Defining the roles of local precipitation and anthropogenic water sources in driving the abundance of Aedes aegypti, an emerging disease vector in urban, arid landscapes

Understanding drivers of disease vectors' population dynamics is a pressing challenge. For short-lived organisms like mosquitoes, landscape-scale models must account for their highly local and rapid life cycles. Aedes aegypti, a vector of multiple emerging diseases, has become abundant in desert population centers where water from precipitation could be a limiting factor. To explain this apparent paradox, we examined Ae. aegypti abundances at > 660 trapping locations per year for 3 years in the urbanized Maricopa County (metropolitan Phoenix), Arizona, USA. We created daily precipitation layers from weather station data using a kriging algorithm, and connected localized daily precipitation to numbers of mosquitoes trapped at each location on subsequent days. Precipitation events occurring in either of two critical developmental periods for mosquitoes were correlated to suppressed subsequent adult female presence and abundance. LASSO models supported these analyses for female presence but not abundance. Precipitation may explain 72% of Ae. aegypti presence and 90% of abundance, with anthropogenic water sources supporting mosquitoes during long, precipitation-free periods. The method of using kriging and weather station data may be generally applicable to the study of various ecological processes and patterns, and lead to insights into microclimates associated with a variety of organisms' life cycles.

Toward a cohesive understanding of ecological complexity

Ecological systems are quintessentially complex systems. Understanding and being able to predict phenomena typical of complex systems is, therefore, critical to progress in ecology and conservation amidst escalating global environmental change. However, myriad definitions of complexity and excessive reliance on conventional scientific approaches hamper conceptual advances and synthesis. Ecological complexity may be better understood by following the solid theoretical basis of complex system science (CSS). We review features of ecological systems described within CSS and conduct bibliometric and text mining analyses to characterize articles that refer to ecological complexity. Our analyses demonstrate that the study of complexity in ecology is a highly heterogeneous, global endeavor that is only weakly related to CSS. Current research trends are typically organized around basic theory, scaling, and macroecology. We leverage our review and the generalities identified in our analyses to suggest a more coherent and cohesive way forward in the study of complexity in ecology.

A new fractal index to classify forest fragmentation and disorder

Context

Forest loss and fragmentation pose extreme threats to biodiversity. Their efficient characterization from remotely sensed data therefore has strong practical implications. Data are often separately analyzed for spatial fragmentation and disorder, but no existing metric simultaneously quantifies both the shape and arrangement of fragments.

Objectives

We present a fractal fragmentation and disorder index (FFDI), which advances a previously developed fractal index by merging it with the Rényi information dimension. The FFDI is designed to work across spatial scales, and to efficiently report both the fragmentation of images and their spatial disorder.

Methods

We validate the FFDI with 12,600 synthetic hierarchically structured random map (HRM) multiscale images, as well as several other categories of fractal and non-fractal test images (4880 images). We then apply the FFDI to satellite imagery of forest cover for 10 distinct regions of the Romanian Carpathian Mountains from 2000–2021.

Results

The FFDI outperformed its two individual components (fractal fragmentation index and Rényi information dimension) in resolving spatial patterns of disorder and fragmentation when tested on HRM classes and other image types. The FFDI thus offers a clear advantage when compared to the individual use of fractal fragmentation index and the Information Dimension, and provided good classification performance in an application to real data.

Conclusions

This work improves on previous characterizations of landscape patterns. With the FFDI, scientists will be able to better monitor and understand forest fragmentation from satellite imagery. The FFDI may also find wider applicability in biology wherever image analysis is used.

Geographic name resolution service: A tool for the standardization and indexing of world political division names, with applications to species distribution modeling

Massive biological databases of species occurrences, or georeferenced locations where a species has been observed, are essential inputs for modeling present and future species distributions. Location accuracy is often assessed by determining whether the observation geocoordinates fall within the boundaries of the declared political divisions. This otherwise simple validation is complicated by the difficulty of matching political division names to the correct geospatial object. Spelling errors, abbreviations, alternative codes, and synonyms in multiple languages present daunting name disambiguation challenges. The inability to resolve political division names reduces usable data, and analysis of erroneous observations can lead to flawed results. Here, we present the Geographic Name Resolution Service (GNRS), an application for correcting, standardizing, and indexing world political division names. The GNRS resolves political division names against a reference database that combines names and codes from GeoNames with geospatial object identifiers from the Global Administrative Areas Database (GADM). In a trial resolution of political division names extracted from >270 million species occurrences, only 1.9%, representing just 6% of occurrences, matched exactly to GADM political divisions in their original form. The GNRS was able to resolve, completely or in part, 92% of the remaining 378,568 political division names, or 86% of the full biodiversity occurrence dataset. In assessing geocoordinate accuracy for >239 million species occurrences, resolution of political divisions by the GNRS enabled the detection of an order of magnitude more errors and an order of magnitude more error-free occurrences. By providing a novel solution to a significant data quality impediment, the GNRS liberates a tremendous amount of biodiversity data for quantitative biodiversity research. The GNRS runs as a web service and is accessible via an API, an R package, and a web-based graphical user interface. Its modular architecture is easily integrated into existing data validation workflows.

Areas of global importance for conserving terrestrial biodiversity, carbon and water

To meet the ambitious objectives of biodiversity and climate conventions, the international community requires clarity on how these objectives can be operationalized spatially and how multiple targets can be pursued concurrently. To support goal setting and the implementation of international strategies and action plans, spatial guidance is needed to identify which land areas have the potential to generate the greatest synergies between conserving biodiversity and nature's contributions to people. Here we present results from a joint optimization that minimizes the number of threatened species, maximizes carbon retention and water quality regulation, and ranks terrestrial conservation priorities globally. We found that selecting the top-ranked 30% and 50% of terrestrial land area would conserve respectively 60.7% and 85.3% of the estimated total carbon stock and 66% and 89.8% of all clean water, in addition to meeting conservation targets for 57.9% and 79% of all species considered. Our data and prioritization further suggest that adequately conserving all species considered (vertebrates and plants) would require giving conservation attention to ~70% of the terrestrial land surface. If priority was given to biodiversity only, managing 30% of optimally located land area for conservation may be sufficient to meet conservation targets for 81.3% of the terrestrial plant and vertebrate species considered. Our results provide a global assessment of where land could be optimally managed for conservation. We discuss how such a spatial prioritization framework can support the implementation of the biodiversity and climate conventions.

How deregulation, drought and increasing fire impact Amazonian biodiversity

Biodiversity contributes to the ecological and climatic stability of the Amazon Basin^1,2, but is increasingly threatened by deforestation and fire^3,4. Here we quantify these impacts over the past two decades using remote-sensing estimates of fire and deforestation and comprehensive range estimates of 11,514 plant species and 3,079 vertebrate species in the Amazon. Deforestation has led to large amounts of habitat loss, and fires further exacerbate this already substantial impact on Amazonian biodiversity. Since 2001, 103,079-189,755 km² of Amazon rainforest has been impacted by fires, potentially impacting the ranges of 77.3-85.2% of species that are listed as threatened in this region⁵. The impacts of fire on the ranges of species in Amazonia could be as high as 64%, and greater impacts are typically associated with species that have restricted ranges. We find close associations between forest policy, fire-impacted forest area and their potential impacts on biodiversity. In Brazil, forest policies that were initiated in the mid-2000s corresponded to reduced rates of burning. However, relaxed enforcement of these policies in 2019 has seemingly begun to reverse this trend: approximately 4,253-10,343 km² of forest has been impacted by fire, leading to some of the most severe potential impacts on biodiversity since 2009. These results highlight the critical role of policy enforcement in the preservation of biodiversity in the Amazon.

The commonness of rarity: Global and future distribution of rarity across land plants

A key feature of life's diversity is that some species are common but many more are rare. Nonetheless, at global scales, we do not know what fraction of biodiversity consists of rare species. Here, we present the largest compilation of global plant diversity to quantify the fraction of Earth's plant biodiversity that are rare. A large fraction, ~36.5% of Earth's ~435,000 plant species, are exceedingly rare. Sampling biases and prominent models, such as neutral theory and the k-niche model, cannot account for the observed prevalence of rarity. Our results indicate that (i) climatically more stable regions have harbored rare species and hence a large fraction of Earth's plant species via reduced extinction risk but that (ii) climate change and human land use are now disproportionately impacting rare species. Estimates of global species abundance distributions have important implications for risk assessments and conservation planning in this era of rapid global change.

Revealing biases in the sampling of ecological interaction networks

The structure of ecological interactions is commonly understood through analyses of interaction networks. However, these analyses may be sensitive to sampling biases with respect to both the interactors (the nodes of the network) and interactions (the links between nodes), because the detectability of species and their interactions is highly heterogeneous. These ecological and statistical issues directly affect ecologists’ abilities to accurately construct ecological networks. However, statistical biases introduced by sampling are difficult to quantify in the absence of full knowledge of the underlying ecological network’s structure. To explore properties of large-scale ecological networks, we developed the software EcoNetGen, which constructs and samples networks with predetermined topologies. These networks may represent a wide variety of communities that vary in size and types of ecological interactions. We sampled these networks with different mathematical sampling designs that correspond to methods used in field observations. The observed networks generated by each sampling process were then analyzed with respect to the number of components, size of components and other network metrics. We show that the sampling effort needed to estimate underlying network properties depends strongly both on the sampling design and on the underlying network topology. In particular, networks with random or scale-free modules require more complete sampling to reveal their structure, compared to networks whose modules are nested or bipartite. Overall, modules with nested structure were the easiest to detect, regardless of the sampling design used. Sampling a network starting with any species that had a high degree (e.g., abundant generalist species) was consistently found to be the most accurate strategy to estimate network structure. Because high-degree species tend to be generalists, abundant in natural communities relative to specialists, and connected to each other, sampling by degree may therefore be common but unintentional in empirical sampling of networks. Conversely, sampling according to module (representing different interaction types or taxa) results in a rather complete view of certain modules, but fails to provide a complete picture of the underlying network. To reduce biases introduced by sampling methods, we recommend that these findings be incorporated into field design considerations for projects aiming to characterize large species interaction networks.

Analysing ecological networks of species interactions

Network approaches to ecological questions have been increasingly used, particularly in recent decades. The abstraction of ecological systems - such as communities - through networks of interactions between their components indeed provides a way to summarize this information with single objects. The methodological framework derived from graph theory also provides numerous approaches and measures to analyze these objects and can offer new perspectives on established ecological theories as well as tools to address new challenges. However, prior to using these methods to test ecological hypotheses, it is necessary that we understand, adapt, and use them in ways that both allow us to deliver their full potential and account for their limitations. Here, we attempt to increase the accessibility of network approaches by providing a review of the tools that have been developed so far, with - what we believe to be - their appropriate uses and potential limitations. This is not an exhaustive review of all methods and metrics, but rather, an overview of tools that are robust, informative, and ecologically sound. After providing a brief presentation of species interaction networks and how to build them in order to summarize ecological information of different types, we then classify methods and metrics by the types of ecological questions that they can be used to answer from global to local scales, including methods for hypothesis testing and future perspectives. Specifically, we show how the organization of species interactions in a community yields different network structures (e.g., more or less dense, modular or nested), how different measures can be used to describe and quantify these emerging structures, and how to compare communities based on these differences in structures. Within networks, we illustrate metrics that can be used to describe and compare the functional and dynamic roles of species based on their position in the network and the organization of their interactions as well as associated new methods to test the significance of these results. Lastly, we describe potential fruitful avenues for new methodological developments to address novel ecological questions.

Effects of anthropogenic wildfire in low-elevation Pacific island vegetation communities in French Polynesia

Anthropogenic (or human-caused) wildfire is an increasingly important driver of ecological change on Pacific islands including southeastern Polynesia, but fire ecology studies are almost completely absent for this region. Where observations do exist, they mostly represent descriptions of fire effects on plant communities before the introduction of invasive species in the modern era. Understanding the effects of wildfire in southeastern Polynesian island vegetation communities can elucidate which species may become problematic invasives with continued wildfire activity. We investigate the effects of wildfire on vegetation in three low-elevation sites (45-379 m) on the island of Mo'orea in the Society Islands, French Polynesia, which are already heavily impacted by past human land use and invasive exotic plants, but retain some native flora. In six study areas (three burned and three unburned comparisons), we placed 30 transects across sites and collected species and abundance information at 390 points. We analyzed each local community of plants in three categories: natives, those introduced by Polynesians before European contact (1767 C.E.), and those introduced since European contact. Burned areas had the same or lower mean species richness than paired comparison sites. Although wildfire did not affect the proportions of native and introduced species, it may increase the abundance of introduced species on some sites. Non-metric multidimensional scaling indicates that (not recently modified) comparison plant communities are more distinct from one another than are those on burned sites. We discuss conservation concerns for particular native plants absent from burned sites, as well as invasive species (including Lantana camara and Paraserianthes falcataria) that may be promoted by fire in the Pacific.

Consumption-Based Conservation Targeting: Linking Biodiversity Loss to Upstream Demand through a Global Wildlife Footprint

Although most conservation efforts address the direct, local causes of biodiversity loss, effective long-term conservation will require complementary efforts to reduce the upstream economic pressures, such as demands for food and forest products, which ultimately drive these downstream losses. Here, we present a wildlife footprint analysis that links global losses of wild birds to consumer purchases across 57 economic sectors in 129 regions. The United States, India, China, and Brazil have the largest regional wildlife footprints, while per-person footprints are highest in Mongolia, Australia, Botswana, and the United Arab Emirates. A US$100 purchase of bovine meat or rice products occupies approximately 0.1 km2 of wild bird ranges, displacing 1-2 individual birds, for 1 year. Globally significant importer regions, including Japan, the United Kingdom, Germany, Italy, and France, have large footprints that drive wildlife losses elsewhere in the world and represent important targets for consumption-focused conservation attention.

Borrelia burgdorferi sensu lato spirochetes in wild birds in northwestern California: associations with ecological factors, bird behavior and tick infestation

Although Borrelia burgdorferi sensu lato (s.l.) are found in a great diversity of vertebrates, most studies in North America have focused on the role of mammals as spirochete reservoir hosts. We investigated the roles of birds as hosts for subadult Ixodes pacificus ticks and potential reservoirs of the Lyme disease spirochete B. burgdorferi sensu stricto (s.s.) in northwestern California. Overall, 623 birds representing 53 species yielded 284 I. pacificus larvae and nymphs. We used generalized linear models and zero-inflated negative binomial models to determine associations of bird behaviors, taxonomic relationships and infestation by I. pacificus with borrelial infection in the birds. Infection status in birds was best explained by taxonomic order, number of infesting nymphs, sampling year, and log-transformed average body weight. Presence and counts of larvae and nymphs could be predicted by ground- or bark-foraging behavior and contact with dense oak woodland. Molecular analysis yielded the first reported detection of Borrelia bissettii in birds. Moreover, our data suggest that the Golden-crowned Sparrow (Zonotrichia atricapilla), a non-resident species, could be an important reservoir for B. burgdorferi s.s. Of 12 individual birds (9 species) that carried B. burgdorferi s.l.-infected larvae, no birds carried the same genospecies of B. burgdorferi s.l. in their blood as were present in the infected larvae removed from them. Possible reasons for this discrepancy are discussed. Our study is the first to explicitly incorporate both taxonomic relationships and behaviors as predictor variables to identify putative avian reservoirs of B. burgdorferi s.l. Our findings underscore the importance of bird behavior to explain local tick infestation and Borrelia infection in these animals, and suggest the potential for bird-mediated geographic spread of vector ticks and spirochetes in the far-western United States.

Maximum information entropy: a foundation for ecological theory

The maximum information entropy (MaxEnt) principle is a successful method of statistical inference that has recently been applied to ecology. Here, we show how MaxEnt can accurately predict patterns such as species-area relationships (SARs) and abundance distributions in macroecology and be a foundation for ecological theory. We discuss the conceptual foundation of the principle, why it often produces accurate predictions of probability distributions in science despite not incorporating explicit mechanisms, and how mismatches between predictions and data can shed light on driving mechanisms in ecology. We also review possible future extensions of the maximum entropy theory of ecology (METE), a potentially important foundation for future developments in ecological theory.

Taxon categories and the universal species-area relationship (a comment on Šizling et al., “between geometry and biology:the problem of universality of the species-area relationship”)

A theory of macroecology based on the maximum information entropy (MaxEnt) inference procedure predicts that the log-log slope of the species-area relationship (SAR) at any spatial scale is a specified function of the ratio of abundance, N(A), to species richness, S(A), at that scale. The theory thus predicts, in generally good agreement with observation, that all SARs collapse onto a specified universal curve when local slope, z(A), is plotted against N(A)/S(A). A recent publication, however, argues that if it is assumed that patterns in macroecology are independent of the taxonomic choices that define assemblages of species, then this principle of "taxon invariance" precludes the MaxEnt-predicted universality of the SAR. By distinguishing two dimensions of the notion of taxon invariance, we show that while the MaxEnt-based theory predicts universality regardless of the taxonomic choices that define an assemblage of species, the biological characteristics of assemblages should under MaxEnt, and do in reality, influence the realism of the predictions.

Potassium buffering in the central nervous system

Rapid changes in extracellular K+ concentration ([K+](o)) in the mammalian CNS are counteracted by simple passive diffusion as well as by cellular mechanisms of K+ clearance. Buffering of [K+](o) can occur via glial or neuronal uptake of K+ ions through transporters or K+-selective channels. The best studied mechanism for [K+](o) buffering in the brain is called K+ spatial buffering, wherein the glial syncytium disperses local extracellular K+ increases by transferring K+ ions from sites of elevated [K+](o) to those with lower [K+](o). In recent years, K+ spatial buffering has been implicated or directly demonstrated by a variety of experimental approaches including electrophysiological and optical methods. A specialized form of spatial buffering named K+ siphoning takes place in the vertebrate retina, where glial Muller cells express inwardly rectifying K+ channels (Kir channels) positioned in the membrane domains near to the vitreous humor and blood vessels. This highly compartmentalized distribution of Kir channels in retinal glia directs K+ ions from the synaptic layers to the vitreous humor and blood vessels. Here, we review the principal mechanisms of [K+](o) buffering in the CNS and recent molecular studies on the structure and functions of glial Kir channels. We also discuss intriguing new data that suggest a close physical and functional relationship between Kir and water channels in glial cells.

Propagation of intercellular calcium waves in retinal astrocytes and Müller cells

Intercellular Ca(2+) waves are believed to propagate through networks of glial cells in culture in one of two ways: by diffusion of IP(3) between cells through gap junctions or by release of ATP, which functions as an extracellular messenger. Experiments were conducted to determine the mechanism of Ca(2+) wave propagation between glial cells in an intact CNS tissue. Calcium waves were imaged in the acutely isolated rat retina with the Ca(2+) indicator dye fluo-4. Mechanical stimulation of astrocyte somata evoked Ca(2+) waves that propagated through both astrocytes and Müller cells. Octanol (0.5 mm), which blocks coupling between astrocytes and Müller cells, did not reduce propagation into Müller cells. Purinergic receptor antagonists suramin (100 microm), PPADS (20-50 microm), and apyrase (80 U/ml), in contrast, substantially reduced wave propagation into Müller cells (wave radii reduced to 16-61% of control). Suramin also reduced wave propagation from Müller cell to Müller cell (51% of control). Purinergic antagonists reduced wave propagation through astrocytes to a lesser extent (64-81% of control). Mechanical stimulation evoked the release of ATP, imaged with the luciferin-luciferase bioluminescence assay. Peak ATP concentration at the surface of the retina averaged 78 microm at the stimulation site and 6.8 microm at a distance of 100 microm. ATP release propagated outward from the stimulation site with a velocity of 41 microm/sec, somewhat faster than the 28 microm/sec velocity of Ca(2+) waves. Ejection of 3 microm ATP onto the retinal surface evoked propagated glial Ca(2+) waves. Together, these results indicate that Ca(2+) waves are propagated through retinal glial cells by two mechanisms. Waves are propagated through astrocytes principally by diffusion of an internal messenger, whereas waves are propagated from astrocytes to Müller cells and from Müller cells to other Müller cells primarily by the release of ATP.

Genetic inactivation of an inwardly rectifying potassium channel (Kir4.1 subunit) in mice: phenotypic impact in retina

The inwardly rectifying potassium channel Kir4.1 has been suggested to underlie the principal K(+) conductance of mammalian Müller cells and to participate in the generation of field potentials and regulation of extracellular K(+) in the retina. To further assess the role of Kir4.1 in the retina, we generated a mouse line with targeted disruption of the Kir4.1 gene (Kir4.1 -/-). Müller cells from Kir4.1 -/- mice were not labeled with an anti-Kir4.1 antibody, although they appeared morphologically normal when stained with an anti-glutamine synthetase antibody. In contrast, in Müller cells from wild-type littermate (Kir4.1 +/+) mice, Kir4.1 was present and localized to the proximal endfeet and perivascular processes. In situ whole-cell patch-clamp recordings showed a 10-fold increase in the input resistance and a large depolarization of Kir4.1 -/- Müller cells compared with Kir4.1 +/+ cells. The slow PIII response of the light-evoked electroretinogram (ERG), which is generated by K(+) fluxes through Müller cells, was totally absent in retinas from Kir4.1 -/- mice. The b-wave of the ERG, in contrast, was spared in the null mice. Overall, these results indicate that Kir4.1 is the principal K(+) channel subunit expressed in mouse Müller glial cells. The highly regulated localization and the functional properties of Kir4.1 in Müller cells suggest the involvement of this channel in the regulation of extracellular K(+) in the mouse retina.

An eyecup preparation for the rat and mouse

The eyecup preparation has traditionally been used to study retinal physiology in lower vertebrates and in some mammals. The procedures for preparing eyecups of the rat and mouse have not been described, however. We now describe methods for preparing and maintaining viable eyecups for these two species. Eyecups were everted over a plastic dome and held in place between the two halves of a superfusion chamber. Fluid exchange in the chamber was rapid, with near total exchange occurring in 9 s. Eyecup viability was tested by monitoring light-evoked retinal responses as the preparation aged. In both rat and mouse, the amplitude of the electroretinogram (ERG) b-wave decreased slowly, declining to 1/2 maximal amplitude in approximately 70 min. Light-evoked spike activity of neurons in the ganglion cell layer remained stable for approximately 3 h and attenuated responses were recorded for an additional 1-2 h. Eyecups were able to dark adapt. Retinal sensitivity, tested by monitoring b-wave amplitude, recovered following exposure to an adapting light.

Tail docking and beliefs about the practice in the Victorian dairy industry

OBJECTIVE

To determine the occurrence of tail docking and beliefs about the practice in the Victorian dairy industry.

DESIGN

Survey responses were analysed using chi-square tests and by correlation and regression analyses to determine associations between husbandry practices and beliefs and to identify possible predictive variables in relation to docking.

PROCEDURE

A survey of the occurrence of docking and beliefs about the practice was conducted in 1997 using face-to-face interviews of 313 respondents at 234 Victorian dairy farms.

RESULTS

On average, 35% of dairy farms routinely docked cattle. The practice varied from 11 to 63% in different regions and 12% of stud farms docked their cows. Rubber rings were used on 75% of farms and the average age of the cow at docking was 18 months. Twenty-two percent of cows were docked at a level above the top of the udder and 54% were docked level with the top of the udder. Respondents that docked believed that milking was finished quicker, the risks of leptospirosis for the operator and mastitis for the cow were reduced, the cows were easier to handle, fly numbers were reduced and milk quality was improved. There was a general belief that intact tails could cause significant discomfort to the operator and that docking resulted in acute but not chronic pain.

CONCLUSIONS

Docking is an entrenched practice in the Victorian dairy industry. Those farmers who docked generally believed that it was a highly desirable farming practice with particular benefits for the operator.

Sodium-bicarbonate cotransport in retinal astrocytes and Müller cells of the rat

Sodium-bicarbonate cotransport in retinal glial cells was studied in the everted eyecup preparation of the rat. Intracellular pH was monitored with the indicator dye BCPCF and fluorescence confocal microscopy. Raising the K+ concentration from 3 to 12 mM in HCO3- -buffered perfusate evoked an intracellular alkalinization in both astrocytes and Müller cells. The alkalinization developed more rapidly and was larger in astrocytes. The K+ -induced alkalinization was HCO3- -dependent; it was reduced by 33% in astrocytes and 71% in Müller cells when HCO3- was removed from the perfusate. The alkalinization was effectively blocked by addition of 0.5 mM 4,4"-diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS). Removal of Na+ from the perfusate evoked a rapid acidification in both types of glial cells. The results indicate that astrocytes and Müller cells in situ in the rat retina possess an electrogenic Na+/HCO3- cotransporter.

Modulation of neuronal activity by glial cells in the retina

Glial-neuronal communication was studied by monitoring the effect of intercellular glial Ca2+ waves on the electrical activity of neighboring neurons in the eyecup preparation of the rat. Calcium waves in astrocytes and Müller cells were initiated with a mechanical stimulus applied to the retinal surface. Changes in the light-evoked spike activity of neurons within the ganglion cell layer occurred when, and only when, these Ca2+ waves reached the neurons. Inhibition of activity was observed in 25 of 53 neurons (mean decrease in spike frequency, 28 +/- 2%). Excitation occurred in another five neurons (mean increase, 27 +/- 5%). Larger amplitude Ca2+ waves were associated with greater modulation of neuronal activity. Thapsigargin, which reduced the amplitude of the glial Ca2+ increases, also reduced the magnitude of neuronal modulation. Bicuculline and strychnine, inhibitory neurotransmitter antagonists, as well as 6-Nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione (NBQX) and D(-)-2-amino-7-phosphonoheptanoic acid (D-AP7), glutamate antagonists, reduced the inhibition of neuronal activity associated with glial Ca2+ waves, suggesting that inhibition is mediated by inhibitory interneurons stimulated by glutamate release from glial cells. The results suggest that glial cells are capable of modulating the electrical activity of neurons within the retina and thus, may directly participate in information processing in the CNS.

Asymmetric gap junctional coupling between glial cells in the rat retina

Gap junctional communication between glial cells is thought to play a role in K+ spatial buffering, in the propagation of inter-astrocytic Ca2+ waves, and in glial-neuronal signaling. In the present study, we characterize dye coupling between astrocytes, and between astrocytes and Müller cells, in the isolated rat retina. Whole-cell patch recordings were obtained from retinal astrocytes and Müller cells and the cells filled with Lucifer Yellow and neurobiotin. Spread of Lucifer Yellow to two to ten neighboring astrocytes occurred in 90% of the astrocyte recordings. After fixation and incubation of the retina with fluorescent conjugated streptavidin, neurobiotin was seen to label clusters of 13-88 astrocytes, as well as > 100 Müller cells. In contrast, when Müller cells were filled with Lucifer Yellow and neurobiotin, both tracers were confined solely to the recorded Müller cell. The uncoupling agents octanol, halothane, and doxyl-stearic acid were tested for their ability to uncouple retinal glia in situ. All three agents eliminated the visible spread of Lucifer Yellow from the injected astrocyte and the spread of neurobiotin into Müller cells. However, only doxyl-stearic acid combined with octanol eliminated the spread of neurobiotin between astrocytes. These results demonstrate that astrocytes in the rat retina are coupled to each other and to Müller cells. The astrocyte-to-Müller cell coupling is asymmetric, allowing transfer of the tracer in the forward direction only. In addition, astrocyte-to-Müller cell coupling is more sensitive to the uncoupling agents tested than is astrocyte-to-astrocyte coupling.

Calcium waves in retinal glial cells

Calcium signals were recorded from glial cells in acutely isolated rat retina to determine whether Ca2+ waves occur in glial cells of intact central nervous system tissue. Chemical (adenosine triphosphate), electrical, and mechanical stimulation of astrocytes initiated increases in the intracellular concentration of Ca2+ that propagated at approximately 23 micrometers per second through astrocytes and Müller cells as intercellular waves. The Ca2+ waves persisted in the absence of extracellular Ca2+ but were largely abolished by thapsigargin and intracellular heparin, indicating that Ca2+ was released from intracellular stores. The waves did not evoke changes in cell membrane potential but traveled synchronously in astrocytes and Müller cells, suggesting a functional linkage between these two types of glial cells. Such glial Ca2+ waves may constitute an extraneuronal signaling pathway in the central nervous system.

Acid efflux from retinal glial cells generated by sodium bicarbonate cotransport

Sodium bicarbonate cotransport was studied in freshly dissociated Müller cells of the salamander retina. Variations in intracellular and extracellular pH evoked extracellular potassium concentration ([K+]o were recorded. Intracellular pH was measured by standard ratio imaging of the pH-sensitive dye BCECF, whereas extracellular pH was monitored by imaging BCECF fixed to coverslips under dissociated cells. Increasing [K+]o from 2.5 to 50 mM resulted in an intracellular alkalinization. The rate of alkalinization, 0.047 pH units/min, was reduced to 42% of control when HEPES was substituted for HCO3- and was reduced to 36% of control by the addition of 0.5 mM DIDS, a Na+/HCO3- cotransport blocker. The K(+)-evoked alkalinization was Cl(-)-independent and was not substantially reduced by amiloride or bumetanide. Increasing [K+]o to 50 mM also produced a rapid extracellular acidification, 0.01 to 0.05 pH units in amplitude. HEPES substitution and addition of 0.5 mM DIDS reduced the acidification to 7-8% of control, respectively. These results confirm the presence of a Na+/HCO3- cotransport system in salamander Müller cells and provide definitive evidence that glial cells can generate an extracellular acidification when [K+]o is increased. The K(+)-evoked extracellular acidification measured beneath cell endfeet was 304% of the amplitude of the acidification beneath cell somata, confirming that cotransporter sites are preferentially localized to the endfoot. The carbonic anhydrase inhibitor benzolamide (2 x 10(-5) M), which is poorly membrane permeant, increased the K(+)-evoked extracellular acidification to 269% of control, demonstrating that salamander Müller cells possess extracellular carbonic anhydrase.

A physiological measure of carbonic anhydrase in Müller cells

Carbonic anhydrase activity was characterized in freshly dissociated Müller cells of the salamander retina. Intracellular pH was monitored using ratio imaging of the indicator dye BCECF as extracellular PCO2 was varied. The extracellular solution was switched rapidly (141 ms rise time) from a HEPES buffered to a CO2-HCO3- buffered solution (both pH 7.4). Introduction of CO2-HCO3- produced a rapid cell acidification. Cell pH dropped from a steady-state pH of 7.02 in HEPES solution to pH 6.81 in CO2-HCO3-. Methazolamide, a carbonic anhydrase inhibitor, dramatically reduced the initial rate of acidification, demonstrating that the acidification was produced by the carbonic anhydrase-catalyzed hydration of CO2. The initial rate of acidification, 52.6 pH units per min (0.88 pH units per s), was reduced approximately 150-fold to 0.36 pH units per min by 10(-3) M methazolamide. Half-maximal inhibition occurred at a methazolamide concentration of 5.6.10(-7) M. The carbonic anhydrase inhibitor acetazolamide (10(-3) M) also greatly reduced the rate of cell acidification. The latency to the onset of carbonic anhydrase inhibition was 660 ms for methazolamide and 7.5 s for acetazolamide. The carbonic anhydrase inhibitor benzolamide (10(-4) M, 4 min exposure), which is poorly membrane permeant, had little effect on the rate of cell acidification, indicating that the site of carbonic anhydrase action was intracellular. The activity of Müller cell carbonic anhydrase may help to buffer extracellular CO2 variations in the retina.

Inward-rectifying potassium channels in retinal glial (Müller) cells

The voltage- and K(+)-dependent properties of Müller cell currents and channels were characterized in freshly dissociated salamander Müller cells. In whole-cell voltage-clamp experiments, cells with endfeet intact and cells missing endfeet both displayed strong inward rectification. The rectification was similar in shape in both groups of cells but currents were 9.2 times larger in cells with endfeet. Ba2+ at 100 microM reduced the inward current to 6.8% of control amplitude. Decreasing external K+ concentration shifted the cell current-voltage (I-V) relation in a hyperpolarizing direction and reduced current magnitude. In multichannel, cell-attached patch-clamp experiments, patches from both endfoot and soma membrane displayed strong inward rectification. Currents were 38 times larger in endfoot patches. In single-channel, cell-attached patch-clamp experiments, inward-rectifying K+ channels were, in almost all cases, the only channels present in patches of endfoot, proximal process, and soma membrane. Channel conductance was 27.8 pS in 98 mM external K+. Reducing external K+ shifted the channel reversal potential in a hyperpolarizing direction and reduced channel conductance. Channel open probability varied as a function of voltage, being reduced at more negative potentials. Together, these observations demonstrate that the principal ion channel in all Müller cell regions is an inward-rectifying K+ channel. Channel density is far higher on the cell endfoot than in other cell regions. Whole-cell I-V plots of cells bathed in 12, 7, 4, and 2.5 mM K+ were fit by an equation including Boltzmann relation terms representing channel rectification and channel open probability. This equation was incorporated into a model of K+ dynamics in the retina to evaluate the significance of inward-rectifying channels to the spatial buffering/K+ siphoning mechanism of K+ regulation. Compared with ohmic channels, inward-rectifying channels increased the rate of K+ clearance from the retina by 23% for a 1 mM K+ increase and by 137% for a 9.5 mM K+ increase, demonstrating that Müller cell inward-rectifying channels enhance K+ regulation in the retina.

Fear of humans and its relationships with productivity in laying hens at commercial farms

1. The relationship between the behavioural responses of laying hens to humans and productivity was determined at 16 commercial sheds from 14 farms. 2. A number of behaviour variables were moderately to highly correlated with production variables; for example, the proportion of birds that moved away from an approaching experimenter in an unfamiliar environment ('shute test') was negatively correlated with peak hen day production, (PKHDP). 3. Behavioural responses to humans accounted for between 23 and 63% of the variation in a number of production variables, including PKHDP and the duration of a high level of production. 4. Inclusion of farm factor variables increased the amount of variation accounted for by the behaviour variables. For example, adding the variable 'time/day spent in the shed by stockpeople' to the behaviour variables 'the proportion of birds that moved away from an approaching human' in the shute test and 'the number of times birds in cages adopted an erect posture' in response to an approaching human increased the variation accounted for in PKHDP from 53 to 61%. 5. The results suggest that fear of humans may be a factor that limits the productivity of commercial laying hens.

Sodium-bicarbonate cotransport in retinal Müller (glial) cells of the salamander

An electrogenic Na+/HCO3- cotransport system was studied in freshly dissociated Müller cells of the salamander retina. Cotransporter currents were recorded from isolated cells using the whole-cell, voltage-clamp technique following the block of K+ conductance with external Ba2+ and internal Cs+. At constant pHo, an outward current was evoked when extracellular HCO3- concentration was raised by pressure ejecting a HCO3(-)-buffered solution onto the surface of cells bathed in nominally HCO3(-)-free solution. The HCO3(-)-evoked outward current was reduced to 4.4% of control by 0.5 mM DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonate), to 28.8% of control by 2 mM DNDS (4,4'-dinitrostilbene-2,2'-disulfonate), and to 28.4% of control by 2 mM harmaline. Substitution of choline for Na+ in bath and ejection solutions reduced the response to 1.3% of control. Bicarbonate-evoked currents of normal magnitude were recorded when methane sulfonate was substituted for Cl- in bath, ejection, and intracellular solutions. Similarly, an outward current was evoked when extracellular Na+ concentration was raised in the presence of HCO3-. The Na(+)-evoked response was reduced to 16.2% of control by 2 mM DNDS and was abolished by removal of HCO3- from bath and ejection solutions. Taken together, these results (block by stilbenes and harmaline, HCO3- and Na+ dependence, Cl- independence) indicate that salamander Müller cells possess an electrogenic Na+/HCO3- cotransport system. Na+/HCO3- cotransporter sites were localized primarily at the endfoot region of Müller cells. Ejection of HCO3- onto the endfoot evoked outward currents 10 times larger than currents evoked by ejections onto the opposite (distal) end of the cell. The reversal potential of the cotransporter was determined by DNDS block of cotransport current. In the absence of a transmembrane HCO3- gradient, the reversal potential varied systematically as a function of the transmembrane Na+ gradient. The reversal potential was -0.1 mV for a [Na+]o:[Na+]i ratio of 1:1 and -25.2 mV for a Na+ gradient ratio of 7.4:1. Based on these values, the estimated stoichiometry of the cotransporter was 2.80 +/- 0.13:1 (HCO3-:Na+). Possible functions of the glial cell Na+/HCO3- cotransporter, including the regulation of CO2 in the retina and the regulation of cerebral blood flow, are discussed.

Localization and stoichiometry of electrogenic sodium bicarbonate cotransport in retinal glial cells

An electrogenic Na+/HCO3- cotransport system was identified and characterized in freshly dissociated salamander Müller (glial) cells. Under voltage-clamp, these cells generated an outward current when external HCO3- concentration [( HCO3-]o) was raised. This current was Na(+)-dependent, Cl(-)-independent, and was blocked by the stilbenes 4,4'-diisothiocyanato-stilbene-2,2'-disulfonate (DIDS) and 4,4'-dinitrostilbene-2,2'-disulfonate (DNDS), and by harmaline, demonstrating that the current was generated by a Na+/HCO3- cotransport system. Substantially larger currents were evoked when [HCO3-]o was raised at the Müller cell endfoot as compared to other cell regions, indicating that cotransporter sites are localized preferentially to the endfoot. The reversal potential of the current, which varied as a function of HCO3- and Na+ transmembrane gradients, indicated that the cotransporter has a HCO3-:Na+ stoichiometry of 3:1.

Potassium conductance block by barium in amphibian Müller cells

The effect of barium on Müller cell K+ conductance was evaluated in the tiger salamander using enzymatically dissociated cells and cells in situ (retinal slice and isolated retina). Barium effects were similar in both cases. In dissociated cells, 50 microM Ba2+ depolarized cells 14.7 mV and raised cell input resistance from a control value of 16.0 to 133 M omega. For cells in situ, 50 microM Ba2+ depolarized cells 6.9 mV and raised cell resistance from 12.5 to 50.4 M omega. At corresponding Ba2+ concentrations, the resistance of cells in situ was somewhat lower than was the resistance of dissociated cells, a phenomenon that may be due to the small degree of electrical coupling present between Müller cells in situ. There was a similar positive correlation between the magnitude of Ba2+-induced depolarization and input resistance in both dissociated cells and in situ cells. The magnitude of depolarizations generated by localized K+ ejections onto Müller cells was reduced substantially by Ba2+. These observations indicate that Ba2+ is an effective K+ channel blocker in Müller cells in situ as well as in enzymatically dissociated cells.

Spatial buffering of light-evoked potassium increases by retinal Müller (glial) cells

Activity-dependent variations in extracellular potassium concentration in the central nervous system may be regulated, in part, by potassium spatial buffering currents in glial cells. The role of spatial buffering in the retina was assessed by measuring light-evoked potassium changes in amphibian eyecups. The amplitude of potassium increases in the vitreous humor was reduced to approximately 10 percent by 50 micromolar barium, while potassium increases in the inner plexiform layer were largely unchanged. The decrease in the vitreal potassium response was accurately simulated with a numerical model of potassium current flow through Müller cells, the principal glial cells of the retina. Barium also substantially increased the input resistance of Müller cells and blocked the Müller cell-generated M-wave, indicating that barium blocks the potassium channels of Müller cells. Thus, after a light-evoked potassium increase within the retina, there is a substantial transfer of potassium from the retina to the vitreous humor by potassium current flow through Müller cells.

Potassium conductance in Müller cells of fish

The distribution of potassium conductance across the surface of retinal glial (Müller) cells was determined in three species of fishes: two teleosts, the goldfish (Carassius auratus) and the alewife (Alosa pseudoharengus), and an elasmobranch, the spiny dogfish (Squalus acanthias). Potassium conductance was measured by monitoring cell depolarizations evoked by focal ejections of a 15 mEq/L K+ solution onto the surface of freshly dissociated cells. The K+ conductance distributions observed in these three species resembled those found previously in other animals with avascular retinas. In both alewife and dogfish, K+ conductance was highest in the endfoot; K+ conductance in the distal half of these cells ranged from 7.0 to 22.9% of the endfoot conductance. In goldfish, in contrast, K+ conductance was highest in the proximal region of the proximal process (114% of the endfoot conductance). As in the two other species, however, K+ conductance in goldfish was low in the distal half of the cell (7.6 to 40.1% of endfoot conductance). Mean input resistance values of isolated cells were as follows: goldfish, 12.5 M omega; alewife, 26.4 M omega; dogfish, 38.0 M omega. The high resistance of dogfish Müller cells lacking their endfeet (749 M omega) indicates that 95% of the cell membrane conductance is located in or near the endfoot in this species.

Generation of the e-wave of the electroretinogram in the frog retina

The e-wave and a delayed-OFF increase in extracellular K+ concentration are both maximum in the distal half of the inner plexiform layer. These responses also have similar latency, time-course, intensity-dependence, surround properties, and sensitivity to tetrodotoxin. Current source-density analysis of the e-wave reveals a current sink through the proximal retina, a source at the retinal surface, and, in some cases, a weaker source in the mid-retina. These results suggest a model for e-wave generation: delayed-OFF activity in proximal neurons releases K+, which enters Muller cells in the inner plexiform layer; a current exists Muller cells primarily via their endfeet, and the return flow through extracellular space produces the e-wave.

Model of potassium dynamics in the central nervous system

A one-dimensional numerical model of potassium dynamics in the central nervous system is developed. The model incorporates the following physiological processes in computing spatial and temporal changes in extracellular K+ concentration, [K+]o: 1) the release of K+ from K+ sources into extracellular space, 2) diffusion of K+ through extracellular space, 3) active uptake of K+ into cells and blood vessels, 4) passive uptake of K+ into a cellular distribution space, and 5) the transfer of K+ by K+ spatial buffer current flow in glial cells. The following tissue parameters can be specified along the single spatial dimension of the model: 1) the volume fraction and tortuosity of extracellular and glial cell spaces, 2) the volume fraction of the cellular distribution space, 3) rate constants of active uptake and passive uptake processes, and 4) glial cell membrane conductance. The model computes variations in [K+]o and current flow through glial cells for three tissue geometries: 1) planar geometry (the retina and the surface of the brain), 2) cylindrical geometry (tissue surrounding a blood vessel), and 3) spherical geometry (tissue surrounding a point source of K+). For simple sources of K+, the performance of the model matches that predicted from analytical equations. Simulations of previous ion dynamics experiments indicate that the model can accurately predict ion diffusion and K+ current flow in the brain. Simulations of electroretinogram generation and K+ siphoning onto blood vessels suggest that unanticipated K+ dynamics mechanisms may be operating in the central nervous system.

Does the release of potassium from astrocyte endfeet regulate cerebral blood flow?

Local increases in neuronal activity within the brain lead to dilation of blood vessels and to increased regional cerebral blood flow. Increases in extracellular potassium concentration are known to dilate cerebral arterioles. Recent studies have suggested that the potassium released by active neurons is transported through astrocytic glial cells and released from their endfeet onto blood vessels. The results of computer simulations of potassium dynamics in the brain indicate that the release of potassium from astrocyte endfeet raises perivascular potassium concentration much more rapidly and to higher levels than does diffusion of potassium through extracellular space, particularly when the site of a potassium increase is some distance from the vessel wall. On the basis of this finding, it is proposed that the release of potassium from astrocyte endfeet plays an important role in regulating regional cerebral blood flow in response to changes in neuronal activity.

Distribution of potassium conductance in mammalian Müller (glial) cells: a comparative study

The distribution of K+ conductance across the surface of retinal Müller cells was determined in 5 mammalian species--rabbit, guinea pig, mouse, owl monkey, and cat--and in tiger salamander. Potassium conductance was measured by monitoring cell depolarizations evoked by focal ejections of a high-K+ solution onto the surface of freshly dissociated cells. This technique measured the total K+ conductance of a given cell region (regional conductance), i.e., the specific K+ conductance times the total surface area in that region. In mammalian species with avascular retinas (rabbit, guinea pig), the regional K+ conductance within the middle portion of the cell was only a fraction (10.6-28.9%) of the endfoot conductance, while the conductance of the distal (photo-receptor) end of the cell was approximately half (41.2-49.8%) the endfoot conductance. In 2 species with vascularized retinas (mouse and owl monkey), by contrast, the regional K+ conductance within the middle portion of the cell was as large as 125.5-129.8% of the endfoot conductance. In these cells the K+ conductance of the distal end was 68.3-82.9% of the endfoot value. In cat, a third vascularized species, the K+ conductance was highest (187.1% of the endfoot value) at the distal end of the cell. In tiger salamander, which has an avascular retina, the regional K+ conductance of all regions distal to the endfoot was only 2.4-15.7% of the endfoot value. Differences in the distributions of regional K+ conductance observed in the 6 species raise the possibility that in vascularized mammalian retinas, the high-K+ conductance of the middle portion of Müller cells is associated with retinal blood vessels. The results are consistent with the hypothesis that, in avascular species, Müller cells aid in regulating extracellular K+ levels by transferring (siphoning) excess K+ principally into the vitreous humor, while in at least some vascularized species (mouse, monkey), excess K+ is transferred by Müller cells into retinal capillaries, as well as into the vitreous.

Regulation of potassium levels by Müller cells in the vertebrate retina

The membrane properties of Müller cells, the principal glial cells of the vertebrate retina, have been characterized in a series of physiological experiments on freshly dissociated cells. In species lacking a retinal circulation (tiger salamander, rabbit, guinea pig), the end-foot of the Müller cell has a much higher K+ conductance than do other cell regions. In species with retinal circulation (mouse, cat, owl monkey) the K+ conductance of the end-foot is greater than the conductance of the proximal process of the cell. In these species, however, the K+ conductance of the soma and distal process is equal to, or greater than, the end-foot conductance. Müller cells also possess four voltage-dependent ion channels, including an inward rectifying K+ channel. These membrane specializations may aid in the regulation of extracellular K+ levels by Müller cells in the retina. High end-foot conductance shunts excess K+ out through the end-foot, where it diffuses into the vitreous humor. In vascularized retinae, excess K+ may also be transferred to the ablumenal wall of capillaries, where it could be transported into the blood.