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Questions and Answers
What is the primary effect of melatonin therapy on elderly insomniacs?
What is the primary effect of melatonin therapy on elderly insomniacs?
How does melatonin differ from other antioxidants regarding redox cycling?
How does melatonin differ from other antioxidants regarding redox cycling?
Which of the following is one of melatonin's roles in cancer prevention?
Which of the following is one of melatonin's roles in cancer prevention?
What is the effect of melatonin on reproductive hormones?
What is the effect of melatonin on reproductive hormones?
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How does melatonin contribute to the brain's protection against reactive oxygen species?
How does melatonin contribute to the brain's protection against reactive oxygen species?
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What is the significance of daylength to reproductive functions mediated by melatonin?
What is the significance of daylength to reproductive functions mediated by melatonin?
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Which species is noted for having distinct breeding seasons influenced by melatonin?
Which species is noted for having distinct breeding seasons influenced by melatonin?
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Why is melatonin referred to as a terminal or suicidal antioxidant?
Why is melatonin referred to as a terminal or suicidal antioxidant?
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What is a primary role of melatonin in seasonal breeders?
What is a primary role of melatonin in seasonal breeders?
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What hormone is primarily responsible for regulating water retention in the body?
What hormone is primarily responsible for regulating water retention in the body?
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How does an increase in plasma osmolarity affect antidiuretic hormone secretion?
How does an increase in plasma osmolarity affect antidiuretic hormone secretion?
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What physiological effect does high concentration of antidiuretic hormone (ADH) have aside from water retention?
What physiological effect does high concentration of antidiuretic hormone (ADH) have aside from water retention?
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Which of the following best describes how osmoreceptors function in regulating antidiuretic hormone secretion?
Which of the following best describes how osmoreceptors function in regulating antidiuretic hormone secretion?
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A practical application of manipulating melatonin concentrations involves which of the following?
A practical application of manipulating melatonin concentrations involves which of the following?
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Which of the following is NOT a function of vasopressin (ADH)?
Which of the following is NOT a function of vasopressin (ADH)?
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What is the primary endocrine source of antidiuretic hormone (ADH)?
What is the primary endocrine source of antidiuretic hormone (ADH)?
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What is the primary effect of antidiuretic hormone (ADH) secretion?
What is the primary effect of antidiuretic hormone (ADH) secretion?
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Which physiological stimuli primarily increase antidiuretic hormone (ADH) secretion?
Which physiological stimuli primarily increase antidiuretic hormone (ADH) secretion?
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How does ADH enhance water reabsorption in the kidneys?
How does ADH enhance water reabsorption in the kidneys?
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Which condition is a potent stimulus for the secretion of antidiuretic hormone (ADH)?
Which condition is a potent stimulus for the secretion of antidiuretic hormone (ADH)?
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What is the function of aquaporins in the cell membrane?
What is the function of aquaporins in the cell membrane?
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What type of receptors does ADH utilize in the kidneys to exert its effects?
What type of receptors does ADH utilize in the kidneys to exert its effects?
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What mechanism increases the level of cAMP following ADH binding?
What mechanism increases the level of cAMP following ADH binding?
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What effect does nausea have on antidiuretic hormone (ADH) secretion?
What effect does nausea have on antidiuretic hormone (ADH) secretion?
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Study Notes
The Pineal Gland
- Also known as the pineal body or "third eye", it's a small endocrine gland within the vertebrate brain.
- Its shape resembles a tiny pine cone, hence the name.
- Located near the center of the brain.
- Size varies across species. Humans have a roughly 1 cm long pineal gland, while dogs have a 1 mm long one.
- Historically, its deep brain location led to philosophical theories of it possessing special importance, and this led to associated myths, superstition, and occult theories.
- René Descartes called it the "seat of the soul", believing it connected intellect and body.
- Produces melatonin, a hormone affecting wake/sleep cycles and seasonal functions.
Main Function of the Pineal Gland
- The pineal gland plays the role of a biological clock, responding to light.
- Melatonin production increases during the night (peaking around 11 PM - 2 AM), and decreases during the day.
- Melatonin production increases in the first three months after birth, peaks at around six years of age, and then generally decreases after puberty.
- Melatonin production is modulated by the amount of light received.
Other Functions of the Pineal Gland
- Melatonin is related to the mechanism by which some amphibians and reptiles change skin color.
- Scientists first discovered the substance in extract of cow pineal glands in 1917 because it lightened frog skin.
- Dermatology professionals used extracts in the hopes of treating skin diseases, isolating and naming the hormone melatonin in 1958.
- Mid-1970s research demonstrated melatonin production follows a circadian rhythm in humans.
- Scientists discovered melatonin's antioxidant properties in 1993.
How Retina Transmits Light-Dark Exposure
- The retina of the eye relays light exposure information to the suprachiasmatic nucleus (SCN) in the hypothalamus.
- Fibers from the hypothalamus travel to the spinal cord and then to the superior cervical ganglia.
- Post-ganglionic neurons carrying this information then pass back and reach the pineal gland.
Melatonin Production
- Melatonin production is inhibited by light and permitted by darkness, hence why it is called "the hormone of darkness".
- Light exposure to the eyes dramatically affects melatonin synthesis.
- Serum melatonin concentrations are low during daylight hours and peak during the night.
- The precursor to melatonin is serotonin, which is derived from the amino acid tryptophan. (see diagram)
Melatonin Synthesis Mechanism (Diagram)
Duration of Melatonin Secretion
- The duration of melatonin secretion each day is proportional to the length of nighttime.
- Serotonin N-acetyltransferase (NAT) enzyme activity, the rate-limiting step in melatonin synthesis, is low during daylight and peaks in the dark.
- In some species, changes in NAT activity closely correlate with NAT messenger RNA levels; in others, post-transcriptional regulation of NAT levels is key. - The other enzyme involved in melatonin synthesis (hydroxyindole-O-methyltransferase, HIOMT) does not exhibit light-exposure pattern regulation.
Melatonin Receptors
- Mammals have two melatonin receptors (Mel1A and Mel1B).
- These are G protein-coupled cell surface receptors, differentially expressed in various tissues.
- The highest receptor density is in the SCN of the hypothalamus, the anterior and posterior pituitary, and in the retina. Receptors are also found in other areas of the brain.
Biological Effects of Melatonin
- Circadian Rhythm (Biological Clock): Melatonin likely has some effect in regulating sleep patterns, although isn't the primary regulator. Melatonin levels can be lower in elderly insomniacs but there is some research showing modest therapeutic benefit.
Melatonin's Antioxidant Activity
- Melatonin easily crosses cell membranes and the blood-brain barrier.
- It scavenges free radicals like OH, O2, and NO.
- Unlike other antioxidants, melatonin does not undergo redox cycling (repeated reduction and oxidation).
- Due to this, melatonin, once oxidized, cannot return to its original state. It forms stable end-products upon reacting with free radicals, becoming a terminal (or suicidal) antioxidant.
Other Melatonin Effects
- Melatonin has been demonstrated to prevent DNA damage from some carcinogens and help protect against brain injury from reactive oxygen species (ROS).
- It enhances immunity and decreases the deposition of beta-amyloid protein, frequently involved in Alzheimer's disease.
Melatonin's Role in Reproductive Function
- Seasonal day length changes significantly affect reproduction in various species. Melatonin plays a key role in regulating these changes.
- In temperate climates, animals (like hamsters, horses, and sheep) have distinct breeding seasons.
- During non-breeding seasons, gonads are inactive and, as the breeding season approaches, they must be rejuvenated.
- Photoperiod (day length) is a crucial cue for animals to determine the right time of year. The pineal gland, by measuring daylight, can adjust melatonin secretion according to the length of the days.
- Animals whose pineal glands have been removed or are unable to receive light cues cannot properly prepare for their breeding seasons.
Effect of Melatonin on Reproductive Systems
- Melatonin's effect is summarized as being anti-gonadotropic, meaning it inhibits the secretion of reproductive hormones (LH and FSH) from the anterior pituitary.
- Inhibition of hypothalamus hormones is the major reason why melatonin has this effect.
- Melatonin can be practically used to adjust reproductive cycles in species that normally reproduce once a year. Examples include controlling sheep reproduction
Antidiuretic Hormone (Vasopressin)
- Also called arginine vasopressin (AVP) or antidiuretic hormone (ADH).
- Found in most mammals.
- A nine-amino-acid hormone.
- Regulates water reabsorption in the kidney, affecting kidney tubule permeability.
- Important in maintaining homeostasis and regulating water, glucose, and salts in the blood.
- Synthesized in the hypothalamus and stored in vesicles in the posterior pituitary, with some also released directly into the brain;
- Plays a role in blood pressure regulation, through moderate vasoconstriction.
Main Biological Function of ADH
- Regulating water retention by the body. Released when the body is dehydrated
- The kidneys conserve water, concentrating urine, and reducing urine volume.
- At high concentrations, it also raises blood pressure by causing moderate vasoconstriction.
Control of Antidiuretic Hormone Secretion
- Plasma osmolarity is the primary regulator.
- Osmoreceptors in the hypothalamus sense changes in plasma osmolarity.
- When osmolarity is below a certain threshold, osmoreceptors don't trigger ADH secretion.
- As osmolarity increases above the threshold, osmoreceptors trigger increased ADH secretion, which increases linearly with osmolarity.
Stimuli for Antidiuretic Hormone Secretion
- Decrease in blood pressure and volume.
- Loss of 15-20% blood volume (as in hemorrhage).
- Nausea and vomiting (regions of the brain controlling these functions are linked to the hypothalamus).
Water Transport Mechanism
- Diffusion: Through permeable plasma membranes, dependent on osmolality differences between both sides of the plasma membrane lipid bilayer. Transfer is slow and not a significant way to move water in the body.
- Aquaporins: Water channels in the plasma membrane. Integral plasma membrane proteins that help the body regulate water absorption. Forms tetramers containing four channels to facilitate water movement in both directions. Various types including AQP-0, AQP-1, etc. are present in different areas like red blood cells, kidney, choroid plexus. AQP-2 is sensitive to vasopressin levels.
Mechanism of ADH Action on the Kidney
- V2 receptors (G-coupled protein receptors) on kidney plasma membranes are acted on by ADH.
- Adenylyl cyclase III and VI are activated, converting ATP to cAMP.
- cAMP increases aquaporin-2 water channel production.
- Protein kinase A (PKA) is then activated to add phosphate groups to aquaporin proteins, stimulating water re-absorption.
Vasopressin Secretion Disorders
- Hypothalamic diabetes insipidus: ADH deficiency from hypothalamus problem. Causes include head trauma, infections, or tumors related to hypothalamus.
- Nephrogenic diabetes insipidus: The kidneys fail to respond to ADH. Causes include kidney disease or mutations in ADH receptor or aquaporin-2 gene.
- Common symptom is excessive urine production (polyuria).
- Treatment typically involves exogenous (outside) ADH.
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Description
Test your knowledge about the pineal gland, its functions, and historical significance. This quiz covers aspects like melatonin production, location, and theories associated with this intriguing gland. Dive into the neuroscience of the 'third eye' and discover its impact on our biology.