Avian adeno-associated virus as an anterograde transsynaptic vector

BACKGROUND

Retrograde and anterograde transsynaptic viral vectors are useful tools for studying the input and output organization of neuronal circuitry, respectively. While retrograde transsynaptic viral vectors are widely used, viral vectors that show anterograde transsynaptic transduction are not common.

NEW METHOD

We chose recombinant avian adeno-associated virus (A3V) carrying the mCherry gene and injected it into the eyeball, cochlear duct, and midbrain auditory center of chickens. We observed different survival times to examine the virus transcellular transport and the resulting mCherry expression. To confirm the transcellular transduction mode, we co-injected A3V and cholera toxin B subunit.

RESULTS

Injecting A3V into the eyeball and cochlea labeled neurons in the visual and auditory pathways, respectively. Second-, and third-order labeling occurred approximately two and seven days, respectively, after injection into the midbrain. The distribution of labeled neurons strongly suggests that A3V transport is preferentially anterograde and transduces postsynaptic neurons.

COMPARISON WITH EXISTING METHOD(S)

A3V displays no extrasynaptic leakage and moderate speed of synapse passage, which is better than other viruses previously reported. Compared with AAV1&9, which have been shown to pass one synapse anterogradely, A3V passes several synapses in the anterograde direction.

CONCLUSIONS

A3V would be a good tool to study the topographic organization of projection axons and their target neurons.

Inhibitory Neural Circuits in the Mammalian Auditory Midbrain

The auditory midbrain is the critical integration center in the auditory pathway of vertebrates. Synaptic inhibition plays a key role during information processing in the auditory midbrain, and these inhibitory neural circuits are seen in all vertebrates and are likely essential for hearing. Here, we review the structure and function of the inhibitory neural circuits of the auditory midbrain. First, we provide an overview on how these inhibitory circuits are organized within different clades of vertebrates. Next, we focus on recent findings in the mammalian auditory midbrain, the most studied of the vertebrates, and discuss how the mammalian auditory midbrain is functionally coordinated.

Inhibitory Projections from the Inferior Colliculus to the Medial Geniculate body Originate from Four Subtypes of GABAergic Cells

GABAergic cells constitute 20-40% of the cells that project from the inferior colliculus [(IC) a midbrain auditory hub] to the medial geniculate body [(MG) the main auditory nucleus of the thalamus]. Four subtypes of GABAergic IC cells have been identified based on their association with perineuronal nets (PNs) and dense rings of axosomatic terminals expressing vesicular glutamate transporter 2 (VGLUT2 rings). These subtypes differ in their soma size and distribution within the IC. Based on previous work emphasizing large GABAergic cells as the origin of GABAergic IC-MG projections, we hypothesized that GABAergic IC cells surrounded by PNs and VGLUT2 rings, which tend to have larger somas, were more likely to project to the MG than smaller cells lacking these extracellular markers. Here, we injected retrograde tract tracers into the MG of guinea pigs of either sex and analyzed retrogradely labeled GABAergic cells in the ipsilateral IC for soma size and association with PNs and/or VGLUT2 rings. We found a range of GABAergic soma sizes present within the IC-MG pathway, which were reflective of the full range of GABAergic soma sizes present within the IC. Further, we found that all four subtypes of GABAergic IC cells participate in the IC-MG pathway, and that GABAergic cells lacking PNs and VGLUT2 rings were more prevalent within the pathway than would be expected based on their overall prevalence in the IC. These results may provide an anatomical substrate for the multiple roles of inhibition in the IC-MG pathway, which have emerged in electrophysiological studies.

Organization of subcortical auditory nuclei of Japanese house bat (Pipistrellus abramus) identified with cytoarchitecture and molecular expression

The auditory system of echolocating bats shows remarkable specialization likely related to analyzing echoes of sonar pulses. However, significant interspecies differences have been observed in the organization of auditory pathways among echolocating bats, and the homology of auditory nuclei with those of non-echolocating species has not been established. Here, in order to establish the homology and specialization of auditory pathways in echolocating bats, the expression of markers for glutamatergic, GABAergic, and glycinergic phenotypes in the subcortical auditory nuclei of Japanese house bat (Pipistrellus abramus) was evaluated. In the superior olivary complex, we identified the medial superior olive and superior paraolivary nuclei as expressing glutamatergic and GABAergic phenotypes, respectively, suggesting these nuclei are homologous with those of rodents. In the nuclei of the lateral lemniscus (NLL), the dorsal nucleus was found to be purely GABAergic, the intermediate nucleus was a mixture of glutamatergic and inhibitory neurons, the compact part of the ventral nucleus was purely glycinergic, and the multipolar part of the ventral nucleus expressed both GABA and glycine. In the inferior colliculus (IC), the central nucleus was found to be further subdivided into dorsal and ventral parts according to differences in the density of terminals and the morphology of large GABAergic neurons, suggesting specialization to sonar pulse structure. Medial geniculate virtually lacked GABAergic neurons, suggesting that the organization of the tectothalamic pathway is similar with that of rodents. Taken together, our findings revealed that specialization primarily occurs with regard to nuclei size and organization of the NLL and IC.

Subtypes of GABAergic cells in the inferior colliculus

The inferior colliculus occupies a central position in ascending and descending auditory pathways. A substantial proportion of its neurons are GABAergic, and these neurons contribute to intracollicular circuits as well as to extrinsic projections to numerous targets. A variety of types of evidence - morphology, physiology, molecular markers - indicate that the GABAergic cells can be divided into at least four subtypes that serve different functions. However, there has yet to emerge a unified scheme for distinguishing these subtypes. The present review discusses these criteria and, where possible, relates the different properties. In contrast to GABAergic cells in cerebral cortex, where subtypes are much more thoroughly characterized, those in the inferior colliculus contribute substantially to numerous long range extrinsic projections. At present, the best characterized subtype is a GABAergic cell with a large soma, dense perisomatic synaptic inputs and a large axon that provides rapid auditory input to the thalamus. This large GABAergic subtype projects to additional targets, and other subtypes also project to the thalamus. The eventual characterization of these subtypes can be expected to reveal multiple functions of these inhibitory cells and the many circuits to which they contribute.

Anatomical characterization of subcortical descending projections to the inferior colliculus in mouse

Descending projections from the thalamus and related structures to the midbrain are evolutionarily highly conserved. However, the basic organization of this auditory thalamotectal pathway has not yet been characterized. The purpose of this study was to obtain a better understanding of the anatomical and neurochemical features of this pathway. Analysis of the distributions of retrogradely labeled cells after focal injections of retrograde tracer into the inferior colliculus (IC) of the mouse revealed that most of the subcortical descending projections originated in the brachium of the IC and the paralaminar portions of the auditory thalamus. In addition, the vast majority of thalamotectal cells were found to be negative for the calcium-binding proteins calbindin, parvalbumin, or calretinin. Using two different strains of GAD-GFP mice, as well as immunostaining for GABA, we found that a subset of neurons in the brachium of the IC is GABAergic, suggesting that part of this descending pathway is inhibitory. Finally, dual retrograde injections into the IC and amygdala plus corpus striatum as well into the IC and auditory cortex did not reveal any double labeling. These data suggest that the thalamocollicular pathway comprises a unique population of thalamic neurons that do not contain typical calcium-binding proteins and do not project to other paralaminar thalamic forebrain targets, and that a previously undescribed descending GABAergic pathway emanates from the brachium of the IC. J. Comp. Neurol. 525:885-900, 2017. © 2016 Wiley Periodicals, Inc.