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Chronic restraint stress is known to affect the morphology and synaptic organization of the hippocampus, predominantly within CA3 but also in CA1 and dentate gyrus. In this study, we provide the first evidence for specific ultrastructural alterations affecting asymmetric axo-spinous synapses in CA1 stratum lacunosum-moleculare following chronic restraint stress (6 h/day, 21 days) in the rat. The structure of asymmetric axo-spinous post-synaptic densities was investigated using serial section three-dimensional reconstruction procedures in control (n=4) and chronic restraint stress (n=3) animals. Dendritic spine profiles (spine head+neck) associated with the sampled synaptic contacts (30 per animal) were also reconstructed in three-dimensions. Morphometric analyses revealed a significant increase in post-synaptic density surface area (+36%; P=0.03) and a highly significant increase in post-synaptic density volume (+79%; P=0.003) in the chronic restraint stress group. These changes were directly associated with 'non-macular' (perforated, complex and segmented) post-synaptic densities. A highly significant overall increase in the 'post-synaptic density surface area/spine surface area' ratio was also detected in the chronic restraint stress group (+27%; P=0.002). In contrast, no quantitative changes in spine parameters were found between groups. The Cavalieri method was used to assess the effects of chronic restraint stress exposure upon CA1 hippocampal volume. The mean volume of total dorsal anterior CA1 hippocampus was significantly lower in the chronic restraint stress group (-16%; P=0.036). However, when corrected for volume changes, no significant alteration in a relative estimate of the mean number of asymmetric axo-spinous synapses was detected in CA1 stratum lacunosum-moleculare between control and chronic restraint stress groups. The data indicate a structural remodeling of excitatory axo-spinous synaptic connectivity in rat CA1 stratum lacunosum-moleculare as a result of chronic restraint stress.

Caloric restriction (CR) is a reduction of total caloric intake without a decrease in micronutrients or a disproportionate reduction of any one dietary component. While CR attenuates age-related cognitive deficits in tasks of hippocampal-dependent memory, the cellular mechanisms by which CR improves this cognitive decline are poorly understood. Previously, we have reported age-related decreases in key synaptic proteins in the CA3 region of the hippocampus that are stabilized by lifelong CR. In the present study, we examined possible age-related changes in the functional microcircuitry of the synapses in the stratum lacunosum-moleculare (SL-M) of the CA3 region of the hippocampus, and whether lifelong CR might prevent these age-related alterations. We used serial electron microscopy to reconstruct and classify SL-M synapses and their postsynaptic spines. We analyzed synapse number and size as well as spine surface area and volume in young (10 months) and old (29 months) ad libitum fed rats and in old rats that were calorically restricted from 4 months of age. We limited our analysis to SL-M because previous work demonstrated age-related decreases in synaptophysin confined to this specific layer and region of the hippocampus. The results revealed an age-related decrease in macular axo-spinous synapses that was not reversed by CR that occurred in the absence of changes in the size of synapses or spines. Thus, the benefits of CR for CA3 function and synaptic plasticity may involve other biological effects including the stabilization of synaptic proteins levels in the face of age-related synapse loss. © 2010 IBRO.

Chronic stress and spatial training have been proposed to affect hippocampal structure and function in opposite ways. Previous morphological studies that addressed structural changes after chronic restraint stress and spatial training were based on two-dimensional morphometry which does not allow a complete morphometric characterisation of synaptic features. Here, for the first time in such studies, we examined these issues by using three-dimensional (3-D) reconstructions of electron microscope images taken from thorny excrescences of hippocampal CA3 pyramidal cells. Ultrastructural alterations in postsynaptic densities (PSDs) of thorny excrescences receiving input from mossy fibre boutons were also determined, as were changes in numbers of multivesicular bodies (endosome-like structures) within thorny excrescences and dendrites. Quantitative 3-D data demonstrated retraction of thorny excrescences after chronic restraint stress which was reversed after water maze training, whilst water maze training alone increased thorny excrescence volume and number of thorns per thorny excrescence. PSD surface area was unaffected by restraint stress but water maze training increased both number and area of PSDs per thorny excrescence. In restrained rats that were water maze trained PSD volume and surface area increased significantly. The proportion of perforated PSDs almost doubled after water maze training and restraint stress. Numbers of endosome-like structures in thorny excrescences decreased after restraint stress and increased after water maze training. These findings demonstrate that circuits involving contacts between mossy fibre terminals and CA3 pyramidal cells at stratum lucidum level are affected conversely by water maze training and chronic stress, confirming the remarkable plasticity of CA3 dendrites. They provide a clear illustration of the structural modifications that occur after life experiences noted for their different impact on hippocampal function.