Light and electron microscopic studies have shown that ovarian steroids regulate the density and number of excitatory synaptic inputs to hippocampal pyramidal cells in the adult female rat; elevated levels of estradiol are associated with a higher density of dendritic spine synapses on CA1 pyramidal cells. Electrophysiological analyses indicate that these hormone-induced synapses increase hippocampal excitability as well as the potential for synaptic plasticity. Importantly, correlation of dendritic spine density and sensitivity to synaptic input of individual CA1 pyramidal cells from estradiol-treated and control animals suggests that synapses induced by estradiol may be a specialized subpopulation that contains primarily the NMDA subtype of glutamate receptor. The apparent NMDA receptor specificity of these synapses may be key to understanding their functional significance. Currently, the behavioral consequences of additional spine synapses are unknown. Numerous studies have aimed at correlating hormone-induced changes in hippocampal connectivity with differences in hippocampus-dependent spatial learning ability in mazes, but the results of these efforts have been equivocal. Anatomical, electrophysiological, and behavioral studies of estradiol-mediated hippocampal plasticity are reviewed. In conclusion, it is suggested that standard behavioral tests of hippocampal function are not sufficient to reveal the behavioral consequences of hormone-induced hippocampal plasticity. Rather, understanding the behavioral consequences of estradiol and progesterone effects on hippocampal connectivity may require analysis of the hippocampus' cognitive and spatial information processing functions in relation to alternative biologically relevant behaviors. A (nonexclusive) proposal that hormone-induced hippocampal plasticity may facilitate appropriate prepartum/maternal behavior is discussed.
Copyright 1998 Academic Press.