Optical Studies of Excitation and Secretion in Vertebrate Nerve Terminals

Abstract

Optical methods that employ potential sensitive molecular probes are shown to monitor rapid changes in membrane potential from populations of nerve terminals in the neurohypophyses of amphibia and mammals. Ca⁺⁺ and Na⁺ contributions to the action potential are demonstrated and, in the mouse, variations in light scattering are detected following membrane potential changes known to trigger the release of peptide hormones. Calcium antagonists such as Cd⁺⁺, Mn⁺⁺, Co⁺⁺, and Ni⁺⁺ block an after-hyperpolarization in the frog that we attribute to the effect of a calcium mediated potassium conductance. Block of sodium and potassium currents with TTX and TEA reveals a pure calcium action potential which is eliminated by 0.5 mN Cd⁺⁺. In the mouse, the magnitude of the light scattering signal depends upon the frequency of stimulation and the extracellular concentration of Ca⁺⁺, as well as on the presence of secretagogues such as 4-aminopyridine. A part of this intrinsic optical signal that seems to be correlated with neuropeptide release is blocked by inorganic calcium antagonists, while a faster component related to the action potential itself remains. The weak wavelength dependence of the light scattering signal, contrasted with the strong wavelength dependence of the extrinsic absorption signal provided by linear potentiometric probes (e.g., merocyanines), may permit one to monitor simultaneoulsy, in a stained preparation, the voltage changes in the nerve terminals and the release of secretory products. The inherently fast responses of the two optical measurements may then improve our ability to resolve early events in the coupling of excitation to secretion. Supported in part by USPHS grant NS 16824.