One drawback to this technique is that it cannot be effectively combined with other staining techniques. Because the goal of the present study was only to address spine density and morphology and not to identify other cell types, we selected this method over other available techniques for spine analysis. The analysis of Golgi stained neurons showed that radiation exposure led to a gradual decrease in spine density in the DG over time. In contrast, spine density in the CA1 basal dendrites decreased at 1 week post irradiation with a trend toward recovery at 1 month. The observed reductions in spine density might indicate early signs of neuronal injury in the hippocampus following irradiation and also suggest that there is a time dependent vulnerability of the two hippocampal sub regions following radiation exposure. A number of factors might account for the observed differences in spine density between the two hippocampal subregions. Numerous studies have demonstrated that spine density is regulated by glutamatergic transmission and glutamate receptor subtypes located on dendritic spine heads. In addition, a series of in vitro studies have shown that N-methyl-Daspartic acid receptors mediate the destabilization of filamentous actin associated with dendritic spine loss. Although the effects of radiation on NMDA receptor dynamics on hippocampal sub-regions are not well understood, studies by Shi et al have shown differential changes in subunits of NMDA receptors in the hippocampal subfields following whole brain irradiation. Thus, it is tempting to speculate that the observed temporal differences in reduction in spine density between DG and CA1. Brain derived neurotrophic factor is another well characterized determinant of dendritic spine number and morphology. Regulation of BDNF and its receptor expression has been reported to be very sensitive to radiation in the hippocampus and such changes vary depending on time after irradiation. Therefore, it is also possible that radiation might differentially alter BDNF and its downstream signaling targets in the dendrites of dentate granule cells and CA1 basal dendrites which may account for the differential changes in spine density at these two regions as a function of time after irradiation. In our earlier studies using the same dose of radiation in the same strain of mice, we found LY294002 increased numbers of activated microglia in the DG 1 week, which became significant at 2 months post irradiation. Therefore, the gradual decrease in spine density over time observed in the DG could be associated with an increase in microglial activation. Other investigators have recently shown that changes in dendritic spines are associated with alterations in microglia, an effect that may be associated with the release of soluble factors. Further studies are in progress to address the molecular mechanisms involved in the observed temporal differences in radiation induced alterations in spine density. In contrast to DG and CA1 basal dendrites, irradiation did not alter spine density in CA1 apical dendrites. Differential vulnerability between basal and apical dendrites due to exogenous or endogenous factors has been reported in the literature although the mechanisms involved are not clear. For instance, Santos et al reported that neonatal rats exposed repetitively to low doses of paroxon lost dendritic spine selectively in basal dendrites with no changes in apical dendrites of CA1 pyramidal neurons. Moreover normal aging also results in decreases of the spine density on basal but not apical dendrites in C57BL/6 mice.