Study of CA1 Pyramidal Cell Dendritic Spines of the Rat Hippocampus by Serial Electron Microscopy Following a Single Ethanol Exposure on Postnatal Day 6

Abstract

Exposure of the developing fetus to ethanol has numerous and wide-ranging effects including hippocampal damage resulting in difficulties with spatial learning and memory. Children and animal models tested on these types of tasks are able to perform them but not to the same standard as controls. This suggests that although the brains of these subjects still contain the same basic circuitry, it appears to have been altered in some way that decreases performance. This has been previously investigated with cell count studies that found ethanol exposure during development significantly decreases the number of CA1 but not CA3 cells in the hippocampus. The axons from CA3 pyramidal neurons are an important input to the CA1 cells and it is likely that the synaptic complement of the CA1 apical dendritic tree will be altered to accommodate this imbalance. The current study investigated whether there were any changes to the spines, synapses and mitochondria of the CA1 dendritic tree in the upper stratum radiatum following ethanol exposure. Rat pups were exposed to a single-binge ethanol dose (6g/kg body weight) on PN6 to investigate brain tissue at PN26-30. The animals underwent intra-cardiac perfusion with a paraformaldehyde/glutaraldehyde fixative solution to obtain hippocampal tissue for transmission electron microscopy. An initial series of fixation trials were performed to optimise the quality of tissue fixation then sets of serial sections were used with 3-dimensional reconstruction technology (Synapse Web) to analyse components of the CA1 dendritic tree. The total number of spines/μm of dendrite was not found to be significantly different between ethanol-exposed and control animals, (p=0.8894) and both groups had significantly more normal spines (F(1, 27)=0.2369, P=0.6304) than other types. The total number of synapses was not significantly different between treatment groups (p=0.5677) and each group had significantly more non-perforated than perforated synapses (F(1,20)=26.15, P<0.0001). There was also no significant difference in the volume fraction of mitochondria within the length of dendrite analysed, between groups (P=0.9593). However, the length of total apical dendritic tree of the CA1 pyramidal cells was significantly longer (p<0.001) in the alcohol treated group (294.1±4.9µm) compared to the control group (267.4±4.3µm). This study has shown that the density of dendritic spines and synapses on the spiny branchlets of CA1 pyramidal cells in the upper stratum radiatum was preserved following a single ethanol exposure on postnatal day six, but that the overall length of the dendrite tree was increased. This is thought to be an attempt to compensate for CA1 pyramidal cell loss with reduced plasticity of the existing dendrite and suggests that instead the total number of spines and synapses per CA1 cell may have been altered.