HYPOTHALAMICPITUITARY GONADAL AXIS

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HYPOTHALAMIC-PITUITARY GONADAL AXIS
THE PULSE GENERATOR AND FEEDBACK INTERACTIONS
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HYPOTHALAMIC PITUITARY FEEDBACK
MODEL SYSTEMS

• . RAT MODEL

• . PRIMATE MODEL

• . SHEEP MODEL

• . HORSE MODEL

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RAT MODEL (female)

• . HALASZ KNIFE 1969 - SEPARATED POA FROM MBH

• . PRODUCED STATE OF CONSTANT VAGINAL
  CORNIFICATION

• . OVARIES FULL OF FOLLICLES

• . NO OVULATION AND NO CORPORA LUTEA

• . FSH AND LH SECRETED IN BASAL AMOUNTS

• . NO OVULATORY LH SURGE

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RAT MODEL (contd)

• . EXOGENOUS LH CAUSED OVULATION

• . ELECTRIC STIMULATION OF POA CAUSES OVULATORY
  SURGE OF LH

• . NOREPINEPHRINE CAUSES OVULATION IN ANIMALS
  WITH AH LESIONS

• . ?-ADRENERGIC BLOCKER (phenoxybenzamine)
  PREVENTS OVULATION

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QUESTION WHY DO MALES NOT EXHIBIT CYCLIC
GONADOTROPIN PATTERNS?

• MALE RATS DO NOT HAVE A FUNCTIONAL SURGE
  CENTER

• FEMALE RATS EXPOSED TO ANDROGEN AT THE TIME OF
  BIRTH DO NOT HAVE A FUNCTIONAL SURGE CENTER
  EITHER (CALLED ANDROGEN STERLIZATION)

• ROGER GORSKY - SEXUALLY DIMORPHIC NUCLEUS

• NEONATAL FEMALE RAT PUPS TREATED WITH ANDROGEN
  EXHIBIT A MALE-SIZE SDN

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PRIMATE MODEL

• . NEURAL DISCONNECTION OF THE ANTERIOR
  HYPOTHALAMUS (AH) FROM THE MEDIAL BASAL
  HYPOTHALAMUS (MBH) DID NOT INTERFERE WITH
  OVULATION

• . COMPLETE MBH ISOLATION (hypothalamic islands)
  STILL DID NOT INTERFERE WITH OVULATION

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HYPOTHALAMIC ISLANDS CONSISTED OF

• . ARCUATE NUCLEUS (ARC)

• . MEDIAN EMINENCE (ME)

• . VENTROMEDIAL NUCLEUS (VMN)

• . PREMAMMILLARY AREA

• . MAMMILLARY BODIES

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BUT EXCLUDED

• . PREOPTIC AREA (POA)

• . SUPRACHIASMATIC NUCLEI (SCN)

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HYPOTHALAMIC ISLANDS

• . WITH HYPOTHALAMIC ISLANDS, LH WAS SECRETED
  IN RESPONSE TO ESTROGEN ADMINISTRATION
  (150 pg gt 36 hours)

• . CONCLUSION NEUROENDOCRINE MECHANISMS
  GOVERNING BASAL GONADOTROPIN SECRETION ARE
  PRESENT WITHIN THE MBH

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MBH PENINSULA

• . ASPIRATION OF

• . CEREBRAL HEMISPHERES

• . ALL BRAIN STRUCTURES DORSAL AND ANTERIOR TO
  THE OPTIC CHIASM

• . CREATED MBH PENINSULA INCLUDING

• . MEDIAN EMINENCE

• . ARCUATE NUCLEUS

• . MEDIAL BASAL HYPOTHALAMUS

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MBH PENINSULA

• . ADMINISTRATION OF ESTROGEN RESULTED IN ? LH

• . ELECTROLYTIC LESIONS TO THE ARC

• . SUDDEN ? IN LH

• . INABILITY TO RESPOND TO E2

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ROLE OF GnRH IN THE PRIMATE MODEL

• . HYPOPHYSIOTROPIC CLAMP MODEL

• . ARC LESIONS

• . GnRH INFUSIONS DID NOT MAINTAIN LH OR FSH

• . GnRH PULSATILE INFUSION (1 pulse/hr)
  RESTORED LH FSH LEVELS

• . E2 INITIALLY ? LH THEN ? LH
• (150 pg/ml 36 HR)

• . ?-ADRENERGIC BLOCKER HAD NO EFFECT

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HYPOTHESIS
THE PATTERN OF LH SECRETION THROUGHOUT THE
MENSTRUAL CYCLE IS DIRECTED BY THE EBB AND FLOW
OF OVARIAN ESTROGENS ACTING DIRECTLY ON THE
PITUITARY AND GnRH PLAYS ONLY A PERMISSIVE (but
obligatory) ROLE
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TEST OF THE HYPOTHESIS

• . INTACT FEMALE RHESUS WITH ARC LESIONS

• . PULSATILE GnRH ADMINISTRATION
• (1 pulse/hr)

• . AFTER 2 TO 3 DAYS, LH AND E2 INCREASED

• . OVULATION OCCURRED

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PROGESTERONE BLOCK

• . P4 ADMINISTRATION WITH E2 BLOCKED LH RELEASE

• . P4 ADMINISTRATION 12 HR AFTER E2 ADVANCED
  SURGE 24 HR

• . HOWEVER, IN HYPOPHYSIOTROPIC CLAMP ANIMALS,
  P4 BLOCK DOES NOT OCCUR, THEREFORE

• . P4 BLOCKS AT CNS LEVEL

• . P4 MODULATES AT PITUITARY LEVEL

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PRIMATE MODEL (contd)
ROLE OF ADRENERGIC SYSTEM

• . ADRENERGIC BLOCKERS DO NOT INTERFERE WITH
  () FEEDBACK OF E2

• . GnRH PULSES INTERRUPTED BY ?
• BUT NOT ? ADRENERGIC BLOCKERS

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SHEEP MODEL - THE PULSE GENERATOR
STEROID FEEDBACK

• . E2 (-) FEEDBACK

• . MICROINJECTIONS OF E2 TO HYPOTHALAMUS BLOCK
  LH PULSE

• . E2 RESULTS IN ? LH PULSE AMPLITUDE

• . SEASONAL CHANGE IN SENSITIVITY
• TO E2 (-) FEEDBACK

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STEROID FEEDBACK (contd)

• . E2 () FEEDBACK

• . IF E2 BECOMES SUFFICIENTLY HIGH, (-) FEEDBACK
  BECOMES INTERRUPTED AND () EFFECT OBSERVED

• . LATENCY - 24 - 48 HR (monkey), 12 - 48 hr
  (sheep)

• . RAT - LATENCY NOT APPLICABLE DUE TO
  CIRCADIAN CONTROL

• ) E2 IMPLANTS CAUSE DAILY LH SURGE FOR WEEKS

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STEROID FEEDBACK (contd)
SITE OF ACTION OF E2 FEEDBACK

• . ENHANCE RESPONSE TO GnRH (pituitary)

• . ? GnRH RECEPTORS

• . ? mRNA FOR LH SUBUNITS

• . ENHANCE GnRH SECRETION (hypothalamus)

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STEROID FEEDBACK (contd)

• . PROGESTERONE (-) FEEDBACK

• . ? LH PULSE FREQUENCY

• ) MEAN LH UNAFFECTED BECAUSE PULSE AMPLITUDE ?

• . IN COMBINATION WITH E2 ?? LH

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SITE OF ACTION OF P4 FEEDBACK

• . HYPOTHALAMUS - REDUCES GnRH

• . INVOLVES INHIBITION BY EOP

• . NALOXONE (EOP receptor inhibitor) REVERSES
  (-) EFFECT OF P4 ON LH

• . ?-ENDORPHIN IS HIGHEST IN PORTAL BLOOD WHEN
  P4 ? AND PULSE FREQUENCY ?

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FRED KARSCH OVARIECTOMIZED, STEROID IMPLANT
SHEEP MODEL

• FIRST TESTED ABILITY OF SILASTIC IMPLANTS TO
  DUPLICATE ESTROUS CYCLE

• USED THE MODEL TO TEST STEROID FEEDBACK EFFECTS
  ON GnRN-LH INTERACTION

• THEN USED THE MODEL TO TEST THE EFFECTS OF
  ESTROGEN FEEDBACK ON SEASONAL LH SECRETION

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LH PULSE FREQUENCY
GONADOTROPINS
ESTRADIOL PROGESTERONE
PROSTAGLANDIN
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LH PULSE FREQUENCY
GONADOTROPINS
ESTRADIOL PROGESTERONE
PROSTAGLANDIN
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P4 LOW FREQUENCY, HIGH AMPLITUDE
E2 HIGH FREQUENCY, LOW AMPLITUDE
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FRED KARSCH OVARIECTOMIZED, STEROID IMPLANT
SHEEP MODEL SUMMARY

• P4 MAINTAINS LOW PULSE FREQUENCY, SIMILAR TO
  LUTEAL PHASE

• WITHDRAW P4 IMPLANTS AND ADD E2 IMPLANTS HAS
  INITIAL NEGATIVE FEEDBACK ON LH (HIGH FREQUENCY,
  LOW AMPLITUDE)

• CONTINUED E2 LEADS TO POSITIVE FEEDBACK, AND
  LH SURGE OCCURS

• QUESTION WHAT MECHANISM GOVERNS THE
  COORDINATED RELEASE OF GnRH FOR THE OVULATORY
  SURGE?

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HORSE MODEL

• . PULSE GENERATOR MODEL DOES NOT FIT

• . PROLONGED LH FSH SECRETION

• . EVIDENCE OF FREQUENT, PREDICTABLE PULSES IS
  QUESTIONABLE

• . TOLERANT OF CONTINUOUS GnRH

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LH Secretory Dynamics
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• TOLERANCE TO CONTINUOUS GnRH STIMULATION

GnRH (100 ug/hr)
GnRH (25 ug/hr)
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• TOLERANCE TO
• CONTINUOUS GnRH

GnRH (100 UG/HR)
CONTROL
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GnRH RECEPTOR
GnRH
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Ligand-induced InternalizationOvine GnRH receptor
Hashizume et al., 2001
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HYPOTHESIS PITUITARY DOWNREGULATION DOES NOT
END THE LH SURGE, THEREFORE (-) FEEDBACK IS
REQUIRED TO END THE LH SURGE
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OVUL
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STEROID FEEDBACK (horse model)

• . E2 (-) FEEDBACK

• . ASIDE FROM 1 REFERENCE, THERE IS NO EVIDENCE
  FOR E2 (-) FEEDBACK

• . OVARIAN FEEDBACK NOT REQUIRED FOR SEASONAL
  DECLINE IN LH FSH SECRETION

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STEROID FEEDBACK (horse model)

• . E2 () FEEDBACK

• . E2 FURTHER STIMULATES LH IN OVX MARES DURING
  THE EQUIVALENT BREEDING SEASON

• . E2 REQUIRED FOR INCREASE IN LH DURING ESTRUS?

• . E2 STIMULATES GnRH SECRETION

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STEROID FEEDBACK (horse model)

• . P4 (-) FEEDBACK

• . P4 HAS STRONG (-) EFFECT ON LH

• . EOP MEDIATED, AS NALOXONE REVERSES

• ) LH ?

• ) GnRH ?

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GnRH CHALLENGE
PROGESTERONE
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GnRH