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Primary role of the serotonergic midline system in synchronized spontaneous activity during development of the embryonic mouse hindbrain.

In the developing embryonic mouse hindbrain, we have shown that previously widespread synchronized spontaneous activity at E11.5 retracts to the initiating zone of the rostral hindbrain by E13.5, and ceases completely by E14.5. We now confirm that at E11.5 and E13.5, the primary driver of spontaneous activity is serotonergic input, while other transmitters (GABA, glutamate, NE, and ATP) have only modulatory roles. Using immunocytochemistry, we also show that at E13.5, 5-HT-positive neurons in the midline extend over a larger rostro-caudal distance than at E11.5, and that in the presumptive initiating zone, cell bodies occupy a band that extends 200 microm laterally on each side of the midline, with extensive axonal processes. The 5-HT2A receptor retains expression in lateral tissue over this developmental time. We find that in addition to being sensitive to 5-HT receptor antagonists, spontaneous activity is also abolished by blockers of gap junctions, and is increased in frequency and lateral spread by application of ammonium, presumably via increased intracellular pH augmenting gap junction conductance. Thus, 5-HT neurons of the midline remain the primary drivers of spontaneous activity at several stages of development in the hindbrain, relying in part on gap junctional communication during initiation of activity.