Transcript Alex`s ppt

Photo-entrainment:
Physiology and Behavior
Alex Harryman
Focal Article: Melanopsin and rod-cone photoreceptive systems account
for all major accessory visual functions in mice
Hattar et al.
2003
Natural Rhythms
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Ultradian rhythms
Examples: Heartbeat, somite deposition during
vertebrate embryogenesis, respiratory oscillations in
yeast
Infradian rhythms
Examples: Female estrus cycles, mating cycles,
emergence of cicadas
Circadian rhythms
– From Latin circa diem, meaning “about a day”
– 24 hour cycle, reset by light cues
Internal Clocks
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Animals possess internal clocks, which
regulate hormonal control, body
temperature, and sleep patterns
Clocks under genetic control, but can be
influenced by external cues.
 Light cues result in photo-entrainment
Components of Circadian Clock
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Central pacemaker with intrinsic rhythm (genetic)
– Regulated by number of genes and transcription
factors
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Input pathway to pacemaker (physiological)
– Exogenous cues (zeitgebers) reset pacemaker
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Output pathway to effector systems (behavioral)
– Synchronized response to cues
This paper…
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Overall Goal: To determine the relationship
between the physiological basis for photoentrainment via light transduction and the
behavioral response of mammals.
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Specific Hypothesis: Melanopsin is the
photopigment responsible for phototransduction
to the circadian pacemaker, and, as such,
mammals without melanopsin (Opn4-) cannot
photo-entrain.
Part I: Physiology of Light Detection
Question: Where are the photoreceptors
responsible for photo-entrainment located in
mammals?
Experiment: Enucleated mice.
Result: Mice could no longer photo-entrain.
Conclusion: Mammalian circadian photoreceptors
located in the retina.
Rods and Cones
What Happens When We Assume?
Question: Are rods and cones the PRCs responsible for
sending light signals to the brain?
Experiment: Investigated ability of mice lacking rods and
cones to photo-entrain.
Results: Mice could photo-entrain normally.
Conclusion: An independent photoreceptor system exists
and is responsible for photo-entrainment.
A New Candidate: Melanopsin
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Melanopsin isolated from
melanophores of Xenopus
laevis
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Melanopsin found in a
subset of retinal ganglion
cells (RGCs)
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Homology to non-vertebrate
opsins
Structure of rhodopsin (found in rods and cones)
Structure of melanopsin (found in melanophores)
Further Evidence
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Melanopsin-containing RGCs are
intrinsically photosensitive to
light with maximal absorption at
480 nm
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Melanopsin-containing RGCs
project to the SCN and other
regions of the brain responsible
for photo-entrainment
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PACAP, a neurotransmitter
thought to have a role in photoentrainment, is found exclusively
in melanopsin-containing RGCs
(From Berson, 2003)
(From Hattar et al., 2003)
Icing on the Cake
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(From Lucas et. al, 2003)
Melanopsin knockout
mice expressed a
reduced pupillary
response to light.
Phototransduction Model
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Photoreceptor cells (PRCs)
– Responsible for detecting light
input and relaying signals to
brain
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Retinohypothalamic Pathway
(RHT)
– Pathway from retina to SCN
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Suprachiasmatic Nucleus
(SCN)
– Portion of brain responsible
for photo-entrainment in
mammals
(From S.M. Reppert & D.R. Weaver, Nature 2002)
Part II: Behavioral Response
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One of best indicators of an animal’s
ability to photo-entrain is its behavioral
response
– Wheel running activity in rodents used
– Rodents will synchronize their activity to
light-dark cycles
“Actograms”
(From Biemans, 2003)
Link Between Physiology and
Behavior
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Determine that melanopsin was necessary
for phototransduction of light signals to
SCN
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Prove that melanopsin is the only
independent non-visual photoreceptor
system in mammalian retina
“Triple Knockout” Mice
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Deleted genes in rod and cone signaling
pathways
- Rods and cones intact, can receive light, but not transmit signal
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Replaced melanopsin gene with tau-LacZ
construct
– RGCs with reporter gene determined by X-gal labeling
Result: No Photo-entrainment
(From Hattar et al., 2003)
Summary of Behavioral Response Studies
(From Panda et al., 2003)
Conclusions
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Rod-cone and melanopsin systems are the
only light detecting systems in mammalian
eye
– Demonstrated by pupillary light reflex
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Presence of melanopsin is essential for
photo-entrainment
– Full range of photoreception with rods and
cones present
Unanswered Questions
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In vivo, melanopsin exhibits a maximum
light absorbance at 484 nm
In vitro, melanopsin exhibits a blue shift in
absorbance, and action spectra peaks at
420 nm
 Unusual chromophore?
 Unstable during purification?
So, why do I want to put a light
behind my knee???
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Concept: Humoral phototransduction
 Circadian rhythms of body temperature and melatonin can be
shifted
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Mechanism
 Irradiation of blood will cause heme photopigments to release
NO, which is known to be necessary in SCN for phase shifts
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Problem
 No evidence of how NO released in knee can reach the brain
and trigger response
SO, TURN OFF THE LIGHT!
References
Focal Article
Hattar S, Lucas RJ, Mrosovsky N, Thompson S, Douglas RH, Hankins MW, Lem J, Hofmann F, Foster RG, Yau KW
(2003) Melanopsin and rod-cone photoreceptive systems account for all major accessory visual functions in mice.
Nature 175: 1 – 5.
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