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the receptive field. Since electrophysiological recordings were made in the superficial layer of the tectum and the extracellular multiunit recordings were generated in the area of the receptive field, we are confident that recordings were acquired from the terminal areas of the optic axons. We must therefore assume that optic axonal terminal arbors increase to cover larger than the normal terminal area of the tectum. We have recently observed (unpublished preliminary data) a qualitative increase in the optic axonal terminal areas in the tectum of glaucomatous animals when compared to the terminal areas (of similar type) seen on the mirror image location of contralateral normal tectum.

Behavioral consequences of glaucoma

Sauve et al. (2004) reported a reasonable correlation of full-field ERG and visual field thresholds recorded from the superior colliculus in rat. These authors showed that magnitude of b-wave could be used to predict the computed area and degree of visual field preservation in the central nervous system (CNS). Grozdanic et al. (2003) reported a similarly functional characterization of retina after ischemia. By contrast, a smaller number of studies have been undertaken to measure changes in visual function after glaucoma treatment (Greve et al., 1975; Flammer and Drance, 1983; Yoshikawa et al., 1989; Rolando et al., 1991). These authors showed moderate improvements in retinal function in humans. However, in animals with experimental glaucoma, hardly any studies have dealt with RGC survival and recovery of function. In rodent’s glaucoma, functional analysis has been hampered due to limited and tedious methodologies to test visual behavior. Recently, Prusky and his colleagues have developed a rapid quantification method for testing spatial vision in rats. The spatial vision (grating acuity and contrast sensitivity) can be measured by a virtual reality optomotor system in which spatial visual thresholds can be measured rapidly and without specific reinforcement training. The profiles of acuity and

contrast sensitivity present a dynamic picture of the functioning of the visual system, and the ability of the rat to reflexively track a moving grating can easily be assessed. Using this procedure, one can characterize the loss of vision in glaucomatous animals and evaluate the effects of various manipulations aimed at saving RGC in glaucoma. The other complimentary aspect of vision is the measurement of visual perceptual thresholds that can be used to follow changes in visual processing in glaucoma. The perceptual threshold is measured by visual water task method. Both tests are necessary to evaluate potential assessment of visual dysfunction.

Slow horizontal head and body rotation occurs in rats when the visual field is rotated around them. These optomotor responses can be readily produced. If one eye is closed, only motion in the temporal-to-nasal direction for the contralateral eye evokes the tracking response. When the maximal spatial frequency capable of driving the response (acuity) was measured under monocular and binocular viewing conditions, the monocular acuity was identical to binocular acuity measured with the same rotation direction. Thus, the visual capabilities through each eye can be measured under binocular conditions simply by changing the direction of rotation. The spatial frequency, contrast, and velocity of stimulus pattern are generated by the computer and can be changed instantaneously.

The grating thresholds of Long-Evans rats are clustered around 1.0 cycle/degree. Using spatial frequencies already described for Long-Evans rats (Prusky et al., 2002), visual thresholds were obtained in normal rat. As described by Prusky et al. (2000), for each eye, motion in the temporal- to-nasal direction evokes tracking, whereas motion in the nasal-to-temporal direction does not. We used frequency of 0.103 cycle/degree sine wave gratings. As grating was rotated, rat ceased body movements and began to track grating with head movements in relation to the rotation. The contrast was increased or decreased to achieve the threshold. In experimental animals where only one eye had elevated IOP and the contralateral eye served as a control, the tests for contrast sensitivity and visual acuity began at 5 weeks after the

surgery. The mean IOP of the control left eye was 16.5 mmHg, whereas the experimental right eye had a mean IOP of 25.15 mmHg. Visual acuity refers to the maximal spatial frequency that evoked an optomotor response. Acuity thresholds were measured for both eyes of normal and experimental animals in which one eye had elevated IOP. The eyes of normal animals had comparable mean acuity thresholds of 0.52; a paired t-test indicates no significant difference between the eyes of normal animals (t ¼ 1, df ¼ 2, p ¼ 0.42). For elevated IOP eyes, the experimental eye had a lower mean acuity value (0.1570.013 SEM) than did the contralateral normal eye (0.5270.001 SEM). A paired t-test indicated that the acuity values for the experimental and normal eye differed significantly (t ¼ 26.9, df ¼ 6, po0.0005).

These animals were further tested for contrast sensitivity as a function of spatial frequency. The experiment formed a 2 6 design (eye vs. spatial frequency) with repeated measures on both variables. A 2 6 repeated-measures ANOVA indicated that the main effect of eye (normal vs. experimental elevated IOP eye) was significant (f ¼ 22, 471.6, df ¼ 1, po0.0005).

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The main effect of spatial frequency was also significant (f ¼ 626.7, df ¼ 5, po0.0005). Examination of the data indicates that the interaction was due to the flattening of the curve for the experimental eye relative to the curve for the normal eye. This flattening was due to a floor effect created by the poor contrast sensitivity of the experimental eye. A two-tailed, paired t-test was done at each spatial frequency to compare the mean contrast sensitivities of the experimental and normal eyes. All six t-tests were significant and the t values ranged from 33.9 to 193.4 (df ¼ 6, po0.0005). Therefore, the contrast sensitivity of the experimental eye differed from that of the normal eye at all six spatial frequencies (Fig. 5).

Once again, the above-described preliminary (unpublished) data in glaucomatous rat stand in contrast to the published data from the primates. Primate glaucomatous eyes showed visual behavior changes where approximately 50% RGC have already died, but in glaucomatous rats the earliest changes in visual acuity were evident within a month following IOP elevation. It seems that about 15% RGC death was sufficient to measure the earliest changes in acuity and contrast

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sensitivity in the rat and these changes may be due to very sensitive measuring equipment.

Glaucoma as a neurodegenerative disease versus neuroplasticity and adaptive changes

Dr. Yucel and his colleagues in Canada have shed light on the effects of elevated IOP in primate and human glaucomatous visual centers. Elevated IOP induces RGC death and as expected leads to the changes in the LGN where cell loss and the shrinkage of LGN was reported (Weber et al., 2000; Yucel et al., 2000, 2001, 2003). Finally, changes in the primary visual cortex were reported. Cytochrome oxidase activity decreased in the ocular dominant columns subserving the glaucomatous eye (Crawford et al., 2001; Lam et al., 2003). These transsynaptic changes in the visual pathways following glaucoma led the authors to describe glaucoma as a neurodegenerative disease. Evidence has further accumulated that IOP elevation induces changes in the LGN without optic nerve axonal loss. It is conceivable that initiating changes both physical (dendritic withdrawal preceding RGC death) and chemical (decrease in cytochrome oxidase activity in LGN) may be linked. Hence, one can conclude that initiation of the elevated IOP may trigger changes in the retina and CNS whose effect continues for a very long time. To date, emphasis has been placed on lowering IOP as a means to reduce RGC death in glaucomatous eyes. However, in many cases RGC death continues even after normal pressure is maintained.

Future directions

The continuous debate on the ‘‘dogma’’ of glaucoma, i.e., whether ganglion cells die first or optic axons are damaged first, is yet to be resolved. In both situations, changes commence in the termination sites of the visual pathways. The dramatic change seen in the visual centers in glaucomatous animal and human raise the question that the functional relationship between the loss or degradation of the visual field and the

changes in visual acuity and visual thresholds must be further explored.

The factors that initiate RGCs death in glaucoma are incompletely defined. Loss of RGCs via apoptotic mechanisms induced by the elevation of IOP leads to degenerative morphological changes and cell death in the LGN and primary visual cortex. Some reorganization of retinotopic cortical maps in adult mammals has been reported following lesions of the retina. Conditions that might lead to cortical neuroplasticity following IOP elevation in monkeys have been reported (Lam et al., 2003).

Rescued RGCs in the optic nerve transection model do not provide any means to assess their functionality, as cut optic axons do not regenerate ordinarily. Furthermore, rescued or still surviving and perhaps uninjured RGCs in glaucoma maintain their nerve connections to the brain centers allowing evaluations of their functionality. At what stage(s) in the progression of glaucomatous RGCs death is there degradation of the central visual field maps? It is important to acquire such information to allow better understanding of the progression of visual loss in glaucoma, and it will provide avenues to assess the effects of neuroprotective agents on the consequence of rescuing RGCs in glaucoma.

The ability of the neuronal system to undergo remodeling and repair following injury is influenced by interactions between the remaining viable RGCs in glaucomatous retina as well as by the response of individual cells to the reduction in IOP and the administration of neuroprotective agents. Another factor that ought to be considered is that the reduction in the overall number of RGC may induce the dendrodendritic interactions of the remaining RGCs in delimiting the size of the dendritic tree. If by reducing the IOP and/or administration of neuroprotective agents one can determine the functional consequences of retina leading to either partial or complete restitution of function, it may offer a new avenue for improving visual function in glaucoma patients.

Future experiments should be directed to explore the effect of lowering the IOP, either surgically or medically, on the receptive field sizes.

Is the degradation of visual receptive field ever restored? Experiments should also be directed to evaluate the progression of changes in visual function by recording from the visual cortex.

Is there functional reorganization in the primary visual cortex in response to glaucoma in human? If the emergence of degradation of visual fields takes a few months of sustained elevated IOP in rats and it happens when ganglion cell death cascade has slowed down, it might take much longer time in humans when IOP in glaucomatous eye has been managed.

Finally, as it is becoming acceptable that glaucomatous changes are well pronounced in central visual nuclei, the question arises that treating the visual centers by neuroprotective agents so that death of LGN and cortical neurons is reduced or controlled might benefit patients with glaucoma. Of course, these treatments should be coupled with lowering IOP treatments of the eye to have any real effect in glaucomatous patients.

Acknowledgment

The work reported has been supported by NIH/ NEI grants.

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