Olfactory learning-induced increase in spine density along the apical dendrites of CA1 hippocampal neurons

We have previously shown that rule learning of an olfactory discrimination task is accompanied by increased spine density along the apical dendrites of piriform cortex pyramidal neurons. The purpose of the present study was to examine whether such olfactory learning task, in which the hippocampus is actively involved, induces morphological modifications in CA1 pyramidal neurons as well. Rats were trained to discriminate positive cues in pairs of odors for a water reward. Morphological modifications were studied in Golgi-impregnated neurons with light microscopy, 1 and 3 days after training completion. Spine densities were measured on the proximal region of apical dendrites and on basal dendrites after rule learning. Three days after training completion, the mean spine density on apical dendrites in neurons from trained rats was significantly higher by 20.5% than in neurons from pseudo-trained and naive animals, which did not differ from each other. By contrast, there was no significant difference in spine density of basal dendrites among the three groups. As length and diameter of spiny dendritic segments did not change after learning, the learning-related increase in spine density in neurons from trained rats may reflect a net increase in the number of excitatory synapses in the hippocampus following olfactory rule learning.