Nicotinic receptor modulation of the default mode network

RATIONALE

Previous neuroimaging studies of cognition involving nicotinic acetylcholine receptor (nAChR) agonist administration have repeatedly found enhanced task-induced deactivation of regions of the default mode network (DMN), a group of brain systems that is more active at rest and mediates task-independent thought processes. This effect may be related to pro-cognitive nAChR agonist effects OBJECTIVES: The present study sought to test whether nAChR modulation of the DMN is bi-directional, i.e., whether a nAChR antagonist would reduce task-induced deactivation.

METHODS

Eighteen healthy non-smokers underwent functional magnetic resonance imaging while performing a letter N-back task. Scans were performed after nicotine administration (7 mg/24 h, transdermally), after administration of the nAChR antagonist mecamylamine (7.5 mg, p.o.), and after double placebo, in counterbalanced sequence. Blood-oxygen-level-dependent (BOLD) signal was analyzed within ventromedial prefrontal cortex (vmPFC) and posterior cingulate cortex (PCC) regions of interest-central hubs of the DMN in which consistent nAChR agonist-induced changes had previously been identified.

RESULTS

Nicotine enhanced hit rate in both the 0-back and 2-back condition, while mecamylamine slowed reaction time in the 2-back condition. Mecamylamine reduced task-induced deactivation of vmPFC and PCC. Nicotine had no significant effects on the BOLD signal.

CONCLUSIONS

The finding that nAChR tone reduction by mecamylamine weakened task-induced DMN deactivation indicates that a constant tone of nAChR activation helps regulate DMN activity in healthy individuals. This suggests that low nAChR tone may play a causal role in DMN dysregulation seen in conditions such as mild cognitive impairment or Alzheimer's disease.

Load-dependent hyperdeactivation of the default mode network in people with schizophrenia

Schizophrenia is associated with impairment in a range of cognitive functions. Neuroimaging studies have reported lower, but also higher, task-induced activation accompanying impaired performance. Differences in task-load and the ability of people with schizophrenia (PSZ) to stay engaged in the cognitive operations probed appear to underlie such discrepancies. Similarly, task-induced deactivation of the default mode network (DMN) was weaker in PSZ relative to healthy control subjects (HCS) in most studies, but some reported greater deactivation. An inability to stay engaged in the cognitive operations could account for these discrepancies, too, as it would lead to more time off-task and consequently less deactivation of DMN functions. The present study employed a change detection paradigm with small to moderate set sizes (SSs) of 1, 2, and 4 items. Task training prior to fMRI scanning abolished the group difference in no-response trials. Task-positive regions of interest (ROIs) displayed greater activation with increasing SS in both groups. PSZ showed greater activation relative to HCS at SSs 1 and 2. DMN ROIs displayed greater deactivation with increasing SS in PSZ, but not in HCS, and PSZ tended to hyperdeactivate DMN regions at SS 4. No hypodeactivation was observed in PSZ. In conclusion, when minimizing differences in task-engagement, PSZ tend to over-recruit task-positive regions during low-load operations, and hyperdeactivate DMN functions at higher load, perhaps reflecting heightened non-specific vigilance or effort when dealing with cognitive challenges. This speaks against an inability to down-regulate task-independent thought processes as a primary mechanism underlying cognitive impairment in schizophrenia.

Hyperdeactivation of the Default Mode Network in People With Schizophrenia When Focusing Attention in Space

When studying selective attention in people with schizophrenia (PSZ), a counterintuitive but replicated finding has been that PSZ display larger performance benefits than healthy control subjects (HCS) by cues that predicts the location of a target stimulus relative to non-predictive cues. Possible explanations are that PSZ hyperfocus attention in response to predictive cues, or that an inability to maintain a broad attentional window impairs performance when the cue is non-predictive. Over-recruitment of regions involved in top-down focusing of spatial attention in response to predictive cues would support the former possibility, and an inappropriate recruitment of these regions in response to non-predictive cues the latter. We probed regions of the dorsal attention network while PSZ (N = 20) and HCS (N = 20) performed a visuospatial attention task. A central cue either predicted at which of 4 peripheral locations a target signal would appear, or it gave no information about the target location. As observed previously, PSZ displayed a larger reaction time difference between predictive and non-predictive cue trials than HCS. Activity in frontoparietal and occipital regions was greater for predictive than non-predictive cues. This effect was almost identical between PSZ and HCS. There was no sign of over-recruitment when the cue was predictive, or of inappropriate recruitment when the cue was non-predictive. However, PSZ differed from HCS in their cue-dependent deactivation of the default mode network. Unexpectedly, PSZ displayed significantly greater deactivation than HCS in predictive cue trials, which may reflect a tendency to expend more processing resources when focusing attention in space.

The relationship between working memory capacity and broad measures of cognitive ability in healthy adults and people with schizophrenia

OBJECTIVE

Working memory (WM) capacity, typically measured with cognitively complex span tasks, is correlated with higher order cognitive abilities in healthy adults. The goals of this study were to determine whether a more focused measure of visual WM storage capacity would show similar higher order ability correlations in healthy adults and in people with schizophrenia (PSZ), thereby demonstrating the importance of simple storage capacity; determine whether the illness alters the pattern of correlations across cognitive domains; and evaluate whether between-groups differences in WM capacity could account for the generalized cognitive impairment in PSZ.

METHOD

Ninety-nine PSZ and 77 healthy controls (HCs) completed a visual WM change-localization task, the Wechsler Abbreviated Scale of Intelligence (WASI), and the MATRICS Consensus Cognitive Battery (MCCB).

RESULTS

PSZ performed more poorly than HCs on all cognitive measures. The between-groups effect size for WM capacity was large (d = 1.11). WM robustly correlated with WASI and MCCB performance, with no significant differences in the magnitude or pattern of correlations across groups. When the groups were pooled, WM capacity correlated at r = .68 with MCCB composite score and at r = .56 with WASI estimated Full Scale IQ. WM capacity accounted for approximately 40% of the between-groups variance across the WASI and MCCB.

CONCLUSIONS

A simple measure of WM storage capacity is robustly associated with the higher order cognitive abilities assessed by the WASI and MCCB in HCs and PSZ. WM capacity reduction may be a critical determinant of the general cognitive impairment in PSZ.

A test of the cognitive self-medication hypothesis of tobacco smoking in schizophrenia

BACKGROUND

Heavier tobacco smoking among people with schizophrenia (SCZ) has been suggested to reflect self-medication of cognitive deficits. The idea that cognitive-enhancing effects of nicotine are a primary motivator of tobacco consumption in SCZ and that abstinence would deprive SCZ of such beneficial effects might explain hesitation among providers to pursue smoking cessation in SCZ. This study tested predictions of the cognitive self-medication hypothesis.

METHODS

In three counterbalanced sessions, 17 SCZ and 17 healthy control subjects (HCS), all smokers, were tested under ad libitum smoking or 3.5 hours after abstaining and receiving a nicotine (14 mg/24 hours) or placebo patch.

RESULTS

Attention task performance was improved by transdermal nicotine relative to placebo, with intermediate performance by ad libitum smoking. These effects were of similar size in SCZ and HCS and did not reflect remediation of functions disproportionately impaired in SCZ. Although more SCZ reported that the need to concentrate influenced their smoking, this was not reflected by the actual behavior of these patients. Self-reported ability to concentrate changed with nicotine status in HCS but not SCZ, suggesting insensitivity of SCZ to nicotine-derived performance benefits. Nicotine plasma concentrations after ad libitum smoking were not associated with performance benefits but instead with the propensity to experience nicotine withdrawal upon abstinence. This association was seen selectively in SCZ, suggesting a possible reason for heavier smoking.

CONCLUSIONS

These findings suggest that subjective or objective attentional benefits are unlikely the primary driving force of tobacco consumption in SCZ and should not discourage providers from supporting quit attempts.

Kraepelin and Bleuler had it right: people with schizophrenia have deficits sustaining attention over time

An inability to sustain attention was noted in the original clinical descriptions of schizophrenia, but the vast majority of experimental studies have failed to report a performance decrement over time, calling this observation into question. To test for such deficits when task conditions conform to basic science taxonomy for the validity of sustained attention tasks, a dynamic stimulus array was presented in which targets, differing subtly from standard stimuli, were presented infrequently and unpredictably. Both people with schizophrenia (PSZ, n=40) and healthy control subjects (HCS, n=29) displayed a reduction in hit rate and an increase in reaction time (RT) from the first to the second 5-min period. Thereafter, the hit rate of HCS recovered and remained stable, while that of PSZ continued to decline. When performance at task onset was equated between groups, the decrement over time in PSZ remained of the same robust magnitude. Thus, when the nature of the task challenges sustaining attention over time, PSZ display a clear deficit in this ability.

Visuospatial attention in schizophrenia: deficits in broad monitoring

Although selective attention is thought to be impaired in people with schizophrenia (PSZ), prior research has found no deficit in the ability to select one location and withdraw attention from another. PSZ and healthy control subjects (HCS) performed a stimulus detection task in which one, two, or all four peripheral target locations were cued. When one or two locations were cued, both PSZ and HCS responded faster when the target appeared at a cued than uncued location. However, increases in the number of validly cued locations had much more deleterious effects on performance for PSZ than HCS, especially for targets of low contrast whose detection was more dependent on attention. PSZ also responded more slowly in trials with four cued locations relative to trials with one or two invalidly cued locations. Thus, visuospatial attention deficits in schizophrenia arise when broad monitoring is required rather than when attention must be focused narrowly.

Failure of schizophrenia patients to overcome salient distractors during working memory encoding

BACKGROUND

Prior demonstrations of impaired attentional control in schizophrenia focused on conditions in which top-down control is needed to overcome prepotent response tendencies. Attentional control over stimulus processing has received little investigation. Here, we test whether attentional control is impaired during working memory encoding when salient distractors compete with less salient task-relevant stimuli.

METHODS

Patients with schizophrenia (n = 28) and healthy control subjects (n = 25) performed a visuospatial working memory paradigm in which half of the to-be-encoded stimuli flickered to increase their salience. After a 2-second delay, stimuli reappeared and participants had to decide whether or not a probed item had shifted location.

RESULTS

In the unbiased condition where flickering and nonflickering stimuli were equally likely to be probed, both groups displayed a trend toward better memory for the flickering items. In the flicker-bias condition in which the flickering stimuli were likely to be probed, both groups displayed a robust selection advantage for the flickering items. However, in the nonflicker-bias condition in which the nonflickering stimuli were likely to be probed, only healthy control subjects showed selection of the nonflickering items. Patients displayed a trend toward preferential memory for the flickering items, as in the unbiased condition.

CONCLUSIONS

Both groups were able to select salient over nonsalient stimuli, but patients with schizophrenia were unable to select nonsalient over salient stimuli, consistent with impairment in the effortful control of attention. These findings demonstrate the generality of top-down control failure in schizophrenia in the face of bottom-up competition from salient stimuli as with prepotent response tendencies.

Chromosomal aberrations induced by defined DNA double-strand breaks: the origin of achromatic lesions

The mechanisms of formation of chromosomal aberrations are poorly understood, despite the common use of aberrations as a measure of the genetic effects of physical and chemical agents. We have used restriction endonucleases to introduce defined DNA double-strand breaks into mammalian cells, and measured chromosomal aberration formation relative to the activity of the endonuclease. The endonucleases AluI and Sau3AI remain active for a relatively short time under simulated cellular conditions and induce achromatic lesions ('gaps') in chromatids only within the first hour or two following treatment. In contrast, the endonuclease MboI (an isoschizomer of Sau3AI) is active for an extremely long time and continues to produce chromatid gaps during the whole 12 hr sampling period. This observation strongly suggests that the aberrations classified as gaps are a manifestation of unrejoined DNA double-strand breaks. The formation of gaps may relate to the opportunities for repair of DNA breaks in relation to cell-cycle position. It is more difficult to relate the formation of structural chromatid aberrations to the endonuclease activity, although at relatively low concentrations all 3 endonucleases gave similar levels of structural aberrations.

Investigating the nature of chromatid breaks produced by restriction endonucleases

It is a basic assumption of the breakage-and-reunion theory that the majority of open chromatid breaks seen at metaphase are the residue of unrejoined primary breaks that have neither restituted nor rejoined illegitimately to form exchange aberrations. If Chinese hamster chromosomes with BrdU sister-chromatid differentiation are irradiated, and chromatid aberrations scored from G2 cells, some 15-20% of open breaks show a colour-jump at the point of discontinuity, indicating a two-lesion intrachange origin. Since we see complete forms of several intrachanges whose incomplete forms will also look like breaks, but devoid of a colour-jump, it appears that a substantial proportion of observed breaks are intrachange derived. Experiments to date show that the colour-jump proportion is constant, irrespective of radiation dose, radiation quality, BrdU concentration and hamster cell origin. It is the same for the very low "spontaneous' breaks found in control samples. Restriction endonucleases (RE) can be introduced into cells by various poration methods, and are highly efficient at producing all types of aberrations. This is taken as strong evidence that DNA dsb are significant lesions triggering aberrations. One might anticipate, therefore, that observed breaks will be predominantly unrejoined dsb, and the proportion of colour-jump break correspondingly low. We tested this supposition using three RE; Alu 1, a blunt-end cutter, Sau3A 1, a cohesive-end cutter, both with a short life-time in vivo, and Mbo 1, an isoschizomer of Sau3A 1, which has a long cutting life-time in vivo. Although there were differences in absolute yields of breaks, and of relative frequencies of aberration types recovered, the proportion of colour-jump breaks was as high as that in a parallel X-ray experiment, and fell well within the range encountered in all our previous experiments. It is difficult to reconcile this universal constancy of colour-jump breaks with the expectations of breakage-and-reunion theory, where the occurrence of two-break events must be a treatment variable. Rather, our results suggest that most open breaks are secondary, resulting from a regular intrachange processing mechanism.

Electroporation and streptolysin O--a comparison of poration techniques

CHO (Chinese hamster ovary), xrs5 (X-ray sensitive Chinese hamster) and HF19 (untransformed human fibroblast) cells, were exposed to a lethal dose of the restriction enzyme Pvu II during electroporation or poration with the bacterial toxin streptolysin O. The uptake of the exclusion dye trypan blue was used as a measure of poration and compared with survival as measured by subsequent colony formation. It was assumed that any surviving cells had not been permeabilized and therefore did not receive any restriction enzyme. Electroporation alone proved to be more cytotoxic to the cells, whilst streptolysin O was more efficient at permeabilizing both hamster and human cells.

Chromatid damage induced by 238Pu alpha-particles in G2 and S phase Chinese hamster V79 cells

The comparative induction of chromatid aberrations by 238Pu alpha-particles, or by 250 kVp X-rays was investigated in V79 Chinese hamster cells. Metaphases were sampled at hourly intervals postirradiation up to 8 h and BrdU/FPG staining methods were used to distinguish G2, S and G1 phase cells. Two experiments were performed. In the first, an alpha-particle dose of 0.41 Gy was compared with an X-ray dose of 1.5 Gy used in a previously published study. In the second, an X-ray dose of 1.2 Gy was used in parallel with 0.41 Gy of alpha-particles to produce a similar overall frequency of interchanges, and allow comparative ratios to be derived for other aberration types. At these isoexchange doses, alpha-particles produce relatively less gaps and breaks, particularly in late G2, and significantly more isochromatid deletions. A very high proportion of the isochromatid deletions were incomplete after alpha-particles compared with X-rays, but no difference in incompleteness was found for interchanges. With X-rays, about 6% of interchanges are complex intra-interchange forms. At similar exchange frequencies this increases to 26.7% for alpha-irradiation, suggesting increased multiple lesion interaction. Differences in dose distribution between alpha-particles and X-rays are discussed and mitotic delay is examined after separation of the analysed cells into damaged and undamaged classes.

A case of caffeine-mediated cancellation of mitotic delay without enhanced breakage in V79 cells

Chinese hamster cells (V79 379A) were grown for 17 h in the presence of 10 micrograms/ml bromodeoxyuridine (BrdU) to obtain cells with potential sister-chromatid differentiation. At this time batches were irradiated (1.5 Gy, 250 kVp X-rays) and the medium of all flasks replaced with one containing 10 micrograms/ml thymidine with or without 400 micrograms/ml caffeine. Metaphases from irradiated and unirradiated batches were sampled every hour for 7 h and FPG stained. All categories of chromatid-type aberrations were scored in G2 and S phase cells. As expected, mitotic delay in the presence of caffeine, (measured by a shift in the portion of the fraction of differentially stained metaphases (FDM) curve and a reduced fall in the mitotic index) was largely cancelled, but there was a negligible increase in chromatid-aberration frequency (all categories), only achieving significance if the consistency of the whole 7-h sampling period was considered. Caffeine had no effect on the frequencies of light/dark chromatid involvement, nor in the completeness of chromatid interchanges. We conclude that the enhanced breakage frequency often observed with post-irradiation caffeine treatment is not necessarily causally related to the cancellation of delay.

Revell revisited

The controversy of the Classic versus the Exchange theories for the origin of simple chromatid breaks is outlined. Using BrdU harlequin sister-chromatid differentiation four Revell ratios can be defined and these have been obtained and tested as a block in V79 hamster cells. The values are quite different from the simple predictions. However, values similar to those observed (taken as a block) can be readily simulated from Revell theory by making the assumption that intra-chromatid events are dominant (0.7:0.3). They can also be obtained from contingency modelling of Classic theory using the same assumption plus the additional constraint that there is no contribution from isolated single lesions (Poisson class 1). If this latter assumption is correct, then the frequency of breaks involving a colour-jump (ratio III) should not decline to zero as the dose falls. A dose-response experiment shows that it does not, but remains approximately constant at about 12%, even in the unirradiated control. An added complication arises when we discover for the TB/BB situation, that whilst neither breaks nor gaps show any excess BB involvement (sensitisation), lesions involved in interchanges show at least a 2-fold BB excess. Clearly, the chromatid discontinuities we are scoring are not behaving as would be expected of a residue of unrejoined primary breaks (Classic theory) and we infer also that they are not 'simple'.