Histamine Promotes Locomotion Recovery After Spinal Cord Injury Via Inhibiting Astrocvtic Scar Formation

Spinal cord injury (SCI) is a devastating disease which can result in complete or incomplete loss of motor and sensory function caudal to the level of injury, and its complications often refer the vascular system, respiratiory system, gastrointestinal system and uriary system. The mechanisms of injury to the spinal cord included the "primary injury" and "secondary injury". The primary injury results in structural disturbance for physical and mechanical trauma, which is irreversible. The secondary injury following the primary injury results from a series of pathophysiological and biochemical changes, including hypoxia & ischemia, excitotoxicity, inflammation, astrogliosis and formation of glial scar, etc. The glial scar formation was the main factor to determine the extent of tissue injury and dysfunction of body. The secondary injury, a dynamic process which occurs in the hours and weeks, is a major determinant of functional deficit, however, its consequences can be reversed. Therefore, targeting to the pathological mechanisms in the secondary injury may be a potential therapeutic strategy for the treatment of SCI.Histamine is an important neurotransmitter and neuromodulator in the central nervous system (CNS). In our previous studies, we found that histamine can up-regulate astrocytic glutamate transporter GLT-1 and glutamine synthetase (GS) expression, resulting in neuroprotection against excitotoxicity. These data indicate that histamine may be a regulator of astrocyte function during CNS injury. A number of studies have examined the role of histamine in ischemic brain pathology. Both in vivo and in vitro studies indicated that histaminergic neurotransmission mediates neuroprotective activity in brain ischemic pathology, probably involving targeting to astrocytes. In SCI, it has been reported that histamine level increases in the traumatized segment at 2 h after injury, which may contribute to the increase of the sensitivity of vessels to other vasoactive substances. However, the role of histamine in the functional recovery after SCI, especially its action on glial scar formation remains unclear.We investigated the role of histamine in SCI by exogenous application of histamine. After T10 hemisection, the behavioural evaluation from Basso, Beattie and Bresnahan (BBB) scale and footprint analysis indicated that histamine significantly improved the chronic locomotor recovery at 28 d after SCI. The results from HE staining, LFB staining and GFAP immunohistochemistry showed that the local administration of histamine significantly reduced the cavity size, demyelination and glial scar formation after SCI. Moreover, the results from immunofluorescent staining, DAB staining and western blot analysis suggested that the astrogliosis and neurocan expression around the lesion were obviously suppressed by histamine, while the NF-200 expression was greatly increased in histamine group, indicating that histamine can support axonal regrowth beyond the glial scar. Histidine decarboxylase knockout (HDC-KO) mice, which are deficient of histamine synthesis, were used to confirm the action of histamine. The results showed that the HDC-KO mice, lacking in endogenous histamine, exhibited lower behavior score, increased lesion size and astrogliosis, compared with the wild-types.Furthermore, selective antagonists for H1, H2 receptors were utilized in vivo and in vitro to investigate receptor properties of histamine’s action on astrogliosis. The results from immunofluorescent staining, DAB staining and western blot showed that pretreatment with anti-H1 antagonist reversed the inhibitory action of histamine on astrogliosis assessed by the number of astrocytes and GFAP expression, and partly reversed the neuroprotective effect of histamine on locomotor recovery measured by open field score. In addition, in vitro "scratch-wound" model, we also found that the anti-proliferation effect of histamine on astrocytes was reversed by anti-H1 antagonist. However, the anti-H2 antagonist had no effect on the action of histamine in vivo and in vitro.In conclusion, our current data firstly provide direct evidence that histamine significantly improved the chronic locomotor recovery via attenuating astrogliosis after SCI by stimulating histamine H1 receptor. This study highlights a therapeutic potential of histamine and its related drugs for SCI.