Download Development of Behavior - McLoon Lab

Development of Behavior
Steven McLoon
Department of Neuroscience
University of Minnesota
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Origin of Behavior
 Invertebrate behavior –
 Many behaviors during development and throughout
adult life are genetically determined and are ‘hardwired’
into neuronal function and circuitry.
 Genes responsible for specific behaviors are being
identified. Homologous genes are expressed in
vertebrates.
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Origin of Behavior
 Vertebrate behavior –
 Innate behaviors – i.e. preprogrammed into the default
neuronal function and circuitry, genetically determined
 Experience dependent behaviors – i.e. learned; uses
neural circuitry and a response to experience that is
formed by a combination of genetic program and
plasticity
 In developing vertebrates, the very first behaviors are
innate, and leaning has a greater role in behaviors later
in development.
 In the adult, most behaviors are the result of innate and
experience-dependent responses.
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Origin of Behavior
 Vertebrate behavior –
 A Black6 mouse reared by a BALBc mother exhibits
exploratory behavior intermediate between that of a
normally born and reared Black6 mouse and a BALBc
mouse.
 This suggests that the exploratory behavior of mice is
partially innate and partially learned.
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Early Movements in Vertebrate Embryos
 First movements are the result of spontaneous activity
of motor neurons.
 A motor neuron can activate neighboring
motor neurons, which results in contraction
of a muscle.
 This movement has no organization.
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Early Movements in Vertebrate Embryos
 Slightly later, activity of dorsal root ganglion
neurons can initiate motor neuron activation via
a local reflex circuit.
 Removal of premigratory neural crest (i.e. no
DRG develop) had no effect on development
of the first movements but reduced this next
phase of movements.
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Early Movements in Vertebrate Embryos
 Later, there is coordinated contraction of extensor
muscles followed by flexor muscles as is typical of many
adult movements.
 Neighboring motor
synchronized.
neuron
activity
becomes
 Extensor motor neurons activate local Renshaw cells
via axon collaterals. Renshaw cells directly inhibit
flexor motor neurons, which results in alternating
activity of extensor then flexor motor neurons.
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Early Movements in Vertebrate Embryos
 Next, movements become coordinated along the length
of the embryo.
 Sensory pathways project up the spinal cord to
commissural neurons in the brainstem that project to
motor neurons on the contralateral side.
 Motor neurons have collaterals to ipsilateral motor
neurons in the next segment down.
 Thus, a stimulus to one side causes bending of the
body to the other side.
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Early Movements in Vertebrate Embryos
 Next, movements become coordinated from side-to-side.
 Commissural neurons connect motor units on each
side of the spinal cord.
 Muscle contraction on one side is followed with a
delay by contraction on the other side.
 This results in swimming-like movements.
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Inducing factors promote expression of specific transcription factors.
e.g. spinal cord
 The relative levels of Shh and BMPs determines the
transcription factors expressed along the dorsal-ventral
axis of the spinal cord.
 The combination of factors expressed at each dorsalventral level determines the cell types that develop
there.
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Early Movements in Vertebrate Embryos
 The excitatory V3 commissural neuron is required for
left-right alternations in muscle contraction.
 The transcription factor Sim1 is required for V3
development.
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Early Movements in Vertebrate Embryos
 Finally, alternating movements of the limbs develop in
quadrupeds (like humans?).
 This requires coordination between the two sides and
between the flexors and extensors of a limb.
 This is similar to the movements required for crawling.
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Early Movements in Vertebrate Embryos
 Newly hatched chicks have synchronous movement of
the forelimbs (i.e. wings) as used to fly and alternating
movement of the legs as used in walking.
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Early Movements in Vertebrate Embryos
 Surgical replacement of the cervical cord with
lumbosacral cord and vice versa in the early chick
embryo resulted in reverse behavior at hatching.
 These behaviors are intrinsic to the neurons and their
connections.
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Activity is not required for early progress of development of behavior.
 Amphibian embryos treated with a drug that blocked
neuronal activity showed stage appropriate movements
when the drug treatment was stopped.
 This suggests that early behaviors are due to intrinsic
programs and are not due to learning based on previous
behaviors.
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Function of many systems improves during development.
 Visual acuity improves.
 Newborn humans resolve ~1 alternating black & white
lines per degree of visual space.
 Normal adults resolve ~30 lines per degree.
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Function of many systems improves during development.
 Sound localization improves.
 Newborn humans require a 25° change in position to
detect relocation of a sound source.
 Normal adults can detect a 1° change.
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Function of many systems improves during development.
 These changes in visual acuity and sound localization
involve both innate and experience dependent
maturation of the nervous system circuitry.
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Preventing normal function during critical periods of development
can permanently alter function in the adult.
 Monocular
deprivation
or
strabismus
during
development will prevent normal formation of ocular
dominance columns in visual cortex. This permanently
alters aspects of adult visual function including
stereopsis.
 Immobilization of a forelimb in a developing primate
reduces fine motor skills in the adult.
 Cortical pyramidal neurons in rats that develop in an
‘enriched’ environment have ~25% more dendritic
spines (i.e. synapses) as adults.
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Some behaviors are required at certain stages of development.
 Chick embryos undergo an innate ‘hatching behavior’ at
a certain stage of development.
 If a newly hatched chick is returned to an artificial egg, it
will immediately exhibit a hatching-like behavior.
 Within a day of hatching, returning the chick to an egglike structure will no longer initiate the same behavior.
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Some behaviors are required at certain stages of development.
 A human newborn turns its face towards a tactile
stimulus to a cheek and opens its mouth in preparation
to nurse.
 This behavior is lost as the baby matures.
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Some behaviors during development may be evolutionary relics.
 All mammalian embryos exhibit rhythmic movements
along their length as a result of spinal pattern generator.
Movement resembles swimming in larval amphibians
and fish. This movement is not used later in
development.
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Some behaviors during development may be evolutionary relics.
 Human infants exhibit a grasp reflex until ~3 months of
age that is normally used by non-human primates to
cling to their mothers’ hair.
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Sex-specific Differentiation of the Nervous System
 Males and females exhibit innate behavioral differences
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Sex-specific Differentiation of the Nervous System
 Males and females exhibit innate behavioral differences:
o e.g. Male mice exhibit a mounting behavior when
encountering female mice and aggression when
encountering male mice. Female mice exhibit lordosis
when they encounter other mice.
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Sex-specific Differentiation of the Nervous System
 Males and females exhibit morphological differences in
their nervous systems:
o e.g. The ‘sexual dimorphic nucleus of the preoptic
area’ (SDN-POA) of the hypothalamus is larger in
males than in females.
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Sex-specific Differentiation of the Nervous System
 What is the fundamental difference between males and
females?
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Sex-specific Differentiation of the Nervous System
 What is the fundamental difference between males and
females?
 Females have two X chromosomes and males have
one X and one Y chromosome.
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Sex-specific Differentiation of the Nervous System
 What is the fundamental difference between males and
females?
 Every cell in the body carries this repertoire of
chromosomes, but mainly the gonadal progenitor cells
exhibit sex-specific gene expression.
 Expression of the Sex-determining Region Y (SRY)
gene on the Y chromosome promotes testes
development and expression of testosterone.
 Testosterone drives
characteristics.
development
of
most
male
 The absence of testosterone results in the female
phenotype.
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Sex-specific Differentiation of the Nervous System
 In the brain, testosterone is converted to estradiol, an
estrogen hormone, by aromatase. Males have much
higher levels of aromatase and estradiol in the brain
than do females.
 Testosterone and androgen receptor are required for
development of male behavior and brain morphology.
 Androgen receptor is expressed in several developing
brain regions including hypothalamus, amygdale, and
cortex.
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Sex-specific Differentiation of the Nervous System
 The
level
of
testosterone/estradiol
development of SDN-POA in mouse.
regulates
 Estradiol prevents developmental death of SDN-POA
neurons.
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Sex-specific Differentiation of the Nervous System
 Juvenile male monkeys exhibit more mock-fighting than
females.
 Administration of testosterone to a pregnant monkey
results in juvenile female monkeys with increased mockfighting behavior.
 The volume of the amygdale and preoptic area of the
hypothalamus correlates with this behavior.
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Sex-specific Differentiation of the Nervous System
 In cultures of embryonic diencephalon harvested prior
to gonad development, tyrosine hydroxylase neurons
from males are 30% larger than from females.
 Also, there are two times more prolactin expressing
neurons in cells harvested from females than from
males.
 This suggests that the male-female chromosome
difference may have cell autonomous effects on some
aspects of brain development.
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Zebrafinch Song
 The male of many bird species (including zebrafinch)
attract a mate by singing a particular song during the
mating season.
 The song nuclei of the brain, RA and HVc, are larger in
male zebrafinch than in females and are essential for
song learning.
 Male zebrafinch learn their song from an adult ‘tutor’
during the first 80 days posthatching.
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Zebrafinch Song
 Females treated with estradiol during this period
develop a larger RA & HVc and learn to sing as well as
males.
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