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

Abstract

1. Membrane currents produced by flashes and steps of light (photo‐current) were recorded from the ventral photoresponsive neurone of Aplysia californica. The effects of background illumination and changes in temperature were also examined. 2. The falling phase of the response wave form may be separated into two components with time constants of 10‐‐12 sec and 50 sec. 3. Background illumination reduced the response amplitude to light impulses without appreciably altering the response wave form. 4. Lowering the temperature greatly reduced the amplitude of the photo‐current with a Q10 of 2.91 (25‐‐15 degrees C) and greatly prolonged the duration of the response. 5. Because of the relatively large distance between the plasma membrane and the pigmented cytoplasmic lipochondria where light is absorbed, a diffusion‐based model with Ca as the internal‐transmitter (Andresen & Brown, 1979) was developed. 6. In this model diffusion of Ca2+ released from the lipochondria upon photon absorption is slowed by the reversible uptake of Ca2+ at cytoplasmic binding sites. Ca2+ interacts with sites at the plasma membrane to increase GK and Ca2+ levels are subsequently restored by irreversible uptake processes. Ca2+ release and its adsorption and desorption from the more numerous plasma membrane binding sites were assumed to be instantaneous with respect to the duration of the light‐evoked response. 7. The linearized model equations adequately predict the experimental response wave forms, the effects of temperature, and saturation of the steady‐state amplitude‐‐stimulus relationship. Aside from amplitude scaling, no curve‐fitting was used. 8. The model also gives realistic values for the cytoplasmic diffusion coefficient of Ca and the net rate of Ca efflux required to restore dark Ca activity.

Calcium current‐dependent and voltage‐dependent inactivation of calcium channels in Helix aspersa

“…The time course at [Ca(t)], may be compared to those expected from the solution ofdiffusion in a sphere with a constant flux at the surface (Gorman & Thomas, 1980;Carslaw & Jaeger, 1959;Crank, 1975). Since one can arrive at an effective diffusion constant which includes a homogeneous, equilibrium chemical reaction inside the cell it would seem that this solution should approximate our physical situation (Anderson, Brown & Yasui, 1979;Crank, 1975;Carslaw & Jaeger, 1959). The time course of the Ca concentration at this inner surface when realistic values are applied (diffusion constant = 10-cm2/sec and cell radius,= 100#tm) shows no sign of reaching a steady value at 50 msec (Gorman & Thomas, 1980) where we observed an almost steady value of Ca-dependent inactivation.…”

Local communication within dendritic spines: Models of second messenger diffusion in granule cell spines of the mammalian olfactory bulb

“…Studies of the diffusion of calcium date back to the pioneering work of Blaustein and Hodgkin (1969) and continue with increasing sophistication as modern computational methods are brought to bear on the problem. Many of the studies for the diffusion of calcium have been primarily concerned with the coupling of calcium input to presynaptic transmitter release and the calcium-activated potassium channel (Andersen et al, 1979;Baker et al, 1971;Fogelson and Zucker, 1985;Hodgkin and Keynes, 1957;Parnas et al, 1989;Sala and Hernandez-Cruz, 1990;Simon and Llinas, 1985;Stockbridge and Moore, 1984). In all cases, the models predict steep, transient calcium gradients from the input site to the interior of the cell.…”

Potassium conductance and internal calcium accumulation in a molluscan neurone

“…2A), but typically the time constant for the final decay phase was slightly longer than that for the decay of the absorbance signal. The dual exponential decay of IK, Ca has been shown previously (Gorman & Hermann, 1979) and is also a characteristic feature of the decay of the light induced K+ current in some molluscan neurones which is caused by release of Ca2+ from internal stores (Andresen, Brown & Yasui, 1979).…”