Optical imaging analysis of neural circuit formation in the embryonic brain

1. Multiple-site optical recording with the voltage-sensitive absorption dye NK2761 was used to investigate functiogenesis of the neural circuits in the embryonic central nervous system (CNS). In the present review, we emphasize the functional development of glossopharyngeal nerve (N. IX)- and vagus nerve (N. X)-related neural circuits in the embryonic chick brainstem. 2. In the nucleus tractus solitarius (NTS), glutamatergic excitatory post-synaptic potentials (EPSPs) evoked by stimulation of N. IX/N. X were first detected at Embryonic Day (E) 7, when morphological differentiation of pre- and post-synaptic neurons is incomplete. The removal of extracellular Mg(2+) elicited small EPSPs at E6, suggesting that synaptic function mediated by N-methyl-d-aspartate (NMDA) receptors (glutamatergic receptors) is latently generated 1 day before the expression of glutamatergic EPSPs. In the nucleus of the glossopharyngeal nerve and the dorsal motor nucleus of the vagus nerve, action potentials were detected from E3.5 and E4, respectively. Similar chronological sequences were observed in the rat embryo and in other cranial nerve-related nuclei and were used to construct basic profiles of embryonic EPSPs in the vertebrate CNS. 3. We further investigated the development of the secondary synaptic pathways from the NTS to higher centres and found that neuronal circuits from the NTS are already generated when the primary afferents form functional synapses with NTS neurons. 4. A widely spreading depolarization wave was found in the embryonic CNS. This wave was triggered by multisensory inputs and spontaneous activities and was suggested to be mediated by a dual network of chemical synapses and gap junctions. It was also accompanied by a Ca(2+) wave, indicating its nurturing effects on CNS development.