In the former group of studies, individual dendritic

spines could be activated by electrical stimulation VE-822 solubility dmso or by photoconversion of caged glutamate (Harvey & Svoboda, 2007; Lee et al., 2009), to find that stimulation of a single spine can cause a nearly immediate expansion of the spine head volume by 3–4-fold (Matsuzaki et al., 2004). This effect was dependent on activation of the NMDA receptor and its maintenance, at a lower level than the original expansion, was dependent on activation of kinases (Yang et al., 2008). The spine expansion preceded the electrophysiological change, which progressed at a slower time course, and the change in spine volume was much smaller than in the original report (Yang et al., 2008). These studies illustrate the ability of spines to change their volume over a short period of time after exposure to a massive excitatory stimulation. On the other hand, such a massive increase in spine volume was not seen by others, who found a slow change in volume following a massive activation of glutamate receptors (Sapoznik et al., 2006), or no change at all, even in conditions in which the activation of the

spine followed a pairing protocol for induction of LTP (Nevian & Sakmann, 2006). The difference between such observations on FK506 solubility dmso spine head expansion may have to do with the insertion of glutamate receptors into the spine heads such that only the spines to which glutamate receptors are added into their heads will expand (Kopec et al., 2007; Korkotian & Segal, 2007)

while others will not. Even this expansion is rather Thymidylate synthase slow, and cannot underlie the nearly immediate expansion of spine heads reported previously (Matsuzaki et al., 2004). More recently, a persistent change in spine number (but not in their volume) in the mouse neocortex has been seen following extensive motor learning; the change lasted over many days after initial training (Yang et al., 2009; Xu et al., 2009). While these results are technologically impressive they do not relate specific spines to specific neuronal activity, to the extent that beyond the correlation between performance and spine number there is no clear indication that these additional spines participate in the enhanced network activity resulting from the training. Still, these studies did not show a dramatic change in spine volume as predicted by the earlier studies. The other approach, which involves comparisons of populations of spines using 3-D electron-microscopic reconstructions of spines, was used extensively both in vivo and in vitro (Stewart et al., 2005; Medvedev et al., 2010).