Na + / K + -pump activity in photoreceptors of the blowfly Calliphora : a model analysis based on membrane potential measurements

Rapid cold hardening increases axonal Na+/K+-ATPase activity and enhances performance of a visual motion detection circuit in Locusta migratoria

Rapid cold hardening (RCH) is a type of phenotypic plasticity that delays the occurrence of chill coma in insects. Chill coma is mediated by a spreading depolarization of neurons and glia in the CNS, triggered by a failure of ion homeostasis. We used biochemical and electrophysiological approaches in the locust, Locusta migratoria, to test the hypothesis that the protection afforded by RCH is mediated by activation of the Na+/K+-ATPase (NKA) in neural tissue. RCH did not affect NKA activity measured in a biochemical assay of homogenized thoracic ganglia. However, RCH hyperpolarized the axon of a visual interneuron (DCMD) and increased the amplitude of an activity-dependent hyperpolarization (ADH) shown previously to be blocked by ouabain. RCH also improved performance of the visual circuitry presynaptic to DCMD to minimize habituation and increase excitability. We conclude that RCH enhances in situ NKA activity in the nervous system but also affects other neuronal properties that promote visual processing in locusts.

Modulation of voltage-dependent K+ conductances in photoreceptors trades off investment in contrast gain for bandwidth

Modulation is essential for adjusting neurons to prevailing conditions and differing demands. Yet understanding how modulators adjust neuronal properties to alter information processing remains unclear, as is the impact of neuromodulation on energy consumption. Here we combine two computational models, one Hodgkin-Huxley type and the other analytic, to investigate the effects of neuromodulation upon Drosophila melanogaster photoreceptors. Voltage-dependent K⁺ conductances in these photoreceptors: (i) activate upon depolarisation to reduce membrane resistance and adjust bandwidth to functional requirements; (ii) produce negative feedback to increase bandwidth in an energy efficient way; (iii) produce shunt-peaking thereby increasing the membrane gain bandwidth product; and (iv) inactivate to amplify low frequencies. Through their effects on the voltage-dependent K⁺ conductances, three modulators, serotonin, calmodulin and PIP2, trade-off contrast gain against membrane bandwidth. Serotonin shifts the photoreceptor performance towards higher contrast gains and lower membrane bandwidths, whereas PIP2 and calmodulin shift performance towards lower contrast gains and higher membrane bandwidths. These neuromodulators have little effect upon the overall energy consumed by photoreceptors, instead they redistribute the energy invested in gain versus bandwidth. This demonstrates how modulators can shift neuronal information processing within the limitations of biophysics and energy consumption.

The properties of neurons and neural circuits can be adjusted by neuromodulators, molecules that alter their ability to respond to future activity. Many neuromodulators target voltage-dependent ion channels, molecular components of cell membranes that influence the electrical activity of neurons. Because of their importance, the action of neuromodulators upon voltage-dependent ion channels and the subsequent changes in neural activity has been studied extensively. However, the properties of voltage-dependent ion channels also influence the energy that neural signalling consumes. Here we assess the impact of neuromodulators upon neuronal energy consumption. We use analytical and computational models to determine the impact of different neuromodulators upon the signalling properties and energy consumption of fly photoreceptors. Our models uncover previously unknown properties of voltage-dependent ion channels in fly photoreceptors, showing how they adjust the membrane properties, gain and bandwidth, to prevailing light levels. Neuromodulators alter voltage-dependent ion channel properties, adjusting the gain and bandwidth. Although neuromodulators do not substantially alter the overall energy consumption of photoreceptors, they redistribute energy investment in gain and bandwidth. Hence, our models provide novel insights into the functions that neuromodulators play in neurons and neural circuits.

Voltage-dependent K+ channels improve the energy efficiency of signalling in blowfly photoreceptors

Voltage-dependent conductances in many spiking neurons are tuned to reduce action potential energy consumption, so improving the energy efficiency of spike coding. However, the contribution of voltage-dependent conductances to the energy efficiency of analogue coding, by graded potentials in dendrites and non-spiking neurons, remains unclear. We investigate the contribution of voltage-dependent conductances to the energy efficiency of analogue coding by modelling blowfly R1-6 photoreceptor membrane. Two voltage-dependent delayed rectifier K⁺ conductances (DRs) shape the membrane's voltage response and contribute to light adaptation. They make two types of energy saving. By reducing membrane resistance upon depolarization they convert the cheap, low bandwidth membrane needed in dim light to the expensive high bandwidth membrane needed in bright light. This investment of energy in bandwidth according to functional requirements can halve daily energy consumption. Second, DRs produce negative feedback that reduces membrane impedance and increases bandwidth. This negative feedback allows an active membrane with DRs to consume at least 30% less energy than a passive membrane with the same capacitance and bandwidth. Voltage-dependent conductances in other non-spiking neurons, and in dendrites, might be organized to make similar savings.

Development and plasticity of mitochondria and electrical properties of the cell membrane in blowfly photoreceptors

Blowfly photoreceptors are highly energy demanding sensory systems. Their information processing efficiency is enabled by the high temporal resolution of the cell membrane, requiring heavy metabolic support by the mitochondria. We studied the developmental changes of the mitochondrial apparatus and electrical properties of the photoreceptor membrane in the white eyed Calliphora vicina Chalky. Using in vivo microspectrophotometry and Western blot analysis, we found an age-dependent increase in the concentration of mitochondrial pigments. The maximal change occurred during the first week. The age-related changes were smaller in dark-bred than in light-bred flies. The mitochondrial pigment content increased after the switch from dark to light rearing and decreased after the switch from light to dark rearing. The electrical parameters of the photoreceptors were investigated with intracellular recordings. The resting membrane resistance and time constant decreased significantly after eclosion. The decrease was again most significant during the first week of adult life, paralleled with changes in the Na/K pump-dependent hyperpolarizing afterpotential. We conclude that the photoreceptor mitochondria exhibit remarkable ontogenetic and phenotypic plasticity, because the quantity of mitochondrial pigments tightly follows the development of the cell membrane as well as the energy demands of the photoreceptors under different rearing conditions.

Fly photoreceptors demonstrate energy-information trade-offs in neural coding

Trade-offs between energy consumption and neuronal performance must shape the design and evolution of nervous systems, but we lack empirical data showing how neuronal energy costs vary according to performance. Using intracellular recordings from the intact retinas of four flies, Drosophila melanogaster, D. virilis, Calliphora vicina, and Sarcophaga carnaria, we measured the rates at which homologous R1–6 photoreceptors of these species transmit information from the same stimuli and estimated the energy they consumed. In all species, both information rate and energy consumption increase with light intensity. Energy consumption rises from a baseline, the energy required to maintain the dark resting potential. This substantial fixed cost, ∼20% of a photoreceptor's maximum consumption, causes the unit cost of information (ATP molecules hydrolysed per bit) to fall as information rate increases. The highest information rates, achieved at bright daylight levels, differed according to species, from ∼200 bits s⁻¹ in D. melanogaster to ∼1,000 bits s⁻¹ in S. carnaria. Comparing species, the fixed cost, the total cost of signalling, and the unit cost (cost per bit) all increase with a photoreceptor's highest information rate to make information more expensive in higher performance cells. This law of diminishing returns promotes the evolution of economical structures by severely penalising overcapacity. Similar relationships could influence the function and design of many neurons because they are subject to similar biophysical constraints on information throughput.

Many animals show striking reductions or enlargements of sense organs or brain regions according to their lifestyle and habitat. For example, cave dwelling or subterranean animals often have reduced eyes and brain regions involved in visual processing. These differences suggest that although there are benefits to possessing a particular sense organ or brain region, there are also significant costs that shape the evolution of the nervous system, but little is known about this trade-off, particularly at the level of single neurons. We measured the trade-off between performance and energetic costs by recording electrical signals from single photoreceptors in different fly species. We discovered that photoreceptors in the blowfly transmit five times more information than the smaller photoreceptors of the diminutive fruit fly Drosophila. The blowfly pays a high price for better performance; its photoreceptor uses ten times more energy to code the same quantity of information. We conclude that, for basic biophysical reasons, neuronal energy consumption increases much more steeply than performance, and this intensifies the evolutionary pressure to reduce performance to the minimum required for adequate function. Thus the biophysical properties of sensory neurons help to explain why the sense organs and brains of different species vary in size and performance.

Evidence from single-neuron recordings supports the law of diminishing returns, i.e., high performance eyes in larger, faster flies have less efficient photoreceptors than those of their small, sluggish counterparts.

Light dependence of oxygen consumption by blowfly eyes recorded with a magnetic diver balance

We measured the oxygen (O2) consumption of isolated blowfly eyes using a magnetic diver balance, a device for high-resolution volumetric O2 consumption measurements. The light-induced O2 consumption is at most three times the value of the dark consumption, which is 0.6 nl O2 s(-1) eye(-1), and is in good agreement with the estimates based on electrophysiological data. With longer stimuli the increase follows a double exponential time course. The respective time constants are approximately 2 and 20 s and show no dependence on light intensity, whereas the dependence of amplitudes can be fitted by a Hill equation. Decreasing the stimulus duration reveals that the peak in O2 consumption overshoots the time course induced by long stimuli. We suggest this may be a general feature of mitochondrial activation. The dependence of the O2 consumption peak on stimulus duration at high light intensity has a hump with stimulus durations of 10-20 ms, coinciding with the stimulus durations that start to induce the adaptation of the receptor potential.

Interactions Between Light-Induced Currents, Voltage-Gated Currents, and Input Signal Properties inDrosophilaPhotoreceptors

Voltage-gated K+channels are important in neuronal signaling, but little is known of their interactions with receptor currents or their behavior during natural stimulation. We used nonparametric and parametric nonlinear modeling of experimental responses, combined with Hodgkin–Huxley style simulation, to examine the roles of K+channels in forming the responses of wild-type (WT) and Shaker mutant ( Sh14) Drosophila photoreceptors to naturalistic stimulus sequences. Naturalistic stimuli gave results different from those of similar experiments with white noise stimuli. Sh14responses were larger and faster than WT. Simulation indicated that, in addition to eliminating the Shaker current, the mutation changed the current flowing through light-dependent channels [light-induced current (LIC)] and increased the delayed rectifier current. Part of the change in LIC could be attributed to direct feedback from the voltage-sensitive ion channels to the light-sensitive channels by the membrane potential. However, we argue that other changes occur in the light detecting machinery of Sh14mutants, possibly during photoreceptor development.

Dendritic calcium accumulation associated with direction-selective adaptation in visual motion-sensitive neurons in vivo

Motion adaptation in directionally selective tangential cells (TC) of the fly visual system has previously been explained as a presynaptic mechanism. Based on the observation that adaptation is in part direction selective, which is not accounted for by the former models of motion adaptation, we investigated whether physiological changes located in the TC dendrite can contribute to motion adaptation. Visual motion in the neuron's preferred direction (PD) induced stronger adaptation than motion in the opposite direction and was followed by an afterhyperpolarization (AHP). The AHP subsides in the same time as adaptation recovers. By combining in vivo calcium fluorescence imaging with intracellular recording, we show that dendritic calcium accumulation following motion in the PD is correlated with the AHP. These results are consistent with a calcium-dependent physiological change in TCs underlying adaptation during continuous stimulation with PD motion, expressing itself as an AHP after the stimulus stops. However, direction selectivity of adaptation is probably not solely related to a calcium-dependent mechanism because direction-selective effects can also be observed for fast moving stimuli, which do not induce sizeable calcium accumulation. In addition, a comparison of two classes of TCs revealed differences in the relationship of calcium accumulation and AHP when the stimulus velocity was varied. Thus the potential role of calcium in motion adaptation depends on stimulation parameters and cell class.

Calcium imaging demonstrates colocalization of calcium influx and extrusion in fly photoreceptors

During illumination, Ca(2+) enters fly photoreceptor cells through light-activated channels that are located in the rhabdomere, the compartment specialized for phototransduction. From the rhabdomere, Ca(2+) diffuses into the cell body. We visualize this process by rapidly imaging the fluorescence in a cross section of a photoreceptor cell injected with a fluorescent Ca(2+) indicator in vivo. The free Ca(2+) concentration in the rhabdomere shows a very fast and large transient shortly after light onset. The free Ca(2+) concentration in the cell body rises more slowly and displays a much smaller transient. After approximately 400 ms of light stimulation, the Ca(2+) concentration in both compartments reaches a steady state, indicating that thereafter an amount of Ca(2+), equivalent to the amount of Ca(2+) flowing into the cell, is extruded. Quantitative analysis demonstrates that during the steady state, the free Ca(2+) concentration in the rhabdomere and throughout the cell body is the same. This shows that Ca(2+) extrusion takes place very close to the location of Ca(2+) influx, the rhabdomere, because otherwise gradients in the steady-state distribution of Ca(2+) should be measured. The close colocalization of Ca(2+) influx and Ca(2+) extrusion ensures that, after turning off the light, Ca(2+) removal from the rhabdomere is faster than from the cell body. This is functionally significant because it ensures rapid dark adaptation.

Potentiation in the first visual synapse of the fly compound eye

In the first visual synapse of the insect compound eye, both the presynaptic and postsynaptic signals are graded, nonspiking changes in membrane voltage. The synapse exhibits tonic transmitter release (even in dark) and strong adaptation to long-lasting light backgrounds, leading to changes also in the dynamics of signal transmission. We have studied these adaptational properties of the first visual synapse of the blowfly Calliphora vicina. Investigations were done in situ by intracellular recordings from the presynaptic photoreceptors, photoreceptor axon terminals, and the postsynaptic first order visual interneurons (LMCs). The dark recovery, the shifts in intensity dependence, and the underlying processes were studied by stimulating the visual system with various adapting stimuli while observing the recovery (i.e., dark adaptation). The findings show a transient potentiation in the postsynaptic responses after intense light adaptation, and the underlying mechanisms seem to be the changes in the equilibrium potential of the transmitter-gated conductance (chloride) of the postsynaptic neurons. The potentiation by itself serves as a mechanism that after light adaptation rapidly recovers the sensitivity loss of the visual system. However, this kind of mechanism, being an intrinsic property of graded potential transmission, may be quite widespread among graded synapses, and the phenomenon demonstrates that functional plasticity is also a property of graded synaptic transmission.

The metabolic cost of neural information

We derive experimentally based estimates of the energy used by neural mechanisms to code known quantities of information. Biophysical measurements from cells in the blowfly retina yield estimates of the ATP required to generate graded (analog) electrical signals that transmit known amounts of information. Energy consumption is several orders of magnitude greater than the thermodynamic minimum. It costs 10(4) ATP molecules to transmit a bit at a chemical synapse, and 10(6)-10(7) ATP for graded signals in an interneuron or a photoreceptor, or for spike coding. Therefore, in noise-limited signaling systems, a weak pathway of low capacity transmits information more economically, which promotes the distribution of information among multiple pathways.

A quantitative estimate of flash-induced Ca(2+)- and Na(+)-influx and Na+/Ca(2+)-exchange in blowfly Calliphora photoreceptors

The flash-induced Ca(2+)- and Na(+)-influx and Na+/Ca(2+)-exchange activity in blowfly Calliphora photoreceptors were investigated. The change in membrane potential, induced by a bright flash, was intracellularly measured in vivo. Based on a biophysical photoreceptor model, the Na(+)- and Ca(2+)-currents and concentration changes were determined from the first transient depolarization phase of the photoreceptor response. The activity of Na+/Ca(2+)-exchange was determined from the after depolarization phase. It appeared that the Na(+)-influx by Na+/Ca(2+)-exchange is about twice that through light-activated channels, suggesting a substantial contribution of Na+/Ca(2+)-exchange to Na(+)-regulation.