Biomedical subjects

Laura J Frishman

Publications and source records attributed to Laura J Frishman.

At least 19 recordsLinked to original sources

PubMed2016

Substituting mouse transcription factor Pou4f2 with a sea urchin orthologue restores retinal ganglion cell development.

Pou domain transcription factor Pou4f2 is essential for the development of retinal ganglion cells (RGCs) in the vertebrate retina. A distant orthologue of Pou4f2 exists in the genome of the sea urchin (class Echinoidea) Strongylocentrotus purpuratus (SpPou4f1/2), yet the photosensory structure of sea urchins is strikingly different from that of the mammalian retina. Sea urchins have no obvious eyes, but have photoreceptors clustered around their tube feet disc. The mechanisms that are associated with the development and function of photoreception in sea urchins are largely unexplored. As an initial approach to better understand the sea urchin photosensory structure and relate it to the mammalian retina, we asked whether SpPou4f1/2 could support RGC development in the absence of Pou4f2. To answer this question, we replaced genomic Pou4f2 with an SpPou4f1/2 cDNA. In Pou4f2-null mice, retinas expressing SpPou4f1/2 were outwardly identical to those of wild-type mice. SpPou4f1/2 retinas exhibited dark-adapted electroretinogram scotopic threshold responses, indicating functionally active RGCs. During retinal development, SpPou4f1/2 activated RGC-specific genes and in S. purpuratus, SpPou4f2 was expressed in photoreceptor cells of tube feet in a pattern distinct from Opsin4 and Pax6. Our results suggest that SpPou4f1/2 and Pou4f2 share conserved components of a gene network for photosensory development and they maintain their conserved intrinsic functions despite vast morphological differences in mouse and sea urchin photosensory structures.

Animals↗

PubMed2006

Effect of experimental glaucoma in primates on oscillatory potentials of the slow-sequence mfERG.

PURPOSE: To determine the effect of experimental glaucoma in macaque monkeys on oscillatory potentials (OPs) in the slow-sequence multifocal electroretinogram (mfERG). METHODS: Photopic slow-sequence mfERGs were recorded from anesthetized adult macaque monkeys and normal human subjects. The stimulus consisted of 103 equal-sized hexagons within 17 degrees of the fovea. The m-sequence was slowed, with 14 blank frames, approximately 200 ms, interleaved between flashes for monkeys and 7 blank frames, approximately 100 ms, for humans, to produce waveforms similar to the photopic full-field flash ERG. Recordings were made under control conditions (24 monkey eyes, 7 human) and after laser-induced experimental glaucoma in monkeys (n = 8). A Fourier fast transform [FFT] was used to determine the frequency ranges of the major OPs. OP amplitudes were quantified by using root mean square (RMS) for two-frequency bands in five horizontal and four vertical locations. Visual field defects were assessed using behavioral static perimetry. Full-field photopic flash ERGs also were recorded. RESULTS: OPs in two distinct frequency bands were discriminated in the monkey mfERG: fast OPs, with a peak frequency of 143 +/- 20 Hz, and slow OPs, with a peak at 77 +/- 8 Hz. There were similar findings in humans and with the flash ERG in monkeys. The fast OP RMS in monkey control eyes was significantly larger in temporal than nasal retina (P < 0.01) and in superior versus inferior retina (P < 0.05) as reported previously. The slow OP RMS was largest in the foveal region. Experimental glaucoma reduced fast OP RMS in all locations studied, even when visual field defects were moderate (MD = -5 to -10 dB; P < 0.05), whereas the slow OP RMS was reduced significantly primarily in the foveal region when field defects were severe (MD < -10 dB; P < 0.05). The fast OP RMS showed a moderate correlation with local visual field sensitivity and with local ganglion cell density (calculated from visual field sensitivity). For the slow OPs the correlation was much poorer. Consistent with previous studies, the photopic negative response (PhNR) amplitude was significantly reduced when the visual sensitivity was minimally affected. CONCLUSIONS: OPs in the ERG of primates fall in two frequency bands: fast OPs with a peak frequency around 143 Hz and slow OPs, with a peak frequency around 77 Hz. The fast OPs, which rely more on the integrity of retinal ganglion cells and their axons than do the slow OPs, have potential utility for monitoring the progression of glaucoma and the effects of treatment.

Animals↗

PubMed2005

Intrinsically photosensitive retinal ganglion cells detect light with a vitamin A-based photopigment, melanopsin.

In mammals, intrinsically photosensitive retinal ganglion cells (ipRGCs) mediate non-image-forming visual functions such as pupillary light reflex (PLR) and circadian photoentrainment. This photosensitivity requires melanopsin, an invertebrate opsin-like protein expressed by the ipRGCs. The precise role of melanopsin remains uncertain. One suggestion has been that melanopsin may be a photoisomerase, serving to regenerate an unidentified pigment in ipRGCs. This possibility was echoed by a recent report that melanopsin is expressed also in the mouse retinal pigment epithelium (RPE), a key center for regeneration of rod and cone pigments. To address this question, we studied mice lacking RPE65, a protein essential for the regeneration of rod and cone pigments. Rpe65-/- ipRGCs were approximately 20- to 40-fold less photosensitive than normal at both single-cell and behavioral (PLR) levels but were rescued by exogenous 9-cis-retinal (an 11-cis-retinal analog), indicating the requirement of a vitamin A-based chromophore for ipRGC photosensitivity. In contrast, 9-cis-retinal was unable to restore intrinsic photosensitivity to melanopsin-ablated ipRGCs, arguing against melanopsin functioning merely in photopigment regeneration. Interestingly, exogenous all-trans-retinal was also able to rescue the low sensitivity of rpe65-/- ipRGCs, suggesting that melanopsin could be a bistable pigment. Finally, we detected no melanopsin in the RPE and no changes in rod and cone sensitivities due to melanopsin ablation. Together, these results strongly suggest that melanopsin is the photopigment in the ipRGCs.

Animals↗

PubMed2005

Ganglion cells are required for normal progenitor- cell proliferation but not cell-fate determination or patterning in the developing mouse retina.

The vertebrate retina develops from an amorphous sheet of dividing retinal progenitor cells (RPCs) through a sequential process that culminates in an exquisitely patterned neural tissue. A current model for retinal development posits that sequential cell-type differentiation is the result of changes in the intrinsic competence state of multipotent RPCs as they advance in time and that the intrinsic changes are influenced by continuous changes in the extracellular environment. Although several studies support the proposition that newly differentiated cells alter the extrinsic state of the developing retina, it is still far from clear what role they play in modifying the extracellular environment and in influencing the properties of RPCs. Here, we specifically ablate retinal ganglion cells (RGCs) as they differentiate, and we determine the impact of RGC absence on retinal development. We find that RGCs are not essential for changing the competence of RPCs, but they are necessary for maintaining sufficient numbers of RPCs by regulating cell proliferation via growth factors. Intrinsic rather than extrinsic factors are likely to play the critical roles in determining retinal cell fate.

Animals↗

PubMed2004

Regulation of retinal cone bipolar cell differentiation and photopic vision by the CVC homeobox gene Vsx1.

Cone bipolar cells of the vertebrate retina connect photoreceptors with ganglion cells to mediate photopic vision. Despite this important role, the mechanisms that regulate cone bipolar cell differentiation are poorly understood. VSX1 is a CVC domain homeoprotein specifically expressed in cone bipolar cells. To determine the function of VSX1, we generated Vsx1 mutant mice and found that Vsx1 mutant retinal cells form but do not differentiate a mature cone bipolar cell phenotype. Electrophysiological studies demonstrated that Vsx1 mutant mice have defects in their cone visual pathway, whereas the rod visual pathway was unaffected. Thus, Vsx1 is required for cone bipolar cell differentiation and regulates photopic vision perception.

Animals↗

PubMed2004

Sampling and interpolation of the a-wave of the electroretinogram.

UNLABELLED: This study was undertaken (a) to determine the minimum sampling frequency required to record a-waves evoked by flashes of very high energy without significant distortion and (b) to demonstrate that data sampled at a minimally adequate frequency can be interpolated to reconstruct the original waveform. METHODS: Dark-adapted ERGs from two anaesthetized macaque monkeys and an adult human were studied. Responses evoked by high-energy flashes that produced a-waves peaking as early as 5 or 6 ms after the flash were sampled at 5 kHz and transformed to obtain their discrete Fourier spectra. The amplitude of all spectral components above some cut-off frequency (e.g., 400 Hz) was then set to zero and the modified spectra transformed back into the time domain. The resulting computed responses, which contained no Fourier components above the cut-off frequency, were compared with the original recorded samples. To assess the validity of one method of interpolation, sample sets consisting of every fifth point of records sampled at 5 kHz (i.e., sets of 1 kHz samples) were subjected to Fourier transformation to give spectra with a frequency range of 0-500 Hz. These spectra were extended from 500 Hz up to a much higher frequency (e.g., 8 kHz) by adding zeros. The extended spectra were transformed back into the time domain to provide sets of interpolated samples at twice the chosen spectral frequency limit (i.e., 16 kHz). RESULTS: Removing all Fourier components above 400 or 500 Hz had no significant effect upon the leading edge or peak of the a-wave. However, removing Fourier components above 500 Hz gave rise to slight distortion of the oscillatory potentials (OPs) that appeared just after the a-wave peak on the leading edge of the b-wave. Except for this small distortion, the original 5 kHz data samples corresponded very well with the interpolated curves that had been generated as described above from a 1 kHz subset of the samples. This provides further confirmation that dark-adapted ERG a-waves evoked by flashes of up to about 50,000 sc. Td sc do not contain Fourier components with frequencies above 500 Hz. CONCLUSION: Human and macaque a-waves are completely represented by 1 kHz samples and Fourier methods can be used to reconstruct the original continuous waveform. However, to capture the OPs with complete fidelity, a higher sampling rate is necessary.

Adult↗

PubMed2004

VSX1 (RINX) mutation with craniofacial anomalies, empty sella, corneal endothelial changes, and abnormal retinal and auditory bipolar cells.

PURPOSE: To present a previously unreported African American family with 1 variation and 1 mutation of the homeobox transcription factor gene, VSX1 (RINX), and to describe the clinical features of family members. DESIGN: Family genotype and clinical studies. PARTICIPANTS: A 3-generation family with 7 available family members. METHODS: Blood was drawn from all available family members, and the VSX1 (RINX) gene was sequenced. Craniofacial abnormalities, central nervous system defects, anterior segment features, and retinal and auditory function were assessed. MAIN OUTCOME MEASURES: Main outcome measures included identification and molecular characterization of 1 variation and 1 mutation in VSX1 (RINX) of 4 affected family members (3 adults and 1 child). Craniofacial features were documented. Central neuroimaging was performed. Ophthalmologic findings were described. Retinal and auditory functions were quantified. RESULTS: Two changes in VSX1 (RINX) were identified: a variation (R131S) not in a critical region and in few controls, and a mutation (A256S) in the critical CVC-domain and not in any controls. Both were present on 1 chromosome at 20p11.2 and were segregated with the 4 affected patients. Clinical features demonstrated extremely variable expressivity. Craniofacial features, including wide interpupillary distance and unusual pinnae, occurred in the 4 affected patients. Neuroimaging demonstrated that the propositus had an empty sella turcica, a posterior fossa cyst, an anterior encephalocele, hypertelorism, and severe hydrocephalus; her mother had a partially empty sella turcica, a small pituitary gland without any subarachnoid extension of fluid, and hypertelorism; and her older sister had hypertelorism but otherwise normal neuroimaging results. Anterior segment anomalies of the corneal endothelium were a constant finding in all affected family members. Electrophysiologic examination provided evidence for abnormal cone bipolar cells (visual evoked response and electroretinogram) in the adult affected patients and for abnormal auditory bipolar cells (audiogram and audio-evoked brainstem response) in the propositus. CONCLUSIONS: The new mutation in the VSX1 (RINX) gene described in this report results in abnormal craniofacial features, absence of the roof of the sella turcica, and anomalous development of the corneal endothelium. This mutation also impacts on the maintenance of cone bipolar cells of the visual system and of bipolar cells of the auditory system.

Abnormalities, Multiple↗

PubMed2004

Effects of pirenzepine on pupil size and accommodation in rhesus monkeys.

PURPOSE: Pirenzepine is suggested to be a relatively selective muscarinic (M(1)) antagonist and is currently under investigation for the treatment of myopia. Atropine, a nonselective M-type antagonist, is used in the treatment of myopia, but has undesired ocular and systemic side effects. An M(1)-specific antagonist may decrease side effects and remain effective at reducing the progression of myopia. In the current study, the effects of pirenzepine on pupil diameter, resting refraction, and accommodation were studied in rhesus monkeys. METHODS: The time course and extent of mydriasis from subconjunctival injection of 2% pirenzepine were determined in five normal rhesus monkeys, and the effects on static and dynamic accommodation were determined in four rhesus monkeys with permanent indwelling electrodes in the Edinger-Westphal (EW) nucleus of the midbrain. Subconjunctival injections of 0.0002% to 0.2% pirenzepine in log unit dilutions were tested in three monkeys to determine the effects on static EW-stimulated accommodation. At 40 to 50 minutes after pirenzepine injection, accommodation was stimulated pharmacologically in both eyes, and the response was measured for 30 minutes. RESULTS: After 2% pirenzepine injection, pupil size increased 2.02 +/- 0.41 mm, there was a hyperopic shift in resting refraction of 1.07 +/- 0.23 D, and nearly complete cycloplegia occurred. Maximum EW-stimulated accommodation was significantly decreased 20 to 40 minutes after 0.02% or greater pirenzepine. Carbachol-stimulated accommodation was significantly decreased after 0.2% or greater pirenzepine. CONCLUSIONS: Subconjunctival injections of 0.02% or greater pirenzepine result in a significant decrease in accommodation and are probably acting through nonselective muscarinic antagonism. Subconjunctival injections of 0.002% or less pirenzepine do not decrease EW-stimulated accommodation.

Accommodation, Ocular↗

PubMed2004

Photopic ERGs in patients with optic neuropathies: comparison with primate ERGs after pharmacologic blockade of inner retina.

PURPOSE: To determine whether anterior ischemic optic neuropathy and compressive optic neuropathy in humans alter the photopic flash ERG and to investigate the cellular origins of the waves that are affected by pharmacologic agents in primates. METHODS: Photopic flash ERGs were recorded differentially, with DTL electrodes, between the two eyes of 22 patients with diagnosed optic neuropathy (n = 17, anterior ischemic optic neuropathy [AION]; n = 5, compressive optic neuropathy) and 25 age-matched control subjects and in 17 eyes of 13 monkeys (Macaca mulatta). The stimulus consisted of brief (<5 ms) red (lambda(max) = 660 nm) Ganzfeld flashes (energy range, 0.5-2.0 log td-s) delivered on a rod-saturating blue background of 3.7 log sc td (lambda(max) = 460 nm). An eye of the patient with ischemic changes at the disc was classified as symptomatic if it showed visual field defects with a mean deviation (MD) of P < 2%. Recordings in macaque monkeys were made before and after inner retinal blockade with tetrodotoxin (TTX) (1.2-2.1 microM; n = 7), TTX+N-methyl-d-aspartate (NMDA; 1.4-6.4 mM; n = 7), and cis-2, 3 piperidine dicarboxylic acid (PDA; 3.3-3.8 mM; n = 3). RESULTS: The PhNR amplitude was significantly reduced in both symptomatic (P = 3.4 x 10(-8)) and asymptomatic (P = 0.036) eyes of patients with AION or compressive optic neuropathy (P = 0.0054) compared with control subjects. The PhNR amplitude in the symptomatic eye showed a moderate correlation with field defects (P < 0.05) similar to previous findings in open-angle glaucoma. The a-wave also was reduced significantly in the symptomatic eye (P = 0.0002) of patients with AION. The i-wave, a positive wave on the trailing edge of the b-wave peaking around 50 ms, became more prominent in eyes in which the PhNR was significantly reduced. In monkeys, the PhNR was eliminated by TTX. The a-wave at the peak and later times was reduced by TTX, further reduced by NMDA, and eliminated after PDA in response to the red stimuli. PDA also eliminated the i-wave. CONCLUSIONS: PhNR amplitude is significantly reduced in eyes with open-angle glaucoma, AION, and compressive optic neuropathy. Experiments in primates indicate that this reduction reflects loss of a spike-driven contribution to the photopic ERG. There also are small spike-driven contributions to the a-wave elicited by full-field red stimuli. The i-wave, which becomes more prominent when the PhNR is reduced, has origins in the off-pathway distal to the ganglion cells.

Adult↗

PubMed2003

Differential distribution of vesicle associated membrane protein isoforms in the mouse retina.

PURPOSE: Many proteins associated with synaptic vesicle exocytosis are differentially distributed among synapses in the retina and elsewhere in the central nervous system. The synapse-specific distribution of these proteins and their isoforms is thought to contribute to synapse-specific functional differences. Vesicle-associated membrane protein (VAMP, also known as synaptobrevin) is an integral synaptic vesicle membrane protein that is part of the fusion core complex needed for docking and fusing of synaptic vesicles at the synaptic active zone. Two VAMP isoforms have been identified that are considered to be synaptic, VAMP-1 and VAMP-2, however their distributions among the various synapses in the mammalian retina have not been characterized. METHODS: Single- and double-labeling immunocytochemistry was used to investigate the distribution of the synaptic VAMP isoforms, VAMP-1 and VAMP-2, in the mouse retina. RESULTS: VAMP-2 was the predominant isoform in both synaptic layers. Double-labeling studies using conventional and ribbon-synapse-specific markers showed that VAMP-2 was broadly distributed among conventional and ribbon synapses. In contrast, the distribution of VAMP-1 was very limited. In the outer retina, only weak labeling was present in photoreceptor terminals. In the inner retina, labeling for VAMP-1 was found in the dendrites, cell bodies, and axons of some ganglion cells, as demonstrated by double labeling with the ganglion cell markers, microtubule-associated protein-1 and Brn-3a. VAMP-1 labeling did not colocalize with amacrine or bipolar cell markers, nor did it colocalize with other pre-synaptic markers, suggesting that VAMP-1 is not associated directly with neurotransmitter release in the inner retina. Labeling for VAMP-1 identified a set of large ganglion cells that ramified in the mid-IPL (inner plexiform layer), suggesting that they may show ON-OFF responses. Some of these cells had cell bodies displaced to the inner nuclear layer. The dendrites of the large VAMP-1-immunoreactive ganglion cells did not co-stratify with the cholinergic plexuses of the starburst amacrine cells (labeled for choline acetyltransferase) and therefore are unlikely to show directional selectivity. However, these cells are likely to receive input from bipolar cells and a population of putative glutamatergic amacrine cells. CONCLUSIONS: VAMP-1 and VAMP-2 are differentially distributed among the synapses of the mouse retina. VAMP-2 is the predominant isoform and is widely expressed at ribbon and conventional synapses in both plexiform layers. VAMP-1 expression in the mouse retina is much more limited and is not restricted to presynaptic terminals. In the OPL, VAMP-1 is co-expressed with VAMP-2 presynaptically in photoreceptor terminals. However, VAMP-1 expression in the IPL is associated with ganglion cells and does not appear to be localized to presynaptic terminals. VAMP-1 is a specific marker for a set of large ganglion cells and displaced ganglion cells that ramify in the mid-IPL and are likely to have ON-OFF physiology.

Animals↗

PubMed2003

Expression of vesicular glutamate transporter 1 in the mouse retina reveals temporal ordering in development of rod vs. cone and ON vs. OFF circuits.

Glutamatergic transmission is crucial to the segregation of ON and OFF pathways in the developing retina. The temporal sequence of maturation of vesicular glutamatergic transmission in rod and cone photoreceptor and ON and OFF bipolar cell terminals is currently unknown. Vesicular glutamate transporters (VGLUTs) that load glutamate into synaptic vesicles are necessary for vesicular glutamatergic transmission. To understand better the formation and maturation of glutamatergic transmission in the rod vs. cone and ON vs. OFF pathways of the retina, we examined the developmental expression of VGLUT1 and VGLUT2 immunocytochemically in the mouse retina. Photoreceptor and bipolar cell terminals showed only VGLUT1-immunoreactivity (-IR); no VGLUT2-IR was present in any synapses of the developing or adult retina. VGLUT1-IR was first detected in cone photoreceptor terminals at postnatal day 2 (P2), several days before initiation of ribbon synapse formation at P4-P5. Rod terminals showed VGLUT1-IR by P8, when they invade the outer plexiform layer (OPL) and initiate synaptogenesis. Developing OFF bipolar cell terminals showed VGLUT1-IR around P8, 2-3 days after bipolar terminals were first identified in the inner plexiform layer (IPL) by labeling for the photoreceptor and bipolar cell terminal marker, synaptic vesicle protein 2B. Although terminals of ON bipolar cells were present in the IPL by P6-P8, most did not show VGLUT1-IR until P8-P10 and increased dramatically from P12. These data suggest a hierarchical development of glutamatergic transmission in which cone circuits form prior to rod circuits in both the OPL and IPL, and OFF circuits form prior to ON circuits in the IPL.

Aging↗

PubMed2003

Differential distribution and developmental expression of synaptic vesicle protein 2 isoforms in the mouse retina.

Synaptic vesicle protein 2 (SV2), a ubiquitous synaptic vesicle protein, is known to participate in the regulation of Ca(2+)-mediated synaptic transmission, although its precise function has not been established. Three SV2 isoforms (SV2A, SV2B, SV2C) have been identified recently, each of which has a unique distribution in brain, suggesting synapse-specific functions. To determine if SV2A, -B, and -C are differentially distributed among synapses in the retina and the sequence of their development, we examined their distribution and expression patterns immunocytochemically in adult and developing mouse retina. The three SV2 isoforms were differentially distributed in the synapses of the two plexiform layers in the adult retina. SV2A was present in cone, but not rod, terminals in the outer plexiform layer (OPL) and in many synaptic terminals in the inner plexiform layer (IPL). SV2B was present only in the ribbon synapse-containing terminals of rod and cone photoreceptors and bipolar cells. SV2C was present in starburst amacrine cells, other conventional synapses in the IPL of unknown origin, and in presumptive interplexiform cell terminals in the INL and OPL. Each SV2 isoform was expressed in its distinct presynaptic terminals early and throughout postnatal development. In addition, SV2A was transiently expressed by developing horizontal cells. The unique distribution of each isoform suggests potentially distinct functions at different types of synapses, with SV2B having ribbon synapse-specific functions, and SV2C being important for the functions of starburst amacrine cells. Rod and cone terminals contain different complements of SV2 isoforms, indicating that ribbon synapses are not all identical. The early expression of SV2 isoforms prior to initiation of synapse formation suggests that they may have important synapse-specific roles during synaptogenesis.

Amacrine Cells↗

PubMed2003

Rod and cone contributions to the a-wave of the electroretinogram of the macaque.

The electroretinogram (ERG) of anaesthetised dark-adapted macaque monkeys was recorded in response to ganzfeld stimulation and rod- and cone-driven receptoral and postreceptoral components were separated and modelled. The test stimuli were brief (< 4.1 ms) flashes. The cone-driven component was isolated by delivering the stimulus shortly after a rod-saturating background had been extinguished. The rod-driven component was derived by subtracting the cone-driven component from the mixed rod-cone ERG. The initial part of the leading edge of the rod-driven a-wave scaled linearly with stimulus energy when energy was sufficiently low and, for times less than about 12 ms after the stimulus, it was well described by a linear model incorporating a distributed delay and three cascaded low-pass filter elements. Addition of a simple static saturating non-linearity with a characteristic intermediate between a hyperbolic and an exponential function was sufficient to extend application of the model to most of the leading edge of the saturated responses to high energy stimuli. It was not necessary to assume involvement of any other non-linearity or that any significant low-pass filter followed the non-linear stage of the model. A negative inner-retinal component contributed to the later part of the rod-driven a-wave. After suppressing this component by blocking ionotropic glutamate receptors, the entire a-wave up to the time of the first zero-crossing scaled with stimulus energy and was well described by summing the response of the rod model with that of a model describing the leading edge of the rod-bipolar cell response. The negative inner-retinal component essentially cancelled the early part of the rod-bipolar cell component and, for stimuli of moderate energy, made it appear that the photoreceptor current was the only significant component of the leading edge of the a-wave. The leading edge of the cone-driven a-wave included a slow phase that continued up to the peak, and was reduced in amplitude either by a rod-suppressing background or by the glutamate analogue, cis-piperidine-2,3-dicarboxylic acid (PDA). Thus the slow phase represents a postreceptoral component present in addition to a fast component of the a-wave generated by the cones themselves. At high stimulus energies, it appeared less than 5 ms after the stimulus. The leading edge of the cone-driven a-wave was adequately modelled as the sum of the output of a cone photoreceptor model similar to that for rods and a postreceptoral signal obtained by a single integration of the cone output. In addition, the output of the static non-linear stage in the cone model was subject to a low-pass filter with a time constant of no more than 1 ms. In conclusion, postreceptoral components must be taken into account when interpreting the leading edge of the rod- and cone-driven a-waves of the dark-adapted ERG.

Animals↗

PubMed2003

Multiple effects of adenosine in the arterially perfused mammalian eye. Possible mechanisms for the neuroprotective function of adenosine in the retina.

It has been postulated that the major physiological role of adenosine is protection of the central nervous system in conditions such as ischemia, hypoxia, or prolonged neuronal excitation. Under these conditions adenosine is released, and exerts multiple effects, including vasodilation, inhibition of neuronal activity, and enhancement of glycogenolysis, resulting in neuroprotection. In this article, published as well as unpublished data on the multiple effects of exogenous adenosine and application of adenosine-related agents, performed using the arterially perfused cat eye, will be reviewed and discussed within the framework of the neuroprotective role of adenosine. The isolated, arterially perfused eye preparation has the advantage of combining integrity of the eye structure, exact control of arterial concentration and timing of applied pharmacological agents, and access to electrophysiological parameters of both retina and optic nerve, as well as the ability to control and monitor perfusate flow. The absence of red blood cells in the perfusate prevents adenosine from being metabolized prior to reaching the eye.

Adenosine↗

PubMed2003

Regional variations in local contributions to the primate photopic flash ERG: revealed using the slow-sequence mfERG.

PURPOSE: To determine the variations with eccentricity of the primate photopic ERG and to separate contributions by different retinal cells by using intravitreal pharmacologic agents. METHODS: Slow-sequence multifocal (mf)ERGs were obtained from 19 anesthetized adult rhesus monkeys and 5 normal human subjects. Recordings in monkeys were obtained before and after injections of tetrodotoxin citrate (TTX) to block sodium-dependent spiking; TTX+N-methyl-D-aspartic acid (NMDA)+picrotoxin (PTX) or gamma-aminobutyric acid (GABA) to block all inner retinal activity; L-2 amino-4-phosphonobutyric acid (APB) to block the On-pathway; and cis-2, 3 piperidine dicarboxylic acid (PDA) to block the Off-pathway and the otherwise unblocked inner retinal activity. The stimulus consisted of 103 equal-sized hexagons within 17 degrees of the fovea; every 200 ms (15 frames), each hexagon had a 50% chance of remaining at 20 cd/m(2) or increasing briefly to 4.7 cd-s/m(2). Oscillatory potentials (OPs; 90-300 Hz) were extracted. RESULTS: The slow-sequence mfERG summed over the stimulated area looked similar to a standard photopic, full-field ERG, with a- and b-waves and OPs. OPs in the foveal and temporal retina were larger than in the nasal retina. This nasotemporal asymmetry was removed by TTX, and the OPs were eliminated, either by blocking inner retina activity or by blocking the On-pathway. The summed mfERG waveform, including OPs, was shaped mainly by the more peripheral retinal regions. The foveal b-wave peak occurred about 5 to 6 ms later than in the periphery, with the depolarizing peak of the On-pathway/bipolar contribution occurring earlier than the depolarizing peak of the Off contribution at all eccentricities. The a-wave was composed of a small photoreceptor contribution and postreceptoral portion originating from hyperpolarizing neurons. CONCLUSIONS: The variations in the primate photopic ERG with eccentricity are due to spike-driven oscillatory activity that is more prominent in central and temporal retina than in nasal retina and to the slower timing of all responses in the central, compared with the peripheral, retina. The full-field, photopic ERG most closely resembles the mfERG responses to stimulation of peripheral regions.

Adolescent↗

PubMed2002

The scotopic threshold response of the dark-adapted electroretinogram of the mouse.

The most sensitive response in the dark-adapted electroretinogram (ERG), the scotopic threshold response (STR) which originates from the proximal retina, has been identified in several mammals including humans, but previously not in the mouse. The current study established the presence and assessed the nature of the mouse STR. ERGs were recorded from adult wild-type C57/BL6 mice anaesthetized with ketamine (70 mg kg(-1)) and xylazine (7 mg kg(-1)). Recordings were between DTL fibres placed under contact lenses on the two eyes. Monocular test stimuli were brief flashes (lambda(max) 462 nm; -6.1 to +1.8 log scotopic Troland seconds(sc td s)) under fully dark-adapted conditions and in the presence of steady adapting backgrounds (-3.2 to -1.7 log sc td). For the weakest test stimuli, ERGs consisted of a slow negative potential maximal approximately 200 ms after the flash, with a small positive potential preceding it. The negative wave resembled the STR of other species. As intensity was increased, the negative potential saturated but the positive potential (maximal approximately 110 ms) continued to grow as the b-wave. For stimuli that saturated the b-wave, the a-wave emerged. For stimulus strengths up to those at which the a-wave emerged, ERG amplitudes measured at fixed times after the flash (110 and 200 ms) were fitted with a model assuming an initially linear rise of response amplitude with intensity, followed by saturation of five components of declining sensitivity: a negative STR (nSTR), a positive STR (pSTR), a positive scotopic response (pSR), PII (the bipolar cell component) and PIII (the photoreceptor component). The nSTR and pSTR were approximately 3 times more sensitive than the pSR, which was approximately 7 times more sensitive than PII. The sensitive positive components dominated the b-wave up to > 5 % of its saturated amplitude. Pharmacological agents that suppress proximal retinal activity (e.g. GABA) minimized the pSTR, nSTR and pSR, essentially isolating PII which rose linearly with intensity before showing hyperbolic saturation. The nSTR, pSTR and pSR were desensitized by weaker backgrounds than those desensitizing PII. In conclusion, ERG components of proximal retinal origin that are more sensitive to test flashes and adapting backgrounds than PII provide the 'threshold' negative and positive (b-wave) responses of the mouse dark-adapted ERG. These results support the use of the mouse ERG in studies of proximal retinal function.

Adaptation, Ocular↗

PubMed2002

Visual field defects and neural losses from experimental glaucoma.

Glaucoma is a relatively common disease in which the death of retinal ganglion cells causes a progressive loss of sight, often leading to blindness. Typically, the degree of a patient's visual dysfunction is assessed by clinical perimetry, involving subjective measurements of light-sense thresholds across the visual field, but the relationship between visual and neural losses is inexact. Therefore, to better understand of the effects of glaucoma on the visual system, a series of investigations involving psychophysics, electrophysiology, anatomy, and histochemistry were conducted on experimental glaucoma in monkeys. The principal results of the studies showed that, (1) the depth of visual defects with standard clinical perimetry are predicted by a loss of probability summation among retinal detection mechanisms, (2) glaucomatous optic atrophy causes a non-selective reduction of metabolism of neurons in the afferent visual pathway, and (3) objective electrophysiological methods can be as sensitive as standard clinical perimetry in assessing the neural losses from glaucoma. These experimental findings from glaucoma in monkeys provide fundamental data that should be applicable to improving methods for assessing glaucomatous optic neuropathy in patients.

Animals↗

PubMed2002

Inner-retinal contributions to the photopic sinusoidal flicker electroretinogram of macaques. Macaque photopic sinusoidal flicker ERG.

The purpose of this study was to examine inner-retinal contributions to the photopic sinusoidal flicker ERG. ERGs were recorded from 5 anesthetized monkeys to sinusoidally modulated (100%, 0.5-120 Hz) red full field flicker at Lmean of 3.2 log phot td on a rod saturating blue background (3.7 log scot td; 3.0 log phot td) before and after intravitreal injections of tetrodotoxin (TTX) to block Na+-dependent spikes of retinal ganglion and amacrine cells, followed by N-methyl-D-aspartate (NMDLA) to suppress all activity of these cells. Recordings also were made after blocking bipolar (and horizontal) cell responses with L-2-amino-4-phosphonobutyric acid (APB) and 2-cis-piperidine-2,3-dicarboxylic acid (PDA) or 6-cyano-nitroquinoxaline-2,3-dione (CNQX). Control fundamental (F1) and second harmonic (F2) amplitudes were large and variable at temporal frequencies up to 2 Hz. At higher frequencies, Fl amplitude was minimal with a phase step at a frequency between 13 and 19 Hz and maximal at 27-33 Hz. F2 was minimal at 2-3 Hz and maximal at 6-8 Hz, again with a phase step near the minimum. TTX, or NMDLA, produced small changes in Fl that shifted the amplitude minimum to a lower and the maximum to a higher frequency. In contrast, F2 was more strongly affected; both the amplitude minimum (and phase step) and maximum were greatly attenuated, leaving a moderate response from 0.5 to 8 Hz, which then declined as frequency was increased to 30 HZ. After APB and PDA or CNQX, F1 decreased continuously with increasing frequency and F2 was generally much smaller. The nearly linear F1 phase plotwas consistent with the presence of a single mechanism (i.e. photoreceptors). Inner-retinal neurons contribute to the photopic sinusoidal flicker ERG. Whereas for F1, inner-retinal contributions are small relative to those from bipolar cells; for F2, they are equal or greater between 2and 16 Hz.

6-Cyano-7-nitroquinoxaline-2,3-dione↗