REDUCTION IN SPINE DENSITY ASSOCIATED WITH LONG-TERM POTENTIATION IN THE DENTATE GYRUS SUGGESTS A SPINE FUSION-AND-BRANCHING MODEL OF POTENTIATION

Approximately 2,700 dendritic spines in Golgi-impregnated hippocampal granule cells were quantified via image analysis 24 h after the unilat eral induction of long-term potentiation in seven rats. Stereological corrections were made using a tilting disector and analytical unfoldin g technique. In the potentiated hemisphere the mean spine density alon g dendrites was reduced by similar to 20%. The relative frequency of s horter, thicker spines was increased in potentiated tissue. Physiologi cal consequences of two morphological changes leading to a reduction i n spine density (retraction or fusion of spines) were examined using a compartmental model of a simplified granule cell. The model was const ructed in the NEURON modeling environment and included a realistic pop ulation of 60 dendritic spines (with dual-component synapses and activ e Ca2+-dependent mechanisms). Simulations demonstrated that potentiati on of postsynaptic responses was compatible with fusion (with branchin g) of a proportion of spines with their neighbors but was not compatib le with retraction of spines. This result held over wide variations of model parameters as long as dendritic membranes were assumed to be ex citable. (C) 1997 Wiley-Liss, Inc.