INVERTEBRATE photoreceptors show electrical changes which apparently result from isomerisation of single rhodopsin molecules by light or thermal energy1-3. Observation of corresponding phenomena in vertebrates has been prevented by intercellular electrical coupling, which averages membrane potential over many photoreceptors4-6. Recently, however, recordings of membrane current from individual rod outer segments have revealed responses to single photons7,8. Here we report that similar electrical events occasionally occur in darkness, perhaps because of thermal isomerisation of rhodopsin.
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