“…The observation that 5HT was not entirely eliminated from the SC after complete, chronic spinal transection from the brain led to the inferred existence of spinally‐intrinsic serotonergic neurons (Carlsson et al, ; Andén et al, ; Shibuya and Anderson, ; Clineschmidt et al, ). Subsequent development of immunohistochemical labeling techniques (Steinbusch et al, ) assisted both in confirming and characterizing intraspinal serotonergic neurons (ISNs) in a wide variety of vertebrates, including teleost and non‐teleost fishes (Parent and Northcutt, ; Ritchie and Leonard, ; Harris‐Warrick et al, ; Van Dongen et al, ; Onstott and Elde, ; Van Raamsdonk et al, ; McLean and Fetcho, ), chicks (Sako et al, ; Wallace et al, ), murids (Newton et al, ; Ballion et al, ), and monkeys (Lamotte et al, ). Robust innervation of the SC with descending serotonergic fibers from the raphe is well‐conserved; however, both the number and location of ISNs are variable between species (Schmidt and Jordan, ; Lillesaar, ).…”