Dendritic spines lost during glutamate receptor activation reemerge at original sites of synaptic contact

During cerebral ischemia, neurons undergo rapid alterations in dendritic st ructure consisting of focal swelling and spine loss. We used time-lapse mic roscopy to determine the fate of dendritic spines that disappeared after br ief, sublethal hypoxic or excitotoxic exposures. Dendrite and spine morphol ogy were assessed in cultured cortical neurons expressing yellow fluorescen t protein or labeled with the fluorescent membrane tracer, DiI, Neurons exp osed to NMDA, kainate, or oxygen-glucose deprivation underwent segmental de ndritic beading and loss of approximately one-half of dendritic spines. Mos t spine loss was observed in regions of local dendritic swelling. Despite w idespread loss, spines recovered within 2 hr after termination of agonist e xposure or oxygen-glucose deprivation and remained stable over the subseque nt 24 hr, Recovery was slower after NMDA than AMPA/kainate receptor activat ion. Time-lapse fluorescence imaging showed that the vast majority of spine s reemerged in the same location from which they disappeared. In addition t o spine recovery, elaboration of dendritic filopodia was observed in new lo cations along the dendritic shaft after dendrite recovery. Spine recovery d id not depend on actin polymerization because it was not blocked by applica tion of latrunculin-A, which eliminated filamentous actin staining in spine s and blocked spine motility. Throughout spine loss and recovery, presynapt ic and postsynaptic elements remained in physical proximity. These results suggest that elimination of dendritic spines is not necessarily associated with loss of synaptic contacts. Rapid reestablishment of dendritic spine sy napses in surviving neurons may be a substrate for functional recovery afte r transient cerebral ischemia.