Ординатура / Офтальмология / Английские материалы / Eye, Retina, and Visual System of the Mouse_Chalupa, Williams_2008

Optic nerve diseases include a variety of blinding disorders such as glaucoma, optic neuritis, ischemic optic neuropathy, and mitochondrial optic neuropathy. Mouse models for these diseases are being developed at an increasing rate to investigate specific pathophysiological mechanisms. Experimental mouse models of optic nerve transection and crush injury are also widely used to better understand molecular mechanisms of retinal ganglion cell (RGC) and axon death, as well as to explore neuroprotective treatments, including gene therapy (Levkovitch-Verbin, 2004). The use of these models may be greatly enhanced by the availability of noninvasive methods able to monitor RGC function longitudinally.

The pattern electroretinogram as a tool to measure retinal ganglion cell function

The pattern electroretinogram (PERG) is a particular kind of ERG obtained in response to contrast reversal of patterned visual stimuli (gratings, checkerboards) rather than traditional flashes of diffuse light (figure 8.1), and has characteristics fundamentally different from the flash ERG.

The pattern stimulus consists of two sets of elements of equal areas whose luminances increase and decrease at a given frequency FHz (flicker). At the retinal level, flickering pattern elements generate local flicker ERGs at frequency FHz. Because adjacent pattern elements generate local flicker ERGs 180 degrees out of phase, these are summed and canceled at the distant electrode. An ERG is recordable in response to pattern reversal because additional, nonlinear ERG components are generated (mainly at 2 FHz frequency, corresponding to the contrast reversal rate) that are in-phase and do not cancel at the electrode. This is what constitutes the PERG. The main generators of local flicker ERGs at FHz are likely the photoreceptors, which have approximately linear behavior, whereas the PERG generators at 2 FHz are likely postreceptoral elements with center-surround receptive field (RF) organization and nonlinear behavior (Baker and Hess, 1984). In sum, photoreceptor activity is necessary for PERG generation but it is not apparent in the PERG

waveform because of cancellation at the electrode. Since RGCs have RFs with strong center-surround antagonism, these seem the best candidates for PERG generation. RGCs are expected to be maximally activated by pattern elements whose dimensions match the average size of the RGC RF center. In keeping with this prediction, the PERG amplitude displays a maximum at a specific spatial frequency that approximately corresponds to the average RGC RF size (Hess and Baker, 1984; Drasdo et al., 1987; Porciatti, 2007). In addition, for a given retinal eccentricity and stimulus area, the PERG amplitude to the peak spatial frequency is linearly proportional to the expected volume of RGCs (Drasdo et al., 1990).

RGC activity is indeed necessary for PERG generation, since RGC retrograde degeneration after optic nerve transection abolishes the response in all mammals tested so far. These include cat (Maffei and Fiorentini, 1981), monkey (Maffei et al., 1985), rat (Berardi et al., 1990), and mouse (Porciatti et al., 1996; Chierzi et al., 1998) (see figure 8.1). In the same experimental animals the a- and b-waves of the conventional bright flash ERG, which originate in the outer retina, are little or not affected. PERG generation also requires physiological integrity of anatomically present RGCs. The PERG amplitude can be reversibly reduced, though not abolished, by intravitreal injections of tetrodotoxin, which block Na+-dependent spiking activity in the inner retina (Trimarchi et al., 1990; Viswanathan et al., 2000). Short-term, moderate elevation of intraocular pressure (IOP) reversibly reduces the PERG amplitude while leaving the flash ERG intact (Siliprandi et al., 1988; Feghali et al., 1991). A reduction in PERG amplitude may therefore reflect both the reduced activity of nonfunctional RGCs and the lack of activity of lost RGCs.

Thus, the PERG may represent an important tool for monitoring the onset and progression of RGC dysfunction in mouse models of optic nerve disease, as well as for probing the effects of neuroprotective treatments. Strong evidence supports the view that functional RGCs are necessary to generate the PERG, whereas less is known about what aspects of RGC activity relate to the response. Although a

A

B

N

Figure 8.1 A, The PERG is recorded from anesthetized mice by means of corneal electrodes, allowing unobstructed viewing of alternating grating patterns. B, Two months after intracranial optic nerve transection, isodensity maps of cell bodies in the retinal ganglion cell layer of flat mount retinas show massive depletion of neurons and loss of PERG response compared with the control eye. Calibration bar = 3 mm. N, nasal quadrant of each retina. For the PERG, calibration is 0.5 μV (vertical) and 100 ms (horizontal). (Modified from Porciatti et al., 1996, 2007, and Chierzi et al., 1998.)

TTX-dependent component suggests that RGC axon spiking activity plays a role in PERG generation, slow electrical activity generated at the level of RGC dendrites, or electrical activity in the inner retina circuitry impinging on RGCs, cannot be excluded. Finally, a Müller cell component in the PERG generation cannot be excluded, since Müller cells can passively generate electric currents in response to extracellular modulation of K+ ions produced by active retinal neurons (Kline et al., 1978).

Methods for pattern electroretinogram recording

An optimized protocol for mouse PERG recording has been recently described (Porciatti et al., 2007). In brief, mice are anesthetized with intraperitoneal (IP) injections (0.5– 0.7 ml/kg) of a mixture of ketamine, 42.8 mg/mL; xylazine, 8.6 mg/mL; and acepromazine, 1.4 mg/mL. Mice are gently restrained using a mouth bite and a nose holder that allows

unobstructed vision, and kept at constant body temperature at 37.0°C using a feedback-controlled heating pad. Under these conditions the eyes of mice are naturally wide open and in a stable position, with pupils pointing laterad and upward. The recording electrode is a thin (0.25 mm diameter) silver wire configured in a semicircular loop of 2 mm radius. It is gently leaned on the corneal surface in such a way as to encircle the undilated pupil without interfering with vision (see figure 8.1). Electrode positioning entails minimal corneal stimulation, which might otherwise induce cataract (Fraunfelder and Burns, 1970) and preclude further PERG testing. Reference and ground electrodes—small stainless steel needles—are inserted into the skin of the back of the head and the back of the body, respectively. Instillations of BSS drops every 30 minutes are sufficient to maintain the cornea and lens in excellent condition for many hours. Pattern stimuli consist of horizontal bars of variable spatial frequency and contrast that alternate at different temporal frequency. Stimuli are displayed on a television monitor whose center is aligned with the projection of the pupil and presented from short distance (typically 20 cm) to stimulate a large retinal area (typically 50–60°) centered on the optic disc (see figure 8.1). Eyes are not refracted for the viewing distance because the mouse eye has a large depth of focus due to the pinhole pupil (Remtulla and Hallett, 1985). PERGs (Porciatti et al., 1996) and pattern visual evoked potentials (VEPs; Porciatti et al., 1999b) are not modified by trial lenses of ±10 spherical diopters placed before the eyes. Compared to the traditional ERG, the amplitude of the PERG is smaller by a factor of about 100. Therefore, robust averaging (1,000–2,000 sweeps) is needed to isolate the response from background noise and reduce variability. At optimal spatial frequency (0.05 c/deg), temporal frequency (1 Hz), and contrast (100%), the PERG signal-to-noise ratio is of the order of 10 : 1 (Porciatti et al., 2007), which represents an adequate dynamic range for application in mouse models of optic nerve disease.

Pattern electroretinogram correlates of visual behaviors

Being an RGC-driven electrical response, the PERG represents an index of the retinal output. By changing the spatialtemporal characteristics of the pattern stimulus, it is possible to obtain estimates of retinal resolution (acuity) (Porciatti et al., 1996; Rossi et al., 2001), contrast threshold (Porciatti et al., 1996), and temporal resolution (Porciatti and Falsini, 2003) (figure 8.2).

Visual thresholds determined at retinal level with PERG have a counterpart in corresponding measures of visual acuity and contrast sensitivity determined with VEPs (Porciatti et al., 1999b), operant psychophysical behavior (Gianfranceschi et al., 1999; Prusky and Douglas, 2004), and passive optomotor responses (Prusky et al., 2004; Schmucker

0.4

0.6

100 C

Contrast (%)

Figure 8.2 Spatial-temporal properties of the mouse PERG. A, Examples of PERG responses recorded in young adult C57BL/6J mice in response to 1 Hz alternating gratings of high contrast (95%) and different spatial frequency (numbers to the left of each waveform expressed in c/deg). B, The mean (±SEM) PERG amplitude in six different mice decreases with increasing spatial frequency and

et al., 2005). PERG acuity develops postnatally (Porciatti and Falsini, 2000; Porciatti et al., 2002) in parallel with visual acuity determined with either VEPs (Huang et al., 1999) or optomotor responses (Prusky et al., 2004). Around eye opening (postnatal day 14–15), the PERG acuity is of the order of 0.2 c/deg and matures during the next 2 weeks to reach the adult acuity (0.6 c/deg) by about 1 month of age (figure 8.3). By combining PERG measures with VEPs, optomotor responses, or psychophysical visual behavior, it is possible to evaluate the relative contribution of retinal and postretinal stages to a particular disease or condition.

Examples of pattern electroretinogram application to study central nervous system plasticity and degeneration

Role of Developmental Spontaneous Activity In the mammalian visual system the formation of eye-specific layers at the thalamic level depends on retinal waves of spontaneous activity (Wong, 1999), which rely on nicotinic acetylcholine

Spatial Frequency (c/deg)

D

5 10 15

Temporal Frequency (Hz)

reaches the noise level (dashed line) at 0.6 c/deg, which represents the retinal acuity. For gratings of 0.05 c/deg, the PERG amplitude decreases with decreasing contrast (C) and increasing temporal frequency (D). The contrast threshold is about 10%, and the temporal resolution is about 13 Hz (26 reversals/s). (From Porciatti and Falsini, 2003.)

1

0.01

Figure 8.3 Postnatal maturation of PERG acuity in C57BL/6J mice. (From Porciatti and Falsini, 2000.)

receptor activation (Feller et al., 1996). In mutant mice lacking the β2 subunit of the neuronal nicotinic receptor (Picciotto et al., 1995), but not in mice lacking the α4 subunit, retinofugal projections do not form eye-specific layers in the dorsolateral geniculate nucleus (Rossi et al., 2001; MuirRobinson et al., 2002). Still, retinogeniculate projections segregate into a patchy distribution (Muir-Robinson et al., 2002), indicating that segregation of left eye and right eye axons can be uncoupled from macroscopic patterning in the visual system. β2−/− mice show an expansion of the binocular subfield of the primary visual cortex and a decrease in visual acuity at the cortical level (VEP) (Rossi et al., 2001). This indicates that the β2 subunit of the nicotinic acetylcholine receptor is necessary for the anatomical and functional development of the postretinal visual system. The retinal acuity determined with PERG, however, is identical in β2−/− and wild-type mice (Rossi et al., 2001). In addition, detailed anatomical analysis of different RGC classes does not reveal significant differences between wild-type and β2−/− animals (Van der List et al., 2006). These results indicate that cholinergic-mediated activity in the developing retina is not required for the normal postnatal development of retinal ganglion cells (figure 8.4).

Role of Developmental Cell Death The BCL2 protein is a potent inhibitor of apoptotic cell death. Transgenic mice have been generated with overexpression of the human Bcl-2 gene in RGCs and in most neurons of the central nervous system (CNS) (Martinou et al., 1994). As a result of inhibition of apoptotic cell death during development, adult Bcl-2- overexpressing mice have 2.6 times more RGCs and optic nerve fibers than normal, and have larger brains (Cenni et al., 1996). Despite the marked neuronal redundancy of

the retinal and postretinal visual pathway, the retinal acuity (as determined by PERG) of the Bcl-2-overexpressing mouse is normal (Porciatti et al., 1996). The cortical (VEP) visual acuity (Porciatti et al., 1999a) and the behavioral visual acuity (Gianfranceschi et al., 1999) are also normal. A detailed anatomical study of the Bcl-2 transgenic mouse (Strettoi and Volpini, 2002) indicates that a compensatory growth of axonal arborizations of bipolar cells ultimately results in an increased divergence on RGCs, thus neutralizing the effect of their higher density. At cortical level, neuronal redundancy causes expansion of the brain, which keeps neuronal density in the normal range (Porciatti et al., 1999a). To account for brain expansion, the cortical representation of the visual coordinates (i.e., the vertical meridian) is shifted laterally (Porciatti et al., 1999a).

Survival of Retinal Ganglion Cells after Lesion CNS neurons of adult Bcl-2 mice are little altered after lesions. The great majority of RGCs survive for a long time after optic nerve section (Cenni et al., 1996), and their physiological response (P-ERG) is spared (Porciatti et al., 1996; Chierzi et al., 1998) (figure 8.5). This suggests that neuroprotective strategies aimed at targeting the apoptotic cascade at mitochondrial level may successfully rescue from death a high number of RGCs with normal physiological response. Mice deficient in BCL2-associated X protein (BAX) can be also used to investigate the role of BAX-mediated cell death in optic nerve lesions. BAX deficiency protects RGCs after axon injury by optic nerve crush (Libby et al., 2005c). BAX deficiency in DBA/2J mice with spontaneous glaucoma (discussed in the next section) protect RGCs from death (Libby et al., 2005c). It remains to be established whether protected RGCs retain normal function.

Figure 8.4 Comparison between cortical and retinal acuity in adult C57BL/6J mice (WT) and in mutant mice lacking the beta2 subunit of the neuronal nicotinic receptor (β2−/−). In β2−/− mice, visual acuity is reduced at cortical level but not at retinal level. (Modified from Rossi et al., 2001, fig. 4.)

Pattern electroretinogram application in mouse models of glaucoma

The DBA/2J mouse is a well-established model of spontaneous glaucoma. Recessive mutations in two genes, Gpnmb and Tyrp1, cause iris atrophy and pigment dispersion ( John et al., 1998). The iris disease is apparent at 6 months and progresses with age, resulting in elevated intraocular pressure (IOP) (Libby et al., 2005a). Young (2- to 4-month-old) DBA/2J mice have normal IOP, a normal PERG, and normal histological appearance of RGCs and optic nerve. Glaucoma damage in the optic nerve is apparent in about 60% of eyes by 10–11 months of age, and in about 90% of eyes by 18 months (Libby et al., 2005a). Between 10 and 11 months of age, the PERG amplitude is markedly reduced in 100% of eyes, including eyes with no signs of nerve damage (figure 8.6). Between 2 and 13 months of age, the outer retina is histologically intact ( Jakobs et al., 2005) and the light-adapted flash ERG displays only minor

Bcl-2 3.5 mo. after ON cut

0.8

Spatial Frequency (c/deg)

Figure 8.5 Effect of intracranial section of one optic nerve on the PERG. In C57BL/6J mice, the mean (±SEM, n = 3) PERG amplitude of axotomized eyes is at noise level (dashed lines) 2 months after

changes (Porciatti et al., 2007; Libby et al., 2006) (see figure 8.6). Taken together, these results suggest that DBA/2J mice develop a progressive functional damage in the inner retina (abnormal PERG) but not in the outer retina (normal flash ERG) that seems to precede anatomical damage of the RGC layer and of the optic nerve. This may suggest that surviving RGCs may not be functional. Therefore, neuroprotection studies using mouse models of glaucoma should include functional endpoints in addition to anatomical endpoints.

The noninvasive nature of PERG allows serial recordings as a function of changing conditions (e.g., age, IOP levels). Recently, we have been able to characterize the natural history of RGC dysfunction and its relationship with IOP in a 12-month longitudinal study of BDA/2J mice (Saleh et al., 2007). On average, the IOP increases moderately between 2 and 6 months, with a progression of 0.92 mm Hg/month. After 6 months the IOP displays a steeper increase, and tends to level off by 11 months at a value of about 30 mm Hg (Saleh et al., 2007). After 3 months, the PERG amplitude decreases linearly with age to reach the noise level at about 10–11 months (Saleh et al., 2007). Histological analysis of eyes with abolished PERG shows that the retinal nerve fiber layer is largely preserved. Between 2 and 11 months the cone-flash ERG does not show significant changes (Saleh et al., 2007). We were also able to characterize the changes

Wild type 2 mo. after ON cut

0.8

PERG control eyes

PERG ON cut eyes

Noise control eyes

Noise ON cut eyes

0.0

Spatial Frequency (c/deg)

surgery. By contrast, in Bcl-2-overexpressing mice, the mean (±SEM, n = 3) PERG amplitude is at control values 3.5 months after axotomy. (Modified from Porciatti et al., 1996, fig. 4.)

Figure 8.6 Amplitude of PERG and light-adapted flash ERG in DBA/2J mice of different ages. The PERG of glaucomatous 12to 14-month-old mice, compared with that of preglaucomatous 2- to 4-month-old mice, is reduced to the noise range (mean 1.18 ± 0.35 μV). The light-adapted flash ERG displays minor changes in glaucomatous 12to 14-month-old mice. (Replotted from Porciatti et al., 2007, fig. 5.)

in IOP and PERG that occur when DBA/2J mice are put in a head-down (60-degree) body position (Aihara et al., 2003). Postural changes cause substantial (+30%–35%) reversible IOP increases in DBA/2J mice of different ages (3–10 months) (Nagaraju et al., 2007). Postural changes also cause PERG amplitude reductions that are strongly age dependent (3-month-old: no change; 5-month-old: −48%; 10-month-old: −67%) (Nagaraju et al., 2007). Finally, when 10-month-old mice with reduced PERG amplitude are treated with mannitol, the IOP decreases by about 50% and the PERG amplitude increases by 80% (Nagaraju et al., 2007). These results indicate that RGC vulnerability in the DBA/2J mouse model of glaucoma is age and IOP dependent, and that existing RGC dysfunction can be restored by reducing the level of IOP.

Use of the pattern electroretinogram in human clinical research

Since Maffei and Fiorentini reported that the ERG with contrast reversal was abolished by optic nerve section in the cat (Maffei and Fiorentini, 1981), PERG has received considerable attention for clinical application in a wide spectrum of optic nerve disorders (reviewed in Holder, 2001; Parisi, 2003; Ventura and Porciatti, 2006). Applications in glaucoma have received by far the largest consideration; the spontaneous DBA/2J mouse model of glaucoma described above, together with mouse genetics and noninvasive functional tests such as PERG, may represent a powerful link with the clinical condition to unlock the mechanisms of glaucoma (John et al., 1999; Libby et al., 2005b). Recent results in human glaucoma indicate that RGC dysfunction, as measured by PERG (Porciatti and Ventura, 2004), exceeds the proportion expected from loss of RGC axons (Ventura et al., 2006). This implies that the population of surviving axons is not functional. Additional results show that RGC dysfunction in patients with early glaucoma may at least in part be restored by reducing IOP with eye drops (Ventura and Porciatti, 2005). Taken together, these results suggest that in glaucoma, RGCs undergo a stage of reversible dysfunction before death, thereby offering a window of opportunity to detect and treat the condition before irreversible damage and loss of vision occur. As described earlier in the chapter, qualitatively similar results have been obtained in the DBA/2J model. The current standard for initiating glaucoma treatment is based on a repeatable abnormality of the psychophysical visual field sensitivity, which typically occurs when at least 30% of RGCs have already degenerated (e.g., Kerrigan-Baumrind et al., 2000). That in glaucomatous optic neuropathy RGCs undergo a phase of reversible dysfunction before irreversible damage and death occur has important implications for the early diagnosis and treatment of a wide variety of progressive neurological disorders.

Limitations of the pattern electroretinogram technique

The main limitation of the technique is that it requires the integrity of the eye optics to be properly recorded. Cataracts may develop in experimental mouse eyes as a result of drugs, cold, anoxia, stress, and dehydration (Fraunfelder and Burns, 1970). Careful manipulation of experimental mice, however, prevents cataract formation (Porciatti et al., 2007). Another potential shortcoming is that PERG is a relatively small signal, since generator sources are limited to cone-driven (light-adapted) postreceptoral activity. Robust averaging, however, permits obtaining PERGs with excellent signal-to- noise ratio and with a variability comparable to that of traditional ERG (Porciatti et al., 2007).

A component of full-field flash ERG, the scotopic threshold response (STR), has been shown to depend on RGCs activity in rats (Bui and Fortune, 2004) and is altered in rat models of glaucoma (Fortune et al., 2004). Compared with PERG, the STR is expected to depend less on eye optics and might represent an alternative to PERG. The STR is also recordable in mice (Saszik et al., 2002), but the contribution of normal and abnormal RGC activity to the response still needs to be demonstrated.

Conclusion

PERG is a valuable tool for characterizing the spatialtemporal retinal output in mice of different ages and genotypes. Its noninvasive nature allows serial recording to evaluate longitudinal changes of normal and abnormal RGC activity in mouse models of optic nerve disease, as well as their response to stress factors. Overall, PERG may represent a powerful tool for neuroprotection studies.

acknowledgments Work was supported by NIH grant no. RO1 EY014957, NIH grant no. RO3 EY016322, NIH center grant no. P30-EY14801, and an unrestricted grant to the University of Miami from Research to Prevent Blindness, Inc.

REFERENCES

Aihara, M., Lindsey, J. D., and Weinreb, R. N. (2003). Episcleral venous pressure of mouse eye and effect of body position. Curr. Eye Res. 27(6):355–362.

Baker, C. L., Jr., and Hess, R. F. (1984). Linear and nonlinear components of human electroretinogram. J. Neurophysiol. 51(5):952–967.

Berardi, N., Domenici, L., Gravina, A., and Maffei, L. (1990). Pattern ERG in rats following section of the optic nerve. Exp. Brain Res. 79(3):539–546.

Bui, B. V., and Fortune, B. (2004). Ganglion cell contributions to the rat full-field electroretinogram. J. Physiol. 555(Pt. 1):153– 173.

Cenni, M. C., Bonfanti, L., Martinou, J.-C., Ratto, G. M., Strettoi, E., and Maffei, L. (1996). Long-term survival of

retinal ganglion cells following optic nerve section in adult I bcl-2 transgenic mice. Eur. J. Neurosci. 8:1735–1745.

Chierzi, S, Cenni, M. C., Maffei, L., Pizzorusso, T., Porciatti, V., Ratto, G. M., and Strettoi, E. (1998). Protection of retinal ganglion cells and preservation of function after optic nerve lesion in bcl-2 transgenic mice. Vision Res. 38:1537–1543.

Drasdo, N., Thompson, D. A., and Arden, G. B. (1990). A comparison of pattern ERG amplitudes and nuclear layer thickness in different zones of the retina. Clin. Vision Sci. 5(4): 415–420.

Drasdo, N., Thompson, D. A., Thompson, C. M., and Edwards, L. (1987). Complementary components and local variations of the pattern electroretinogram. Invest. Ophthalmol. Vis. Sci. 28(1):158–162.

Feghali, J. G., Jin, J. C., and Odom, J. V. (1991). Effect of shortterm intraocular pressure elevation on the rabbit electroretinogram. Invest. Ophthalmol. Vis. Sci. 32(8):2184–2189.

Feller, M. B., Wellis, D. P., Stellwagen, D., Werblin, F. S., and Shatz, C. J. (1996). Requirement for cholinergic synaptic transmission in the propagation of spontaneous retinal waves. Science 272(5265):1182–1187.

Fortune, B., Bui, B. V., Morrison, J. C., Johnson, E. C., Dong, J., Cepurna, W. O., Jia, L., Barber, S., and Cioffi, G. A. (2004). Selective ganglion cell functional loss in rats with experimental glaucoma. Invest. Ophthalmol. Vis. Sci. 45(6):1854–1862.

Fraunfelder, F. T., and Burns, R. P. (1970). Acute reversible lens opacity: Caused by drugs, cold, anoxia, asphyxia, stress, death and dehydration. Exp. Eye Res. 10(1):19–30.

Gianfranceschi, L., Fiorentini, A., and Maffei, L. (1999). Behavioural visual acuity of wild-type and bcl2 transgenic mouse. Vision Res. 39(3):569–574.

Hess, R. F., and Baker, C. L., Jr. Human pattern-evoked electroretinogram. J. Neurophysiol. 51(5):939–951.

Holder, G. E. (2001). Pattern electroretinography (PERG) and an integrated approach to visual pathway diagnosis. Prog. Retin. Eye Res. 20(4):531–561.

Huang, Z. J., Kirkwood, A., Pizzorusso, T., Porciatti, V., Morales, B., Bear, M. F., Maffei, L., and Tonegawa, S. (1999). BDNF regulates the maturation of inhibition and the critical period of plasticity in mouse visual cortex. Cell 98(6): 739–755.

Jakobs, T. C., Libby, R. T., Ben, Y., John, S. W., and Masland, R. H. (2005). Retinal ganglion cell degeneration is topological but not cell type specific in DBA/2J mice. J. Cell Biol. 171(2): 313–325.

John, S. W., Anderson, M. G., and Smith, R. S. (1999). Mouse genetics: A tool to help unlock the mechanisms of glaucoma. J. Glaucoma 8(6):400–412.

John, S. W., Smith, R. S., Savinova, O. V., Hawes, N. L., Chang, B., Turnbull, D., Davisson, M., Roderick, T. H., and Heckenlively, J. R. (1998). Essential iris atrophy, pigment dispersion, and glaucoma in DBA/2J mice. Invest. Ophthalmol. Vis. Sci. 39(6):951–962.

Kerrigan-Baumrind, L. A., Quigley, H. A., Pease, M. E., Kerrigan, D. F., and Mitchell, R. S. (2000). Number of ganglion cells in glaucoma eyes compared with threshold visual field tests in the same persons. Invest. Ophthalmol. Vis. Sci. 41(3):741–748.

Kline, R. P., Ripps, H., and Dowling, J. E. (1978). Generation of b-wave currents in the skate retina. Proc. Natl. Acad. Sci. U.S.A. 75(11):5727–5731.

Levkovitch-Verbin, H. (2004). Animal models of optic nerve diseases. Eye 18(11):1066–1074.

Libby, R. T., Anderson, M. G., Pang, I. H., Robinson, Z. H., Savinova, O. V., Cosma, I. M., Snow, A., Wilson, L. A., Smith, R. S., et al. (2005a). Inherited glaucoma in DBA/2J mice: Pertinent disease features for studying the neurodegeneration. Vis. Neurosci. 22(5):637–648.

Libby, R. T., Gould, D. B., Anderson, M. G., and John, S. W. (2005b). Complex genetics of glaucoma susceptibility. Annu. Rev. Genomic. Hum. Genet. 6:15–44.

Libby, R. T., Li, Y., Savinova, O. V., Barter, J., Smith, R. S., Nickells, R. W., and John, S. W. (2005c). Susceptibility to neurodegeneration in a glaucoma is modified by Bax gene dosage. PLoS. Genet. 1(1):17–26.

Libby, R. T., Porciatti, V., Tapia, M., Lee, R. K., and John, S. W. M. (2006). PERG analysis detects physiological dysfunction prior to ganglion cell loss in DBA/2J glaucoma. ARVO e-abstract 4005.

Maffei, L., and Fiorentini, A. (1981). Electroretinographic responses to alternating gratings before and after section of the optic nerve. Science 211(4485):953–955.

Maffei, L., Fiorentini, A., Bisti, S., and Hollander, H. (1985). Pattern ERG in the monkey after section of the optic nerve. Exp. Brain Res. 59(2):423–425.

Martinou, J. C., Dubois-Dauphin, M., Staple, J. K., Rodriguez, I., Frankowski, H., Missotten, M., Albertini, P., Talabot, D., Catsicas, S., et al. (1994). Overexpression of BCL-2 in transgenic mice protects neurons from naturally occurring cell death and experimental ischemia. Neuron 13(4):1017–1030.

Muir-Robinson, G., Hwang, B. J., and Feller, M. B. (2002). Retinogeniculate axons undergo eye-specific segregation in the absence of eye-specific layers. J. Neurosci. 22(13):5259–5264.

Nagaraju, M., Saleh, M., and Porciatti, V. (2007). IOPdependent retinal ganglion cell dysfunction in glaucomatous DBA/2J mice. Invest. Ophthalmol. Vis. Sci. 48:4573–4579.

Parisi, V. (2003). Correlation between morphological and functional retinal impairment in patients affected by ocular hypertension, glaucoma, demyelinating optic neuritis and Alzheimer’s disease. Semin. Ophthalmol. 18(2):50–57.

Picciotto, M. R., Zoli, M., Lena, C., Bessis, A., Lallemand, Y., Le Novere, N., Vincent, P., Pich, E. M., Brulet, P., et al. (1995). Abnormal avoidance learning in mice lacking functional high-affinity nicotine receptor in the brain. Nature 374(6517): 65–67.

Porciatti, V. (2007). The mouse pattern electroretinogram. Doc. Ophthalmol., epub ahead of print.

Porciatti, V., and Falsini, B. (2000). Maturation of flash-cone ERG and pattern ERG in the mouse. ARVO abstract 500.

Porciatti, V., and Falsini, B. (2003). Physiological properties of the mouse pattern electroretinogram. ARVO abstract 2705.

Porciatti, V., Pizzorusso, T., Cenni, M. C., and Maffei, L. (1996). The visual response of retinal ganglion cells is not altered by optic nerve transection in transgenic mice overexpressing

Bcl-2. Proc. Natl. Acad. Sci. U.S.A. 93(25):14955–14959. Porciatti, V., Pizzorusso, T., and Maffei, L. (1999a). Vision in

mice with neuronal redundancy due to inhibition of developmental cell death. Vis. Neurosci. 16(4):721–726.

Porciatti, V., Pizzorusso, T., and Maffei, L. (1999b). The visual physiology of the wild type mouse determined with pattern VEPs. Vision Res. 39(18):3071–3081.

Porciatti, V., Pizzorusso, T., and Maffei, L. (2002). Electrophysiology of the postreceptoral visual pathway in mice. Doc. Ophthalmol. 104(1):69—82.

Porciatti, V., Saleh, M., and Nagaraju, M. (2007). The pattern electroretinogram as a tool to monitor progressive retinal

ganglion cell dysfunction in the DBA/2J mouse model of glaucoma. Invest. Ophthalmol. 48(2):745–751.

Porciatti, V., and Ventura, L. M. (2004). Normative data for a user-friendly paradigm for pattern electroretinogram recording. Ophthalmology 111(1):161–168.

Prusky, G. T., Alam, N. M., Beekman, S., and Douglas, R. M. (2004). Rapid quantification of adult and developing mouse spatial vision using a virtual optomotor system. Invest. Ophthalmol. Vis. Sci. 45(12):4611–4616.

Prusky, G. T., and Douglas, R. M. (2004). Characterization of mouse cortical spatial vision. Vision. Res. 44(28):3411–3418.

Remtulla, S., and Hallett, P. E. (1985). A schematic eye for the mouse, and comparisons with the rat. Vision Res. 25(1): 21–31.

Rossi, F. M., Pizzorusso, T., Porciatti, V., Marubio, L. M., Maffei, L., and Changeux, J. P. (2001). Requirement of the nicotinic acetylcholine receptor beta 2 subunit for the anatomical and functional development of the visual system. Proc. Natl. Acad. Sci. U.S.A. 98(11):6453–6458.

Saleh, M., Nagaraju, M., and Porciatti, V. (2007). Longitudinal evaluation of retinal ganglion cell function and IOP in the DBA/2J mouse model of glaucoma. Invest. Ophthalmol. Vis. Sci. 48:4564–5472.

Saszik, S. M., Robson, J. G., and Frishman, L. J. (2002). The scotopic threshold response of the dark-adapted electroretinogram of the mouse. J. Physiol. 543(Pt. 3):899–916.

Schmucker, C., Seeliger, M., Humphries, P., Biel, M., and Schaeffel, F. (2005). Grating acuity at different luminances in wild-type mice and in mice lacking rod or cone function. Invest. Ophthalmol. Vis. Sci. 46(1):398–407.

Siliprandi, R., Bucci, M. G., Canella, R., and Carmignoto, G. (1988). Flash and pattern electroretinograms during and after acute intraocular pressure elevation in cats. Invest. Ophthalmol. Vis. Sci. 29(4):558–565.

Strettoi, E., and Volpini, M. (2002). Retinal organization in the bcl-2-overexpressing transgenic mouse. J. Comp. Neurol. 446(1):1–10.

Trimarchi, C., Biral, G., Domenici, L., Porciatti, V., and Bisti, S. (1990). The flashand pattern electroretinogram generators in the cat: A pharmacological approach. Clin. Vision Sci. 6: 19–24.

Van der List, D. A., Coombs, J. L., and Chalupa, L. M. (2006). Normal development of retinal ganglion cell morphological properties in mice lacking the beta2 subunit of the nicotinic acetylcholine receptor. ARVO abstract 3313.

Ventura, L. M., and Porciatti, V. (2005). Restoration of retinal ganglion cell function in early glaucoma after intraocular pressure reduction: A pilot study. Ophthalmology 112(1):20–27.

Ventura, L. M., and Porciatti, V. (2006). Pattern electroretinogram in glaucoma. Curr. Opin. Ophthalmol. 17(2):196–202.

Ventura, L. M., Sorokac, N., De Los Santos, R., Fener, W. J., and Porciatti, V. (2006). The relationship between retinal ganglion cell function and retinal nerve fiber thickness in early glaucoma. Invest. Ophthalmol. Vis. Sci. 47(9):3904–3911.

Viswanathan, S., Frishman, L. J., and Robson, J. G. (2000). The uniform field and pattern ERG in macaques with experimental glaucoma: Removal of spiking activity. Invest. Ophthalmol. Vis. Sci. 41(9):2797–2810.

Wong, R. O. (1999). Retinal waves and visual system development.

Annu. Rev. Neurosci. 22:29–47.