Rapid changes of dendritic spine morphology during induction of long-term potentiation. Cynthia Lang, Leonard Zablow, Steven A. Siegelbaum and Stanislav S. Zakharenko. Howard Hughes Medical Institute, Center for Neurobiology and Behavior, Columbia University, 722 West 168th street (Kolb Annex), rm. 622, New York, NY 10032. Spine motility is implicated in mechanisms of long-term potentiation (LTP) at CA3-CA1 excitatory synapses in the hippocampus. Although long-term spine motility lasting from one day to several weeks has recently been described [1, 2], little is known about changes in spine morphology minutes after LTP induction. To address this issue, we used two-photon imaging in acute hippocampal slices from adult mice that express green fluorescent protein in a small population of pyramidal neurons. We show that in the absence of synaptic stimulation, all dendritic spines remained stable over several hours. However, after synaptic stimulation approximately 2% of spines underwent very rapid (on the order of minutes) growth and retraction (see figure). Only previously established spines changed their shape and size, as we never observed de novo spine growth. Spine modifications were evoked by stimulation at frequencies

ranging from 10 Hz to 200 Hz, the same frequencies conventionally used to induce LTP at CA3-CA1 synapses in the hippocampus. Furthermore, these changes were blocked by application of the NMDA receptor antagonist D-AP5, suggesting that, similar to the induction mechanisms of LTP, NMDA receptors play a crucial role for short-term alterations in

spine morphology. Conversely, we showed that L-type voltage-gated calcium channels involved in presynaptic mechanisms of LTP at CA3-CA1 synapses [3] are not involved in mechanisms of spine modification. We also tested the importance of actin in this phenomenon. Latrunculin A, which binds monomeric actin to cause depolymerization, completely blocked activity-dependent alteration of spines. Protein synthesis, on the other hand, does not play a role in the short-term spine modifications, as evidenced by their resistance to anisomycin. Our findings show for the first time that spines are capable of very fast transformation in response to synaptic stimulation and that this phenomenon may be part of induction machinery that ultimately generates long-term synaptic plasticity in the hippocampus. References: 1. Grutzendler, J., Kasthuri, N. & Gan, W. B. (2002) Nature 420, 812-6. 2. Trachtenberg, J. T., Chen, B. E., Knott, G. W., Feng, G., Sanes, J. R., Welker, E. &

Svoboda, K. (2002) Nature 420, 788-94. 3. Zakharenko, S. S., Zablow, L. & Siegelbaum, S. A. (2001) Nat Neurosci 4, 711-7.