Stimulus-Evoked Intrinsic Optical Signals in the Retina: Pharmacologic Dissection Reveals Outer Retinal Origins

Abstract

Purpose To elucidate the anatomic origins of stimulus-evoked intrinsic optical signals in the mammalian retina by using selective pharmacologic blockade of specific retinal layers. Methods Four adult cats were used to investigate the stimulus-evoked intrinsic signals. The retinas were visually stimulated with a liquid crystal display (LCD) integrated into a modified fundus camera. The evoked signals in the near infrared (NIR) were recorded with a digital camera to image the changes in the optical reflectance of the retinas. Variants of the electroretinogram (pattern ERG and long-pulse ERG) were also recorded as additional measures of retinal function. Specific retinal layers were inactivated via intravitreal injections of the voltage-gated sodium channel blocker, tetrodotoxin (TTX), the metabotropic glutamate receptor (mGluR6) agonist, 2-amino-4-phosphonobutyric acid (APB), and/or the ionotropic glutamate receptor antagonist cis-2,3 piperidinedicarboxylic acid (PDA). The stimulus-evoked intrinsic signals were imaged before and after drug injection. Results ERG recordings and tests of the consensual pupillary response confirmed the effectiveness of each drug. Yet despite the pharmacologic blockade of the inner retina (TTX) and postreceptoral retinal circuitry (APB and PDA), the stimulus-evoked intrinsic signals remained essentially unaltered from preinjection conditions. Similarly, the time course of the signal did not appreciably shift in time or shape. Conclusions The findings demonstrate that stimulus-evoked intrinsic signals persist after injection of APB, PDA, and TTX, drugs that work to suppress inner and postreceptoral retinal circuitry. The persistence of the intrinsic signals after administration of these drugs indicates that the dominant intrinsic signals are likely to arise from the outer retina.

Stimulus-Evoked Intrinsic Optical Signals in the Retina: Spatial and Temporal Characteristics

“…Our accompanying paper shows that there was little or no signal tuning when different spatial frequencies were probed. 15 We find that the signal is driven primarily by stimulus intensity and duration (Figs. 5, 6), properties much more akin to photoreceptors than ganglion cells.…”

“…Perhaps foremost is the expectation that the signals should reflect a decrease in metabolic demand with light stimuli, at least to the extent to which these signals are due to photoreceptor activity. 15 Similarly, it has been shown that oxygen utilization decreases and oxygen tension increases in the outer retina with light stimulation. 27 These facts do not seem to be compatible with the notion of an increase in blood volume with light stimulation.…”

“…An accompanying paper in this journal demonstrates that the intrinsic signals show little or no spatial frequency tuning. 15 …”

“…We focused our efforts on characterizing the stimulus-dependent signals, to better elucidate their origins. 15 …”

Imaging translucent cell bodies in the living mouse retina without contrast agents

“…The capability to optically distinguish this cell class in the living retina also provides a new and exciting way to optically interrogate horizontal cell function in the retina using near infrared intrinsic signals [48][49][50][51]. Previous studies have analyzed signals from the full thickness of the retina or that of photoreceptors [48], now that we can spatially localize these cells, new studies may investigate endogenous optical changes within this cell class to a variety of visual stimulation patterns.…”