Spatial Distribution of Serotonergic Raphespinal Neurons in the Neonatal Mouse Brainstem
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Descending serotonergic (5-HT) neurons located in the brainstem are known to influence mammalian locomotion. Neonates have poorer locomotor abilities than adults. Theoretically, a lesser contribution from 5-HT descending neurons in neonates is possible. However, a detailed quantification of 5-HT descending neurons in the neonate has yet to be performed. In this study, we quantify 5-HT descending neurons in each caudal raphe nuclei of the brainstem in the neonatal Pet1-GCaMP6 mouse (P1-P2). Descending 5-HT neurons were present in only three raphe nuclei, the raphe magnus (RMg), the raphe pallidus (RPa), and the raphe obscurus (ROB). Results show that 11% of all 5-HT neurons present in these nuclei project down the brainstem into the spinal cord. The largest majority of 5-HT descending neurons was found in the raphe pallidus (RPa), followed by the raphe magnus (RMg) nuclei, and finally, the raphe obscurus (ROB) nuclei.