Plasticity and neural coding in the hippocampus

Hippocampal pyramidal cells are characterized by location-specific firing. A given pyramidal cell (“place cell”) will fire strongly when the head of the animal is in a particular part of the environment (“place field”) and will remain silent when the animal is outside the field. This firing pattern is preserved while the animal explores a familiar environment and is changed by exposure to a novel environment. It is believed that the formation and stabilization of this “place” correlate of neuronal activity is accomplished in the hippocampus by a long-term potentiation (LTP)-like mechanism. However, it is not known what effect a long-term increase in the strength of a limited number of synaptic inputs may have on the “place”-specific firing of a hippocampal pyramidal cell as well as on the hippocampal network. The most frequently used paradigm to affect neuronal connections is tetanic stimulation. Here I tested whether place cell activity can be influenced by high frequency stimulation applied to commissural or perforant path fibers. Place field maps were computed from CA3 and CA1pyramidal cells of rats exploring a familiar environment before and at various time intervals after the induction of LTP. An array of 8 independently moveable tetrodes was used to monitor the spontaneous unit and spontaneous and evoked field activity. About half of the recorded place cells significantly changed their place-related firing, and half of the latter changed the preferred location of maximal firing. The changes were direction-sensitive, stable throughout recording and independently affected by the tetanization of the two pathways. I observed no significant effect on the hippocampal network activity after LTP. In contrast, stimulation of the intrahippocampal fibers leading to seizure-like afterdischarges was associated with changes in the network activity with less stable changes in place fields. These observations suggest that LTP is able to induce changes in place cell activity that are similar to those induced by exposure to a novel environment. A limited amount of change in synaptic weight appears to be sufficient in order to induce stable, non-pathological changes in the representation of the environment.