a gene promoter region

    • bind specific transcription factors & regulate transcription of genes

• secretion/excretion

  1. steroid secreted by gonad

  2. steroid enters blood and goes to target tissue

  3. steroid causes change in target tissue

  4. steroid in blood passes through liver

  5. liver renders steroid H2O soluble (glucuronides and sulfates)

  6. Reenters blood and enters kidney or bile

  7. excreted in urine and/or feces as glucuronide or sulfate

Order and ½ Life (Bovine)

Hormones and Intermediates:

cholesterol (C27) → pregnenolone (C21) → progesterone (C21) → androstenedione (C19) → testosterone (C19) → estrogen (C18) ~all accomplished through enzymatic conversion~ Two Cell Theory:

Cholesterol (C27) → Progesterone (C21) → Testosterone (C19) → Estrogen (C18) ± 18 - 59 min. ± 2-4 hrs. ± 6-24 hrs. aromatase → enzyme converting testosterone to estrogen

Congenital Adrenal Hyperplasia

• deficiency for enzyme (21-hydroxylase)

  • responsible for conversion of progesterone to cortisol

• results in an overproduction of androgens and a deficiency in cortisol

Estrogen

• E₁ (Estrone)

  • 66% efficacy

  • 1 OH group

  • major end-product in swine

  • “the estrogen of menopause”

• E₂ (Estradiol)

  • two OH groups

  • E₂a = 50% efficacy

  • E₂b = 100% efficacy

• E₃ (Estriol)

  • 16% efficacy

  • 3 OH groups

  • weak estrogen found in female urine

  • dominant in pregnant women

Selective Estrogen Receptor Modulators (SERMs)

occupies estrogen receptors → blocks action of estrogen in the breast and other tissues Applications:

• Estrogen binds to its receptor to its receptor to stimulate oxytocin receptors in myometrium while progesterone blocks estrogen receptor synthesis

  • thereby removing the effects of oxytocin

• FSH stimulates increase in FSH receptors on Granulosa Cells to prepare for ovulation by…

  • increasing estrogen synthesis

  • increasing LH receptors

Example:

• Tamoxifen

  • SERM medication used to treat breast cancer in men and women

  • does the job, but at what cost?

    • agonist at bone and uterus

    • antagonist at breast

Lack of Androgen Receptor in Male → Testicular Feminization

• During Gestation:

  • Testis Determining Factor (TDF) →

  • Testes develop →

  • Sertoli Cells secrete Anti-Mullerian Hormone (AMH) - regresses feminization characteristics →

    • AMH causes leydig cells to differentiate

    • no AMH?????? →

      • degeneration of paramesonephric duct →

        • testicular feminization

Prostaglandin Pathway

Prostaglandin

Basics:

• produced by all tissues of the body

• can have a local effect on tissues

  • same tissue which produced it

• inhibited by corticosteroids

• cow - rapidly degraded in lungs

• mare - slowly degraded

• Includes: A, B, C, D, E, F, G, H, I & J

Precursors and Pathway

underlined and italicized are precursors to the pathway

• Fatty Acid Phospholipids (diacylglycerol or phospholipid) →

  • rate limiting (Phospholipase A₂)

• arachidonic acid (precursor to prostaglandins) →

  • COX1, COX2

• Prostaglandin G₂ (PGG) →

• Prostaglandin H₂ (PGH)→

  • Prostaglandin D₂ →

    • Prostaglandin J₂

    • Or Prostaglandin F2

  • Prostaglandin I₂

  • Prostaglandin E₂ (foal makes) →

    • Prostaglandin F₂

      • inactive PG metabolite (PGFM)

    • Prostaglandin A₂ →

      • Prostaglandin B₂

PGF_2a

• causes vasoconstriction of blood vessels

• effects luteal cell of CL

  • causes CL regression

• involved with ovulation, parturition, and sperm transport

• Hydroxyl (OH) Group

PGE₂

• causes vasodilation of blood vessels

• maintain CL

• cumulus expansion

• embryo implantation

• ovulation cascade (follicle rupture)

• meiotic maturation

• Oxygen double bond

Cyclo-Oxygenase Enzymes (COX1 and COX2)

General:

• two enzymes involved in conversion of Arachidonic Acid to Prostaglandin

• target for many drugs

COX1

• maintains prostaglandin level at a homeostatic level

  • tonic

• example: aspirin

COX2

• inducible

  • inflammatory response (surge)

• upregulates

• example: arthritis drugs

Inhibitory Compounds

Aspirin

• irreversibly inhibits COX1 and COX2

  • primarily COX1

Corticosteroids (Dexamethasone)

• inhibits COX2

• caution during use with pregnant animals

  • mimics fetal stress → cause parturition

NSAIDs

• inhibit the activity COX1 and/or COX2

  • primarily COX2

Hormones

Classifications and Breakdown

Steroid (from cholesterol)

Estrogen (Estradiol 17B)

• Gland: Graafian Follicle

• Principle Function(s): secondary sex characteristics, begins ovulation cascade, female mating behavior

Progesterone

• Gland: Corpus Luteum

• Principle Function(s): caps the surge, inhibits estrus, pregnancy hormone

Testosterone

• Gland: leydig cells

• Principle Function(s): male sex characteristics, spermatogenesis synthesis

Glucocorticoids & Corticosteroids (Cortisol)

• Gland: adrenal cortex

• Principle Function(s): induction of parturition by fetus, stress response

Estrogen/Progestins

• Gland: placenta

• Principle Function(s): regulate placental blood flow, maintain pregnancy

Glycoprotein (protein hormone with CHO molecules)

longer life than standard protein hormones some have large amounts of sialic acid sialic acid: acetylated neuraminic acid a 9C sugar (present on FSH, very little on LH)

Luteinizing Hormone (1-2% CHO)

• Gland: anterior pituitary

• Principle Function(s): surge = ovulation, tonic = formation of CL

Follicle Stimulating Hormone (8% CHO)

• Gland: anterior pituitary

• Principle Function(s): stimulate follicle growth

Equine Choriogonadotropin (45% CHO)

• Gland: placenta

• Principle Function(s): equids-LH like activity, formation of accessory CL, non-equids-FSH activity

Human Choriogonadotropin (30% CHO)

• Gland: placenta

• Principle Function(s): establishment of pregnancy in women (maternal recognition signal), LH-like activity

  • home pregnancy test → hCG is degraded and passed in urine

Peptide (few to several amino acids, hypothalamus derived)

neurohormones: produced in specific neural cell & released at its neural terminal

Oxytocin (octapeptide - 8aa)

• Gland: CL, hypothalamus (stored in post pit)

• Principle Function(s): binds to uterine endometrium to release PGF2a, muscle contractions, milk let-down

Gonadotropin Releasing Hormone (decapeptide - 10aa)

• Gland: hypothalamus

• Principle Function(s): release gonadatrops (LH & FSH)

Growth Hormone Releasing Hormone (GHRH)

• Gland: hypothalamus

• Principle Function(s): stimulates release of growth hormone from Anterior Pituitary

Corticotropic Releasing Hormone

• Gland: hypothalamus

• Principle Function(s): stimulates release of ACTH from Adrenal Cortex

Lipid (from arachidonic acid)

Prostaglandin F2a, Prostaglandin E

• Gland: uterus endometrium, Graafian Follicle, Seminal Vesicles

• Principle Function(s): kill CL, stimulate myometrial contractions

Biogenic Amine (derived from Tyrosine or Tryptophan)

Melatonin

• Gland: pineal gland

• Principle Function(s): control of seasonal reproduction

Dopamine

• Gland: hypothalamus

• Principle Function(s): inhibits release of Prolactin from Anterior Pituitary

Protein (long chains of amino acids)

degraded in kidney and liver by breaking peptide bonds (cleaving amino acids)

Prolactin (198aa)

• Gland: Anterior Pituitary

• Principle Function(s): stimulate milk synthesis

Placental Lactogen

• Gland: placenta

• Principle Function(s): stimulates mammary growth and milk secretion

Relaxin

• Gland: Graafian Follicle (ovary), placenta

• Principle Function(s): expansion of pelvis, dilation of cervix at parturition

Activin

• Gland: Graafian Follicle (ovary)

• Principle Function(s): stimulates FSH secretion

Inhibin

• Gland: Graafian Follicle (ovary)

• Principle Function(s): regulates release of FSH from Ant Pit

Insulin-Like Growth Factor (IGF1 & IGF2)

• Gland: Liver

• Principle Function(s): stimulates steroidogenesis, mammary growth, fetal development, inhibits programmed cell death

Adrenocorticotropic Hormone (ACTH)

• Gland: anterior pituitary

• Principle Function(s): release of corticosteroids and glucocorticoids from adrenal cortex, initiates parturition

Growth Hormone (GH) (Somatotropin)

• Gland: anterior pituitary

• Principle Function(s): stimulates milk synthesis through IGF1 secretion from liver

Factors Affecting Strength of Hormone Action

Pattern and Duration of Secretion

Luteinizing Hormone Surge: ovulation Tonic: maintenance/development of CL

COX1 & COX2: tonic vs surge

½ Life

Shortest ½ life: GnRH - straight shot Hypothalamus → Anterior Pituitary via portal system, Oxytocin dumped in periphery, PGF2a - endometrium → myometrium (stimulate contractions) or ovary (kill CL) one trip = 90% metabolized Longest ½ life: eCG & hCG - high CHO amount (carbohydrate molecule moiety) takes longer to cleave off all carbohydrates

Receptor Density

the more there, the more sensitive is. upregulate receptors to

throw baseball in a crowd, more people there → better chances of ball being caught

upregulation better be at the right time or little to no effect (less chances of ball being caught)

estrogen sets stage for progesterone and vice versa

Receptor-Hormone Affinity

agonist - binds and gives biological response. antagonist - binds but does not illicit biological response. estrogen (estrone will fit, but does not send signal because it binds but does not cause response due to switched OH and H on the chain.

Structural Analysis/Determination of Biological Activity

LH, FSH, RSH, eCG and hCG

Two Chains → Held Together by Disulfide Bonds need both for biological activity

• alpha - a

  • identical (ALWAYS THE SAME)

  • common among hormones

  • a- subunits all exhibit the same amino-acid sequence in a given species

  • human = 92aa, other mammals = 96aa

• beta - b

  • gives the hormone its specific action

  • unique for each hormone

  • specific hormone and amino acids

    • FSHb - 111aa

    • LHb - 120aa

    • TSHb - 138aa

    • hCGb - 145aa

    • eCGb - 145aa

Inhibin (inhibits FSH)

Consists of:

• 1 a chain

• 1 of 4 b chains

  • b_A, b_B, b_C, b_E

Activin (causes release of FSH)

Consists of:

• 2 b chains

• Activin A is a homodimer with two b_A chains

• Activin B is a homodimer with two b_B chains

• Activin AB is a heterodimer with b_A and b_B chains

Synthetic Hormones (Delta-4 (5)Pathway)

long ½ life in blood system → not degraded in liver quickly can be given orally or implanted have strong affinity for receptors

Progestins

• Melengestrol Acetate (MGA)

  • feed additive

  • prevents cyclicity in feedlot heifers

• Norgestimate

• Altrenogest (Matrix/Regumate)

• CIDR

  • 1.38g Progesterone

Testosterone (Androgens)

Estrogens

• Zeranol (Ralgro)

  • estrogenic

  • slow release

• Synovex Implant

  • combination of estrogen and androgen

    • testosterone proprionate

    • estradiol benzoate

Kisspeptin

• discovered as a metastasis inhibitor in melanoma cell lines

• later found to be stimulator of the reproductive system

  • lack kisspeptin or receptor → remain infertile

  • lack functioning kisspeptin system → don’t reach puberty

• used to super-ovulate in human IVF procedures

• KNDy neurons involved with the positive feedback actions of E2 to induce LH surge

½ Life of Hormones in the Body

as CHO increases → ½ Life Increases

What affects the ½ life of hormones in the body?

• % CHO moieties on molecule

• structure/length of molecule

Disappearance = % In Blood After Injection

GnRH/Oxytocin = 0% in 10 min.

LH = 0% in 45 min.

FSH = 0% in 4 hrs.

hCG = 75% in 6 hrs.

eCG = 95% in 6 hrs.

Fescue Toxicity

Estimated losses attributed to fescue toxicosis: 2001: >$800 million/year 2015: >$2 billion/year

Cause: Ergovaline

• Ergovaline: ergot alkaloid produced by fungal endophyte in tall fescue

• endophyte fungus is in leaf sheath cell

  • not in leaf blade

• at levels of 200 to 300 ppb

  • symptoms begin to be observed

Implications on Production

• rate of gain decreased

  • partially from reduced consumption → animals sense or taste toxins

  • affects on the microbiome

• reproductive problems

  • low conception rates

  • poor offspring survival

  • agalactia

    • the absence (or diminished) milk production

    • especially severe when cows are grazing tall fescue during last trimester

    • leads to:

      • aborted fetuses & lack of colostrum/milk for young

• elevated body temperature & loss of blood flow to extremities

  • cause: rough appearing hair coat, loss of hoof, ears, nose & tail

• thickened placentas ** in mares

  • cause foals to suffocate after birth

Preventative Measures

• don’t graze tall fescue when temperatures are above 75F

• do not fertilize an infected strand with high N fertilizer rates

• remove seed head

  • at maturity, endophyte levels are highest in seed head

• over seeding clover

  • symbiotic N-fixation throughout the season

  • cattle consuming more than just infected forage

    • diluting amount of toxin consumed

Key Terms:

Antagonist

• does not initiate a biological response upon binding to its receptor

Agonist

• does initiate a biological response upon binding to its receptor

Anterior Pituitary

• adenohypophysis, non-neural tissue, connected to hypothalamus via blood supply

• synthesizes and stores: FSH, LH, ACTH, TSH, MSH, GH, PRL

Posterior Pituitary

• neurohypophysis, neural tissue, connected to hypothalamus structurally

• does not synthesize

• only stores: oxytocin, vasopressin

5a-reductase

• enzyme converting Testosterone to dihydrotestosterone (DHT)

  • aromatase converts DHT to Estrone

21-hydroxylase

• enzyme converting 17a-Hydroxyprogesterone to cortisol

  • not there? goes to andostenedione

Dopamine Agonist

• activates dopamine receptors

Ergo Alkaloid

• bromocriptine

  • blocks release of Prolactin from Anterior Pituitary

  • affects menstrual cycle and milk production

• ergovaline

  • produced by the fungal endophyte in tall fescue that causes fescue toxicosis

Selective Estrogen Receptor Modulator (SERM)

• occupies estrogen receptors → blocks action of estrogen in the breast and other tissues

Estriol

• E3

  • 3 OH groups

• weak estrogen found in female urine

• dominant in pregnant women

Estrone

• E1

  • 1 OH group

• 66% efficacy

• estrogen of menopause

• major end-product in swine

Estradiol

• E2

  • 2 OH groups

• result of conversion of testosterone by aromatase

• beta chain = 100% efficacy

• alpha chain = 50% efficacy

NSAIDS

• non-steroidal anti-inflammatory drugs

• inhibit the activity of COX1 and/or COX2

Chimera

• fusion of morula (two sets of DNA)

• 1984: 1st chimeric sheep-goat pregnancy

Phosphodiesterase

• activated by kinase

• inactivates cAMP by conversion to 5AMP

• shut-off mechanism

Enzyme-Linked ImmunoSorbent Assay (ELISA)

• an immunological assay commonly used to measure antibodies, antigens, proteins and glycoproteins in biological samples

• process:

  • antigen binds to surface

  • antigen-enzyme conjugate attaches to enzyme

  • substrate and enzyme interaction creates color change detection

• receptor made for specific molecule

• tagged antigen, hormone-receptor affinity

Kisspeptin

• stimulant of the reproductive tract

• used to super-ovulate women for IVF procedures

• lack of system → no puberty

• lack of kisspeptin and/or receptor at maturity → no fertility

• discovered as a metastasis inhibitor in melanoma cell lines

• going to be very important for future reproduction techniques

• involved in long vs short day breeders

Up-Regulation

• increase in number of target receptors as a response to continuous exposure to antagonist

Down-Regulation

• decrease in number of target receptors as a response to continuous exposure to agonist

Positive Feedback

• stimulation

• example: Estrogen

Negative Feedback

• suppression

• example: Progesterone