Justin S. Klein

Inhibition to retinal rod bipolar cells is regulated by light levels

The retina responds to a wide range of light stimuli by adaptation of retinal signaling to background light intensity and the use of two different photoreceptors: rods that sense dim light and cones that sense bright light. Rods signal to rod bipolar cells that receive significant inhibition from amacrine cells in the dark, especially from a rod bipolar cell-activated GABAergic amacrine cell. This inhibition modulates the output of rod bipolar cells onto downstream neurons. However, it was not clear how the inhibition of rod bipolar cells changes when rod signaling is limited by an adapting background light and cone signaling becomes dominant. We found that both light-evoked and spontaneous rod bipolar cell inhibition significantly decrease with light adaptation. This suggests a global decrease in the activity of amacrine cells that provide input to rod bipolar cells with light adaptation. However, inhibition to rod bipolar cells is also limited by GABAergic connections between amacrine cells, which decrease GABAergic input to rod bipolar cells. When we removed this serial inhibition, the light-evoked inhibition to rod bipolar cells remained after light adaptation. These results suggest that decreased inhibition to rod bipolar cells after light adaptation is due to decreased rod pathway activity as well as an active increase in inhibition between amacrine cells. Together these serve to limit rod bipolar cell inhibition after light adaptation, when the rod pathway is inactive and modulation of the signal is not required. This suggests an efficiency mechanism in the retina to limit unnecessary signaling.

Different types of retinal inhibition have distinct neurotransmitter release properties

Neurotransmitter release varies between neurons due to differences in presynaptic mechanisms such as Ca(2+) sensitivity and timing. Retinal rod bipolar cells respond to brief dim illumination with prolonged glutamate release that is tuned by the differential release of GABA and glycine from amacrine cells in the inner retina. To test if differences among types of GABA and glycine release are due to inherent amacrine cell release properties, we directly activated amacrine cell neurotransmitter release by electrical stimulation. We found that the timing of electrically evoked inhibitory currents was inherently slow and that the timecourse of inhibition from slowest to fastest was GABAC receptors > glycine receptors > GABAA receptors. Deconvolution analysis showed that the distinct timing was due to differences in prolonged GABA and glycine release from amacrine cells. The timecourses of slow glycine release and GABA release onto GABAC receptors were reduced by Ca(2+) buffering with EGTA-AM and BAPTA-AM, but faster GABA release on GABAA receptors was not, suggesting that release onto GABAA receptors is tightly coupled to Ca(2+). The differential timing of GABA release was detected from spiking amacrine cells and not nonspiking A17 amacrine cells that form a reciprocal synapse with rod bipolar cells. Our results indicate that release from amacrine cells is inherently asynchronous and that the source of nonreciprocal rod bipolar cell inhibition differs between GABA receptors. The slow, differential timecourse of inhibition may be a mechanism to match the prolonged rod bipolar cell glutamate release and provide a way to temporally tune information across retinal pathways.

Ultrahigh resolution all-reflective optical coherence tomography system with a compact fiber-based supercontinuum source

We report the construction and characterization of an all-reflective optical coherence tomography (OCT) system using a newly developed compact fiber-based broadband supercontinuum source. The use of only reflective optical components has enabled us to avoid chromatic dispersion effects and to obtain ultrahigh resolution OCT images of biological samples. We achieved an axial resolution of 2 μm in air with 87 dB dynamic range at a center wavelength around 1300 nm.

Slow changes in Ca2+ cause prolonged release from GABAergic retinal amacrine cells

The timing of neurotransmitter release from neurons can be modulated by many presynaptic mechanisms. The retina uses synaptic ribbons to mediate slow graded glutamate release from bipolar cells that carry photoreceptor inputs. However, many inhibitory amacrine cells, which modulate bipolar cell output, spike and do not have ribbons for graded release. Despite this, slow glutamate release from bipolar cells is modulated by slow GABAergic inputs that shorten the output of bipolar cells, changing the timing of visual signaling. The time course of light-evoked inhibition is slow due to a combination of receptor properties and prolonged neurotransmitter release. However, the light-evoked release of GABA requires activation of neurons upstream from the amacrine cells, so it is possible that prolonged release is due to slow amacrine cell activation, rather than slow inherent release properties of the amacrine cells. To test this idea, we directly activated primarily action potential-dependent amacrine cell inputs to bipolar cells with electrical stimulation. We found that the decay of GABAC receptor-mediated electrically evoked inhibitory currents was significantly longer than would be predicted by GABAC receptor kinetics, and GABA release, estimated by deconvolution analysis, was inherently slow. Release became more transient after increasing slow Ca(2+) buffering or blocking prolonged L-type Ca(2+) channels and Ca(2+) release from intracellular stores. Our results suggest that GABAergic amacrine cells have a prolonged buildup of Ca(2+) in their terminals that causes slow, asynchronous release. This could be a mechanism of matching the time course of amacrine cell inhibition to bipolar cell glutamate release.

Using optical coherence tomography to characterize thick-glaze structure: Chinese Southern Song Guan glaze case study

Abstract This study explores the structure characteristics of thick glaze, in terms of the case study of Chinese Southern Song Guan (SSG) ware, focusing on the bubble and its media structure, using a novel focus-tracking optical coherence tomography (OCT) system. The OCT images we obtained not only unveil the structural uniqueness of the thick-glaze SSG sample, but also establish a distinguishable structural pattern for aiding authentication. In addition, information revealed in our images provides a logical explanation for the subtle texture and tone of SSG glaze as well as insights into the technologies used in layering and firing these thick glazes.

Ultrahigh resolution all-reflective OCT system with a compact fiber-based supercontinuum source

We report the construction and characterization of an all-reflective OCT (R-OCT) system using a newly developed compact fiber-based broadband supercontinuum source. We achieved an axial resolution of 1.5 μm in tissue (2 μm in air) with 87 dB dynamic range at a center wavelength around 1300 nm.

Endoscopic spectral domain optical coherence tomography of murine colonic morphology to determine effectiveness of chemopreventive and chemotherapeutic agents in colorectal cancer

Optical coherence tomography (OCT) is a minimally-invasive imaging modality capable of tracking the development of individual colonic adenomas. As such, OCT can be used to evaluate the mechanisms and effectiveness of chemopreventive and chemotherapeutic agents in colorectal cancer models. The data presented here represent part of a larger study evaluating α-difluoromethylornithine (DFMO) and Sulindac as chemopreventive and chemotherapeutic agents using mice treated with the carcinogen azoxymethane (AOM). 27 A/J mice were included in the chemoprevention study, subdivided into four treatment groups (No Drug, DFMO, Sulindac, DFMO/Sulindac). 30 mm lateral images of each colon at eight different rotations were obtained at five different time points using a 2 mm diameter spectral domain OCT endoscopy system centered at 890 nm with 3.5 μm axial resolution in air and 5 μm lateral resolution. Images were visually analyzed to determine number and size of adenomas. Gross photos of the excised colons and histology provided gold standard confirmation of the final imaging time point. Preliminary results show that 100% of mice in the No Drug group developed adenomas over the course of the chemoprevention study. Incidence was reduced to 71.43% in mice given DFMO, 85.71% for Sulindac and 0% for DFMO/Sulindac. Discrete adenoma size did not vary significantly between experimental groups. Additional experiments are currently under way to verify these results and evaluate DFMO and Sulindac for chemotherapeutic applications.