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Frolov RV. Non-inactivating voltage-activated K+ conductances can increase photoreceptor signaling bandwidth beyond the bandwidth set by phototransduction. PLoS One 2023; 18:e0289466. [PMID: 37527242 PMCID: PMC10393161 DOI: 10.1371/journal.pone.0289466] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/04/2023] [Accepted: 07/19/2023] [Indexed: 08/03/2023] Open
Abstract
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.
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Affiliation(s)
- Roman V Frolov
- Laboratory of Comparative Sensory Physiology, Sechenov Institute of Evolutionary Physiology and Biochemistry, Russian Academy of Sciences, Saint Petersburg, Russia
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Frolov RV, Severina I, Novikova E, Ignatova II, Liu H, Zhukovskaya M, Torkkeli PH, French AS. Opsin knockdown specifically slows phototransduction in broadband and UV-sensitive photoreceptors in Periplaneta americana. J Comp Physiol A Neuroethol Sens Neural Behav Physiol 2022; 208:591-604. [PMID: 36224473 DOI: 10.1007/s00359-022-01580-z] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/24/2022] [Revised: 09/27/2022] [Accepted: 09/27/2022] [Indexed: 12/14/2022]
Abstract
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.
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Affiliation(s)
- Roman V Frolov
- Laboratory of Comparative Sensory Physiology, Sechenov Institute of Evolutionary Physiology and Biochemistry, Russian Academy of Sciences, Pr. Thorez 44, 194223, Saint-Petersburg, Russia.
| | - Irina Severina
- Laboratory of Comparative Sensory Physiology, Sechenov Institute of Evolutionary Physiology and Biochemistry, Russian Academy of Sciences, Pr. Thorez 44, 194223, Saint-Petersburg, Russia
| | - Ekaterina Novikova
- Laboratory of Comparative Sensory Physiology, Sechenov Institute of Evolutionary Physiology and Biochemistry, Russian Academy of Sciences, Pr. Thorez 44, 194223, Saint-Petersburg, Russia
| | - Irina I Ignatova
- Laboratory of Comparative Sensory Physiology, Sechenov Institute of Evolutionary Physiology and Biochemistry, Russian Academy of Sciences, Pr. Thorez 44, 194223, Saint-Petersburg, Russia
| | - Hongxia Liu
- Department of Physiology and Biophysics, Dalhousie University, P.O. BOX 15000, Halifax, NS, B3H 4R2, Canada
| | - Marianna Zhukovskaya
- Laboratory of Comparative Sensory Physiology, Sechenov Institute of Evolutionary Physiology and Biochemistry, Russian Academy of Sciences, Pr. Thorez 44, 194223, Saint-Petersburg, Russia
| | - Päivi H Torkkeli
- Department of Physiology and Biophysics, Dalhousie University, P.O. BOX 15000, Halifax, NS, B3H 4R2, Canada
| | - Andrew S French
- Department of Physiology and Biophysics, Dalhousie University, P.O. BOX 15000, Halifax, NS, B3H 4R2, Canada
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Speed of phototransduction in the microvillus regulates the accuracy and bandwidth of the rhabdomeric photoreceptor. PLoS Comput Biol 2020; 16:e1008427. [PMID: 33196643 PMCID: PMC7704055 DOI: 10.1371/journal.pcbi.1008427] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/07/2020] [Revised: 11/30/2020] [Accepted: 10/07/2020] [Indexed: 12/16/2022] Open
Abstract
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.
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Electrophysiological adaptations of insect photoreceptors and their elementary responses to diurnal and nocturnal lifestyles. J Comp Physiol A Neuroethol Sens Neural Behav Physiol 2019; 206:55-69. [PMID: 31858215 PMCID: PMC6995784 DOI: 10.1007/s00359-019-01392-8] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/25/2019] [Revised: 11/18/2019] [Accepted: 12/03/2019] [Indexed: 12/16/2022]
Abstract
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 (Cm), 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 Cm, so that nocturnal species had relatively large and prolonged current bumps. Consequently, although the voltage bump amplitude correlated negatively with Cm, the strength of the total voltage signal increased. Importantly, the background voltage noise decreased strongly with increasing Cm, yielding a notable increase in signal-to-noise ratio for voltage bumps. A similar decrease in the background noise with increasing Cm was found in intracellular recordings in vivo. Morphological measurements of rhabdomeres were consistent with our Cm estimates. Our results indicate that the increased photoreceptor Cm 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.
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Ignatova II, Saari P, Frolov RV. Latency of phototransduction limits transfer of higher-frequency signals in cockroach photoreceptors. J Neurophysiol 2019; 123:120-133. [PMID: 31721631 DOI: 10.1152/jn.00365.2019] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
Visual transduction in rhabdomeric photoreceptors is compartmentalized and discretized. Signals of individual microvilli, the quantum bumps, are electrotonically summed, producing a graded response. Intrinsic dispersion of quantum bump latencies is thought to introduce noise and degrade signal transfer. Here, we found profound differences in the information rate and signaling bandwidth between in vitro patch-clamp and in vivo intracellular recordings of Periplaneta americana photoreceptors and traced them to the properties of quantum bumps and membrane resistance. Comparison of macroscopic and elementary light responses revealed differences in quantum bump summation and membrane resistance in vivo versus in vitro. Modeling of voltage bumps suggested that current bumps in vivo should be much bigger and faster than those actually recorded in vitro. Importantly, the group-average latency of dark-adapted photoreceptors was 30 ± 8 ms in intracellular (n = 34) versus 70 ± 19 ms in patch-clamp (n = 57) recordings. Duration of composite responses increased with mean latency because bump dispersion depended on mean latency. In vivo, latency dispersion broadened the composite response by 25% on average and slowed its onset. However, in the majority of photoreceptors, the characteristic durations of multiphoton impulse responses to 1-ms stimuli did not exceed the durations of mean voltage bumps. Consistently, we found strong associations between the latency and onset kinetics of the macroscopic response, on the one hand and the higher-frequency signal gain and information rate of the photoreceptor, on the other hand, indicating a direct connection between quantum bump latency and its dispersion and the signaling bandwidth.NEW & NOTEWORTHY When stimulated by light, microvilli of rhabdomeric photoreceptors produce discrete signals characterized by variable latencies. We show that this intrinsic latency dispersion restricts signaling bandwidth and information rate of photoreceptors in Periplaneta americana. Profound differences are found between the properties of photoreceptor responses in vivo and in vitro.
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Affiliation(s)
- Irina I Ignatova
- Biophysics Group, Nano and Molecular Systems Research Unit, University of Oulu, Oulu, Finland
| | - Paulus Saari
- Biophysics Group, Nano and Molecular Systems Research Unit, University of Oulu, Oulu, Finland
| | - Roman V Frolov
- Biophysics Group, Nano and Molecular Systems Research Unit, University of Oulu, Oulu, Finland
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