The hippocampus as a decision-maker

Synapses are modified through learning. Up until now, scientists believed that a particular form of synaptic plasticity in the brain's hippocampus was responsible for learning spatial relations. This was based on a receptor type for the neurotransmitter glutamate: the NMDA receptor.

Researchers at the Max Planck Institute for Medical Research in Heidelberg and Oxford University have now observed that mice develop a spatial memory, even when the NMDA receptor-transmitted plasticity is switched off in parts of their hippocampus.

However, if these mice have to resolve a conflict while getting their bearings, they are not successful in resolving it; the hippocampal NMDA receptors are clearly needed to detect or resolve the conflict. This has led the researchers involved in this experiment to refute a central tenet of neuroscience regarding the function of hippocampal NMDA

The hippocampus is part of the forebrain and processes a large amount of information from various parts of the brain. Incoming signals are transmitted by granule cells in the dentate gyrus to pyramid cells in the CA3 region and from these to pyramid cells in the CA1 region.

NMDA receptors can optimise or weaken the transmission efficiency of the neurotransmitter glutamate at the synapses involved at the signal flow. It has long been speculated that this form of synaptic plasticity is needed when learning about spatial associations. Rolf Sprengel and Peter H. Seeburg from the Max Planck Institute for Medical Research worked with colleagues from Oxford and Oslo to refute this theory.

The scientists studied genetically modified mice lacking NMDA receptors in dentate gyrus granule cells and pyramid cells in the CA1 region. They were thus able to observe for the first time ever what happens when NMDA receptor-dependent plasticity is switched off almost exclusively at these synapses in the hippocampus.