Rod transduction parameters from the a wave of local receptor populations

Extracellular ATP induces retinal photoreceptor apoptosis through activation of purinoceptors in rodents

We have previously demonstrated that photoreceptors express P2X(7) purinoceptors. These excitatory receptors are activated by extracellular adenosine 5'-triphosphate (ATP) and have been implicated in neurodegeneration in other parts of the central nervous system (CNS). In this study we examined whether extracellular ATP could contribute to photoreceptor degeneration in rodents through excessive activation of P2 purinoceptors. Intravitreal injection of high concentrations of extracellular ATP into normal rat eyes induced extensive and selective apoptosis of photoreceptors within 18 hours of injection. Five days after injection the outer nuclear layer was severely degenerated and electroretinographic responses were impaired. Preinjection of the purinergic antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) protected against ATP-mediated apoptosis. The initial phase of ATP-induced photoreceptor death did not temporally coincide with retinal pigment epithelium degeneration or microglial activation, suggesting that cell death was due to direct activation of purinergic receptors on photoreceptors. Finally, we demonstrate that intravitreal injection of PPADS results in a 30% increase in photoreceptor survival in the rd1 mouse, a model of human recessive retinitis pigmentosa (RP). These findings highlight the importance of extracellular ATP in retinal neurodegeneration and provide a potential new avenue for therapeutic intervention in RP.

Contribution of voltage-gated sodium channels to the b-wave of the mammalian flash electroretinogram

Voltage-gated sodium channels (Na(v) channels) in retinal neurons are known to contribute to the mammalian flash electroretinogram (ERG) via activity of third-order retinal neurons, i.e. amacrine and ganglion cells. This study investigated the effects of tetrodotoxin (TTX) blockade of Na(v) channels on the b-wave, an ERG wave that originates mainly from activity of second-order retinal neurons. ERGs were recorded from anaesthetized Brown Norway rats in response to brief full-field flashes presented over a range of stimulus energies, under dark-adapted conditions and in the presence of steady mesopic and photopic backgrounds. Recordings were made before and after intravitreal injection of TTX (approximately 3 microm) alone, 3-6 weeks after optic nerve transection (ONTx) to induce ganglion cell degeneration, or in combination with an ionotropic glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 200 microm) to block light-evoked activity of inner retinal, horizontal and OFF bipolar cells, or with the glutamate agonist N-methyl-D-aspartate (NMDA, 100-200 microm) to reduce light-evoked inner retinal activity. TTX reduced ERG amplitudes measured at fixed times corresponding to b-wave time to peak. Effects of TTX were seen under all background conditions, but were greatest for mesopic backgrounds. In dark-adapted retina, b-wave amplitudes were reduced only when very low stimulus energies affecting the inner retina, or very high stimulus energies were used. Loss of ganglion cells following ONTx did not affect b-wave amplitudes, and injection of TTX in eyes with ONTx reduced b-wave amplitudes by the same amount for each background condition as occurred when ganglion cells were intact, thereby eliminating a ganglion cell role in the TTX effects. Isolation of cone-driven responses by presenting test flashes after cessation of a rod-saturating conditioning flash indicated that the TTX effects were primarily on cone circuits contributing to the mixed rod-cone ERG. NMDA significantly reduced only the additional effects of TTX on the mixed rod-cone ERG observed under mesopic conditions, implicating inner retinal involvement in those effects. After pharmacological blockade with CNQX, TTX still reduced b-wave amplitudes in cone-isolated ERGs indicating Na(v) channels in ON cone bipolar cells themselves augment b-wave amplitude and sensitivity. This augmentation was largest under dark-adapted conditions, and decreased with increasing background illumination, indicating effects of background illumination on Na(v) channel function. These findings indicate that activation of Na(v) channels in ON cone bipolar cells affects the b-wave of the rat ERG and must be considered when analysing results of ERG studies of retinal function.

Temporal response properties of the primary and secondary rod-signaling pathways in normal and Gnat2 mutant mice

Multiple signaling pathways have been proposed for rod vision in the mammalian retina. The primary and secondary rod pathways have been characterized in humans with the scotopic 15-Hz flicker electroretinogram (ERG). The purpose of this study was to determine whether the response properties of these pathways in the mouse are similar to those of humans. C57BL/6J and Gnat2(cpfl3) mutant mice lacking functional cones were used in these experiments. Standard ERG recording techniques were employed. Response functions were obtained for a range of flash intensities (-4.7logcd-s/m(2) to -0.2logcd-s/m(2)) and temporal modulation frequencies (1-30Hz). The mouse intensity-response functions to 15-Hz flickering stimuli possessed the same features as that of humans - a local amplitude minimum and a rapid phase change in the intensity region where the primary and secondary pathways are mutually inhibitory. However, the secondary pathway in the mouse did not achieve the same level of sensitivity as previously shown for humans, suggesting inter-species differences in post-receptoral signal processing. In Gnat2(cpfl3) mutant mice, the secondary pathway was completely abolished. Measurements of temporal acuity indicated that the primary and secondary rod pathways could mediate temporal frequencies as high as 30 and 50Hz, respectively. The response functions for mice are similar to those of humans, although the evidence suggests that the primary rod pathway dominates all rod-mediated signal processing in the mouse. Nevertheless, these results demonstrate the feasibility of measuring non-invasively the performance characteristics of the primary and secondary rod retinal pathways in the mouse and provide a mechanism for testing hypotheses about the action of disease where post-receptoral cells are differentially affected.

Evidence for the involvement of purinergic P2X receptors in outer retinal processing

Extracellular ATP mediates fast excitatory neurotransmission in many regions of the central nervous system through activation of P2X receptors. Although several P2X receptor subunits have been identified in the mammalian retina, little is known about the functional role of these receptors in retinal signalling. The purpose of the present study was to investigate whether purinergic P2X(7) receptors are involved in outer retinal processing by assessing receptor localization, degradation of extracellular ATP and the effect of functional activation of P2X(7) receptors on the electroretinogram (ERG). Using light and electron microscopy, we demonstrated that P2X(7) receptors are expressed postsynaptically on horizontal cell processes as well as presynaptically on photoreceptor synaptic terminals in both the rat and marmoset retina. Using an enzyme cytochemical method, we showed that ecto-ATPases are active in the outer plexiform layer of the rat retina, providing a mechanism by which purinergic synaptic transmission can be rapidly terminated. Finally, we evaluated the role of P2X(7) receptors in retinal function by assessing changes to the ERG response of rats after intravitreal delivery of the P2X(7) receptor agonist benzoyl benzoyl ATP (BzATP). Intravitreal injection of BzATP resulted in a sustained increase (up to 58%) in the amplitude of the photoreceptor-derived a-wave of the ERG. In contrast, BzATP caused a transient reduction in the rod- and cone-derived postreceptoral responses. These results provide three lines of evidence for the involvement of extracellular purines in outer retinal processing.

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.