Abstract
Purpose: :
The ipRGCs are inner retinal photoreceptors that mediate subconscious physiological responses to light, such as circadian photoentrainment and the pupillary light reflex. These cells are thought to encode light intensity over very long time scales. However, the vast majority of studies published to date have used short-duration light stimuli (typically ≤1 min), and we remain ignorant of how these ganglion-cell photoreceptors respond to prolonged photic stimulation. In this study, we investigated the intrinsic responses of ipRGCs to 1-hour light exposure.
Methods: :
Retinas were isolated from dark-adapted Long-Evans rats and multielectrode-array recordings of ganglion cell action potentials were performed. Rod/cone input to ipRGCs and to all other ganglion cells was blocked with L-AP4, DNQX and D-AP5, thereby allowing the melanopsin-mediated photoresponses of ipRGCs to be analyzed in isolation. All stimuli were 1-hour step increases in intensity of full-field 480nm light and were presented under dark-adapted conditions.
Results: :
The lowest light intensity that could evoke robust, excitatory melanopsin responses was found to be ~1010 photons cm-2 s-1. At near-threshold intensities, melanopsin photoresponses had extremely slow kinetics, often with onset latencies exceeding 5 minutes and peak latencies longer than 30 minutes. At these relatively low intensities, melanopsin light responses were remarkably sustained and usually outlasted the 1-hour light stimulus. By contrast, the responses to intensities above ~1013 photons cm-2 s-1 were more transient and most cells' spike frequencies fully returned to prestimulus levels within about 20 minutes of light stimulation.
Conclusions: :
We have found that for 1-hour light exposure, the threshold intensity of the intrinsic ipRGC photoresponse is about an order of magnitude lower than that determined previously using 1-min light stimuli (Tu et al. 2005, Neuron 48:987-99). This finding suggests that melanopsin phototransduction can integrate light stimuli over many minutes, and that as stimulus intensity decreases, a longer integration time is required to generate a response. We have also provided the first electrophysiological proof that ipRGCs are capable of continuously responding to light for at least an hour. However, this capability appears to be limited to near-threshold and moderate light intensities, because melanopsin photoresponses display pronounced adaptation at higher intensities.
Keywords: photoreceptors • ganglion cells • retina: proximal (bipolar, amacrine, and ganglion cells)