GABAergic regulation of proliferation in the postnatal spinal cord

Ependymal cells (ECs) of the central canal (CC) are a quiescent population of neural stem cells (NSCs) present in the intact spinal cord. Normally dormant ECs are activated by injury; exhibiting increased proliferation, migration, and differentiation. Astrocytes and oligodendrocytes also generate new progeny in the intact and injured postnatal spinal cord. Understanding how the microenvironment of the spinal cord modulates proliferation and differentiation is essential if we are to consider harnessing endogenous mechanisms such as these to aid spinal cord repair. This project aims to investigate the role that the neurotransmitter GABA may play in modulating proliferation and differentiation in the adult spinal cord. Alterations in the levels of ambient γ-aminobutyric acid (GABA) in the spinal cord in vigabatrin- treated or GAD67-GFP mice resulted in either a decrease or an increase in the number of proliferating EdU+ cells in the white matter (WM), grey matter (GM), and CC compared to control, respectively. In the postnatal spinal cord there appears to be an inverse relationship between GABAergic signalling and the number of proliferating cells. Potentiation of GABAaR by the central benzodiazepine (BZ) recognition site (CBR) ligands etifoxine and midazolam also increased the number of EdU+ cells compared to vehicle. The endogenous CBR site ligand diazepam binding inhibitor (DBI) was found to be expressed in the spinal cord, with robust expression in ECs, suggesting the presence of an intrinsic mechanism which modulates the basal inhibitory GABAergic tone to restrict proliferation. Indeed, animals with alteration in ligand binding at the CBR site as a result of either flumazenil treatment, Ro15-4513 treatment, or G2F77I mutation possessed greater numbers of EdU+ cells compared to control animals. Flumazenil treatment also increased proliferation after lysophosphatidylcholine (LPC) -induced demyelination in the dorsal column. The effect of GABAergic modulation upon differentiation was varied, and its contribution to differentiation of specific cell types in the spinal cord was unclear. This study shows that GABA both positively and negatively influences proliferation in the postnatal spinal cord and that endogenous ligands such as DBI may be instrumental in the restrictive nature of GABA. Work presented here provides a basis for further study into how modulation of GABAaR by endogenous ligands such as DBI may also influence spinal cord self-regeneration and recovery.