Physiological function of granule cells: a hypothesis.

In this chapter we review the physiological properties of granule cells in vivo and in vitro. We conclude from the literature that in intact rats granule cells fire rhythmic bursts of action potentials concurrent with exploration-associated theta waves. The population discharge of granule cells coincides with the maximum probability of firing of CA3 pyramidal cells on the positive phase of focally recorded theta waves. During consummatory behaviors, immobility and anesthesia the firing rate of granule cells substantially decreases. We propose that the conjoint activity of the discharging CA3 cells and tetanization of these same cells by mossy fibers during exploratory (theta) behavior will temporarily increase synaptic efficacy among the active CA3 neurons even after the termination of exploration. As a result, the very same CA3 cells that carried information during exploration now become the burst-initiator cells of the sharp-wave associated population bursts during consummatory behaviors and sleep. The creation of new burst-initiator neurons is hypothesized to be essential for memory trace formation. From this perspective the main physiological function of granule cells is to 'tetanize' CA3 pyramidal neurons during exploratory behaviors and induce a meaningful reorganization of the functional connectivity of the CA3 network.