AN INCREASE IN DENDRITIC SPINE DENSITY ON HIPPOCAMPAL CA1 PYRAMIDAL CELLS FOLLOWING SPATIAL-LEARNING IN ADULT-RATS SUGGESTS THE FORMATION OF NEW SYNAPSES

The search for cellular correlates of learning is a major challenge in neurobiology. The hippocampal formation is important for learning spa tial relations. A possible long-lasting consequence of such spatial le arning is alteration of the size, shape, or number of excitatory synap ses. The dendritic spine density is a good index for the number of hip pocampal excitatory synapses. By using laser-scanning confocal microsc opy, we observed a significantly increased spine density in CA1 basal dendrites of spatially trained rats when compared to nontrained contro ls. With unchanged dendritic length, the higher spine density reflects an increased number of excitatory synapses per neuron associated with spatial learning.