Temperature dependency of light adaptation in bullfrog cone photoreceptors

Isolated, superfused bullfrog retinas were stimulated in such a manner as to allow examination of the dynamics of the shift in the stimulus-response curve that accompanies light adaptation of cone photoreceptors. Observed phenomena were highly temperature dependent with complex Arrhenius relationships, suggesting that the mechanisms responsible for the shift involve multiple enzyme systems.     

Calcium and the mechanism of light adaptation in vertebrate photoreceptors.

Vertebrate photoreceptors transduce the absorption of light into a hyperpolarizing change in membrane potential. The mechanism of transduction is becoming fairly well understood and has been shown to occur via a G protein-coupled decrease in cyclic GMP. Attention is now turning to the way the enzymatic machinery in the outer segment of the photoreceptor cell is modulated during light adaptation. Recent studies show that light adaptation cannot occur if changes in the concentration of cytoplasmic free calcium in the outer segment are prevented, suggesting that calcium functions as a second messenger in sensitivity regulation.     

Visuomotor adaptation during inactivation of the dentate nucleus.

Recent experiments have suggested that the process of visuomotor adaptation depends on how a visual distortion is introduced. The cerebellum is thought to be involved in adapting to rapidly introduced visual distortions; however its role in adapting to a gradually introduced distortion is unknown. We tested adaptation to a sudden or a gradual introduction of a visual distortion, during reversible inactivation of a monkey's dentate nucleus. There was significant adaptation in both of these tasks without any lignocaine infusion and during saline infusions. However after inactivation the ability to adapt to either visual distortion was slightly impaired. This dysfunction was significant when the visuomotor distortion was introduced over several trials, suggesting that the cerebellum has a differential contribution to visual adaptation depending on the type of visuo-motor disturbance encountered.     

Spatial and temporal parameters of cortical inactivation by GABA

Inactivation by GABA is a powerful tool for studying the function of specific cortical regions. It is especially useful in electrophysiology, because inactivation is reversible within short time periods, and because the extent of the inactivated region can be accurately controlled. Iontophoresis of GABA inactivates neurons up to 300 microm around the micropipette. Pressure injection of GABA inactivates neurons further away, but the spatial and temporal characteristics of inactivation by this method have been poorly studied. In order to address this question, we built devices made of micropipettes and microelectrodes glued at various distances. We experienced that repetition of small injections of 100 mM GABA inactivate cortex in a more homogenous way than bolus injections. Diffusion of GABA after pressure injection does not seem to follow a point spread diffusion model as in the case of iontophoresis: GABA probably goes up along the micropipette shaft, and the volume of inactivation has an ellipsoidal form. In order to precisely determine the extent of the inactivated region, we built a mathematical model to fit the experimental data of inactivations obtained above and below the pipette tip. The model provides estimates of the inactivated region for volumes smaller than 60 nl of GABA 100 mM. Limits of inactivation are between 250 and 500 microm lateral to the tip of the pipette. The geometry of inactivation is difficult to predict beyond 60 nl and it seems hazardous to try to inactivate neurons beyond 800 microm with pressure injections of GABA 100 mM.     

Temporal and spatial adaptation to food restriction in mice under naturalistic conditions

Free-living female laboratory mice, adapted to outdoor life in large pens providing a naturalistic environment, were tested for their ability to modify their foraging habits to controlled food supply. An automatic feeder box delivered a small portion of the daily quantity of seeds to each individual mouse. Eight such boxes were placed into an outdoor pen. Each day, mice had to visit all boxes to gather the daily amount of food and were rewarded only at the first visit to each box. Mice were individually recognised by an implanted microchip. Throughout a 16-day period, feeding activity concentrated in an interval time around the beginning of the daily session. During the same period, the number of different feeders visited every day by mice increased irrespective of variation in exploratory activity. The experimental set-up allowed detecting temporal and spatial adaptations to the food restriction, as well as behavioural differences due to territorial and social factors. These data permit the design of novel tests assessing behavioural changes, memory and learning in normal and genetically modified mice, both in the laboratory and in naturalistic settings.     

Seeing the light: photobehavior in fruit fly larvae

Understanding how sensory stimuli drive behavior requires a detailed understanding of the molecular and neural nature through which the stimuli are received and processed. The visual system of the fruit fly Drosophila melanogaster shares marked similarities to that of mammals. Although much focus has been given to the fly visual system, an even further simplified eye and brain makes the visual system of Drosophila larvae an excellent model for dissecting sensory processing and behavioral responses to light. Recent work has identified sensory and central brain neurons required for larval visual behaviors, including circadian rhythms. Here, we review the genes and neurons regulating visual processing in Drosophila larvae and discuss the implications of this work for furthering understanding of more complex visual systems.     

An analysis of the number and composition of the synaptic populations formed by photoreceptors of the fly

The photoreceptor terminals of newly eclosed female flies, Musca domestica, have been sampled in the first optic neuropile (or lamina) in one of two ways: first, in large number (n = 760) from single sections and second, from serial electron micrographs of the six terminals within each of three cartridges. Both sampling methods concur in assessing the number of synapses established with the two principal monopolar relay interneurons, L1 and L2, within each cartridge. Each receptor is calculated to be presynaptic at about 200 ± 40 (2 SE) synapses. This value considerably exceeds previous estimates, primarily because we took careful account of the appearance of synapses in different section planes. The number of these synapses correlates highly with the area of receptor terminal presynaptic membrane, so that each synapse is allotted, on average, about 1.6 μm². The synapses are evenly graded in their distribution with an unexplained 23% decrease in both membrane perimeter and synapse number halfway along their receptor terminal's length. The numbers of synapses per receptor did not vary systematically within two horizontal (3 × 20 cartridge) strips of frontal, equatorial lamina sampled. Individual synapses are elongate tetrads (Burkhardt and Braitenberg, ′76) with two pairs of postsynaptic elements. The first pair is invariably contributed by the interneurons L1 and L2 (one each). The second pair comes either from the α processes of an amacrine cell or from a glial cell. In the distal lamina, however, L3 contributes one of the two postsynaptic processes, the second being αor glial. The overall ratio of postsynaptic involvement at distal synapses (α:glial:L3) is 55%, 20%, and 12% respectively, the remainder being unidentified.     

Spatial transfer of adaptation of scanning voluntary saccades in humans

The properties and neural substrates of the adaptive mechanisms that maintain over time the accuracy of voluntary, internally triggered saccades are still poorly understood. Here, we used transfer tests to evaluate the spatial properties of adaptation of scanning voluntary saccades. We found that an adaptive reduction of the size of a horizontal rightward 7 degrees saccade transferred to other saccades of a wide range of amplitudes and directions. This transfer decreased as tested saccades increasingly differed in amplitude or direction from the trained saccade, being null for vertical and leftward saccades. Voluntary saccade adaptation thus presents bounded, but large adaptation fields, suggesting that at least part of the underlying neural substrate encodes saccades as vectors.     

Influence of relative blood flow restriction pressure on muscle activation and muscle adaptation

Introduction: The aim of this study was to investigate the acute and chronic skeletal muscle response to differing levels of blood flow restriction (BFR) pressure. Methods: Fourteen participants completed elbow flexion exercise with pressures from 40% to 90% of arterial occlusion. Pre/post torque measurements and electromyographic (EMG) amplitude of each set were quantified for each condition. This was followed by a separate 8‐week training study of the effect of high (90% arterial occlusion) and low (40% arterial occlusion) pressure on muscle size and function. Results: For the acute study, decreases in torque were similar between pressures [–15.5 (5.9) Nm, P = 0.344]. For amplitude of the first 3 and last 3 reps there was a time effect. After training, increases in muscle size (10%), peak isotonic strength (18%), peak isokinetic torque (180°/s = 23%, 60°/s = 11%), and muscular endurance (62%) changed similarly between pressures. Conclusion: We suggest that higher relative pressures may not be necessary when exercising under BFR. Muscle Nerve 53: 438–445, 2016     

Basic fibroblast growth factor and local injury protect photoreceptors from light damage in the rat.

Injection of basic fibroblast growth factor (bFGF) into the eye, intravitreally or subretinally, delays photoreceptor degeneration in inherited retinal dystrophy in the rat, as does local injury to the retina (Faktorovich et al., 1990). To determine whether this heparin-binding peptide or local injury is effective in any other form of photoreceptor degeneration, we examined their protective roles in light damage. Albino rats of the F344 strain were exposed to 1 or 2 weeks of constant fluorescent light (115-200 footcandles), either with or without 1 microliter of bFGF solution (1150 ng/microliters in PBS) injected intravitreally or subretinally 2 d before the start of light exposure. Uninjected and intravitreally PBS-injected controls showed the loss of a majority of photoreceptor nuclei and the loss of most inner and outer segments after 1 week of light exposure, while intravitreal injection of bFGF resulted in significant photoreceptor rescue. The outer nuclear layer in bFGF-injected eyes was two to three times thicker than in controls, and the inner and outer segments showed a much greater degree of integrity. Following recovery in cyclic light for 10 d after 1 week of constant light exposure, bFGF-injected eyes showed much greater regeneration of photoreceptor inner and outer segments than did the controls. bFGF also increased the incidence of presumptive macrophages, located predominantly in the inner retina, but the evidence suggests they are not directly involved in photoreceptor rescue. Subretinal injection of bFGF resulted in photoreceptor rescue throughout most of the superior hemisphere in which the injection was made, with rescue extending into the inferior hemisphere in many of the eyes. Remarkably, the insertion of a dry needle or injection of PBS into the subretinal space also resulted in widespread photoreceptor rescue, extending through 70% or more of the superior hemisphere, and sometimes into the inferior hemispheres. This implicates the release and widespread diffusion of some endogenous survival-promoting factor from the site of injury in the retina. Our findings indicate that the photoreceptor rescue activity of bFGF is not restricted to inherited retinal dystrophy in the rat, and that light damage is an excellent model for studying the cellular site(s), kinetics, and molecular mechanisms of both the normal function of bFGF and its survival-promoting activity. Moreover, the injury-related rescue suggests that survival-promoting factors are readily available to provide a protective role in case of injury to the retina, presumably comparable to those that mediate the "conditioning lesion" effect in other neuronal systems.     

Multiple vulnerability of photoreceptors to mesopic ambient light in the P23H transgenic rat

The P23H transgenic rat was engineered to mimic a human form of retinal degeneration caused by a mutation in rhodopsin. We have tested whether the P23H transgene influences the vulnerability of photoreceptors to modest variations in ambient light, well within the physiological range. P23H-3 (P23H line 3) and control Sprague-Dawley (SD) rats were raised in cyclic light (12 h light, 12 h dark), with the light phase set at either 5 lx ('scotopic-reared') or 40-60 lx ('mesopic-reared'). Mesopic rearing reduced the length of outer segments (OSs) in both SD and P23H-3 strains, but the shortening was more marked in the P23H-3 strain. Mesopic rearing was associated with thinning of the ONL, again more prominently in the P23H-3. Correspondingly, mesopic rearing increased the rate of photoreceptor death (assessed by TUNEL labelling), the increase occurring during early postnatal life. Mesopic rearing upregulated FGF-2 (basic fibroblast growth factor) levels in photoreceptors and glial fibrillary acidic protein (GFAP) in Müller cells in both SD and P23H-3 strains; again the changes were more marked in the P23H-3. Finally, mesopic rearing decreased the amplitude of the a-wave of the ERG in both strains; again the effect was greater in the P23H-3 strain. The ERG decline induced in both strains by mesopic-rearing can be explained by a reduction of functional OS membrane, due to a combination of photoreceptor death and OS shortening. The P23H-3 transgene makes photoreceptors abnormally vulnerable to modest levels of ambient light, their vulnerability being evident in multiple ways. In humans suffering photoreceptor degeneration from comparable genetic causes, light restriction may preserve the number and the function of photoreceptors.     

Octopaminergic modulation of contrast gain adaptation in fly visual motion‐sensitive neurons

Locomotor activity like walking or flying has recently been shown to alter visual processing in several species. In insects, the neuromodulator octopamine is thought to play an important role in mediating state changes during locomotion of the animal [K.D. Longden & H.G. Krapp (2009) J. Neurophysiol., 102 , 3606–3618; (2010) Front. Syst. Neurosci., 4 , 153; S.N. Jung et al. (2011) J. Neurosci., 31 , 9231–9237]. Here, we used the octopamine agonist chlordimeform (CDM) to mimic effects of behavioural state changes on visual motion processing. We recorded from identified motion‐sensitive visual interneurons in the lobula plate of the blowfly Calliphora vicina. In these neurons, which are thought to be involved in visual guidance of locomotion, motion adaptation leads to a prominent attenuation of contrast sensitivity. Following CDM application, the neurons maintained high contrast sensitivity in the adapted state. This modulation of contrast gain adaptation was independent of the activity of the recorded neurons, because it was also present after stimulation with visual motion that did not result in deviations from the neurons’ resting activity. We conclude that CDM affects presynaptic inputs of the recorded neurons. Accordingly, the effect of CDM was weak when adapting and test stimuli were presented in different parts of the receptive field, stimulating separate populations of local presynaptic neurons. In the peripheral visual system adaptation depends on the temporal frequency of the stimulus pattern and is therefore related to pattern velocity. Contrast gain adaptation could therefore be the basis for a shift in the velocity tuning that was previously suggested to contribute to state‐dependent processing of visual motion information in the lobula plate interneurons.     

Chloride enters glial cells and photoreceptors in response to light stimulation in the retina of the honey bee drone

Double-barrelled ion-selective microelectrodes were used to measure free [Cl-] in photoreceptors, extracellular space, and glial cells in superfused slices of drone retina. Tests indicated that with normal superfusate the intracellular electrode signal was due essentially to Cl- and not to some other interfering anion. The results indicate that Cl- is more concentrated in both photoreceptors and glial cells than would be predicted for a passive electrochemical distribution. When the photoreceptors were stimulated by a standard train of 20 ms flashes, 1/s for 90 s, their intracellular free [Cl-] (Cli) rose by 8 +/- 1 mM. At the end of stimulation Cli usually continued to rise for up to a further 2 min and then returned toward the baseline over about 10 min. During light stimulation Cli in the glia rose. The magnitude of the increase was 5.1 +/- 0.4 mM, about half the increase in Ki. In some extracellular recording sites, light stimulation caused [Cl-] to increase and in others to decrease. The mean change was -0.7 mM, SD 6.5 mM. The Cl- that entered the photoreceptors and the glia was presumably made available by the shrinking of the extracellular space. When the cells were depolarized by increasing [K+] in the superfusate from 7.5 mM to 18 mM, Cli increased. The half-time of the change in Cli was longer than the half-time of the depolarization by 10-30 s in the glia and 50-250s in the photoreceptors. During superfusion with 0 Cl- Ringer's solution, the light-induced rise in extracellular [K+] was greater by a factor of 1.4-2.7, and the clearance after the end of the stimulation was slower. The rate of increase in glial Ki during light stimulation fell; the rate of increase of glial Ki caused by superfusion with raised [K+] (in the absence of Cl-) fell more. We conclude that when extracellular [K+] is increased, entry of Cl- into the glia is necessary for part, but not all, of the net uptake of K+. During light stimulation, the observed movement of CL- into glia contributes to homeostasis of extracellular [K+], and the cell swelling associated with movement of Cl- into both glia and photoreceptors contributes to homeostasis of extracellular [Na+].     

Characteristics of the coding of light stimulus intensity by visual cortex neurons during light adaptation in the cat

Response of 70 neurons in area 17 of the visual cortex to optimal stimuli of different intensity in the receptive field under conditions of photopic adaptation were analyzed in unanesthetized cats. The reaction threshold, differential sensitivity, optimal intensity and the width of the brightness range were estimated. No intensity detectors were found in this area. 70% of neurons studied had inhibitory distortion in the range of their intensity functions. The neurons differed in their threshold reactions by 5-6 orders, in dynamic range--by 3-4 orders, and in differential sensitivity--by 2-3 orders. The visual cortex neurons with receptive fields in central and periphery parts of the visual field had different intensity functions.     

Localization of hydroxyindole-O-methyltransferase-like immunoreactivity in photoreceptors and cone bipolar cells in the human retina: a light and electron microscope study

The localization of the melatonin-synthesizing enzyme hydroxyindole-O-methyltransferase (HIOMT) was examined by light and electron microscopic immunocytochemistry in the human retina. HIOMT-like immunoreactivity was observed in the photoreceptor layers and the inner nuclear layer (INL). The immunoreactive cells in the INL were more numerous in the central retina than in the peripheral retina and sent processes to both the outer plexiform and inner plexiform layers. The HIOMT immunoreactivity in the inner plexiform layer (IPL) appeared as punctate terminals in the proximal and distal one-thirds of that layer. At the ultrastructural level, HIOMT-like immunoreactivity was localized to the cytoplasm of rod and cone photoreceptors and to a population of cone bipolar cells. HIOMT-immunoreactive bipolar cell dendrites were observed to make both invaginating and flat synaptic contacts with cone pedicles. No immunoreactive invaginating contacts in rod spherules were observed. HIOMT immunoreactivity was observed in the bipolar cell cytoplasm in the INL, and in the bipolar synaptic terminals in the IPL. These terminals contained synaptic ribbons, which formed synaptic contacts with unlabeled cells in the IPL. HIOMT radioenzymatic assays confirmed the presence of HIOMT in the human retina. Average HIOMT activity of eight donors was determined to be 15.0 pmol/mg protein/hour +/- 7.2 S.D. The ultrastructural localization of HIOMT observed in this study, combined with reports from other laboratories, suggests that the cytoplasm of the photoreceptors and a population of cone bipolar cells may be the sites of melatonin synthesis in the human retina.     

Four of the six Drosophila rhodopsin‐expressing photoreceptors can mediate circadian entrainment in low light

Light is the major stimulus for the synchronization of circadian clocks with day–night cycles. The light‐driven entrainment of the clock that controls rest–activity rhythms in Drosophila relies on different photoreceptive molecules. Cryptochrome (CRY) is expressed in most brain clock neurons, whereas six different rhodopsins (RH) are present in the light‐sensing organs. The compound eye includes outer photoreceptors that express RH1 and inner photoreceptors that each express one of the four rhodopsins RH3–RH6. RH6 is also expressed in the extraretinal Hofbauer–Buchner eyelet, whereas RH2 is only found in the ocelli. In low light, the synchronization of behavioral rhythms relies on either CRY or the canonical rhodopsin phototransduction pathway, which requires the phospholipase C‐β encoded by norpA (no receptor potential A). We used norpA^P24 cry⁰² double mutants that are circadianly blind in low light and restored NORPA function in each of the six types of photoreceptors, defined as expressing a particular rhodopsin. We first show that the NORPA pathway is less efficient than CRY for synchronizing rest–activity rhythms with delayed light–dark cycles but is important for proper phasing, whereas the two light‐sensing pathways can mediate efficient adjustments to phase advances. Four of the six rhodopsin‐expressing photoreceptors can mediate circadian entrainment, and all are more efficient for advancing than for delaying the behavioral clock. In contrast, neither RH5‐expressing retinal photoreceptors nor RH2‐expressing ocellar photoreceptors are sufficient to mediate synchronization through the NORPA pathway. Our results thus reveal different contributions of rhodopsin‐expressing photoreceptors and suggest the existence of several circuits for rhodopsin‐dependent circadian entrainment. J. Comp. Neurol. 524:2828–2844, 2016. © 2016 Wiley Periodicals, Inc.     

Benzodiazepine and GABA receptors in the rat retina: effect of light and dark adaptation

Benzodiazepine and GABA receptors in the rat retina are influenced by environmental lighting. In dark-adapted animals, the affinities of [³H]GABA and [³H]diazepam for their specific binding sites are markedly higher than in light-adapted rats. No differences were found in the maximal number of binding sites (B_max) between dark- and light-adapted rats. The results suggest that GABA and benzodiazepine receptors are involved in the physiological mechanisms of vision.     

Ghrelin receptor in agouti‐related peptide neurones regulates metabolic adaptation to calorie restriction

Ghrelin is a gut hormone that signals to the hypothalamus to stimulate growth hormone release, increase food intake and promote fat deposition. The ghrelin receptor, also known as growth hormone secretagogue receptor (GHS‐R), is highly expressed in the brain, with the highest expression in agouti‐related peptide (AgRP) neurones in the hypothalamus. Compelling evidence indicates that ghrelin serves as a survival hormone with respect to maintaining blood glucose and body weight during nutritional deficiencies. Recent studies have demonstrated that AgRP neurones are involved in metabolic and behavioural adaptation to an energy deficit to improve survival. In the present study, we used a neuronal subtype‐specific GHS‐R knockout mouse (AgRP‐Cre;Ghsr^f/f) to investigate the role of GHS‐R in hypothalamic AgRP neurones in metabolic and behavioural adaptation to hypocaloric restricted feeding. We subjected the mice to a restricted feeding regimen of 40% mild calorie restriction (CR), with one‐quarter of food allotment given in the beginning of the light cycle and three‐quarters given at the beginning of the dark cycle, to mimic normal mouse intake pattern. The CR‐fed AgRP‐Cre;Ghsr^f/f mice exhibited reductions in body weight, fat mass and blood glucose. Metabolic profiling of these CR‐fed AgRP‐Cre;Ghsr^f/f mice showed a trend toward reduced basal metabolic rate, significantly reduced core body temperature and a decreased expression of thermogenic genes in brown adipose tissue. This suggests a metabolic reset to a lower threshold. Significantly increased physical activity, a trend toward increased food anticipatory behaviour and altered fuel preferences were also observed in these mice. In addition, these CR‐fed AgRP‐Cre;Ghsr^f/f mice exhibited a decreased counter‐regulatory response, showing impaired hepatic glucose production. Lastly, hypothalamic gene expression in AgRP‐Cre;Ghsr^f/f mice revealed increased AgRP expression and a decreased expression of genes in β‐oxidation pathways. In summary, our data suggest that GHS‐R in AgRP neurones is a key component of the neurocircuitry involved in metabolic adaptation to calorie restriction.     

Cytochrome oxidase histochemistry in the effect of light deprivation on the fly visual system

Deprivation of visual experience for a short time during the early period postemergence in the fly influences strongly the development of visual pattern discrimination. Cytochrome oxidase histochemistry was used to examine the effect of visual deprivation on the development of the neuronal network in the optic lobe. Flies in which one compound eye was covered immediately after emergence were raised under normal light and dark conditions (LD). Other flies were raised without covering of the compound eyes under LD or continuous darkness (DD) as controls. At various periods of postemergence flies from the 3 groups were used for histochemistry. The results indicated that the deprivation caused by unilateral eye covering produced a decrease in cytochrome oxidase staining of the covered side of the retina and optic lobe, but not in the central part of the brain. This asymmetry in staining was not seen in flies of 1-5 days postemergence but became evident after the sixth day postemergence. When flies were raised in DD until the 5th day postemergence and then in LD, the development of asymmetry in cytochrome oxidase staining was delayed. These results confirm previous behavioral experiments on visual deprivation. Both suggest that visual deprivation during the early period of postemergence leads to a long-lasting decrease in neuronal activity.