ORFEUS echelle spectra: Molecular hydrogen in disk, IVC, and HVC gas in front of the LMC

In front of the LMC molecular hydrogen is found in absorption near 0 km/s, being local disk gas, near +60 km/s in an intermediate velocity cloud, and near +120 km/s, in a high velocity halo cloud. The nature of the gas is discussed based on four ORFEUS far UV spectra of LMC stars and including data from the ground and from the IUE satellite. The local gas is cool and, given a span of sight lines of only 2.5 deg, rather fluffy. The fractional abundance of H_2 varies from log(f)=-5.4 to -3.3. Metal depletions (up to -1.7 dex for Fe) are typical for galactic disk gas. In the IV and HV gas an apparent underabundance of neutral oxygen points to an ionization level of the gas of about 90%. H_2 is detected in IV and HV gas toward HD 269546. In the IV gas we find an H_2 column density of log(N)\simeq15.6. The H_2 excitation indicates that the line of sight samples a cloud at a temperature below 150 K. Column densities are too small to detect the higher UV pumped excitation levels. The high velocity H_2 (log(N)\simeq15.6) is highly excited and probably exposed to a strong radiation field. Its excitation temperature exceeds 1000 K. Due to the radial velocity difference between the halo gas and the Milky Way disk, the unattenuated disk radiation is available for H_2 excitation in the halo. We do not find evidence for an intergalactic origin of this gas; a galactic as well as a Magellanic Cloud origin is possible.Comment: 12 pages, 5 figures, accepted for publication in A&

Design concepts for the Cherenkov Telescope Array CTA: an advanced facility for ground-based high-energy gamma-ray astronomy

Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5-10 improvement in sensitivity in the 100 GeV-10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA

Neural circuit mechanisms of memory coding in the Drosophila mushroom body

Learning allows animals to adapt their behaviour to changes in the environment. In humans and other mammals, memories are stored in the hippocampus and cerebellum, whereas in insects, they are stored inside the mushroom bodies (MB). Here, MB-intrinsic Kenyon cells (KCs) form plastic synapses to MB output neurons (MBONs) that are modulated by the reinforcing action of dopaminergic neurons (DANs). Despite decades of research on the MB, the main neurotransmitter underlying the plastic KC → MBON synapse has remained a mystery. Here, I show that this synapse is cholinergic in the fruit fly Drosophila melanogaster. MBONs show fast excitatory responses to direct acetylcholine (ACh) application. KCs synthesise ACh-related proteins ChAT and VAChT. MBONs express and require nicotinic ACh receptors (nAChRs) to become fully activated by odour presentation. Lastly, artificial activation of KCs leads to MBON calcium responses that are blocked by nicotinic antagonists and genetic reduction of VAChT in KCs. Short neuropeptide F (sNPF) may play a role as a modulatory co-transmitter that can either excite or inhibit specific MBONs and DANs. The retrieval of memories is state-dependent and known to potentially change the original memory. Fruit flies need to be hungry to express appetitive memories. Hunger state depends on insulin signalling that activates the GABAergic MBON MVP2, while appetitive memory retrieval depends on decreased activity in M4/6 MBONs. Here, I show that optogenetic MVP2 activation acutely inhibits M4/6 odour responses, rendering MVP2 an inhibitory MBON interneuron. I also show that other MBONs are functionally connected to DANs, thus linking memory reinforcement and retrieval pathways in a way that enables the updating of the original memory. These findings show that associative memories in Drosophila are initially formed at cholinergic KC–MBON synapses, and can be retrieved and modified through an intricate KC–MBON–DAN network.</p

Neural transposition in the Drosophila brain: is it all bad news?

Transposition of mobile genetic elements can radically alter genome structure and sequence. In doing so, they can alter gene expression and cellular function. Perhaps unsurprisingly, this potentially catastrophic process is heavily constrained, especially in the germ line where aberrations lead to sterility or could be passed onto the next generation. However, recent studies in mammals and fruit flies suggest that transposition happens at measurable levels in the brain, and possibly more so in some cell types than in others. This has led to the suggestion that certain cell types may utilize transposable elements to diversify cellular properties. In this review, we discuss these findings and ideas in light of our current understanding of transposons and their control in the fly, and the growing evidence for an involvement of transposition in neurological disease in humans

Functional Microarchitecture of the Mouse Dorsal Inferior Colliculus Revealed through In Vivo Two-Photon Calcium Imaging

The inferior colliculus (IC) is an obligatory relay for ascending auditory inputs from the brainstem and receives descending input from the auditory cortex. The IC comprises a central nucleus (CNIC), surrounded by several shell regions, but the internal organization of this midbrain nucleus remains incompletely understood. We used two-photon calcium imaging to study the functional microarchitecture of both neurons in the mouse dorsal IC and corticocollicular axons that terminate there. In contrast to previous electrophysiological studies, our approach revealed a clear functional distinction between the CNIC and the dorsal cortex of the IC (DCIC), suggesting that the mouse midbrain is more similar to that of other mammals than previously thought. We found that the DCIC comprises a thin sheet of neurons, sometimes extending barely 100 μm below the pial surface. The sound frequency representation in the DCIC approximated the mouse's full hearing range, whereas dorsal CNIC neurons almost exclusively preferred low frequencies. The response properties of neurons in these two regions were otherwise surprisingly similar, and the frequency tuning of DCIC neurons was only slightly broader than that of CNIC neurons. In several animals, frequency gradients were observed in the DCIC, and a comparable tonotopic arrangement was observed across the boutons of the corticocollicular axons, which form a dense mesh beneath the dorsal surface of the IC. Nevertheless, acoustically responsive corticocollicular boutons were sparse, produced unreliable responses, and were more broadly tuned than DCIC neurons, suggesting that they have a largely modulatory rather than driving influence on auditory midbrain neurons

Re-evaluation of learned information in Drosophila.

Animals constantly assess the reliability of learned information to optimize their behaviour. On retrieval, consolidated long-term memory can be neutralized by extinction if the learned prediction was inaccurate. Alternatively, retrieved memory can be maintained, following a period of reconsolidation during which it is labile. Although extinction and reconsolidation provide opportunities to alleviate problematic human memories, we lack a detailed mechanistic understanding of memory updating. Here we identify neural operations underpinning the re-evaluation of memory in Drosophila. Reactivation of reward-reinforced olfactory memory can lead to either extinction or reconsolidation, depending on prediction accuracy. Each process recruits activity in specific parts of the mushroom body output network and distinct subsets of reinforcing dopaminergic neurons. Memory extinction requires output neurons with dendrites in the α and α' lobes of the mushroom body, which drive negatively reinforcing dopaminergic neurons that innervate neighbouring zones. The aversive valence of these new extinction memories neutralizes previously learned odour preference. Memory reconsolidation requires the γ2α'1 mushroom body output neurons. This pathway recruits negatively reinforcing dopaminergic neurons innervating the same compartment and re-engages positively reinforcing dopaminergic neurons to reconsolidate the original reward memory. These data establish that recurrent and hierarchical connectivity between mushroom body output neurons and dopaminergic neurons enables memory re-evaluation driven by reward-prediction error

Rapid adaptive remote focusing microscope for sensing of volumetric neural activity

The ability to record neural activity in the brain of a living organism at cellular resolution is of great importance for defining the neural circuit mechanisms that direct behavior. Here we present an adaptive two-photon microscope optimized for extraction of neural signals over volumes in intact Drosophila brains, even in the presence of specimen motion. High speed volume imaging was made possible through reduction of spatial resolution while maintaining the light collection efficiency of a high resolution, high numerical aperture microscope. This enabled simultaneous recording of odor-evoked calcium transients in a defined volume of mushroom body Kenyon cell bodies in a live fruit fly

Re-evaluation of learned information in Drosophila.

Animals constantly assess the reliability of learned information to optimize their behaviour. On retrieval, consolidated long-term memory can be neutralized by extinction if the learned prediction was inaccurate. Alternatively, retrieved memory can be maintained, following a period of reconsolidation during which it is labile. Although extinction and reconsolidation provide opportunities to alleviate problematic human memories, we lack a detailed mechanistic understanding of memory updating. Here we identify neural operations underpinning the re-evaluation of memory in Drosophila. Reactivation of reward-reinforced olfactory memory can lead to either extinction or reconsolidation, depending on prediction accuracy. Each process recruits activity in specific parts of the mushroom body output network and distinct subsets of reinforcing dopaminergic neurons. Memory extinction requires output neurons with dendrites in the α and α' lobes of the mushroom body, which drive negatively reinforcing dopaminergic neurons that innervate neighbouring zones. The aversive valence of these new extinction memories neutralizes previously learned odour preference. Memory reconsolidation requires the γ2α'1 mushroom body output neurons. This pathway recruits negatively reinforcing dopaminergic neurons innervating the same compartment and re-engages positively reinforcing dopaminergic neurons to reconsolidate the original reward memory. These data establish that recurrent and hierarchical connectivity between mushroom body output neurons and dopaminergic neurons enables memory re-evaluation driven by reward-prediction error

Rapid adaptive remote focusing microscope for sensing of volumetric neural activity

The ability to record neural activity in the brain of a living organism at cellular resolution is of great importance for defining the neural circuit mechanisms that direct behavior. Here we present an adaptive two-photon microscope optimized for extraction of neural signals over volumes in intact Drosophila brains, even in the presence of specimen motion. High speed volume imaging was made possible through reduction of spatial resolution while maintaining the light collection efficiency of a high resolution, high numerical aperture microscope. This enabled simultaneous recording of odor-evoked calcium transients in a defined volume of mushroom body Kenyon cell bodies in a live fruit fly