Women comprise more than half of the population of stroke survivors. Upper extremity motor impairments are an especially prevalent and disabling consequence of stroke. Improvements in the functional capacity of the paretic upper limb can be driven with motor rehabilitative training, an effect that is linked with its promotion of new synapse formation and the resurrection and reorganization of upper limb motor maps in the motor cortex of the injured hemisphere. There are currently many investigational treatments focused on facilitating motor cortical plasticity to improve post-stroke function. However, the mechanistic understanding that is guiding these directions is based largely on studies in young adult males. We have recently uncovered a striking difference in the effect of age on motor cortical organization in female, compared with male, rodents. Aging in males is characterized by shifts in the relative areas of distal and proximal forelimb movement representations, without reductions in the overall size of motor maps. Our preliminary data in females indicate that, beginning around middle age, there are reductions in the cortical territory devoted to forelimb movement, an effect that is not easily explained by declines in ovarian estradiol release. This includes a drastic reduction in distal forelimb (hand) movement representations in the rostral motor cortex, a region related to premotor cortex in humans. These data and related findings lead us to suspect that just as stroke incidence begins to rise dramatically, there could be substantial alterations in the cortical substrates of motor skill learning and of post-stroke functional improvements in females that are not present in males. We will begin to test this possibility using an established mouse model of post-stroke chronic upper extremity impairments and motor rehabilitative training. We will investigate in females and males with matched infarct severities the effects of chronological aging, but do so in a manner that is sensitive to influences of reproductive aging in females. Repeated in vivo cranial window imaging of synaptic elements motor cortex will be used in combination with sensitive behavioral measures and high resolution mapping of motor cortical organization to detect age-dependencies in the structural and functional reorganization of cortex linked with motor skill learning (Aim 1), motor recovery (Aim2) and motor rehabilitative training efficacy (Aim 3) and differences in the patterns of these age-related effects across sexes. The studies are intended to support and guide the mechanistic direction of future studies to understand age by sex interactions in the neural mechanisms of recovery and rehabilitation efficacy. At minimum, the results will begin to fill a huge knowledge gap surrounding aging effects on neural events in the chronic post-stroke period in the female brain. The results also have the potential to be of major importance for post-stroke treatment tailoring efforts.
Rehabilitative training approaches remain the primary means of improving function in the chronic period after stroke, but they are currently far from perfect and there is ongoing effort to improve them by facilitating cortical plasticity. There is growing awareness of the need to tailor treatments for different stroke subpopulations. This project will reveal if there is likely to be a need to tailor treatments for older women in a manner that is different from men. The studies will also advance basic understanding of aging effects on post-stroke recovery processes in females.