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Biomedical subjects

M D Hunter

Publications and source records attributed to M D Hunter.

11 recordsLinked to original sources
PubMed2005

Neural activity in speech-sensitive auditory cortex during silence.

That auditory hallucinations are voices heard in the absence of external stimuli implies the existence of endogenous neural activity within the auditory cortex responsible for their perception. Further, auditory hallucinations occur across a range of healthy and disease states that include reduced arousal, hypnosis, drug intoxication, delirium, and psychosis. This suggests that, even in health, the auditory cortex has a propensity to spontaneously "activate" during silence. Here we report the findings of a functional MRI study, designed to examine baseline activity in speech-sensitive auditory regions. During silence, we show that functionally defined speech-sensitive auditory cortex is characterized by intermittent episodes of significantly increased activity in a large proportion (in some cases >30%) of its volume. Bilateral increases in activity are associated with foci of spontaneous activation in the left primary and association auditory cortices and anterior cingulate cortex. We suggest that, within auditory regions, endogenous activity is modulated by anterior cingulate cortex, resulting in spontaneous activation during silence. Hence, an aspect of the brain's "default mode" resembles a (preprepared) substrate for the development of auditory hallucinations. These observations may help explain why such hallucinations are ubiquitous.

Acoustic Stimulation↗
PubMed2001

Algal extracellular products suppress Anabaena flos-aquae heterocyst spacing.

Intra- and interspecific chemical signals allow bacteria to respond to environmental conditions by regulating gene transcription. In cyanobacteria, gene products and the presence of fixed nitrogen regulate heterocyst frequency. In this paper, we describe a chemical made by a green alga, Chlamydomonas reinhardtii, that suppresses heterocyst formation in the co-occurring cyanobacterium, Anabaena flos-aquae. Cyanobacterial heterocyst frequencies were reduced in the presence of water-soluble, proteinase- and heat-resistant molecules greater than 15 kDa in molecular size. Green algal cells in all phases of growth made the suppressor. Ammonium and nitrate concentrations in the medium did not correlate with this change in phenotype. In addition, growth rate was not enhanced by the extracellular products. Therefore, C. reinhardtii extracellular products acted as a heterocyst inhibitor, not as a fixed nitrogen source. Chemical interactions between green algae and cyanobacteria influence heterocyst formation, an important consideration in understanding the outcome of competition between these organisms and the dynamics of phytoplankton communities.

Anabaena↗
PubMed2001

Elevational trends in defense chemistry, vegetation, and reproduction in Sanguinaria canadensis.

Evaluation of biotic interactions along geographic gradients reveals that pressure on plant populations by herbivores and pathogens increases as latitude decreases, and is accompanied by a parallel increase in the number and toxicity of alkaloid-bearing plants. We compared rhizome alkaloid content with plant reproductive and vegetative characters in Sanguinaria canadensis (Papaveraceae) along an elevational gradient over two growing seasons to ascertain 1) if alkaloid production in bloodroot varies among populations and systematically with elevation, and 2) if there exists a correlation between isoquinoline alkaloid, vegetative and reproductive production. In general, alkaloid content in bloodroot rhizomes declines with elevation, increases with rhizome water content, varies by site, and fluctuates seasonally with plant growth and reproduction. Alkaloid content was positively correlated with vegetative and reproductive effort with few exceptions. Analysis of total protopine and benzophenanthridine alkaloid concentrations revealed generally similar patterns as those of individual alkaloid concentrations, although significant differences did appear between individual alkaloid concentrations.

Adaptation, Physiological↗
PubMed2001

Environmental and genotypic influences on isoquinoline alkaloid content in Sanguinaria canadensis.

In a common garden, we investigated genetic and environmental influences on alkaloid production using Sanguinaria canadensis as a model. Nutrient and shade regimes were applied to replicated clones over one growing season, and induction of alkaloid production in bloodroot was tested on a whole-plant basis using jasmonic acid as an elicitor. Alkaloid concentrations increased with decreasing light intensity and fertilizer levels. Induction was not achieved by foliar application of jasmonic acid. Genetic influences represented by clone effects may be indicated by variation in alkaloid concentration by clone, but this experimental design did not allow us to distinguish genetic from pre-experiment environmental influences on the rhizomes.

Adaptation, Physiological↗
PubMed2001

Rickettsia associated with male-killing in a buprestid beetle.

Many populations of the buprestid leaf-mining beetle, Brachys tessellatus, from central South Carolina, USA, show highly skewed sex ratios, ranging from 1.3 to 6.0 females per male. We have identified a Rickettsia bacterium that is associated with sex ratio distortion (SRD) and selective killing of male embryos in B. tessellatus. Molecular assays of infection by this bacterium are highly associated with SRD within families, and treatment with an antibiotic (tetracycline) increases the number of male eggs that hatch and develop. The 16S rDNA sequence indicates that this is a novel Rickettsia, most closely related to Rickettsia bellii (a tick-associated bacterium) and a pea-aphid Rickettsia. It is also related to a Rickettsial bacterium that causes male-killing in an unrelated ladybird beetle species. Low levels of parthenogenesis are also observed in this system (about 10% of females) and may be the result of selection due to male rarity, or a direct result of infection by the Rickettsia.

Animals↗
PubMed2000

Green algal extracellular products regulate antialgal toxin production in a cyanobacterium.

Many bacterial genes and virulence factors are regulated by interbacterial and/or host-parasite chemical signals. We demonstrate that toxin production by a free-living freshwater cyanobacterium is regulated in part by the presence of extracellular products of a eukaryotic green alga. In growth experiments, extracellular products made by the cyanobacterium Anabaena flos-aquae contained both anatoxin and microcystin, and significantly reduced the yield of Chlamydomonas reinhardtii, a green alga. Based on experiments in which we added purified toxins to C. reinhardtii cultures, we believe that microcystin was responsible for the growth reduction. A. flos-aquae produced anatoxin constitutively when grown alone, but anatoxin concentration increased in the presence of C. reinhardtii elicitors. Microcystin accumulation depended on the growth phase; however, high concentrations of C. reinhardtii extracellular products completely inhibited microcystin accumulation. Our results demonstrate that cyanobacterial toxin production may be regulated by complex growth phase-dependent and environmental chemical cues, and suggest that secreted chemicals can mediate the outcome of competition between the cyanobacterium A. flos-aquae and the green alga C. reinhardtii.

Anabaena↗
PubMed2000

Identification and characterization of a membrane permease involved in iron-hydroxamate transport in Staphylococcus aureus.

Staphylococcus aureus was shown to transport iron complexed to a variety of hydroxamate type siderophores, including ferrichrome, aerobactin, and desferrioxamine. An S. aureus mutant defective in the ability to transport ferric hydroxamate complexes was isolated from a Tn917-LTV1 transposon insertion library after selection on iron-limited media containing aerobactin and streptonigrin. Chromosomal DNA flanking the Tn917-LTV1 insertion was identified by sequencing of chromosomal DNA isolated from the mutant. This information localized the transposon insertion to a gene whose predicted product shares significant similarity with FhuG of Bacillus subtilis. DNA sequence information was then used to clone a larger fragment of DNA surrounding the fhuG gene, and this resulted in the identification of an operon of three genes, fhuCBG, all of which show significant similarities to ferric hydroxamate uptake (fhu) genes in B. subtilis. FhuB and FhuG are highly hydrophobic, suggesting that they are embedded within the cytoplasmic membrane, while FhuC shares significant homology with ATP-binding proteins. Given this, the S. aureus FhuCBG proteins were predicted to be part of a binding protein-dependent transport system for ferric hydroxamates. Exogenous iron levels were shown to regulate ferric hydroxamate uptake in S. aureus. This regulation is attributable to Fur in S. aureus because a strain containing an insertionally inactivated fur gene showed maximal levels of ferric hydroxamate uptake even when the cells were grown under iron-replete conditions. By using the Fur titration assay, it was shown that the Fur box sequences upstream of fhuCBG are recognized by the Escherichia coli Fur protein.

ATP-Binding Cassette Transporters↗
PubMed1997

Estimating the relative roles of top-down and bottom-up forces on insect herbivore populations: a classic study revisited.

Although most ecologists agree that both top-down and bottom-up forces (predation and resource limitation, respectively) act in concert to influence populations of herbivores, it has proven difficult to estimate the relative contributions of such forces in terrestrial systems. Using a combination of time-series analysis of population counts recorded over 16 years and experimental data, we present the first estimates of the relative roles of top-down and bottom-up forces on the population dynamics of two terrestrial insect herbivores on the English oak (Quercus robur). Data suggest that temporal variation in winter moth, Operophtera brumata, density is dominated by time-lagged effects of pupal predators. By comparison, spatial variation in O. brumata density is dominated by host-plant quality. Overall, top-down forces explain 34.2% of population variance, bottom-up forces explain 17.2% of population variance, and 48.6% remains unexplained. In contrast, populations of the green oak tortrix, Tortrix viridana, appear dominated by bottom-up forces. Resource limitation, expressed as intraspecific competition among larvae for oak leaves, explains 29.4% of population variance. Host quality effects explain an additional 5.7% of population variance. We detected no major top-down effects on T. viridana populations. An unknown factor causing a linear decline in T. viridana populations over the 16-year study period accounts for most of the remaining unexplained variance. We discuss the observed differences between the insect species and the utility of time-series analysis as a tool in assessing the relative importance of top-down and bottom-up forces on herbivore populations.

Journal Article↗
PubMedDate not supplied

Heritability of phenolics in Quercus laevis inferred using molecular markers.

Studies of quantitative inheritance of phenotypes do not generally encompass the range of environmental conditions to which a population may be exposed in a natural setting and are rarely conducted on long-lived species due to the time required for traditional crossing experiments. We used a marker-based method to estimate relatedness with microsatellite markers in a natural population of a long-lived oak, then used this inferred relatedness to examine quantitative genetic variation in the concentration of foliar phenolics. Estimating heritability using this method requires both significant relatedness and variance in relatedness over distance. However, this population did not show significant variance of relatedness, so only the presence of heritability, and its ranking among traits and environments, could be estimated. Seven foliar phenolics showed a significant relationship between phenotypic similarity and relatedness. The significance of this relationship varied among individual phenolic compounds, as well as by season. Genetic factors appeared to have a more measurable influence on the production of secondary compounds early in the season. After leaf expansion, covariance of relatedness and phenotypic variance appear to become less significant. Therefore heritability may vary seasonally for these traits.

Genetic Heterogeneity↗
PubMedDate not supplied

The functional and psychological status of patients with amyotrophic lateral sclerosis: some implications for rehabilitation.

Amyotrophic lateral sclerosis (ALS), known as motor neurone disease (MND) in Britain, poses special problems in rehabilitation by virtue of its nature, trajectory and the age of patients with the disease. Many practical difficulties of ALS are well known, but there has been little research on the psychological parameters of the disease. This study of 181 ALS patients, from a national register in England and Wales, indicates that, contrary to some previous research, psychological distress (measured by the General Health Questionnaire) is widespread among patients at all stages of the disease. Severity of functional impairment is significantly related to psychological distress, but explains only a small part of the variance. Rehabilitation in relation to ALS must take account of the high incidence of psychological difficulties, as well as the considerable functional problems associated with the disease.

Activities of Daily Living↗