Two-Photon Bidirectional Control and Imaging of Neuronal Excitability with High Spatial Resolution In Vivo

Summary: Sensory information is encoded within the brain in distributed spatiotemporal patterns of neuronal activity. Understanding how these patterns influence behavior requires a method to measure and to bidirectionally perturb with high spatial resolution the activity of the multiple neuronal cell types engaged in sensory processing. Here, we combined two-photon holography to stimulate neurons expressing blue light-sensitive opsins (ChR2 and GtACR2) with two-photon imaging of the red-shifted indicator jRCaMP1a in the mouse neocortex in vivo. We demonstrate efficient control of neural excitability across cell types and layers with holographic stimulation and improved spatial resolution by opsin somatic targeting. Moreover, we performed simultaneous two-photon imaging of jRCaMP1a and bidirectional two-photon manipulation of cellular activity with negligible effect of the imaging beam on opsin excitation. This all-optical approach represents a powerful tool to causally dissect how activity patterns in specified ensembles of neurons determine brain function and animal behavior. : Forli et al. developed an all-optical method to image and bidirectionally manipulate brain networks with high spatial resolution and minimal crosstalk in the intact mammalian brain. They validate the method across cell types and layers in the mouse neocortex. Keywords: optogenetics, two-photon excitation, digital holography, patterned illumination, two-photon imagin

SHARP - I. A high-resolution multi-band view of the infra-red Einstein ring of JVAS B1938+666

We present new mass models for the gravitational lens system B1938+666, using multi-wavelength data acquired from Keck adaptive optics (AO) and Hubble Space Telescope (HST) observations. These models are the first results from the Strong-lensing at High Angular Resolution Program (SHARP), a project designed to study known quadruple-image and Einstein ring lenses using high-resolution imaging, in order to probe their mass distributions in unprecedented detail. Here, we specifically highlight differences between AO- and HST-derived lens models, finding that -- at least when the lens and source galaxies are both bright and red, and the system has a high degree of circular symmetry -- AO-derived models place significantly tighter constraints on model parameters. Using this improved precision, we infer important physical properties about the B1938+666 system, including the mass density slope of the lensing galaxy (gamma = 2.045), the projected dark matter mass fraction within the Einstein radius (M_dark/M_lens = 0.55), and the total magnification factor of the source galaxy (~ 13). Additionally, we measure an upper-limit constraint on luminous substructure (M_V > 16.2), based on the non-detection of bright satellite galaxies in all data sets. Finally, we utilize the improved image resolution of the AO data to reveal the presence of faint arcs outside of the primary Einstein ring. The positions and orientations of these arcs raise the intriguing possibility that B1938+666 has a second source galaxy, located at a more distant redshift. However, future work is needed to verify this hypothesis.Comment: 12 pages, 7 figures; Accepted for publication in MNRA

The radio source population at high frequency: follow-up of the 15-GHz 9C survey

We have carried out extensive radio and optical follow-up of 176 sources from the 15 GHz 9th Cambridge survey. Optical identifications have been found for 155 of the radio sources; optical images are given with radio maps overlaid. The continuum radio spectrum of each source spanning the frequency range 1.4 - 43 GHz is also given. Two flux-limited samples are defined, one containing 124 sources complete to 25 mJy and one of 70 sources complete to 60 mJy. Between one fifth and one quarter of sources from these flux-limited samples display convex radio spectra, rising between 1.4 and 4.8 GHz. These rising-spectrum sources make up a much larger fraction of the radio source population at this high selection frequency than in lower frequency surveys. We find that by using non-simultaneous survey flux density measurements at 1.4 and 15 GHz to remove steep spectrum objects, the efficiency of selecting objects with spectra rising between 1.4 and 4.8 GHz (as seen in simultaneous measurements) can be raised to 49 percent without compromising the completeness of the rising spectrum sample.Comment: 40 pages, 223 figures. Accepted for publication in MNRAS. Version with full size images (A4 paper) avaliable at http://www.mrao.cam.ac.uk/publications/papers files ME777.ps and ME777.pd

There Is No Safe Dose of Prions

Understanding the circumstances under which exposure to transmissible spongiform encephalopathies (TSEs) leads to infection is important for managing risks to public health. Based upon ideas in toxicology and radiology, it is plausible that exposure to harmful agents, including TSEs, is completely safe if the dose is low enough. However, the existence of a threshold, below which infection probability is zero has never been demonstrated experimentally. Here we explore this question by combining data and mathematical models that describe scrapie infections in mice following experimental challenge over a broad range of doses. We analyse data from 4338 mice inoculated at doses ranging over ten orders of magnitude. These data are compared to results from a within-host model in which prions accumulate according to a stochastic birth-death process. Crucially, this model assumes no threshold on the dose required for infection. Our data reveal that infection is possible at the very low dose of a 1000 fold dilution of the dose that infects half the challenged animals (ID50). Furthermore, the dose response curve closely matches that predicted by the model. These findings imply that there is no safe dose of prions and that assessments of the risk from low dose exposure are right to assume a linear relationship between dose and probability of infection. We also refine two common perceptions about TSE incubation periods: that their mean values decrease linearly with logarithmic decreases in dose and that they are highly reproducible between hosts. The model and data both show that the linear decrease in incubation period holds only for doses above the ID50. Furthermore, variability in incubation periods is greater than predicted by the model, not smaller. This result poses new questions about the sources of variability in prion incubation periods. It also provides insight into the limitations of the incubation period assay

The quasar proximity effect at redshift ~= 2.6 with the From Lines to Overdensities approach

We revisit the proximity effect produced by quasars (QSOs) at redshifts 2.1-3.3 applying the From Lines to Overdensities (FLO) approach to a sample of ~6300 Lyalpha lines fitted in 21 high resolution, high signal-to-noise spectra. This new technique allows to recover the hydrogen-density field from the HI column densities of the lines in the Lyalpha forest, on the basis of simple assumptions on the physical state of the gas. To minimize the systematic uncertainties that could affect the density recovering in the QSO vicinity, we carefully determined the redshifts of the QSOs in our sample and modelled in detail their spectra to compute the corresponding ionizing fluxes. The mean density field obtained from the observed spectra shows a significant overdensity in the region within 4 proper Mpc from the QSO position, confirming that QSOs are hosted in high-density peaks. The absolute value of rho/<rho> for the peak is uncertain by a factor of ~3, depending on the assumed QSO spectral slope and the minimum HI column density detectable in the spectra. We do not confirm the presence of a significant overdensity extending to separations of ~15 proper Mpc from the QSO, claimed in previous works at redshifts <z> ~= 2.5 and 3.8. Our best guess for the ultraviolet background (UVB) ionization rate based on the intergalactic medium (IGM) mean density recovered by FLO is GammaUVB ~= 10-12s-1. However, values of GammaUVB ~= 3 × 10-12s-1 could be viable if an inverted temperature-density relation with index alpha ~= -0.5 is adopted. Based on observations collected at the European Southern Observatory (ESO) Very Large Telescope (VLT), Cerro Paranal, Chile - Programs 166.A-0106(A) and during commissioning and science verification of UVES. E-mail: [email protected]

Tracing the gas at redshift 1.7-3.5 with the Lyman-alpha forest: the FLO approach

[Abridged] We present FLO (From Lines to Over-densities), a new technique to reconstruct the hydrogen density field for the Lya forest lines observed in high resolution QSO spectra. The method is based on the hypothesis that the Lya lines arise in the low to intermediate density intergalactic gas and that the Jeans length is the typical size of the Lya absorbers. The reliability of FLO is tested against mock spectra obtained from cosmological simulations. The recovering algorithm gives satisfactory results in the range from the mean density to over-densities of ~30 and reproduces correctly the correlation function of the density field and the 1D power spectrum on scales between ~20 and 60 comoving Mpc. A sample of Lya forests from 22 high resolution QSO spectra is analysed, covering the redshift range 1.7<z<3.5. For each line of sight, we fit Voigt profiles to the lines of the Lya forest, providing the largest, homogeneous sample of fitted Lya lines ever studied. The line number density evolution with redshift follows a power-law relation: dn/dz=(166 +/- 4) [(1+z)/3.5]^{(2.8 +/- 0.2)} (1 sigma errors). The two-point correlation function of lines shows a signal up to separations of ~2 comoving Mpc; weak lines (log N(HI)<13.8) also show a significant clustering but on smaller scales (r<1.5 comoving Mpc). We estimate with FLO the hydrogen density field toward the 22 observed lines of sight. The redshift distribution of the average densities computed for each QSO is consistent with the cosmic mean hydrogen density in the analysed redshift range. The two-point correlation function and the 1D power spectrum of the delta field are estimated. The correlation function shows clustering signal up to ~4 comoving Mpc.Comment: 17 figures and 2 tables, accepted for publication in MNRA

What explains rare and conspicuous colours in a snail? A test of time-series data against models of drift, migration or selection

It is intriguing that conspicuous colour morphs of a prey species may be maintained at low frequencies alongside cryptic morphs. Negative frequency-dependent selection by predators using search images ('apostatic selection') is often suggested without rejecting alternative explanations. Using a maximum likelihood approach we fitted predictions from models of genetic drift, migration, constant selection, heterozygote advantage or negative frequency-dependent selection to time-series data of colour frequencies in isolated populations of a marine snail (Littorina saxatilis), re-established with perturbed colour morph frequencies and followed for >20 generations. Snails of conspicuous colours (white, red, banded) are naturally rare in the study area (usually <10%) but frequencies were manipulated to levels of ~50% (one colour per population) in 8 populations at the start of the experiment in 1992. In 2013, frequencies had declined to ~15-45%. Drift alone could not explain these changes. Migration could not be rejected in any population, but required rates much higher than those recorded. Directional selection was rejected in three populations in favour of balancing selection. Heterozygote advantage and negative frequency-dependent selection could not be distinguished statistically, although overall the results favoured the latter. Populations varied idiosyncratically as mild or variable colour selection (3-11%) interacted with demographic stochasticity, and the overall conclusion was that multiple mechanisms may contribute to maintaining the polymorphisms.Heredity advance online publication, 21 September 2016; doi:10.1038/hdy.2016.77

A Simple, Versatile and Sensitive Cell-Based Assay for Prions from Various Species

Detection and quantification of prion infectivity is a crucial step for various fundamental and applied aspects of prion research. Identification of cell lines highly sensitive to prion infection led to the development of cell-based titration procedures aiming at replacing animal bioassays, usually performed in mice or hamsters. However, most of these cell lines are only permissive to mouse-adapted prions strains and do not allow titration of prions from other species. In this study, we show that epithelial RK13, a cell line permissive to mouse and bank vole prion strains and to natural prion agents from sheep and cervids, enables a robust and sensitive detection of mouse and ovine-derived prions. Importantly, the cell culture work is strongly reduced as the RK13 cell assay procedure designed here does not require subcultivation of the inoculated cultures. We also show that prions effectively bind to culture plastic vessel and are quantitatively detected by the cell assay. The possibility to easily quantify a wider range of prions, including rodent experimental strains but also natural agents from sheep and cervids, should prompt the spread of cell assays for routine prion titration and lead to valuable information in fundamental and applied studies