Hebbian modification of a hippocampal population pattern in the rat

1. The study of the physiological role of long-term potentiation (LTP) is o ften hampered by the challenge of finding a physiological event that can be used to assess synaptic strength. We explored the possibility of utilising a naturally occurring event, the hippocampal sharp wave (SPW), for the ass essment of synaptic strength and the induction of LTP in vivo. 2. We used two methods in which hippocampal cells were either recorded intr acellularly or extracellularly in vivo. In both cases, a linear association between the magnitude of the SPW and cellular responsiveness was observed. 3. LTP was induced by depolarising cells during SPWs by either direct intra cellular current injection or extracellular microstimulation adjacent to th e cell body. Both of these approaches led to an increase in the slope of th e linear association between SPWs and cellular responsiveness. 4. This change was achieved without a rise in overall cell excitability, im plying that the synapses providing input to CA1 cells during sharp waves ha d undergone potentiation. 5. Our findings show that the Hebbian pairing of cellular activation with s pontaneous, naturally occurring synaptic events is capable of inducing LTP.