The Unit on Neurocytology and Physiology is investigating the molecules and processes involved in regulating the structure and function of the nervous system in response to appropriate patterns of neural activity, and exploring the cellular mechanisms that regulate gene expression in response to neuronal firing. A combination of molecular, imaging, and electrophysiological techniques is employed in mammalian neurons in vivo and in vitro. Findings in the last year concern four areas: (1) activity-dependent neuron-glial communication, (2) intracellular signaling from action potentials, (3) the role of gene expression in LTP, and (4) control of myelination by action potentials. Using confocal calcium imaging in co-cultures of Schwann cells and DRG neurons, we find that Schwann cells can detect impulse activity in premyelinating neurons. We have determined that a signaling molecule is secreted from non-synaptic regions of axons, which binds receptors on Schwann cells and activates intracellular signaling cascades that regulate the expression of genes controlling differentiation and proliferation. In other studies to explore how gene expression can be regulated by the pattern of neural impulse firing, we are investigating how information coded in the temporal pattern of action potential firing is transduced across the cell membrane and decoded within intracellular signaling cascades. Theoretical modeling and in vitro studies have predicted that the frequency of neuronal impulses could be decoded by autophosphorylation of a calcium-dependent kinase CaM KII. In the first test of this theory in intact neurons, we find that the spike frequency decoding of CaM KII is weak, and that the enzyme is remarkably sensitive to the concentration of extracellular calcium. Electrophysiological studies in rat hippocampus, in combination with molecular methods, are revealing how signals reach the nucleus to activate genes necessary for the conversion of short-term to long-term memory. These studies show that the transcription factor CREB and a gene associated with LTP can be induced by action potentials in postsynaptic CA1 neurons in the complete absence of synaptic activity. This result contradicts the currently accepted view that a signaling molecule generated at the synapse must translocate to the nucleus to activate transcription factors necessary for long-term memory. In a continuation of our previous work showing that neural impulse activity could regulate the expression of a cell adhesion molecule, L1, on the surface of axons, we now report that the formation of myelin is inhibited by impulse activity at a frequency that lowers axonal expression of L1. Another frequency of firing that does not affect L1 expression has no affect on myelination. These studies are providing a better understanding of the molecular mechanisms involved in regulating development and synaptic plasticity in the nervous system according to functional activity. - myelin, LTP, calcium, Schwann cell, memory, gene expression, activity- dependent development, CREB, cell adhesion molecules,