Common strategies for light adaptation in the peripheral visual systems of fly and dragonfly

Light suppresses the activity of serotonin-immunoreactive neurons in the optic lobe of the swallowtail butterfly

Intracellular recordings and stainings were made from 5-HT-immunoreactive neurons with a large soma at the anterior edge of the second optic neuropile of the swallowtail butterfly Papilio xuthus. The neurons have a dendritic arborization in the third optic neuropile and project axonal processes to the first optic neuropile and the dorsal protocerebrum. The neurons discharged impulses in the dark and the impulse discharge was inhibited by illumination of the ipsilateral eye. The anatomical and physiological profiles suggest that the neurons may have a role in controlling the light-dark adaptation or the overall activity of the optic lobe and the midbrain.

Protein kinase C is required for light adaptation in Drosophila photoreceptors

Protein kinase C (PKC) is a key enzyme for many cellular processes but its physiological roles are poorly understood. An excellent opportunity to investigate the function of PKC has been provided by the identification of an eye-specific PKC in Drosophila and a null PKC mutant, inaCP209 (refs 5,6). Bright conditioning lights delivered to inaC photoreceptors lead to an abnormal loss of sensitivity in whole cell recordings from dissociated ommatidia; this has been interpreted as 'hyper-adaptation' and PKC's role has been suggested to be distinct from light adaptation. A presumably related finding is that during intense light, the response of inaC declines to baseline. Invertebrate photoreceptors use the phosphoinositide signalling cascade, responding to single photons with so-called quantum bumps which sum to form the macroscopic response to light. Light adaptation allows photoreceptors to adjust their sensitivity over the enormous range of ambient intensities. Although the molecular mechanism of light adaptation remains obscure, it is a negative-feedback process mediated by a rise in cytosolic calcium and a decrease in bump size. We now show that under physiological conditions light adaptation is severely reduced in inaC, suggesting that eye-specific PKC, itself activated by a rise in cytosolic calcium and diacylglycerol, is required for adaptation. Furthermore, we show that in the absence of PKC individual bumps fail to terminate normally, an effect that can account for the pleiotropic manifestations of the inaC phenotype.

Real and optimal neural images in early vision

It has been suggested that the first steps in visual processing strive to compress as much information as possible about the outside world into the limited dynamic range of the visual channels. Here I compare measured neural images with theoretical calculations based on maximizing information, taking into account the statistical structure of natural images. Neural images were obtained by scanning an image while recording from a second-order neuron in the fly visual system. Over a 5.5-log-units-wide range of mean intensities, experiment and theory correspond well. At high mean intensities, redundancy in the image is reduced by spatial and temporal antagonism. At low mean intensities, spatial and temporal low-pass filtering combat noise and increase signal reliability.

Are there separate ON and OFF channels in fly motion vision?

Visual information is processed in a series of subsequent steps. The performance of each of these steps depends not only on the computations it performs itself but also on the representation of the visual surround on which it operates. Here we investigate the consequences of signal preprocessing for the performance of the motion-detection system of the fly. In particular, we analyze whether the retinal input signals are rectified and segregate into separate ON and OFF channels, which then feed independent parallel motion-detection pathways. We recorded the activity of an identified directionally selective interneuron (H1-cell) in response to apparent motion stimuli, i.e. sequential brightness changes at two neighboring locations in the visual field, as well as to brightness changes at only a single location. For apparent motion stimuli, the motion-dependent response component was determined by subtracting from the overall response to the individual stimulus components when presented alone. The following conclusions could be derived: (1) Apparent motion consisting of a sequence of increased or decreased brightness at two locations in the visual field have the same optimum interstimulus time interval (Fig. 3). (2) Sequences of brightness steps of like polarity (either increments or decrements) elicit positive and negative motion-dependent response components when mimicking motion in the cell's preferred and null direction, respectively. The motion-dependent response components are inverted in sign when the brightness steps of a stimulus sequence have a different polarity (Fig. 7). (3) The responses to the beginning and the end of a brightness pulse depend on the pulse duration. For pulse durations of less than 2 s, both events interact with each other (Fig. 9). All of these results do not provide any indication that the fly processes motion information in independent ON and OFF motion detectors. Brightness changes of both signs are rather represented at the input of the same movement detectors, and interactions between signals resulting from both brightness increments and decrements take their sign into account. This type of preprocessing of the retinal input is argued to render a motion-detection system particularly robust against noise.

A molecular basis for Weber's law

A mathematical model is presented that obeys a strong form of Weber's law--over a range of adapting and stimulus intensities, equal contrast stimuli evoke identical responses. To account for the strong Weber's law, the adaptive stage in the proposed model employs a "delayed" reverse reaction along with a power-law input. It is suggested that this Weber's law mechanism is responsible for a slow, voltage-uncorrelated component of adaptation in the vertebrate photoreceptor. A plausible biochemical mechanism is the G-protein cycle with phosphorylation of photoactivated photopigment (and binding of arrestin to the phosphorylated photopigment) as the adaptive process. In an Appendix, features of the general model and implications of a specific biochemical model are examined by computer simulation.

Synaptic organization of columnar elements in the lamina of the wild type in Drosophila melanogaster

The synaptic connections within the lamina, the first of the optic neuropiles underlying the insect's compound eye, have been little studied in Drosophila melanogaster until now, despite the genetic advantages of this animal. Here we report the reconstruction through its entire depth of one of the lamina modules, or cartridges, of a female wild-type Drosophila, for which a series of EM cross sections was analysed at levels extending from the retinal basement membrane to the first optic chiasma. A complete, comprehensive catalogue of the synaptic connections of all columnar elements has been compiled from this single series, confirmed from comparisons with less completely photographed cartridges. Combinations of the 12 types of cartridge neurons form divergent multiple-contact synapses (dyads, triads, and tetrads) throughout the lamina's depth. These 12 neuron types include 11 narrow-field elements (one class of receptor terminal, R1-R6, providing input to the cartridge; two types of long visual fiber from the ommatidium, R7 and R8; five types of monopolar cell, L1-5; and three types of medulla cell--two centrifugal neurons C2 and C3, and a third, T1) as well as a wide-field intrinsic or amacrine cell. Connections within the lamina formed by L4 from two adjacent cartridges (posterodorsal and posteroventral) contribute to the matrix of connections. In addition, connections of at least one other wide-field element have also been incorporated.

Molecular analysis of no-on-transient A, a gene required for normal vision in Drosophila

Mutation of the Drosophila melanogaster gene no-on-transient A (nonA) results in reduced visual acuity, behavior abnormalities, and an electrophysiological defect for which the mutant is named. We mapped the nonA gene genetically to a 20 kb interval within the 14C1,2 region of the X chromosome, isolated this chromosomal region, and used P element-mediated transformation to delimit the nonA gene to a 9 kb region. Analysis of cDNA clones indicates that this region encodes alternatively spliced transcripts encoding protein products of approximately 77 kd that differ only in their C-terminal 35 amino acids. Analysis of mutations generated in vitro in this transcription unit confirm that these transcripts are the products of the nonA gene.

Biochemical and physiological evidence that histamine is the transmitter of barnacle photoreceptors

We tested the hypothesis that histamine is the transmitter released by barnacle photoreceptors. Median and lateral ocelli were incubated with 3H-histidine and found to synthesize 3H-histamine, identified by high-voltage electrophoresis. Synthesis could be blocked by the histidine decarboxylase inhibitor (S)-alpha-fluoromethylhistidine. Histamine was applied to I-cells either by superfusion or by pressure ejection from a pipette (100 microM or 1 mM histamine) positioned close to the I-cell's soma. When bath-applied at concentrations ranging from 5-100 microM, histamine hyperpolarized the I-cell in a dose-dependent fashion and increased its conductance. At 100 microM, histamine abolished the I-cell's response to light. The response to a pulse of pressure-applied histamine was a hyperpolarization whose amplitude was graded with dose (determined by the duration of the pulse). This response persisted in concentrations of Co2+ and Cd2+ that blocked synaptic transmission from the photoreceptors. Cimetidine, an antagonist of mammalian H2 receptors, markedly decreased the cell's responses both to HA and to light at 100 microM and blocked both responses at 1 mM. Pyrilamine and triprolidine, H1 antagonists, had a complex effect on the I-cell's responses to histamine and to light. Neither H1 nor H2 antagonists, nor histamine itself, affected the voltage or light responses recorded in the presynaptic terminal region, or any phase of calcium-dependent action potentials induced in the terminal in the presence of tetraethylammonium ion. Thus, biochemical, immunocytochemical, and physiological evidence suggests that HA is the transmitter from these photoreceptors to the I-cells. Although gamma-aminobutyric acid (GABA) is also present in the photoreceptors, it did not affect the I-cell's responses to light or to histamine when bath-applied at 100 microM. Thus, GABA does not appear to modulate transmission from the photoreceptor to the I-cell.

A histamine-activated chloride channel involved in neurotransmission at a photoreceptor synapse

Compared with the variety of neuromodulatory agents acting through second messenger systems, the number of fast neurotransmitters which directly activate ion channels is limited. Thus, synaptic receptors that act as ligand-gated ion channels have been firmly established only for acetylcholine, glycine, GABA and glutamate, with the first three of these belonging to the same molecular superfamily. Recently, however, a possible addition to this list has been suggested as a result of evidence implicating histamine as the neurotransmitter released by a variety of arthropod photoreceptors. Neurotransmission at this synapse has been studied extensively, particularly in the fly. The postsynaptic elements, large monopolar cells, respond to light with a rapid, chloride-mediated hyperpolarization that can be mimicked by the application of histamine. In this report I document some basic properties of the histamine receptors present on large monopolar cells isolated from blowfly optic lobes. The receptor is a ligand-gated chloride channel showing properties consistent with its presumed role of mediating neurotransmission at the photoreceptor synapse.

Light and dark adaptation in fly photoreceptors: duration and time integral of the impulse response

The properties of impulse responses of blowfly photoreceptors in light and dark adaptation were studied with brief (30 mu sec) white light flashes applied with different repetition frequencies, thereby producing a range of interstimulus periods (ISPs) from 200 msec to 100 sec. The amplitude of the impulse response as intracellularly recorded membrane voltage recovered fully with ISP greater than 300 msec. The duration of the impulse response (as half-repolarization time) increased beyond that, saturating with ISPs of about 30 sec with durations of about 60 msec, which is to be compared with a duration of 14 msec with ISP of 200 msec. This finding is not associated with saturation of the membrane potential, as it is also found with subsaturating responses. It is also different from the prolonged depolarizing afterpotential (PDA). The findings could in principle be explained by calcium-activated potassium conductance in the photoreceptor membrane.

Is histamine a neurotransmitter in insect photoreceptors?

Intracellular recordings were made from the large monopolar cells (LMC's) in the first visual neuropil (lamina) of the fly Musca, whilst applying pharmacological agents from a three-barrelled ionophoretic pipette. Most of the known neurotransmitter candidates (except the neuropeptides) were tested. The LMC's were most sensitive to histamine, saturating with ionophoretic pulses of less than 2 nC. The responses to histamine were fast hyperpolarizations with maximum amplitudes similar to that of the light-induced response. Like the light response, the histamine response was associated with a conductance increase. The histamine responses were not blocked by a synaptic blockade induced by ionophoretic application of cobalt ions. Several histamine antagonists, and also atropine, were effective at blocking or reducing both the response to histamine and the response to light. Other transmitter candidates having marked effects on the LMC's were: a) the acidic amino-acids, L-aspartate and L-glutamate, which evoked slower hyperpolarizations that could be blocked by cobalt; b) GABA, which induced a depolarization associated with an inhibition of the light response; and c) acetylcholine which also caused a depolarization. Substances with no obvious effect on the LMC's included serotonin (5-HT), beta-alanine, dopamine, octopamine, glycine, taurine and noradrenalin. Together with the evidence of Elias and Evans (1983), which shows the presence, synthesis and inactivation of histamine in the retina and optic lobes of the locust, the data suggest that histamine is a neurotransmitter in insect photoreceptors.

Saturation in a wide-field, directionally selective movement detection system in fly vision

In the third optic lobe of the fly large-field spiking neurons are found which detect movement in a directionally selective way. For a wide variety of stimuli the responses of one of these, the H1 neuron, can very well be described and predicted by Reichardt's correlation model which is based on behavioural optomoter responses. However, when the spike rate is driven to large values with intense stimuli, the correlation model needs to be extended in order to account for the saturation phenomena that occur. Saturation can take place in all the elements of the large and extensive movement detecting system which contains interneurons in the optic lobes which process and guide the signals from the peripheral receptors to the central H1 neuron. To separate saturation at the peripheral site from that at the central level of the H1 neuron, a special stimulation technique was chosen. In measuring the saturation at the central level the stimulus parameter was the size of the stimulus field, while the modulation depth of the moving grating in this field stayed constant. Saturation at the peripheral site of the system was studied with a stimulus in which the modulation depth of the moving grating was the parameter and the size of the stimulus field was small and constant. When a simple feedback loop is incorporated in the final stage of the correlation model the saturation phenomena at the central level for steady-state stimulus conditions can very well be described. Saturation phenomena at the peripheral site of the system can also be explained by the same kind of feedback mechanisms in the input channels.