The Nmda Receptor Supramolecular Complex

Probably the most-studied molecule in the short history of molecular studies of learning and memory is the NMDA subtype of glutamate receptor. As described in the Chapter 4, the breakthrough discovery in this area was made by Graham Collingridge and his colleagues, who found a necessity for the NMDA receptor for LTP induction at Schaffer-collateral synapses in hippocampal area CA1. Many studies subsequent to this work have brought into sharp relief the importance of the NMDA receptor in synaptic plasticity and memory formation. Recent work by Seth Grant and his colleagues has shown that the "NMDA receptor" is in fact a large multiprotein complex (90, 91). This complex includes a striking representation of many different types of scaffolding proteins and signal transduction molecules (see table). In fact, the NMDA receptor supramolecular complex includes a number of proteins whose function has been directly implicated in human learning, including NF1, PKA, raf-1, MEK, ERK, and RSK-2. These gene products have all directly or indirectly been implicated in human learning because they are associated with various types of human mental retardation syndromes (see Chapter 10). As

Grant and his colleagues mentioned in their report (90), the presence of gene products linked to human mental retardation within the NMDA receptor complex was intriguing and consistent with a role for this complex in human cognition.

The NMDA receptor supramolecular complex has been referred to as the Hebbosome. I am not particularly fond of this nomenclature because Hebbian plasticity involves much more than just the NMDA receptor in my opinion. One of the major themes of this book is that we need to begin to think of Hebbian plasticity as not sufficiently described by just considering the function of the NMDA receptor and its associated proteins in isolation. In particular, regulation of membrane excitability properties are being emphasized here, so I don't want to do myself a disservice by adapting the Hebbosome nomenclature.

Of course, the Hebbosome terminology also makes a variation of this point as well. The Hebbosome nomenclature emphasizes the enormously complex molecular machine that is involved in NMDA receptor-dependent processes. I do not wish to take away from the importance of this concept. It just seems to me that what we might call the

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