Donald J. Stehouwer - US grants

During the metamorphic transition from aquatic to terrestrial life in the frog, a corresponding shift in larval swimming to adult-like hindlimb locomotion are functionally mature no later than midlarval stages, or about a year before hindlimb locomotor behavior occurs. I have shown in electrophysiological studies in vitro that the spinal circuits for hindlimb locomotor behavior commences. This fact and the results of other experiments I have performed suggest that hindlimb locomotor activity is suppressed in the larva, probably by supraspinal structures. A second change associated with metamorphosis is a precipitous decline in spontaneous activity of locomotor circuits in vitro (fictive locomotion). It is hypothesized that the change in locomotor topography and gross levels of fictive locomotion during metamorphosis are the result of the dramatic development of sensorimotor structures (e.g., cerebellum and optic tectum) associated with this period of development. It is specifically hypothesized that spinal hindlimb locomotor circuits are tonically inhibited in both larval and adult life and are disinhibited in response to particular sensory stimuli. The relative increase in use of the hindlimbs in locomotion and the decline of fictive locomotion in vitro are both assumed to occur because changes in peripheral sensory structures and associated changes in sensorimotor integration increase the specificity and complexity of the adequate stimuli for release of locomotion. This hypothesis is consistent with a diverse set of manipulations that elicit precocial hindlimb use, the behavior of adult anura, concepts of locomotor development as derived from developing chicks and mammals, anatomical studies of metamorphic changes in the frog's brain and mechanisms of locomotor control in invertebrates. Because of the unique advantages of the bullfrog larva as a model system this hypothesis is easily testable, and the experiments should add to our understanding of vertebrate locomotor development and organization, help bridge the gap between invertebrate and vertebrate studies and provide a basis for more sophisticated analyses of motor development and control. Tests of alternative hypotheses are also included.

Locomotor systems of the frog undergo dramatic morphological and functional change during metamorphosis. The research will investigate the contribution of changing sensory inputs, neuropharmacological change, and hormonal events on development of locomotor behavior and its underlying neural circuitry. The basic hypothesis is that the interneuronal network producing rhythmic undulations of the tadpole is conserved, at least in part, to coordinate the hindlimbs of the postmetamorphic frog. Mechanisms underlying pedal locomotion and undulatory swimming coexist throughout much of the larval period, even though hindlimb locomotor behavior is not normally observed until metamorphic climax. Preliminary results suggest that hindlimb locomotor behavior is suppressed by descending inputs prior to metamorphosis. Therefore, research will focus on factors that experimentally facilitate activity of the hindlimbs in tadpoles and could normally "turn on" pedal locomotion at metamorphosis. This activation of hindlimb motoneurons could be accomplished by specifically gating the of output of locomotor pattern generators to hindlimb motoneurons or by non-specifically increasing the excitability of hindlimb motoneurons. The problem will be attacked behaviorally by using single-frame analyses of locomotor behavior. It will be attacked electrophysiologically by recording, in vitro, the output of motoneurons innervating larval axial muscles and motoneurons innervating the hindlimbs, using an isolated central nervous system preparation. Answers to these questions may lead to a better understanding of mechanisms of locomotor development in other limbed vertebrates, including mammals. This research may also yield insight into general mechanisms of motor development. Are the same circuits used to produce different, functionally-related behaviors at different times during development? For example, are circuits that underlie suckling behavior of the infant later used to produce mastication? If so, what are the factors that determine whether those circuits should produce sucking or mastication? Why does brain damage in adulthood sometimes result in the return of the rooting reflex and sucking responses?

The proposed research will use a new paradigm, drug-elicited air- stepping, to study the development of locomotion and its neuropharmacological and neuroanatomical substrates in the preweanling rat. The first aim is to describe in detail the locomotor behavior elicited by I-DOPA at different stages of ontogeny. L-DOPA-induced air-stepping has revealed largely unsuspected capacities for highly patterned locomotor movements in the neonatal rat. Air-stepping will be compared with treadmill walking and swimming to determine how biomechanical factors, reflexes, and sensory feedback interact with central nervous system (CNS) maturation in the production of locomotor behavior at different stages of maturity. At present, there are no quantitative data bearing on these issues. The second aim is to identify the neuropharmacological substrate(s) critical for 1-DOPA-induced air-stepping. The effects of agonists for receptor subtypes of both dopamine and noradrenaline will be tested. Thus the role(s) of the catecholamines in locomotor development will be fully described for the first time. The third aim is to identify the subdivision of the central nervous system necessary for drug-induced air-stepping at each age studied during preweanling ontogeny. Midthoracic spinal transection and precollicular decerebration will be used to grossly localize locomotor mechanisms activated by catecholamine agonists. These experiments will enable us to estimate the contributions of different levels of the CNS to the maturation of locomotion. The long-term goal of this research program is to identify and quantify developmental changes in locomotor parameters, and to identify their anatomical and pharmacological substrates. Because the principles of locomotor development in rats and humans are remarkably similar, the results will be relevant to our understanding of human locomotor development.

9322052 Rowland This award provides support to the Department of Psychology at the University of Florida to establish an REU site in the field of psychobiology. Applicants will be solicited from schools in the Southeastern United States, with special emphasis on smaller undergraduate and/or minority institutions at which students do not otherwise have an opportunity for gaining research experience. The eight selected applicants will spend 12 weeks at UF, during which time they will complete a small research project under the direct supervision of a faculty mentor. Ten primary faculty, all with active research programs in the general field, will participate along with several affiliated faculty and graduate assistants. A variety of observational, surgical, biochemical, computer, and other analytical techniques are available to the students. In addition to research, students will attend a classroom course which will cover a range of relevant issues including faculty research, ethical issues, experimental design, and principles of psychobiology. The students will learn communication skills by giving interim and final oral reports of their research projects. ***