Ayako Yamaguchi - US grants

DESCRIPTION (provided by applicant): The long-term goal of the proposed project is to understand how steroid hormones modify the functional properties of neurons and neuronal circuits to produce sexually distinct behavior patterns. In this project, the link between steroid hormones, neuronal function, and behavior will be investigated by using the vocal behavior of the African clawed frog as a model. Male and female frogs produce sexually distinct vocalizations that are regulated by steroid hormones. The vocalizations of male and female Xenopus are produced by the sex-specific firing rates of laryngeal motoneurons that control the vocal organ. Thus, in this system, the challenge of understanding the neural mechanisms of behavior can be reduced to a problem of understanding generation of stereotyped rhythmic spike patterns by motoneurons. The proposed research will investigate how biophysical and structural properties of laryngeal motoneurons and premotor neurons are modified by steroid hormones, and how these changes help shape the motor output that underlies sex-specific vocal production. Ultimately, understanding how steroids modulate the excitability of neurons will be important in unraveling the mechanisms of numerous human diseases including such common and important problems as epilepsy. Recent data suggests that certain steroids may be used as anti-epileptic drugs while in some patients, steroid hormones clearly increase the incidence of epileptic seizure. In addition, seizures can be linked to the menstrual cycle. However, there is very little known about how steroids can induce a sudden synchronous firing among neurons that causes seizure nor how they can be used to prevent such a storm of activity. By studying a well-defined neural system in which neuronal excitability is clearly regulated by androgen, as proposed here, we can better understand the cellular mechanisms that trigger bursts of neuronal activity. This more detailed understanding of cellular activity is necessary for further progress to be made in developing therapeutic approaches to epilepsy.

The most salient output of brain function is behavior. However, how the nervous system produces behavior is not well understood, largely because most of the neural pathways underlying behavior are complicated. In this research project, vocal behavior of African clawed frogs is used as a model because their vocal neural pathways are simple and straight forward, and the pathways in action can be studied using techniques that were previously developed in the PI's laboratory. In addition to their simplicity and accessibility, the frog vocal pathways provide a unique opportunity to study how female and male brains function differently; male and female frogs produce sex-specific vocalizations during the breeding season, and the injection of male-specific hormones into an adult female results in male-like vocalizations within thirteen weeks. In this study, the focus is placed on one group of cells that are known to play a critical role in the operation of the pathways. A variety of experimental techniques will be used to understand where these neurons are, how these neurons function, and how they respond to male-specific hormones. The results of the study will not only provide us with the understanding of how behaviors are generated in the two sexes, but also provide us with an insight into how human brains generates rhythmic activity such as alpha and gamma waves, many of which are known to underlie cognitive processes, and known to be disrupted in diseased states.

A fundamentally important question in neuroscience is how neural networks function to generate motor programs that underlie behavior. Although analyses of a complete neural network that generates behavior is a formidable task, the relative simplicity of the Xenopus vocal network combined with the development of the fictive preparation (a "singing brain in a dish" preparation) and the application of behavioral, electrophysiological, anatomical, and newly developed optogenetic techniques allows detailed investigation of the dynamic organization of brain in action. The results of the proposed study will not only provide insight into the structure, function, and plasticity of the rhythm-generating neural network at the cellular levels, but also allow us to understand the logic of how a feedback loop should be engineered into a network to generate stable rhythms. Rhythmic neuronal activity is not limited to motor systems, but is prevalent across the entire CNS and is considered to underlie important functions such as perception and cognition. Thus, understanding the biophysical principles that govern rhythm generation using a simple neural network has a potential to elucidate mechanisms underlying neuronal oscillations in general. On a technical level, successful application of optogenetic tools to the Xenopus fictive preparation in vitro fills an important gap between research efforts conducted on genetic vs non-genetic model organisms. There are many non-genetic model organisms that present unique questions. The ability to express genetically encoded tools in non-model organisms represents a revolutionary change in the field of comparative neuroscience.