The increasingly plastic, hormone-responsive adult brain

Pity the poor neuroscientist. Drilled in details of neuroanatomy that have been gleaned from over a century's intensive research, we come to believe we have a reasonably accurate portrait of what the cells constituting the brain look like, and roughly what their connections are with one another. By 1949, Donald Hebb (1) persuasively described how the behavioral plasticity we see in adult animals, such as that exemplified in learning and memory, could be accomplished, in theory, by simply changing the strength of existing synapses, without any rewiring of the circuits. But it has become clear that, in fact, synaptic connections come and go even in the adult brain, and that our snapshot views of the brain at a single point in time may be missing the point. Indeed, reports from the annual assembly of some 25,000 congregants at the Society for Neurosciences meeting suggest that the adult brain is each year more plastic than it was the year before. In several systems, hormones play a role, fine tuning nervous system connections to facilitate, at the appropriate time, adaptive behaviors such as singing (2), copulation (3), pheromone detection (4), stress response (5), etc. In this issue of PNAS, Yankova and colleagues (6) report that the hormonally driven appearance of new synapses in one brain region, already shown to wax and wane in the course of just 4 days in the estrous cycle of female rats, is not a mere duplication of connections between two established partners. Rather, it appears that the new synapses represent a divergence of connections that may serve to synchronize neural activity, a circuit change that might prove relevant to epilepsy and/or the cognitive benefits of estrogen treatment. The new synapses represent a divergence of connections that may serve to synchronize neural activity, a circuit change that might …