Importance of Calcium And Magnesium Ions for Postexcitatory Hypersensitivity in the Jumping Spider (Menemerus) Eye

Photoreceptor cells of the anterior median eye of the jumping spider Menemerus are more sensitive for a brief period following illumination than they are during complete dark adaptation (Yamashita & Tateda, 1976). It was suggested that such hypersensitivity may be related to the ionic composition of the saline, since calcium ions are important in controlling the sensitivity of photoreceptor cells in other arthropods (Lisman & Brown, 1972, 1975; Brown & Lisman, 1972, 1975; Brown & Blinks, 1974; Fein & Lisman, 1975; Bader, Baumann & Bertrand, 1976; Hanani & Hillman, 1976; Fein & Charlton, 1977; Brown, Brown & Pinto, 1977; Lisman & Strong, 1979).

In the experiments reported here, the effects of the four major cations on hypersensitivity in Menemerus have been studied.

Jumping spiders Menemerus confusus Boes et Str. were used throughout this study. Preparation and recording methods were similar to those described previously (Yamashita & Tateda, 1976). Electroretinograms (ERGs) were recorded from the anterior median eyes with glass pipette microelectrodes or suction electrodes. Two 6-8 V tungsten lamps (lamp I, II) placed side by side were used for white light stimulation. Lamp I was used as the control light and the test light, and lamp II was used as the conditioning light. Initially, the control light, serving also as the test light, was presented. This was followed by the conditioning light and after various time intervals by a test light (Fig. 1 A). The control light was used on the completely dark-adapted eye. In some cases, the conditioning light was used also for background illumination. The composition of the normal physiological saline was as follows: NaCl, 217 mM; KC1, 5 mM; CaCl₂, 4mM; MgCl₂, I-I HIM; NaHCO₃, 3 mM (Rath-mayer, 1965). Low-Na+ salines containing 112 mM, 46 mM and 3 mM-Na⁺ were made by replacing NaCl with equimolar choline chloride, and K⁺-free saline by replacing KC1 with choline chloride. Low-Ca²+ saline (0·1mM-Ca²⁺) was made by replacing CaCl₂ with equivalent NaCl, and high-Ca²⁺ saline (10 mM Ca²+) by replacing NaCl with CaCl₂. Mg²+-free saline was made by replacing MgCl₂ with equivalent NaCl. After a change in the perfusing solution, a period of 3 – 10 min was usually required to establish the steady-state effects of the new solution upon the eye’s light response. Reported results were obtained repeatedly under these steady-state conditions.

ERG intensity-response curves to white light stimuli were obtained from completely dark-adapted eyes and from eyes 5 s after a constant conditioning stimulus of 1 s duration (Fig. 1B). The intensity-response curves obtained before and after conditioning stimuli were almost parallel ; any one of them could be superimposed on the other by a shift along the abscissa.

Sensitivity was defined as the reciprocal of the relative intensity required to elicit a response amplitude 50% that of the saturated response, and the sensitivity of the completely dark-adapted eye was taken as 1·0. After the conditioning stimulus, relative sensitivity was initially very low but then increased rapidly above the dark-adapted level and fell back slowly (Fig. 1C). In fresh preparations, maximum relative sensitivity was about 1-5, and occurred 3–5 s following the end of illumination.

ERG light responses before and 5 s, 10 s and 30 s after a constant conditioning stimulus were obtained in normal and in low-Na+ saline (Fig. 2). In normal saline (220 mM-Na+) and in 112 mM-Na⁺ saline, the amplitude of the responses increased after the conditioning stimulus. In 46 mM-Na+ saline, the response showed little increase after the conditioning stimulus, and in 3 mM-Na+ saline, it markedly decreased.

In 46 mM and in 3 mM-Na+ saline, the intensity-response function for the dark-adapted eye and that for the eye 5 s after the cessation of the conditioning stimulus show that the saturated values markedly decreased after illumination compared with those before illumination (Fig. 3). Note that sensitivities increased after the conditioning stimulus at both reduced Na+ levels. The relative sensitivity 5 s after conditioning was 1·51 in 46 mM-Na+ saline and 1·31 in 3 mM-Na+ saline. These values were approximately the same as those in normal saline (cf. Fig. 1).

In 112 mM-Na+ saline both the intensity-response curve during the dark and the change in sensitivity after illumination were almost identical with those in normal line (Fig. 4). Therefore, it is unlikely that Na⁴² plays a significant role in the hypersensitivity reported above. The role of Na+ in regulating the saturated value of the ERG after the conditioning illumination was not investigated.

In K⁺-free saline, the dark-adapted intensity-response curve and the change in sensitivity after illumination were about the same as with those in normal saline (Fig. 5). Therefore, it is unlikely too that K⁺ plays a significant role in the hypersensitivity.

In contrast, Ca²⁺ had marked effects on the hypersensitivity. Fig. 6 shows the effects of changes in external Ca²⁺ concentration on ERG light responses to control light (C) presented to the completely dark-adapted eye, and test light (T) presented 5 s after the onset of a conditioning stimulus (T₁) and 5 s after the cessation of the conditioning stimulus (T₂). The amplitudes of the responses to C, T₁ and T₂ are given in Table 1.

Changes in external Ca²⁺ concentration had a small effect on light responses during the completely dark-adapted state, but a large effect during and after illumination. The intensity-response curves show that the sensitivity of the dark-adapted eye was somewhat greater in low-Ca²+ saline, and somewhat lower in high-Ca²⁺ saline (Fig. 7 A). Hypersensitivity after illumination markedly increased to a maximum of about 2·0 in low-Ca²⁺ saline and markedly decreased to about 1 · 1 in high-Ca²⁺ saline (Fig. 7B). Maximum sensitivity was about 1·5 in normal saline. These results show that external Ca²+ plays an important role in determining the sensitivity of the eye especially during and after illumination.

Intensity-response relations during the dark (Fig. 8 A) and the changes in sensitivities after illumination (Fig. 8B) obtained with normal and Mg²⁺-free saline show that Mg²⁺ had no effect in the dark-adapted eye. Mg²⁺, however, had a marked effect on hypersensitivity. In normal saline, maximum sensitivity after illumination was about 1·4 but in Mg²⁺-free saline, it was only about 1·05. These results show that Mg²⁺ is necessary for maintenance of hypersensitivity in the Menemerus eye.

It has been reported previously that Ca²⁺ plays important roles in controlling sensitivity of invertebrate photoreceptors. Lisman & Brown (1972) showed that during intracellular iontophoretic injection of Ca²+ into Limulus ventral photoreceptors, there was a progressive dimunution of the light response. They postulated that light stimulation leads to an increase in intracellular Ca²⁺ which in turn reduces responsiveness to light stimulation. Direct measurement of intracellular Ca²⁺ concentration in Limulus ventral photoreceptors and Balanus photoreceptors indicates that illumination does induce an increase in the intracellular Ca²⁺ concentration (Brown & Blinks, 1974). Hanani & Hillman (1976) showed that the barnacle photoreceptor sensitivity either decreased or increased after exposure to illumination, and both phenomena were influenced by external Ca²+ concentration. In a study on the retina of the honey bee drone, Bader et al. (1976) showed that an increase in intracellular Ca²⁺ concentration played a central role in light adaptation.

As shown in the present study, sensitivity of the spider anteromedian eye was also greatly affected by Ca²⁺. When the external Ca²⁺ concentration is low, the sensitivity of the dark-adapted eye increases a little and hypersensitivity after illumination markedly increases. Yamashita & Tateda (1976) suggested that there were two antagonistic processes in photoreceptor cell of the AM eye. One process decreased the sensitivity during illumination (process I), the other process increased the sensitivity (process II), and a saturation of process II occurred more slowly than that of process I. The actual time course of light and dark adaptation corresponded to the summated time courses of processes I and II. They (1976, 1978) showed that respiration was necessary to maintain hypersensitivity, and process II appeared to be affected by respiratory activity suggesting that process II might be related to an active mechanism. The hypersensitivity phenomenon in spider eye may be explained by the following hypothesis. A light-dependent Ca²⁺-pump which may be related to process II is present in the photoreceptor cells. Efflux of Ca²+ from the photoreceptor cells mediated by the light-dependent Ca²⁺-pump is greater than influx of Ca²⁺ during illumination. The resulting decrease of intracellular Ca²⁺ concentration may continue for a short period after the cessation of illumination. As a result, the sensitivity of the photoreceptor cells following illumination may increase for a short period. When the external Ca²⁺ concentration is high, or Mg²⁺ concentration is low, influx of Ca²⁺ may be greater and/or the Ca²⁺-pump may be less activated than in normal saline.

The authors are deeply indebted to Professor T. H. Waterman, Yale University, for critical revision of the manuscript. This research was supported in part by grants from the Ministry of Education of Japan.