TABLE OF CONTENTS
KEY CONCEPTS IN THIS LECTURE 1. At temperate latitudes, most species are seasonal breeders. Breeding occurs during seasons when food is available and there is an enhanced chance of survival for the young. Seasonal breeding is determined by either: 1) an endogenous annual timer or 2) an ecologically relevant environmental cue. Annual timers (=circannual clocks) are important in the timing of seasonal migration of some birds and in the hibernation cycle of some rodents. Ecologically relevant environmental cues could include 1) ambient temperature (warm temperatures stimulate reproduction in lizards), 2) quantity or quality of food (fresh grass stimulates reproduction in voles), 3) water availability (rainfall stimulates reproduction in amphibians), or 4) photoperiod (increasing spring daylengths stimulates gonadal growth in birds). Species using photoperiod are classified as either long-day breeders (mice, hamsters) or short-day breeders (elk, deer, sheep). Short day breeders have longer gestation periods and have their young in the spring when abundant food is available. 2. Biological clocks are internal timing mechanisms which can have a period of several hours, a day, or a year. The circadian clock runs with a period of about 24 hours. Circadian clocks have two functional characteristics: 1) they will persist (=free-run) with a period of about a 24 hours in the absence of environmental cues; and 2) they will synchronize (=entrain) to a 24 hour environmental cue, such as the light-dark cycle. Entrainment is important because it permits animals to synchronize to a changes in the seasonal photocycle. How they synchronize to the cue is determined by the phase response curve (PRC). 3. Circadian clocks are important in photoperiod time measurement. There are two models for how clocks might be important in measurement of photoperiod: 1) External coincidence model (external light occurs at a critical phase in the circadian oscillation) and 2) Internal coincidence model (internal phase of multiple circadian oscillators is set by dusk and dawn). Circadian clocks are also important in animal orientation. Many species of bird and fish use the sun for orientation. To be successful, however, adjustment to the sun's daily movement is necessary. A circadian clock makes this adjustment. 4. In mammals, the Suprachiasmatic Nucleus (SCN) in the hypothalamus is an important site of the circadian clock, although other clocks have been described, such as the Food-entrainable oscillator (FEO). In other vertebrates, the pineal gland also exhibits clock-like properties. In mammals, the pineal gland is driven by the SCN. The efferent path from the SCN to the pineal gland is known. In response, the pineal gland releases pulsatile melatonin at night. The duration of this nocturnal melatonin pulse controls reproduction in a number of photoperiod-sensitive species. 5. Sleep is a readily reversible state of reduced responsiveness/interaction with the environment. The sleep-wake cycle is also under circadian control and free-runs under constant conditions. Sleep is divided into Slow Wave Sleep (SWS, or non-REM) and Rapid Eye Movement (REM sleep). REM sleep is less common and is characterized by a lack of movement, dreaming, and greater sleep depth. Sleep is regulated in part by the reticular activating system (RAS). The neural circuits controlling REM and non-REM sleep probably involve an antagonism between Acetylcholine and Serotonin/Norepinephrine. All vertebrates sleep. Sleep is probably needed as part of a recovery process, although the precise reasons for sleep remain unknown. LECTURE OBJECTIVES 1. Establish the fundamental characteristics
of all biological clocks
2. Describe the location and operation of the neural clock 3. Explain how biological clock function is important in the timing of daily and annual events and in animal behavior 4. Describe the sleep cycle and its neural control in humans and animals LECTURE OUTLINE I. INTRODUCTION TO SEASONALITY
A. Latitude and Seasonality
1. Timing reproduction to enhance survival of offspring
a. Short day and long day breeders
2. Example of a representative seasonal cycle--Indigo Bunting
B. Role of endogeneous timers (circannual clocks) or exogenous cues
II. AN ANNUAL CLOCK (=CIRCANNUAL CLOCK)
A. Characteristics of an circannual clock
1. Endogenous timer with a period of about a year
2. Synchronized to an environmental cue
3. Examples of circannual timing
a. Annual migration of the Willow Warbler
b. Hibernation of ground squirrels
III. DIVERSITY OF EXOGENOUS CUES THAT EXIST
A. Temperature
1. Example: Chameleon reproduction
B. Food Quantity and/or Quality
1. Example: Vole reproduction
a. Importance of fresh grass (6-MBOA) and phenols
2. Example: House mouse reproduction
C. Water Availability
1. Example: Peromyscus
D. Photoperiod (Change in daylength)
1. Photoperiod changes with latitude
2. How is daylength (or nightlength) measured?
a. Hourglass model measures the absolute length of the night
b. A daily endogenous clock is involved (=Circadian Clock)
IV. WHAT ARE BIOLOGICAL CLOCKS AND WHY HAVE THEM?
A. Timing of physiological events in your circadian schedule
1. The importance of processing various body functions efficiently
V. CHARACTERISTICS OF A BIOLOGICAL CLOCK
A. Clocks free-run under constant environmental conditions such as LL
1. Circadian = about 24 hr
2. Circannual = about 1 year
3. Tidal = about 7 days (= a lunar cycle)
4. Ultradian = in hours
B. Clocks entrain (=synchronize) to a Zeitgeber (=an exogenous cue)
1. Phase Response Curve (PRC) defines entrainment properties
a. PRC structure
b. PRC function **
1) How entrainment works: Flying squirrel activity in a
natural setting
2) Jet Lag is a disruption of human entrainment
a) Melatonin and jet lag?
VI. SCN: AN IMPORTANT MAMMALIAN CIRCADIAN CLOCK
A. SCN Anatomy and Neural Circuitry
B. Some experimental evidence that the SCN is a clock
1. SCNx makes animals arrhythmic
2. 2-DG metabolism is greatest during the day
3. SCN neural activity expresses a 24 hr rhythm
4. Fetal SCN implants re-instate rhythmicity in SCNx animals
a. tau mutant
C. Are there other candidate clocks in vertebrates?
1. Food Entrainable Oscillator in rats
2. Pineal gland in birds
a. Pineal transplant studies
b. Interaction with SCN
3. Third eye in ectotherms **
VII. THE MOLECULAR CLOCK
A. Drosophila mutants: An opening toward understanding
molecular clocks
1. per and tim mutants
a. How per and tim interact
b. Phase shifting action of light
2. Other clock mutants (clk) and what they do
VI. CIRCADIAN CLOCKS, PHOTOPERIODISM, AND SEASONALITY
A. How might daily clocks be involved in photoperiod time
measurement (PTM)
1. Internal coincidence verses External coincidence models
2. "Critical photoperiod"
3. Experimental evidence from skeleton photoperiods **
B. Seasonal breeding cycles in birds and mammals
1. Short (sheep, elk) and long day breeders (mouse, hamster)
2. Role of photorefractoriness
C. Physiology of seasonal breeding cycles in rodents
1. Light is received by the eye
2. Role of the SCN in PTM
3. Role of the pineal gland
a. effect of pinealectomy on short-day-induced regression
4. Duration of nocturnal melatonin induces gonadal regression
a. Melatonin infusion studies **
IX. ULTRADIAN CLOCKS AND ANIMAL BEHAVIOR
A. Timing of Animal Foraging Behavior
1. Vole foraging behavior exhibits an ultradian rhythm
2. Vole foraging behavior is synchronized within the group
a. Why have such synchronization?
X. SLEEP: AN OVERVIEW
A. Introduction
1. Sleep is a readily reversible state of reduced responsiveness/
interaction with the environment
2. Sleep is a circadian rhythm which persists under constant conditions
3. A clock and a homeostat control the sleep-wake cycle.
B. Sleep structure
1. Rapid Eye Movement (REM) and Non-REM
a. Stages of sleep: REM and Non-REM
2. Age affects the sleep cycle
a. Sleep patterns in a newborn
b. Sleep structure changes as we age **
C. We need to sleep!
1. What happens to a person's behavior when they lack sleep?
2. How much sleep do we need? Is it the same for all mammals?
a. Unique sleep patterns: Dolphins
D. Why Sleep?
1. Sleep is essential for normal function and even survival
2. Possible reasons for sleep usually involve some "recovery"
process
a. Tissue repair
b. Resting the body and brain
c. Brain anabolism (e.g., synthesis of glycogen)
d. Consolidation of memory and daily experiences
1) Experimental evidence
E. Neural mechanisms of sleep
1. Sleep is not fatigue, but an active process
a. Evidence
2. Role of the reticular formation in sleep
a. RAS activates the cerebral cortex via multiple
ascending tracts
b. Involvement of 5-HT, NE, and Ach
3 Role of the hypothalamus in sleep
a. Preoptic area stimulates sleep (GABA)
a. Orexin (=Hypocretin) stimulates wakefulness
b. Brain adenosine stimulates sleep
4. Neural control of REM and Non-REM
a. Interaction of cholinergic (+) & aminergic (-) pathways
b. A neuronal model for REM sleep **
F. What are dreams and why dream?
1. Most dreams occur during REM sleep
2. Hobson's idea of dreams
a. Role of acetylcholine and the PGO
b. Extrastriate visual cortex and limbic system
are active; Prefrontal cortex and V1 are quiet
G. Sleep disorders
1. REM sleep behavior disorder
2. Cataplexy and sleep attacks
A
Web Guide to Biological Clocks. A refresher course on the properties
of biological clocks, including entrainment, PRC's, photoperiodism,
etc. from the University of Warwick.
What undergraduates should know about sleep. Sleep and Learning from the Society of Neuroscience Larks
and Owls: Clock function in "morning" and "night" people from
the New Scientist.
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