R Feiler

A photostable pigment within the rhabdomere of fly photoreceptors no. 7

SummaryThe population of the centrally located rhabdomeres no. 7 in the ommatidia of flies (Musca, Calliphora, Drosophila) is inhomogeneous: approximately 2/3 of them contain — besides a photoisomerizable rhodopsin — a photostable pigment. Its extinction spectrum has a maximum at 460 nm and two shoulders at 430 and 485 nm respectively. Extinction is maximal for theE-vector perpendicular to the microvilli. Whereas the functional role of the photostable pigment for receptor 7 has still to be worked out, its functional consequence for receptors no. 8, which are located proximal to receptors 7, is obvious: it modifies their spectral sensitivity by selectively absorbing blue light. Due to this “screening”-effect, a shift of the maximal sensitivity of receptors no. 8 is predicted from 490 nm (maximal sensitivity of unscreened receptor 8, Harris et al., 1976) to 520 to 540 nm. This is in agreement with recent electrophysiological data (Hardie, 1977). The results show that spectral sensitivities of receptors no. 8, as determined by means of the ERG of white-eyed mutants or of mutants lacking receptor 7, do not represent the spectral sensitivities of most of these receptors in wild-type flies.

Spectral tuning of rhodopsin and metarhodopsin in vivo

Color vision is dependent upon the expression of spectrally distinct forms of rhodopsin in different photoreceptor cells. To identify the structural features of rhodopsin that regulate spectral sensitivity and absorption in vivo, we have constructed a series of chimeric Drosophila rhodopsin molecules, derived from a blue- and a violet-sensitive rhodopsin, and used P element-mediated germline transformation to generate transgenic flies that express the modified pigments in the R1-R6 photoreceptor cells of the compound eye. Our analysis of these animals indicates that multiple regions of the opsin protein are involved in regulating rhodopsin spectral sensitivity and that the native and photoactivated forms of rhodopsin can be tuned independently of each other. These results demonstrate the feasibility of designing receptor molecules with specifically modified activated states.

Evidence for a sensitizing pigment in the ocellar photoreceptors of the fly (Musca, Calliphora)

SummaryA method is described that allows the spectral sensitivity of photoreceptors to be measured with high spectral resolution. It is shown that the sensitivity of the ocelli ofMusca andCalliphora has a vibrational fine structure in the ultraviolet, strongly indicative of a sensitizing pigment. The visual pigment has its absorption maximum close to 425 nm.

Cortical oscillations and the origin of express saccades

The latencies of visually guided saccadic eye movements can form bimodal distributions. The ‘express saccades’ associated with the first mode of the distribution are thought to be generated via an anatomical pathway different from that for the second mode, which comprises regular saccades. The following previously published observations are the basis for a new alternative model of these effects: (i) visual stimuli can cause oscillations to appear in the electroencephalogram ; (ii) visual stimuli can cause a negative shift in the electroencephalogram that lasts for several hundreds of milliseconds; and (iii) negativity in the electroencephalogram can be associated with reduced thresholds of cortical neurons to stimuli. In the new model both express and regular saccades are generated by the same anatomical structures. The differences in saccadic latency are produced by an oscillatory reduction of a threshold in the saccade-generating pathway that is transiently produced under certain stimulus paradigms. The model has implications regarding the functional significance of spontaneous and stimulus-induced oscillations in the central nervous system.

The kinetics of formation of metarhodopsin in intact photoreceptors of the fly.

The formation of metarhodopsin in fly photoreceptor no. 1 - 6 occurs at room temperature with a time constant of 125 μs (Q10 ≈2.5). The formation of rhoclopsin is faster by factor of 1/10 to 1/100 at least.

Identification of optic lobe neurons of locusts by video films

The response characteristic of visual interneurons of the brain was studied in Locusta migratoria and Schistocerca gregaria. Alternating light and dark, moving dots, bars and striped patterns were used for stimulation (Fig. 3). These stimuli were recorded with a video system and replayed on TV-screens during the experiment to allow fast testing of the sensitivity of a neuron to different stimuli during the limited time of intracellular recording. Data were stored and analysed by computer. The neurons were anatomically identified by intracellular injection of Lucifer yellow. Neutral (“non-visual”) and several classes of spiking interneurons of the medulla and lobula sensitive to visual stimuli could be distinguished by anatomical and physiological characteristics (Figs. 1, 2). The visual cells respond either to light-on, or to light-off, flicker, moving small dots, bars or striped patterns (Figs. 2–6). One class is directionally sensitive to pattern movement either from back to front or into the reverse direction (horizontal cells; Figs. 7, 8) and may therefore be involved in optomotor flight control.