Dissociating working memory from task difficulty in human prefrontal cortex

A functional magnetic resonance imaging (fMRI) study was conducted to determine whether prefrontal cortex (PFC) increases activity in working memory (WM) tasks as a specific result of the demands placed on WM, or to other processes affected by the greater difficulty of such tasks. Increased activity in dorsolateral PFC (DLPFC) was observed during task conditions that placed demands on active maintenance (long retention interval) relative to control conditions matched for difficulty. Furthermore, the activity was sustained over the entire retention interval and did not increase when task difficulty was manipulated independently of WM requirements. This contrasted with the transient increases in activity observed in the anterior cingulate, and other regions of frontal cortex, in response to increased task difficulty but not WM demands. Thus, this study established a double-dissociation between regions responsive to WM versus task difficulty, indicating a specific involvement of DLPFC and related structures in WM function.     

Y chromosome polymorphisms in native American and Siberian populations: identification of native American Y chromosome haplotypes

We have initiated a study of ancient male migrations from Siberia to the Americas using Y chromosome polymorphisms. The first polymorphism examined, a C-->T transition at nucleotide position 181 of the DYS199 locus, was previously reported only in Native American populations. To investigate the origin of this DYS199 polymorphism, we screened Y chromosomes from a number of Siberian, Asian, and Native American populations for this and other markers. This survey detected the T allele in all five Native American populations studied at an average frequency of 61%, and in two of nine native Siberian populations, the Siberian Eskimo (21%) and the Chukchi (17%). This finding suggested that the DYS199 T allele may have originated in Beringia and was then spread throughout the New World by the founding populations of the major subgroups of modern Native Americans. We further characterized Native American Y chromosome variation by analyzing two additional Y chromosome polymorphisms, the DYS287 Y Alu polymorphic (YAP) element insertion and a YAP-associated A-->G transition at DYS271, both commonly found in Africans. We found neither African allele associated with the DYS199 T allele in any of the Native American or native Siberian populations. However, we did find DYS287 YAP+ individuals who harbored the DYS199 C allele in one Native American population, the Mixe, and in one Asian group, the Tibetans. A correlation of these Y chromosome alleles in Native Americans with those of the DYS1 locus, as detected by the p49a/p49f (p49a,f) probes on TaqI-digested genomic DNA, revealed a complete association of DYS1 alleles (p49a,f haplotypes) 13, 18, 66, 67 and 69 with the DYS199 T allele, while DYS1 alleles 8 and 63 were associated with both the DYS199 C and T allele.     

HLA class I and II polymorphisms and trachomatous scarring in a Chlamydia trachomatis-endemic population

Immune responses to Chlamydia trachomatis contribute to protection from infection and to immunopathologic disease. To test whether subjects' HLA class I (A, B, and Cw) or class II (DRbeta1 and DQbeta1) types influence risk of trachomatous scarring from chronic infection with C trachomatis, 153 cases and pair-matched controls in Gambia were studied. No HLA type was associated with protection from scarring, indicating that protective immune responses are not limited to only one or a few HLA-restricted epitopes in C. trachomatis antigens. One class I antigen, HLA-A28, was significantly more common among cases than controls (25.8% vs. 15.9%, respectively; McNemar's odds ratio [OR], 1.88; 95% confidence interval [CI] = 1.01-3.49; P = .046). In DNA subtyping of the A28 specificity, the A*6801 allele was equally common among cases and controls, but the A*6802 allele was significantly overrepresented among cases (McNemar's OR, 3.14; 95% CI = 1.32-7.44; P = .009). This association may be due to an immunopathologic HLA-A*6802-restricted cytotoxic T lymphocyte response.     

Lens epithelium: a primary target of UVB irradiation

Cultured rabbit lenses and cultured rabbit lens epithelial cells were irradiated with UV to correlate morphological changes in the epithelium with physiological changes in the whole lens during the development of UV-induced cataract. Two UV spectral ranges were utilized; one spanned 290 to 340 nm and was designated near-UV, the other was a narrower, pure UVB region: 303 to 313 nm, designated UVB. Irradiation with either spectrum of the anterior surface of whole lenses caused opacification and a dose-dependent loss of ion homeostasis as measured by Na+ and Ca2+ concentrations in whole lenses. It was determined that cation pump activity, assessed by 86Rb uptake, continued to decline steadily during culture after UV irradiation. Whole mount preparations of the epithelial cell layer of UVB-irradiated lenses revealed morphological changes within 2 hr of irradiation and cell death after 20 hr. Following posterior irradiation of whole lenses, the epithelial cells remained viable and lenses remained transparent during 3 days of culture, presumably because UV photons did not reach the epithelium. Absorption of UV photons by posterior fiber cell membranes and proteins did not cause opacification. To learn more about the epithelial damage, cultured rabbit lens epithelial cells were irradiated, UVB treatment retarded growth over a 7-day period in cultured cells. The surviving cells at day 7 were abnormal in appearance and the potassium concentration was approximately 50% less than controls, a finding which may explain the previously reported reduction in protein synthesis by UVB irradiation. Collectively, the data suggest that UV cataract is initiated by damage to the epithelium, including a change in membrane permeability leading to loss of ion homeostasis in the lens.     

Effect of long-term lead exposure on hematology, blood biochemistry, and growth curves in monkeys

A total of 90 monkeys (Macaca fascicularis), comprising four study cohorts born over a seven-year period, were hand reared and dosed orally with lead or vehicle according to one of several protocols, in most cases from birth to 9-14 years. Blood lead concentrations of lead-exposed groups ranged from 10 to 90 micrograms/dl depending on dose and age. Routine hematology and blood biochemistry analyses were performed regularly. Comparison of treated groups at various ages to the appropriate control group revealed no strong indication of lead-related effects. In addition, body weight increase was modeled from days 30-3500 of age in subset of this larger group, including 52 monkeys exposed to vehicle or lead during development according to one of four regimens: vehicle, lead from birth onward, lead to 400 days of age, or lead beginning at 300 days of age. No effect of lead on body weight was found. These results suggest that lead exposure beginning early in life and continuing for as long as 14 years resulted in no overt toxicity, as measured by these parameters.     

Effect of dopamine receptor antagonists on cocaine subjective effects: a naturalistic case study

Schizophrenic patients on neuroleptic medications abuse cocaine and report cocaine-induced euphoria. This study was undertaken to provide better clinical characterization of these phenomena by administering the POMS and a custom-designed questionnaire. A group of heavy cocaine users who were not mentally ill served as the control group. The results clearly suggest that schizophrenic patients report cocaine-induced euphoria and post-use craving despite being treated with therapeutic doses of haloperidol or fluphenazine. The responses of the control group were similar to that of the schizophrenic group except that the latter subjects reported a greater degree of anxiety. These results suggest that blockade of D2 receptors is not sufficient to block cocaine-induced subjective effects in humans.     

Broad spectrum aminoglycoside phosphotransferase type III from Enterococcus: overexpression, purification, and substrate specificity

The aminoglycoside phosphotransferases (APHs) are responsible for the bacterial inactivation of many clinically useful aminoglycoside antibiotics. We report the characterization of an enterococcal enzyme, APH(3')-IIIa, which inactivates a broad spectrum of aminoglycosides by ATP-dependent O-phosphorylation. Overproduction of APH(3')-IIIa has permitted the isolation of 30-40 mg of pure protein/(L of cell culture). Purified APH(3')-IIIa is a mixture of monomer and dimer which is slowly converted to dimer only over time. Dimer could be dissociated into monomer by incubation with 2-mercaptoethanol, suggesting that dimerization is mediated by formation of disulfide bond(s). Both monomer and dimer show Km values in the low micromolar range for good substrates such as kanamycin and neomycin, and kcat values of 1-4 s-1. All aminoglycosides show substrate inhibition except amikacin and kanamycin B. Determination of minimum inhibitory concentrations indicates a positive correlation between antibiotic activity and kcat/Km, but not with Km or kcat. NMR analysis of phosphorylated kanamycin A has directly demonstrated regiospecific phosphoryl transfer to the 3'-hydroxyl of the 6-aminohexose ring of the antibiotic. Analysis of structure-activity relationships with a variety of aminoglycosides has revealed that the deoxystreptamine aminocyclitol ring plays a critical role in substrate binding. This information will form the basis for future design of inhibitors of APH(3')-IIIa.     

Cancer incidence and mortality: the priority of screening frequency and population coverage

In addition to the usual measures of screening-test performance, it is important to consider testing frequency when evaluating a screening program. Data on which to base recommendations for the timing of screening tests are urgently needed. For example, in the cases of cervical and colon cancer, when the target is a precursor lesion, research indicates that less frequent screening may be appropriate. This finding may not apply, however, to screening for breast cancer by mammography, which requires currently recommended intervals for the early detection of malignancies. Resources now allocated to breast cancer might more effectively be applied to the construction of tests that would permit longer intervals between screenings. To achieve the National Cancer Institute's goal of reducing cancer mortality in the United States by the year 2000, it will be important to review the balance between population coverage and individual screening for each cancer and to emphasize prevention strategies that maximize population coverage while minimizing expenditures.