melanopsin 332
TRANSCRIPT
Melanopsin, Pinopsin and Encephalic Photoreception in
BirdsSimon BishopAlice Cowie
Emily PurcellJeannette Shipman
Gemma Sykes
Outline
• Photoreception in vertebrates
• Importance of the pineal gland and melatonin
• Melanopsin and pinopsin – what are they?
• How melanopsin and pinopsin entrain melatonin production by the pineal gland to light
Photoreception in Vertebrates• Most vertebrates (birds
included) have both retinal and extra retinal photoreceptors
• These include:– Lateral eyes– Deep brain
photoreceptors– Intracranial pineal organ– Intracranial parapineal
organ (fish only)– Extra-retinal “third eye”
(reptiles and amphibians only)
Birds and Mammals Compared
Mammals and Birds compared
• Mammals:– Eyes are the only photoreceptors. – Signals sent from eyes along the retinohypothalamic tract
to the SCN, which acts as a MASTER CLOCK. – SCN sends inhibitory or stimulatory information to the
pineal gland to rhythmically control its production of melatonin.
• Birds:– Much more complex!– Eyes, hypothalamus and pineal gland all act as
photoreceptors AND circadian oscillators.– Melatonin production by the pineal gland can be directly
entrained to the environmental light/dark cycle.
The Pineal Gland• Particularly important in avian photoreception.• Small endocrine gland in the brain, developmentally
derived from diencephalic tissue.• Primary function to rhythmically synthesize and
release melatonin.
Melatonin• Indoleamine hormone. • Rhythmically synthesised and
released by cells in the pineal gland.
• Production of melatonin by the pineal gland is stimulated by darkness and inhibited by light.
• Secretion of melatonin peaks in the middle of the night
• Output must be entrained to the light dark cycle.
• Entrainment achieved by photopigments.
Phototransduction and Entrainment
• Experimental evidence suggests two distinct transduction pathways mediate the effects of light on pineal gland melatonin output:– One causes the acute suppression of
melatonin output– One mediates phase shift entrainment of
the pineal clock
• Each pathway is possibly controlled by a different photopigment.
What Evidence is there for 2 Phototransduction Pathways?• Experiment 1:
– Deprive cultured pineal cells of Vitamin A– Acute effect of light on melatonin production is
reduced– Phase shifts are unaffected
• Experiment 2:– Apply pertussis toxin to cultured chick pineal cells
(interferes with G proteins which are often coupled with photoreceptors)
– Blocks acute, but not phase-shifting effects of light on melatonin production
Pinopsin• mRNA rhythmically expressed.• Daily rhythm of expression regulated by light
and an intrapineal circadian oscillator.• Gene expression initially thought to be purely
light-controlled.• However when chickens/isolated pineal glands
were kept in constant darkness rhythmic pinopsin expression continued (albeit at a reduced level).
• Therefore now know that circadian oscillators must also play a role in its control.
Melanopsin
• Rhythmically expressed (like pinopsin).• In constant darkness, daily amplitudes of
melanopsin gene expression are not reduced, in some cases seem even to increase.
• Therefore seems that regulation of melanopsin production is primarily controlled by the pineal circadian oscillator (unlike pinopsin).
Daily Cycles in Pinopsin and Melanopsin Levels
• Pinopsin and melanopsin levels low in early morning (ZT 0 – 6).
• Increase in middle of day (ZT 6) by approx. 5-fold.
• Reach a peak between ZT 10 – 12.
• mRNA levels decrease after lights-off.
• Return to low nocturnal levels within 4 – 6 hours.
Circadian Variations of Melanopsin and Pinopsin mRNA levels in Chick Pineal Glands
under LD 12:12
In vivo In vitro
Holthues H. et al (2004).
Pinopsin – what is it?• An opsin-like photopigment. • Related to, but distinct from,
other visual opsins.• First isolated from the pineal
gland of the chicken (Gallus domesticus).
• Expressed exclusively in the pineal gland (key difference with melanopsin).
• Precise role still unclear.• Involved in ‘acute
suppression’ pathway???
Melanopsin – what is it?• Opsin-like photopigment, also
called Opn4, first isolated from photosensitive skin and retinal cells in the African Claw frog (Xenopus laevis).
• In birds, found in:– Specialised photosensitive
ganglion cells in the retina– Iris muscles– Deep brain regions– Pineal gland– Skin cells
• Involved in ‘phase-shift entrainment’ pathway???
How do they work?• Melanopsin
– Knockout mice have attenuated phase shifting light response
– Transfection makes non-photosensitive cells respond to light
– Linked to Gq-type G-proteins and neuron depolarisation
– But… G-proteins are indiscriminate and use varies between species
• Pinopsin– PTX blocks Gi- and Gt-type
G-proteins, blocking the acute effect of light on pinealocytes in vitro.
– Transducin (Gt1α) is coupled with pinopsin in vivo.
– Pinopsin activated Gt1 in vitro when illuminated
– Therefore pinopsin-Gt1 pathway contributes to the acute pathway
– But… Gq/11α also localises with pinopsin, speculated to be involved with phase-shifting
In Summary…
• Birds have multiple photoreceptors• Some involved in vision, others in
temporal physiology• Pinopsin and melanopsin – two
photopigments with a role in controlling daily melatonin output by the pineal gland
• Exact functions still unknown but:– Pinopsin – involved in ‘acute effect’…?– Melanopsin – involved in ‘phase-shift
entrainment’ effect…?
References• Natesan A. et al (2002). Rhythm and Soul in the Avian Pineal. Cell
Tissue Res 309 35 – 45.
• Holthues H. et al (2004). Circadian gene expression patterns of melanopsin and pinopsin in the chick pineal gland. Biochem and Biophys Res Comm 326 160 – 165.
• Oishi T. et al (2001). Multiphotoreceptor and multioscillator system in avian circadian organization Micros Res and Tech 53 43 – 47.
• Peirson S. and Foster R.G. (2006). Melanopsin: Another Way of Signalling Light. Neuron 49 331-339
• Okano T. and Fukada Y. (2001). Photoreception and Circadian Clock System of the Chicken Pineal Gland. Micros Res and Tech 53 72-80
• Wada Y. et al (2000). Phototransduction Molecules in the Pigeon Deep Brain. J. Comp. Neuro. 428 138-144
• Kumar Nayak S. et al (2007). Role of a Novel Photopigment, Melanopsin, in Behavioural Adaptation to Light. Cell. Mol. Life. Sci 64 144-154
• Bailey M.J. and Cassone V.M. (2005) Melanopsin Expression in the Chick Retina and Pineal Gland Molecular Brain Research 134 345-398
• Foster R.G. and Soni B.G. (1998) Extraretinal Photoreceptors and Their Regulation of Temporal Physiology. Reviews of Reproduction 3 145-150
• Takanaka Y. et al (1998) Light-Dependent Expression of Pinopsin Gene in Chicken Pineal Gland. J. Neurochem. 70 908-913
• Fu Z. et al. (1998) Vitamin A Deficiency Reduces the Responsiveness of Pineal Gland to Light in Japanese Quail (Coturnix japonica) Comp. Biochem. Physiol. 119 593-598