Tress Suppresses and Learning Induces Lasticity in Ca3 of Rat Hippocampus: Three-dimensional Ultrastructural Study of Thorny Xcrescences and Their Postsynaptic Densities

bstract—Chronic stress and spatial training have been proosed to affect hippocampal structure and function in oppoite ways. Previous morphological studies that addressed tructural changes after chronic restraint stress and spatial raining were based on two-dimensional morphometry which oes not allow a complete morphometric characterisation of ynaptic features. Here, for the first time in such studies, we xamined these issues by using three-dimensional (3-D) reonstructions of electron microscope images taken from horny excrescences of hippocampal CA3 pyramidal cells. ltrastructural alterations in postsynaptic densities (PSDs) of horny excrescences receiving input from mossy fibre bouons were also determined, as were changes in numbers of ultivesicular bodies (endosome-like structures) within horny excrescences and dendrites. Quantitative 3-D data emonstrated retraction of thorny excrescences after hronic restraint stress which was reversed after water maze raining, whilst water maze training alone increased thorny xcrescence volume and number of thorns per thorny exrescence. PSD surface area was unaffected by restraint tress but water maze training increased both number and rea of PSDs per thorny excrescence. In restrained rats that ere water maze trained PSD volume and surface area inreased significantly. The proportion of perforated PSDs alost doubled after water maze training and restraint stress. umbers of endosome-like structures in thorny excresences decreased after restraint stress and increased after ater maze training. These findings demonstrate that circuits nvolving contacts between mossy fibre terminals and CA3 yramidal cells at stratum lucidum level are affected conersely by water maze training and chronic stress, confirmng the remarkable plasticity of CA3 dendrites. They provide clear illustration of the structural modifications that occur fter life experiences noted for their different impact on hip-