Ionic Dependence of Membrane Potential and Glutamate Receptor‐Linked Responses in Synaptoneurosomes as Measured with a Cyanine Dye, DiS‐C2‐(5)

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Abstract
Membrane potentials of particles present in a sub-cellular brain preparation, called synaptoneurosomes, have been monitored by measurement of changes in the absor-bance of a cyanine dye, DiS-C2-5. The membrane potential of the particles seems to be dependent on both Cl− and K+ diffusion potentials, as judged from dependence of the ab-sorbance changes on the K+ equilibrium potential across the membrane in the presence of Ba2+ or when Cl− was replaced with gluconate. The apparent high Cl− permeability of the membrane preparation was reduced in the presence of pic-rotoxin, a finding suggesting endogenous activation of receptor-linked Cl− channels. Glutamate and kainate caused depolarization of the membranes present in the preparation. This effect was only seen if K+ channels had been blocked in the presence of Ba2+ or 4-aminopyridine. No responses were observed with other glutamate receptor agonists (quisqualate or N-methyl-d-aspartate). The membrane potential of particles present in conventional synaptosomal preparations neither had a high Cl− permeability nor reacted to glutamate or kainate in the present conditions. The results suggest that synaptoneurosome preparations may be used for functional studies on postsynaptic neurotransmit-ter receptor-linked membrane potential changes with optical probes of membrane potential.