Adaptation and facilitation in the barnacle photoreceptor

The barnacle photoreceptor sensitivity may either decrease (light adaptation) or increase (facilitation) after exposure to a conditioning light. The balance between adaptation and facilitation is influenced by at least three factors: initial sensitivity state of the cell, external calcium concentration, and conditioning intensity. Cells of very high sensitivity show mainly adaptation, which appears only for higher conditioning intensities and is suppressed in low-calcium media. Less sensitive cells, or those whose sensitivity is reduced by injury or metabolic decay, exhibit facilitation, expecially in low-calcium media and at intermediate conditioning intensities. Both phenomena show recovery time-courses of seconds-to-minutes. Models are proposed which relate light adaptation, as previously suggested, to increased internal calcium concentration, and facilitation either to decreased internal calcium concentration or to decreased activation "affinity" of ion-channel-blocking sites.     

Sensitivity facilitation in the insect eye

Summary ERG amplitude facilitation, observed in the eye ofAtta sexdens after light adaptation, was studied as a function of duration and intensity of adaptation, of dark interval between adapting and test stimuli, and of level of steady background illumination. Results show that sensitivity facilitation in this eye cannot be regarded as a minor effect since it covers a 2 log unit range, the same as that obtained for conditions that produce sensitivity reduction. Maximum facilitation occurs with short and intense light adaptation. The time span of the effect is close to 2 min, and its maximum amplitude may be attained up to 20 s after light adaptation. Increase in background illumination gradually erases facilitation. However, the facilitated response is less sensitive to background illumination than the dark adapted response. Long durations of light adaptation cause ERG decrease, or inhibition. A comparison of these two end results of light adaptation suggests that they arise from different processes, perhaps with distinct origins.Supported by a grant from Fundação de Amparo à Pesquisa do Estado de São Paulo, to the senior author (Contract n 71/1141)With a Fellowship from Fundação de Amparo à Pesquisa do Estado de São Paulo (N 74/388)We wish to express our appreciation to Henrique Fix for his editorial assistance, and to Celia Jablonka for laboratory help.     

The contribution of pigment transitions to sensitivity changes in the barnacle photoreceptor and the correlation with the prolonged depolarizing afterpotential

A conditioning light can cause a decrease (adaptation) or an increase (facilitation) in the sensitivity of barnacle photoreceptors, as measured by the amplitude of the late receptor potential (LRP). We show that a net transfer of visual pigment from the rhodopsin (R) to the metarhodopsin (M) state induces a large facilitation whereas the reverse transfer results in a much smaller facilitation or even an adaptation. These effects were not due to the response to the conditioning light but to the pigment reactions. When the conditioning light did not alter the pigment population (i.e., M M, R R) it was followed by an intermediate degree of facilitation. These conclusions are correct for cells which have relatively low sensitivity. In sensitive cells, all pigment transitions produce adaptation.LRP facilitation and the prolonged depolarizing afterpotential (PDA) show several common characteristics with respect to pigment transitions: 1.Their magnitude increases with the amount of pigment transferred from R to M. 2. Both are depressed by the M R transition. 3. Their production is impeded by the M R transition. 4. The PDA itself is facilitated by the R M transition and this facilitation decays with a time course comparable to that of LRP facilitation. These results suggest that there may be an underlying process common to LRP facilitation and PDA.     

Electrical studies on the compound eye of Ligia occidentalis Dana (Crustacea: Isopoda)

The ERG of the compound eye in freshly collected Ligia occidentalis, in response to high intensity light flashes of ⅛ second or longer duration, begins with a negative on-effect quickly followed by an early positive deflection, rapidly returns to the baseline during illumination, and ends with a positive off-effect. As the stimulus intensity is decreased the early positivity progressively decreases and the rapid return to the baseline is replaced by a slowing decline of the negative on-effect. Responses were recorded with one active electrode subcorneally situated in the illuminated eye, the reference electrode in the dark eye. The dark-adapted eye shows a facilitation of the amplitude and rates of rise and fall of the on-effect to a brief, high intensity light stimulus. This facilitation may persist for more than 2 minutes. Following light adaptation under conditions in which the human eye loses sensitivity by a factor of almost 40,000 the Ligia eye loses sensitivity by a factor of only 3. The flicker fusion frequency of the ERG may be as high as 120/second with a corneal illumination of 15,000 foot-candles. Bleeding an otherwise intact animal very rapidly results in a decline of amplitude, change of wave form, and loss of facilitation in the ERG. When the eye is deganglionated without bleeding the animal the isolated retina responds in the same manner as the intact eye. Histological examination of the Ligia receptor layer showed that each ommatidium contains three different retinula cell types, each of which may be responsible for a different aspect of the ERG.     

Detection of red and green flashes: evidence for cancellation and facilitation.

Green and red flashes of light will differentially stimulate the middle- and long-wavelength sensitive cones. Interaction of cone signals was studied by measuring increment thresholds for combinations of green and red flashes on a yellow adapting field. When the yellow adapting field was at 10.000 trolands (td), green and red incremental flashes (1 degree, 200-msec duration) produced cancellation when presented simultaneously and facilitation when presented sequentially. A green incremental flash (1.15 degrees, 200 msec, 5000-td adaptation field) and red decremental flash, or vice versa, produced facilitation when presented simultaneously. The results can be explained by color-differencing, opponent-mechanisms. The cancellation effect for the simultaneous incremental flashes largely disappeared when the flashes were exposed briefly (10 msec) or reduced in size (0.04 degrees). It is unlikely that the stimuli were exclusively detected by achromatic, luminance channels, as suggested by previous work, since observers could partially distinguish the hue of threshold flashes of 570- and 590-nm light (0.04 degrees, 10 msec) on a bright yellow field.     

Evolution of local facilitation in arid ecosystems

In harsh environments, sessile organisms can make their habitat more hospitable by buffering environmental stress or increasing resource availability. Although the ecological significance of such local facilitation is widely established, the evolutionary aspects have been seldom investigated. Yet addressing the evolutionary aspects of local facilitation is important because theoretical studies show that systems with such positive interactions can exhibit alternative stable states and that such systems may suddenly become extinct when they evolve (evolutionary suicide). Arid ecosystems currently experience strong changes in climate and human pressures, but little is known about the effects of these changes on the selective pressures exerted on the vegetation. Here, we focus on the evolution of local facilitation in arid ecosystems, using a lattice-structured model explicitly considering local interactions among plants. We found that the evolution of local facilitation depends on the seed dispersal strategy. In systems characterized by short-distance seed dispersal, adaptation to a more stressful environment leads to high local facilitation, allowing the population to escape extinction. In contrast, systems characterized by long-distance seed dispersal become extinct under increased stress even when allowed to adapt. In this case, adaptation in response to climate change and human pressures could give the final push to the desertification of arid ecosystems.     

ON THE MECHANISM OF ADAPTATION TO PROTEIN SYNTHESIS INHIBITORS BY TETRAHYMENA : Facilitation, Cross Adaptation, and Resensitization

Tetrahymena is able to adapt to the presence of sublethal concentrations of many drugs which inhibit a wide variety of cellular functions. In spite of the generality of this phenomenon in Tetrahymena, the mechanism of adaptation at the cellular and molecular levels is unknown. This study deals mainly with adaptation to the protein synthesis inhibitors, cycloheximide and emetine. The physiological response of Tetrahymena to sublethal concentrations of these drugs is an immediate cessation of cell division for a period of time dependent on the drug concentration, followed by an abrupt resumption of exponential growth at a constant rate. By measuring the length of the growth lags under a variety of experimental conditions, we have confirmed several observations made by Frankel and coworkers, and provide evidence for two new phenomena associated with adaptation to cycloheximide: (a) adaptation to cycloheximide also results in adaptation of cells to emetine, another protein synthesis inhibitor not closely related structurally to cycloheximide. We have termed this phenomenon cross adaptation, (b) exposure to concentrations of cycloheximide too low to cause any growth lags or inhibition of protein synthesis significantly shortens the time required by cells to adapt to higher concentrations of cycloheximide. We have termed this phenomenon facilitation. Facilitation shows some degree of specificity in that facilitation with cycloheximide has no effect on adaptation to emetine. From this, we infer the existence of two distinct systems involved in adaptation to cycloheximide, one of which shows a higher degree of specificity towards cycloheximide than the other. We also show that transfer of adapted or facilitated cells to drug-free medium results in a gradual but complete resensitization. The kinetics of resensitization suggest that the cellular machinery responsible for adaptation and facilitation does not leave the cell, but is simply diluted out during cell division.     

Interactions between mountain birch seedlings from differentiated populations in contrasting environments of subarctic Russia

So far very few experiments have accounted for the combined effect of two phenomena co-occurring in stress gradients: local adaptation to stress and the increase in facilitation with increasing stress (predicted by the stress-gradient hypothesis, SGH). Mountain birch (Betula pubescens subsp. czerepanovii) facilitates conspecific seedlings in subarctic high stress sites and is capable of rapid evolutionary adaptation, being therefore a good model species for a study combining local ecotypes and SGH. A within-species experiment was conducted to test SGH in three stress gradients, detect potential local adaptations between low and high stress populations, and assess their effects on seedling-seedling interactions. Although no evidence for local adaptation was detected, high and low stress populations showed some differentiation, possibly explained by decreasing phenotypic plasticity in high stress conditions and/or neutral evolutionary mechanisms. Weak support for SGH was detected. While facilitation was unaffected by seedling origin, low stress populations showed better competitive ability.     

Effect of light on the feeding rates of pelagic and littoral mysid shrimps: a trade-off between feeding success and predation avoidance

It has been shown that freshwater pelagic mysid shrimps capture zooplankton at a higher rate in light than in darkness. This has been suggested to be due to facilitation of visual predation on evasive zooplankton prey. To test this hypothesis with Baltic mysid shrimps, and to see whether pelagic (migrating) and littoral (non-migrating) mysids differ in this respect, we compared the feeding rates of Mysis mixta and Praunus flexuosus on the copepod Acartia spp. As light conditions change radically from the beginning of summer to late autumn at the Baltic latitudes, we conducted the experiments during three different times of the year to determine if there was a seasonal response to light in mysids. The feeding rates of pelagic mysids were significantly higher in total darkness than in light. In contrast, the feeding rates of littoral mysids did not differ in the dark and the light in the three different seasons. The decreased feeding of pelagic mysids under well-lit conditions may be an adaptation to avoid visual predation by pelagic fish. In contrast, littoral mysids, which live in the well-lit layer throughout the day, do not show a similar response. The fact that light did not increase feeding in either of the mysid species indicates that these mysid species do not use vision in capturing prey, but rely mainly on mechano-reception.     

Long-term facilitation and long-term adaptation at synapses of a crayfish phasic motoneuron

Stimulation of the phasic (fast) motor axon of the isolated crayfish claw preparation at relatively low frequency (0.1 Hz) leads to depression of the excitatory junction potential (EJP) recorded from single muscle fibers. When the same stimulation is delivered following depression of the EJP at a higher frequency (5 Hz), a potentiated EJP appears, which is more resistant to low frequency depression. The potentiation appears to be analogous to "long-term facilitation" observed after stimulation of a tonic motor axon in crayfish and crabs. Long-term facilitation can be detected in preparations made from claws of animals in which the phasic motoneuron was stimulated at 5 Hz for 2 h in situ. This effect lasts for at least one day after one conditioning trial. Long-term facilitation is observed after stimulation of decentralized axons in situ, indicating that the change is attributable to local changes in terminal regions of the axon, and does not require the cell body. When electrodes are implanted in situ and the phasic motoneuron stimulated at 5 Hz for 2 h each day, synaptic depression becomes less pronounced and initial EJP amplitude becomes smaller over a period of several days. The latter changes, which adapt the neuron to a more tonic activity pattern, usually require several days for completion. Adaptation of fatigability occurs more rapidly than adaptation of initial EJP amplitude, and once established, remains for many days without further superimposed activity. Long-term adaptation does not occur in decentralized axons. Long-term facilitation and long-term adaptation are different responses of the neuron to enhanced activity. The former can occur in isolated or decentralized axons and leads to enhancement of EJP amplitude for a period of several hours to at least one day after a single episode of conditioning. The latter requires more time to be established, and leads to reduction of initial EJP amplitude and to lessened fatigability which persists for many days.     

Synaptic plasticity in a regenerated crayfish phasic motoneuron.

Crustacean neuromuscular systems provide many advantages for the study of synaptic transmission and plasticity. The present study examines aspects of synaptic transmission in the phasic, fast closer excitor (FCE) motoneuron of regenerated crayfish claws. Excitatory postsynaptic potentials (EPSPs) fatigued rapidly and showed poor long-term facilitation (LTF) in the smallest of regenerating claws. EPSPs in larger regenerating claws fatigued less and showed pronounced facilitation. These observations were not the same as those previously made during primary development of this motoneuron (Lnenicka and Atwood, 1985a, J. Neuroscience 5:459-467). Hence, regeneration is not the recapitulation of primary development. In situ stimulation of the FCE is known to lead to long-lasting adaptation of synaptic performance. This adaptation is age dependent; it is expressed in young but not old animals. In the regenerated FCE of old animals, we observed a novel form of long-lasting adaptation to imposed activity: EPSPs showed large initial EPSPs and did not exhibit resistance to fatigue during maintained stimulation. This indicates that aged motoneurons can express adaptive changes to increased activity following axonal regeneration, but that the adaptive changes are the opposite to what is observed in nonregenerated motoneurons.     

Synaptic plasticity in a regenerated crayfish phasic motoneuron

Crustacean neuromuscular systems provide many advantages for the study of synaptic transmission and plasticity. The present study examines aspects of synaptic transmission in the phasic, fast closer excitor (FCE) motoneuron of regenerated crayfish claws. Excitatory postsynaptic potentials (EPSPs) fatigued rapidly and showed poor long-term facilitation (LTF) in the smallest of regenerating claws. EPSPs in larger regenerating claws fatigued less and showed pronounced facilitation. These observations were not the same as those previously made during primary development of this motoneuron (Lnenicka and Atwood, 1985a, J. Neuroscience 5:459–467). Hence, regeneration is not the recapitulation of primary development. In situ stimulation of the FCE is known to lead to long-lasting adaptation of synaptic performance. This adaptation is age dependent; it is expressed in young but not old animals. In the regenerated FCE of old animals, we observed a novel form of long-lasting adaptation to imposed activity: EPSPs showed large initial EPSPs and did not exhibit resistance to fatigue during maintained stimulation. This indicates that aged motoneurons can express adaptive changes to increased activity following axonal regeneration, but that the adaptive changes are the opposite to what is observed in nonregenerated motoneurons. © 1992 John Wiley & Sons, Inc.     

Inhibition and efferent facilitation of sensory activity in the isolated labyrinth of the frog

The functioning modalities of the efferent system were analysed in the isolated frog labyrinth. The efferent synapses of the posterior canal were activated via an axon reflex by antidromic electrical shocks (10-200 Hz) applied for increasing times (250 ms-10 s) to the anterior-horizontal nerves. Either decrease (inhibition) or increase (facilitation) in the resting discharge rate were observed in the majority of the units examined. Inhibition and facilitation, however, are peculiar to any given unit since inhibition does not reverse to facilitation or vice-versa. This fact as well as the long response latency (not less than 10 ms) and the linear dependence of both effects on the stimulation frequency suggest that inhibition and facilitation are due to the repetitive activation of two different types of efferent fibres synapsing on the hair cells. The drastic modifications in the afferent synaptic discharge produced by full activation of the efferent system indicate that the static properties (response asymmetry) as well as the dynamic properties (response adaptation) of the mechanically driven afferent response can be substantially controlled by the central nervous system at the receptor level.     

Schooling behaviour and retinomotor response of juvenile Pacific bluefin tuna Thunnus orientalis under different light intensities

Schooling behaviour and histological retinal light adaptation in juvenile Pacific bluefin tuna Thunnus orientalis were examined under various light intensities to determine the effect of light intensity on behaviour. After monitoring the schooling behaviour of juveniles 35–36 and 45–46 days post hatching, schooling variables such as nearest neighbour distance and separation swimming index were measured under different light intensities. Furthermore, retinal indices of light adaptation were investigated histologically for each experimental light intensity. Under intensities >5 lx, schooling variables in the two juvenile growth stages were nearly constant, allowing schooling. In contrast, the schooling variables indicated that the fish gradually swam more widely and randomly with decreasing light intensities <5 lx. The retinal indices also showed a shift from light adaptation to dark adaptation at light levels <5 lx. From 5 to 0·01 lx, retinal adaptation and fish schooling behaviour changed with light intensity. These data suggest that the schooling behaviour of juvenile Pacific bluefin tuna is greatly affected by retinal adaptation.     

THE COURSE OF ROD DARK ADAPTATION AS INFLUENCED BY THE INTENSITY AND DURATION OF PRE-ADAPTATION TO LIGHT

An increase in the degree of light adaptation causes a decrease in the slope of the subsequent rod dark adaptation function and a displacement of the function to the right on the time axis. Over a wide range, these changes occur to the same extent whether the increase in the degree of light adaptation is produced by raising the intensity or by prolonging the exposure. Within these limits, the Bunsen-Roscoe reciprocity law applies to the intensity and duration of pre-exposure. Over a still wider range, dark adaptation has the same course following brief exposure to a bright light as it has following prolonged exposure to a dim light, provided the degree of light adaptation is the same in both instances (as indicated by identical initial dark adaptation thresholds).     

The role of cytoplasmic calcium in photoreceptor light adaptation.

The process of light adaptation in vertebrate rod and cone photoreceptors is believed to involve a diffusible cytoplasmic messenger. Two lines of evidence indicate that photoreceptor light adaptation is mediated by a light-induced fall in cytoplasmic calcium concentration (Ca2+i). First, if changes in calcium concentration are slowed by the incorporation of calcium chelators into the photoreceptor cytoplasm then light adaptation is slowed also. Second, if the normal control of Ca2+i is prevented by simultaneously minimising calcium influx and efflux across the outer segment membrane by means of external solution changes, then all of the manifestations of light adaptation are abolished. Furthermore, recent results show that changes in Ca2+i imposed in the absence of light are sufficient to cause at least some of the manifestations of light adaptation. Together these results indicate that calcium acts as the messenger of light adaptation in the photoreceptors of both lower and higher vertebrates.     

Designing and implementing two facilitation interventions within the ‘Facilitating Implementation of Research Evidence (FIRE)’ study: a qualitative analysis from an external facilitators’ perspective

Background

The ‘Facilitating Implementation of Research Evidence’ study found no significant differences between sites that received two types of facilitation support and those that did not on the primary outcome of documented compliance with guideline recommendations. Process evaluation highlighted factors that influenced local, internal facilitators’ ability to enact the roles as envisaged. In this paper, the external facilitators responsible for designing and delivering the two types of facilitation intervention analyse why the interventions proved difficult to implement as expected, including the challenge of balancing fidelity and adaptation.

Methods

Qualitative data sources included notes from monthly internal-external facilitator teleconference meetings, from closing events for the two facilitation interventions and summary data analyses from repeated interviews with 16 internal facilitators. Deductive and inductive data analysis was led by an independent researcher to evaluate how facilitation in practice compared to the logic pathways designed to guide fidelity in the delivery of the interventions.

Results

The planned facilitation interventions did not work as predicted. Difficulties were encountered in each of the five elements of the logic pathway: recruitment and selection of appropriate internal facilitators, preparation for the role, ability to apply facilitation knowledge and skills at a local level, support and mentorship from external facilitators via monthly teleconferences, working collaboratively and enabling colleagues to implement guideline recommendations. Moreover, problems were cumulative and created tensions for the external facilitators in terms of balancing the logic pathway with a more real-world, flexible and iterative approach to facilitation.

Conclusion

Evaluating an intervention that is fluid and dynamic within the methodology of a randomised controlled trial is complex and challenging. At a practical level, relational aspects of facilitation are critically important. It is essential to recruit and retain individuals with the appropriate set of skills and characteristics, explicit support from managerial leaders and accessible mentorship from more experienced facilitators. At a methodological level, there is a need for attention to the balance between fidelity and adaptation of interventions. For future studies, we suggest a theoretical approach to fidelity, with a focus on mechanisms, informed by prospective use of process evaluation data and more detailed investigation of the context-facilitation dynamic.

     

Adaptation in the Ventral Eye of Limulus is Functionally Independent of the Photochemical Cycle, Membrane Potential, and Membrane Resistance

The early receptor potential (ERP), membrane potential, membrane resistance, and sensitivity were measured during light and/or dark adaptation in the ventral eye of Limulus. After a bright flash, the ERP amplitude recovered with a time constant of 100 ms, whereas the sensitivity recovered with an initial time constant of 20 s. When a strong adapting light was turned off, the recovery of membrane potential and of membrane resistance had time-courses similar to each other, and both recovered more rapidly than the sensitivity. The receptor depolarization was compared during dark adaptation after strong illumination and during light adaptation with weaker illumination; at equal sensitivities the cell was more depolarized during light adaptation than during dark adaptation. Finally, the waveforms of responses to flashes were compared during dark adaptation after strong illumination and during light adaptation with weaker illumination. At equal sensitivities (equal amplitude responses for identical flashes), the responses during light adaptation had faster time-courses than the responses during dark adaptation. Thus neither the photochemical cycle nor the membrane potential nor the membrane resistance is related to sensitivity changes during dark adaptation in the photoreceptors of the ventral eye. By elimination, these results imply that there are (unknown) intermediate process(es) responsible for adaptation interposed between the photochemical cycle and the electrical properties of the photoreceptor.     

Increased intracellular sodium mimics some but not all aspects of photoreceptor adaptation in the ventral eye of Limulus

The effects of the intracellular iontophoretic injection of Na+ ions have been quantitatively compared with adaptation in ventral photoreceptors of Limulus. We find that: (a) both light adaptation and sodium injection are associated with a decrease in the variability of the threshold response amplitued; (b) both light adaptation and sodium injection are associated with a decrease in the absolute value of the temporal dispersion of the threshold response time delay; (c) the same template curve adequately fits the intensity response relationships measured under light adaptation and Na+ injection; (d) both light adaptation and Na+ injection produce a fourfold decrease in response time delay for a desensitization of 3 log units; (e) the time coures of light adaptation and dark adaptation is significantly faster than the onset of and recovery from desensitization produced by Na+ injection; (f) unlike local illumination, Na+ injection does not produce localized desensitization of the photoreceptor. These findings suggest that a rise in intracellular Na+ concentration makes at most only a minor contribution (probably less than 5%) to the total adaptation of these receptors in the intensity range we have examined (up to 3 log units above absolute threshold). However, changes in intracellular Na+ concentration may contribute to certain components of light and dark adaptation in these receptors.     

Light Adaptation in the Ventral Photoreceptor of Limulus

Light adaptation in both the ventral photoreceptor and the lateral eye photoreceptor is a complex process consisting of at least two phases. One phase, which we call the rapid phase of adaptation, occurs whenever there is temporal overlap of the discrete waves that compose a light response. The recovery from the rapid phase of adaptation follows an exponential time-course with a time constant of approximately 75 ms at 21°C. The rapid phase of adaptation occurs at light intensities barely above discrete wave threshold as well as at substantially higher light intensities with the same recovery time-course at all intensities. It occurs in voltage-clamped and unclamped photoreceptors. The kinetics of the rapid phase of adaptation is closely correlated to the photocurrent which appears to initiate it after a short delay. The rapid phase of adaptation is probably identical to what is called the "adapting bump" process. At light intensities greater than about 10 times discrete wave threshold another phase of light adaptation occurs. It develops slowly over a period of ½ s or so, and decays even more slowly over a period of several seconds. It is graded with light intensity and occurs in both voltage-clamped and unclamped photoreceptors. We call this the slow phase of light adaptation.