LIAD-fs scheme for studies of ultrafast laser interactions with gas phase biomolecules.

Laser induced acoustic desorption (LIAD) has been used for the first time to study the parent ion production and fragmentation mechanisms of a biological molecule in an intense femtosecond (fs) laser field. The photoacoustic shock wave generated in the analyte substrate (thin Ta foil) has been simulated using the hydrodynamic HYADES code, and the full LIAD process has been experimentally characterised as a function of the desorption UV-laser pulse parameters. Observed neutral plumes of densities >10(9) cm(-3) which are free from solvent or matrix contamination demonstrate the suitability and potential of the source for studying ultrafast dynamics in the gas phase using fs laser pulses. Results obtained with phenylalanine show that through manipulation of fundamental femtosecond laser parameters (such as pulse length, intensity and wavelength), energy deposition within the molecule can be controlled to allow enhancement of parent ion production or generation of characteristic fragmentation patterns. In particular by reducing the pulse length to a timescale equivalent to the fastest vibrational periods in the molecule, we demonstrate how fragmentation of the molecule can be minimised whilst maintaining a high ionisation efficiency.

Approaches to intestinal failure in Crohn's disease.

Crohn's disease is one of the leading causes of intestinal failure. The term 'type 2' intestinal failure is used to describe the relatively rare type of intestinal failure that occurs in association with septic, metabolic and complex nutritional complications, typically following surgical resection and/or laparostomy for intra-abdominal sepsis. A multidisciplinary approach to the management of patients with type 2 intestinal failure is crucial, and it is helpful to approach patient care in a structured manner using the 'sepsis-nutrition-anatomy-plan' algorithm: resolution of sepsis is required before adequate nutritional repletion can be achieved, and it is crucial to optimise nutritional status, and define intestinal anatomy before delineating a definitive medical or surgical plan. A structured approach to the management of patients with inflammatory bowel disease, who have developed type 2 intestinal failure, should reduce the likelihood of these patients developing 'type 3' intestinal failure, which is characterised by the need for long-term parenteral nutrition. However, Crohn's disease is still the commonest indication for home parenteral nutrition in the UK.

A comb-sampling method for enhanced mass analysis in linear electrostatic ion traps.

In this paper an algorithm for extracting spectral information from signals containing a series of narrow periodic impulses is presented. Such signals can typically be acquired by pickup detectors from the image-charge of ion bunches oscillating in a linear electrostatic ion trap, where frequency analysis provides a scheme for high-resolution mass spectrometry. To provide an improved technique for such frequency analysis, we introduce the CHIMERA algorithm (Comb-sampling for High-resolution IMpulse-train frequency ExtRAaction). This algorithm utilizes a comb function to generate frequency coefficients, rather than using sinusoids via a Fourier transform, since the comb provides a superior match to the data. This new technique is developed theoretically, applied to synthetic data, and then used to perform high resolution mass spectrometry on real data from an ion trap. If the ions are generated at a localized point in time and space, and the data is simultaneously acquired with multiple pickup rings, the method is shown to be a significant improvement on Fourier analysis. The mass spectra generated typically have an order of magnitude higher resolution compared with that obtained from fundamental Fourier frequencies, and are absent of large contributions from harmonic frequency components.

Controlled redistribution of vibrational population by few-cycle strong-field laser pulses.

The use of strong-field (i.e. intensities in excess of 10(13) Wcm(-2)) few-cycle ultrafast (durations of 10 femtoseconds or less) laser pulses to create, manipulate and image vibrational wavepackets is investigated. Quasi-classical modelling of the initial superposition through tunnel ionization, wavepacket modification by nonadiabatically altering the nuclear environment via the transition dipole and the Stark effect, and measuring the control outcome by fragmenting the molecule is detailed. The influence of the laser intensity on strong-field ultrafast wavepacket control is discussed in detail: by modifying the distribution of laser intensities imaged, we show that focal conditions can be created that give preference to this three-pulse technique above processes induced by the pulses alone. An experimental demonstration is presented, and the nuclear dynamics inferred by the quasi-classical model discussed. Finally, we present the results of a systematic investigation of a dual-control pulse scheme, indicating that single vibrational states should be observable with high fidelity, and the populated state defined by varying the arrival time of the two control pulses. The relevance of such strong-field coherent control methods to the manipulation of electron localization and attosecond science is discussed.

Excited ions in intense femtosecond laser pulses: laser-induced recombination.

Electron-ion recombination in a laser-induced electron recollision is of fundamental importance as the underlying mechanism responsible for the generation of high-harmonic radiation and hence for the production of attosecond pulse trains in the extreme ultraviolet and soft x-ray spectral regions. By using an ion beam target, remotely prepared to be partially in long-lived excited states, the recombination process has for the first time been directly observed and studied.

NMDA receptor function in mouse models of Huntington disease.

Huntington disease (HD) is an autosomal dominant disorder in which degeneration of medium-sized spiny striatal neurons occurs. The HD gene and the protein it encodes, huntingtin, have been identified but their functions remain unknown. Transgenic mouse models for HD have been developed and we examined responses of medium-sized striatal neurons recorded in vitro to application of N-methyl-D-aspartate (NMDA) in two of these. The first model (R6/2) expresses exon 1 of the human HD gene with approximately 150 CAG repeats. In the R6/2 an enhancement of currents induced by selective activation of NMDA receptors as well as an enhancement of intracellular Ca(2+) flux occurred in both presymptomatic and symptomatic mice. These alterations appeared specific for the NMDA receptor because alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor-mediated currents were reduced in symptomatic R6/2s. In R6/2 animals there were parallel increases in NMDA-R1 and decreases in NMDA-R2A/B subunit proteins as established by immunohistochemistry. The second model (YAC72) contains human genomic DNA spanning the full-length gene and all its regulatory elements with 72 CAG repeats. The phenotypical expression of the disorder develops more gradually than in the R6/2. In YAC72 mice we found similar but less marked increases in responses of medium-sized striatal neurons to NMDA. These findings indicate that alterations in NMDA receptor function may predispose striatal neurons to excitotoxic damage, leading to subsequent neuronal degeneration and underscore the functional importance of NMDA receptors in HD.

Changes in cortical and striatal neurons predict behavioral and electrophysiological abnormalities in a transgenic murine model of Huntington's disease.

Neurons in Huntington's disease exhibit selective morphological and subcellular alterations in the striatum and cortex. The link between these neuronal changes and behavioral abnormalities is unclear. We investigated relationships between essential neuronal changes that predict motor impairment and possible involvement of the corticostriatal pathway in developing behavioral phenotypes. We therefore generated heterozygote mice expressing the N-terminal one-third of huntingtin with normal (CT18) or expanded (HD46, HD100) glutamine repeats. The HD mice exhibited motor deficits between 3 and 10 months. The age of onset depended on an expanded polyglutamine length; phenotype severity correlated with increasing age. Neuronal changes in the striatum (nuclear inclusions) preceded the onset of phenotype, whereas cortical changes, especially the accumulation of huntingtin in the nucleus and cytoplasm and the appearance of dysmorphic dendrites, predicted the onset and severity of behavioral deficits. Striatal neurons in the HD mice displayed altered responses to cortical stimulation and to activation by the excitotoxic agent NMDA. Application of NMDA increased intracellular Ca(2+) levels in HD100 neurons compared with wild-type neurons. Results suggest that motor deficits in Huntington's disease arise from cumulative morphological and physiological changes in neurons that impair corticostriatal circuitry.

Facilitated glutamatergic transmission in the striatum of D2 dopamine receptor-deficient mice.

Dopamine (DA) receptors play an important role in the modulation of excitability and the responsiveness of neurons to activation of excitatory amino acid receptors in the striatum. In the present study, we utilized mice with genetic deletion of D2 or D4 DA receptors and their wild-type (WT) controls to examine if the absence of either receptor subtype affects striatal excitatory synaptic activity. Immunocytochemical analysis verified the absence of D2 or D4 protein expression in the striatum of receptor-deficient mutant animals. Sharp electrode current- and whole cell patch voltage-clamp recordings were obtained from slices of receptor-deficient and WT mice. Basic membrane properties were similar in D2 and D4 receptor-deficient mutants and their respective WT controls. In current-clamp recordings in WT animals, very little low-amplitude spontaneous synaptic activity was observed. The frequency of these spontaneous events was increased slightly in D2 receptor-deficient mice. In addition, large-amplitude depolarizations were observed in a subset of neurons from only the D2 receptor-deficient mutants. Bath application of the K+ channel blocker 4-aminopyridine (100 microM) and bicuculline methiodide (10 microM, to block synaptic activity due to activation of GABA(A) receptors) markedly increased spontaneous synaptic activity in receptor-deficient mutants and WTs. Under these conditions, D2 receptor-deficient mice displayed significantly more excitatory synaptic activity than their WT controls, while there was no difference between D4 receptor-deficient mice and their controls. In voltage-clamp recordings, there was an increase in frequency of spontaneous glutamate receptor-mediated inward currents without a change in mean amplitude in D2 receptor-deficient mutants. In WT mice, activation of D2 family receptors with quinpirole decreased spontaneous excitatory events and conversely sulpiride, a D2 receptor antagonist, increased activity. In D2 receptor-deficient mice, sulpiride had very little net effect. Morphologically, a subpopulation of medium-sized spiny neurons from D2 receptor-deficient mice displayed decreased dendritic spines compared with cells from WT mice. These results provide evidence that D2 receptors play an important role in the regulation of glutamate receptor-mediated activity in the corticostriatal or thalamostriatal pathway. These receptors may function as gatekeepers of glutamate release or of its subsequent effects and thus may protect striatal neurons from excessive excitation.

Differential sensitivity of medium- and large-sized striatal neurons to NMDA but not kainate receptor activation in the rat.

Infrared videomicroscopy and differential interference contrast optics were used to identify medium- and large-sized neurons in striatal slices from young rats. Whole-cell patch-clamp recordings were obtained to compare membrane currents evoked by application of N-methyl-d-aspartate (NMDA) and kainate. Inward currents and current densities induced by NMDA were significantly smaller in large- than in medium-sized striatal neurons. The negative slope conductance for NMDA currents was greater in medium- than in large-sized neurons and more depolarization was required to remove the Mg2+ blockade. In contrast, currents induced by kainate were significantly greater in large-sized neurons whilst current densities were approximately equal in both cell types. Spontaneous excitatory postsynaptic currents occurred frequently in medium-sized neurons but were relatively infrequent in large-sized neurons. Excitatory postsynaptic currents evoked by electrical stimulation were smaller in large- than in medium-sized neurons. A final set of experiments assessed a functional consequence of the differential sensitivity of medium- and large-sized neurons to NMDA. Cell swelling was used to examine changes in somatic area in both neuronal types after prolonged application of NMDA or kainate. NMDA produced a time-dependent increase in somatic area in medium-sized neurons whilst it produced only minimal changes in large interneurons. In contrast, application of kainate produced significant swelling in both medium- and large-sized cells. We hypothesize that reduced sensitivity to NMDA may be due to variations in receptor subunit composition and/or the relative density of receptors in the two cell types. These findings help define the conditions that put neurons at risk for excitotoxic damage in neurological disorders.