Light response of a giant Aplysia neuron

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 E(K) 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 E(K). 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 Ca(2+) into this neuron also hyperpolarizes the cell membrane. This effect is also due largely to an increase in K(+) conductance. The light response after Ca(2+) 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 Ca(2+) which in turn increases K(+) conductance, thereby hyperpolarizing the neuronal membrane.