Serotonergic axon development in the medulla oblongata in post-natal mice
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Sudden Infant Death Syndrome (SIDS) is the sudden death of an infant younger than one year of age that remains unexplained after a complete investigation. For these infants, many different reasons have been hypothesized as to the cause of these deaths including: inherent vulnerability and improper hypoxic arousal. Studies done in other laboratories have shown that there seems to be a reduction in the levels of the neurotransmitter serotonin (5-HT) in the neurons of the raphe, extra-raphe, and ventral populations along with projection sites of these neurons. The huge implications of 5-HT in the control of respiration, prompted animal model studies to further investigate a potential connection between 5-HT and SIDS. 5-HT deficient mice were engineered by knocking out the Pet-1 transcription factor so that knockout mice only retained 30-40% of their brainstem 5-HT neurons. By comparing these 5-HT deficient Pet-1 knockout mice to wild-type mice, it was demonstrated that 5-HT deficient mice failed to autoresuscitate themselves after repeated bouts of hypoxia. Intriguingly, these mice only experienced an autoresuscitation deficit during a specific time period during development. To further evaluate the pathological development behind this behavior issue, in the current study we utilized mice that have modified Pet-1-Flpe driver, Egr2-Cre driver, along with a knock-in RC::FPSit allele to observe 5-HT development in the brainstem in a mature adult and across the critical period (postnatal days 8 and 13- P8 and P13). The transgenic mouse model Pet1-Krox20 gives us a way of exploring a specific subset of 5-HT neurons that rise from the developmental rhombomeres r3 and r5. The use of the knock-in RC::FPSit allele allows us to view the axonal projections of these specific 5-HT neurons by utilizing the presynaptic marker synaptophysin-GFP. This model (PKSit) will allow us to target 5-HT neurons that are implicated in respiration. We chose to compare two projection targets of the PKSit 5-HT neuron subtype through the vulnerable period of development and mature adult mouse: the Locus Coeruleus (LC) and the Nucleus Tractus Solitarius (NTS). In this study we tested the amount of colabeling between 5-HT and GFP in the LC and NTS at P8, P13, as well as the mature adult. We hypothesize that the LC undergoes significant serotonergic axon development and increases colocalization with GFP labeled axon projections between the ages of P8 and P13. We sliced mouse brains and ran immunofluorescence before taking confocal images. By utilizing ImageJ software to run colocalization analysis on the images obtained, we were able to quantify the amount of 5-HT labeled axon projections that are colocalized with GFP labeled axon projections. The parameters we used to quantify the amount of colocalization include the Pearson's Coefficient (PC), Mander's Coefficient (M1/M2), Cytofluorograms, Costes' Method, and van Steensel's Cross-Correlation Coefficient (CCF). We found that the LC shows significant changes with age in the colocalization of 5-HT with GFP while the NTS does not exhibit significant changes with age. The significant changes found in the LC 5-HT/GFP expression between the ages of P8 and P13 suggest one possible cause of failure of arousal. At P8, this lack of 5-HT colabeling with GFP projections suggests that there is some development occurring, which prevents the proper function of 5-HT. At P13, there is a significant increase in the colabeling of 5-HT with GFP, which indicates that the Pet1-Krox20 lineage is actively using 5-HT. The colocalization studies demonstrate that as the mouse ages, the amount of 5-HT labeling with GFP-synaptophysin in the NTS stays the same. The lack of overlap even in mature adult mice suggests that the expression of 5-HT in GFP labeled projections is not necessary. This colocalization study shows that there is an effect of age on the development of the serotonergic system in the LC, but no effect of age in the NTS. While this demonstrates that there is a critical period of development in relation to the LC, it is only one aspect of why mice pups failed to respond to repeated bouts of hypoxia.