Phenotypic plasticity in Periplaneta americana 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.

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.

Electrophysiological adaptations of insect photoreceptors and their elementary responses to diurnal and nocturnal lifestyles

Nocturnal vision in insects depends on the ability to reliably detect scarce photons. Nocturnal insects tend to have intrinsically more sensitive and larger rhabdomeres than diurnal species. However, large rhabdomeres have relatively high membrane capacitance (C_m), which can strongly low-pass filter the voltage bumps, widening and attenuating them. To investigate the evolution of photoreceptor signaling under near dark, we recorded elementary current and voltage responses from a number of species in six insect orders. We found that the gain of phototransduction increased with C_m, so that nocturnal species had relatively large and prolonged current bumps. Consequently, although the voltage bump amplitude correlated negatively with C_m, the strength of the total voltage signal increased. Importantly, the background voltage noise decreased strongly with increasing C_m, yielding a notable increase in signal-to-noise ratio for voltage bumps. A similar decrease in the background noise with increasing C_m was found in intracellular recordings in vivo. Morphological measurements of rhabdomeres were consistent with our C_m estimates. Our results indicate that the increased photoreceptor C_m in nocturnal insects is a major sensitivity-boosting and noise-suppressing adaptation. However, by requiring a compensatory increase in the gain of phototransduction, this adaptation comes at the expense of the signaling bandwidth.

The sources of electrophysiological variability in the retina ofPeriplaneta americana

Variability in the electrophysiological properties of homotypic photoreceptors is widespread and is thought to facilitate functioning under disparate illumination conditions. Compound eyes of insects have three sources of variability: inter-individual, intra-individual, and intra-ommatidial, the latter two overlapping. Here, I explored the causes of variability inPeriplaneta americana, a nocturnal insect characterized by highly variable photoreceptor responses. By recording from photoreceptors in dissociated ommatidia, including consecutive recordings from photoreceptors in the same ommatidium (SO), I studied the variability of six properties: whole-cell membrane capacitance (Cm), phototransduction latency, maximal conductance (Gmax) and the slope factor of the sustained Kv current, absolute sensitivity in dim light, and sustained light-induced current (LIC) amplitude in bright light. Coefficient of variation (CV) metrics were used to compare variances in four experimental groups: SO, same animal (SA), all data combined “full sample” (FS), and full sample of all SO recordings (FSSO). For the normally distributed parametersCm,Gmax, slope factor, and latency, the highest CV values were found in FS and FSSO, intermediate in SA, and the lowest in SO. On average, SO variance accounted for 47% of the full-sample variance in these four parameters. Absolute sensitivity and LIC values were not normally distributed, and the differences in variability between SO and FS/FSSO groups were smaller than for the other four parameters. These results indicate two main sources of variability, intra-ommatidial and inter-individual. Inter-individual variability was investigated by exposing adult cockroaches to constant light or dark for several months. In both groups, the majority of CV measures for the six parameters decreased compared to control, indicating substantial contribution of phenotypic plasticity to inter-individual differences. Analysis of variability of resting potential and elementary voltage responses revealed that resting potential is mainly determined by the sustained Kv conductance, whereas voltage bump amplitude is mainly determined by current bump amplitude andCm.

Changes in electrophysiological properties of photoreceptors in Periplaneta americana associated with the loss of screening pigment

Absence of screening pigment in insect compound eyes has been linked to visual dysfunction. We investigated how its loss in a white-eyed mutant (W-E) alters the photoreceptor electrophysiological properties, opsin gene expression, and the behavior of the cockroach, Periplaneta americana. Whole-cell patch-clamp recordings of green-sensitive photoreceptors in W-E cockroaches gave reduced membrane capacitance, absolute sensitivity to light, and light-induced currents. Decreased low-pass filtering increased voltage-bump amplitudes in W-E photoreceptors. Intracellular recordings showed that angular sensitivity of W-E photoreceptors had two distinct components: a large narrow component with the same acceptance angle as wild type, plus a relatively small wide component. Information processing was evaluated using Gaussian white-noise modulated light stimulation. In bright light, W-E photoreceptors demonstrated higher signal gain and signal power than wild-type photoreceptors. Expression levels of the primary UV- and green-sensitive opsins were lower and the secondary green-sensitive opsin significantly higher in W-E than in wild-type retinae. In behavioral experiments, W-E cockroaches were significantly less active in dim green light, consistent with the relatively low light sensitivity of their photoreceptors. Overall, these differences can be related to the loss of screening pigment function and to a compensatory decrease in the rhabdomere size in W-E retinae.