Optical mapping of neural responses in the embryonic rat brainstem with reference to the early functional organization of vagal nuclei.

We examined the functional organization of the vagal nuclei of the rat embryo during morphogenesis, using multiple-site optical recording with a voltage-sensitive dye. Slice preparations with vagus nerve fibers were dissected from 13- to 16-d-old embryonic (E13-E16) rat brainstems, and they were stained with the dye. Electrical activity in response to vagal stimulation was recorded optically from many sites. In the E13-E14 preparations, two types of spike-like optical signals were recorded: one was a narrow signal (type I), and the other was a broader signal (type II). Comparison with the morphology revealed by DiI labeling suggests that the type I signal response area corresponds to the nucleus of the tractus solitarius, and the type II signal response area corresponds to the dorsal motor nucleus of the vagus nerve. In the E15-E16 preparations, type I signals were followed by a slow signal related to glutamate-mediated excitatory postsynaptic potentials, suggesting that synaptic function is organized in the nucleus of the tractus solitarius by the 15-d-old embryonic stage. In the E14 preparation, a small, slow signal was evoked only in Mg2+-free solution, implying that postsynaptic function related to NMDA receptors emerges, in latent form, at the 14-d-old embryonic stage. In the E15 and E16 preparations, although the nucleus ambiguus is identified morphologically, no neural response-related optical signal was observed there, indicating that the embryonic organization of morphology and physiological function is not necessarily temporally coincident. We have mapped the dynamic spatiotemporal patterns of the evoked optical signals and have outlined the early phase of the functional organization of the cranial nuclei related to the vagus.