Multiple stimuli induce expression of IEGs that include trophic factors downstream of receptor tyrosine kinases (RTKs, e.g., EGFR, TrkA, TrkB), calcium channels (e.g., NMDAR), neurotransmitters (glutamine, dopamine), and viral infection (Bahrami and Drabløs, 2016; Minatohara et al., 2016; Gallo et al., 2018). Here, NTRK2 is linked to viral infectious disease.