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2.  Endocannabinoids in the Dentate Gyrus 
Progress in brain research  2007;163:319-337.
Recent years have produced rapid and enormous growth in our understanding of endocannabinoid-mediated signalling in the CNS. While much of the recent progress has focused on other areas of the brain, a significant body of evidence has developed that indicates the presence of a robust system for endocannabinoid-mediated signalling in the dentate gyrus. This chapter will provide an overview of our current understanding of that system based on available anatomical and physiological data.
PMCID: PMC2846697  PMID: 17765727
3.  GABAA receptors 
Progress in brain research  2007;160:21-41.
γ-Aminobutyric acid type A (GABAA) receptors, the major inhibitory neurotransmitter receptors responsible for fast inhibition in the basal ganglia, belong to the superfamily of “cys-cys loop” ligand-gated ion channels. GABAA receptors form as pentameric assemblies of subunits, with a central Cl− permeable pore. On binding of two GABA molecules to the extracellular receptor domain, a conformational change is induced in the oligomer and Cl−, in most adult neurons, moves into the cell leading to an inhibitory hyperpolarization. Nineteen mammalian subunit genes have been identified, each showing distinct regional and cell-type-specific expression. The combinatorial assembly of the subunits generates considerable functional diversity. Here we place the focus on GABAA receptor expression in the basal ganglia: striatum, globus pallidus, substantia nigra and subthalamic nucleus, where, in addition to the standard α1β2/3γ2 receptor subtype, significant levels of other subunits (α2, α3, α4, γ1, γ3 and δ) are expressed in some nuclei.
PMCID: PMC2648504  PMID: 17499107
GABA; GABAA receptor; basal ganglia; striatum; globus pallidus; substantia nigra; benzodiazepines
4.  Remyelination of the injured spinal cord 
Progress in brain research  2007;161:419-433.
Contusive spinal cord injury (SCI) can result in necrosis of the spinal cord, but often long white matter tracts outside of the central necrotic core are demyelinated. One experimental strategy to improve functional outcome following SCI is to transplant myelin-forming cells to remyelinate these axons and improve conduction. This review focuses on transplantation studies using olfactory ensheathing cell (OEC) to improve functional outcome in experimental models of SCI and demyelination. The biology of the OEC, and recent experimental research and clinical studies using OECs as a potential cell therapy candidate are discussed.
PMCID: PMC2605400  PMID: 17618995
spinal cord injury; remyelination; olfactory ensheathing cells
5.  Neural mechanisms underlying somatic tinnitus 
Progress in brain research  2007;166:107-123.
Somatic tinnitus is clinically observed modulation of the pitch and loudness of tinnitus by somatic stimulation. This phenomenon and the association of tinnitus with somatic neural disorders indicate that neural connections between the somatosensory and auditory systems may play a role in tinnitus. Anatomical and physiological evidence supports these observations. The trigeminal and dorsal root ganglia relay afferent somatosensory information from the periphery to secondary sensory neurons in the brainstem, specifically, the spinal trigeminal nucleus and dorsal column nuclei, respectively. Each of these structures has been shown to send excitatory projections to the cochlear nucleus. Mossy fibers from the spinal trigeminal and dorsal column nuclei terminate in the granule cell domain while en passant boutons from the ganglia terminate in the granule cell domain and core region of the cochlear nucleus. Sources of these somatosensory–auditory projections are associated with proprioceptive and cutaneous, but not nociceptive, sensation. Single unit and evoked potential recordings in the dorsal cochlear nucleus indicate that these pathways are physiologically active. Stimulation of the dorsal column and the cervical dorsal root ganglia elicits short- and long-latency inhibition separated by a transient excitatory peak in DCN single units. Similarly, activation of the trigeminal ganglion elicits excitation in some DCN units and inhibition in others. Bimodal integration in the DCN is demonstrated by comparing responses to somatosensory and auditory stimulation alone with responses to paired somatosensory and auditory stimulation. The modulation of firing rate and synchrony in DCN neurons by somatatosensory input is physiological correlate of somatic tinnitus.
PMCID: PMC2566901  PMID: 17956776
somatic tinnitus; somatosensory; trigeminal; nonauditory pathways; cochlear nucleus
Progress in brain research  2007;165:383-394.
From a computational perspective, the act of using a tool and making a movement involves solving three kinds of problems: we need to learn the costs that are associated with our actions as well as the rewards that we may experience at various sensory states. We need to learn how our motor commands produce changes in things that we can sense. Finally, we must learn how to actually produce the motor commands that are needed so that we minimize the costs and maximize the rewards. The various computational problems appear to require different kinds of error signals that guide their learning, and might rely on different kinds of contextual cues that allow their recall. Indeed, there may be different neural structures that compute these functions. Here we use this computational framework to review the motor control capabilities of two important patients: HM, who suffered from severe amnesia, and BG, who suffered from apraxia. When viewed from a computational perspective, the capabilities and deficits of these patients provide insights into the neural basis of our ability to willfully move our limbs and interact with the objects around us.
PMCID: PMC2553852  PMID: 17925259
7.  The dentate gyrus: fundamental neuroanatomical organization (dentate gyrus for dummies) 
Progress in brain research  2007;163:3-22.
The dentate gyrus is a simple cortical region that is an integral portion of the larger functional brain system called the hippocampal formation. In this review, the fundamental neuroanatomical organization of the dentate gyrus is described, including principal cell types and their connectivity, and a summary of the major extrinsic inputs of the dentate gyrus is provided. Together, this information provides essential information that can serve as an introduction to the dentate gyrus — a “dentate gyrus for dummies.”
PMCID: PMC2492885  PMID: 17765709
neuroanatomy; circuits; connections; cell types
8.  Spatial organization and state-dependent mechanisms for respiratory rhythm and pattern generation 
Progress in brain research  2007;165:201-220.
The brainstem respiratory network can operate in multiple functional states engaging different state-dependent neural mechanisms. These mechanisms were studied in the in situ perfused rat brainstem–spinal cord preparation using sequential brainstem transections and administration of riluzole, a pharmacological blocker of persistent sodium current (INaP). Dramatic transformations in the rhythmogenic mechanisms and respiratory motor pattern were observed after removal of the pons and subsequent medullary transactions down to the rostral end of pre-Bötzinger complex (pre-BötC). A computational model of the brainstem respiratory network was developed to reproduce and explain these experimental findings. The model incorporates several interacting neuronal compartments, including the ventral respiratory group (VRG), pre-BötC, Bötzinger complex (BötC), and pons. Simulations mimicking the removal of circuit components following transections closely reproduce the respiratory motor output patterns recorded from the intact and sequentially reduced brainstem preparations. The model suggests that both the operating rhythmogenic mechanism (i.e., network-based or pacemaker-driven) and the respiratory pattern generated (e.g., three-phase, two-phase, or one-phase) depend on the state of the pre-BötC (expression of INaP-dependent intrinsic rhythmogenic mechanisms) and the BötC (providing expiratory inhibition in the network). At the same time, tonic drives from pons and multiple medullary chemoreceptive sites appear to control the state of these compartments and hence the operating rhythmogenic mechanism and motor pattern. Our results suggest that the brainstem respiratory network has a spatial (rostral-to-caudal) organization extending from the rostral pons to the VRG, in which each functional compartment is controlled by more rostral compartments. The model predicts a continuum of respiratory network states relying on different contributions of intrinsic cellular properties versus synaptic interactions for the generation and control of the respiratory rhythm and pattern.
PMCID: PMC2408750  PMID: 17925248
respiratory CPG; brainstem; medulla; pons; pre-Bötzinger complex; computational modeling; respiratory rhythm generation
9.  Modeling the mammalian locomotor CPG: insights from mistakes and perturbations 
Progress in brain research  2007;165:235-253.
A computational model of the mammalian spinal cord circuitry incorporating a two-level central pattern generator (CPG) with separate half-center rhythm generator (RG) and pattern formation (PF) networks is reviewed. The model consists of interacting populations of interneurons and motoneurons described in the Hodgkin-Huxley style. Locomotor rhythm generation is based on a combination of intrinsic (persistent sodium current dependent) properties of excitatory RG neurons and reciprocal inhibition between the two half-centers comprising the RG. The two-level architecture of the CPG was suggested from an analysis of deletions (spontaneous omissions of activity) and the effects of afferent stimulation on the locomotor pattern and rhythm observed during fictive locomotion in the cat. The RG controls the activity of the PF network that in turn defines the rhythmic pattern of motoneuron activity. The model produces realistic firing patterns of two antagonist motoneuron populations and generates locomotor oscillations encompassing the range of cycle periods and phase durations observed during cat locomotion. A number of features of the real CPG operation can be reproduced with separate RG and PF networks, which would be difficult if not impossible to demonstrate with a classical single-level CPG. The two-level architecture allows the CPG to maintain the phase of locomotor oscillations and cycle timing during deletions and during sensory stimulation. The model provides a basis for functional identification of spinal interneurons involved in generation and control of the locomotor pattern.
PMCID: PMC2408748  PMID: 17925250
spinal cord; CPG; rhythm generation; locomotion; afferent control
10.  Object and event representation in toddlers 
Progress in brain research  2007;164:227-235.
Mental representation of absent objects and events is a major cognitive achievement. Research is presented that explores how toddlers (2- to 3-year-old children) search for hidden objects and understand out-of-sight events. Younger children fail to use visually obvious cues, such as a barrier that blocks a moving object’s path. Spatiotemporal information provided by movement cues directly connected to the hidden object is more helpful. A key problem for toddlers appears to be difficulty in representing a spatial array involving events with multiple elements.
PMCID: PMC2394671  PMID: 17920434
object search; reasoning; cognitive development; toddlers
Progress in brain research  2007;163:63-799.
PMCID: PMC1989689  PMID: 17765712
12.  The CA3 “Backprojection” to the Dentate Gyrus 
Progress in brain research  2007;163:627-637.
The hippocampus is typically described in the context of the trisynaptic circuit, a pathway that relays information from the perforant path to the dentate gyrus, dentate to area CA3, and CA3 to area CA1. Associated with this concept is the assumption that most hippocampal information processing occurs along the trisynaptic circuit. However, the entorhinal cortex may not be the only major extrinsic input to consider, and the trisynaptic circuit may not be the only way information is processed in hippocampus. Area CA3 receives input from a variety of sources, and may be as much of an “entry point” to hippocampus as the dentate gyrus. The axon of CA3 pyramidal cells targets diverse cell types, and has commissural projections, which together make it able to send information to much more of the hippocampus than granule cells. Therefore, CA3 pyramidal cells seem better designed to spread information through hippocampus than the granule cells. From this perspective, CA3 may be a point of entry that receives information which needs to be “broadcasted,” whereas the dentate gyrus may be a point of entry that receives information with more selective needs for hippocampal processing.
One aspect of the argument that CA3 pyramidal cells have a widespread projection is based on a part of its axonal arbor that has received relatively little attention, the collaterals that project in the opposite direction to the trisynaptic circuit, “back” to the dentate gyrus. The evidence for this “backprojection” to the dentate gyrus is strong, particularly in area CA3c, the region closest to the dentate gyrus, and in temporal hippocampus. The influence on granule cells is indirect, through hilar mossy cells and GABAergic neurons of the dentate gyrus, and appears to include direct projections in the case of CA3c pyramidal cells of ventral hippocampus. Physiological studies suggest that normally area CA3 does not have a robust excitatory influence on granule cells, but serves instead to inhibit it by activating dentate gyrus GABAergic neurons. Thus, GABAergic inhibition normally controls the backprojection to dentate granule cells, analogous to the way GABAergic inhibition appears to control the perforant path input to granule cells. From this perspective, the dentate gyrus has two robust glutamatergic inputs, entorhinal cortex and CA3, and two “gates,” or inhibitory filters that reduce the efficacy of both inputs, keeping granule cells relatively quiescent. When GABAergic inhibition is reduced experimentally, or under pathological conditions, CA3 pyramidal cells activate granule cells reliably, and do so primarily by disynaptic excitation that is mediated by mossy cells. We suggest that the backprojection has important functions normally that are dynamically regulated by nonprincipal cells of the dentate gyrus. Slightly reduced GABAergic input would lead to increased polysynaptic associative processing between CA3 and the dentate gyrus. Under pathological conditions associated with loss of GABAergic interneurons, the backprojection may support reverberatory excitatory activity between CA3, mossy cells, and granule cells, possibly enhanced by mossy fiber sprouting. In this case, the backprojection could be important to seizure activity originating in hippocampus, and help explain the seizure susceptibility of ventral hippocampus.
PMCID: PMC1986638  PMID: 17765742
13.  Sex Steroids and the Dentate Gyrus 
Progress in brain research  2007;163C:399-816.
In the late 1980s, the finding that the dentate gyrus contains more granule cells in the male than in the female of certain mouse strains provided the first indication that the dentate gyrus is a significant target for the effects of sex steroids during development. Gonadal hormones also play a crucial role in shaping the function and morphology of the adult brain. Besides reproduction-related processes, sex steroids participate in higher brain operations such as cognition and mood, in which the hippocampus is a critical mediator. Being part of the hippocampal formation, the dentate gyrus is naturally involved in these mechanisms and as such, this structure is also a critical target for the activational effects of sex steroids. These activational effects are the results of three major types of steroid-mediated actions. Sex steroids modulate the function of dentate neurons under normal conditions. In addition, recent research suggests that hormone-induced cellular plasticity may play a larger role than previously thought, particularly in the dentate gyrus. Specifically, the regulation of dentate gyrus neurogenesis and synaptic remodeling by sex steroids received increasing attention lately. Finally, the dentate gyrus is influenced by gonadal hormones in the context of cellular injury, and the work in this area demonstrates that gonadal hormones have neuroprotective potential. The expression of estrogen, progestin and androgen receptors in the dentate gyrus suggests that sex steroids, which could be of gonadal origin and/or synthesized locally in the dentate gyrus, may act directly on dentate cells. In addition, gonadal hormones could also influence the dentate gyrus indirectly, by subcortical hormone-sensitive structures such as the cholinergic septohippocampal system. Importantly, these three sex steroid-related themes, functional effects in the normal dentate gyrus, mechanisms involving neurogenesis and synaptic remodeling, as well as neuroprotection, have substantial implications for understanding normal cognitive function, with clinical importance for epilepsy, Alzheimer's disease and mental disorders.
PMCID: PMC1964752  PMID: 17765731
androgen; estrogen; progesterone; sex difference; electrophysiology; neurogenesis; synaptic remodeling; neuroprotection

Results 1-13 (13)