Ary S. Ramoa - US grants

9421983 Ramoa Very recently it was discovered that, before the time that the nervous tissue of the eye (the retina) can respond to light, the nerve cells in the eye generate waves of activity that are transmitted to the cells in the eye's central nervous system target (the dorsal lateral geniculate nucleus). Importantly, the waves of retinal activity are patterned, and they occur during a time of development when the patterns of connections between the eye and the brain are being established and refined. This research is designed to determine whether these spontaneous waves of activity contribute to the definition of the connection patterns between the eye and the brain, as well as how this activity interacts with other central nervous system inputs to the cells in the dorsal lateral geniculate nucleus. It has long been known that patterns of activity generated by light stimuli play an important role in visual system development. This research will provide important information about how intrinsic, spontaneous activity contributes to specifying the connectivity in the central nervous system.

The mammalian visual system requires experience to develop normally. Lack of sensory experience due to degradation of the visual input of one eye during development may lead to irreversible loss of visual function mediated by this eye. This condition, known as amblyopia, is a major cause of visual disability in children. There is no doubt regarding the substantial scientific and clinical relevance of this type of neural plasticity. However, the cellular and molecular mechanisms underlying this crucial developmental process remain unknown. In recent years much attention has been focused on the N-methyl-D-aspartate (NMDA) subtype of excitatory amino acid receptor because of its proposed role in brain development, learning and memory. Its specific role in visual plasticity has, nevertheless, remained elusive. Recent molecular studies have provided an exciting new opportunity to examine the contribution of the NMDA receptor to visual function and plasticity. These studies have shown that this receptor is composed of different subunit proteins and that different combinations of subunits exhibit distinct functional properties. It is proposed here that developmental changes in subunit composition of the NMDA receptor play a critical role in visual plasticity. A novel approach is proposed to examine this hypothesis. Intracortical infusion of antisense DNA will be used to suppress selected NMDA receptor subunits. The receptors should then be assembled from the remaining subunits. The resultant modifications in functional properties of cortical NMDA receptors will be examined by intracellular recordings in cortical slices maintained living in vitro and the changes in visual responses will be assessed in normal animals as well as in those which were monocularly deprived of vision during antisense treatment. This approach offers several significant advantages over currently used pharmacological procedures, such as the possibility of selectively manipulating individual subunits. Moreover, transgenic animals cannot be used to conduct these experiments, since animals lacking some NMDA receptor subunits are not viable. The results should delineate links between the molecular composition of the NMDA receptor and its functional role in visual cortex. In addition, since molecular interventions may one day be used for therapeutic purposes in visual disorders, these studies may provide a new dimension to our clinical armamentarium at the level of the synapse.

DESCRIPTION: Fetal alcohol syndrome (FAS) is characterized by a constellation of behavioral and physiological abnormalities in children, including learning, sensory and motor deficits. There is growing evidence that abnormalities of neocortical function and plasticity underlie these deficits. However, the cellular and molecular mechanisms by which prenatal alcohol exposure disrupts neocortical development remain elusive. Neuronal electrophysiological activity involving the N-methyl-D-aspartate (NMDA) type of glutamate receptor is thought to have a critical function in the circuit rearrangements that characterize the developing sensory neocortex. Moreover, inflow of calcium through the NMDA receptor activates the transcription factor cAMP/Calcium-dependent response element binding protein (CREB), which regulates gene expression required in neural plasticity. Alcohol is known to block NMDA receptors and there is increasing evidence that CREB activation is reduced following chronic alcohol exposure. The proposed studies will examine a series of interrelated hypotheses focused on effects of alcohol on NMDA receptor- and CREB-dependent mechanisms of neocortical development and plasticity. The studies will use molecular techniques, including in vivo antisense techniques and viral vectors for gene transfer, to examine the molecular mechanisms by which alcohol disrupts cortical development and plasticity. The first goal is to characterize abnormalities of function in sensory neocortex of animals exposed to alcohol during the third trimester equivalent of human gestation. Animals will be studied electrophysiologically and behaviorally following a period of one alcohol-free month. The second goal is to examine whether effects of alcohol on NMDA receptors underlie the disruption of cortical development. Alcohol is known to decrease, but not completely block, NMDA receptor function. Similarly to alcohol, antisense DNA suppresses but does not block cortical NMDA receptor function. Therefore, antisense DNA injected intracortically will be used to verify whether a partial blockade of cortical NMDA receptor function is sufficient to disrupt cortical development. The third goal of these studies is to elucidate whether reduction of CREB activation underlies the decreased cortical plasticity in FAS. To examine this hypothesis, a herpes simplex viral vector will be used to induce overexpression of CREB, compensating for the downregulation caused by the chronic alcohol treatment. Electrophysiological recordings will then be used to determine whether cortical plasticity is restored to its normal level by the overexpression of CREB. Finally, these studies will examine whether alcohol affects development of peripheral sites that influence cortical development. The visual cortex will be used as a model in these studies because it has been the most studied area of the neocortex. Moreover, visual cortical plasticity is thought to share some basic mechanisms with learning and memory, and there is substantial evidence that NMDA receptors and CREB are involved in this type of plasticity. The studies on this system should assess the effects of alcohol on prenatal cortical plasticity in general and the results should not be restricted to the visual cortex. This information may one day contribute to devise therapeutic interventions that will prevent or alleviate morbidity in FAS.