Susan B. Udin - US grants

Visual experience is one of the factors which influences the formation of nerve connections in the developing brain. A dramatic example of this effect occurs in the development of binocular input to the tectum of the frog, Xenopus laevis. In this species, normal vision is necessary for proper positioning of terminals from the nucleus isthmi (NI), a midbrain structure which projects to the tectum. If eye position is abnormal, then connections from the NI form in the wrong locations. Anterograde labeling of isthmo-tectal axons show that most normal axons travel straight to their proper locations but that axons in abnormal animals often follow very circuitous routes and put our branches at inappropriate sites. In order to understand what leads to these patterns, axons will be labeled in developing animals as well as in adults. In particular we will examine whether axons initially terminate at random and then later restrict their connections to a limited region. Another topographic input to the tectum is the uncrossed isthmo-tectal projection. Lesion studies and electrophysiological recordings will be used to determine whether this input is necessary for formation of an appropriately oriented map from the crossed isthmo-tectal projection. The synaptic interrelationships of retino-tectal axons, isthmo-tectal axons and tectal cells will be studied to determine how information may be conveyed from retinotectal terminals to growing isthmo-tectal terminals. The insights which we gain from studying this relatively simple system in the frog brain will help us to understand comparable phenomena in the more complex brains of mammals. In particular, we know that connections in the developing mammalian brain cortex are affected by abnormal eye position (strabismus), and previous experience indicates that any development principles which we find in frogs will apply to mammals as well.

Our long-term goal is to understand the factors that govern the ability or inability of axons to respond to altered sensory input by reorganizing their connections. The visual system of Xenopus frogs is an appropriate system for studying such effects for several reasons: 1) Binocular connections to the optic tectum undergo major reorganization during development, and successful establishment of matching connections from the two eyes depends upon visual input during a critical period of early life. 2) Connections can be systematically reorganized in response to abnormal visual input, such as from rotation of one eye. 3) The ability to reorganize connections normally is lost in adults. This difference provides a tool for identifying the permissive characteristics present in young but not older animals 4) This plasticity can be restored in adults by application of low doses of N-methyl-D-aspartate. In order to understand the mechanisms underlying plasticity, we need a more complete pharmacological description of the tectum. The glutamatergic character of the axons that bring input directly from the contralateral retina has been established by several labs, with particular attention to the N-methyl-D-aspartate receptor and its role in control of intradendritic calcium. However, the cholinergic axons that bring input indirectly from the ipsilateral eye have been much less well studied. Since their activity is important for the activity-mediated establishment of ipsilateral maps, we need to know which tectal cells are their targets, what sorts of acetylcholine receptors are present on those cells, and whether cholinergic activation of those cells results in increases in intracellular calcium. Electrophysiological and imaging studies will reveal the nature of the acetylcholine receptors and their influence on intracellular calcium. These data will also help to clarify the possible role of acetylcholine receptors that are found on retinotectal terminals and that may modulate transmitter release by those synapses. This research will contribute to our understanding of the mechanisms by which reorganization of the brain occurs during early life and after alterations of sensory input or various types of trauma in later life. In particular, the growing evidence of the importance of glutamate receptors will be enhanced by a better characterization of the role of acetylcholine in control of stabilization of synapses.

DESCRIPTION (Adapted from Applicant's abstract): The hormone melatonin is a molecule secreted by the brain on a circadian basis. Thee is evidence that it plays a role in governing sleep cycles and, in some species, reproductive readiness, but it also has been purported to influence everything from aging to cancer. It is consumed by many people, but our knowledge of what melatonin really does and where it exerts it effects is still quite incomplete. The tectum of Xenopus laevis frogs is an appropriate system in which to study melatonin's effects, since the tectum is richly endowed with melatonin receptors; in addition, visual input plays a dramatic role in the Xenopus tectum in the formation of orderly binocular projections (plasticity and preliminary data indicate that chronic melatonin prolongs the critical period during which visual input can alter axonal projections. The experiments proposed here focus on exploring the role of melatonin on tectal function in normal Xenopus and on testing further whether chronic melatonin treatment prolongs the critical period of development of binocular maps in the tectum. Levels of melatonin, melatonin receptors, and melatonin receptor mRNA will be assessed at different times of day, and under different conditions of plasticity in order to determine whether any of those parameters change in correlation with plasticity. The anatomical consequences of chronic melatonin treatment on axonal morphology will be assessed using horseradish-peroxidase filling of isthmotectal axons in order to determine where the axons' development and responses to visual input are altered by chronic melatonin. To assess whether melatonin alters neuronal transmission in the tectum, fluorescent indicators will be used to assess melatonin's effects on calcium levels in axons and cells; also patch-clamp recording from tectal cells will be used to test whether melatonin changes presynaptic transmitter release and/or postsynaptic responses to glutamate or acetylcholine. The results of these studies will provide new information on both the acute and chronic effects of melatonin in the developing and mature nervous system. These data will be relevant to our understanding of how normal patterns of melatonin secretion affect the brain and will serve to alter us to possible consequences of long-term ingestion of melatonin.