Volume 12 Issue 4, April 2009

Neural stem cells transition through several progenitor stages before finally generating postmitotic neurons. New work shows that one of these steps, the generation of neuroblasts from transient amplifying precursors in the adult subventricular zone, requires downregulation of the transcription factor Sox9 by the microRNA miR-124.

How can the multifunctional factor Sonic Hedgehog (SHH) elicit specific responses from its target cells? A study now pinpoints proteoglycans as crucial anchors and modulators of SHH signaling, eliciting a proliferation response.

What controls the functional connections between sending and receiving neurons? A new model suggests that each receiver circuit has a local switch that is controlled by the balance between excitation and inhibition.

A subset of neurons in rat barrel cortex integrate information about the object a whisker contacts with the motion of the whisker at the time of contact, setting the stage for a highly specialized object localization system.

The small size and high resistance of C. elegans neurons makes them sensitive to the random opening of single ion channels, probably rendering codes that are based on classical, all-or-none action potentials unworkable. The recent discovery in C. elegans of a special class of regenerative events known as plateau potentials introduces the possibility of digital neural codes. Such codes would solve the problem of representing information in nervous systems in which action potentials are unreliable.

Recent work suggests that endoplasmic reticulum stress in myelinating cells is important in the pathogenesis of various disorders of myelin, including multiple sclerosis. In this review, Lin and Popko review our current understanding of the role of endoplasmic reticulum stress in myelinating cells.

Axon degeneration is an active process. Miller and colleagues show in fly and mouse models that axon degeneration requires the kinase DLK and its downstream target JNK to proceed.

To understand how monocular eyelid closure and retinal inactivation can affect cortical ocular dominance, Linden et al. examined the firing activity of the dorsal lateral geniculate nucleus. They report that the firing pattern, but not the firing rate, is differentially affected by manipulations of retinal activity.

The striatum contains two distinct types of GABAergic neurons, striatonigral and striatopallidal cells. Durieux and colleagues genetically ablated the striatopallidal population in mice, uncovering specific roles for these neurons in the control of locomotion and the response to the addictive drug amphetamine.

Previous work suggests that post-learning rapid eye movement (REM) sleep is crucial for memory consolidation, but this study shows that suppressing REM sleep via serotonin/norepinephrine reuptake inhibitors instead enhances memory consolidation of a motor skill task.

This study shows that the microRNA miR-124 is required for neuronal differentiation of adult neural progenitors in vitro and in vivo. The transcription factor Sox9 opposes neuronal differentiation and is suppressed by miRNA-124.

Sonic hedgehog (Shh) functions as both a mitogen and a patterning agent during development. Looking mainly at the developing mouse cerebellum, this study shows that the mitogenic activity, but not the patterning role, requires Shh to interact with proteoglycans.

This study shows that the transcription factor SRF, expressed in neurons, crucially affects oligodendrocyte maturation and myelination through a non–cell autonomous mechanism. SRF appears to act, at least in part, by repressing transcription of the paracrine growth factor CTGF. Overexpression of CTGF reduced oligodendrocyte differentiation.

Although the receptor tyrosine phosphatase LAR is known to regulate the devolvement of excitatory synapse and to direct proper guidance of axons, the extracellular ligand for its activation has remained unknown. This study identifies postsynaptic netrin G-ligand 3 (NGL-3) as the trans-synaptic adhesion ligand of LAR and demonstrates a bidirectional regulation of excitatory synapse formation by the LAR/NGL-3 interaction.

The release of stress hormone from the hypothalamic paraventricular nucleus (PVN) is regulated by nearby GABAergic input. This study shows that behavioral stress affects chloride ion homeostasis and thus tapers GABAergic inhibition, thereby suggesting disinhibition of the PVN as a mechanism for stress response initiation.

In the cochlea, ribbon synapses are used to transmit acoustic information from inner hair cells to spiral ganglion cells. Here the authors find that the properties of these synapses vary along the tonotopic axis, providing a candidate presynaptic mechanism for modulating the dynamics of ganglion cell spiking.

Phosphodiesterase (PDE) activity in olfactory sensory neuron cilia is thought to be responsible for degrading odor-induced signaling, resulting in rapid response termination. However, disrupting the variety of PDE found only in cilia unexpectedly turns out not to result in a rapid response termination deficit. Only eliminating both cilia-localized and cell body–localized varieties of PDEs resulted in prolonged termination.

The authors use patch-clamp recordings and two-photon microscopy to characterize monosynaptic connections between Purkinje cells of the juvenile cerebellum. They then construct a network model that generates traveling waves of activity between connected Purkinje cells and validate their model with observations in juvenile cerebellar cortex.

The organization of the olfactory bulb has been extensively studied, but much less is known about downstream areas. The authors compare activity patterns in the zebrafish olfactory bulb and two of its targets. They find that the subpallial area has overlapping odor representations, whereas those in the area homologous to the olfactory bulb are more sharply tuned.

The balance of excitation and inhibition across large populations of spiking neurons has been suggested to be important. Here the authors model the effects of a more detailed balance between incoming excitation and local inhibition on the transmission of signals through a neural network.

Active somatosensory perception requires the integration of signals arising from both external stimuli and motor activity. The authors found a population of neurons in rat somatosensory cortex that responded to touch only when contact occurred at a specific phase in the whisk cycle, providing information about the position of the object relative to the rat's face.

Previous work has implicated prefrontal and parietal cortex in time perception and the temporal domain in decision making. Single-cell recordings in monkeys performing an interval-generation task now reveal that neurons in the medial motor areas can also represent the passage of time.

People can form opinions of others during an initial encounter. Neuroimaging results suggest that these first impressions are mediated by the amygdala and posterior cingulate cortex.

The frontal lobes are critical for cognitive control over both abstract actions and motor plans. On the basis of the behavioral deficits of lesions patients, the authors report that there is a hierarchical organization of cognitive control, with rostral areas being required for decisions about more abstract actions and caudal areas being required for decisions about more concrete actions.