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Optical survey of neural circuit formation in the embryonic chick vagal pathway

The multiple‐site optical recording technique with a voltage‐sensitive dye, NK2761, was used to survey functional organization of neural circuits related to the vagus nerve in the embryonic chick brainstem. When we stimulated the vagus nerve, in addition to the responses in the vagal sensory nucleus (nucleus of the tractus solitarius (NTS)) and motor nucleus (dorsal motor nucleus of the vagus nerve (DMNV)) on the stimulated side, another response area was discriminated at the level of the pons/rostral medulla on the contralateral side. Characteristics of the contralateral optical signals suggested that they correspond to the neural activity in the second/higher‐ordered nucleus of the vagal pathway, possibly the parabrachial nucleus, which receives inputs from the NTS. Blockade of non‐N‐methyl‐d‐aspartate (NMDA) receptors abolished the responses on the contralateral side, together with the postsynaptic firing in the NTS, suggesting the significance of non‐NMDA receptor function in sensory information transfer via the NTS. The responses on the contralateral side were first detected from the 7‐day‐old embryonic stage, when the glutamatergic excitatory postsynaptic potentials were first expressed in the NTS. The results suggest that the synaptic pathway from the NTS to the contralateral nucleus is already generated when the primary vagal afferents make functional synapses on NTS neurons.