AXOTOMY OF EXTRAOCULAR MOTONEURONS LEADS TO CHANGES IN THE EXPRESSION OF KCC2 AND NKCC1

KCC2 and NKCC1 are key chloride cotransporters mediating Cl^- efflux and influx, respectively, in neurons. They play a crucial role in modulating the activity of inhibitory neurotransmitters (GABA and glycine), as low levels of KCC2 and high levels of NKCC1 are associated with an increase in intracellular Cl^- concentration, leading to excessive neuronal activity. We therefore studied whether these alterations occur in extraocular motoneurons after axotomy. Using immunofluorescence techniques, we observed that 15 days after injury, the decrease in KCC2 expression and the increase in NKCC1 reached their maximum levels. Along with these changes, we also observed alterations in cell volume, potentially related to the capacity of these cotransporters to drive osmotic water movement coupled to ion transport. To determine whether the protein changes reflected transcriptional regulation, we assessed mRNA expression using fluorescent in situ hybridization. As expected, KCC2 mRNA levels decreased, in parallel with protein expression; however, NKCC1 mRNA levels remained unchanged after axotomy, despite the increase observed at the protein level. Together, these findings provide a foundation for dissecting the signaling pathways that regulate KCC2 and NKCC1 expression, and for testing whether the observed changes affect motoneuron excitability and firing properties.