The Time-course And Mechanism Of Ocular Dominance Plasticity In Adult Mice

The nervous system can adjust its connection with external experience(termed plasticity),which is essential for the survival of animals in complex environment.During a special time window(the critical period)after birth,animals show strong ocular dominance plasticity.This plasticity reduces and even disappears in adult animals.It is well known that restoration of plasticity in adulthood would be of benefit to amblyopia treatment and injury recovery.However,the mechanisms underlying adult plasticity are largely unknown.In this study,optical imaging of intrinsic signals,in vivo extracellular single-unit recording and pharmacological method were used to reveal the time-course of adult ocular dominance plasticity.Red light(630± 10 nm)and near-infrared light(720 ± 10 nm)were used as incident light in optical imaging of intrinsic signals recording.The detected response by near-infrared light was significantly lower than that by red light,and the ocular dominance index obtained with two illuminations was similar in adult non-deprived mice.Following long-term monocular deprivation(7-8 days),the ocular dominance shift was observed similarly by both incident wavelengths.Following shortterm deprivation,however,the ocular dominance shift was detected by near-infrared light rather than red light.This plasticity was mediated by the potentiation of non-suture eye response,suggesting this is adult ocular dominance plasticity.Then we used in vivo extracellular single-unit recording to find out the origin of short-term deprivation induced plasticity.Consistent with previous reports,long-term monocular deprivation biased the ocular dominance to the nondeprived eye,which was mediated by potentiation of nondeprived eye response and relatively weak depression of deprived eye response.Ocular dominance shifts for the total cell population were not observed after short-term deprivation(4 days).Then the cell population was divided into superficial(150 μm-480 μm under the pia,mainly L2/3 and L4 cells)and deep(520μm-800 μm under the pia mainly L5 cells and a few L6 cells)groups.In short-term deprived mice,ocular dominance shift was not observed in superficial neurons but the ocular dominance distribution of deep neurons was biased toward open eye.The difference between ocular dominance plasticity in the deep and superficial layers was not observed in neither the nondeprived mice nor the mice with long-term visual deprivation.The adult ocular dominance plasticity depends on NMDA receptor.We intraperitoneally administered CPP to mice with short term deprivation.This treatment blocked the ocular dominance shift in deep layers measured by electrophysiological recording,as well as the optical imaging with near-infrared light.These results suggested that short-term deprivation in adulthood is enough to induce NMDA receptor-dependent ocular dominance shift,which occurs in the deep layers and can be detected by optical imaging of intrinsic signals of near-infrared light.Excitatory neurons in visual cortex in both hemispheres connect each other through the corpus callosum and be involved in visual signal processing and ocular dominance plasticity.Previous studies have suggested that a certain proportion of callosal inhibitory neurons occurred in the early developmental stage and rapidly disappeared.We combined optogenetics and intrinsic signals optical imaging to investigate the potential role of inhibitory interneurons to contralateral visual cortex.When the inhibitory neurons were activated on one side of the hemisphere,it would reduce the basal spikes in the binocular region of another side of the hemisphere.Furthermore,it also inhibited the visually evoked response in the contralateral cortex.In the binocular region,we observed a stronger effect on the response to the ipsilateral eye than the contralateral eye.The effect was relatively weak in the monocular region.Similar phenomena were observed when two subpopulations of interneurons(parvalbumin-positive neurons and somatostatin-positive neurons)were activated.These results suggest that there may be a pathway between the two visual cortexes mediating the inhibition of interneurons to contralateral side.In summary,we found near-infrared intrinsic signal optical imaging can detecet the ocular dominane plasticity in deep layers after brief deprivation,which overthrow the conclusion that the ocular dominance plasticity can not be induced by short-term deprivation in adult mice.The inhibition between the binocular regions of two hemispheres effects the ocular dominace distribution,paving the way for future reaserch about interaction between hemispheres.