Non-Linear Transmission of Single-Photon Signals From Rods to Rod-Bipolar Cells in Mouse and Macaque Retina

Abstract

Abstract: : Purpose: To examine the possibility that there is a threshold in the synaptic mechanism linking rods to rod-bipolar cells that can reduce the transmission of continuous noise from the rods without blocking the transmission of significant numbers of single-photon responses (cf. Field & Rieke, 2002). Methods: Corneal electroretinogram (ERG) recordings were made from dark-adapted mice and macaques anesthetized with ketamine/xylazine or urethane(some macaques) after inner-retinal components had been suppressed by intravitreal injections of GABA (mice) or PDA/TTX or NMDA/DNQX (macaques) to isolate PII, the response of rod-bipolar cells. Results: All energy-scaled records of isolated PII obtained with ganzfeld stimuli that gave rise to much less than one photoisomerization per rod (and therefore insignificant numbers of multiple isomerizations) had an identical waveform. Stronger stimuli caused a reduction in the peak amplitude of energy-scaled records (saturation) and stimuli strong enough to produce multiple isomerizations in individual rods resulted in a shortening of the response latency and an increase of the energy-scaled amplitude at early times (supra-linearity). The shape of the rising edge of isolated PII changed with flash energy in a way that was consistent with the existence of a synaptic threshold whose level was less than one third of the amplitude of single-photon signals and a continuous pre-synaptic noise whose rms amplitude was even lower. Conclusions: There is a threshold nonlinearity operating at the rod to rod-bipolar cell synapse that can greatly reduce the transmission of continuous rod noise without significantly affecting the transmission of single-photon signals.

Keywords: bipolar cells • retina: distal(photoreceptors, horizontal cell • electroretinography: non-clinical