Vladimir Brezina - US grants

Description of Application for an ADAMHA Scientist Development Award. I am committed to a scientific career of research into the neural basis and cellular mechanisms of behavior and its plasticity. I am aware, however, that I do not yet possess the broad-ranging technical expertise that would allow me to independently undertake the kind of integrated, multidisciplinary investigation necessary to understand these complex questions. By permitting me to work with and learn from a technically expert Preceptor already active in the field, award of the SDA will allow me to begin such a project nevertheless. I have chosen to work with Dr. K. Weiss, whose use of a relatively simple invertebrate model system, the marine snail Aplysia, has enabled him to make considerable progress toward revealing the neural and cellular mechanisms of behavioral plasticity (specifically, plasticity of the animal's feeding behavior) that arises from changes in motivational state (food-induced arousal and satiation). this work, focusing on a representative feeding muscle and its innervating motorneurons, has revealed that, on the cellular level, much of this plasticity is due to release, under the appropriate behavioral conditions, of modulatory neuropeptide cotransmitters from the same motorneurons whose primary transmitters mediate the behavior. Preliminary evidence indicates that one family of these peptides, the buccalins, act at autoreceptors on the very same presynaptic terminals from which they were released to cause feedback inhibition of release of the neurons' primary transmitter, acetylcholine, but not of themselves or the other peptide cotransmitters. The general hypothesis that I propose to test in this system is that a major function of such differential presynaptic autoinhibition is to match to behavioral demands the ratio of transmitters and cotransmitters released from the same neuron and thus optimize the behavioral output of the organism during changes in its motivational state. Specifically, I shall determine the patterns of activity of the motorneurons in freely feeding animals, and then similarly stimulate the motorneurons in vitro to determine if with these behaviorally relevant stimulation parameters the buccalins are actually released. To determine whether the buccalins are released in sufficient amounts to produce behavioral effects, antibodies and receptor blockers will be used to interfere with their actions. Experiments to confirm the presynaptic site and the differential nature of the inhibition, and studies of the intracellular mechanisms of buccalin action will complete the project. Apart from its scientific rationale, I propose this logically developing sequence of research so that it will naturally involve my technical training as it becomes necessary to the research. In this way, my wish to answer the scientific questions will become the motivation and goal of the technical training, and my ability to answer them successfully will be the validation of its adequacy. By the end of this project I hope to have become expert in the great variety of approaches and techniques required to study the neural basis of behavior, memory and learning, with relevance to issues of human mental health and illness.

DESCRIPTION:(from applicant's abstract) The long-term goal of this research is to understand the basic computational and control principles which the central nervous system uses to generate functional behavior. Some fundamental principles are implicit in the interaction of the central controller with its peripheral effectors, most importantly muscles. The motor commands of the nervous system and the peripheral response characteristics of the neuromuscular system must be mutually matched for optimal performance. In many systems this matching is accomplished by peripheral modulation which dynamically tunes the properties of the muscle so as to enable it to perform the behavior being commanded by the nervous system. But, although set up as part of the behavior, the modulation generally has much slower dynamics than those of the behavior. In effect, the modulatory state represents a memory, maintained peripherally in the muscle, of past behavior. This memory then prepares the muscle to perform future behavior. It facilitates performance especially of the same kind of behavior as in the past, but may complicate performance if the nervous system commands a different behavior without its presence into account. This peripheral memory and its consequences for control of motor performance and behavior by the nervous system will be studied in a well known, experimentally advantageous model neuromuscular system. The system participates in several behaviors and exhibits a rich variety of neuromuscular modulation on a wide range of time scales. Preliminary studies demonstrate prominent peripheral memory in the system. A strategy combining experiments with mathematical modeling will be used to address the following questions: What motor commands does the nervous system send in the different behaviors? What corresponding modulation occurs? How do the commands and modulation interact to produce functional movement? How does the functional movement change when on the one hand the motor commands, and on the other hand the modulation, are altered? Altogether, this work will test a two-part hypothesis, reflecting the mutual interdependence of controller and effector: that the peripheral memory is required for smooth, efficient integration of successive cycles of a behavior and even for transitions from one behavior to another; but that, at the same time, its existence requires modification of the commands sent by the central nervous system.

Great efforts are being made to map the "connectome" the neuronal wiring diagram of brain circuits. But mapping the static connectome will not suffice for an understanding of the computations that these circuits perform, because during these computations the static connectome is dynamically modified by a complex network of diffusible modulatory neurochemicals. As yet, no experimental system has come close to providing a full understanding of this modulatory system and its functional role. The problem is that there are invariably many modulators present simultaneously, with mutually interacting effects. To understand the operation of the entire modulatory system, a global overview of the simultaneous effects of all possible modulator combinations is required. By brute force, this would take a vast number of experiments.

This project will test a simple yet powerful solution to this problem. The heart of the solution lies in a combinatorial algorithm that compacts all of the modulator combinations to a tractably small number of combinations whose testing is sufficient. Using mathematical modeling in conjunction with experimental neurophysiological methods, this approach will be developed and tested in a neuromuscular system that is simple and experimentally tractable, yet can yield generally relevant principles. The larger goal of the work is to demonstrate a coherent, integrated strategy for the study of highly multidimensional modulatory systems. This strategy will allow the full modulatory complexity of brain circuits to be studied for the first time and answer broader questions about the design of neural systems and their modulatory control. This project will integrate experiments with mathematical modeling and theory, and train students, including minorities and women, in both areas. Furthermore, cross-fertilization is anticipated with areas of technology in which a compact combinatorial test set can simplify work with the complex, dynamic, modulated systems that are becoming common also in today's technological environment.