Intracellular calcium and extra‐retinal photoreception in Aplysia giant neurons

Abstract

The early or "instantaneous" current-voltage relationship for the light-activated potassium current in Aplysia giant neurons was linear during the first second of illumination. However, the light current was greatly reduced or abolished by prolonged hyperpolarization. It was also greatly reduced by the injection of calcium EGTA buffers having calcium activities of 5.6 X 10(-8) M and simulated by injecting buffers with calcium activities of 2.8-5.6 X 10(-7) M. Removal of calcium from the extracellular fluid had no effect. Both the light- and calcium-activated outward potassium currents were reduced by tetraethylammonium (TEA) ions. The light current was not affected by substituting rubidium for potassium nor by substituting either lithium or Tris for sodium. The calcium-activated potassium current persisted when the neuron was cooled to 5 degrees C. However, the light response could no longer be elicited. Light hyperpolarizes Aplysia neurons probably by increasing intracellular calcium activity two-to six-fold which activates a membrane potassium conductance. Calcium levels appear to be restored within the cell and are energy dependent. The light-activated release of calcium is inhibited by cooling. The body wall of Aplysia transmits enough visible or 500 nm light to hyperpolarize some Aplysia giant neurons under ambient conditons. These neurons may be involved in the extraretinal light entrainment that occurs in Aplysia.

Voltage, temperature and ionic dependence of the slow outward current in Aplysia burst‐firing neurones.

“…The slope of the dotted line was 11 x 10-8 mhos. The reversal potentials measured in this manner are similar to the potassium equilibrium potential reported by Russell & Brown (1972) and the reversal of the light-activated K+ current in the giant cell, R2, demonstrated by Brown et al (1977). Thus the holding current in these cells is the sum of several ionic currents (most likely C1-, K+, and some slow Na+/Ca2+), while the current following the depolarizing conditioning command is the sum of these same currents plus a K+ current that has been activated by the depolarization.…”

“…The experiments presented here were intended to focus only on the slow outward current that is activated by depolarization in R15 of Aplysia. This current is probably identical, or at least analogous, to the slow outward currents described in a variety of other neurones (Brodwick & Junge, 1972;Meech, 1972;Brown et al 1977;Thompson, 1977). The function of I"o for bursting pace-maker activity is to shut off the depolarizing phase and to hyperpolarize the cell between cycles (Carnevale, 1973;Junge & Stephens, 1973;Meech, 1974).…”

Voltage, temperature and ionic dependence of the slow outward current in Aplysia burst‐firing neurones.

“…The slope of the dotted line was 11 x 10-8 mhos. The reversal potentials measured in this manner are similar to the potassium equilibrium potential reported by Russell & Brown (1972) and the reversal of the light-activated K+ current in the giant cell, R2, demonstrated by Brown et al (1977). Thus the holding current in these cells is the sum of several ionic currents (most likely C1-, K+, and some slow Na+/Ca2+), while the current following the depolarizing conditioning command is the sum of these same currents plus a K+ current that has been activated by the depolarization.…”

“…The experiments presented here were intended to focus only on the slow outward current that is activated by depolarization in R15 of Aplysia. This current is probably identical, or at least analogous, to the slow outward currents described in a variety of other neurones (Brodwick & Junge, 1972;Meech, 1972;Brown et al 1977;Thompson, 1977). The function of I"o for bursting pace-maker activity is to shut off the depolarizing phase and to hyperpolarize the cell between cycles (Carnevale, 1973;Junge & Stephens, 1973;Meech, 1974).…”

Intracellular calcium accumulation during depolarization in a molluscan neurone.

“…The average intracellular Ca2+ concentration (140 nM) estimated from the suppression potential in normal (10 mM) external Ca2+ is 300-600 times smaller than the value estimated from the Ca2+ current suppression potential in intact molluscan neurones (Adams & Gage, 1979a;Connor, 1979), but these estimates are likely to be complicated by the presence of outward K+ currents which are incompletely blocked at positive membrane potentials. The estimated free intracellular Ca2+ concentration is, however, about 2 times greater than indirect estimates from other molluscan neurones (Meech & Standen, 1975;Brown, Brodwick & Eaton, 1977) and 2-4 times greater than direct estimates from squid axon (Brinley, 1978), but there are reasons for suspecting that our estimate is on the high side. Our estimate is based on measurements of the suppression potential for the change in [Ca]i and not on the change in Ca2+ current.…”

The role of diffusion in the photoresponse of an extraretinal photoreceptor of Aplysia.

“…The experimental observation that the flash induced membrane current returns to its original level suggests that in addition to the previous binding effect, there is also an irreversible sink similar to those known to transport Ca2+ out of cells (Blaustein & Russell, 1975). This membrane pump acts to restore cytoplasmic Ca2+ activities to low levels (-6 x 10-8 M, Brown, Brodwick & Eaton, 1977). A similar transport mechanism might also be located in the lipochondrial membranes since we have shown that the lipochondria accumulate Ca2+ (Baur,Brown,Rogers & Brower, 291 M. C. ANDRESEN, A. M. BROWN AND S. YASUI 1977).…”

“…The light response of Aplysia neurones with its high threshold and slow response may be appropriate for extraretinal photoresponses such as those involved in circadian locomotion (Block & Lickey, 1973;Brown et al 1977). However, diffusion alone cannot account for the wave form of the photoresponse in all photoreceptors, particularly the much smaller retinal photoreceptors.…”

Voltage, temperature and ionic dependence of the slow outward current in Aplysia burst‐firing neurones.

“…The slope of the dotted line was 11 x 10-8 mhos. The reversal potentials measured in this manner are similar to the potassium equilibrium potential reported by Russell & Brown (1972) and the reversal of the light-activated K+ current in the giant cell, R2, demonstrated by Brown et al (1977). Thus the holding current in these cells is the sum of several ionic currents (most likely C1-, K+, and some slow Na+/Ca2+), while the current following the depolarizing conditioning command is the sum of these same currents plus a K+ current that has been activated by the depolarization.…”

“…The experiments presented here were intended to focus only on the slow outward current that is activated by depolarization in R15 of Aplysia. This current is probably identical, or at least analogous, to the slow outward currents described in a variety of other neurones (Brodwick & Junge, 1972;Meech, 1972;Brown et al 1977;Thompson, 1977). The function of I"o for bursting pace-maker activity is to shut off the depolarizing phase and to hyperpolarize the cell between cycles (Carnevale, 1973;Junge & Stephens, 1973;Meech, 1974).…”

Intracellular calcium accumulation during depolarization in a molluscan neurone.

“…The average intracellular Ca2+ concentration (140 nM) estimated from the suppression potential in normal (10 mM) external Ca2+ is 300-600 times smaller than the value estimated from the Ca2+ current suppression potential in intact molluscan neurones (Adams & Gage, 1979a;Connor, 1979), but these estimates are likely to be complicated by the presence of outward K+ currents which are incompletely blocked at positive membrane potentials. The estimated free intracellular Ca2+ concentration is, however, about 2 times greater than indirect estimates from other molluscan neurones (Meech & Standen, 1975;Brown, Brodwick & Eaton, 1977) and 2-4 times greater than direct estimates from squid axon (Brinley, 1978), but there are reasons for suspecting that our estimate is on the high side. Our estimate is based on measurements of the suppression potential for the change in [Ca]i and not on the change in Ca2+ current.…”

The role of diffusion in the photoresponse of an extraretinal photoreceptor of Aplysia.

“…The experimental observation that the flash induced membrane current returns to its original level suggests that in addition to the previous binding effect, there is also an irreversible sink similar to those known to transport Ca2+ out of cells (Blaustein & Russell, 1975). This membrane pump acts to restore cytoplasmic Ca2+ activities to low levels (-6 x 10-8 M, Brown, Brodwick & Eaton, 1977). A similar transport mechanism might also be located in the lipochondrial membranes since we have shown that the lipochondria accumulate Ca2+ (Baur,Brown,Rogers & Brower, 291 M. C. ANDRESEN, A. M. BROWN AND S. YASUI 1977).…”

“…The light response of Aplysia neurones with its high threshold and slow response may be appropriate for extraretinal photoresponses such as those involved in circadian locomotion (Block & Lickey, 1973;Brown et al 1977). However, diffusion alone cannot account for the wave form of the photoresponse in all photoreceptors, particularly the much smaller retinal photoreceptors.…”

Voltage, temperature and ionic dependence of the slow outward current in Aplysia burst‐firing neurones.

“…The slope of the dotted line was 11 x 10-8 mhos. The reversal potentials measured in this manner are similar to the potassium equilibrium potential reported by Russell & Brown (1972) and the reversal of the light-activated K+ current in the giant cell, R2, demonstrated by Brown et al (1977). Thus the holding current in these cells is the sum of several ionic currents (most likely C1-, K+, and some slow Na+/Ca2+), while the current following the depolarizing conditioning command is the sum of these same currents plus a K+ current that has been activated by the depolarization.…”

“…The experiments presented here were intended to focus only on the slow outward current that is activated by depolarization in R15 of Aplysia. This current is probably identical, or at least analogous, to the slow outward currents described in a variety of other neurones (Brodwick & Junge, 1972;Meech, 1972;Brown et al 1977;Thompson, 1977). The function of I"o for bursting pace-maker activity is to shut off the depolarizing phase and to hyperpolarize the cell between cycles (Carnevale, 1973;Junge & Stephens, 1973;Meech, 1974).…”

Intracellular calcium accumulation during depolarization in a molluscan neurone.

“…The average intracellular Ca2+ concentration (140 nM) estimated from the suppression potential in normal (10 mM) external Ca2+ is 300-600 times smaller than the value estimated from the Ca2+ current suppression potential in intact molluscan neurones (Adams & Gage, 1979a;Connor, 1979), but these estimates are likely to be complicated by the presence of outward K+ currents which are incompletely blocked at positive membrane potentials. The estimated free intracellular Ca2+ concentration is, however, about 2 times greater than indirect estimates from other molluscan neurones (Meech & Standen, 1975;Brown, Brodwick & Eaton, 1977) and 2-4 times greater than direct estimates from squid axon (Brinley, 1978), but there are reasons for suspecting that our estimate is on the high side. Our estimate is based on measurements of the suppression potential for the change in [Ca]i and not on the change in Ca2+ current.…”

The role of diffusion in the photoresponse of an extraretinal photoreceptor of Aplysia.

“…The experimental observation that the flash induced membrane current returns to its original level suggests that in addition to the previous binding effect, there is also an irreversible sink similar to those known to transport Ca2+ out of cells (Blaustein & Russell, 1975). This membrane pump acts to restore cytoplasmic Ca2+ activities to low levels (-6 x 10-8 M, Brown, Brodwick & Eaton, 1977). A similar transport mechanism might also be located in the lipochondrial membranes since we have shown that the lipochondria accumulate Ca2+ (Baur,Brown,Rogers & Brower, 291 M. C. ANDRESEN, A. M. BROWN AND S. YASUI 1977).…”

“…The light response of Aplysia neurones with its high threshold and slow response may be appropriate for extraretinal photoresponses such as those involved in circadian locomotion (Block & Lickey, 1973;Brown et al 1977). However, diffusion alone cannot account for the wave form of the photoresponse in all photoreceptors, particularly the much smaller retinal photoreceptors.…”