Improved short-term spatial memory but impaired reversal learning following the dopamine D(2) agonist bromocriptine in human volunteers.

RATIONALE: Studies in humans of cognitive effects of dopaminergic drugs have largely focused on tasks of working memory, with a few studies also examining executive function. OBJECTIVES: This study was designed to investigate the effects of 1.25 mg of the dopamine D(2) agonist bromocriptine on spatial working memory, planning and discrimination reversal learning in young healthy volunteers. METHODS: Twenty volunteers were tested in a double-blind, placebo-controlled, cross-over design. The cognitive assessment included tests taken from the Cambridge Neuropsychological Test Automated Battery (CANTAB) designed to test visuo-spatial recognition memory and spatial working memory. In addition, tests of spatial planning and discrimination reversal learning were used to assess the more general effects of bromocriptine. Tests of subjective feelings and motivation were also incorporated into the battery. RESULTS: Bromocriptine enhanced the spatial memory span of subjects, whilst impairing their ability to reverse a learned probabilistic discrimination. Tests of recognition memory and planning were unaffected by the drug. The findings were not explained by changes in subjective mood or motivational measures. CONCLUSIONS: The pattern of findings observed here mirror medication-dependent observations seen in Parkinson's disease. The results are discussed with reference to the different anatomical networks known to subserve performance of the differentially affected tasks.

Aging of the liver: proteolysis of oxidatively modified glutamine synthetase.

During aging cells accumulate altered forms of proteins, notably oxidatively modified proteins. The multicatalytic protease selectively degrades oxidized proteins, suggesting that the age-related accumulation of oxidized proteins might be a consequence of decreased activity of this protease. The protease activity of liver homogenates was assayed with an improved fluorimetric method, using oxidatively modified glutamine synthetase as substrate. Application of this assay to extracts from liver of Fischer 344 rats from both Japan and the United States demonstrated a marked preference for the oxidized substrates, as expected. Extracts from animals ages 8 to 26 months maintain both total proteolytic activity and the ability to distinguish between native and oxidized substrates. Oxidatively modified hepatocyte extracts were also employed as substrate, and older animals again maintained proteolytic activity. The multicatalytic protease was purified from liver of young and old rats, and the specific activity of the preparations were comparable when assayed with oxidatively modified glutamine synthetase. We conclude that the intrinsic neutral or alkaline proteolytic activity of rat liver is maintained during aging.

Oxidative dissociation of human alpha 2-macroglobulin tetramers into dysfunctional dimers.

Human alpha 2-macroglobulin is a broad-spectrum, homotetrameric antiproteinase that can maximally bind up to two proteinase molecules in a ternary complex. Proteinases cleave the inhibitor within a peptide stretch termed the bait region and induce the emergence of internal thiol esters whose nucleophilic scission precede a major conformational change which entraps enzymes within molecular cages. In a previous study, leukocyte-generated hypohalous acids and N-haloamines were identified as the first examples of physiologically relevant inactivators of the antiproteolytic activity of alpha 2-macroglobulin (Reddy, V. Y., Pizzo, S. V., and Weiss, S. J. (1989) J. Biol. Chem. 264, 13801-13809), but the mechanisms whereby the oxidants damaged the inhibitor remained undefined. We now demonstrate that N-chloramines (RNCl) destroy the antiproteolytic activity of alpha 2-macroglobulin in an unusual biphasic process that results in the formation of inactive alpha 2-macroglobulin half-molecules. In the first phase, 8 eq of RNCl reacted with each alpha 2-macroglobulin subunit to generate a partially oxidized antiproteinase containing 8 methionyl sulfoxide residues/monomer. Structure-function analyses demonstrated that the oxidized inhibitor retained its homotetrameric structure as well as its ability to entrap proteinases. In marked contrast, the oxidation of an additional 6 methionyl residues and a single tryptophanyl residue fractured the alpha 2 M homotetramer across its non-covalent axis into two pairs of disulfide-linked dimers. Despite the fact that the oxidized dimers displayed normal bait regions whose cleavage by proteinases initiated thiol ester scission, all antiproteolytic activity was lost. Furthermore, the oxidized dimers were unable to undergo the critical conformational changes normally associated with bait region cleavage or thiol ester scission. Together, these results demonstrate that chlorinated oxidants destroy the antiproteolytic activity of alpha 2-macroglobulin by attacking a subset of methionyl and tryptophanyl residues whose oxidation mediates the dissociation of the native homotetramer into conformationally locked dimers.

Diagnosis of subclinical rickets.

44 randomly selected infants under age one year with suspected lower respiratory infections were investigated for the presence of subclinical rickets. Seven infants had metaphyseal changes at the wrist compatible with a diagnosis of rickets and all of these infants had 25-hydroxy-vitamin D (25-OHD) concentrations less than 12 ng/ml. Serum calcium and phosphorus concentrations were normal in all 44 children. Alkaline phosphatase concentrations did not correlate with the presence of metaphyseal changes. The clinical presence of craniotabes or splaying and loss of definition of the anterior ends of the ribs on x-rays did not correlate with metaphyseal changes at the wrist or with 25-OHD concentrations. An x-ray of the wrist is essential to confirm the presence of subclinical rickets and the at-risk infant can be detected by measuring serum 25-OHD concentrations.