Avian adeno-associated virus as an anterograde transsynaptic vector

Recombinase-independent AAV for anterograde transsynaptic tracing

Viral transsynaptic labeling has become indispensable for investigating the functional connectivity of neural circuits in the mammalian brain. Adeno-associated virus serotype 1 (AAV1) allows for anterograde transneuronal labeling and manipulation of postsynaptic neurons. However, it is limited to delivering an AAV1 expressing a recombinase which relies on using transgenic animals or genetic access to postsynaptic neurons. We reasoned that a strong expression level could overcome this limitation. To this end, we used a self-complementary AAV of serotype 1 (scAAV1) under a strong promoter (CAG). We demonstrated the anterograde transneuronal efficiency of scAAV1 by delivering a fluorescent marker in mouse retina-superior colliculus and thalamic-amygdala pathways in a recombinase-independent manner in the mouse brain. In addition to investigating neuronal connectivity, anterograde transsynaptic AAVs with a strong promoter may be suitable for functional mapping and imaging.

High resolution recording of local field currents simultaneously with sound-evoked calcium signals by a photometric patch electrode in the auditory cortex field L of the chick

BACKGROUND

Functioning of the brain is based on both electrical and metabolic activity of neural ensembles. Accordingly, it would be useful to measure intracellular metabolic signaling simultaneously with electrical activity in the brain in vivo.

NEW METHOD

We innovated a PhotoMetric-patch-Electrode (PME) recording system that has a high temporal resolution incorporating a photomultiplier tube as a light detector. The PME is fabricated from a quartz glass capillary to transmit light as a light guide, and it can detect electrical signals as a patch electrode simultaneously with a fluorescence signal.

RESULTS

We measured the sound-evoked Local Field Current (LFC) and fluorescence Ca²⁺ signal from neurons labeled with Ca²⁺-sensitive dye Oregon Green BAPTA1 in field L, the avian auditory cortex. Sound stimulation evoked multi-unit spike bursts and Ca²⁺ signals, and enhanced the fluctuation of LFC. After a brief sound stimulation, the cross-correlation between LFC and Ca²⁺ signal was prolonged. D-AP5 (antagonist for NMDA receptors) suppressed the sound-evoked Ca²⁺ signal when applied locally by pressure from the tip of PME.

COMPARISON WITH EXISTING METHODS

In contrast to existing multiphoton imaging or optical fiber recording methods, the PME is a patch electrode pulled simply from a quartz glass capillary and can measure fluorescence signals at the tip simultaneously with electrical signal at any depth of the brain structure.

CONCLUSION

The PME is devised to record electrical and optical signals simultaneously with high temporal resolution. Moreover, it can inject chemical agents dissolved in the tip-filling medium locally by pressure, allowing manipulation of neural activity pharmacologically.

Viral Tools for Neural Circuit Tracing

Neural circuits provide an anatomical basis for functional networks. Therefore, dissecting the structure of neural circuits is essential to understanding how the brain works. Recombinant neurotropic viruses are important tools for neural circuit tracing with many advantages over non-viral tracers: they allow for anterograde, retrograde, and trans-synaptic delivery of tracers in a cell type-specific, circuit-selective manner. In this review, we summarize the recent developments in the viral tools for neural circuit tracing, discuss the key principles of using viral tools in neuroscience research, and highlight innovations for developing and optimizing viral tools for neural circuit tracing across diverse animal species, including nonhuman primates.

Convergence of bilateral auditory midbrain inputs on neurons in the auditory thalamus of chicken

In the avian ascending auditory pathway, the nucleus mesencephalicus lateralis pars dorsalis (MLd; the auditory midbrain center) receives inputs from virtually all lower brainstem auditory nuclei and sends outputs bilaterally to the nucleus ovoidalis (Ov; the auditory thalamic nucleus). Axons from part of the MLd terminate in a particular domain of Ov, thereby suggesting a formation of segregated pathways point-to-point from lower brainstem nuclei via MLd to the thalamus. However, it has not yet been demonstrated whether any spatial clustering of thalamic neurons that receive inputs from specific domains of MLd exists. Ov neurons receive input from bilateral MLds; however, the degree of laterality has not been reported yet. In this study, we injected a recombinant avian adeno-associated virus, a transsynaptic anterograde vector into the MLd of the chick, and analyzed the distribution of labeled postsynaptic neurons on both sides of the Ov. We found that fluorescent protein-labeled neurons on both sides of the Ov were clustered in domains corresponding to subregions of the MLd. The laterality of projections was calculated as the ratio of neurons labeled by comparing ipsilateral to contralateral projections from the MLd, and it was 1.86 on average, thereby indicating a slight ipsilateral projection dominance. Bilateral inputs from different subdomains of the MLd converged on several single Ov neurons, thereby implying a possibility of a de novo binaural processing of the auditory information in the Ov.

Towards Tabula Gallus

The Tabula Gallus is a proposed project that aims to create a map of every cell type in the chicken body and chick embryos. Chickens (Gallus gallus) are one of the most recognized model animals that recapitulate the development and physiology of mammals. The Tabula Gallus will generate a compendium of single-cell transcriptome data from Gallus gallus, characterize each cell type, and provide tools for the study of the biology of this species, similar to other ongoing cell atlas projects (Tabula Muris and Tabula Sapiens/Human Cell Atlas for mice and humans, respectively). The Tabula Gallus will potentially become an international collaboration between many researchers. This project will be useful for the basic scientific study of Gallus gallus and other birds (e.g., cell biology, molecular biology, developmental biology, neuroscience, physiology, oncology, virology, behavior, ecology, and evolution). It will eventually be beneficial for a better understanding of human health and diseases.