Light Response of a Giant Aplysia Neuron

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Abstract
Illumination of an Aplysia giant neuron evokes a membrane hyperpolarization which is associated with a membrane conductance increase of 15%. The light response is best elicited at 490 nM: the neuron also has an absorption peak at this wavelength. At the resting potential (-50 to -60 mV) illumination evokes an outward current in a voltage-clamped cell. This current reverses sign very close to EK calculated from direct measurements of internal and external K+ activity. Increases in external K+ concentration shift the reversal potential of the light-evoked response by the same amount as the change in EK. Decreases in external Na+ or Cl- do not affect the response. Therefore, the response is attributed to an increase in K+ conductance. Pressure injection of Ca2+ into this neuron also hyperpolarizes the cell membrane. This effect is also due largely to an increase in K+ conductance. The light response after Ca2+ injection does not appear to be altered. Pressure injection of EGTA abolished or greatly reduced the light response. The effect was reversible. We suggest that light acts upon a single pigment in this neuron, releasing Ca2+ which in turn increases K+ conductance, thereby hyperpolarizing the neuronal membrane.