Developmental changes of serotonin 4(a) receptor expression in the rat pre-Botzinger complex

Serotonin receptors (5-HTRs) are known to be involved in the regulation of breathing behavior and to mediate neurotrophic actions that exert a significant function in network formation during development. We studied neuronal 5-HT4(a)R-immunoreactivity (-IR) at developmental ages from E14 to P10. Within the pre-Botzinger complex (pre-BotC), a part of the respiratory network important for rhythmogenesis, 5-HT4(a)R-IR was most extensive in rats at an age of E18. The 5-HT4(a)-IR was found predominantly in the neuropil, whereas somatic staining was sporadic at late embryonic (E18-E20) stages. At birth, we observed a dramatic change to a predominantly somatic staining, and neuropil staining was greatly reduced and disappeared at an age of P4. In all developmental stages, 5-HT4(a) and mu-opioid receptors were strongly coexpressed in neurons of the pre-BotC, whereas 5-HT4(a)R expression was absent in neurons within the dorsal horn. Nestin, a marker for CNS progenitor cells, was used to obtain information about the degree of pre-BotC differentiation. Nestin-positive cells did not appear within the pre-BotC before age E20. At E16, nestin-expressing cells were absent in the nucleus ambiguus (NA) and its ventral periphery. The number of nestin-positive cells increased after birth within and outside the pre-BotC, the majority of cells being glial. Coexpression of nestin and 5-HT4(a)R was localized predominantly within the NA and appeared only sporadically within the pre-BotC. We conclude that 5-HT(4(a))Rs are important not only for neuromodulation of cellular excitability but also for respiratory network formation.