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Neuro 4623: Behavioral & Molecular Biological Clocks > Lecture 4 > Flashcards

Lecture 4 Flashcards

1
Q

SCN in mammals

A
  1. Evidence that SCN is circadian oscillator
  2. Features of SCN
  3. Transplantation of SCN
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2
Q

Cockroaches and Biological Clocks

A

The circadian system of cockroaches has been well-studied.

Roaches are large, hardy insects that display good rhythms in laboratory studies and tolerate surgical procedures well.

In addition, claims made during the 1950’s concerning the
localization of the roach circadian pacemaker were the first such claims in any animal.

Although these claims were later disproved, they stimulated
a great deal of interest in the insects and their circadian
systems.

The result of this interest is a large and fascinating body of
information on the neural regulation of circadian systems in roaches and related species.

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3
Q

Cockroach Optic Lobes

A

The optic lobes are bilateral parts of the roach brain that are connected to the compound eyes via the optic nerves.

The lobes are large structures comprised of several layers of cell bodies and neuropil, which receive the retinal input and themselves project into the central part of the brain called the protocerebrum.

Output from the protocerebral lobes runs via the
circumesophageal connectives to the subesophageal ganglion and then out to the other ganglia of the thorax & abdomen.

If one cuts the neural connection between the optic lobes and the rest of the nervous system, the ability to generate circadian rhythms in behavior is lost.

This can be shown by severing the optic lobes from the protocerebrum, or by cutting the
circumesophageal connectives that connect the protocerebrum to the thoracic ganglia that have direct control over motor activity.

By contrast, if one simply cuts the optic nerves that bring light information from the eyes to the
optic lobes, the locomotor activity rhythm remains intact, but it free-runs under all lighting
conditions.

The hypothesis that emerges from these results is that the oscillator is somewhere within the optic lobes.

Selective lesion studies aimed at various parts of the brain have shown that even very small lesions in the medulla near the base of the optic lobes can cause arrhythmicity.

The idea that the connection of the pacemaker to the rest of the nervous system is neural is reinforced by the lack of rhythmicity if the brain is excised and simply implanted in the abdomen.

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4
Q

Cockroach Optic Lobes:

Figure 43:

A

????

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5
Q

Cockroach Optic Lobes:

Terry Page

A

Terry Page found that if one studied roaches with transected optic lobes left in situ for about 40 days, all of the arrhythmic roaches became rhythmic again with no further intervention.

The period of the restored rhythm was similar to that of the original rhythm.

Page first showed that the restoration of rhythmicity was coincident with and dependent on the regrowth of fibers from the optic lobes to the rest of the brain.

The restoration of behavioral rhythmicity is coincident with the restoration of light-evoked responses in the rest of the brain, which depend on an intact neural route from the retina to the optic lobe to the
protocerebrum.

These results indicate that the oscillator in the optic lobe continues to function after being separated from the brain and that the output mechanism from the pacemaker to the driven systems in the brain can
regenerate spontaneously.

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6
Q

MAMMALIAN CIRCADIAN

CLOCK: THE SCN

A

Discovery that the SCN is the master circadian pacemaker in
mammals:
a. early attempts of Curt Richter to find circadian clock in rats
b. knowledge that eyes are the only photoreceptors in mammals
c. discovery of retinohypothalamic tract (RHT) innervating SCN

Glutamate is the main neurotransmitter in the RHT—released at synapses in SCN in response to light stimulation

Intergeniculate leaflet (IGL) as an additional source of photic information for entrainment of SCN. Projections from IGL to SCN release NPY

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7
Q

Characteristics of SCN

A
  1. bilateral, paired nuclei
  2. a cluster of about 10,000 neurons
  3. different cell types
  4. presence of neuropeptides
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8
Q

____ is the main neurotransmitter in the ____, released at synapses in SCN in response to _____.

A

*Glutamate is the main neurotransmitter in the *RHT, released at synapses in SCN in response to *Light Stimulation

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9
Q

INPUT into SCN

A

Monosynaptic pathway: Retinohypothalamic Tract (RHT)

The Intergeniculate Leaflet (IGL)

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10
Q

Retinohypothalamic Tract (RHT)

A
  • INPUT into SCN
  • Monosynaptic pathway
  • This derives from specialized photoreceptors that sense light.
  • The RHT connects to the core of the SCN.
  • The detection of light is transmitted via the RHT.
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11
Q

Intergeniculate Leaflet (IGL)

A

INPUT into SCN

• Receives data directly from the retina, but the route
separates from the RHT.

• The IGL terminates in the geniculo-hypothalamic tract (GHT)

• The IGL path appears to play an important role in the
entrainment, synchronization, by non-photic zeitgebers such as motor activity.

  • Another major input route originates in the raphe nuclei.
  • Other inputs come from cholinergic projections from the basal forebrain and histaminergic projections from the hypothalamus.
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12
Q

Output from SCN

A

• The suprachiasmic nuclei project to the anterior
hypothalamus, the thalamus, the lateral and dorsal medial hypothalamus, the IGL, and lateral septal nucleus.

• The Subparaventricular Zone of the hypothalamus.
From this region, projections arise that innervate the
upper thoracic intermediolateral cell column and from there projections extend to the superior cervical ganglion
sympathetic neurons which in turn innervate the pineal gland.
Thus, the SCN is able to regulate the production of melatonin, which is secreted in the pineal gland

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13
Q

Types of evidence that SCN is

circadian pacemaker:

A

a. SCNx (destruction of SCN by lesion)
- —> arrhythmicity

b. electrical or chemical stimulation of SCN
- —> phase shifts

c. rhythm of neural firing activity and 2DG uptake in SCN in vivo
1. rhythm of neural firing persists after surgical isolation of SCN from neural connections with brain
2. rhythm of neural firing persists in vitro
3. rhythm of 2DG (2-deoxyglucose) uptake indicates
rhythm in metabolic rate

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14
Q

Intergeniculate leaflet (IGL) is an additional source of _____ information for ______.

A

Intergeniculate leaflet (IGL) ais an additional source of *photic information for *entrainment of SCN.

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15
Q

SCNx …

A

(destruction of SCN by lesion)

results in arrhythmicity

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16
Q

Electrical or chemical stimulation of SCN….

A

results in phase shifts

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17
Q

Rhythm of neural firing activity and 2DG uptake in SCN in vivo

A
  1. rhythm of neural firing persists after surgical isolation of SCN from neural connections with brain
  2. rhythm of neural firing persists in vitro
  3. rhythm of 2DG (2-deoxyglucose) uptake indicates
    rhythm in metabolic rate
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18
Q

Effects of SCN Lesion om Activity Rhythm in Sparrow

A

…..

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19
Q

Lesions of the SCN abolish _____ .

A

Lesions of the SCN abolish circadian rhythms

20
Q

Multiunit Activity in Hypothalamus Island Containing SCN and in Caudate Nucleus of Rat Brain

A

…..

21
Q

SCN maintains rhythms in vitro

A

….

22
Q

Circadian Rhythms are a

Property of Individual SCN Cells

A

…..

23
Q

SCN transplant studies

5 Findings

A
  1. Tau mutant hamster has tau(dd) = 20h (very different from wild-type hamster tau(dd), which is very close to 24h
  2. Lesion SCN of wild-type hamster and implant SCN taken
    from tau mutant hamster: Transplant restores rhythm and tau = 20h, similar to tau of donor hamster.
  3. Lesion SCN of tau mutant and implant SCN taken from wildtype hamster: rhythmicity is restored and tau is close to
    24h (like that of donor)
  4. Transplanted SCN restores locomotor activity rhythm, but
    rhythm can no longer be entrained to light, probably
    because connections from RHT to SCN are not restored.
  5. Transplanted SCN does not restore pineal melatonin rhythm. Neural connections (output) from SCN to pineal are
    not restored.
24
Q

Restoration of Activity Rhythm by SCN Transplant from Wild-Type Hamsters to SCN-Ablated Tau-Mutant Hamster

A

…..

25
Q

Restoration of Activity Rhythm by Wild-Type Hamster SCN Transplant Into Tau-Mutant Hamster with Partial Ablation of SCN

A

/////

26
Q

There is some evidence that a humoral factor may be involved in SCN output to drive locomotor activity rhythm.

A

SCN transplants enclosed inside a capsule that does not permit neural connections to the brain still restored locomotor activity rhythms

27
Q

SCN Transplants Do Not Restore the Glucocorticoid Rhythm

A

…..

28
Q

Circadian Behavioral Rhythms Do Not Require Neural Efferents

A

…..

29
Q

Seasonal Gonadal Response Not Restored by SCN Transplants

A

….

30
Q

The SCN is structurally

heterogeneous:

A

…..

31
Q

The SCN: Neurochemically
Heterogeneous
Functionally Homogenous?

A

…..

32
Q

Light-Induced Per1 is Seen in the CalB Region of the SCN

A

….

33
Q

Monosynaptic
Projections
from the SCN:

A

> > > >

34
Q

sPVZ

A

???

35
Q

8 Peripheral Targets of the SCN

A
  • Kidney
  • Liver
  • Thyroid
  • Bladder
  • Spleen
  • Adrenal
  • Pancreas
  • Adipose Tissue
36
Q

Extra-SCN Brain Regions and

Peripheral Organs Contain Clock Genes

A

Pituitary
Arcuate
Lung
Liver

37
Q

Zeitgebers

A

German for “time-giver,” pronounced “zyte gay-burr.”

Environmental factors such as light-dark cycles, meals, clocks etc. that give clues as to
external cycles are called zeitgebers.

In the absence of any zeitgeber, behaviors that show rhythmicity are driven by an internal timing device - a biological clock.

38
Q

What happens if an animal is removed from

exposure to zeitgebers?

A

Many studies in humans and other mammals have shown that, in the absence of zeitgebers, (i.e. when the biological clock is allowed to free-run), activity has a period of 22.5–26 hrs.

This period is incredibly robust; it does not deviate by more than a few minutes in a year and is unaffected by food/water intake, exposure to LSD, removal of endocrine glands or electroconvulsive shock (Richter, 1967).

However, the clock can be reset by brief exposure to light, a phenomenon called entraining.

39
Q

Phase shifts

A

There a number of ways in which the environment or changes in behavior can make the internal clock ‘out of synch’ with the external world.

Sudden mismatches between ‘subjective’ time and the external LD cycle are called phase shifts these occur in situations such as jet lag and shift work.

40
Q

Advance Shift

A

It is difficult for humans to phase-advance (West to East).

When traveling from East to West, the subjective day is lengthened, reentraining to light occurs later (delay shift) and adaptation is easier (although it can still take several days):

41
Q

Delay shift

A

Some shift work schedules involve phase shifts which are more difficult to adapt to than others.

Example: Someone who works 8h shifts that start 8h earlier on subsequent weeks is undergoing large advance shifts with little time to re-entrain (and often is trying to adapt to the competing activity cycle of society).

42
Q

Skeleton Photoperiod

A

A light-dark cycle in which dawn and dusk are represented by two separate
light periods, e.g, LDLD1:7:1:15 (the skeleton of LD9:15 or of LD17:7)

If the 2 light periods are of equal length, then the skeleton is considered to be symmetric.

43
Q

Subjective Day

A

The time during which the organism exhibits the behavior usually associated with light in an LD cycle.

For example, a diurnal sparrow is usually active during its subjective day, whereas a nocturnal hamster usually rests during its subjective day.

44
Q

Subjective Night

A

The time during which the organism exhibits the behavior usually associated with dark in an LD cycle.

For example, a sparrow usually rests in its subjective night whereas a hamster is active in its subjective night.

45
Q

Splitting

A

Hamsters housed in LL show ‘splitting’ of their locomotor activity patterns.

Often the 2 circadian
oscillations appear to free-run
with different τau’s.

46
Q

Splitting:

Two oscillators

A

Pittendrigh & Daan introduced the hypothesis of a pacemaker consisting of 2 coupled oscillators.

Their proposal of an evening oscillator (E) and a morning oscillator (M) was based on the splitting of activity rhythms in hamsters under constant conditions.

Recently, Illnerova & Vanecek explained the different sensitivity of onset and offset of Nacetyltransferase activity towards light pulses with this model of 2 coupled oscillators.

47
Q

A schematic representation of the effects of an 8h phase
advance on both E and M (left panel) and the resulting
melatonin profile (right panel). Grey areas represent the dark period.

A

……….

Neuro 4623: Behavioral & Molecular Biological Clocks (11 decks)

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