Complex sentence: The nonpeptidergic nociceptors switch off the TrkA and begin expressing Ret, which is a transmembrane signaling component that allows the expression of glial-cell-derived growth factor (GDNF).
Split into simpler sentences: The nonpeptidergic nociceptors switch off the TrkA and begin expressing Ret. Ret is a transmembrane signaling component which allows for the expression of another growth factor--glial cell-derived growth factor (GDNF).