Ernest Nordeen - US grants

This proposal is aimed at establishing neural sites and cellular mechanisms by which gonadal hormones promote sex differences in vocal ability and neural organization in passerine birds. In zebra finches, song is an androgen-dependent behavior that is normally produced only by males. Estrogens act during development to determine the organization, and, importantly, the extent androgen accumulation within song regions. Androgens induce further neural growth in adulthood, and promote song development. This proposal focuses on three important issues regarding the secual differentiation of the song system. The first regards where estrogen (E2) acts to masculinize developing regions. Steroid autoradiography will be used to assess which developing song region accumulate E2. Intracranial E2 implants will be used to determine which of these neural sites are primary targets for E2 action, and which are influenced indirectly, or trans-synaptically. The second issue concerns how early E2 exposure determines the extent of androgen accumulation and sensitivity in the adult brain. Steroid autoradiography will be used to assess androgen accumulation within developing song regions and hence, whether E2 induces, or preserves a masculine pattern of androgen accumulation. The existence of a critical period for this aspect of E2 action will be determine by examining the pattern of androgen accumulation in birds given systemic E2 in adulthood. Thymidine autoradiography will be used to determine if neurogenesis occurs during the critical period of E2 action, and if E2 stimulates this neurogenesis. The third issue regards the neural sites and functional significance of androgen action in the adult brain. Using light microscopic analyses of cell morphology in song regions, and behavioral analyses, the timecourse of androgen-induced neural change will be assessed and related to vocal development. Projections of androgen-accumulating cells will be established throught the simultaneous use of fluorescent retrograde tracers and steroid autoradiography, and intracranial antiandrogen implants will be used to determine if androgen-accumulating neurons in certain song regions mediate androgen-induced neural growth in their efferent targets. A better understanding of the cellular mechanisms of sexual differentiation hold great promise for therapeutic remedy of abnormalities in sexual differentiation in humans. It will also increase our understanding of cellular mechanisms that permit aspects of neural plasticity that are manifest in birds, yet less robust in humans.

This proposal is aimed at establishing neural sites and cellular mechanisms by which steroids promote sex differences in vocal ability and neural organization. In zebra finches, song behavior is androgen-dependent and normally produced only by males. Estrogens act during development to determine the organization and extent of androgen accumulation within song regions. Androgens induce further neural growth in adulthood, and promote song development. This proposal focuses on three important issues regarding the sexual differentiation of the song system. The first regards where estrogen (E2) acts to masculinize developing song regions. Steroid autoradiography will be used to assess which developing song regions accumulate E2. Intracranial E2 implants will be used to determine which of these neural sites are primary targets for E2 action, and which are influenced indirectly, or transynaptically. The second issue concerns how early E2 exposure determines the extent of androgen uptake in the adult brain. Recent studies indicate that in one song region, early E2 exposure promotes the addition of androgen target cells during adolescence. Thymidine autoradiography will be used to determine if this cell addition involves neurogenesis, and whether E2 enhances either the proliferation or survival of cells in this region. The existence of a critical period for the estrogenic regulation of androgen accumulation will be determined by examining the pattern of androgen uptake in birds given E2 in adulthood. The third issue regards the neural sites and functional significance of androgen action in the adult brain. Using light microscopic analyses of cell morphology in sone regions, and behavioral analyses, the time course of androgen-induced nerual change will be assessed and related to vocal development. Projections of androgen-accumulating cells will be established through the simultaneous use of fluorescent retrograde tracers and steroid autoradiography, and intracranial antiandrogen implants will be used to determine if androgen-accumulating neurons in certain song regions mediate androgen-induced neural growth in their efferent targets. A better understanding of the sexual differentiation process holds great promise for therapeutic remedy of abnormalities in sexual differentiation in humans. It will also increase our understanding of cellular mechanisms that permit aspects of neural plasticity that are manifest in birds, yet less robust in humans.

The aims of this proposal are to (1) identify cellular changes in the developing avian brain that underlie "critical" or "sensitive" periods for vocal learning and, (2) to determine how such changes are influenced by the learning of song-like vocalizations. Most birds learn song memorizing a suitable song model (sensory learning) and then using auditory feedback to mimic that model (sensorimotor learning). These two phases of vocal learning are often restricted to species-specific developmental period believed to coincide with pivotal changes in the organization of song-related brain regions. In zebra finches, for instance, sensory and sensorimotor learning overlap with drastic changes in the number, size, and connectivity of song-related neurons. The first study proposed will employ a comparative neuroanatomical approach to define better the relationship between neural change and sensory or sensorimotor learning. Developmental changes in the anatomy of vocal nuclei will be measured in swamp sparrows, a species in which the two phases of vocal learning are well separated in time. Next, neural changes that may be necessary for memorizing a vocal model will be identified. Acoustic isolation will be used to extend the critical period for sensory learning in zebra finches. It will then be determined how this manipulation influences the timing of neural changes in developing song regions. Finally, to determine how song learning influences the organization of song nuclei, neuroanatomical measurements will be correlated with individual differences in song complexity. Also, the ability of auditory experiences to influence the growth, retention, or addition of neurons during sensory or sensorimotor learning will be assessed. Critical learning periods exist for phenomenon as diverse as language acquisition, social attachment and imprinting. The proposal's long-term goal is to understand the neural mechanisms underlying these periods of unique susceptibility and to determine how information may be stored through experience-dependent modifications of the developing nervous system.

The neural system controlling avian song has proved a powerful model for elucidating hormonal influence on neural organization and behavior. Song is an androgen-dependent behavior produced predominately in males. Sex differences in song behavior are mirrored in the anatomy of brain regions that control song. Neuron size and number, androgen binding activity, and the projections of song-related brain regions are all much greater in males than in females. The striking anatomical dimorphisms result from differences in exposure to gonadal steroids during development. Dr. Nordeen will use sophisticated anatomical techniques to examine how gonadal steroids act on developing song regions during the first few weeks after hatching. He will determine how hormones regulate the production, migration and/or survival of neurons within these specific brain nuclei. These experiments will provide insights into the cellular mechanisms of sexual differentiation. Possibly the most exciting contribution resulting from work on this model system is the recognition of the plasticity of the brain during development and in adulthood. A major outcome of these studies will be to increase our understanding of those mechanisms that permit aspects of neural plasticity and thus, may eventually prove important in developing treatments for enhancing the recovery of function in humans that have been incapacitated by accidents or strokes.

IBN-9507979 Nordeen, Ernest J. Understanding how the production, specification and survival of nerve cells are regulated to generate variation in neuron number is an important goal of developmental neurobiology. In many instances, these processes are influenced by steroid hormones such as estrogen, which act on the developing nervous system to regulate neuron number and growth within specific brain regions. This proposal concerns the biological mechanisms and cellular populations through which these hormones exert control over brain cell number. Several proposed studies will test the hypothesis that hormones prevent cell death to influence neuron number within specific brain regions. Other studies are aimed at identifying the cell populations actually mediating these neurotrophic effects of steroids. More specifically, experiments will test the notion that some neurons transiently express estrogen receptors during their migration, and depend on this hormone for their survival during this period. In addition, the work will address how glia contribute to the neurotrophic effects of steroid hormones during development. The long term aim of these studies is to use the hormonal regulation of neuron growth and survival as a tool with which to explore the underlying cellular and molecular changes influencing a neuron's decision to live or die. In so doing, a greater understanding of the factors contributing to neuron death during development and in disease will be attained.

Instances of early learning (e.g. language acquisition, imprinting, birdsong learning) illustrate that the nervous system's ability to change in response to experience is exaggerated during distinct sensitive periods in development. This proposal is aimed at identifying biological conditions that foster this developmental plasticity by investigating the molecular mechanisms by which gonadal androgens influence the timing of vocal learning in birds. Drawing on earlier work demonstrating that activation of forebrain N-Methyl-D aspartate receptors is critical for vocal learning, and the closure of the sensitive learning period is associated with a decrease in this receptor subtype, the proposed studies will employ quantitative measures of gene expression to determine if androgens regulate developmental changes in NMD receptor expression and composition to control when learning occurs. The work will broaden our understanding of how hormones affect neural and behavioral plasticity, and also may reveal strategies for augmenting learning and memory in later life.

This project focuses on the neural processes by which learned motor skills are adjusted when feedback from an animal's actions deviates from expectations. Studies utilize a well-described neural circuit controlling learned birdsong, and will investigate brain mechanisms that promote behavioral change when auditory feedback from an animal's vocalizations deviates from previously learned patterns. The focus is on a forebrain circuit implicated in motor learning across a wide range of vertebrates. Lesions and/or reversible neuronal inactivation will be used to determine if this forebrain circuit is directly responsible for vocal experimentation when auditory feedback signals the need for vocal change. Also, neuroanatomical measures will establish if such feedback-driven behavioral plasticity entails changes in neuronal connections within motor pathways, and whether the propensity for such neural change decreases in older animals. The project will provide training in behavioral, neuropharmacological, and neuroanatomical methods for both undergraduate and graduate students. Additionally, knowledge derived from these studies will find its way quickly into the classroom, as the investigators are involved heavily in both undergraduate and graduate neuroscience programs. The broad aims are to understand better how the brain evaluates the consequences of behavior, and to identify some of the mechanisms it uses to adaptively adjust behavior. This information will provide insights into general mechanisms of motor learning and development, as well as adult neural and behavioral plasticity. The work should impact a variety of disciplines ranging from cognitive and sensory-motor neuroscience to neural reorganization.