Estimation of the total number of hippocampal CA1 pyramidal neurons: new methodology applied to helpless rats

We have recently reported that in the learned helplessness model of depression, the less hippocampal spine synapses rats have, the more helpless they become. It remains unclear, however, whether the observed synaptic changes are associated with the loss of CA1 pyramidal cells. Cell bodies in the CA1 pyramidal layer are very densely packed, making cell counting difficult in this hippocampal subregion. To address this issue, we developed a new approach that (1) yields excellent preservation of the three-dimensional tissue structure; (2) utilizes osmium tetroxide to unambiguously label nucleoli; and (3) facilitates and accelerates unbiased, reliable counting of densely packed cell bodies. Our method provides an improved tool for studies aiming to evaluate hippocampal atrophy and cell loss, the most characteristic features in many neurodegenerative diseases, such as Alzheimer's disease, temporal lobe epilepsy and ischemia, as well as in several psychiatric disorders. Using this new method, we demonstrated no significant changes in the number of CA1 pyramidal cells in the rat learned helplessness paradigm. In addition, volumes of the CA1 pyramidal cell layer and the entire CA1 subfield remained unchanged among treatment groups. We conclude that previously observed synaptic alterations in helpless rats are not associated with CA1 pyramidal cell loss. This finding suggests that behavioral outcome in the learned helplessness paradigm is related to plastic events at the synaptic level, rather than at the level of principal cells.