Electroretinographic determination of human rod flash response in vivo

Rod photoreceptor activation and deactivation in early-stage diabetic eye disease

PURPOSE

To infer rod phototransduction activation and deactivation characteristics in diabetics who have mild or no clinically-apparent retinopathy.

METHODS

Fifteen non-diabetic controls, 15 diabetics with no clinically-apparent diabetic retinopathy (NDR), and 15 diabetics with mild non-proliferative diabetic retinopathy (MDR) participated. Dark-adapted flash electroretinograms (3.2 to 4.4 log scot td-s) were recorded to assess rod activation. The a-waves were fit with a Gaussian model to derive R_mp3 (maximum photoreceptor response amplitude) and S (phototransduction sensitivity). Rod deactivation was assessed with a paired flash paradigm, in which a-waves were measured for two flashes separated by inter-stimulus intervals (ISIs) of 0.125 to 16 s. The ISI needed for the a-wave amplitude of the second flash to recover to 50% of the first flash (t₅₀) was determined. The effect of stimulus retinal illuminance on activation and deactivation was evaluated in a subset of control subjects.

RESULTS

Analysis of variance indicated that both diabetic groups had significant log S reductions compared to controls (p < 0.001). Mean S was reduced by approximately 49% and 78% for the NDR and MDR groups, respectively. In contrast, log R_mp3 and log t₅₀ did not differ significantly among the groups (both p > 0.08). Reducing stimulus retinal illuminance significantly reduced S, but did not significantly affect R_max or t₅₀.

CONCLUSIONS

Only phototransduction sensitivity was abnormal in this sample of diabetic subjects. The normal deactivation kinetics suggests that circulating rod current is normal. These findings begin to constrain possible explanations for abnormal rod function in early diabetic retinal disease.

Dominant cone-rod dystrophy: a mouse model generated by gene targeting of the GCAP1/Guca1a gene

Cone dystrophy 3 (COD3) is a severe dominantly inherited retinal degeneration caused by missense mutations in GUCA1A, the gene encoding Guanylate Cyclase Activating Protein 1 (GCAP1). The role of GCAP1 in controlling cyclic nucleotide levels in photoreceptors has largely been elucidated using knock-out mice, but the disease pathology in these mice cannot be extrapolated directly to COD3 as this involves altered, rather than loss of, GCAP1 function. Therefore, in order to evaluate the pathology of this dominant disorder, we have introduced a point mutation into the murine Guca1a gene that causes an E155G amino acid substitution; this is one of the disease-causing mutations found in COD3 patients. Disease progression in this novel mouse model of cone dystrophy was determined by a variety of techniques including electroretinography (ERG), retinal histology, immunohistochemistry and measurement of cGMP levels. It was established that although retinal development was normal up to 3 months of age, there was a subsequent progressive decline in retinal function, with a far greater alteration in cone than rod responses, associated with a corresponding loss of photoreceptors. In addition, we have demonstrated that accumulation of cyclic GMP precedes the observed retinal degeneration and is likely to contribute to the disease mechanism. Importantly, this knock-in mutant mouse has many features in common with the human disease, thereby making it an excellent model to further probe disease pathogenesis and investigate therapeutic interventions.

Deactivation of the rod response in retinopathy of prematurity

It is known that retinopathy of prematurity (ROP) alters the activation of rod photoreceptors, but the effect of ROP on deactivation has not been investigated. We studied deactivation using an electroretinographic (ERG) paired flash procedure in 22 subjects (12 infants and 10 older subjects) with a history of preterm birth and ROP. The amplitude of the rod-isolated a-wave response to a flash presented 2-120 s after a test flash was measured, and the time at which it reached 50% of the single flash amplitude (t(50)) was determined by linear interpolation. Deactivation results were compared to those in former preterms who never had ROP (n = 6) and term-born controls. In infants, t(50) values of ROP subjects did not differ from those in subjects who never had ROP or term-born controls. Among mature ROP subjects, eight of 12 had t(50) values longer than any control subject. Prolonged deactivation in these mature ROP subjects may indicate lack of maturation of the deactivation process (t(50)) or progressive compromise of retinal function with increasing age.

A novel GCAP1(N104K) mutation in EF-hand 3 (EF3) linked to autosomal dominant cone dystrophy

The GUCA1A gene encodes a guanylate cyclase activating protein (GCAP1) that is involved in regulation of phototransduction in the vertebrate retina. We discovered a novel C312A transversion in exon 2 of the human GUCA1A gene, replacing Asn-104 (N104) in GCAP1 with Lys (K), in two affected members of a family with dominant cone dystrophy. The mutation N104K is located in the third EF-hand motif (EF3) shown previously to be instrumental in converting Ca2+-free GCAP1 to a GC inhibitor in the Ca2+-bound form. In one patient, rod ERGs were fairly stable over a 12-year-period whereas 30 Hz flicker ERG and single-flash cone ERGs declined. In both patients, double-flash ERGs showed that rod recovery from an intense test flash was significantly delayed. The EC(50) for GC stimulation shifted from approximately 250 nM in wild-type GCAP1 to approximately 800 nM in the GCAP1(N104K) mutant suggesting inability of the mutant to assume an inactive form under physiological conditions. The replacement of N104 by K in GCAP1 is the first naturally occurring mutation identified in the EF3 loop. The rod recovery delays observed in double-flash ERG of affected patients suggest a novel dominant-negative effect that slows GC stimulation.

Augmented rod bipolar cell function in partial receptor loss: an ERG study in P23H rhodopsin transgenic and aging normal rats

Physiological consequences of early stages of photoreceptor degeneration were examined in heterozygous P23H rhodopsin transgenic (Tg) and in aging normal Sprague-Dawley rats. Rod photoreceptor and rod bipolar (RB) cell function were estimated with maximum value and sensitivity parameters of P3 and P2 components of the electroretinogram. In both Tg and aging normal rats, the age-related rate of decline of P3 amplitude was steeper than that of the P2 amplitude. Tg rats showed greater than normal sensitivity of the rods. A new model of distal RB pathway connectivity suggested photoreceptor loss could not be the sole cause of physiological abnormalities; there was an additional increase of post-receptoral sensitivity. We propose that changes at rod-RB synapses compensate for the partial loss of rod photoreceptors in senescence and in early stages of retinal degeneration.

Dynamic and steady-state light adaptation of mouse rod photoreceptors in vivo

1. Electroretinographic (ERG) methods were used to investigate the effects of background illumination on the responses of mouse rod photoreceptors in vivo. A paired-flash procedure, involving the recording and analysis of the ERG a-wave response to a bright probe flash presented after a brief test flash, was used to derive the rod response to the test flash in steady background light. A related, step-plus-probe procedure was used to derive the step response of the rods to backgrounds of defined strength. 2. Steady background light produced a maintained derived response that was graded with background strength. Determinations of the full time course of the derived weak-flash response in steady background light, and of the effect of background strength on the flash response at fixed post-test-flash times, showed that moderate backgrounds reduce the peak amplitude and duration of the flash response. 3. The response to stepped onset of an approximately half-saturating background (1.2 sc cd m(-2)) exhibited a gradual rise over the first 200-300 ms, and an apparent subsequent relaxation to plateau amplitude within 1 s after background onset. Determinations of normalized amplitudes of the derived response to a test flash presented at 50 or 700 ms after background onset indicated substantial development of background-induced shortening of the test flash response within this 1 s period. These findings indicate a time scale of approximately 1 s or less for the near-completion of light adaptation at this background strength. 4. Properties of the derived response to a stepped background and to test flashes presented in steady background light are in general agreement with photocurrent data obtained from mammalian rods in vitro and suggest that the present results describe, to good approximation, the in vivo desensitization of mouse rods by background light.

Time course of the flash response of dark- and light-adapted human rod photoreceptors derived from the electroretinogram

1. The a-wave of the electroretinogram was recorded from human subjects with normal vision, using a corneal electrode and ganzfeld stimulation. We applied the paired-flash technique, in which an intense 'probe' flash was delivered at different times after a 'test' flash. The amplitude of the probe-flash response provided a measure of the circulating current remaining at the appropriate time after the test flash. 2. We extended previous methods by measuring not at a fixed time, but at a range of times after the probe flash, and then calculating the ratio of the 'test-plus-probe' response to the 'probe-alone' response, as a function of time. 3. Under dark-adapted conditions the rod response derived by the paired-flash technique (in response to a relatively dim test flash) peaked at ca 120 ms, with a fractional sensitivity at the peak of ca 0.1 Td(-1) s(-1). 4. As reported previously, background illumination reduced the maximal response, reflecting a reduction in rod circulating current. In addition, it shortened the time to peak (to ca 70 ms at an intensity of 170 Td), and reduced the flash sensitivity measured at the peak. The flash sensitivity declined approximately according to Weber's Law, with a 10-fold reduction occurring at an intensity of 100-200 Td. We could not reliably measure responses at significantly higher background intensities because the circulating current became so small. 5. In order to investigate the phototransduction process after correction for response compression, we expressed the derived response as a fraction of the maximal response that could be elicited in the presence of the background. The earliest rising phase of this 'fractional response per unit intensity' was little affected by background illumination, suggesting that the amplification constant of transduction was unaltered by light adaptation.