Visualization and molecular characterization of whole-brain vascular networks with capillary resolution.

Structural elucidation and molecular scrutiny of cerebral vasculature is crucial for understanding the functions and diseases of the brain. Here, we introduce SeeNet, a method for near-complete three-dimensional visualization of cerebral vascular networks with high signal-to-noise ratios compatible with molecular phenotyping. SeeNet employs perfusion of a multifunctional crosslinker, vascular casting by temperature-controlled polymerization of hybrid hydrogels, and a bile salt-based tissue-clearing technique optimized for observation of vascular connectivity. SeeNet is capable of whole-brain visualization of molecularly characterized cerebral vasculatures at the single-microvessel level. Moreover, SeeNet reveals a hitherto unidentified vascular pathway bridging cerebral and hippocampal vessels, thus serving as a potential tool to evaluate the connectivity of cerebral vasculature.

Quick visualization of neurons in brain tissues using an optical clearing technique.

Neurons are classified into several morphological types according to the locations of their somata and the branching patterns of their axons and dendrites. Recent studies suggest that these morphological features are related to their physiological properties, including firing characteristics, responses to neuromodulators, and wiring patterns. Therefore, rapid morphological identification of electrophysiologically recorded neurons promises to advance our understanding of neuronal circuits. One of the most common anatomical cell identification methods is neuronal reconstruction with biocytin delivered through whole-cell patch-clamp pipettes. However, conventional reconstruction methods usually take longer than 24 h and limit the throughput of electrophysiological experiments. Here, we developed a quick, simple cell reconstruction method by optimizing the tissue clearing protocol ScaleSQ. We found that adding 200 mM NaCl almost entirely prevented tissue swelling without compromising optical clearing ability. This solution, termed IsoScaleSQ, allowed us to increase the transparency of the gray matter of 500-µm-thick slices within 30 min, meaning that the total time required to reconstruct whole-cell recorded neurons was reduced to 1 h. This novel method will improve the efficacy and effectiveness of electrophysiological experiments linked to cell morphology.

Dopamine receptor activation reorganizes neuronal ensembles during hippocampal sharp waves in vitro.

Hippocampal sharp wave (SW)/ripple complexes are thought to contribute to memory consolidation. Previous studies suggest that behavioral rewards facilitate SW occurrence in vivo. However, little is known about the precise mechanism underlying this enhancement. Here, we examined the effect of dopaminergic neuromodulation on spontaneously occurring SWs in acute hippocampal slices. Local field potentials were recorded from the CA1 region. A brief (1 min) treatment with dopamine led to a persistent increase in the event frequency and the magnitude of SWs. This effect lasted at least for our recording period of 45 min and did not occur in the presence of a dopamine D1/D5 receptor antagonist. Functional multineuron calcium imaging revealed that dopamine-induced SW augmentation was associated with an enriched repertoire of the firing patterns in SW events, whereas the overall tendency of individual neurons to participate in SWs and the mean number of cells participating in a single SW were maintained. Therefore, dopaminergic activation is likely to reorganize cell assemblies during SWs.

Subicular activation preceding hippocampal ripples in vitro.

Sharp wave-ripple complexes (SW-Rs), a transient form of high-frequency field oscillations observed in the hippocampus, are thought to mediate memory consolidation. They are initiated mainly in hippocampal CA3 area and propagate to the entorhinal cortex through the subiculum; however, little is known about how SW-Rs are initiated and propagate. Here, we used functional multineuronal calcium imaging to monitor SW-R-relevant neuronal activity from the subiculum at single-cell resolution. An unexpected finding was that a subset of subicular neurons was activated immediately before hippocampal SW-Rs. The SW-R-preceding activity was not abolished by surgical lesion of the CA1-to-subiculum projection, and thus, it probably arose from entorhinal inputs. Therefore, SW-Rs are likely to be triggered by entorhinal-to-CA3/CA1 inputs. Moreover, the subiculum is not merely a passive intermediate region that SW-Rs pass through, but rather, it seems to contribute to an active modification of neural information related to SW-Rs.