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…?

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