Characterisation of the axon initial segment and intrinsic excitability in the sub-acute phase post-ischemic stroke

BackgroundStroke is a leading cause of disability and stroke-induced changes in cortical excitability are thought to impede functional recovery. Identifying cellular targets that contribute to maladaptive excitability holds great potential for the development of therapeutic interventions to improve stroke outcomes. One potential target is the axon initial segment (AIS), the specialised cellular domain where action potentials are initiated. In the acute phase post stroke, neurons in the peri-infarct zone display abnormal AIS structural properties which is assumed to contribute to altered neuronal excitability. However, whether this continues into the sub-acute phase post stroke, a period with heightened plasticity and when physical rehabilitation typically begins is unknown. MethodsWe induced a photothrombotic ischemic stroke to the right motor cortex of 13-week-old mice alongside adeno-associated virus labelling of layer 2/3 and layer 5 pyramidal neurons in the peri-infarct zone and contralesional motor cortex. Immunofluorescence staining for Ankyrin-G and whole-cell patch clamp electrophysiology measures were made at 28-days post stroke to assess changes in AIS structure and function. Additionally, we investigated potential hemispheric-, cortical layer-, and sex-dependent differences in AIS and intrinsic excitability properties. ResultsWe found that normal AIS structure and function are preserved in the sub-acute phase post ischemic stroke. However, we found evidence of reduced input resistance across both hemispheres and reduced evoked spike firing frequency in the peri-infarct zone in both sexes. In addition, we found stroke reduced the evoked spike firing frequencies in the contralesional hemisphere, but only in males. ConclusionDespite the preservation of normal AIS structure and function in the sub-acute phase post ischemic stroke, cortical pyramidal neuron excitability is reduced through other intrinsic membrane mechanisms. Additionally, we show that changes to neuronal excitability spread to the contralesional hemisphere in males. These findings provide novel insight into the maladaptive changes to neural excitability in the sub-acute phase of ischemic stroke and further highlight the need to develop sex-specific stroke treatments.