, 2003). It is therefore

not likely that these neurons lose their afferents once their spines disappear or are not formed. The reverse case has been documented in vivo; when the cell loses its afferents the relevant spines disappear, only to reappear when see more a new pathway innervates the vacated region on the dendritic shaft (Frotscher et al., 2000). Once again, this reported formation of new spines is not associated with an increase in filopodia extension, indicating that spines can form anew or extend from existing shaft synapses. The need for ongoing activity in the maintenance of dendritic spines has also been demonstrated in cultured slices, where chronic blockade of AMPA receptors led to disappearance of spines, but this was apparently compensated for by the appearance of shaft synapses Bcl2 inhibitor and by an increase in efficacy of synaptic

transmission (Mateos et al., 2007), similar to our observations in dissociated cultures of cortical neurons (Fishbein & Segal, 2007). There is no consistent relationship between spine formation and afferent activity. In some cases (e.g. cerebellum) the lack of afferent innervation does not deter formation of spines, which seem to develop naturally in a preprogrammed fashion (Cesa & Strata, 2005). On the Phospholipase D1 other hand, we have shown that striatal neurons, about the spiniest cells in the brain, do not form dendritic spines if grown in culture in the absence of excitatory cortical afferents. Only the addition of such afferents enables the formation of dendritic spines in striatal neurons (Segal et al., 2003). Furthermore, blockade of electrical activity in these co-cultured striatal andd cortical neurons chronically exposed to TTX also prevents formation of spines, indicating that ongoing network activity is necessary for the formation

and maintenance of dendritic spines in at least these striatal neurons (Segal et al., 2003). An interesting deviation from this tentative rule is the finding that long-term sensory deprivation prevents rather than enhances spine pruning (Zuo et al., 2005). The interpretation of this disparity is complicated by the fact that sensory deprivation produced four synapses away from the monitored neuron in the barrel cortex is not equivalent to a local continuous blockade of activity with TTX, especially as the extrinsic sensory afferents constitute only a fraction of the excitatory innervation of the cortical neuron.