Optical imaging of the spatiotemporal patterning of neural responses in the embryonic chick superior cervical ganglion.

Multiple-site optical recording of transmembrane potential changes with a voltage-sensitive dye was used to reveal the functional expression and developmental changes of the postsynaptic potentials in the early embryonic chick superior cervical ganglion. The ganglia were isolated from five- to 12-day-old chick embryos with preganglionic nerve fibres (vertebral and/or cervical carotic nerves) attached. The preparations were stained with a voltage-sensitive merocyanine-rhodanine dye (NK2761). Voltage-related optical (absorbance) changes were recorded simultaneously from 127 contiguous loci in the preparation, using a 12 x 12-element photodiode array. Optical changes having two components were evoked by preganglionic nerve stimulation. One component was the fast spike-like signal and another the delayed slow signal. The amplitude of the slow signal was decreased by repetitive stimulation, reduced by low external calcium ion concentrations and eliminated in the presence of manganese or cadmium ions. The slow signals were also eliminated in the presence of D-tubocurarine. Accordingly, we concluded that the slow signal corresponds to cholinergic excitatory postsynaptic potentials. In the five- and six-day-old superior cervical ganglia, only the fast optical signals (referred to as the action potentials) were recorded. Slow optical signals (referred to as the excitatory postsynaptic potentials) were detected from preparations older than seven days. The amplitude of the slow optical signal gradually increased, together with an expansion of the response area, as the developmental stage proceeded from seven to 10 days. To compare the distribution patterns of the neural responses evoked by stimuli applied to the cervical carotic and vertebral nerves, we have mapped and imaged the spatial patterning of the synaptic responses. In the maps, the positions of the peak size regions of the slow signals were assessed, and we found that there were differences in the location of these areas for the cervical carotic vs vertebral nerves. From these experimental results, we conclude that synaptic function within the chick superior cervical ganglion is initiated at the seven-day-old embryonic stage, and reaches a maximum level at 10 days. Synaptic transmission at these stages is mediated solely by nicotinic acetylcholine receptors. The spatial mapping of the synaptic responses reveals that the neural populations related synaptically to the cervical carotic and vertebral nerves are located separately within the ganglion, even at an early developmental stage.