Potentiometric probes are small (300–500 Mr) amphipatic molecules that bind to, but do not cross, cell membranes and behave as fast linear transducers of membrane voltage. Their optical properties, particularly absorbance and fluorescence, respond to changes in potential in less than 2 microseconds, and they may be used to follow electrical events in membranes which are inaccessible to microelectrodes. We have used these dyes to study the properties of the action potential in the neurosecretory terminals of vertebrate neurohypophyses and, in particular, to investigate the behaviour of the local population of calcium channels. These channels are sensitive to the peptide toxin omega-conotoxin GVIA, derived from the venom of the marine snail Conus geographicus, but insensitive to dihydropyridine channel modulators. In the neurohypophysis of the mouse, it is possible to demonstrate that the calcium channels that are blocked by omega-conotoxin are those that are required for secretion of peptide hormones. In the terminals of the neurohypophysis, excitation is coupled to secretion, and the secretory event is accompanied by large and rapid changes in light scattering. These intrinsic optical signals provide a millisecond time-resolved monitor of events in the terminal that follow the entry of calcium, and may precede the release of hormones. We will consider how the changes in light scattering can be related to secretion, and how the extrinsic (absorption) and intrinsic optical signals may provide complementary information about excitation-secretion coupling.

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