Regulation of 5-HT Receptors and the Hypothalamic-Pituitary-Adrenal Axis

Disturbances in the serotonin (5-HT) system is the neurobiological abnormality most consistently associated with suicide. Hyperactivity of the hypothalmic-pituitary-adrenal (HPA) axis is also described in suicide victims. The HPA axis is the classical neuroendocrine system that responds to stress and whose final product, corticosteroids, targets components of the limbic system, particularly the hippocampus. We will review resulsts from animal studies that point to the possibility that many of the 5-HT receptor changes observed in suicide brains may be a result of, or may be worsened by, the HPA overactivity that may be present in some suicide victims. The results of these studies can be summarized as follows: (1) chronic unpredictable stress produces high corticosteroid levels in rats; (2) chronic stress also results in changes in specific 5-HT receptors (increases in cortical 5-HT2A and decreases in hipocampal 5-HT1A and 5-HT1B); (3) chronic antidepressant administration prevents many of the 5-HT receptor changes observed after stress; and (4) chronic antidepressant administration reverses the overactivity of the HPA axis. If indeed 5-HT receptors have a partial role in controlling affective states, then their modulation by corticosteroids provides a potential mechanism by which these hormones may regulate mood. These data may also provide a biological understanding of how stressful events may increase the risk for suicide in vulnerable individuals and may help us elucidate the neurobiological underpinnings of treatment resistance.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73437/1/j.1749-6632.1997.tb52357.x.pd

Soil Respiration on an Aging Managed Heathland: Identifying an Appropriate Empirical Model for Predictive Purposes

Heathlands are cultural landscapes which are managed through cyclical cutting, burning or grazing practices. Understanding the carbon (C) fluxes from these ecosystems provides information on the optimal management cycle time to maximise C uptake and minimise C output. The interpretation of field data into annual C loss values requires the use of soil respiration models. These generally include model variables related to the underlying drivers of soil respiration, such as soil temperature, soil moisture and plant activity. Very few studies have used selection procedures in which structurally different models are calibrated, then validated on separate observation datasets and the outcomes critically compared. We present thorough model selection procedures to determine soil heterotrophic (microbial) and autotrophic (root) respiration for a heathland chronosequence and show that soil respiration models are required to correct the effect of experimental design on soil temperature. Measures of photosynthesis, plant biomass, photosynthetically active radiation, root biomass, and microbial biomass did not significantly improve model fit when included with soil temperature. This contradicts many current studies in which these plant variables are used (but not often tested for parameter significance). We critically discuss a number of alternative ecosystem variables associated with soil respiration processes in order to inform future experimental planning and model variable selection at other heathland field sites. The best predictive model used a generalized linear multi-level model with soil temperature as the only variable. Total annual soil C loss from the young, middle and old communities was calculated to be 650, 462 and 435 g C m−2 yr−1, respectively

The Age of Managed Heathland Communities: Implications for Carbon Storage?

Background and aims Shrublands are ecosystems vulnerable to climate changes, with key functions such as carbon storage likely to be affected. In dwarf shrublands dominated by Calluna vulgaris, the aboveground carbon allocation is associated with community age and phase of development. As the Calluna community grows older, a shift to net biomass loss occurs and it was hypothesized this would result in carbon stock increases within the soil. Methods The interaction of community age with ecosystem carbon stocks was investigated through a chronosequence study on three Calluna communities, aged 11, 18 and 27 years. Results Aboveground Calluna carbon stock increased significantly from the 11 year community (0.73 kg C m−2) to the 18 year community (1.11 kg C m−2) but did not significantly change from 18 to 27 years (1.0 kg C m−2), indicating a net carbon gain that corresponded with the growth phase of the Calluna plants. Moss was also found to be a relatively large contributor to aboveground carbon stock (e.g. 30 % in the Young community). Moss has often been excluded in aboveground assessments on Calluna heathlands which may have led to previous stock underestimation. Belowground carbon stocks to 25 cm were six to nine times greater than in the aboveground pools. For example in the Young community, 8 % of the carbon stock was located aboveground, 35 % in the organic layer and 55 % in the mineral soil. Conclusions Increased heathland age resulted in increased aboveground carbon stock until peak production was reached at approximately 18 years of age. However, the proportionally large belowground carbon stock eclipsed any aboveground effect when total carbon stocks were considered. The investigation emphasized both the importance of including the mineral soil in sampling programs and of consider all major species, such as bryophytes, and vegetation age in carbon stock assessments

Fourteen Annually Repeated Droughts Suppressed Autotrophic Soil Respiration and Resulted in an Ecosystem Change

Predictions of future climate over the next 100 years show that the frequency of long periods of droughts in summer will increase in the Netherlands. This study investigated the effect of 14 annually repeated droughts on soil respiration at a Dutch heathland. Field measurements of total soil respiration (RS) and microbial respiration (RH) were modeled to determine annual C losses and to derive root respiration (RA) C losses. The application of repeated droughts resulted in suppression of the total soil C loss from 392 to 332 g C m-2 year-1 in 2010-2011 and from 427 to 358 g C m-2 year-1 in 2011-2012. The RH was the greatest contributor to heathland soil C loss (74-76%) and this was suppressed when directly exposed to drought conditions, although not significantly reduced on an annual basis. Annual RA was suppressed by 42% (2010-2011) and 45% (2011-2012) under repeated drought, indicating there was a greater effect of the repeated annual drought in roots than in microbes. Field observations of photosynthesis (PG) showed paradoxical results, with significantly greater ecosystem PG on the drought treatment than the control treatment. Inclusion of plant activity (PG) as a variable did not improve the fit of the models used in this study. However, other changes in plant composition and structure, such as increasing moss cover on the drought treatment, were noted to have occurred during the 14 years of annually repeated drought and these long term trends may help explain the effects of climate change (drought) on soil processes