Why is a guide to the dentate gyrus necessary

As more knowledge is acquired about the nervous system, one sometimes is lost in a sea of literature. The current generation surveys the literature primarily from PubMed, which provide a vast resource for data, but often there is little context for it, and even less of a guide to seminal but pre-1990 papers. Presumably this context, and a historical overview, can be gained by courses, lectures, and from reviews, but this is often difficult. One example of this problem is our understanding of the dentate gyrus. This part of the brain has always been a topic of intense interest, and pioneering studies from the distant and recent past attest to it. But where in a textbook or review can one find a comprehensive overview some basic information about the dentate gyrus? The primary goal of the book is to address this problem by bringing together in an organized manner a series of chapters that address, in an approachable format, the fundamental concepts about dentate gyrus structure, function, and their implications. Some have more detail than others, some provide distinct perspectives on a similar topic, but to some extent this is welcome because each reader may desire different degrees of detail, and some may want to compare viewpoints of different experts in the field.

Often a region of the brain is best understood by first outlining its components, so the first section of the book deals with the fundamental structure of the dentate gyrus. In the first chapter, David Amaral, Helen Scharfman, and Pierre Lavenex describe the neuronal organization and intrinsic circuitry. Laszlo Seress discusses this further by comparing the dentate gyrus of different species, emphasizing those that are primarily used in research (mouse, rat, monkey). Menno Witter outlines the topography of arguably the most important afferent system, the input from entorhinal cortex (the perforant path). Csaba Leranth and Tibor Hajszan discuss other extrinsic afferents, such as those from septum, mammillary bodies, and other areas that are poorly understood compared to the perforant path, particularly from the physiological perspective. Morten Blaabjerg and Jens Zimmer discuss in detail one of the most impressive and complex aspects of the dentate gyrus, the mossy fibers axons of the dentate gyrus granule cells. The remarkable structural complexity of the mossy fiber system, including the unique specializations that characterize their terminal arbors, is like no other in the hippocampus. Following this overview is a complementary discussion of the expression, plasticity, and physiology of the mossy fiber system, by David Jaffe and Rafael Gutierrez. This section then ends with two overviews about the development of the dentate gyrus, the first by Michael Frotscher, Shanting Zhao, and Eckart Forster, and the second by Guangnan Li and Sam Pleasure. The emphasis of both is on the unique set of signals that orchestrate the development of the normal laminar organization and cytoarchitecture of the adult dentate gyrus.

The second section of the book addresses the major neuronal cell types in the dentate gyrus, mostly in the rodent. Charles Ribak and Lee Shapiro begin this section with an overview of the structural characteristics of the neuronal cell types, emphasizing their unique ultrastructural "signatures." Omid Rahimi and Brenda Claiborne discuss the granule cell from a developmental and morphological perspective. Anne Williamson and Peter Patrylo review characteristics of granule cells from an electrophysiological viewpoint, both in rodent and in human tissue studies. The non-granule cells are then addressed in two chapters that cover the two primary subtypes of non-granule cells: the hilar mossy cells and the GABAergic interneurons. Darrell

Henze and Gyuri Buzsaki provide an overview about the mossy cells. Carolyn Houser discusses the GABAergic neurons, which are heterogeneous in anatomy, expression, and functional organization.

Section three attempts to cover the numerous transmitters and neuromodulators that influence the dentate gyrus. Glutamatergic inputs are discussed in other parts of the book (the perforant path in Section 1, extrinsic inputs in Section 1, the CA3 input in Section 5), so this leaves the first chapter to consider GABA (Doug Coulter and Greg Carlson), followed by a series of reviews about non-classical neurotransmitters. Carrie Drake, Charlie Chavkin, and Teri Milner review the expression and organization of the opioid system, a complex topic because of the numerous types of opioid peptides, receptors, and actions. Melanie Tallent addresses the neuropeptide somatostatin, which is normally co-localized with GABA in a subset of interneurons. Giinther Sperk, Trevor Hamilton, and William Colmers review neuropeptide Y and its receptors, functions, and plasticity. Carolyn Harley provides an overview of norepinephrine, a neuromodulator that has been known to have a robust influence in the dentate gyrus for decades. Jason Frazier addresses a topic that has been appreciated relatively recently, endocannabinoids in the dentate gyrus. Mark Pickering and John O'Connor discuss the pro-inflammatory cytokines, neuromodulators that have not been widely acknowledged until recent years. Marian Joels reviews the glucocorticoids, and Devin Binder discusses the neurotrophins, a family of "growth factors'' that are expressed in high concentration in the dentate gyrus, and modulate function in a profound manner. The reproductive steroids, estrogen, progesterone, and androgen, are addressed by Tibor Hajszan, Teri Milner, and Csaba Leranth.

One of the more remarkable characteristics of the dentate gyrus is its plasticity, and this is the focus of the 4th part of the volume. Brian Derrick begins with a perspective that ties together studies of in vitro and in vivo synaptic plasticity, circuitry, and the behaving animal. Clive Bramham reviews long-term potentiation (LTP) and its candidate mechanisms. Beatrice Poschel and Patric Stanton address long-term depression (LTD). Other examples of plasticity follow, such as lesion-induced plasticity. One of the most widely studied examples involves the response to a lesion of the entorhinal cortex, and this large literature is reviewed by Thomas Deller, Domenico Del Turco, Angelika Rappert, and Ingo Bechmann. They also provide a modern context by updating the field with what is currently understood from studies of mice. On a separate subject, Jack Parent provides a review of one of the most exciting developments in the history of dentate gyrus research, and could not illustrate plasticity any better: that neurogenesis occurs in the dentate gyrus throughout the lifespan of mammals. Finally, another remarkable type of plasticity, is discussed by Tom Sutula and Ed Dudek: mossy fiber sprouting. This reorganization of the mossy fiber pathway has captured the attention of many, because it involves dramatic anatomical and physiological changes, and can occur in response to many types of insults or injury.

In the 5th part of the volume, network considerations are addressed. This is a rich and diverse area of research that is reflected by many perspectives, each suggesting distinct roles of the dentate gyrus as a structure. Ray Kesner begins, followed by Laszlo Acsady and Szabolcs Kali. David Hsu discusses the concept that the dentate gyrus is a "gate" or "filter." John Lisman turns to the question of the perforant path input, and specifically how the medial vs. lateral perforant path provide distinct roles. Helen Scharfman discusses the evidence that CA3 is a major input to the dentate gyrus, by virtue of axon collaterals that innervate diverse dentate neurons. This section ends with a comprehensive model of the dentate gyrus network, presented by Robert Morgan, Vijy Santhakumar, and Ivan Soltesz.

In the last section of the book, an effort is made to review some of the literature that addresses how abnormalities within the dentate gyrus contribute to disease, or aging. Monica Chawla and Carol Barnes discuss how granule cell function changes with age, drawing upon the wealth of information from electrophysiology, imaging, and other approaches. Peter Patrylo and Anne Williamson also provide a review of this large field, but with a different perspective. The potential role of dentate gyrus neurogenesis in depression is reviewed by Amar Sahay, Michael Drew, and Rene Hen, whose lab was one of the first to provide evidence that deficits in neurogenesis in the dentate gyrus might be a reason for depression. Thomas

Ohm discusses the diverse anatomical changes in the dentate gyrus that occur in Alzheimer's disease. Leyla DeToledo-Morrell, Travis Stoub, and Changsheng Wang review their studies of Alzheimer's disease, providing a persuasive argument that a major contributing factor is pathology that develops in the entorhinal area. Finally, Ed Dudek and Tom Sutula review a large and active area of research: epileptogenesis in the dentate gyrus.

Although a book such as this can never cover all the topics and all information, and certainly cannot do justice to the multitude of contributors both from the past and present, its purpose is primarily as a guide, and readers will be led to references that can address topics that are not covered. Particularly for those who are not well versed in "hippocampology," it is hoped that this volume can help - and in doing so, provide new impetus to resolve the many outstanding questions that remain about the dentate gyrus.

Helen E. Scharfman Columbia University and Helen Hayes Hospital

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