Network adaptation improves temporal representation of naturalistic stimuli in drosophila eye: II Mechanisms

Retinal networks must adapt constantly to best present the ever changing visual world to the brain. Here we test the hypothesis that adaptation is a result of different mechanisms at several synaptic connections within the network. In a companion paper (Part I), we showed that adaptation in the photoreceptors (R1-R6) and large monopolar cells (LMC) of the Drosophila eye improves sensitivity to under-represented signals in seconds by enhancing both the amplitude and frequency distribution of LMCs' voltage responses to repeated naturalistic contrast series. In this paper, we show that such adaptation needs both the light-mediated conductance and feedback-mediated synaptic conductance. A faulty feedforward pathway in histamine receptor mutant flies speeds up the LMC output, mimicking extreme light adaptation. A faulty feedback pathway from L2 LMCs to photoreceptors slows down the LMC output, mimicking dark adaptation. These results underline the importance of network adaptation for efficient coding, and as a mechanism for selectively regulating the size and speed of signals in neurons. We suggest that concert action of many different mechanisms and neural connections are responsible for adaptation to visual stimuli. Further, our results demonstrate the need for detailed circuit reconstructions like that of the Drosophila lamina, to understand how networks process information

Impacts of climate warming on Alpine lake biota over the past decade

Alpine temperatures have risen at twice the rate compared to the northern-hemispheric average during the past century. This can be expected to affect Alpine lake ecosystems via, for example, intensified thermal stratification, shorter ice cover periods, and altered catchment processes. Our study assesses changes in some main constituents of the plank tic and benthic communities of five mid-Alpine lakes in the Niedere Tauern region in Austria in relation to climatic warming, by comparing community and environmental data from 1998-1999 to data from 2010-2011. Although lake chemistry remained relatively stable between the study periods, we observed an increase in lake water temperatures and a decrease in ice cover durations. Several of the dominant diatom species and chrysophyte cyst types show relatively clear changes; the responses of the whole communities, however, are less evident. Yet, in particular, diatoms show distinct assemblage changes along the climatic gradients in the two lakes with the largest decrease in ice-cover duration. Chironomid communities appear to be less sensitive compared to diatoms and chrysophyte cysts, which are known for reacting quickly to changes in their environment. Finally, Alpine lakes, which are moderately nutrient-enriched because of human activities in the catchment area, are likely to experience increases in their productivity with climate warming.Peer reviewe

Cellular elements for seeing in the dark: voltage-dependent conductances in cockroach photoreceptors

BACKGROUND: The importance of voltage-dependent conductances in sensory information processing is well-established in insect photoreceptors. Here we present the characterization of electrical properties in photoreceptors of the cockroach (Periplaneta americana), a nocturnal insect with a visual system adapted for dim light. RESULTS: Whole-cell patch-clamped photoreceptors had high capacitances and input resistances, indicating large photosensitive rhabdomeres suitable for efficient photon capture and amplification of small photocurrents at low light levels. Two voltage-dependent potassium conductances were found in the photoreceptors: a delayed rectifier type (KDR) and a fast transient inactivating type (KA). Activation of KDR occurred during physiological voltage responses induced by light stimulation, whereas KA was nearly fully inactivated already at the dark resting potential. In addition, hyperpolarization of photoreceptors activated a small-amplitude inward-rectifying (IR) current mediated at least partially by chloride. Computer simulations showed that KDR shapes light responses by opposing the light-induced depolarization and speeding up the membrane time constant, whereas KA and IR have a negligible role in the majority of cells. However, larger KA conductances were found in smaller and rapidly adapting photoreceptors, where KA could have a functional role. CONCLUSIONS: The relative expression of KA and KDR in cockroach photoreceptors was opposite to the previously hypothesized framework for dark-active insects, necessitating further comparative work on the conductances. In general, the varying deployment of stereotypical K+ conductances in insect photoreceptors highlights their functional flexibility in neural coding