Reconstitution of immune cell populations in multiple sclerosis patients after autologous stem cell transplantation

Multiple sclerosis is an inflammatory T cell-mediated autoimmune disease. In a Phase II clinical trial, high-dose immunosuppressive therapy combined with autologous CD34⁺ haematopoietic stem cell transplant resulted in 69·2% of subjects remaining disease-free without evidence of relapse, loss of neurological function or new magnetic resonance imaging (MRI) lesions to year 5 post-treatment. A combination of CyTOF mass cytometry and multi-parameter flow cytometry was used to explore the reconstitution kinetics of immune cell subsets in the periphery post-haematopoietic cell transplant (HSCT) and the impact of treatment on the phenotype of circulating T cells in this study population. Repopulation of immune cell subsets progressed similarly for all patients studied 2 years post-therapy, regardless of clinical outcome. At month 2, monocytes and natural killer (NK) cells were proportionally more abundant, while CD4 T cells and B cells were reduced, relative to baseline. In contrast to the changes observed at earlier time-points in the T cell compartment, B cells were proportionally more abundant and expansion in the proportion of naive B cells was observed 1 and 2 years post-therapy. Within the T cell compartment, the proportion of effector memory and late effector subsets of CD4 and CD8 T cells was increased, together with transient increases in proportions of CD45RA-regulatory T cells (T_regs ) and T helper type 1 (Th1 cells) and a decrease in Th17·1 cells. While none of the treatment effects studied correlated with clinical outcome, patients who remained healthy throughout the 5-year study had significantly higher absolute numbers of memory CD4 and CD8 T cells in the periphery prior to stem cell transplantation.

The interaction between physical activity and obesity gene variants in association with BMI: Does the obesogenic environment matter?

Little is known about how obesity susceptibility single nucleotide polymorphisms (SNPs) interact with moderate to vigorous physical activity (MVPA) in relation to BMI during adolescence, once obesogenic neighborhood factors are accounted for. In race stratified models, including European (EA; N=4977), African (AA; N=1726), and Hispanic Americans (HA; N=1270) from the National Longitudinal Study of Adolescent to Adult Health (1996; ages 12-21), we assessed the evidence for a SNPxMVPA interaction with BMI-for-age Z score, once accounting for obesogenic neighborhood factors including physical activity amenities, transportation and recreation infrastructure, poverty and crime. Eight SNPxMVPA interactions with suggestive significance (p<0.10; three in each EA, and AA, two in HA) were observed showing attenuation on BMI-for-age Z score in adolescents with ≥5 versus <5 bouts/week MVPA, except for rs10146997 (near NRXN3). Findings were robust to the inclusion of neighborhood-level variables as covariates. These findings suggest that any attenuation from MVPA on a genetic susceptibility to obesity during adolescence is likely not operating through obesogenic neighborhood factors.

Influence of SNP*SNP interaction on BMI in European American adolescents: findings from the National Longitudinal Study of Adolescent Health

BACKGROUND

Adolescent obesity is predictive of future weight gain, obesity and adult onset severe obesity (body mass index [BMI] ≥40 kg m(-2) ). Despite successful efforts to identify Single Nucleotide Polymorphisms (SNPs) influencing BMI, <5% of the 40-80% heritability of the phenotype has been explained. Identification of gene-gene (G-G) interactions between known variants can help explain this hidden heritability as well as identify potential biological mechanisms affecting weight gain during this critical developmental period.

OBJECTIVE

We have recently shown distinct genetic effects on BMI across the life course, and thus it is important to examine the evidence for epistasis in adolescence.

METHODS

In adolescent participants of European descent from wave II of the National Longitudinal Study of Adolescent Health (Add Health, n = 5072, ages 12-21, 52.5% female), we tested 34 established BMI-related SNPs for G-G interaction effects on BMI z-score. We used mixed-effects regression, assuming multiplicative interaction models adjusting for age, sex and geographic region, with random effects for family and school.

RESULTS

For 28 G-G interactions that were nominally significant (P < 0.05), we attempted to replicate our results in an adolescent sample from the Childhood European American Cohort from Philadelphia. In the replication study, one interaction (PRKD1-FTO) was significant after correction for multiple testing.

CONCLUSIONS

Our results are suggestive of epistatic effects on BMI during adolescence and point to potentially interactive effects between genes in biological pathways important in obesity.

Moderate to vigorous physical activity interactions with genetic variants and body mass index in a large US ethnically diverse cohort

BACKGROUND

Little is known about the interaction between genetic and behavioural factors during lifecycle risk periods for obesity and how associations vary across race/ethnicity.

OBJECTIVE

The objective of this study was to examine joint associations of adiposity-related single-nucleotide polymorphisms (SNPs) and moderate to vigorous physical activity (MVPA) with body mass index (BMI) in a diverse adolescent cohort.

METHODS

Using data from the National Longitudinal Study of Adolescent Health (n = 8113: Wave II 1996; ages 12-21, Wave III; ages 18-27), we assessed interactions of 41 well-established SNPs and MVPA with BMI-for-age Z-scores in European Americans (EA; n = 5077), African-Americans (AA; n = 1736) and Hispanic Americans (HA; n = 1300).

RESULTS

Of 97 assessed, we found nominally significant SNP-MVPA interactions on BMI-for-age Z-score in EA at GNPDA2 and FTO and in HA at LZTR2/SEC16B. In EA, the estimated effect of the FTO risk allele on BMI-for-age Z-score was lower (β = -0.13; 95% confidence interval [CI]: 0.08, 0.18) in individuals with ≥5 vs. <5 (β = 0.24; CI: 0.16, 0.32) bouts of MVPA per week (P for interaction 0.02). Race/ethnicity-pooled meta-analysis showed nominally significant interactions for SNPs at TFAP2B, POC5 and LYPLAL1.

CONCLUSIONS

High MVPA may attenuate underlying genetic risk for obesity during adolescence, a high-risk period for adult obesity.

Screen time behaviours may interact with obesity genes, independent of physical activity, to influence adolescent BMI in an ethnically diverse cohort

BACKGROUND

There has been little investigation of gene-by-environment interactions related to sedentary behaviour, a risk factor for obesity defined as leisure screen time (ST; i.e. television, video and computer games).

OBJECTIVE

To test the hypothesis that limiting ST use attenuates the genetic predisposition to increased body mass index (BMI), independent of physical activity.

DESIGN

Using 7642 wave II participants of the National Longitudinal Study of Adolescent Health, (Add Health; mean = 16.4 years, 52.6% female), we assessed the interaction of ST (h week(-1) ) and 41 established obesity single nucleotide polymorphisms (SNPs) with age- and sex-specific BMI Z-scores in 4788 European-American (EA), 1612 African-American (AA) and 1242 Hispanic American (HA) adolescents.

RESULTS

Nominally significant SNP*ST interaction were found for FLJ35779 in EA, GNPDA2 in AA and none in HA (EA: beta [SE] = 0.016[0.007]), AA: beta [SE] = 0.016[0.011]) per 7 h week(-1) ST and one risk allele in relation to BMI Z-score.

CONCLUSIONS

While for two established BMI loci, we find evidence that high levels of ST exacerbate the influence of obesity susceptibility variants on body mass; overall, we do not find strong evidence for interactions between the majority of established obesity loci. However, future studies with larger sample sizes, or that may build on our current study and the growing published literature, are clearly warranted.

Estimation of genetic effects on BMI during adolescence in an ethnically diverse cohort: The National Longitudinal Study of Adolescent Health

OBJECTIVE

The contribution of genetic variants to body mass index (BMI) during adolescence across multiethnic samples is largely unknown. We selected genetic loci associated with BMI or obesity in European-descent samples and examined them in a multiethnic adolescent sample.

DESIGN AND SAMPLE

In 5103 European American (EA), 1748 African American (AfA), 1304 Hispanic American (HA) and 439 Asian American (AsA) participants of the National Longitudinal Study of Adolescent Health (Add Health; ages 12-21 years, 47.5% male), we assessed the association between 41 established obesity-related single-nucleotide polymorphisms (SNPs) with BMI using additive genetic models, stratified by race/ethnicity, and in a pooled meta-analysis sample. We also compared the magnitude of effect for BMI-SNP associations in EA and AfA adolescents to comparable effect estimates from 11 861 EA and AfA adults in the Atherosclerosis Risk in Communities study (ages 45-64 years, 43.2% male).

RESULTS

Thirty-five of 41 BMI-SNP associations were directionally consistent with published studies in European populations, 18 achieved nominal significance (P<0.05; effect sizes from 0.19 to 0.71 kg m(-2) increase in BMI per effect allele), while 4 (FTO, TMEM18, TFAP2B, MC4R) remained significant after Bonferroni correction (P<0.0015). Of 41 BMI-SNP associations in AfA, HA and AsA adolescents, nine, three and five, respectively, were directionally consistent and nominally significant. In the pooled meta-analysis, 36 of 41 effect estimates were directionally consistent and 21 of 36 were nominally significant. In EA adolescents, BMI effect estimates were larger (P<0.05) for variants near TMEM18, PTER and MC4R and smaller for variants near MTIF3 and NRXN3 compared with EA adults.

CONCLUSION

Our findings suggest that obesity susceptibility loci may have a comparatively stronger role during adolescence than during adulthood, with variation across race/ethnic subpopulation.

Transesophageal echocardiographic evaluation of aortitis

Aortitis is an uncommon infection which may occur in patients with preexisting atherosclerotic disease of the aorta. The clinical features in two patients presenting with fever and nonspecific symptoms are reviewed. In these patients, transesophageal echocardiographic features of wall thickening and false aneurysm formation were suggestive of the diagnosis of aortitis. Both patients were taken for surgical excision of the descending aorta and subsequently improved.

Dense core vesicles resemble active-zone transport vesicles and are diminished following synaptogenesis in mature hippocampal slices

Large dense core vesicles (approximately 100 nm) contain neuroactive peptides and other co-transmitters. Smaller dense core vesicles (approximately 80 nm) are known to contain components of the presynaptic active zone and thought to transport and deliver these components during developmental synaptogenesis. It is not known whether excitatory axons in area CA1 contain such dense core vesicles, and whether they contribute to synaptic plasticity of mature hippocampus. Serial section electron microscopy was used to identify dense core vesicles in presynaptic axons in s. radiatum of area CA1 in adult rat hippocampus. Comparisons were made among perfusion-fixed hippocampus and hippocampal slices that undergo synaptogenesis during recovery in vitro. Dense core vesicles occurred in 26.1+/-3.6% of axonal boutons in perfusion fixed hippocampus, and in only 17.6+/-4.5% of axonal boutons in hippocampal slices (P<0.01). Most of the dense core vesicle positive boutons contained only one dense core vesicle, and no reconstructed axonal bouton had more than a total of 10 dense core vesicles in either condition. Overall the dense core vesicles had average diameters of 79+/-11 nm. These small dense core vesicles were usually located near nonsynaptic membranes and rarely occurred near the edge of a presynaptic active zone. Their size, low frequency, locations, and decrease following recuperative synaptogenesis in slices are novel findings that merit further study with respect to small dense core vesicle content and possible contributions to synapse assembly and plasticity in the mature hippocampus.

Dendritic spines disappear with chilling but proliferate excessively upon rewarming of mature hippocampus

More dendritic spine synapses occur on mature neurons in hippocampal slices by 2 h of incubation in vitro, than in perfusion-fixed hippocampus. What conditions initiate this spinogenesis and how rapidly do the spines begin to proliferate on mature neurons? To address these questions, CA1 field of the hippocampus neurons expressing green fluorescent protein in living slices from mature mice were imaged with two-photon microscopy. Spines disappeared and dendrites were varicose immediately after slice preparation in ice-cold artificial cerebrospinal fluid (ACSF). Electron microscopy (EM) revealed disrupted dendritic cytoplasm, enlarged or free-floating postsynaptic densities, and excessive axonal endocytosis. Upon warming dendritic varicosities shrank and spines rapidly reappeared within a few minutes illustrating the remarkable resilience of mature hippocampal neurons in slices. When membrane impermeant sucrose was substituted for NaCl in ACSF dendrites remained spiny at ice-cold temperatures and EM revealed less disruption. Nevertheless, spine number and length increased within 30 min in warm ACSF even when the extracellular calcium concentration was zero and synaptic transmission was blocked. When slices were first recovered for several hours and then chilled in 6 degrees C ACSF many spines disappeared and the dendrites became varicose. Upon re-warming varicosities shrank and spines reemerged in the same position from which they disappeared. In addition, new spines formed and spines were longer suggesting that chilling, not the initial injury from slicing, caused the spines to disappear while re-warming triggered the spine proliferation on mature neurons. The new spines might be a substrate for neuronal recovery of function, when neurons have been chilled or exposed to other traumatic conditions that disrupt ionic homeostasis.

Confirmation by DNA analysis that Contarinia maculipennis (Diptera: Cecidomyiidae) is a polyphagous pest of orchids and other unrelated cultivated plants

The cytochrome oxidase subunit I gene in mitochondrial DNA of 53 larvae of Contarinia maculipennis Felt from flower buds of various host plants, collected from Hawaii, Japan and Thailand was analysed. Monophyly of the clade including C. maculipennis from Hawaii, Thailand and Japan was supported. There was no sequential variation within the specimens from Hawaii and Japan, which differed from one another by 6 bp (1.37%). Three haplotypes were recognized in specimens from Thailand but differences from Hawaiian and Japanese specimens were small. Overall, there were no differences in the 146 deduced amino acid residues. It is therefore concluded that C. maculipennis is a polyphagous species that can develop on plant hosts representing at least seven botanical families. This pest of Dendrobium flower buds in glasshouses is considered to have entered Hawaii, Florida and Japan from Southeast Asia, and was recently intercepted in the Netherlands. Infestations have established and spread in orchid glasshouses, causing concern about the possibility of more extensive damage to orchids and to crops, such as bitter gourd, grown in close proximity to orchid glasshouses in Japan. The potential usefulness of DNA analysis in determining host plant ranges of morphologically identical cecidomyiid species that are currently identified solely on differences of host plant is emphasized.

Effect of endoscopic ultrasonography on the management of 100 consecutive patients with oesophageal and junctional carcinoma

BACKGROUND

Endoscopic ultrasonography (EUS) offers very accurate tumour and node staging information for oesophagogastric cancer. The aim was to determine whether the addition of EUS directly influenced the definitive management plan for individual patients.

METHODS

Personal and staging information from 100 consecutive patients with carcinoma of the oesophagus or oesophagogastric junction were summarized and blinded. Three consultant oesophagogastric surgeons independently made a management decision for each patient, in the presence and absence of the EUS data. All scored their perceived value of the EUS staging data for each patient.

RESULTS

EUS was deemed useful in 63-87 per cent of patients and its addition resulted in an increased number of concordant management plans (from 53 to 62 per cent), and increased agreement between surgeons. The greatest change in concordant management was an increased referral of patients for non-surgical palliation.

CONCLUSION

The addition of EUS to the staging of patients with oesophageal and oesophagogastric junction cancer significantly altered the management strategy for some of these patients.

The initial United States experience with the ATS mechanical cardiac valve prosthesis

From January 1, 1997 through June 30, 2000, 224 patients underwent valve replacement with the ATS Medical cardiac valve prosthesis under a USFDA-approved investigational device exemption study. Aortic valve replacement (AVR) was conducted in 152 patients (39 with coronary bypass) and mitral replacement (MVR) in 72 patients (18 with coronary bypass). Overall operative mortality was 1.8% (AVR = 2.8%, MVR = 0%), with only one valve-related death. In 372 patient-years of follow-up, there were an additional four patient deaths, two of which were valve related following a stroke. Valve-related complications included: thromboembolism (linearized rate = 3.8% per patient year), of which 3/11 had chronic deficits (0.8% per patient year); thrombosis (1 MVR = 0.8% per patient year); paravalvular leak (1 AVR = 0.4% per patient year); anticoagulant-related hemorrhage (1 AVR and 5 MVR = 1.6% per patient year) with no patient mortality; prosthetic valve endocarditis (1 MVR = 0.8% per patient year); and valve dysfunction (0%). Echocardiographic gradients were proportional to valve size and did not significantly change over the follow-up period. This study documented the ATS Medical prosthesis to be a valuable addition to the surgeon's armamentarium in the treatment of cardiac valvular disease.

A systematic review of the staging performance of endoscopic ultrasound in gastro-oesophageal carcinoma

BACKGROUND

Endoscopic ultrasound (EUS) may be used for preoperative staging of gastro-oesophageal carcinoma but performance values given in the literature differ.

AIMS

To identify and synthesise findings from all articles on the performance of EUS in tumour, node, metastasis (TNM) staging of gastro-oesophageal carcinoma.

SOURCE

Published and unpublished English language literature, 1981-1996.

METHODS

Data on the staging performance of EUS were retrieved and evaluated. Summary receiver operator characteristic methodology was used for synthesis, and a summary estimate of performance, Q*, obtained. Multiple regression analysis was used to assess study validity and investigate reasons for differences in performance.

RESULTS

Twenty seven primary articles were assessed in detail. Thirteen supplied results for staging oesophageal cancer, 13 for gastric cancer, and four for cancers at the gastro-oesophageal junction. For gastric T staging, Q*=0.93 (95% confidence interval (CI) 0.91-0.95) and for oesophageal T staging, Q*=0.89 (95% CI 0.88-0.92). For gastro-oesophageal T staging, including cancers at the gastro-oesophageal junction, Q*=0.91 (95% CI 0.89-0.93). Inclusion of cases with non-traversable stenosis was found to slightly reduce staging performance. For N staging, Q*=0.79 (95% CI 0.75-0.83). In articles that compared EUS directly with incremental computed tomography, EUS performed better. None of the variables assessed in the regression analysis was significant using a Bonferroni correction. Three variables (anatomical location, traversability, and blinding) showed strong relationships for future research and validation.

CONCLUSIONS

EUS is highly effective for discrimination of stages T1 and T2 from stages T3 and T4 for primary gastro-oesophageal carcinomas. The failure rate of EUS from non-traversability of a stenotic cancer may be a limitation in some patient groups.

Three-dimensional comparison of ultrastructural characteristics at depressing and facilitating synapses onto cerebellar Purkinje cells

Cerebellar Purkinje cells receive two distinctive types of excitatory inputs. Climbing fiber (CF) synapses have a high probability of release and show paired-pulse depression (PPD), whereas parallel fiber (PF) synapses facilitate and have a low probability of release. We examined both types of synapses using serial electron microscopic reconstructions in 15-d-old rats to look for anatomical correlates of these differences. PF and CF synapses were distinguishable by their overall ultrastructural organization. There were differences between PF and CF synapses in how many release sites were within 1 microm of a mitochondrion (67 vs 84%) and in the degree of astrocytic ensheathment (67 vs 94%). However, the postsynaptic density sizes for both types of synapses were similar (0.13-0.14 microm(2)). For both types of synapses, we counted the number of docked vesicles per release site to test whether this number determines the probability of release and synaptic plasticity. PF and CF synapses had the same number of anatomically docked vesicles (7-8). The number of docked vesicles at the CF does not support a simple model of PPD in which release of a single vesicle during the first pulse depletes the anatomically docked vesicle pool at a synapse. Alternatively, only a fraction of anatomically docked vesicles may be release ready, or PPD could result from multivesicular release at each site. Similarities in the number of docked vesicles for PF and CF synapses indicate that differences in probability of release are unrelated to the number of anatomically docked vesicles at these synapses.

Sibling, peer, neighbor, and schoolmate correlations as indicators of the importance of context for adolescent development

We use nationally representative data to calculate correlations in achievement and delinquency between genetically differentiated siblings within a family, between peers as defined by adolescents' "best friend" nominations, between schoolmates living in the same neighborhood, and between grademates within a school. We find the largest correlations between siblings, especially identical twins. Grademate and neighbor correlations are small. Peer-based correlations are considerably larger than grademate and neighbor correlations but not larger than most sibling correlations. The data suggest that family-based factors are several times more powerful than neighborhood and school contexts in affecting adolescents' achievement and behavior.

Cylindrical diameters method for calibrating section thickness in serial electron microscopy

An estimate of section thickness is required for measuring structures in serial section microscopy. Mean section thickness is estimated reliably by averaging the ratios of the diameters of cylindrical objects, such as mitochondria, to the number of sections they span. This cylindrical diameters method improves the accuracy of section thickness as inferred from the colour of sections floating in water. The cylindrical diameters method gives the same answer as that obtained by the minimal folds

METHOD

It is preferable because it can be done in a series that has no folds that can distort and obscure the objects that are being measured.

Overview on the structure, composition, function, development, and plasticity of hippocampal dendritic spines

There has been an explosion of new information on the neurobiology of dendritic spines in synaptic signaling, integration, and plasticity. Novel imaging and analytical techniques have provided important new insights into dendritic spine structure and function. Results are accumulating across many disciplines, and a step toward consolidating some of this work has resulted in Dendritic Spines of the Hippocampus. Leaders in the field provide a discussion at the level of advanced under-graduates, with sufficient detail to be a contemporary resource for research scientists. Critical reviews are presented on topics ranging from spine structure, formation, and maintenance, to molecular composition, plasticity, and the role of spines in learning and memory. Dendritic Spines of the Hippocampus provides a timely discussion of our current understanding of form and function at these excitatory synapses. We asked authors to include areas of controversy in their papers so as to distinguish results that are generally agreed upon from those where multiple interpretations are possible. We thank the contributors for their insights and thoughtful discussions. In this paper we provide background on the structure, composition, function, development, plasticity, and pathology of hippocampal dendritic spines. In addition, we highlight where each of these subjects will be elaborated upon in subsequent papers of this special issue of Hippocampus.

Extending unbiased stereology of brain ultrastructure to three-dimensional volumes

OBJECTIVE

Analysis of brain ultrastructure is needed to reveal how neurons communicate with one another via synapses and how disease processes alter this communication. In the past, such analyses have usually been based on single or paired sections obtained by electron microscopy. Reconstruction from multiple serial sections provides a much needed, richer representation of the three-dimensional organization of the brain. This paper introduces a new reconstruction system and new methods for analyzing in three dimensions the location and ultrastructure of neuronal components, such as synapses, which are distributed non-randomly throughout the brain.

DESIGN AND MEASUREMENTS

Volumes are reconstructed by defining transformations that align the entire area of adjacent sections. Whole-field alignment requires rotation, translation, skew, scaling, and second-order nonlinear deformations. Such transformations are implemented by a linear combination of bivariate polynomials. Computer software for generating transformations based on user input is described. Stereological techniques for assessing structural distributions in reconstructed volumes are the unbiased bricking, disector, unbiased ratio, and per-length counting techniques. A new general method, the fractional counter, is also described. This unbiased technique relies on the counting of fractions of objects contained in a test volume. A volume of brain tissue from stratum radiatum of hippocampal area CA1 is reconstructed and analyzed for synaptic density to demonstrate and compare the techniques.

RESULTS AND CONCLUSIONS

Reconstruction makes practicable volume-oriented analysis of ultrastructure using such techniques as the unbiased bricking and fractional counter methods. These analysis methods are less sensitive to the section-to-section variations in counts and section thickness, factors that contribute to the inaccuracy of other stereological methods. In addition, volume reconstruction facilitates visualization and modeling of structures and analysis of three-dimensional relationships such as synaptic connectivity.

The mechanisms mediating the effects of poverty on children's intellectual development

Although adverse consequences of poverty for children are documented widely, little is understood about the mechanisms through which the effects of poverty disadvantage young children. In this analysis we investigate multiple mechanisms through which poverty affects a child's intellectual development. Using data from the NLSY and structural equation models, we have constructed five latent factors (cognitive stimulation, parenting style, physical environment, child's ill health at birth, and ill health in childhood) and have allowed these factors, along with child care, to mediate the effects of poverty and other exogenous variables. We produce two main findings. First, the influence of family poverty on children's intellectual development is mediated completely by the intervening mechanisms measured by our latent factors. Second, our analysis points to cognitive stimulation in the home, and (to a lesser extent) to parenting style, physical environment of the home, and poor child health at birth, as mediating factors that are affected by lack of income and that influence children's intellectual development.

Identifying studies for systematic reviews. An example from medical imaging

OBJECTIVES

To determine if published figures on the proportion of articles included in systematic reviews and identified in electronic databases are applicable to an example from medical imaging.

METHODS

A systematic review was performed. Additionally, sensitivity and precision of a MEDLINE search were compared with values from three published searches, each customized for a specific field.

RESULTS

All articles included in the systematic review were in electronic databases. The MEDLINE search had low precision compared with searches in other fields.

CONCLUSIONS

In a specific area of medical imaging, electronic databases, including MEDLINE, are reliable sources of articles.

Congenital myasthenic syndrome with sleep hypoventilation

We report the case of a 13-year-old boy with acetylcholinesterase deficiency, a congenital myasthenic syndrome, who developed sleep hypoventilation syndrome during a period of rapid growth. His symptoms were insidious and life-threatening despite changes in strength or lung volume measurements that were not marked. He responded well to noninvasive nocturnal ventilation, with reversal of symptoms and normalization of blood gases. His lung volumes, but not motor function, improved after treatment.

Magnitude of left ventricular hypertrophy and risk of sudden death in hypertrophic cardiomyopathy

BACKGROUND

Sudden death is known to be a possible consequence of hypertrophic cardiomyopathy. Quantification of the risk of sudden death, however, remains imprecise for most patients with this disease.

METHODS

We assessed the relation between the magnitude of left ventricular hypertrophy and mortality in 480 consecutive patients with hypertrophic cardiomyopathy. The patients were categorized into five subgroups according to maximal wall thickness: 15 mm or less, 16 to 19 mm, 20 to 24 mm, 25 to 29 mm, and 30 mm or more. Their ages ranged from 1 to 89 years (median, 47).

RESULTS

Over a mean follow-up period of 6.5 years, 65 of the 480 patients (14 percent) died: 23 suddenly, 15 of heart failure, and 27 of noncardiac causes or stroke. The risk of sudden death increased progressively and in direct relation to wall thickness (P=0.001), ranging from 0 per 1000 person-years (95 percent confidence interval, 0 to 14.4) for a wall thickness of 15 mm or less to 18.2 per 1000 person-years (95 percent confidence interval, 7.3 to 37.6) for a wall thickness of 30 mm or more and almost doubling from each wall-thickness subgroup to the next. The cumulative risk 20 years after the initial evaluation was close to zero for patients with a wall thickness of 19 mm or less but almost 40 percent for wall thicknesses of 30 mm or more. As compared with the other subgroups, patients with extreme hypertrophy were the youngest (mean age, 31 years), and most (41 of 43) had mild symptoms or no symptoms; of the 12 patients who were less than 18 years old at the initial evaluation, 5 died suddenly.

CONCLUSIONS

In hypertrophic cardiomyopathy, the magnitude of hypertrophy is directly related to the risk of sudden death and is a strong and independent predictor of prognosis. Young patients with extreme hypertrophy, even those with few or no symptoms, appear to be at substantial long-term risk and deserve consideration for interventions to prevent sudden death. The majority of patients with mild hypertrophy are at low risk and can be reassured regarding their prognosis.

A case of confirmed nonbacterial thrombotic endocarditis with correlative transesophageal echocardiographic findings

The diagnosis of nonbacterial thrombotic endocarditis is rarely made during life. We describe a patient with non-Hodgkin's lymphoma with evidence of systemic embolism in which transesophageal echocardiography was useful in establishing the diagnosis. The clinical and echocardiographic features of nonbacterial thrombotic endocarditis should be remembered when a valvular mass is seen on echocardiography.

Valvular heart disease. Identifying and managing mitral and aortic lesions

Evaluation and treatment of valvular heart disease (table 1) are in evolution. Echocardiography is the principal diagnostic tool. In general, mitral and aortic stenotic lesions warrant surgical treatment when symptoms appear. Such intervention is also indicated in symptomatic patients with a regurgitant mitral or aortic lesion. Some patients with regurgitant lesions require surgical treatment even before the onset of symptoms, such as when left ventricular dysfunction or dilatation supervenes. Surgical treatment of valvular lesions consists of repair or replacement, except in the case of mitral stenosis, where PBMC is a reasonable alternative. In general, medical therapy has a very limited role in the treatment of valvular heart disease. Severe aortic insufficiency is an exception. In such cases, afterload reduction has proved beneficial in the long-term prognosis.

Dendrites are more spiny on mature hippocampal neurons when synapses are inactivated

Dendrites of CA1 pyramidal neurons in mature rat hippocampal slices were exposed to different levels of synaptic activation. In some slices, synaptic transmission was blocked with glutamate receptor antagonists, sodium and calcium channel blockers and/or a nominally calcium-free medium with high magnesium. In other slices, synapses were activated with low-frequency control stimulation or repeated tetanic stimulation. In slices with blocked synaptic transmission, dendrites were spinier than in either of the activated states. Thus, mature neurons can increase their numbers of spines, possibly compensating for lost synaptic activity.

Three-dimensional relationships between hippocampal synapses and astrocytes

Recent studies show that glutamate transporter-mediated currents occur in astrocytes when glutamate is released from hippocampal synapses. These transporters remove excess glutamate from the extracellular space, thereby facilitating synaptic input specificity and preventing neurotoxicity. Little is known about the position of astrocytic processes at hippocampal synapses. Serial electron microscopy and three-dimensional analyses were used to investigate structural relationships between astrocytes and synapses in stratum radiatum of hippocampal area CA1 in the mature rat in vivo and in slices. Only 57 +/- 11% of the synapses had astrocytic processes apposed to them. Of these, the astrocytic processes surrounded less than half (0.43 +/- 22) of the synaptic interface. Other studies suggest that astrocytes extend processes toward higher concentrations of glutamate; thus the presence of astrocytic processes at particular hippocampal synapses might signal which ones are releasing glutamate. The distance between nearest neighboring synapses was usually (approximately 95%) <1 microgram. Astrocytic processes occurred along the extracellular path between 33% of the neighboring synapses, neuronal processes occurred along the path between another 66% of the neighboring synapses, and only 1% of the synapses were close enough such that neither astrocytic nor neuronal processes occurred between them. These morphological arrangements suggest that the glutamate released at approximately two-thirds of hippocampal synapses might diffuse to other synapses, unless neuronal glutamate transporters are more effective than previously reported. The findings also suggest that physiological recordings made from hippocampal astrocytes do not uniformly sample the glutamate released from all hippocampal synapses.

Structure, development, and plasticity of dendritic spines

Dendritic spines are distinguished by their shapes, subcellular composition, and synaptic receptor subtypes. Recent studies show that actin-dependent movements take place in spine heads, that spines emerge from stubby and shaft synapses after dendritic filopodia disappear, and that spines can form without synaptic activation, are maintained by optimal activation, and are lost with excessive activation or during degeneration.

Slices have more synapses than perfusion-fixed hippocampus from both young and mature rats

Hippocampal slices have long been used to investigate properties of synaptic transmission and plasticity. Here, for the first time, synapses in slices have been compared quantitatively with synapses occurring in perfusion-fixed hippocampus, which is presumed to represent the natural in vivo state. Relative to perfusion-fixed hippocampus, a remarkable 40-50% increase in spine number occurs in adult hippocampal slices, and a 90% increase occurs in slices from postnatal day 21 rats. Serial EM shows that all of the dendritic spines have normal synapses with presynaptic and postsynaptic elements; however, not all spine types are affected uniformly. Stubby and mushroom spines increase in the adult slices, and thin, mushroom, and branched spines increase in the immature slices. More axonal boutons with multiple synapses occur in the slices, suggesting that the new synapses form on preexisting axonal boutons. The increase in spine and synapse number is evident within a couple of hours after preparing the slices. Once the initial spine induction has occurred, no further change occurs for up to 13 hr in vitro, the longest time investigated. Thus, the spine increase is occurring during a period when there is little or no synaptic activity during the first hour, and the subsequent stabilization in spine synapse numbers is occurring after synaptic activity returns in the slice. These findings suggest that spines form in response to the loss of synaptic activity when slices are removed from the rest of the brain and during the subsequent 1 hr recovery period.

Radial approach: a new concept in surgical treatment for atrial fibrillation. II. Electrophysiologic effects and atrial contribution to ventricular filling

BACKGROUND

In a previous study the atrial incisions that follow the concept of the radial approach were designed according to the activation sequence during sinus rhythm and the atrial coronary artery anatomy in normal dogs. The purpose of the present study was to determine whether the radial approach provides a more physiologic activation sequence and atrial transport function than the maze procedure.

METHODS

Ten dogs that had undergone the radial approach (n = 5) or the maze procedure (n = 5) were studied 6 weeks postoperatively. Sinus node function and inducibility of atrial fibrillation were examined before and after operation. The atria were mapped endocardially with 212 electrodes, and atrial activation sequences during sinus rhythm and right atrial pacing were examined. Atrial transport function was assessed by transepicardial Doppler echocardiography.

RESULTS

No dogs developed sinus node dysfunction postoperatively. Both the radial approach and the maze procedure equally prevented sustained atrial fibrillation. The atrial activation sequence was more synchronous after the radial approach than after the maze procedure. There was no electrically isolated region after the radial approach. The total activation time of the left atrium was significantly shorter after the radial approach than after the maze procedure (53.6+/-9.8 versus 70.5+/-9.6 ms, p<0.05). The ratio of peak flow velocity of the E wave to the A wave (peak E/A) of the transmitral Doppler flow was significantly smaller after the radial approach than after the maze procedure (1.7+/-0.4 versus 3.5+/-1.7, p<0.05). The atrial filling fraction of the transmitral Doppler flow was significantly larger after the radial approach than after the maze procedure (29.9%+/-7.3% versus 14.8%+/-5.0%, p<0.01). There was no significant difference in peak E/A and atrial filling fraction of the transtricuspid Doppler flow between the two procedures.

CONCLUSIONS

The radial approach provides a more synchronous activation sequence and atrial transport function, and thus may represent a more physiologic alternative to the maze procedure as a surgical treatment for atrial fibrillation.

Who is the marginal patient? Understanding instrumental variables estimates of treatment effects

OBJECTIVE

To clarify the issues of generalizability arising from the use of instrumental variable (IV) methods to estimate treatment effects in nonexperimental medical outcome studies.

DATA SOURCE

We generate Monte Carlo data designed to resemble typical data sets where detailed health status information is unavailable and the treatment assignment process is unobserved. The model used to generate our data makes the realistic assumption that unobservable health status characteristics of patients influence the treatment assignment process and the effectiveness of treatment.

STUDY DESIGN

We use Monte Carlo data to illustrate the circumstances where IV estimates generalize to an unobservable patient subpopulation and those where IV estimates generalize to the entire patient population represented by the sample used in the analysis. We also simulate the effect of two policy changes that affect practice patterns. Further, we show that IV estimates are useful for predicting the effect of these changes on treatment effectiveness when the subpopulation to which the IV estimate refers is the same or very similar to the population whose treatment status is affected by the policy change.

CONCLUSIONS

Health services researchers cannot take for granted that IV estimates generalize to the same population represented by the sample used for analysis. Instead, researchers must rely on their knowledge of clinical practice and theory regarding the treatment assignment process in interpreting their results and in predicting the effect of changes in practice patterns.

Hepatic Doppler perfusion index: measurement in nine healthy volunteers

Nine healthy volunteers were studied to validate the reproducibility of the Doppler perfusion index--the ratio of hepatic arterial to total liver blood flow--and to evaluate the method of its derivation and the influence of the variable parameters necessary for its calculation. Wide intraobserver variability was observed, and Doppler perfusion index values were consistently outside the previously reported normal range.

Synaptogenesis via dendritic filopodia in developing hippocampal area CA1

To determine the role of dendritic filopodia in the genesis of excitatory synaptic contacts and dendritic spines in hippocampal area CA1, serial section electron microscopy and three-dimensional analysis of 16 volumes of neuropil from nine male rat pups, aged postnatal day 1 (P1) through P12, were performed. The analysis revealed that numerous dendritic filopodia formed asymmetric synaptic contacts with axons and with filopodia extending from axons, especially during the first postnatal week. At P1, 22 +/- 5.5% of synapses occurred on dendritic filopodia, with 19 +/- 5.9% on filopodia at P4, 20 +/- 8.0% at P6, decreasing to 7.2 +/- 4.7% at P12 (p < 0.02). Synapses were found at the base and along the entire length of filopodia, with many filopodia exhibiting multiple synaptic contacts. In all, 162 completely traceable dendritic filopodia received 255 asymmetric synaptic contacts. These synapses were found at all parts of filopodia with equal frequency, usually occurring on fusiform swellings of the diameter. Most synaptic contacts (53 +/- 11%) occurred directly on dendritic shafts during the first postnatal week. A smaller but still substantial portion (32 +/- 12%) of synapses were on shafts at P12 (p < 0.036). There was a highly significant (p < 0.0002) increase in the proportion of dendritic spine synapses with age, rising from just 4.9 +/- 4.3% at P1 to 37 +/- 14% at P12. The concurrence of primarily shaft and filopodial synapses in the first postnatal week suggests that filopodia recruit shaft synapses that later give rise to spines through a process of outgrowth.

Three-dimensional structure and composition of CA3-->CA1 axons in rat hippocampal slices: implications for presynaptic connectivity and compartmentalization

Physiological studies of CA3-->CA1 synaptic transmission and plasticity have revealed both pre- and postsynaptic effects. Understanding the extent to which individual presynaptic axonal boutons could provide local compartments for control of synaptic efficacy and microconnectivity requires knowledge of their three-dimensional morphology and composition. In hippocampal slices, serial electron microscopy was used to examine a nearly homogeneous population of CA3-->CA1 axons in the middle of stratum radiatum of area CA1. The locations of postsynaptic densities (PSDs), vesicles, and mitochondria were determined along 75 axon segments (9.1 +/- 2.0 micrometer in length). Synapses, defined by the colocalization of PSDs and vesicles, occurred on average at 2.7 micrometer intervals along the axons. Most varicosities (68%) had one PSD, 19% had 2-4 PSDs, and 13% had none. Synaptic vesicles occurred in 90% of the varicosities. One-half (53%) of the varicosities lacked mitochondria, raising questions about their regulation of ATP and Ca2+, and 8% of varicosities contained only mitochondria. Eleven axons were reconstructed fully. The varicosities were oblong and varied greatly in both length (1.1 +/- 0.7 micrometer) and volume (0.13 +/- 0.14 micrometer 3), whereas the intervaricosity shafts were narrow, tubular, and similar in diameter (0.17 +/- 0.04 micrometer) but variable in length (1.4 +/- 1.2 micrometer). The narrow axonal shafts resemble dendritic spine necks and thus could promote biochemical compartmentalization of individual axonal varicosities. The findings raise the intriguing possibility of localized differences in metabolism and connectivity among different axons, varicosities, and synapses.

Homocysteine and cardiovascular disease

After a thorough review of the available literature, it appears that hyperhomocysteinemia is an independent risk factor for CHD. Furthermore, folic acid has been shown to reduce homocysteine concentration. Nevertheless, CHD is a multifactorial process, and many risk factors play a role in its pathogenesis. Several unanswered questions remain regarding the role of folic acid supplementation in hyperhomocysteinemia (Table 3). The absolute homocysteine concentration at which cardiovascular risk increases is not certain, and the magnitude of homocysteine-lowering needed to prevent events is unknown. Consequently, the number needed to treat cannot be calculated for folic acid supplements. Based on these data, the populations in whom to evaluate a homocysteine concentration have yet to be described. Because the POEMs are not yet available, it is unknown whether supplemental folic acid to lower homocysteine concentration will reduce CHD morbidity and mortality. It will take several years before any randomized, controlled trials are done, and primary prevention trials will need to be of very long duration to show any change in outcomes. Widespread use of folic acid supplementation has been recommended, however, and the need for clinical outcomes might be precluded. Even in the absence of outcome data, the potential benefits of using folic acid appear to outweigh any risks. A diet high in folic acid should be encouraged in everyone (Table 4). The FDA-mandated folic acid fortification of enriched grain products is most likely insufficient to lower homocysteine concentrations meaningfully, and a daily multivitamin that contains 400 microg of folic acid should be considered for patients who have documented CHD (especially when other risk factors are absent or in patients with premature atherosclerosis) and men and women who have cardiovascular risk factors, in addition to women of childbearing potential. Folic acid supplementation in the form of a multivitamin once daily is safe and inexpensive and might prevent the development and progression of CHD.

Critical assessment of the involvement of perforations, spinules, and spine branching in hippocampal synapse formation

Several studies propose that long-term enhancement of synaptic transmission between neurons results from the enlargement, perforation, and splitting of synapses and dendritic spines. Unbiased analyses through serial electron microscopy were used to assess the morphological basis for synapse spilitting in hippocampal area CA1. Few perforated synapses and almost no split (i.e., branched) spines occurred at postnatal day 15, an age of high synaptogenesis; thus, synapse splitting is unlikely to be important during development. The synapse splitting hypothesis predicts an intermediate stage of branched spines with both heads sharing the same presynaptic bouton. Ninety-one branched dendritic spines were traced through serial sections, and the different branches never synapsed with the same presynaptic bouton. Projections from spines, called "spinules," have been thought to extend from perforations in the postsynaptic density (PSD), thereby dividing the presynaptic bouton. Forty-six spinules were traced, and only 13% emerged from perforations in the PSD. Most spinules emerged from the edges of nonperforated PSDs, or from spine necks, where they extended into boutons that were not presynaptic to the spine. In summary, these morphological characteristics are inconsistent with synapse and spine splitting. An alternative is discussed whereby perforated synapses and spinules are transient components of synaptic activation, and branched spines appear from synapses forming in close proximity to one another.

Paternal involvement with adolescents in intact families: the influence of fathers over the life course

We measure the quality and quantity of fathers' involvement with adolescent children in intact families over time using longitudinal data from The National Survey of Children. We examine differentials in fathers' involvement by children's and family characteristics and model the long-term effects of fathers' involvement on children's outcomes in the transition to adulthood. Fathers are more involved with sons than with daughters and they disengage from adolescents with increasing marital conflict. We find beneficial effects for children of father's involvement in three domains: educational and economic attainment, delinquent behavior, and psychological well-being. The course of affective relations throughout adolescence also has a beneficial effect on delinquent behavior and psychological well-being.

Retrograde infusion of lidocaine or L-arginine before reperfusion reduces myocardial infarct size

BACKGROUND

Retrograde perfusion preserves ischemic myocardium when initiated shortly after coronary artery occlusion. However, benefits diminish as the delay increases. In this study, we used this technique to deliver agents known to reduce the injury associated with the reperfusion of ischemic myocardium. We proposed that the local delivery of lidocaine or L-arginine before reperfusion would reduce the damage caused during reperfusion, even after a delay between onset of ischemia and intervention designed to approximate clinical reality.

METHODS

In a porcine model of myocardial ischemia, the left anterior descending coronary artery was snared immediately distal to its second diagonal branch. After 1 hour of occlusion, 34 animals were randomized into six groups: no intervention (control) (n = 6); administration of normal saline solution into the great cardiac vein (Retro-NS) (n = 6); administration of lidocaine either intravenously (i.v.-LID) (n = 6) or retrograde (Retro-LID) (n = 6); and administration of L-arginine either intravenously (i.v.-L-ARG) (n = 5) or retrograde (Retro-L-ARG) (n = 5). After 90 minutes of ischemia, the snare was released, and the myocardium was reperfused for 3 hours. Two-dimensional echocardiograms were made prior to occlusion and 60, 150, 210, and 270 minutes after occlusion. The infarct size and the area at risk were determined by lissamine green and triphenyltetrazolium chloride staining with computer planimetric quantification. Regional wall motion was assessed by a wall motion score: normal = 1; mild hypokinesia = 2.0; severe hypokinesia = 2.5; and akinesia = 3.

RESULTS

The area of the left ventricle at risk for infarction was similar in all groups and represented 25.4% (5.2% [standard deviation]) of the left ventricular mass (p = 0.63). The percent area of infarction in the area at risk after 3 hours of reperfusion was 76.7% (7.1% for the control group, 73.9% (5.7%) for the Retro-NS group, 72.1% (8.7%) for the i.v.-LID group, 54.5% (10.2%) for the Retro-LID group, 58.8% (4.0%) for the i.v.-L-ARG group, and 54.3% (4.0%) for the Retro-L-ARG group p < 0.005, Retro-LID and Retro-L-ARG versus Control, Retro-NS, and i.v.-LID; p < 0.03, i.v.-L-ARG versus control and Retro-NS). No significant difference in wall motion scores between groups was detected by echocardiography (p = 0.578).

CONCLUSIONS

Retrograde delivery of lidocaine or L-arginine before reperfusion reduces infarct size without acutely affecting wall motion after 90 minutes of ischemia and 3 hours of reperfusion. Lidocaine must be present before reperfusion to have an effect, whereas L-arginine is beneficial if it is delivered at the time of reperfusion.

Three-dimensional organization of cell adhesion junctions at synapses and dendritic spines in area CA1 of the rat hippocampus

Recent work has emphasized the role of adhesion molecules in synaptic plasticity, including long-term potentiation in the hippocampus. Such adhesion molecules are concentrated in junctions that are characterized by dense thickenings on both sides of the junction and are called puncta adhaerentia (PA). Reconstruction from serial electron microscopy was used to determine the location and size of PA in the stratum radiatum of hippocampal area CA1, where many of the previous functional studies have been performed. PAs were found at the edges of synapses on 33% of dendritic spines. The areas occupied by PA were variable across different types of synapses, occupying 0.010+/-0.005 microm2 at macular synapses and 0.034+/-0.031 microm2 at perforated synapses. Another zone, called a vesicle-free transition zone (VFTZ), was identified. Like the PA, this zone also had no presynaptic vesicles and was located at the edges of synapses; however, unlike the PA, the presynaptic thickening was less than the postsynaptic thickening. Together, 45% of spine synapses had PA and/or VFTZ occupying 23+/-11% of the total junctional area between axons and spines. PA also occurred at nonsynaptic sites involving neuronal as well as glial elements. Most (64%) of these PAs occurred between nonsynaptic portions of dendritic spines and neighboring astrocytic processes. Smooth endoplasmic reticulum was often apposed to one or both sides of the synaptic and the nonsynaptic PA. These findings provide further data as a structural basis for understanding the roles of cell adhesion junctions in hippocampal synaptic function and plasticity.

Ultrasound interactions with free silicone in a tissue-mimicking phantom

This study attempts to explain the physical basis for the sonographic appearance of different stages of free silicone in breast tissue by laboratory simulations. A tissue-mimicking breast phantom was constructed, into which silicone inclusions were introduced, to simulate various forms of silicone leakage within the body. These simulations suggest that silicone leakage into surrounding body tissues can cause three primary physical interactions with an ultrasound beam: (1) distortions due to changes in speed of sound; (2) refraction, causing a "lens" effect; and (3) multiple scattering, producing the "snowstorm" effect described previously as a signature pattern for detection of silicone migration in breast tissue.

Computed radiography versus screen-film mammography in detection of simulated microcalcifications: a receiver operating characteristic study based on phantom images

RATIONALE AND OBJECTIVES

The authors compare a 43-micron computed radiographic system with a mammographic screen-film system for detection of simulated microcalcifications in an observer-performance study.

MATERIALS AND METHODS

The task of detecting microcalcifications was simulated by imaging aluminum wire segments (200-500 microns in length; 100, 125, or 150 microns in diameter) that overlapped with tissue background structures produced by beef brisket. A total of 288 such simulations were generated and examined with both computed radiography and conventional screen-film mammography techniques. Computed radiography was performed with high-resolution plates, a 43-micron image reader, and a 43-micron laser film printer. Computed radiographic images were printed with simple contrast enhancement and compared with screen-film images in a receiver operating characteristic study in which experienced readers detected and scored the simulated microcalcifications. Observer performance was quantitated and compared by computing the area under the receiver operating characteristic curve.

RESULTS

Although the resolution of the computed radiography system was better than that of commercial systems, it fell short of that of screen-film systems. For the 100-micron microcalcifications, the difference in the average area under the curve was not statistically significant, but it was significant for the larger simulated microcalcifications: the average area under the curve was 0.58 for computed radiography versus 0.76 for screen-film imaging for the 125-micron microcalcifications and 0.83 versus 1.00, respectively, for the 150-micron microcalcifications.

CONCLUSION

Observer performance in the detection of small simulated microcalcifications (100-150 microns in diameter) is better with screen-film images than with high-resolution computed radiographic images.

Stability in synapse number and size at 2 hr after long-term potentiation in hippocampal area CA1

Long-term potentiation (LTP) is an important model for examining synaptic mechanisms of learning and memory. A key question is whether the enhanced synaptic transmission occurring with LTP involves the addition of new synapses, the enlargement of existing synapses, or a redistribution in synaptic weight among synapses. Two experimental designs were used to address this question. In the first experimental design three conditions were evaluated across hippocampal slices maintained in vitro, including slices with LTP analyzed at 2 hr post-tetanus, slices tetanized in the presence of APV, and control slices receiving test stimulation only. In the second experimental design independent LTP and control (low-frequency stimulation) sites were examined. Synapse density was estimated by an unbiased volume sampling procedure. Synapse size was computed by three-dimensional reconstruction from serial electron microscopy (EM). Serial EM also was used to compute synapse number per unit length of dendrite. In both experimental designs there were no significant effects of LTP on total synapse number, on the distribution of different types of synapses (thin, mushroom, stubby, or branched dendritic spines and macular, perforated, or segmented postsynaptic densities), on the frequency of shaft synapses, nor on the relative proportion of single or multiple synapse axonal boutons. There was also no increase in synapse size. These results suggest that LTP does not cause an overall formation of new synapses nor an enlargement of synapses at 2 hr post-tetanus in hippocampal area CA1, and these results support the hypothesis that LTP could involve a redistribution of synaptic weights among existing synapses.