Photodynamic Inactivation of the Single Crayfish Nerve Cell: Dynamics of Electrophysiological Responses and Comparison of Photosensitisers

. The study of single neuron response to photodynamic effect provides a means for the study of the dynamics of cytotoxic events leading to cell death and allows comparison of the phototoxicity of different photosensitisers. Isolated crayfish stretch receptor neurons were photosensitised for 30 min, then irradiated with a He-Ne laser (632.8 nm; 0.3 W/cm²) until irreversible firing cessation. The dynamics of neuron firing frequency were continuously recorded throughout. The following photosensitisers were studied: methylene blue, janus green B, protoporphyrin IX, chlorins e ₆ and p ₆, haematoporphyrin derivative (Photoheme) and sulphonated aluminium phthalocyanine (Photosens). Nerve cells were found to be insensitive to either He-Ne laser irradiation or photosensitisation alone, but very vulnerable to the photodynamic effect: neurons changed firing rate and died at nanomolar concentrations of photosensitisers. The dynamics of neuron responses was found to depend on photosensitiser type and concentration. The current approach provides a means of evaluation of initial threshold cell membrane alteration and cytotoxic events leading to cell death. The dependence of firing acceleration rate and neuron lifetime on photosensitiser concentration additionally allowed comparison of efficiencies of different photosensitisers. Photosens, Photoheme and chlorin p ₆ were found to be the most potent photosensitisers: neurons responded to their photodynamic effects at concentrations as low as 1–5 nM. Paper received 23 February 1998; accepted following revision 7 December 1998.