Loss and subsequent recovery of local cerebral glucose use in visual targets after controlled optic nerve crush in adult rats

NGF Eye Administration Recovers the TrkB and Glutamate/GABA Marker Deficit in the Adult Visual Cortex Following Optic Nerve Crush

Eye-drop recombinant human nerve growth factor (ed-rhNGF) has proved to recover the retina and optic nerve damage in animal models, including the unilateral optic nerve crush (ONC), and to improve visual acuity in humans. These data, associated with evidence that ed-rhNGF stimulates the brain derived neurotrophic factor (BDNF) in retina and cortex, suggests that NGF might exert retino-fugal effects by affecting BDNF and its receptor TrkB. To address these questions, their expression and relationship with the GABAergic and glutamatergic transmission markers, GAD65 and GAD67, vesicular inhibitory amino acid transporter (VGAT), and vesicular glutamate transporters 1 and 2 (VGLUT-1 and VGLUT-2) were investigated in adult ONC rats contralateral and ipsilateral visual cortex (VCx). Ed-rhNGF recovers the ONC-induced alteration of GABAergic and glutamatergic markers in contralateral VCx, induces an upregulation of TrkB, which is positively correlated with BDNF precursor (proBDNF) decrease in both VCx sides, and strongly enhances TrkB+ cell soma and neuronal endings surrounded by GAD65 immuno-reactive afferents. These findings contribute to enlarging the knowledge on the mechanism of actions and cellular targets of exogenously administrated NGF, and suggest that ed-rhNGF might act by potentiating the activity-dependent TrkB expression in GAD+ cells in VCx following retina damage and/or ONC.

Mesoscopic cortical network reorganization during recovery of optic nerve injury in GCaMP6s mice

As the residual vision following a traumatic optic nerve injury can spontaneously recover over time, we explored the spontaneous plasticity of cortical networks during the early post-optic nerve crush (ONC) phase. Using in vivo wide-field calcium imaging on awake Thy1-GCaMP6s mice, we characterized resting state and evoked cortical activity before, during, and 31 days after ONC. The recovery of monocular visual acuity and depth perception was evaluated in parallel. Cortical responses to an LED flash decreased in the contralateral hemisphere in the primary visual cortex and in the secondary visual areas following the ONC, but was partially rescued between 3 and 5 days post-ONC, remaining stable thereafter. The connectivity between visual and non-visual regions was disorganized after the crush, as shown by a decorrelation, but correlated activity was restored 31 days after the injury. The number of surviving retinal ganglion cells dramatically dropped and remained low. At the behavioral level, the ONC resulted in visual acuity loss on the injured side and an increase in visual acuity with the non-injured eye. In conclusion, our results show a reorganization of connectivity between visual and associative cortical areas after an ONC, which is indicative of spontaneous cortical plasticity.

Unrecognized Potential of Surviving Neurons: : Within-systems Plasticity, Recovery of Function, and the Hypothesis of Minimal Residual Structure

When brain structures are incompletely injured, even a very small number of surviving neurons is sufficient for recovery of the lost functions to occur. Several examples are presented: 1) After partial lesions of the adult rat optic nerve an initial, complete blindness is followed, within a few weeks, by significant recovery of visual functions to near-normal levels. This is possible despite the presence of only about 10% of surviving retinal gan glion cells. 2) In Parkinson's disease, clinical signs manifest only when more than 70%-80% of the dopamine (DA)-containing cells are lost. When the lesion is greater than about 95%, locomotor performance can no longer be compensated, but with the implantation of as little as 5%-10% of the original DA-cell number, significant motor improvements may be achieved. 3) Similarly, in spinal cord-injured rats, recovery of locomotor performance is still observed in animals with lesions greater than 95% where only a small rim of tissue survives the injury. It is therefore proposed that a certain "minimal residual structure" (MRS), often 10% or less, is required for functions to recover following CNS injury. In the context of this "within-systems" plasticity, the MRS-hypoth esis emphasizes the role of spared neuronal tissue in brain plasticity and points to an unrecognized potential of residual neurons to contribute to recovery of function. Future therapeutic studies should make use of behavioral assays "at threshold," in which the MRS is defined so as to avoid the inappropriate rejection of potentially useful therapeutic strategies. NEUROSCIENTIST 3:366-370, 1997

Changes of central visual receptive fields in experimental glaucoma

Retinal ganglion cell apoptotic death in experimental glaucoma is protracted over several months and it leads to the visual dysfunction. In the rat with increased intraocular pressure (IOP), the lack of visual scotoma was observed where visual field was determined electrophysiologically on the contralateral optic tectum in the early stages of the disease. Increases in the sizes of receptive fields on the periphery represented early stage of glaucomatous dysfunction. The relationship of duration and magnitude of IOP elevation had a significant correlation between percentages of receptive field sizes in the tectum. Large increases in receptive field sizes noted in the glaucomatous retinal terminal areas suggest the ability of the remaining retinal axons to compete and compensate for the loss of retinal axons. This compensatory adaptation leads to the degradation of the visual acuity and visual thresholds when measured psychophysically.

Anatomical correlations of intrinsic axon repair after partial optic nerve crush in rats

About 15% of retinal ganglion cells survive diffuse axonal injury of the optic nerve in adult rats. Following initial blindness, discrimination of visual stimuli in behavioral tests recovers within three weeks. To investigate the mechanisms promoting this functional recovery the axonal transport and the neurofilaments were studied. Intraocularly applied MiniRuby is transported until the place of crush and accumulated in enlarged axon terminals. Three weeks after lesion the anterograde transport of MiniRuby recovers distal to the place of crush. At the same point in time the retrograde transport of surviving retinal ganglion cells is restored which was visualized by horseradish peroxidase injected into the superior colliculus. The heavy neurofilament was stained immunohistochemically and analyzed statistically up to three weeks after optic nerve crush. The stained filaments in the axon fibers of retinal ganglion cells appear wavelike and/or fragmented up to day 8, but first signs of heavy neurofilament restitution in the fibers of the optic nerve are seen at day 12 after axonal injury. Because these results cannot be explained by longlasting axon regeneration, the present results provide convincing evidence for intrinsic axon repair soon after diffuse axonal injury that correlates in time with recovery of vision.

Blindsight after optic nerve injury indicates functionality of spared fibers

Some patients with lesions in the geniculostriate pathway (GSP) can respond to visual stimuli in the blind field without conscious acknowledgement. The substrate for this "blindsight" is controversial: whether it is the uninjured extrastriate pathway (EXP), which bypasses the lesion site, or residual fibers within damaged visual cortex ("islands of vision"). Using stimulus detection, localization, and spatial summation tasks, we have found blindsight in patients with damage both in the optic nerve (ON) and EXP. The prevalence and functional characteristics of their blindsight are indistinguishable from that in patients with GSP lesions, so blindsight does not require a completely intact EXP. The present findings support the view that a few surviving ON axons within an area of primary damage are sufficient to mediate blindsight: Several combinations of partially intact pathways can transmit information to the extrastriate cortex and the sum of activation of all visual fibers surviving the injury determines if and to what extent blindsight occurs.

Effects of increased intraocular pressure on rat retinal ganglion cells

The effects of elevated intraocular pressure (IOP) on the morphology of rat retinal ganglion cells (RGCs) was analyzed in this study. After cauterizing two limbal derived episcleral veins, IOP in experimental eyes was elevated 1.5--1.8 times that of control. RGCs of experimental and control eyes were analyzed after: bilateral tectal injections of Fluoro-Gold, and application of fluorescent dye crystals, 4-Di-10-ASP to the proximal stump of the cut optic nerve, at different time intervals after IOP elevation. The RGCs in control and experimental eyes were evaluated at 4, 6, 8, and 10 weeks by counting, as well as by determining the soma diameter. The dendritic field of three types (I, II, III) of RGCs between control and experimental eyes were also studied at 4,6,10 weeks after IOP elevation. At every time point, the number of cells in experimental eyes were significantly less than those of the control eyes. The average retinal ganglion cell death was 3--4% per week in the eyes with elevated IOP. The soma and dendritic field diameter of the RGCs in the experimental eyes were significantly larger in all cell types. However, types I and III cells expanded their dendritic fields more rapidly than type II cells. Furthermore, dendritic fields of surviving RGCs in experimental eyes occupied about the same extent of the retina as the controls. The increase in soma diameter and expansion of dendritic fields in the remaining RGCs in eyes with elevated IOP suggests the existence of plasticity in adult retina.

NMDA-receptor antagonist protects neurons from secondary degeneration after partial optic nerve crush

Damage resulting from a partial acute lesion of white matter in the central nervous system (CNS) gradually spreads also to neurons that escaped the primary injury, resulting in their degeneration. Such spreading has been referred to as secondary degeneration. In order to demonstrate that this degeneration is indeed secondary to that caused by the acute insult, as well as to investigate the mechanism underlying the spread of damage and ways in which to protect neurons from such damage, we have proposed the use of partial lesion of the rodent optic nerve as a model. In this model we examined whether an antagonist of a receptor-mediated channel, shown to be beneficial in gray matter lesions, can protect neurons from undergoing secondary degeneration following white matter lesion. A well-calibrated partial crush lesion inflicted on the optic nerve of adult rats was immediately followed by a single intraperitoneal injection of the N-methyl-D-aspartate receptor antagonist, MK-801 (1 mg/kg). Protection of neurons from secondary degeneration was assessed by retrograde labeling and by measurement of the visual evoked potential (VEP) response to light. Two weeks after the injury, the mean number of neurons that were still intact was about threefold higher in the MK-801-treated group than in the saline-treated control group, indicating a treatment-induced protection of neurons that had escaped primary injury. A positive VEP response to light was obtained in 90% of the MK-801 treated animals and in only 50% of injured controls. The questions regarding whether the secondary degeneration of initially spared neurons starts in their cell bodies or in their axons, and consequently the identity of the primary site of their protection by MK-801, are discussed in relation to the absence of N-methyl-D-aspartate receptors on nerve fibers. The present findings may have implications for both acute and chronic injuries of the CNS.

MK-801 reduces retinal ganglion cell survival but improves visual performance after controlled optic nerve crush

Excitotoxicity is implicated in secondary cell death after ischemic or traumatic brain injury. We therefore evaluated the role of excitotoxicity mediated by the NMDA glutamate receptor subtype on retinal ganglion cell (RGC) survival and visual performance after optic nerve injury in adult rats. To monitor visual deficits after mild optic nerve crush, rats were trained in a two-choice pattern discrimination task. Immediately after the crush and on postoperative day 1, MK-801 (1 nmol), a noncompetitive open channel blocker of the NMDA-receptor, was injected intraocularly. Within the first few days after crush, all rats showed a loss of their discrimination ability that was followed by a significant recovery within a 3-week testing period. Although animals treated with MK-801 had a significantly smaller initial deficit compared with PBS-injected controls, anatomic investigations using retrograde HRP tracing revealed a significant retrograde loss of RGC in lesioned rats that was significantly exacerbated by MK-801. These results confirm our earlier studies suggesting that neuronal damage does not uniformly match behavioral defects in CNS injury paradigms and that near-normal visual performance occurs in rats with only about 10% of RGC being connected to their target. The observation that after traumatic injury MK-801 is neuroprotective functionally while being neurotoxic anatomically is a structural-functional paradox that needs to be explored further.