Wednesday, April 4, 2012 CC-BY-NC
Circadian rhythms and neuroendocrine regulation II

Maintainer: admin


  • suprachiasmatic nuclues (SCN) is the central clock in mammals.
    • located next to 3rd ventricle, right above optic chiasm, where the two optic nerves cross each other.
    • lesioning SCN disrupt circadian rhythm as shown on the actograms.
      • SCN graft restores circadian rhythm... to that of the donor!!!!
    • circadian rhythm of electrical activity is shown in isolated SCN neurons
      • which suggests that mechanisms of regulation is at the cellular level.
  • however, SCN is not the only clock in the organism. also in other brain regions and periphery organs.
    • different clock genes could also have different functions/regulations/circadian rhythms in different locations.
      • as demonstrated by Per1-luciferase transgene's expression rhythms in different tissue cultures.
  • clock genes:
    • conserved among species (ex: rat's and drosophila's clock genes have similar names)
    • conserved feature:
      • oscillating mRNAs, proteins, or activities
        • protein level lags behind mRNA level (both are in rhythms)
      • autoregulatory feedback loops
        • ex: Bmal1 and Clock form dimer and act as TF(transcription factor)
        • the TF activate transcription for more clock genes: Per1/2/3, Cyr1/2
        • Cyr1/2 and P1/2/3, in conjunction w/ CKI (a kinase), dimerize and suppress Bmal+Clock complex and thereby their own expression
        • as Cyr1/2 and P1/2/3 are degraded over time, suppression on Bmal+Clock becomes less, so transcription occurs again on the next day.
        • hence the rhythm
  • Bmal-knockout shows very low expression of Per1 in SCN
    • locomotor activity is still in control, but when environmental clue is removed (put in DD cycle), circadian rhythm is completely disrupted.
    • so rhythm in Bmal1-knockout is due to masking.



  • CKI does many things:
    • 1.phosphorylate Per1 and thereby regulate Per1's location in the cell.
    • 2.phosphorylate PER1 and targets it for degradation
    • 3.phosphoryalte Per1/Cry complex and the complex then inhibit Clock/Bmal1.
    • 4.phosphoryalte BMAL1 to stimulate BMAL1/Clock-induced transcription
  • tau mutation in hamster: mutation of CKI
    • less stalb ePER protein and thus decreased duration of inhibition of CLOCK/BMAL1.
    • heterogenous mutation: period decrease from 24hr to 22hr
    • homozygous mutation : period is 20 hr. (even worse)

2.2Per2, CKI

  • familiar advanced sleep phase disorder (FASPD)
    • otherwise healthy individuals.
    • caused by mutation in PER2 and CK1-gamma
    • shorter period than normal (sleep from 6pm to 2am)
  • problems may lie in the 1st and the 2nd functions of CKI.

3Components of a circadian clock

  • input (light, clues, etc)
  • pacemaker (SCN genes)
  • output (physiological rhythms)
    • there must be a link between pacemaker and clock mechanism --> ans: clock-controlled genes (ccg)

3.1(ccg)Clock-controlled Genes

  • ccgs are controlled by block machinery and thus the RNAs are expressed rhythmically.
  • they can encode neuropeptides such as vasopressin, transcription factors (DBP), receptors, ion channels, metabolic enzymes, etc.
  • ex: vasopressin as a ccg
    • In front of vasopressin gene, there's a Ebox in promoter. the Ebox binds Clock/Bmal1.
  • regulation of ccgs can be different in different tissues
    • in SCN, mutant mice's amp of vasopressin is flattens and has little rhythms comparing to wild type
    • in SON, mutant mice's amp and rhythm of VP is pretty much the same as the wild type.
    • so there are many clock-controlled genes and most ccgs are different in SCN and periphery (like liver).
      • less than 10% of ccgs shows the same rhythm in more than 1 tissue, so genes have different functions in different tissues (whether it's rhythmic or non-rhythmic).
      • this is also proven w/ micrarray analysis, showing that different clock genes are expressed in different tissues.

3.2SCN-periphery communication

  • hierarchy :
    • central clock
      • peripheral clocks, which drives rhythms.
      • directly drives rhythms.
    • environments gives direct input to central clock and indirectly to peripheral clock
  • putative signals
    • glucocorticoids, retinoic acid, neuronal pathway (autonomic nervous systems)
    • temperature cycles (by peripheral clock, not central)
    • feeding rhythms (also directly by SCN, by peripheral clock.
  • SCN sends projections to many nuclei in the brain: PVN, PVT, DMH, CRH, GnRH,etc.

4Melatonin and Cortisol


  • melatonin is secreted by pineal gland, which must have connection w/ SCN cuz lesion at SCN abolishes melatonin production.
  • pathway of projections: SCN --> PVN --> IMC --> SCG --(via noradrenaline)--> (alpha-1 beta-1 adrenergic receptors) PIN (pineal gland) --> high malatonin at night and low during a day.
  • melatonin production:
    • serotonin --(AANAT)--> N-acelylserotonin) --(HIOMT)--> Melatonin
      • AANAT: has a dramatic rhythm : very high at night and very low during the day.
      • HIOMT: more or less level throughout the day.
      • (therefore AANAT is the rate-limiting step)
  • noradrenaline regulation of AANAT via alpha-1/beta-1-adrenergic receptors lead to more transcription of AANAT and less degradation.
    • regulation at transcription and degradation differ among species.

4.2cortisol ##

  • regulated by 3 factors:
    • neuroendocrine: SCN --> PVN --> CRH (aka CRF) --> pituitary (produces ACTH) --> adrenal gland (produces cortisol)
    • parasympathetic" SCN --> PVN --> DMV --> adrenal gland
    • sympathetic: SCN --> PVN --> IML --> adrenal gland.
  • but there are also circaidan clock in the adrenal gland itself. so SCN is not the only source of rhythm for cortisol.
    • ex: Bmal1 and Per1 genes expression oscillates in the cortex and medulla of the adrenal gland.
    • many other clock-controlled genes are involved in the control of adrenal cortisol biosynthesis and secretion.

4.2.1Adrenal clock experiment

  • rodents with mutated Per2/Cry1 do not show difference between level of cortisol during early day and early dark.
  • whereas wild type rodents, cortisol is much higher in early dark than in early day.
    • (rodents are active at night)
  • therefore, while same amount of ACTH act on adrenal gland, different amount of cortisol were secreted in wild type and mutant.
    • so clock in adrenal cortext has gating effect on the ability of ACTH to evoke cortisol release
    • clock determines how strongly tissue would response to ACTh and secrete cortisol.

5Circadian and photoperiodism

  • photoperiod: longer in the summer. duration of light phase.
  • photoperiodic regulation of prolactin and pelage/horn growth
    • hamsters secrete high prolactin in LD and low in SD (short day)
      • they're white in winte rand agouti in summer
    • but if SD number of days persist, there's a SD refractory: prolactin level goes up again and animals become agouti.
      • due to intrinsic regulation
    • same goes with sheep and their prolactin level
      • summer/LD: high prolactin/horn growth
      • winter/SD: low prolatin/no horn growth
      • prolonged LD: refractoriness (intrinsic regulation)
  • photoperiod on fertility of male hamster:
    • wild type: testis atrophy if night >12hr. more fertile when days are long
    • mutant : testis atrophy if night >10hr.
      • mutation in the circadian clock alters photoperiodic time measurement
  • photoperiod on oscillation of Per genes in SCN
    • Per1 is high in light phase
    • in SD: higher and narrower peak
    • in LD: similar behavior, but lower and broader peak.
      • seems to mimics length of day.
  • photoperiod and pineal melatonin rhythms
    • just like Per genes, but opposite
      • melatonin peaks at night and low during day.
    • pituitary cells response to melatonin from pineal gland, determine whether it's SD or LD, and influence hormonal release
      • sheep are reproductive in SD while hamsters aren't
      • hamsters are reproductive in LD while sheep aren't.