Synaptic influence of hippocampus on pyramidal cells of the rat prefrontal cortex: an in vivo intracellular recording study.

The hippocampus and prefrontal cortex are two structures implicated in learning and memory and are related through a direct excitatory pathway. The characteristics of the synaptic influence of the hippocampus on pyramidal cells of the prefrontal cortex were determined using intracellular recordings in anesthetized rats. Single-pulse stimulation of the hippocampus induced an early EPSP of fixed latency in most of the recorded pyramidal cells (n = 106/116) thereby demonstrating a monosynaptic connection between hippocampal neurons and pyramidal cells of the prefrontal cortex. Furthermore, the EPSP was followed by a prolonged IPSP and suggests a simultaneous engagement of pyramidal and non-pyramidal neurons that may ultimately constrain the spread of excitation in response to hippocampal input. Paired-pulse stimulation induced short-term modifications in the synaptic responses and this short-term plasticity may contribute to the temporal filtering of information. Finally, tetanic stimulation of the hippocampus produced long-term potentiation of the monosynaptic EPSP with a concomitant potentiation of the IPSP, indicating that the hippocampo-prefrontal network can participate in the formation and consolidation of memories. In conclusion, the characteristics of the synaptic transmission in the hippocampo-prefrontal cortex pathway further supports the existence of a cooperative relationship between two structures known to be involved in higher cognitive processes.