OVARIAN STEROIDAL CONTROL OF CONNECTIVITY IN THE FEMALE HIPPOCAMPUS -AN OVERVIEW OF RECENT EXPERIMENTAL FINDINGS AND SPECULATIONS ON ITS FUNCTIONAL CONSEQUENCES

Experimental evidence accumulated over the past 5 years clearly indica tes that ovarian steroids regulate the number of synapses in the rat h ippocampal CA1 region. When estradiol levels are high such as during p roestrus and ovulation, the number of synapses is high; when estradiol levels are low such as during estrus, the number of synapses is low. Here we address three questions that are frequently raised by these ph asic fluctuations in synapse number in a brain region to which cogniti ve functions are classically attributed. First, what neuronal signals might produce the changes in synapse number? Second, how are the hippo campal functions of memory encoding and cognitive mapping affected by fluctuating levels of ovarian steroids? Third, for mammals in general, what might be the ecological/cognitive significance of such changes? In this last section, we integrate some of the relevant human and rode nt cognitive/behavioral literature and propose a hypothesis. Namely, b y altering its quantitative connectivity, the female hippocampus is op timized for different cognitive/behavioral functions when the female i s sexually receptive and ovarian steroid levels are high rather than w hen she is not receptive and steroid levels are low. The hippocampus t hus shifts its optimal computational functions across the estrous/mens trual cycle. (C) 1997 Wiley-Liss, Inc.