PDT effect of different photo sensitizers on a single nerve cell: electrophysiological and pharmacological study

The study of the single neuron response to photodynamic effect provides information on the dynamics of cytotoxic events leading to cell death and allows comparison of the phototoxicity of different photosensitizers. Isolated crayfish stretch receptor neurons were incubated 30 min with different concentrations of various photosensitizers and then irradiated with a He-Ne laser (632.8 nm; 0.3 W/cm²) until irreversible firing abolition. The dynamics of neuron impulse response was continuously recorded throughout the experiment. The following photosensitizers were studied: hematoporphyrin derivatives Photoheme and Photofrin II, 6 deuteroporphyrin IX derivatives and sulphonated aluminum and zinc phthalocyanines. Nerve cells were found to be insensitive to either He-Ne laser irradiation or photosensitization alone, but very vulnerable to photodynamic effect: neurons changed their firing rate and irreversibly ceased their impulse activity at nanomolar (phthalocyanines and hematoporphyrin derivatives) and even pikomolar (deuteroporphyrin derivatives) concentrations of photosensitizers. The dynamics of the neuron responses to photodynamic effects included stages of firing activation and/or inhibition followed by irreversible firing abolition. It depended on the photosensitizer type and concentration. Decrease of extracellular pH or action of pharmacological agents increasing intracellular Ca²⁺ concentration or inhibiting ATP synthesis exacerbated photodynamic neuron injury. In contrast, agents decreasing intracellular calcium concentration or bioenergetic substrates protected the neuron against photodynamic injury. The dependencies of neuron lifetimes on the photosensitizer concentration provide comparison of photodynamic efficiencies of different photosensitizers