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Hereditary retinal degenerations result from varied pathophysiologic mechanisms, all ultimately characterized by photoreceptor dysfunction and death. Hence, much research on these diseases has concentrated on the outer retina. Over the past decade or so increasing attention has focused on concomitant changes in complex inner retinal neural circuits that process visual signals for transmission to the brain. One striking abnormality develops before the ultimately profound anatomic disruption of the inner retina. Highly elevated spontaneous activity was first demonstrated in central nervous system visual centers in vivo by Drager and Hubel (1978), and subsequently has been confirmed in vitro, now in multiple animal models and by multiple investigators (see other contributions to this Research Topic). What evidence exists that this phenomenon occurs in human patients with retinal degeneration, and what is the ultimate effect of spontaneous hyperactivity in the output neurons, the retinal ganglion cells? Here I summarize abnormalities of visual perception among patients with retinal degeneration that may arise from hyperactivity. Next, I consider the disruption of neural encoding and anatomic connectivity that may result within the retina and in downstream visual centers of the brain. I then consider how specific characteristics of hyperactivity may distinguish various forms or stages of retinal degeneration, potentially helping in the near future to refine diagnosis and/or treatment choices for different patients. Finally, I review how consideration of these features may help optimize pharmacologic, gene, stem cell, prosthetic or other therapies to forestall visual loss or restore sight.

In a subset of hereditary retinal diseases, early photoreceptor degeneration causes rapidly progressive blindness in children. To better understand how retinal development may interact with degenerative processes, we compared spontaneous and light-evoked activity among retinal ganglion cells in rd1 and rd10 mice, strains with closely related retinal disease. In each, a mutation in the Pde6b gene causes photoreceptor dysfunction and death, but in rd10 mice degeneration starts after a peak in developmental plasticity of retinal circuitry and thereafter progresses more slowly. In vitro multielectrode action potential recordings revealed that spontaneous waves of correlated ganglion cell activity comparable to those in wild-type mice were present in rd1 and rd10 retinas before eye opening [postnatal day (P) 7 to P8]. In both strains, spontaneous firing rates increased by P14–P15 and were many times higher by 4–6 wk of age. Among rd1 ganglion cells, all responses to light had disappeared by ∼P28, yet in rd10 retinas vigorous ON and OFF responses were maintained well beyond this age and were not completely lost until after P60. This difference in developmental time course separates mechanisms underlying the hyperactivity from those that alter light-driven responses in rd10 retinas. Moreover, several broad physiological groups of cells remained identifiable according to response polarity and time course as late as P60. This raises hope that visual function might be preserved or restored despite ganglion cell hyperactivity seen in inherited retinal degenerations, particularly if treatment or manipulation of early developmental plasticity were to be timed appropriately.

Complex alterations in the anatomy of outer retinal pathways accompany photoreceptor degeneration in the rd1 mouse model of retinitis pigmentosa, whereas inner retinal neurons appear relatively preserved. However, the progressive loss of photoreceptor input likely alters the neural circuitry of the inner retina. This study investigated resulting changes in the activity of surviving ganglion cells. Multielectrode recording monitored spontaneous and light-evoked extracellular action potentials simultaneously from 30 to 90 retinal ganglion cells of wild-type ( wt) or rd1 mice. In rd1 mice, this activity evolves through three phases. First, normal spontaneous “waves” of correlated firing are seen at postnatal day 7 (P7) and last until shortly after eye opening. Second, at P14, full-field light flashes evoke reliable responses in many cells, with preferential preservation of off responses. These diminish as photoreceptor degeneration progresses. Third, once light-evoked responses have disappeared in early adulthood, surviving rd1 ganglion cells fire at a much higher spontaneous frequency than normal, sometimes in rhythmic bursts that are distinct from the developmental “waves.” This hyperactivity is sustained well into adulthood, for weeks after photoreceptors have disappeared. Thus striking alterations occur in inner retinal physiology as retinal degeneration progresses in the rd1 mouse. Blindness occurs in the face of sustained hyperactivity among ganglion cells, which remain viable for months despite this activity. On and off responses are differentially affected in early stages of degeneration. While the source of these changes remains to be learned, such features should be considered in designing more effective treatments for these disorders.

The distributions of excitatory and inhibitory synapses upon the dendritic arbor of a direction-selective retinal ganglion cell were compared by triple-labeling techniques. The dendrites were visualized by confocal microscopy after injection of Lucifer yellow. Excitatory inputs from bipolar cells were located by using antibodies against kinesin II, a component of synaptic ribbons. Inhibitory inputs were identified by antibodies against γ-aminobutyric acid-A receptors. The combined images were examined by visual inspection and by formal, automated analyses, in a search for anisotropies that might contribute to a directional preference of the ganglion cell. Within the limits of our analysis, none was found. If an anatomic specialization underlies direction selectivity, it appears to lie in the geometry and spatial positioning of the neurons afferent to the ganglion cell and/or the microcircuitry among its afferent synapses. J. Comp. Neurol. 449:195–205, 2002. © 2002 Wiley-Liss, Inc.

Purpose Traumatic brain injury (TBI) frequently leads to chronic visual dysfunction. The purpose of this study was to investigate the effect of TBI on retinal ganglion cells (RGCs), and to test whether treatment with the novel neuroprotective compound P7C3-S243 could prevent in vivo functional deficits in the visual system. Methods Blast-mediated TBI was modeled using an enclosed over-pressure blast chamber. The RGC physiology was evaluated using a multielectrode array and pattern electroretinogram (PERG). Histological analysis of RGC dendritic field and cell number were evaluated at the end of the study. Visual outcome measures also were evaluated based on treatment of mice with P7C3-S243 or vehicle control. Results We show that deficits in neutral position PERG after blast-mediated TBI occur in a temporally bimodal fashion, with temporary recovery 4 weeks after injury followed by chronically persistent dysfunction 12 weeks later. This later time point is associated with development of dendritic abnormalities and irreversible death of RGCs. We also demonstrate that ongoing pathologic processes during the temporary recovery latent period (including abnormalities of RGC physiology) lead to future dysfunction of the visual system. We report that modification of PERG to provocative postural tilt testing elicits changes in PERG measurements that correlate with a key in vitro measures of damage: the spontaneous and light-evoked activity of RGCs. Treatment with P7C3-S243 immediately after injury and throughout the temporary recovery latent period protects mice from developing chronic visual system dysfunction. Conclusions Provocative PERG testing serves as a noninvasive test in the living organism to identify early damage to the visual system, which may reflect corresponding damage in the brain that is not otherwise detectable by noninvasive means. This provides the basis for developing an earlier diagnostic test to identify patients at risk for developing chronic CNS and visual system damage after TBI at an earlier stage when treatments may be more effective in preventing these sequelae. In addition, treatment with the neuroprotective agent P7C3-S243 after TBI protects from visual system dysfunction after TBI.

Localized injections of 50 μM tetrodotoxin (TTX) in rat hippocampal slices blocked stimulus train-evoked electrographic seizures (EGSs) for several hours. Responses to single stimuli were minimally altered during TTX block of the EGSs. This selective reduction of epileptiform activity could result from general blockade of action potentials in an anatomically distinct group of neurons in the slice. To test this hypothesis, we systematically mapped TTX injection sites in the hippocampal slice, and found that TTX injections that blocked EGSs were nearly always located in or invaded CA2/3 stratum radiatum and/or stratum lacunosum-moleculare. A high degree of recurrent activity in this region contributes to both epileptiform activity and responses to single stimuli; hence our selective inhibition of EGSs suggests a more pharmacologically specific anticonvulsant effect of TTX. Consistent with this hypothesis, we found that low concentrations of TTX (5, 10, or 20 nM) in the perfusion medium blocked EGSs without decreasing the amplitude of extracellular responses to single stimuli. Polysynaptic activity and/or antidromic firing may be particularly vulnerable to TTX action on voltage-gated sodium channels, due to their lower the safety factor for action potential propagation. Selective reduction of this activity may disrupt the abnormal neuronal activity underlying EGSs.

1. Intracellular and extracellular recording techniques were used to study the increase in ectopic (i.e., nonsomatic) action-potential generation occurring among CA3 pyramidal cells during the kindling-like induction of electrographic seizures (EGSs) in this subpopulation of the hippocampal slice. Kindling-like stimulus trains (60 Hz, 2 s) were delivered to s. radiatum of CA3 at 10-min intervals. As EGSs developed, the frequency of ectopic firing increased markedly (by 10.33 +/- 3.29 spikes/min, mean +/- SE, P << 0.01). Several methods were applied to determine the initiation site for these action potentials within the cell (axons vs. dendrites). 2. Collision tests were conducted between known antidromic and orthodromic action potentials in CA3 cells to determine the critical period, c, for collision. Attempts were then made to collide ectopic spikes with known antidromic action potentials. At intervals less than c, ectopic spikes failed to collide with antidromic ones, in 5 of 10 cases. In these cells, this clearly indicates that the ectopic spikes were themselves of axonal origin. In the remaining five cases, ectopic spikes collided with antidromic action potentials at intervals approximately equal to c, most likely because of interactions within the complex system of recurrent axon collaterals in CA3. 3. Action potentials of CA3 pyramidal cells were simulated with the use of a compartmental computer model, NEURON. These simulations were based on prior models of CA3 pyramidal neurons and of the motoneuron action potential. Simulated action potentials generated in axonal compartments possessed a prominent inflection on their rising phase (IS-SD break), which was difficult to appreciate in those spikes generated in somatic or dendritic compartments. 4. An analysis of action potentials recorded experimentally from CA3 pyramidal cells also showed that antidromic spikes possess a prominent IS-SD break that is not present in orthodromic spikes. In addition to identified antidromic action potentials, ectopic spikes also possess such an inflection. Together with the predictions of computer simulations, this analysis also indicates that ectopic spikes originate in the axons of CA3 cells. 5. Tetrodotoxin (TTX, 50 microM) was locally applied by pressure injection while monitoring ectopic spike activity. Localized application of TTX to regions of the slice that could include the axons but not the dendrites of recorded cells abolished or markedly reduced the frequency of ectopic spikes (n = 5), further confirming the hypothesis that these action potentials arise from CA3 axons.(ABSTRACT TRUNCATED AT 400 WORDS)

We studied changes in the characteristics of action potentials of CA3 pyramidal cells accompanying electrographic seizure (EGS) induction, using an in vitro model similar to kindling. We found an increased propensity to fire action potentials which lack EPSPs or other triggering depolarizing potentials. Such ‘baseline spikes’ appear to be initiated at a site electrically distant from the soma.

Visual perceptions seen with retinal and optic nerve disease may overlap with those resulting from retrochiasmal disorders. A few disorders typically present with distinctive perceptions, but the majority have less specific symptoms. Features include whether or not the visual phenomena are negative or positive, monocular or binocular, and the location and form of any deficits. Among negative phenomena, transient visual loss usually is the result of ischemic disease, but particular precipitants may suggest demyelination or photoreceptor degeneration. The pattern and location of visual field defects may help localize disorders to the level of the macula, papillomacular or other inner retina nerve fiber bundles, optic nerve, or chiasm. Altered brightness perception may point to optic nerve or photoreceptor disease. Decreased acuity is among the most common and least specific symptoms, but association with other symptoms may help to narrow the differential diagnosis. Dyschromatopsia points to either a photoreceptor or optic nerve pathologic condition (Table 7). Among positive phenomena, hallucinations resulting from anterior visual system disorders typically are unformed, although deafferentation of retrochiasmal pathways may produce formed hallucinations. The common "floaters" frequently are benign, but occasionally herald more concerning disorders. Various types of photopsias commonly occur with vitreal disorders or photoreceptor disorders. Macular disease typically leads to distortions of the central visual field, and other particular disorders lead to a host of characteristic distortions of color, form, or brightness. Careful attention to the ophthalmologic examination, visual fields, and subtle variance in symptomatology also help to distinguish among various disorders.

PURPOSE In animal models of degenerative photoreceptor disease, there has been some success in restoring photoreception by transplanting stem cell-derived photoreceptor cells into the subretinal space. However, only a small proportion of transplanted cells develop extended outer segments, considered critical for photoreceptor cell function. The purpose of this study was to determine whether photoreceptor cells that lack a fully formed outer segment could usefully contribute to vision. METHODS Retinal and visual function was tested in wild-type and Rds mice at 90 days of age (Rds(P90)). Photoreceptor cells of mice homozygous for the Rds mutation in peripherin 2 never develop a fully formed outer segment. The electroretinogram and multielectrode recording of retinal ganglion cells were used to test retinal responses to light. Three distinct visual behaviors were used to assess visual capabilities: the optokinetic tracking response, the discrimination-based visual water task, and a measure of the effect of vision on wheel running. RESULTS Rds(P90) mice had reduced but measurable electroretinogram responses to light, and exhibited light-evoked responses in multiple types of retinal ganglion cells, the output neurons of the retina. In optokinetic and discrimination-based tests, acuity was measurable but reduced, most notably when contrast was decreased. The wheel running test showed that Rds(P90) mice needed 3 log units brighter luminance than wild type to support useful vision (10 cd/m(2)). CONCLUSIONS Photoreceptors that lack fully formed outer segments can support useful vision. This challenges the idea that normal cellular structure needs to be completely reproduced for transplanted cells to contribute to useful vision.

Detection of light serves many tasks. Independent of form vision, the detection of brightness (irradiance) is used to regulate pupil size,1 entrain circadian rhythms to the day-night cycle,2 and modulate endocrine function.3 Irradiance information is transmitted to central nuclei by a specialized subset of intrinsically photosensitive retinal ganglion cells (ipRGCs).4,5 The response of ipRGCs to light is a composite of their intrinsic melanopsin-generated activity and rod-cone photoreceptor-dependent activation mediated by interneurons of the inner retina.6 However, morphologic and functional specialization among ipRGCs has been identified, and these “ipRGC types” show variation in their projections to central nuclei.7–10 How these differences in ipRGC form, function, and central projections relate to a given irradiance response is not well understood. For instance, the pupillary light reflex and negative masking (a suppression of activity by light) both respond to “lights on” in an irradiance-dependent manner.1,11 However, the effect of outer retina degeneration differs dramatically between these irradiance responses, with a paradoxical enhancement of irradiance sensitivity in negative masking12 and a severe loss of sensitivity in the pupillary light reflex.1,13,14 At the level of the retina, such phenotypic variability could arise from secondary effects of outer retina disease on ipRGCs or from differences in rod-cone input to the different ipRGC types. Although inner retinal circuitry is diverse, all vertical transmission of rod and cone signals to ganglion cells is, at some point, mediated by bipolar cells (BCs).15 Therefore, differences between responses could arise from specificity in bipolar cells mediating rod-cone input. The aim of this study was to determine whether there are differences in inner retinal pathways mediating rod-cone input to functionally distinct irradiance responses and whether such differences could contribute to the divergent effects of outer retinal disease on the different irradiance responses. Negative masking and pupillary light reflex were compared between mice with intact inner-retinal circuitry (wild-type), mice selectively lacking the ON-BC pathway of rod-cone input (Nob4), and mice with degenerate rods and cones (rd1), effectively lacking input to both ON- and OFF-BCs (Fig. 1). Figure 1. Vertical transmission of rod-cone input to ganglion cells. Green: ON pathways. Blue: OFF pathways. Loss of function is indicated by gray shading or cell absence. (A) When rods and cones respond to light, metabotropic or ON-BCs depolarize, and ionotropic ...

1. The preceding report presented evidence that the kindling-like induction of electrographic seizures (EGSs) in the hippocampal slice is accompanied by a lasting increase in the excitability of CA3 axon terminals, which is manifested by an increase in action-potential initiation at this site. In this report we explore the role of the N-methyl-D-aspartate (NMDA) receptor in the induction and maintenance of this antidromic firing, as well as the role of the gamma-aminobutyric acid type A (GABAA) receptor in regulating this activity once it has been induced. 2. Kindling-like stimulus trains (60 Hz, 2 s) were delivered to s. radiatum of CA3 at 10-min intervals. As EGSs developed in control artificial cerebrospinal fluid (ACSF), the frequency of axon terminal firing increased markedly (by 10.33 +/- 3.29 spikes/min, mean +/- SE P << 0.01). The prior application of the competitive NMDA antagonist D-2-amino-5-phosphonovaleric acid (D-APV, 50 or 100 microM) prevented the induction of EGSs and suppressed the increase in terminal firing seen in control ACSF (mean increase 1.06 +/- 1.11 spikes/min, P < 0.02). However, when D-APV was applied only after EGSs and antidromic spikes were induced in control ACSF, it failed to alter the frequency of terminal firing (mean 6.44 +/- 2.03 in control ACSF, 8.89 +/- 2.31 in APV; P >> 0.1). Thus the NMDA receptor is required for the induction but not maintenance of increased axon terminal firing, as we previously have shown to be the case for EGSs.(ABSTRACT TRUNCATED AT 250 WORDS)

In the study of epilepsy, a great deal of attention has been devoted to synaptic mechanisms of seizure induction and expression. Many efforts have focused on understanding changes in the balance between synpatic excitation and inhibition. For example, the application of a number of excitatory amino acids can evoke epileptiform activity (Lehman et al., 1987; McCaslin and Morgan, 1986;. Meldrum, 1986; Piredda and Gale, 1986; Turski et al., 1987a,b), and, conversely, antagonists of excitatory amino acid receptors can suppress seizures or seizurelike activity in some models (Czuczwar et al., 1985; Czuczwar and Meldrum, 1982; De Sarro et al., 1985; Heinemann et al., 1985; Lehman et al., 1987; McNamara et al., 1988; Meldrum, 1986; Sagratella et al., 1987; Traynelis and Dingledine, 1988; Turski et al., 1987a; Walther et al., 1986). Changes in responses to excitatory amino acids are associated with pathological neuroplasticity and accompany both in vivo kindling and similar in vitro stimulation models of epileptogenesis (Mody et al., 1988; Stelzer et al., ss1987). In addition to altering the receptor mediation of synaptic responses, excitation within a neural network may be increased by altering the frequency and probability of synaptic transmission. Thus, in the hippocampal slice, the application of convulsant drugs, disturbances in extracellular ion concentrations, and tetanic stimulation which leads to epileptiform activity can all result in a greater number of excitatory synaptic interactions between cells.

All-or-none electrographic seizures (EGSs) were studied in hippocampal slices from young (21- to 38-day-old) rats in medium containing low (0 mM) or physiological (0.9 mM) levels of magnesium, with and without the GABAB agonist baclofen. Extracellular recording and stimulation were performed in stratum pyramidale and stratum radiatum of CA3, respectively. EGS activity was induced by exposure to low-Mg medium or by delivering repetitive stimulus trains in physiological Mg medium. After EGS activity had stabilized, the EGSs were tested for all-or-none behavior by varying the number of pulses in a train. An EGS was considered all-or-none if subthreshold stimulation produced no afterdischarge bursts, and if the EGS duration was largely independent of the number of suprathreshold stimulus pulses. According to this measure, EGSs in Mg-free + baclofen medium were all-or-none. EGSs evoked in physiological Mg medium were also all-or-none, although the threshold was higher, and the EGS duration lower, than in Mg-free medium. This all-or-none characteristic was observed whether the EGSs were induced by prior exposure to Mg-free medium or by repetitive stimulation, and in the presence and absence of baclofen. The all-or-none characteristic suggests that while the triggering mechanism for EGSs is strongly dependent on stimulus intensity, regenerative mechanisms — independent of stimulus intensity — are responsible for the maintenance of EGSs. EGSs are also terminated by mechanisms not dependent on stimulus intensity.

In this paper we present an in vitro model of epileptogenesis based on electrical stimulation rather than pharmacological or ionic manipulations. Hippocampal slices given a series of stimulus trains similar to those used in kindling exhibited 3 types of epileptiform activity in CA3: afterdischarges immediately following the trains; spontaneous bursts of multiple population spikes; and bursts triggered by single stimuli. The afterdischarges and spontaneous bursts may be comparable to those seen in vivo during kindling; also, the progression of these features in this model was similar to their progression during kindling. All epileptiform activities were long-lasting, persisting for up to 3.5 h following the last train. This stimulus train-induced population bursting should be valuable as an acute model of hippocampal epileptogenesis, and may also help elucidate hippocampal participation in the kindling process.