Latency of phototransduction limits transfer of higher-frequency signals in cockroach photoreceptors

Non-inactivating voltage-activated K+ conductances can increase photoreceptor signaling bandwidth beyond the bandwidth set by phototransduction

Evolution produced a large variety of rhabdomeric photoreceptors in the compound eyes of insects. To study effects of morphological and electrophysiological differences on signal generation and modulation, we developed models of the cockroach and blow fly photoreceptors. The cockroach model included wide microvilli, large membrane capacitance and two voltage-activated K+ conductances. The blow fly model included narrow microvilli, small capacitance and two sustained voltage-activated K+ conductances. Our analysis indicated that membrane of even the narrowest microvilli of up to 3 μm long can be measured fully from the soma. Attenuation of microvillar quantum bump (QB)-like signals at the recording site in the soma increased with the signal amplitude in the microvillus, due to the decreasing driving force. However, conductance of the normal-sized QBs can be detected in the soma with minimal attenuation. Next, we investigated how interactions between the sustained voltage-activated K+ and light-induced conductances can shape the frequency response. The models were depolarized by either a current injection or light-induced current (LIC) and probed with inward currents kinetically approximating dark- or light-adapted QBs. By analyzing the resulting voltage impulse responses (IR), we found that: (1) sustained K+ conductance can shorten IRs, expanding the signaling bandwidth beyond that set by phototransduction; (2) voltage-dependencies of changes in IR durations have minima within the physiological voltage response range, depending on the activation kinetics of K+ conductance, the presence or absence of sustained LIC, and the kinetics of the probing current stimulus; and (3) sustained LIC lowers gain of IRs and can exert dissimilar effects on their durations. The first two findings were supported by experiments. It is argued that improvement of membrane response bandwidth by parametric interactions between passive, ligand-gated and voltage-dependent components of the membrane circuit can be a general feature of excitable cells that respond with graded voltage signals.

Opsin knockdown specifically slows phototransduction in broadband and UV-sensitive photoreceptors in Periplaneta americana

Photoreceptors with different spectral sensitivities serve different physiological and behavioral roles. We hypothesized that such functional evolutionary optimization could also include differences in phototransduction dynamics. We recorded elementary responses to light, quantum bumps (QBs), of broadband green-sensitive and ultraviolet (UV)-sensitive photoreceptors in the cockroach, Periplaneta americana, compound eyes using intracellular recordings. In addition to control photoreceptors, we used photoreceptors from cockroaches whose green opsin 1 (GO1) or UV opsin expression was suppressed by RNA interference. In the control broadband and UV-sensitive photoreceptors average input resistances were similar, but the membrane capacitance, a proxy for membrane area, was smaller in the broadband photoreceptors. QBs recorded in the broadband photoreceptors had comparatively short latencies, high amplitudes and short durations. Absolute sensitivities of both opsin knockdown photoreceptors were significantly lower than in wild type, and, unexpectedly, their latency was significantly longer while the amplitudes were not changed. Morphologic examination of GO1 knockdown photoreceptors did not find significant differences in rhabdom size compared to wild type. Our results differ from previous findings in Drosophila melanogaster rhodopsin mutants characterized by progressive rhabdomere degeneration, where QB amplitudes were larger but phototransduction latency was not changed compared to wild type.

Distinct mechanisms of light adaptation of elementary responses in photoreceptors of Dipteran flies and American cockroach

Of many light adaptation mechanisms optimizing photoreceptor functioning in the compound eyes of insects, those modifying the single photon response, the quantum bump (QB), remain least studied. Here, by recording from photoreceptors of the blow fly Protophormia terraenovae, the hover fly Volucella pellucens and the cockroach Periplaneta americana, we investigated mechanisms of rapid light adaptation by examining how properties of QBs change after light stimulation and multiquantal impulse responses during repetitive stimulation. In P. terraenovae, light stimulation reduced latencies, characteristic durations and amplitudes of QBs in the intensity- and duration-dependent manner. In P. americana, only QB amplitudes decreased consistently. In both species, time constants of QB parameters' recovery increased with the strength and duration of stimulation, reaching about 30 s after bright prolonged 10 s pulses. In the blow fly, changes in QB amplitudes during recovery correlated with changes in half-widths but not latencies, suggesting at least two separate mechanisms of light adaptation: acceleration of QB onset by sensitizing transduction channels, and acceleration of transduction channel inactivation causing QB shortening and diminishment. In the cockroach, light adaptation reduced QB amplitude by apparently lowering the transduction channel availability. Impulse response data in the blow fly and cockroach were consistent with the mechanistic inferences from the QB recovery experiments. However, in the hover fly V. pellucens, impulse response latencies and durations decreased simultaneously whereas amplitudes decreased little, even when bright flashes were applied at high frequencies. These findings indicate existence of dissimilar mechanisms of light adaptation in the microvilli of different species.

Speed of phototransduction in the microvillus regulates the accuracy and bandwidth of the rhabdomeric photoreceptor

Phototransduction reactions in the rhabdomeric photoreceptor are profoundly stochastic due to the small number of participating molecules and small reaction space. The resulting quantum bumps (QBs) vary in their timing (latency), amplitudes and durations, and these variabilities within each cell are not correlated. Using modeling and electrophysiological recordings, we investigated how the QB properties depend on the cascade speed and how they influence signal transfer. Parametric analysis in the model supported by experimental data revealed that faster cascades elicit larger and narrower QBs with faster onsets and smaller variabilities than slower cascades. Latency dispersion was stronger affected by modification of upstream than downstream activation parameters. The variability caused by downstream modifications closely matched the experimental variability. Frequency response modeling showed that corner frequency is a reciprocal function of the characteristic duration of the multiphoton response, which, in turn, is a non-linear function of QB duration and latency dispersion. All QB variabilities contributed noise but only latency dispersion slowed and spread multiphoton responses, lowering the corner frequency. Using the discovered QB correlations, we evaluated transduction noise for dissimilar species and two extreme adaptation states, and compared it to photon noise. The noise emitted by the cascade was non-additive and depended non-linearly on the interaction between the QB duration and the three QB variabilities. Increased QB duration strongly suppressed both noise and corner frequency. This trade-off might be acceptable for nocturnal but not diurnal species because corner frequency is the principal determinant of information capacity. To offset the increase in noise accompanying the QB narrowing during light adaptation and the response-expanding effect of latency dispersion, the cascade accelerates. This explains the widespread evolutionary tendency of diurnal fliers to have fast phototransduction, especially after light adaptation, which thus appears to be a common adaptation to contain stochasticity, improve SNR and expand the bandwidth.

Vision does not impact walking performance in Argentine ants

Many walking insects use vision for long-distance navigation, but the influence of vision on rapid walking performance that requires close-range obstacle detection and directing the limbs towards stable footholds remains largely untested. We compared Argentine ant (Linepithema humile) workers in light versus darkness while traversing flat and uneven terrain. In darkness, ants reduced flat-ground walking speeds by only 5%. Similarly, the approach speed and time to cross a step obstacle were not significantly affected by lack of lighting. To determine whether tactile sensing might compensate for vision loss, we tracked antennal motion and observed shifts in spatiotemporal activity as a result of terrain structure but not illumination. Together, these findings suggest that vision does not impact walking performance in Argentine ant workers. Our results help contextualize eye variation across ants, including subterranean, nocturnal and eyeless species that walk in complete darkness. More broadly, our findings highlight the importance of integrating vision, proprioception and tactile sensing for robust locomotion in unstructured environments.

Suppression of Gq and PLC gene expression has a small effect on quantum bumps in vivo in Periplaneta americana

Visual signal transmission by Drosophila melanogaster photoreceptors is mediated by a Gq protein that activates a phospholipase C (PLC). Mutations and deficiencies in expression of either of these proteins cause severe defects in phototransduction. Here we investigated whether these proteins are also involved in the cockroach, Periplaneta americana, phototransduction by silencing Gq α-subunit (Gqα) and phosphoinositide-specific phospholipase C (PLC) by RNA interference and observing responses to single photons (quantum bumps, QB). We found (1) non-specific decreases in membrane resistance, membrane capacitance and absolute sensitivity in the photoreceptors of both Gqα and PLC knockdowns, and (2) small changes in QB statistics. Despite significant decreases in expressions of Gq and PLC mRNA, the changes in QB properties were surprisingly modest, with mean latencies increasing by ~ 10%, and without significant decrease in their amplitudes. To better understand our results, we used a mathematical model of the phototransduction cascade. By modifying the Gq and PLC abundances, and diffusion rates for Gq, we found that QB latencies and amplitudes deteriorated noticeably only after large decreases in the protein levels, especially when Gq diffusion was slow. Also, reduction in Gq but not PLC lowered quantum efficiency. These results suggest that expression of the proteins may be redundant.