The influence of extracellular calcium on the response of fly photoreceptors

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.

Calcium homeostasis in fly photoreceptor cells

In fly photoreceptor cells, two processes dominate the Ca2+ homeostasis: light-induced Ca2+ influx through members of the TRP family of ion channels, and Ca2+ extrusion by Na+/Ca2+ exchange. Ca2+ release from intracellular stores is quantitatively insignificant. Both, the light-activated channels and the Ca2+-extruding exchangers are located in or close to the rhabdomeric microvilli, small protrusions of the plasma membrane. The microvilli also contain the molecular machinery necessary for generating quantum bumps, short electrical responses caused by the absorption of a single photon. Due to this anatomical arrangement, the light-induced Ca2+ influx results in two separate Ca2+ signals that have different functions: a global, homogeneous increase of the Ca2+ concentration in the cell body, and rapid but large amplitude Ca2+ transients in the microvilli. The global rise of the Ca2+ concentration mediates light adaptation, via regulatory actions on the phototransduction cascade, the voltage-gated K+ channels and small pigment granules controlling the light intensity. The local Ca2+ transients in the microvilli are responsible for shaping the quantum bumps into fast, all-or-nothing events. They achieve this by facilitating strongly the phototransduction cascade at early stages ofthe light response and subsequently inhibiting it. Many molecular targets of these feedback mechanisms have been identified and characterized due to the availability of numerous Drosophila mutant showing defects in the phototransduction.

Effect of Ca(2+)-free and Mg(2+)-free BSS Plus solution on the retinal pigment epithelium and retina in rabbits

PURPOSE

To determine whether intravitreal irrigation with Ca(2+)-free and Mg(2+)-free BSS Plus (Alcon Laboratory, Fort Worth, Texas) solution alters the adhesiveness between the retinal pigment epithelium and the retina of rabbits.

METHODS

Thirty-four eyes of 34 Dutch pigmented rabbits underwent lensectomy and vitrectomy. Subsequently, the vitreous cavity of 24 eyes was irrigated with Ca(2+)-free and Mg(2+)-free BSS Plus solution for 10 or 20 minutes. The other 10 eyes were irrigated with BSS Plus solution for 20 minutes as controls. To determine the adhesiveness between the retinal pigment epithelium and retina, a retinal detachment was produced in 12 of the 34 eyes. The apical surface of the retinal pigment epithelium and the photoreceptor outer segments were examined by scanning electron microscopy. Retinal physiology was assessed by electroretinography and retinal morphology by light microscopy.

RESULTS

After retinal detachment was produced, the number of cone sheaths on the apical surface of the retinal pigment epithelium after irrigation with Ca(2+)-free and Mg(2+)-free BSS Plus solution for 20 minutes (33 +/- 15, mean +/- SD) was significantly less than the number of cone sheaths on the apical surface of the retinal pigment epithelium of eyes after irrigation with BSS Plus solution for 20 minutes (120 +/- 50) or the number of cone sheaths on the apical surface of the retinal pigment epithelium of eyes after 10 minutes of irrigation with Ca(2+)-free and Mg(2+)-free BSS Plus solution (115 +/- 49; P =.02). The b-wave amplitudes in the eyes irrigated with Ca(2+)-free and Mg(2+)-free BSS Plus solution for 20 minutes were depressed compared with the b-waves in eyes irrigated with BSS Plus solution for 20 minutes on the first postoperative day (P =.03). After the third postoperative day, there was no significant difference in the b-waves (P >.06). Light microscopy demonstrated no morphologic abnormalities after the use of both solutions.

CONCLUSIONS

Intravitreal irrigation with Ca(2+)-free and Mg(2+)-free BSS Plus solution for 20 minutes altered the adhesion between the retinal pigment epithelium microvilli and retinal outer segments and made the creation of retinal detachment less traumatic. These results suggest that Ca(2+)-free and Mg(2+)-free BSS Plus solution may be of clinical value for the creation of an intentional retinal detachment for foveal translocation surgery.

Single photon responses in Drosophila photoreceptors and their regulation by Ca2+

1. Discrete events (quantum bumps) elicited by dim light were analysed in whole-cell voltage clamp of photoreceptors from dissociated Drosophila ommatidia. Bumps were automatically detected and analysed for amplitude, rise and decay times, and latency. 2. The bump interval and amplitude distributions, and the 'frequency of seeing' curve conformed to Poisson predictions for the absorption of single photons. 3. At resting potential (-70 mV), bumps averaged 10 pA in peak amplitude with a half-width of ca 20 ms, representing simultaneous activation of ca 15 channels. 4. The macroscopic response to flashes containing up to at least 750 photons were predicted by the linear summation of quantum bumps convolved with their latency dispersion. 5. Bump duration was unaffected by lowering the extracellular Ca2+ concentration ([Ca2+]o) from 1.5 to 0.5 mM, but increased >10-fold between 0.5 mM Ca2+ and 0 Ca2+. Bump amplitude was constant over the range 1.5-100 microM, but decreased ca 5- to 10-fold at lower Ca2+ concentrations. Bump latency increased by ca 50 % between 1.5 mM and 100 microM Ca2+o but returned to near control levels in Ca2+-free solutions. At intermediate [Ca2+]o bumps were biphasic with a slow rising phase followed by rapid amplification and inactivation. This behaviour was mimicked in high [Ca2+]o by internal buffering with BAPTA, but not EGTA. This suggests that Ca2+ influx through the light-sensitive channels must first raise cytosolic Ca2+ to a threshold level before initiating a cycle of positive and negative feedback mediated by molecular targets within the same microvillus. Quantum bumps in trp mutants lacking the major class of light-sensitive channel were reduced in size (mean 3.5 pA) representing simultaneous activation of only one or two channels; however, a second rarer (10 %) class of large bump had an amplitude similar to wild-type (WT) bumps. Bumps in trpl mutants lacking the second class of light-sensitive channel were very similar to WT bumps, but with slightly slower decay times. In InaDP215 mutants, in which the association of the TRP channels with the INAD scaffolding molecule is disrupted, bumps showed a defect in quantum bump termination, but their amplitudes and latencies were near normal.

Pigment granule migration in crustacean photoreceptors requires calcium

We have investigated the role of calcium in the regulation of pigment granule migration in photoreceptors of the semi-terrestrial crab, Sesarma cinereum. Isolated crab eyes (eyecup plus eyestalk) were maintained in crustacean Ringer either prepared normally or calcium-free plus 50 mM EGTA. Pigment granule movement was indirectly observed by monitoring reflectance from the eye during light stimuli using intracellular optical physiological techniques. Electroretinograms (ERGs) were also measured during light stimuli. EGTA treatment caused gradual loss of centripetal migration of pigment granules (normally leading to pupillary closure), and photoreceptors eventually became locked in the open-pupil, dark-adapted state despite repeated stimuli. In contrast, ERG responses continued throughout EGTA treatment, although the size and shape of the response was altered. Normal ERG responses and pigment granule movements returned after replacing EGTA-Ringer with normal-calcium medium. These results suggest that centripetal migration of pigment granules in crustacean photoreceptors requires calcium.

Intracellular free calcium concentration in the blowfly retina studied by Fura-2

The retinal tissue of blowflies was loaded with the fluorescent Ca2+ indicator Fura-2 by incubating cut heads in saline solutions which contained the membrane permeable acetoxymethylester of Fura-2 (Fura-2/AM). The spectral analysis of the tissue fluorescence showed that Fura-2/AM was intracellularly hydrolysed to Fura-2. In order to monitor the intracellular free Ca2+ concentration ([Ca2+]i) the Fura-2 fluorescence was excited by short light flashes. The fluorescence was calibrated by incubating the tissue in Ca2+ buffers of high buffering capacity and subsequent disruption of the cell membranes by freeze/thawing, which gave a dissociation constant for the Ca(2+)-Fura-2 complex of 100 nM. When the extracellular Ca2+ concentration ([Ca2+]o) was altered [Ca2+]i reversibly changed. The changes were most pronounced when [Ca2+]o was varied in the millimolar range, e.g. [Ca2+]i increased from 0.07 microM at [Ca2+]o = 0.1 mM to 1 microM at [Ca2+]o = 10 mM. When extracellular Na+ was replaced by Li+ or other monovalent ions, [Ca2+]i rapidly increased which supports the view that electrogenic Na+/Ca2+ exchange contributes to the control of [Ca2+]i. However, [Ca2+]i decreased again when the tissue was superfused with Na(+)-free media for longer periods, which points to a Ca(2+)-transporting system different from Na+/Ca2+ exchange. Light adaptation had only a small effect on [Ca2+]i. Even after intense stimulation [Ca2+]i increased by a factor of 1.5 only, which is in line with results obtained in the photoreceptors of Balanus and Apis.

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.