Optical detection of postsynaptic potentials evoked by vagal stimulation in the early embryonic chick brain stem slice.

1. A voltage-sensitive dye and multiple-site optical recording of changes in membrane potential were used to reveal the postsynaptic potentials in the early embryonic chick brain stem slice preparation. 2. Vagus-brain stem preparations were isolated from 8-day-old chick embryos and then transverse slice preparations were prepared with both the right and left vagus nerve fibres intact. The slice preparations were stained with a voltage-sensitive merocyanine-rhodanine dye (NK2761). 3. Voltage-related optical (absorbance) changes evoked by vagus nerve stimulation with positive square current pulses using a suction electrode were recorded simultaneously from 127 contiguous loci in the preparation, using a 12 x 12-element photodiode array. Optical responses appeared in a limited area near the dorsal surface of the stimulated side. 4. When relatively large stimulating currents were applied, optical changes having two (or sometimes three) components were recorded. One component was the fast spike-like signal and another the delayed, long-lasting slow signal. 5. The size of the slow signal was decreased by continuous stimulation, reduced by low external calcium ion concentrations and eliminated in the presence of manganese or cadmium ions. 6. The slow signals were eliminated in the presence of kynurenic acid, and they were reduced by 2-APV (DL-2-amino-5-phosphono-valeric acid) and by CNQX (6-cyano-7-nitroquinoxaline-2,3-dione). We conclude that the slow signals correspond to excitatory postsynaptic potentials which are glutamate mediated.