SLC6A4 and central nervous system cancer: To assess for activity-regulated secreted factors in brainstem, we prepared acute hindbrain slices containing the raphe nuclei from glioma-bearing SERT-Cre+/wt × Ai230flx/wt mice (Fig. 3A–B), enabling ex vivo optogenetic stimulation, or mock-stimulation (identical manipulation but without light), of ChRmine-oScarlet-expressing serotonergic neurons (fig.