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

Abstract

It is widely held that injured neurons in the central nervous system do not undergo axonal regrowth. However, there is mounting evidence that serotonin axons are a notable exception. Serotonin axons undergo long-distance regrowth in the neocortex after amphetamine lesion and, following a penetrating stab injury, they can regrow from cut ends to traverse the stab rift. Traumatic brain injury (TBI) is clinically prevalent and can lead to pathologies such as depression that are related to serotonergic dysfunction. Thus, whether serotonin axons can regrow after TBI is an important question. We used two models for TBI, a persistent open skull condition and controlled cortical impact, to evoke injury in adult female mouse neocortex and assessed serotonin axon density one week, one month, and three months after injury by serotonin transporter immunohistochemistry. We found that following both forms of TBI, serotonin axon density is decreased posterior but not anterior to the injury site when measured in layer 1 at one week post-surgery, and that serotonin axons are capable of regrowing into the distal zone to increase density by one month post-surgery. This pattern is consistent with the anterior-to-posterior course of serotonin axons in the neocortex. TBI in these models is associated with significant reactive astrogliosis both anterior and posterior to the impact, but the degree of reactive astrogliosis is not correlated with serotonin axon density when measured one week after TBI. Microglial density remains constant following both types of injuries, but microglial condensation was detected one week after controlled cortical impact.

Catecholaminergic axons in the neocortex of adult mice regrow following brain injury

“…Might the apparent loss and regrowth of TH+ axons following CCI, indicated by TH+ immunohistochemistry, be an artifact produced by nonspecific tissue damage? The observation that, at the one month postinjury time point, the tissue anterior to the CCI injury showed significantly reduced TH+ axon density (Figure 3) but not serotonin axon density (Kajstura et al, 2018), argues against a non-specific reduction in axonal immunopositive signals. Likewise, the failure of CCI injury to produce significant reductions in cholinergic axon density, indicated by VAChT immunohistochemistry, in any neocortical layer, either anterior or posterior to the CCI injury site (Figure 7 and Supplementary Figure 1) argues that the loss and recovery of TH+ axons seen here following CCI was a specific effect.…”

“…Controlled Cortical Impact CCI injury was performed as previously described (Kajstura et al, 2018). Briefly, mice were anesthetized with 3% isoflurane and placed in a stereotaxic device (Stoelting) with continued isoflurane delivery.…”

“…In damaged rodent spinal cord, repaired with grafts and treated with chondroitinase, immunohistochemical studies have shown that some serotonin axons originating from the caudal raphe nuclei have crossed the graft and contributed to the restoration of locomotion, as well as control of the bladder and diaphragm (Zhou et al, 1995;Lee et al, 2010;Alilain et al, 2011;Lee et al, 2013;Yoo et al, 2013;Kanno et al, 2014, see Perrin andNoristani, 2019, for review). Similarly, histological studies have shown that serotonin axons can regrow following several types of rodent brain injury including a thermal lesion (Hawthorne, et al 2011), a stab injury or a controlled cortical impact (CCI) delivered to the neocortex (Kajstura et al, 2018). Using repeated in vivo two-photon imaging of adult serotonin transporter-EGFP BAC transgenic mice (Gong et al, 2003), we found that, in the weeks following neocortical stab injury, a large fraction (>80 %) of the severed ends of serotonin axons display new growth and in many cases this regrowth is sufficient to completely cross the glial scar-laden stab rift .…”

“…Here, to test the hypothesis that catecholamine axons can regrow in the neocortex following physical brain injury, we have subjected adult mice to CCI or neocortical stab injuries, using the protocols we previously employed in studies of serotonin axon regeneration Kajstura et al, 2018). We have performed double immunohistochemistry for TH and NeuN, a marker of neuronal cell bodies, at various time points after injury to assess the damage and potential regrowth of catecholamine axons.…”

Serotonergic Axons as Fractional Brownian Motion Paths: Insights into the Self-organization of Regional Densities

“…They begin synthesizing serotonin around embryonic day 11-13 in the mouse and rat brains (Lidov and Molliver, 1982;Hendricks et al, 1999;Hawthorne et al, 2010) and around 5 weeks of gestation in the human brain (Sundstrom et al, 1993;Mai and Ashwell, 2004). In the adult mammalian brain, serotonergic axons are unusual in their ability to regenerate, with potential implications for the efforts to restore other axon systems after injury (Hawthorne et al, 2011;Jin et al, 2016;Kajstura et al, 2018). A recent study has shown that they may share this property with other axons in the ascending reticular activating system (Dougherty et al, 2019).…”