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Development of Behavior Steven McLoon Department of Neuroscience University of Minnesota 1 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. 2 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. 3 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. 4 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. 5 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. 6 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. 7 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. 8 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. 9 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. 10 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. 11 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. 12 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. 13 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. 14 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. 15 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. 16 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. 17 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. 18 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. 19 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. 20 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. 21 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. 22 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. 23 Sex-specific Differentiation of the Nervous System Males and females exhibit innate behavioral differences 24 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. 25 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. 26 Sex-specific Differentiation of the Nervous System What is the fundamental difference between males and females? 27 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. 28 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. 29 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. 30 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. 31 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. 32 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. 33 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. 34 Zebrafinch Song Females treated with estradiol during this period develop a larger RA & HVc and learn to sing as well as males. 35