News tips from the Journal of Neuroscience

December 16, 2008

1. Intrinsic Membrane Properties Vary Across Cortical Regions
Mark N. Miller, Benjamin W. Okaty, and Sacha B. Nelson

A neuron's function is determined by many factors, including its connections, morphology, and molecular makeup, and these characteristics, together with anatomical positions, are used to draw ever finer distinctions between neuronal classes. Miller and colleagues have found that expression of YFP under control of the Thy-1 promoter labels ~25% of pyramidal neurons in layer 5 of cortex. The authors now report that although YFP-labeled cells present in primary motor and sensory cortices have similar morphology, projections (in the pyramidal tract), and transcriptional sets, they respond differently to sustained current injection. Cells in sensory cortex fired at a constant rate upon current injection, whereas cells in motor cortex exhibited delayed and then accelerating spiking. These differences were attributed to expression of voltage-dependent, slowly inactivating outward currents mediated by potassium Kv1 channels in motor cortex. Such differences in intrinsic membrane properties are likely to contribute to the functional specification of cortical regions.

2. Five Distinct Glial Types Inhabit Adult Drosophila CNS
Takeshi Awasaki, Sen-Lin Lai, Kei Ito, and Tzumin Lee

Awasaki et al. have conducted an extensive analysis of glia in adult Drosophila CNS, laying groundwork for further elucidation of the functions of five identified classes. Each class has a distinct morphology and anatomical position, and each is likely to express different proteins and develop from a distinct lineage. The outer surface of the brain contained oblong glia, which proliferated postembryonically by symmetric cell division, likely on the brain surface. Large, sheet-like glia were present below this outer layer, and these probably form the blood- brain barrier. The cortex of the brain contained mesh-like glia, each of which encapsulated several neuronal somata. These are likely generated embryonically, and may provide metabolic support to neurons. The neuropile contained two types of glia, which proliferate postembryonically and probably migrate after proliferation. Fibrous, lamellar glia lined the boundaries of neuropile subcompartments and may provide electrical insulation. Dendritic glia filled the interior of the neuropile and may perform astrocyte-like functions.

3. ACC and OFC Have Distinct Roles in Decision Making
Peter H. Rudebeck, Timothy E. Behrens, Steven W. Kennerley, Mark G. Baxter, Mark J. Buckley, Mark E. Walton, and Matthew F. S. Rushworth

The anterior cingulate cortex (ACC) and orbitofrontal cortex (OFC) are involved in making decisions based on expectations of reward, but their distinct roles have been unclear. Previous studies have suggested that ACC may be involved in predicting reward associated with a specific action, whereas the OFC estimates the reward associated with a given stimulus. To test this hypothesis, Rudebeck et al. directly compared monkeys with lesions to one of these regions on tasks that required a choice between either actions or visual stimuli. The probability of receiving a reward for each action or stimulus varied, so monkeys had to integrate information over several trials to make the optimal choice. As predicted, only ACC lesions impaired a monkey's ability to choose appropriate actions based on reward probability, whereas only OFC impaired decisions about visual stimuli. Both areas appear to be specifically involved in updating estimates of reward probability based on the outcome of multiple trials.

4. LIF Cytokine Delays Photoreceptor Degeneration
Sandrine Joly, Christina Lange, Markus Thiersch, Marijana Samardzija, and Christian Grimm

Neural stress or injury activates opposing signaling pathways that promote survival and cell death. A thorough understanding of these pathways could allow researchers to effectively shift the balance in favor of survival without exacerbating damage. Leukemia inhibitory factor (LIF) is one of many pro-survival cytokines that are induced by neural injury. This week, Joly et al. demonstrate the importance of LIF in protecting photoreceptors from degeneration resulting from excessive light exposure or from a mutation that causes retinal degeneration. LIF was upregulated in a subset of Müller glia cells under both conditions compared to control, and LIF knock-out accelerated photoreceptor degeneration. LIF knock-out also prevented upregulation of other proteins in response to injury, notably fibroblast growth factor 2, which is thought to be a key neuroprotective factor. Injecting LIF into the eyes reversed the effect of knock-out on protein expression, but its effect on degeneration has yet to be demonstrated.
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