Aims: 

It is known that UV and IR laser-light can stimulate populations of neurons, but the mechanism of activation is unknown. We decided to analyse effects of laser light in the mid visible range (532 nm) on individual neurons. We chose to work on specified neurons with known properties and selected an invertebrate model preparation.

Methods: 

Ganglionic neurons from Helix aspersa were exposed in a chamber containing physiological solution. Neurons were impaled with glass microelectrodes containing 0.7M KCl, K₂SO₄ or Kacetate. Current- and voltage-clamp recordings were made. Light was applied via an 80 mm diameter optical fibre.

Results: 

Neuron-specific effects on excitability were observed that resulted in each case from an increase in membrane conductance. Responses were intensity-dependent, over the range of 25mW to 1200mW, reversible and repeatable. Some neurons were depolarized and excited, with an extrapolated E_rev of + 60mV of the current response. Others were inhibited, with an E_rev close to the resting E_m. Detailed experiments on the C1 neuron, showed that inhibition resulted from an increase in Cl^- ion conductance, that occurs most probably along the axon.

Conclusions: 

532 nm light consistently excited some neurons, probably by increasing membrane permeability to Na⁺ and/ or Ca²⁺ ions, but inhibited the C1 neuron by a specific action on Cl^- ion channels.