Regrowth of Serotonin Axons in the Adult Mouse Brain Following Injury

Abstract

Summary It is widely believed that damaged axons in the adult mammalian brain have little capacity to regrow, thereby impeding functional recovery after injury. Studies using fixed tissue have suggested that serotonin neurons might be a notable exception, but remain inconclusive. We have employed in vivo two-photon microscopy to produce time-lapse images of serotonin axons in the neocortex of the adult mouse. Serotonin axons undergo massive retrograde degeneration following amphetamine treatment and subsequent slow recovery of axonal density which is dominated by new growth with little contribution from local sprouting. A stab injury that transects serotonin axons running in the neocortex is followed by local regression of cut serotonin axons and followed by regrowth from cut ends into and across the stab rift zone. Regrowing serotonin axons do not follow the pathways left by degenerated axons. The regrown axons release serotonin and their regrowth is correlated with recovery in behavioral tests.

Serotonin axons in the neocortex of the adult female mouse regrow after traumatic brain injury

“…Importantly, when serotonin axon density was measured in layer 1 of the posterior region, significant recovery was seen at one month and three months after injury when compared with the one week time-point, indicating that serotonin axon regrowth occurred (Figures 4 and 5). This is consistent with previous reports of damage and regrowth of neocortical serotonin axons following thermal lesion (Hawthorne et al, 2011), amphetamine lesion (O’Hearn et al, 1988; Molliver et al, 1990; Mamounas et al, 2000; Jin et al, 2016) and a neocortical stab injury (Jin et al, 2016). Thus, it appears that serotonin axons have an unusual capacity for regrowth and that this is evident following four different forms of brain injury.…”

“…This finding replicates previous reports using serotonin immunohistochemistry (O’Hearn et al, 1988; Molliver et al, 1990; Mamounas et al, 2000), SERT immunohistochemistry (Jin et al, 2016), or in vivo two-photon observation of serotonin axons in SERT-EGFP BAC transgenic mice (Jin et al, 2016). Importantly, when serotonin axon density was measured in layer 1 of the posterior region, significant recovery was seen at one month and three months after injury when compared with the one week time-point, indicating that serotonin axon regrowth occurred (Figures 4 and 5).…”

“…Importantly, CCI produced a strong reduction in the level of both EYFP and SERT immunofluorescence when measured one week later, posterior to the site of CCI injury (Figure 6). This argues against the hypothesis that apparent serotonin axon degradation was merely an artifact of reduced SERT expression and is consistent with a previously published result in which similar EYFP-expressing serotonin axons in the neocortex were degenerated by amphetamine treatment (Jin et al, 2016)…”

“…However, following injury, serotonin axons can regrow in the central nervous system, both in the spinal cord (Lee et al, 2010; Alilain et al, 2011; Lee et al, 2013; Yoo et al, 2013; Kanno et al, 2014) and in the brain (O’Hearn et al, 1988; Molliver et al, 1990; Mamounas et al, 2000; Hawthorne et al, 2011; Jin et al, 2016). However, the injury models used in these experiments reflect unique insults.…”

“…However, the injury models used in these experiments reflect unique insults. For example, amphetamine lesion (O’Hearn et al, 1988; Molliver et al, 1990; Mamounas et al, 2000) does not create a glial scar (Wilson and Molliver, 1994), while stab injury (Jin et al, 2016) is less common clinically and leaves a relatively narrow rift of damaged tissue to be crossed by regrowing axons. Therefore, whether serotonin axons are able to regrow following the more clinically relevant forms of TBI remains unknown.…”