Slow progression of sciatic nerve degeneration and regeneration after loose ligation through microglial activation and decreased KCC2 levels in the mouse spinal cord ventral horn

Peripheral nerve injury affects motor functions. To reveal the mechanisms underlying motor dysfunction and recovery after nerve compression, which have not been precisely examined, we investigated the temporal relationship among changes in motor function, nerve histopathology, and marker molecule expression in the spinal cord after loose ligation of the mouse sciatic nerve. After ligation, sciatic motor function suddenly declined, and axons gradually degenerated. During degeneration, galanin was localized in motor neuron cell bodies. Then, in the ventral horn, microglia were activated, and expression of choline acetyltransferase (ChAT), a synthetic enzyme of acetylcholine, and potassium chloride co-transporter 2 (KCC2), which shifts the action of γ-amino butyric acid (GABA) and glycine to inhibitory, decreased. Motor function recovery was insufficient although axonal regeneration was complete. ChAT levels gradually recovered during axonal regeneration. When regeneration was nearly complete, microglial activation declined, and KCC2 expression started to increase. The KCC2 level sufficiently recovered when axonal regeneration was complete, suggesting that the excitatory action of GABA/glycine may participate in axonal regeneration. Furthermore, these changes proceeded slower than those after severance, suggesting that loose ligation, compression, may mediate slower progression of degeneration and regeneration than severance, and these changes may cause the motor dysfunction and its recovery.

Specific Expression of KCC2 in the α Cells of Normal and Type 1 Diabetes Model Mouse Pancreatic Islets

Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter in the mature brain; however, it acts excitatory during development. This difference in action depends on the intracellular chloride ion concentration, primarily regulated by potassium chloride co-transporter2 (KCC2). Sufficient KCC2 expression results in its inhibitory action. GABA is also abundant in pancreatic islets, where it acts differentially on the islet cells, and is involved in carbohydrate metabolism. However, the mechanisms underlying the differential action remain unknown. We performed immunohistochemistry for glutamic acid decarboxylase (GAD), a synthetic enzyme for GABA, and KCC2 in normal adult islets. GAD was co-localized with insulin in β cells, whereas KCC2 was expressed in glucagon-positive α cells. These results are in line with previous observations that GABA decreases glucagon release but increases insulin release, and suggest that GABA and insulin may work together in reducing blood glucose levels under hyperglycemia. Next, we examined the streptozotocin-induced type1 diabetes mellitus mouse model. GAD and insulin expression levels were markedly decreased. KCC2 was expressed in glucagon-positive cells, whereas insulin- and somatostatin-positive cells were KCC2-negative. These findings suggest that in diabetes model, reduced GABA release may cause disinhibition of glucagon release, resulting in increased blood sugar levels and the maintenance of hyperglycemic state.