Some Capacities for Structural Growth and Functional Change in the Neuronal Circuitries of the Adult Hippocampus

It has become increasingly clear that the various circuits of the hippocampal formation are capable of dramatic structural reorganization and extremely long-lasting changes in their operating characteristics. Axons and dendrites in this structure will on occasion generate large numbers of new connections while brief periods of repetitive electrical stimulation cause semi-permanent increases in synaptic strength. Structural and physiological plasticites of these types are of interest because they offer possible explanations for the extreme flexibility which is the hallmark of behavior in the “higher” vertebrates. However, attempts to test for such relationships between neuronal and behavioral change are impeded by an almost complete lack of information about the cellular machinery responsible for the former as well as the physiological events which accompany the latter. The present review is concerned with these issues and specifically with the cellular substrates of neuronal plasticity. We shall first describe the “sprouting” response of intact fibers in the partially deafferented hippocampus and use these findings to discuss various hypotheses about the factors which initiate and regulate axonal growth in the adult central nervous system. Following this, we will turn to the remarkable long-term potentiation of synaptic response found in several hippocampal pathways following repetitive stimulation. The emphasis of this section will be on the types of biochemical and structural mechanisms which might be responsible for rapidly appearing, stable changes in synaptic physiology.