Matching stimulation paradigms resolve apparent differences between optogenetic and electrical VTA stimulation.

BACKGROUND: Optogenetic stimulation has grown into a popular brain stimulation method in basic neuroscience while electrical stimulation predominates in clinical applications. In order to explain the effects of electrical stimulation on a cellular level and evaluate potential advantages of optogenetic therapies, comparisons between the two stimulation modalities are necessary. This comparison is hindered, however, by the difficulty of effectively matching the two fundamentally different modalities. OBJECTIVE: Comparison of brain-wide activation patterns in response to intensity-matched electrical and optogenetic VTA stimulation. METHODS: We mapped optogenetic and electrical self-stimulation rates in the same mice over stimulation intensity and determined iso-behavioral intensities. Using functional 99mTc-HMPAO SPECT imaging of cerebral blood flow in awake animals, we obtained brain-wide activation patterns for both modalities at these iso-behavioral intensities. We performed these experiments in two mouse lines commonly used for optogenetic VTA stimulation, DAT::Cre and TH::Cre mice. RESULTS: We find iso-behavioral intensity matching of stimulation gives rise to similar brain activation patterns. Differences between mouse lines were more pronounced than differences between modalities. CONCLUSIONS: Previously found large differences of electrical and optogenetic stimulation might be due to unmatched stimulation intensity, particularly relative electrical overstimulation. These findings imply that therapeutic electrical VTA stimulation might be relatively specific if employed with optimized parameters.

CAVE: An Open-Source Tool for Combined Analysis of Head-Mounted Calcium Imaging and Behavior in MATLAB.

Calcium imaging in freely behaving rodents using head-mounted miniature microscopes is currently becoming an increasingly popular technique in neuroscience. Due to the large amounts of complex data that the technique produces, user friendly software is needed for quick and efficient processing. Here, we present a new tool for analyzing calcium imaging data from head-mounted microscopes together with simultaneously acquired behavioral data: CAVE (Calcium ActiVity Explorer). CAVE bundles a unique set of algorithms specifically tailored to the analysis of single-photon imaging data from awake behaving animals including efficient motion correction and automatic ROI selection with manual audit and refinement. For behavioral analysis, CAVE can automatically track animal position and orientation. Individual behavioral epochs and external events can then be analyzed in correlation to calcium imaging and tracking data. Our program is written in MATLAB, the source code is open source and particularly focuses on providing a streamlined workflow for novice users while also retaining detailed configuration options for advanced users. We evaluate the performance of CAVE by investigating neural activity in hippocampus and somatosensory cortex. The fast analysis provided by CAVE allowed us to track activity in a large set of animals over the course of several months during exploration behavior, detailing the properties of onset and offset of observable activity and the visible cells per imaging location.

Optogenetic silencing of locus coeruleus activity in mice impairs cognitive flexibility in an attentional set-shifting task.

The locus coeruleus (LC) is the sole source of noradrenergic projections to the cortex and essential for attention-dependent cognitive processes. In this study we used unilateral optogenetic silencing of the LC in an attentional set-shifting task (ASST) to evaluate the influence of the LC on prefrontal cortex-dependent functions in mice. We expressed the halorhodopsin eNpHR 3.0 to reversibly silence LC activity during task performance, and found that silencing selectively impaired learning of those parts of the ASST that most strongly rely on cognitive flexibility. In particular, extra-dimensional set-shifting (EDS) and reversal learning was impaired, suggesting an involvement of the medial prefrontal cortex (mPFC) and the orbitofrontal cortex. In contrast, those parts of the task that are less dependent on cognitive flexibility, i.e., compound discrimination (CD) and the intra-dimensional shifts (IDS) were not affected. Furthermore, attentional set formation was unaffected by LC silencing. Our results therefore suggest a modulatory influence of the LC on cognitive flexibility, mediated by different frontal networks.

Photoreceptor projections and receptive fields in the dorsal rim area and main retina of the locust eye.

In many insect species, photoreceptors of a small dorsal rim area of the eye are specialized for sensitivity to the oscillation plane of polarized skylight and, thus, serve a role in sky compass orientation. To further understand peripheral mechanisms of polarized-light processing in the optic lobe, we have studied the projections of photoreceptors and their receptive fields in the main eye and dorsal rim area of the desert locust, a model system for polarization vision analysis. In both eye regions, one photoreceptor per ommatidium, R7, has a long visual fiber projecting through the lamina to the medulla. Axonal fibers from R7 receptors of the dorsal rim area have short side branches throughout the depth of the dorsal lamina and maintain retinotopic projections to the dorsal medulla following the first optic chiasma. Receptive fields of dorsal rim photoreceptors are considerably larger (average acceptance angle 33°) than those of the main eye (average acceptance angle 2.04°) and, taken together, cover almost the entire sky. The data challenge previous reports of two long visual fibers per ommatidium in the main eye of the locust and provide data for future analysis of peripheral networks underlying polarization opponency in the locust brain.

Opsin expression, physiological characterization and identification of photoreceptor cells in the dorsal rim area and main retina of the desert locust, Schistocerca gregaria.

For compass orientation many insects rely on the pattern of sky polarization, but some species also exploit the sky chromatic contrast. Desert locusts, Schistocerca gregaria, detect polarized light through a specialized dorsal rim area (DRA) in their compound eye. To better understand retinal mechanisms underlying visual navigation, we compared opsin expression, spectral and polarization sensitivities and response-stimulus intensity functions in the DRA and main retina of the locust. In addition to previously characterized opsins of long-wavelength-absorbing (Lo1) and blue-absorbing visual pigments (Lo2), we identified an opsin of an ultraviolet-absorbing visual pigment (LoUV). DRA photoreceptors exclusively expressed Lo2, had peak spectral sensitivities at 441 nm and showed high polarization sensitivity (PS 1.3-31.7). In contrast, ommatidia in the main eye co-expressed Lo1 and Lo2 in five photoreceptors, expressed Lo1 in two proximal photoreceptors, and Lo2 or LoUV in one distal photoreceptor. Correspondingly, we found broadband blue- and green-peaking spectral sensitivities in the main eye and one narrowly tuned UV peaking receptor. Polarization sensitivity in the main retina was low (PS 1.3-3.8). V-log I functions in the DRA were steeper than in the main retina, supporting a role in polarization vision. Desert locusts occur as two morphs, a day-active gregarious and a night-active solitarious form. In solitarious locusts, sensitivities in the main retina were generally shifted to longer wavelengths, particularly in ventral eye regions, supporting a nocturnal lifestyle at low light levels. The data support the role of the DRA in polarization vision and suggest trichromatic colour vision in the desert locust.