The parabigeminal nucleus is a source of "retinogeniculate replacement terminals" in mice that lack retinofugal input

In the dorsal lateral geniculate nucleus (LGN) of mice that lack retinal input, a population of large terminals supplants the synaptic arrangements normally made by the missing retinogeniculate terminals. To identify potential sources of these "retinogeniculate replacement terminals," we used mutant mice (math5-/- ) which lack retinofugal projections due to the failure of retinal ganglion cells to develop. In this line, we labeled LGN terminals that originate from the primary visual cortex (V1) or the parabigeminal nucleus (PBG), and compared their ultrastructure to retinogeniculate, V1 or PBG terminals in the dLGN of C57Blk6 (WT) mice (schematically depicted above graph). Corticogeniculate terminals labeled in WT and math5-/- mice were similar in size and both groups were significantly smaller than WT retinogeniculate terminals. In contrast, the PBG projection in math5-/- mice was extensive and there was considerable overlap in the sizes of retinogeniculate terminals in WT mice and PBG terminals in math5-/- mice (summarized in histogram). The data indicate that V1 is not a source of "retinogeniculate replacement terminals" and suggests that large PBG terminals expand their innervation territory to replace retinogeniculate terminals in their absence.

Double Cones and the Diverse Connectivity of Photoreceptors and Bipolar Cells in an Avian Retina

Double cones are the most common photoreceptor cell type in most avian retinas, but their precise functions remain a mystery. Among their suggested functions are luminance detection, polarized light detection, and light-dependent, radical pair-based magnetoreception. To better understand the function of double cones, it will be crucial to know how they are connected to the neural network in the avian retina. Here we use serial sectioning, multibeam scanning electron microscopy to investigate double-cone anatomy and connectivity with a particular focus on their contacts to other photoreceptor and bipolar cells in the chicken retina. We found that double cones are highly connected to neighboring double cones and with other photoreceptor cells through telodendria-to-terminal and telodendria-to-telodendria contacts. We also identified 15 bipolar cell types based on their axonal stratifications, photoreceptor contact pattern, soma position, and dendritic and axonal field mosaics. Thirteen of these 15 bipolar cell types contacted at least one or both members of the double cone. All bipolar cells were bistratified or multistratified. We also identified surprising contacts between other cone types and between rods and cones. Our data indicate a much more complex connectivity network in the outer plexiform layer of the avian retina than originally expected.SIGNIFICANCE STATEMENT Like in humans, vision is one of the most important senses for birds. Here, we present the first serial section multibeam scanning electron microscopy dataset from any bird retina. We identified many previously undescribed rod-to-cone and cone-to-cone connections. Surprisingly, of the 15 bipolar cell types we identified, 11 received input from rods and 13 of 15 received at least part of their input from double cones. Therefore, double cones seem to play many different and important roles in avian retinal processing, and the neural network and thus information processing in the outer retina are much more complex than previously expected. These fundamental findings will be very important for several fields of science, including vertebrate vision, avian magnetoreception, and comparative neuroanatomy.

Outsourcing a visual neuropil - The central visual system of the median eyes of Galeodes granti Pocock, 1903 (Arachnida: Solifugae)

Only a few studies have examined the central visual system of Solifugae until now. To get new insights suitable for phylogenetic analysis we studied the R-cell (or retinula cell) projections and visual neuropils of Galeodes granti using various methods. G. granti possesses large median eyes and rudimentary lateral eyes. In this study, only the R-cells and neuropils of the median eyes were successfully stained. The R-cells terminate in two distinct visual neuropils. The first neuropil is located externally to the protocerebrum directly below the retina, the second neuropil lies in the cell body rind of the protocerebrum, and immediately adjacent is the arcuate body. This layout of the median eye visual system differs from Arachnopulmonata (Scorpiones + Tetrapulmonata). However, there are several similarities with Opiliones. In both, (1) the R-cells are connected to a first and second visual neuropil and not to any other region of the brain, (2) the first neuropil is not embedded in the cell body rind of the protocerebrum, it is rather external to the protocerebrum, (3) the second visual neuropil is embedded in the cell body rind, and (4) the second neuropil abuts the arcuate body. These findings may provide important new characters for the discussion on arachnid phylogeny.

Major Feedforward Thalamic Input Into Layer 4C of Primary Visual Cortex in Primate

One of the underlying principles of how mammalian circuits are constructed is the relative influence of feedforward to recurrent synaptic drive. It has been dogma in sensory systems that the thalamic feedforward input is relatively weak and that there is a large amplification of the input signal by recurrent feedback. Here we show that in trichromatic primates there is a major feedforward input to layer 4C of primary visual cortex. Using a combination of 3D-electron-microscopy and 3D-confocal imaging of thalamic boutons we found that the average feedforward contribution was about 20% of the total excitatory input in the parvocellular (P) pathway, about 3 times the currently accepted values for primates. In the magnocellular (M) pathway it was around 15%, nearly twice the currently accepted values. New methods showed the total synaptic and cell densities were as much as 150% of currently accepted values. The new estimates of contributions of feedforward synaptic inputs into visual cortex call for a major revision of the design of the canonical cortical circuit.

A crustacean lobula plate: Morphology, connections, and retinotopic organization

The lobula plate is part of the lobula complex, the third optic neuropil, in the optic lobes of insects. It has been extensively studied in dipterous insects, where its role in processing flow-field motion information used for controlling optomotor responses was discovered early. Recently, a lobula plate was also found in malacostracan crustaceans. Here, we provide the first detailed description of the neuroarchitecture, the input and output connections and the retinotopic organization of the lobula plate in a crustacean, the crab Neohelice granulata using a variety of histological methods that include silver reduced staining and mass staining with dextran-conjugated dyes. The lobula plate of this crab is a small elongated neuropil. It receives separated retinotopic inputs from columnar neurons of the medulla and the lobula. In the anteroposterior plane, the neuropil possesses four layers defined by the arborizations of such columnar inputs. Medulla projecting neurons arborize mainly in two of these layers, one on each side, while input neurons arriving from the lobula branch only in one. The neuropil contains at least two classes of tangential elements, one connecting with the lateral protocerebrum and the other that exits the optic lobes toward the supraesophageal ganglion. The number of layers in the crab's lobula plate, the retinotopic connections received from the medulla and from the lobula, and the presence of large tangential neurons exiting the neuropil, reflect the general structure of the insect lobula plate and, hence, provide support to the notion of an evolutionary conserved function for this neuropil.

Dynamic Modulation of Myelination in Response to Visual Stimuli Alters Optic Nerve Conduction Velocity

Myelin controls the time required for an action potential to travel from the neuronal soma to the axon terminal, defining the temporal manner in which information is processed within the CNS. The presence of myelin, the internodal length, and the thickness of the myelin sheath are powerful structural factors that control the velocity and fidelity of action potential transmission. Emerging evidence indicates that myelination is sensitive to environmental experience and neuronal activity. Activity-dependent modulation of myelination can dynamically alter action potential conduction properties but direct functional in vivo evidence and characterization of the underlying myelin changes is lacking. We demonstrate that in mice long-term monocular deprivation increases oligodendrogenesis in the retinogeniculate pathway but shortens myelin internode lengths without affecting other structural properties of myelinated fibers. We also demonstrate that genetically attenuating synaptic glutamate neurotransmission from retinal ganglion cells phenocopies the changes observed after monocular deprivation, suggesting that glutamate may constitute a signal for myelin length regulation. Importantly, we demonstrate that visual deprivation and shortened internodes are associated with a significant reduction in nerve conduction velocity in the optic nerve. Our results reveal the importance of sensory input in the building of myelinated fibers and suggest that this activity-dependent alteration of myelination is important for modifying the conductive properties of brain circuits in response to environmental experience.

SIGNIFICANCE STATEMENT

Oligodendrocyte precursor cells differentiate into mature oligodendrocytes and are capable of ensheathing axons with myelin without molecular cues from neurons. However, this default myelination process can be modulated by changes in neuronal activity. Here, we show, for the first time, that experience-dependent activity modifies the length of myelin internodes along axons altering action potential conduction velocity. Such a mechanism would allow for variations in conduction velocities that provide a degree of plasticity in accordance to environmental needs. It will be important in future work to investigate how these changes in myelination and conduction velocity contribute to signal integration in postsynaptic neurons and circuit function.

Dissecting a neuron network: FIB-SEM-based 3D-reconstruction of the visual neuropils in the sea spider Achelia langi (Dohrn, 1881) (Pycnogonida)

BACKGROUND

The research field of connectomics arose just recently with the development of new three-dimensional-electron microscopy (EM) techniques and increasing computing power. So far, only a few model species (for example, mouse, the nematode Caenorhabditis elegans, and the fruit fly Drosophila melanogaster) have been studied using this approach. Here, we present a first attempt to expand this circle to include pycnogonids, which hold a key position for the understanding of arthropod evolution. The visual neuropils in Achelia langi are studied using a focused ion beam-scanning electron microscope (FIB-SEM) crossbeam-workstation, and a three-dimensional serial reconstruction of the connectome is presented.

RESULTS

The two eyes of each hemisphere of the sea spider's eye tubercle are connected to a first and a second visual neuropil. The first visual neuropil is subdivided in two hemineuropils, each responsible for one eye and stratified into three layers. Six different neuron types postsynaptic to the retinula (R-cells) axons are characterized by their morphology: five types of descending unipolar neurons and one type of ascending neurons. These cell types are also identified by Golgi impregnations. Mapping of all identifiable chemical synapses indicates that the descending unipolar neurons are postsynaptic to the R-cells and, hence, are second-order neurons. The ascending neurons are predominantly presynaptic and sometimes postsynaptic to the R-cells and may play a feedback role.

CONCLUSIONS

Comparing these results with the compound eye visual system of crustaceans and insects - the only arthropod visual system studied so far in such detail - we found striking similarities in the morphology and synaptic organization of the different neuron types. Hence, the visual system of pycnogonids shows features of both chelicerate median and mandibulate lateral eyes.

Complex I inhibition in the visual pathway induces disorganization of the node of Ranvier

Mitochondrial defects can have significant consequences on many aspects of neuronal physiology. In particular, deficiencies in the first enzyme complex of the mitochondrial respiratory chain (complex I) are considered to be involved in a number of human neurodegenerative diseases. The current work highlights a tight correlation between the inhibition of complex I and the state of axonal myelination of the optic nerve. Exposing the visual pathway of rats to rotenone, a complex I inhibitor, resulted in disorganization of the node of Ranvier. The structure and function of the node depend on specific cell adhesion molecules, among others, CASPR (contactin associated protein) and contactin. CASPR and contactin are both on the axonal surfaces and need to be associated to be able to anchor their myelin counterpart. Here we show that inhibition of mitochondrial complex I by rotenone in rats induces reactive oxygen species, disrupts the interaction of CASPR and contactin couple, and thus damages the organization and function of the node of Ranvier. Demyelination of the optic nerve occurs as a consequence which is accompanied by a loss of vision. The physiological impairment could be reversed by introducing an alternative NADH dehydrogenase to the mitochondria of the visual system. The restoration of the nodal structure was specifically correlated with visual recovery in the treated animal.

ON cone bipolar cell axonal synapses in the OFF inner plexiform layer of the rabbit retina

Analysis of the rabbit retinal connectome RC1 reveals that the division between the ON and the OFF inner plexiform layer (IPL) is not structurally absolute. ON cone bipolar cells make noncanonical axonal synapses onto specific targets and receive amacrine cell synapses in the nominal OFF layer, creating novel motifs, including inhibitory crossover networks. Automated transmission electron microscopic imaging, molecular tagging, tracing, and rendering of ~400 bipolar cells reveals axonal ribbons in 36% of ON cone bipolar cells, throughout the OFF IPL. The targets include γ-aminobutyrate (GABA)-positive amacrine cells (γACs), glycine-positive amacrine cells (GACs), and ganglion cells. Most ON cone bipolar cell axonal contacts target GACs driven by OFF cone bipolar cells, forming new architectures for generating ON-OFF amacrine cells. Many of these ON-OFF GACs target ON cone bipolar cell axons, ON γACs, and/or ON-OFF ganglion cells, representing widespread mechanisms for OFF to ON crossover inhibition. Other targets include OFF γACs presynaptic to OFF bipolar cells, forming γAC-mediated crossover motifs. ON cone bipolar cell axonal ribbons drive bistratified ON-OFF ganglion cells in the OFF layer and provide ON drive to polarity-appropriate targets such as bistratified diving ganglion cells (bsdGCs). The targeting precision of ON cone bipolar cell axonal synapses shows that this drive incidence is necessarily a joint distribution of cone bipolar cell axonal frequency and target cell trajectories through a given volume of the OFF layer. Such joint distribution sampling is likely common when targets are sparser than sources and when sources are coupled, as are ON cone bipolar cells.

Dysfunctional uveoscleral pathway in a rat model of congenital glaucoma

The purpose of the study was to investigate the presence of the uveoscleral pathway in the normotensive rat (NR) and in a rat model of congenital glaucoma (CGR). We injected the fluorescent tracer 70-kDa dextran rhodamine B in the anterior chamber of four NRs and four CGRs. At 10 and 60 minutes after injection, rats were euthanized by CO₂ inhalation and eyes were enucleated. Cryosections were prepared and analyzed using fluorescent microscopy. Hematoxylin-eosin staining and electron microscopy of the anterior chamber angle (ACA) were performed. At 10 minutes after injection, fluorescent tracer was detected in the iris root and ciliary processes of NRs and CGRs. At 60 minutes, NRs showed prominent signal in the suprachoroidal, whereas, in the CGRs, tracer was barely detectable. Histology of the anterior chamber revealed the presence of an open ACA and electron microscopy confirmed the normal structure of the ciliary body in CGRs.

CONCLUSIONS

Our results document the presence of an uveoscleral pathway in the normotensive rat. The rat model of congenital glaucoma shows severe impairment of the uveoscleral pathway, suggesting that alterations of the uveoscleral outflow might play a role in the pathogenesis of CG.

Building retinal connectomes

Understanding vertebrate vision depends on knowing, in part, the complete network graph of at least one representative retina. Acquiring such graphs is the business of synaptic connectomics, emerging as a practical technology due to improvements in electron imaging platform control, management software for large-scale datasets, and availability of data storage. The optimal strategy for building complete connectomes uses transmission electron imaging with 2 nm or better resolution, molecular tags for cell identification, open-access data volumes for navigation, and annotation with open-source tools to build 3D cell libraries, complete network diagrams and connectivity databases. The first forays into retinal connectomics have shown that even nominally well-studied cells have much richer connection graphs than expected.

Serial EM analysis of a copepod larval nervous system: Naupliar eye, optic circuitry, and prospects for full CNS reconstruction

The medial eye and optic center of the first nauplius of Dactylopusia (=Dactylopodia) tisboides, a harpacticoid copepod, were reconstructed from serial EM micrographs. Axons from the eye project to a set of matching cartridges defined by glial cells processes, and input is then processed in sequence through two synaptic fields. A single class of local relay neurons provides the main pathway between these, subject to modulatory input from a class of densely stained neurons with distinctive dense terminals. The importance of other outside sources of synaptic input to the second synaptic field indicates that the latter is a major site for integrating the optic input with signals originating elsewhere in the CNS. This accords with physiological data on the shadow response in barnacles, whose visual system is also derived from a naupliar eye. With a body length of ca. 80microns, copepod larvae like that of Dactylopusia are arguably among the smallest functional metazoans with a complex nervous system. Hence they are promising subjects for full reconstruction of neural circuitry at the EM level that could, in principle, reveal where key decision-making functions are localized.

Histological findings at the boundary of craniopharyngiomas

Although a craniopharyngioma is grossly well circumscribed, microscopically the borders are frequently irregular and may be associated with gliosis in the adjacent brain tissue. In the current study, we investigated the histology of the interface between craniopharyngiomas and surrounding normal structures such as the hypothalamus and pituitary gland. Histologically, we classified the findings at the boundary of craniopharyngiomas into three types. In type 1, a relatively thick capsule-like tissue was identified at the boundary between the craniopharyngioma and surrounding normal structure composed of tumor cells and inflammatory changes. In type 2, a craniopharyngioma had a relatively clear cleavage between the surrounding gliosis. In type 3, the boundary had some interdigitation of the tumor in the surrounding gliotic layer adjacent to the craniopharyngioma. In types 1 and 3, surgeons may fail to accomplish complete resection of the tumor. These histological features may result in recurrence of craniopharyngioma even after gross total resection.

[Evolutionary significance of reciprocal connections in the turtle tectofugal visual system]

In two turtle species--Emys orbicularis and Testudo horsfieldi--by the method of anterograde and retrograde traicing method at the light and electron microscopy level, the existence is proven of direct descending projections from the thalamic nucleus of the tectofugal visual system n. rotunds (Rot) to the optic tectum. After injection of tracers into Rot alone and into Rot with involvement of the tectothalamic tract (Trtth), occasional labeled fibers with varicosities and terminals are revealed predominantly in the deep sublayers of SGFS of the rostral optic tectum, while in the lower amount in other tectal layers. After the tracer injections into the optic tectum, a few retrogradely labeled neurons were found mainly in the Rot ventral parts and within Trtth. Their localization coincides with that of GABA-immunoreactive cells. Electron microscopy showed the existence of many retrogradely labeled dendrites throughout the whole Rot; a few labeled cell bodies were also present there, some of them being also GABA-immunoreactive. These results allow us to conclude about the existence of reciprocal connections between the optic tectum and Rot in turtles, these connections being able to affect processing of visual information in tectum. We suggest that reciprocity of tectothalamic connections might be the ancestral feature of the vertebrate brain; in the course of amniote evolution the functional significance of this feature can be decreased and even lost in parallel with a rise of the role of direct corticotectal projections.

Axon tracking in serial block-face scanning electron microscopy

Electron microscopy is an important modality for the analysis of neuronal structures in neurobiology. We address the problem of tracking axons across large distances in volumes acquired by serial block-face scanning electron microscopy (SBFSEM). Tracking, for this application, is defined as the segmentation of an axon that spans a volume using similar features between slices. This is a challenging problem due to the small cross-sectional size of axons and the low signal-to-noise ratio in our SBFSEM images. A carefully engineered algorithm using Kalman-snakes and optical flow computation is presented. Axon tracking is initialized with user clicks or automatically using the watershed segmentation algorithm, which identifies axon centers. Multiple axons are tracked from slice to slice through a volume, updating the positions and velocities in the model and providing constraints to maintain smoothness between slices. Validation results indicate that this algorithm can significantly speed up the task of manual axon tracking.

Synaptic circuits of the Drosophila optic lobe: the input terminals to the medulla

Understanding the visual pathways of the fly's compound eye has been blocked for decades at the second optic neuropil, the medulla, a two-part relay comprising 10 strata (M1-M10), and the largest neuropil in the fly's brain. Based on the modularity of its composition, and two previous reports, on Golgi-impregnated cell types (Fischbach and Dittrich, Cell Tissue Res.,1989; 258:441-475) and their synaptic circuits in the first neuropil, the lamina, we used serial-section electron microscopy to examine inputs to the distal strata M1-M6. We report the morphology of the reconstructed medulla terminals of five lamina cells, L1-L5, two photoreceptors, R7 and R8, and three neurons, medulla cell T1 and centrifugal cells C2 and C3. The morphology of these conforms closely to previous reports from Golgi impregnation. This fidelity provides assurance that our reconstructions are complete and accurate. Synapses of these terminals broadly localize to the terminal and provide contacts to unidentified targets, mostly medulla cells, as well as sites of connection between the terminals themselves. These reveal that R8 forms contacts upon R7 and thus between these two spectral inputs; that L3 provides input upon both pathways, adding an achromatic input; that the terminal of L5 reciprocally connects to that of L1, thus being synaptic in the medulla despite lacking synapses in the lamina; that the motion-sensing input cells L1 and L2 lack direct interconnection but both receive input from C2 and C3, resembling lamina connections of these cells; and that, as in the lamina, T1 provides no output chemical synapses.

Anterograde axonal transport of chicken cellular prion protein (PrPc) in vivo requires its N-terminal part

The cellular isoform of prion protein (PrP(c)) can exist in membrane-bound and secreted forms. Both forms of PrP(c) can be transported by retinal ganglion cell (RGC) axons along the optic nerve in the anterograde direction. In this study we determined which part of chicken PrP(c) is required for its anterograde axonal transport within the optic nerve of embryonic chicken. We intraocularly injected radio-iodinated fragments of recombinant chicken PrP(c) and then examined their anterograde axonal transport from retina into optic tectum. Using gamma-counting and different autoradiographic techniques we quantified anterograde axonal transport of the N-terminal part of chicken PrP(c) (amino acid residues 1-116) in this model system. The transport of the N-terminal part has similar properties as the anterograde transport of full-length chicken PrP(c) (Butowt et al., 2006) described previously (e.g., has similar efficiency, is microtubule-dependent, and is saturable). Moreover, the pattern of ultrastructural distribution of the N-terminal fragment within RGCs is similar to the distribution of full-length PrP(c). The C-terminal fragment of chicken PrP(c) (residues 118-246) and different PrP-derived peptides were not transported. Moreover, PrP(c)-derived peptides were sorted into different endocytotic pathways in neurons, indicating that they cannot substitute for full-length PrP(c) to study its internalization and trafficking. These data indicate that the N-terminal half of chicken PrP(c) contains the necessary information to drive the internalization and subsequent sorting of extracellular PrP(c) in RGCs soma into the anterograde axonal transport pathway.

Conventional kinesin-I motors participate in the anterograde axonal transport of neurotrophins in the visual system

Retinal ganglion cells (RGCs) anterogradely transport neurotrophins to the midbrain tectum/superior colliculus with significant downstream effects. The molecular mechanism of this type of axonal transport of neurotrophins is not well characterized. We identified kinesin-I proteins as a motor participating in the anterograde axonal movement of vesicular structures containing radiolabeled neurotrophins along the optic nerve. RT-PCR analysis of purified murine RGCs showed that adult RGCs express all known members of the kinesin-I family. After intraocular injection of (125)I-brain-derived neurotrophic factor (BDNF) into the adult mouse or (125)I-neurotrophin-3 (NT-3) into the embryonic chicken eye, radioactivity was efficiently immunoprecipitated from the optic nerve lysates by anti-kinesin heavy chain and anti-kinesin light chain monoclonal antibodies (H2 and L1). Immunoreactivity for the BDNF receptor trkB is also present in the immunoprecipitates obtained by the anti-kinesin-I antibodies. The delivery of the H2 antibody in vivo into the mouse RGCs substantially reduced anterograde axonal transport of (125)I-BDNF. Anterograde transport of BDNF was not diminished in kinesin light chain 1 (KLC1) knockout mice. However, this may be due to redundancy in functions between two different isoforms of KLC present in the RGCs, as it was described previously for kinesin heavy chains (Kanai et al. [ 2000] J Neurosci 20:6374-6384). These data indicate that kinesin-I is a protein motor that participates in the anterograde axonal transport of neurotrophins in the chicken and mouse visual pathways.

External and internal inputs affecting plasticity of dendrites and axons of the fly's neurons

Neurons and glial cells in the fly's visual system exhibit circadian rhythms through changes in shape and size. Moreover, the number of synaptic contacts between these cells changes during the day and night and in the case of one type of synapses, feedback synapses, is maintained under constant conditions indicating an endogenous origin of this rhythm. The structural changes described above, involving the oscillations in the number of synapses and the size of interneurons and glial cells, are examples of plasticity in the central nervous system driven by internal inputs from a circadian clock and by external stimuli such as light. They are also modulated by visual and other sensory stimuli and by motor activity.

Type 3a and type 3b OFF cone bipolar cells provide for the alternative rod pathway in the mouse retina

The mammalian retina provides several pathways to relay the information from the photoreceptors to the ganglion cells. Cones feed into ON and OFF cone bipolar cells that excite ON and OFF ganglion cells, respectively. In the "classical" rod pathway, rods feed into rod bipolar cells that provide input to both the ON and the OFF pathway via AII amacrine cells. Recent evidence suggests an alternative rod pathway in which rods directly contact some types of OFF cone bipolar cells. The mouse has become an important model system for retinal research. We performed an immunohistochemical analysis on the level of light and electron microscopy to identify the bipolar cells and ganglion cells that are involved in the alternative rod pathway of the mouse retina. 1) We identify a new bipolar cell type, showing that type 3 OFF cone bipolar cells comprise two distinct cell types, that we termed 3a and 3b. Type 3a cells express the ion channel HCN4. Type 3b bipolar cells represent a hitherto unknown cell type that can be identified with antibodies against the regulatory subunit RIIbeta of protein kinase A. 2) We show that both 3a and 3b cells form flat contacts at cone pedicles and rod spherules. 3) Finally, we identify an OFF ganglion cell type whose dendrites costratify with type 3a and 3b bipolar cell axon terminals. These newly identified cell types represent the basis of a neuronal circuit in the mammalian retina that could provide for an alternative fast rod pathway.

Synaptic organization of lobula plate tangential cells inDrosophila: γ-Aminobutyric acid receptors and chemical release sites

In flies, the large tangential cells of the lobula plate represent an important processing center for visual navigation based on optic flow. Although the visual response properties of these cells have been well studied in blowflies, information on their synaptic organization is mostly lacking. Here we study the distribution of presynaptic release and postsynaptic inhibitory sites in the same set of cells in Drosophila melanogaster. By making use of transgenic tools and immunohistochemistry, our results suggest that HS and VS cells of Drosophila express gamma-aminobutyric acid (GABA) receptors in their dendritic region within the lobula plate, thus being postsynaptic to inhibitory input there. At their axon terminals in the protocerebrum, both cell types express synaptobrevin, suggesting the presence of presynaptic specializations there. HS- and VS-cell terminals additionally show evidence for postsynaptic GABAergic input, superimposed on this synaptic polarity. Our findings are in line with the general circuit for visual motion detection and receptive field properties as postulated from electrophysiological and optical recordings in blowflies, suggesting a similar functional organization of lobula plate tangential cells in the two species.

The superficial layers of the superior colliculus are cytoarchitectually and myeloarchitectually disorganized in the reelin-deficient mouse, reeler

The causative gene for the reeler mouse is reelin which encodes Reelin protein, an extracellular molecule. In the present study, we have examined the cytoarchitecture, myeloarchitecture, and afferent/efferent systems of the superior colliculus (SC) of the reeler mouse. In the reeler, the laminar structures of the superficial three layers of the SC were disorganized and intermingled into a single layer, i.e., the superficial fused layer (SuF), as previously reported in the reelin-deficient SRK rat (Sakakibara et al., Develop. Brain Res. 141:1-13). Next, we have investigated the course and terminals of visual corticotectal and retinotectal projections with an injection of biocytin into the visual cortex or an injection of cholera toxin subunit B into the retina, respectively. In the reeler, anterogradely labeled visual corticotectal and retinotectal fibers took an aberrant course within the SuF, resulting in abnormal myeloarchitecture of the superficial SC of the reeler. Retrograde labeling of tectospinal tract neurons could not show any differences between the normal and reeler mice, suggesting that the deep layers of the reeler SC are cytoarchitectually normal. In situ hybridization and immunohistochemical studies have shown that reelin mRNA and Reelin protein were both recognized in the normal SC. These results suggest that Reelin protein plays some roles in histogenesis of the superficial layers of the SC.

Do first order and higher order regions of the thalamic reticular nucleus have different developmental timetables?

The thalamic reticular nucleus (TRN) can been subdivided into sectors based on thalamic and cortical input. Additionally, in carnivores the visual sector of the TRN can be subdivided into first order (perigeniculate nucleus: PGN) and higher order (TRN) regions. This report examines whether TRN development reflects the nature of its higher order visual connections. 170 cells from 12 kittens aged between postnatal day 0 (P0) and P125 were fully analysed after single cell injections in 400-500 microm fixed brain slices. TRN cells have a period of exuberant dendritic branching that peaks between P3 and P12, around the time of eye opening (P7), followed by branch pruning until P68. Similarly, most dendritic appendages are added between P12 and P22 followed by pruning, which is also largely complete by P68. Most branch points occur within the first 10-30% of the dendritic arbor, peaking between 10 and 20% (roughly equivalent to 100 mum from the soma), while appendages were concentrated between 20 and 30% of the arbour; appendages tend to be distributed over a larger proportion of the arbor up to P14 compared to later ages. TRN and PGN maturation were not significantly different. The present data suggest that clear distinctions cannot be made between the maturation of first and higher order pathways and indicate that GABAergic cells of the ventral thalamus may mature earlier than relay cells of the dorsal thalamus. Furthermore, dendritic development in the TRN may be less dependent on extrinsic factors than an intrinsic growth pattern or factors other than a functional hierarchy within the visual pathway.

Activity-independent prespecification of synaptic partners in the visual map of Drosophila

Specifying synaptic partners and regulating synaptic numbers are at least partly activity-dependent processes during visual map formation in all systems investigated to date . In Drosophila, six photoreceptors that view the same point in visual space have to be sorted into synaptic modules called cartridges in order to form a visuotopically correct map . Synapse numbers per photoreceptor terminal and cartridge are both precisely regulated . However, it is unknown whether an activity-dependent mechanism or a genetically encoded developmental program regulates synapse numbers. We performed a large-scale quantitative ultrastructural analysis of photoreceptor synapses in mutants affecting the generation of electrical potentials (norpA, trp;trpl), neurotransmitter release (hdc, syt), vesicle endocytosis (synj), the trafficking of specific guidance molecules during photoreceptor targeting (sec15), a specific guidance receptor required for visual map formation (Dlar), and 57 other novel synaptic mutants affecting 43 genes. Remarkably, in all these mutants, individual photoreceptors form the correct number of synapses per presynaptic terminal independently of cartridge composition. Hence, our data show that each photoreceptor forms a precise and constant number of afferent synapses independently of neuronal activity and partner accuracy. Our data suggest cell-autonomous control of synapse numbers as part of a developmental program of activity-independent steps that lead to a "hard-wired" visual map in the fly brain.

Synaptic input to OFF parasol ganglion cells in macaque retina

A Neurobiotin-injected OFF parasol cell from midperipheral macaque retina was studied by reconstruction of serial ultrathin sections and compared with ON parasol cells studied previously. In most respects, the synaptic inputs to the two subtypes were similar. Only a few of the amacrine cell processes that provided input to the labeled OFF parasol ganglion cell dendrites made or received inputs within the series, and none of these interactions were with the bipolar cells or other amacrine cells presynaptic to the OFF parasol cell. These findings suggest that the direct inhibitory input to OFF parasol cells originates from other areas of the retina. OFF parasol cells were known to receive inputs from two types of diffuse bipolar cells. To identify candidates for the presynaptic amacrine cells, OFF parasol cells were labeled with Lucifer yellow by using a juxtacellular labeling technique, and amacrine cells known to costratify with them were labeled via immunofluorescent methods. Appositions were observed with amacrine cells containing immunoreactive calretinin, parvalbumin, choline acetylatransferase, and G6-Gly, a cholecystokinin precursor. These findings suggest that the inhibitory input to parasol cells conveys information about several different attributes of visual stimuli and, particularly, about their global properties.

Comparison of the ultrastructure of cortical and retinal terminals in the rat superior colliculus

We compared the ultrastructure and synaptic targets of terminals of cortical or retinal origin in the stratum griseum superficiale and stratum opticum of the rat superior colliculus. Following injections of biotinylated dextran amine into cortical area 17, corticotectal axons were labeled by anterograde transport. Corticotectal axons were of relatively small caliber with infrequent small varicosities. At the ultrastructural level, corticotectal terminals were observed to be small profiles (0.44 +/- 0.27 microm(2)) that contained densely packed round vesicles. In tissue stained for gamma amino butyric acid (GABA) using postembedding immunocytochemical techniques, corticotectal terminals were found to contact small (0.51 +/- 0.69 microm(2)) non-GABAergic dendrites and spines (93%) and a few small GABAergic dendrites (7%). In the same tissue, retinotectal terminals, identified by their distinctive pale mitochondria, were observed to be larger than corticotectal terminals (3.34 +/- 1.79 microm(2)). In comparison to corticotectal terminals, retinotectal terminals contacted larger (1.59 +/- 1.70 microm(2)) non-GABAergic dendrites and spines (73%) and a larger proportion of GABAergic profiles (27%) of relatively large size (2.17 +/- 1.49 microm(2)), most of which were vesicle-filled (71%). Our results suggest that cortical and retinal terminals target different dendritic compartments within the neuropil of the superficial layers of the superior colliculus.

Ommatidial type-specific interphotoreceptor connections in the lamina of the swallowtail butterfly, Papilio xuthus

The eye of the butterfly Papilio xuthus contains a random array of three types of ommatidia (types I-III), each bearing nine photoreceptors, R1-R9. Of the six spectral classes of photoreceptors identified, types I, II, and III ommatidia contain four, three, and two classes, respectively: the ommatidia are thus spectrally heterogeneous. The photoreceptors send their axons to the lamina where, together with some large monopolar cells (LMCs), the nine from a single ommatidium contribute to a module called a lamina cartridge. We recently reported that among different photoreceptor axon terminals visualized by confocal microscopy, the number and length of axon collaterals differ for different spectral receptors, suggesting that lamina circuits are specific for each ommatidial type. Here we studied the distribution of synapse-like structures in the cartridges, first characterizing a photoreceptor by measuring its spectral sensitivity and then injecting Lucifer yellow (LY). We subsequently histologically identified the type of ommatidium to which the injected photoreceptor belonged, cut serial ultrathin sections of the entire lamina, labeled these with anti-LY immunocytochemistry, and then localized synapse-like structures. We found numerous interphotoreceptor contacts both within and between cartridges, the combination of which was again specific for the ommatidial type. R3 and R4, which are green-sensitive photoreceptors in all ommatidia, have thick axons lacking collaterals. We found that these cells exclusively make contacts with LMCs and not with photoreceptors. We therefore presume that R3 and R4 construct a system for motion vision, whereas other randomly distributed spectral types provide inputs for color vision.

Patterns of expression of brain-derived neurotrophic factor and tyrosine kinase B mRNAs and distribution and ultrastructural localization of their proteins in the visual pathway of the adult rat

We have examined the cellular and subcellular distribution and the patterns of expression of brain-derived neurotrophic factor (BDNF), and of its high affinity receptor, tyrosine kinase B (TrkB), in retinorecipient regions of the brain, including the superior colliculus, the lateral geniculate nucleus and the olivary pretectal nucleus. In the retinorecipient layers of the superior colliculus, BDNF protein and mRNA were present in the cell bodies of a subpopulation of neurons, and BDNF protein was present in the neuropil as punctate or fiber-like structures. In the lateral geniculate nucleus, however, BDNF mRNA was not detected, and BDNF protein was restricted to punctate and fiber-like structures in the neuropil, especially in the most superficial part of the dorsal lateral geniculate nucleus, just below the optic tract. At the ultrastructural level, BDNF protein was localized predominantly to axon terminals containing round synaptic vesicles and pale mitochondria with irregular cristae, which made asymmetric (Gray type I) synaptic specializations (R-boutons). Enucleation of one eye was followed by loss of BDNF immunoreactivity and disappearance of BDNF-positive R-boutons in the contralateral visual centers, confirming the retinal origin of at least most of these terminals. TrkB was present in postsynaptic densities apposed to immunoreactive R-boutons in the superior colliculus and lateral geniculate nucleus, and was also associated with axonal and dendritic microtubules. These findings suggest that BDNF is synthesized by a subpopulation of retinal ganglion cells and axonally transported to visual centers where this neurotrophin is assumed to play important roles in visual system maintenance and/or in modulating the excitatory retinal input to neurons in these centers.

Ultrastructural Study of Normal and Degenerating Nerve Fibers in the Protocerebral Tract of the Crab Ucides cordatus

Wallerian degeneration is a very well described phenomenon in the vertebrate nervous system. In arthropods, and especially in crustaceans, nerve fiber degeneration has not been described extensively. In addition, literature shows that the events do not follow the same patterns as in vertebrates. In this study we report, by qualitative and quantitative ultrastructural analyses, the features and time course of the protocerebral tract degeneration following extirpation of the optic stalk. No remarkable changes were observed seven days after lesion. After 28 days the protocerebral tracts presented apparently preserved small and large diameter axons and some degenerating medium axons, with irregular contours and empty-looking aspect of the axoplasm. Forty days after the ablation of the optic stalks, both small (type I) and medium (type II and III) axons revealed signs of partial or total degeneration, but large nerve fibers (type IV) were still intact. After 45 days, the tract showed signs of advanced stage of degeneration and, apart from large axons, normal-looking fibers were almost absent. At these 3 last time points, degenerating axons displayed different electron densities and aspects, probably correlating to different onset times of the process. In addition, cells with granules in their cytoplasm, possibly hemocytes, were quite distinct, especially at 40 and 45 days after axotomy. These cells might share with glial cells the function of phagocytosis of cellular debris during the protocerebral tract degeneration. Quantitative analysis showed that the number of degenerating fibers increased significantly from 28 to 40 days after lesion, whereas the number of normal fibers decreased accordingly. Measurements of cross-sectional areas of normal and degenerating axons showed that types II and III (medium) start to degenerate before type I (small). Type IV (large) axons do not degenerate, even after 40 days. Therefore, we can conclude that degeneration in these afferent fibers starts late after axotomy, but proceeds at a faster rate afterwards until the complete degeneration of small and medium axons.

Connections of contralaterally projecting isthmotectal axons and GABA-immunoreactive neurons in Xenopus tectum: An ultrastructural study

To investigate the circuitry that mediates binocular interactions in the tectum of Xenopus frogs, we have begun to identify the tectal cells that receive ipsilateral eye input relayed via the nucleus isthmi. Isthmotectal axons were labeled with horseradish peroxidase, and thin sections were labeled by postembedding immunogold reaction with antibodies to γ-aminobutyric acid (GABA). Ultrastructural examination reveals that many isthmotectal axons terminate on GABA-immunoreactive dendrites. Other isthmotectal axons contact postsynaptic structures that are unlabeled but have an appearance consistent with previously described GABA-poor zones of GABA-immunoreactive dendrites. We also examined the unlabeled inputs to the dendrites that were postsynaptic to filled isthmotectal axons. The most common nonisthmic inputs to those dendrites were GABA-immunoreactive processes with symmetric morphology. Surprisingly, we found only one input with the retinotectal characteristics of densely packed round, clear vesicles and minimal GABA immunoreactivity. These results indicate that isthmotectal axons synapse onto inhibitory interneurons, that retinotectal and isthmotectal axons do not synapse close to each other on the same dendrites, and that inhibitory connections are the closest neighbors to isthmotectal synapses.

Photoreceptor projection and termination pattern in the lamina of gonodactyloid stomatopods (mantis shrimp)

The apposition compound eyes of gonodactyloid stomatopods are divided into a ventral and a dorsal hemisphere by six equatorial rows of enlarged ommatidia, the mid-band (MB). Whereas the hemispheres are specialized for spatial vision, the MB consists of four dorsal rows of ommatidia specialized for colour vision and two ventral rows specialized for polarization vision. The eight retinula cell axons (RCAs) from each ommatidium project retinotopically onto one corresponding lamina cartridge, so that the three retinal data streams (spatial, colour and polarization) remain anatomically separated. This study investigates whether the retinal specializations are reflected in differences in the RCA arrangement within the corresponding lamina cartridges. We have found that, in all three eye regions, the seven short visual fibres (svfs) formed by retinula cells 1-7 (R1-R7) terminate at two distinct lamina levels, geometrically separating the terminals of photoreceptors sensitive to either orthogonal e-vector directions or different wavelengths of light. This arrangement is required for the establishment of spectral and polarization opponency mechanisms. The long visual fibres (lvfs) of the eighth retinula cells (R8) pass through the lamina and project retinotopically to the distal medulla externa. Differences between the three eye regions exist in the packing of svf terminals and in the branching patterns of the lvfs within the lamina. We hypothesize that the R8 cells of MB rows 1-4 are incorporated into the colour vision system formed by R1-R7, whereas the R8 cells of MB rows 5 and 6 form a separate neural channel from R1 to R7 for polarization processing.

Synaptic organization of complex ganglion cells in rabbit retina: type and arrangement of inputs to directionally selective and local-edge-detector cells

The type and topographic distribution of synaptic inputs to a directionally selective (DS) rabbit retinal ganglion cell (GC) were examined and were compared with those received by two other complex GC types. The percentage of cone bipolar cell (BC) input, presumably an index of sustained responses and simple receptive field properties, is much higher than expected for complex GCs in reference to previous reports in other species: approximately 20% for the type 1 bistratified ON-OFF DS GC and for a multistratified GC, and approximately 40% for the small-tufted local-edge-detector GC. Consistent with a previous study (Famiglietti [1991] J. Comp. Neurol. 309:40-70), no ultrastructural evidence is found for inhibitory synapses from starburst amacrine cells to the ON-OFF DS GC. The density of inputs to the ON-OFF DS GC is high and rather evenly distributed over the dendritic tree. Clustering of inputs brings excitatory and inhibitory inputs into proximity, but the strict on-path condition of more proximal inhibitory inputs, favoring shunting inhibition, is not satisfied. Prominent BC input and its regional variation suggest that BCs play key roles in DS neural circuitry, both pre- and postsynaptic to the ON-OFF DS GC, according to a bilayer model (Famiglietti [1993] Invest. Ophthalmol. Vis. Sci. 34:S985). Asymmetry of inhibitory amacrine cell input may signify a region on the preferred side of the receptive field, the inhibition-free zone (Barlow and Levick [1965] J. Physiol. (Lond.) 178:477-504), supporting a role for postsynaptic integration in the DS mechanism. Prominent BC input to the local-edge-detector, often without accompanying amacrine cell input, indicates presynaptic integration in forming its trigger feature.

VGluT2 immunochemistry identifies thalamocortical terminals in layer 4 of adult and developing visual cortex

A vesicular glutamate transporter, VGluT2, has been suggested to be the transporter utilized in the thalamocortical pathway. We examined the reliability of this marker in identifying and discriminating thalamic terminals in adult and developing ferret visual cortex. We studied brain sections stained for the transporter protein and/or anterogradely filled thalamocortical or intracortical axons, by using light, confocal, and electron microscopy. Under light microscopy, VGluT2 immunoreactivity (ir) in adult animals [past postnatal day (P)90] and in neonatal animals as early as P27 formed a dense band in layer 4 and appeared as scattered puncta in layers 6 and 1. Confocal dual-labeling analyses of P46 and adult striate cortices indicated that VGluT2 was present in thalamocortical axons, suggesting that thalamic projections utilize this transporter during postnatal development as well as adulthood. In contrast, extracellularly filled intracortical axons failed to colocalize with VGluT2-ir, suggesting that no significant terminal population originating in cortex contained VGluT2 in layer 4. Electron microscopic analysis revealed that, in adult layer 4, VGluT2-ir was present in large terminals, forming asymmetric synapses. Similar to anterogradely labeled thalamocortical terminals, VGluT2-ir synaptic terminals were different from their unlabeled counterparts in terms of terminal area (0.6 vs. 0.3 microm), synaptic length (486 vs. 353 nm), and preference for synapsing on spines (77% vs. 59%). Moreover, no significant differences were found between VGluT2-ir and anterogradely labeled thalamocortical terminals. Comparable similarities were also demonstrated at P46. These results indicate that thalamocortical terminals in layer 4 of visual cortex utilize VGluT2 and suggest that this marker can be used to identify thalamic axons specifically in adult and developing animals.

Neurofilament transport is dependent on actin and myosin

Real-time analyses have revealed that some newly synthesized neurofilament (NF) subunits translocate into and along axonal neurites by moving along the inner plasma membrane surface, suggesting that they may translocate against the submembrane actin cortex. We therefore examined whether or not NF axonal transport was dependent on actin and myosin. Perturbation of filamentous actin in NB2a/d1 cells with cytochalasin B inhibited translocation of subunits into axonal neurites and inhibited bidirectional translocation of NF subunits within neurites. Intravitreal injection of cytochalasin B inhibited NF axonal transport in optic axons in a dose-response manner. NF subunits were coprecipitated from NB2a/d1 cells by an anti-myosin antibody, and myosin colocalized with NFs in immunofluorescent analyses. The myosin light chain kinase inhibitor ML-7 and the myosin ATPase inhibitor 2,3-butanedione-2-monoxime perturbed NF translocation within NB2a/d1 axonal neurites. These findings suggest that some NF subunits may undergo axonal transport via myosin-mediated interactions with the actin cortex.

The axon arbourisation of nuclei isthmi neurons in the optic tectum of the chick and pigeon. A Golgi and anterograde tracer-study

The optic tectum is reciprocally connected to the nuclei isthmi pars magnocellularis (Imc) and pars parvocellularis (Ipc), which have different modulatory effects on optic transmission. We studied the axon arbourisation of these isthmic nuclei in the optic tectum in order to differentiate between them using Golgi-impregnated preparations both in chickens and pigeons. In addition, sections from animals injected with the anterograde tracer biotinylated dextran-amine (BDA) into the Imc were examined in the bright-field and electron microscope to identify the axon arbourisations and terminals. Also, GABA immunogold stained sections were examined in the electron microscope. In Golgi preparations, slab-like (or poplar tree-like) axon terminal arbourisations of both magnocellular and parvocellular isthmic nuclei neurons were found extending to the tectal surface, with similar branching patterns, but different lengths. The axon arbourisations extending from layer 5 of the optic tectum to the surface were termed type 1, whereas those extending from the internal (12-11) layers to the tectal surface were termed type 2. Type 2 arbourisations very closely matched arbourisations observed in BDA injected material, indicating that Imc neurons gave rise to type 2 arbourisations. The two kinds of axon arbourisation in the external tectal layers were alike in both types of bird, except for the width, which was about 10 mum larger in the type 2 axon arbour. Controlling for size, there was no significant difference between chicks and pigeons. The significance of these afferents in the optic tectum is discussed.

Morphological identification of ganglion cells expressing the alpha subunit of type II calmodulin-dependent protein kinase in the macaque retina

Expression of the alpha subunit of type II calmodulin-dependent protein kinase (alphaCamKII) distinguishes the koniocellular neurons of the primate lateral geniculate nucleus (LGN) from the primary parvo- and magnocellular neurons, but whether the same neurochemical signature distinguishes the retinal ganglion cells providing them input is not known. We find that, in the retina, alphaCamKII expression also differentiates two primary groups of ganglion cell, both characterized by broad, sparsely branching dendritic trees and cell bodies intermediate in size between the parvo- and magnocellular-projecting ganglion cells. Cells in the first group have three or four primary dendrites, a thick axon, and a rounded cell body and likely are made up of multiple types. In contrast, ganglion cells in the second group demonstrate a highly regular morphology, with strictly two primary dendrites and a thinner axon emanating from a smaller, elliptical cell body. This cell resembles the "large sparse" ganglion cell identified by others in retrograde labeling from the LGN and represents about 2% of all ganglion cells. In the optic nerve, alphaCamKII+ axons are also intermediate in size and form a bimodal distribution, correlating with the axonal sizes of the two groups of ganglion cell. For the LGN, we describe a group of alphaCamKII+ axon terminals with morphology consistent with terminals from retinal ganglion cells. These terminals form long, filamentous contacts with alphaCamKII+ relay cells and increase in frequency from the dorsal to the ventral koniocellular regions. Our results indicate that ganglion cells expressing alphaCamKII represent multiple projections to the brain, at least one of which provides input to one or more koniocellular regions of the LGN.

Immunoreactivity for the group III receptor subtype mGluR4a in the visual layers of the rat superior colliculus

Several studies indicate that metabotropic glutamate receptors (mGluRs) participate in the transmission of visual stimuli in optic layers of the superior colliculus (SC). We examined the cellular and subcellular distribution of the group III mGluR4a in superficial layers of the rat SC by means of a specific antiserum and a preembedding immunogold method for electron microscopy. Deposits of mGluR4a immunoparticles were mostly observed on presynaptic membranes of large synaptic terminals, which made asymmetrical synapses and contained abundant spherical, clear synaptic vesicles and numerous electron translucent mitochondria. These characteristic ultrastructural features correspond to retinocollicular synaptic terminals. Also, chains of synaptic retinal terminals along dendrites were labeled for mGluR4a. About 70% of morphologically identified retinal terminals were mGluR4a immunopositive. Furthermore, mGluR4a immunoreactivity in SC greatly disappeared following retinal ablation. About 28% of cortical terminals identified by anterograde tracing showed mGluR4a labeling, whereas only 2% of collicular GABAergic profiles were labeled for mGluR4a. These results reveal that retinal terminals are the major contributors to the mGluR4a immunoreactivity observed in the superior collicular circuitry.

Postnatal development of alkaline phosphatase activity correlates with the maturation of neurotransmission in the cerebral cortex

We have shown previously that the tissue nonspecific alkaline phosphatase (TNAP) is selectively expressed in the synaptic cleft of sensory cortical areas in adult mammals and, by using sensory deprivation, that TNAP activity depends on thalamocortical activity. We further analyzed this structural functional relationship by comparing the developmental pattern of TNAP activity to the maturation of the thalamocortical afferents in the primate brain (Callithrix jacchus). Cortical expression of alkaline phosphatase (AP) activity reflects the sequential maturation of the modality-specific sensory areas. Within the visual cortex, the regional and laminar distribution of AP correlates with the differential maturation of the magno- and parvocellular streams. AP activity, which is transiently expressed in the white matter, exhibits a complementary distributional pattern with myelin staining. Ultrastructural analysis revealed that AP activity is localized exclusively to the myelin-free axonal segments, including the node of Ranvier. It was also found that AP activity is gradually expressed in parallel with the maturation of synaptic contacts in the neuropile. These data suggest the involvement of AP, in addition to neurotransmitter synthesis previously suggested in the adult, in synaptic stabilization and in myelin pattern formation and put forward a role of AP in cortical plasticity and brain disorders.

Identification of a neurofilament-like protein in the protocerebral tract of the crab Ucides cordatus

Neurofilaments (NFs) have not been observed in crustaceans using conventional electron microscopy, and intermediate filaments have never been described in crustaceans and other arthropods by immunocytochemistry. Since polypeptides, labeled by the NN18-clone antibody, were revealed on microtubule side-arms of crayfish, we have tested, in this study, whether proteins similar to mammalian NFs are present in the protocerebral tract (PCT) of the crab Ucides cordatus. We used immunohistochemistry for light microscopy with monoclonal antibodies against three different NF subunits, high (NF-H), medium (NF-M), and light (NF-L). Labeling was observed with the NN18-clone, which recognizes NF-M. In order to confirm the results obtained with the immunohistochemical reactions, Western blotting, using the three primary antibodies, was performed and the presence of NF-M was confirmed. The NN18-clone monoclonal antibody recognized a protein of approximately 160 kDa, similar to the mammalian NF-M protein, but NF-L and NF-H were not recognized. Conventional transmission electron microscopy was used to observe the ultrastructural components of the axons and immunoelectron microscopy was used to show the distribution of the NF-M-like polypeptides along cytoskeletal elements of the PCT. Our results agree with previous studies on crustacean NF proteins that have reported negative immunoreactions against NF-H and NF-L subunits and positive immunoreactions against the mammalian NF-M subunit. However, the protein previously referred to as P600 and recognized by the NN18-clone, has a very high molecular weight, thus, being different from mammalian NF-M subunit and from the protein revealed now in our study.

Neuronal pathway finding: from neurons to initial neural networks

Neuronal pathway finding is crucial for structured cellular organization and development of neural circuits within the nervous system. Neuronal pathway finding within the visual system has been extensively studied and therefore is used as a model to review existing knowledge regarding concepts of this developmental process. General principles of neuron pathway finding throughout the nervous system exist. Comprehension of these concepts guides neuroscience nurses in gaining an understanding of the developmental course of action, the implications of different anomalies, as well as the theoretical basis and nursing implications of some provocative new therapies being proposed to treat neurodegenerative diseases and neurologic injuries. These therapies have limitations in light of current ethical, developmental, and delivery modes and what is known about the development of neuronal pathways.

Neural organisation in the first optic ganglion of the nocturnal bee Megalopta genalis

Each neural unit (cartridge) in the first optic ganglion (lamina) of the nocturnal bee Megalopta genalis contains nine receptor cell axons (6 short and 3 long visual fibres), and four different types of first-order interneurons, also known as L-fibres (L1 to L4) or lamina monopolar cells. The short visual fibres terminate within the lamina as three different types (svf 1, 2, 3). The three long visual fibres pass through the lamina without forming characteristic branching patterns and terminate in the second optic ganglion, the medulla. The lateral branching pattern of svf 2 into adjacent cartridges is unique for hymenopterans. In addition, all four types of L-fibres show dorso-ventrally arranged, wide, lateral branching in this nocturnal bee. This is in contrast to the diurnal bees Apis mellifera and Lasioglossum leucozonium, where only two out of four L-fibre types (L2 and L4) reach neighbouring cartridges. In M. genalis, L1 forms two sub-types, viz. L1-a and L1-b; L1-b in particular has the potential to contact several neighbouring cartridges. L2 and L4 in the nocturnal bee are similar to L2 and L4 in the diurnal bees but have dorso-ventral arborisations that are twice as wide. A new type of laterally spreading L3 has been discovered in the nocturnal bee. The extensive neural branching pattern of L-fibres in M. genalis indicates a potential role for these neurons in the spatial summation of photons from large groups of ommatidia. This specific adaptation in the nocturnal bee could significantly improve reliability of vision in dim light.

Involvement of glial cells in rhythmic size changes in neurons of the housefly's visual system

In the housefly's first optic neuropile, or lamina, the axons of two classes of monopolar cell interneurons, L1 and L2, exhibit a daily rhythm of size changes: swelling during the day, and shrinking by night. At least for the L2 cells this rhythm is circadian. Moreover, epithelial glial cells that enwrap each lamina cartridge, its monopolar cell axons, and their surrounding crown of input photoreceptor terminals also change size, but in the opposite direction to the changes in L1 and L2-swelling by night and shrinking by day. The rhythmic changes in glia indicate the possible involvement of these cells in the lamina's circadian system. To examine their role in regulating the rhythmic changes of L1 and L2's axon sizes we have injected three chemicals into the haemolymph of the fly's head: fluorocitrate (FL) and iodoacetate (IAA), which affect the metabolism of glial cells, and octanol (OC), which closes gap junction channels. All chemicals exerted an effect on L1 and L2, which depended on the time of injection, the drug concentration, and the postinjection times at which we examined the fly's brains. Moreover, day/night changes in the axon sizes of L1 and L2 were increased in FL- and IAA-treated flies, indicating that glial cells may normally inhibit these changes by regulating the sizes of L1 and L2's axons during the day and night. In turn, lack of a day/night rhythm in L1 and L2 after OC injections shows that the rhythm's persistence depends on communication between the lamina cells through gap junction channels.

Differential expression of synapsin in visual neurons of the locustSchistocerca gregaria

In many taxa, photoreceptors and their second-order neurons operate with graded changes in membrane potential and can release neurotransmitter tonically. A common feature of such neurons in vertebrates is that they have not been found to contain synapsins, a family of proteins that indicate the presence of a reserve pool of synaptic vesicles at synaptic sites. Here, we provide a detailed analysis of synapsin-like immunoreactivity in the compound eye and ocellar photoreceptor cells of the locust Schistocerca gregaria and in some of the second-order neurons. By combining confocal laser scanning microscopy with electron microscopy, we found that photoreceptor cells of both the compound eye and the ocellus lacked synapsin-like immunostaining. In contrast, lamina monopolar cells and large ocellar L interneurons of the lateral ocellus were immunopositive to synapsin. We also identified the output synapses of the photoreceptors and of the L interneurons, and, whereas the photoreceptor synapses lacked immunolabeling, the outputs of the L interneurons were clearly labeled for synapsin. These findings suggest that the photoreceptors and the large second-order neurons of the locust differ in the chemical architecture of their synapses, and we propose that differences in the time course of neurotransmission are the reason for this.

Neuroarchitecture of the color and polarization vision system of the Stomatopod haptosquilla

The apposition compound eyes of stomatopod crustaceans contain a morphologically distinct eye region specialized for color and polarization vision, called the mid-band. In two stomatopod superfamilies, the mid-band is constructed from six rows of enlarged ommatidia containing multiple photoreceptor classes for spectral and polarization vision. The aim of this study was to begin to analyze the underlying neuroarchitecture, the design of which might reveal clues how the visual system interprets and communicates to deeper levels of the brain the multiple channels of information supplied by the retina. Reduced silver methods were used to investigate the axon pathways from different retinal regions to the lamina ganglionaris and from there to the medulla externa, the medulla interna, and the medulla terminalis. A swollen band of neuropil-here termed the accessory lobe-projects across the equator of the lamina ganglionaris, the medulla externa, and the medulla interna and represents, structurally, the retina's mid-band. Serial semithin and ultrathin resin sections were used to reconstruct the projection of photoreceptor axons from the retina to the lamina ganglionaris. The eight axons originating from one ommatidium project to the same lamina cartridge. Seven short visual fibers end at two distinct levels in each lamina cartridge, thus geometrically separating the two channels of polarization and spectral information. The eighth visual fiber runs axially through the cartridge and terminates in the medulla externa. We conclude that spatial, color, and polarization information is divided into three parallel data streams from the retina to the central nervous system.

Pretectotectal pathway: an ultrastructural quantitative analysis in cats

Both the pretectum (PT) and the superior colliculus (SC) play an important role in directing eye movements and in sensorimotor coupling. A reciprocal connection between the PT and the SC has been described, which suggests a strong interplay between these two structures. We injected the cat SC with retrograde tracers and examined the labeled pretectotectal (PTT) cells at the light and electron microscopic level. PTT cells were distributed mostly in the nucleus of the optic tract and 93.1% contained gamma amino butyric acid (GABA). We also observed that PTT cells are located outside of pretectal regions distinguished by dense retinal terminals and clusters of cells that contain calbindin. This suggests that the GABAergic PTT cells are distinct from the GABAergic pretectogeniculate cells that have been previously described as being distributed within these regions. Finally, to determine the synaptic targets of PTT terminals, we injected the PT with anterograde tracers and examined terminals labeled in the SC at the ultrastructural level. The labeled PTT terminals were beaded fibers that were distributed mainly within the stratum griseum superficiale (SGS) of the SC. Using postembedding immunocytochemistry, 94.5% were found to be GABAergic. The PTT terminals were mostly small in size and primarily contacted GABA-negative dendrites (88.1%) and in some cases somata (4.7%). The remainder terminated on GABAergic dendrites (7.2%). Our results suggest that the PTT cells constitute a separate population of GABAergic efferent cells in the PT, which may function to inhibit the activity of non-GABAergic SC efferent cells in the SGS.

Binding of an antibody against a noncompact myelin protein to presumptive glial cells in the visual system of the crab Ucides cordatus

Glial cells, in both vertebrate and invertebrate nervous systems, provide an essential environment for developmental, supportive, and physiological functions. However, information on glial cells themselves and on glial cell markers, with the exception of those of Drosophila and other insects, is not abundant in invertebrate organisms. A common ultrastructural feature of invertebrate nervous systems is that layers of glial cell cytoplasm-rich processes ensheath axons and neuronal and glial somata. In the present study, we have examined the binding of a monoclonal antibody to 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) in the compound eye and optic lobe of the crab Ucides cordatus using both light and electron microscopy. CNPase is a noncompact myelin protein that is a phenotypic marker of oligodendroglial and Schwann cells, is apparently involved in the ensheathment step prior to myelin compaction, and is also expressed by the potentially myelinating olfactory ensheathing glia. CNPase has raised much interest, first by virtue of its unusual enzymatic activity and more recently by its membrane-skeletal features and possible involvement in migration or expansion of membranes. We have found CNPase-like immunoreactivity in most cells of the compound eye basement membrane and both in optic cartridges of the synaptic layer and cells of the outer sublayer of the lamina ganglionaris. The results suggest that in the crab visual system some, but not all, glial cells, including some adaxonal glia, may express the noncompact myelin protein CNPase or a related protein.

A direct projection from superior colliculus to substantia nigra for detecting salient visual events

Midbrain dopaminergic neurons respond to unexpected and biologically salient events, but little is known about the sensory systems underlying this response. Here we describe, in the rat, a direct projection from a primary visual structure, the midbrain superior colliculus (SC), to the substantia nigra pars compacta (SNc) where direct synaptic contacts are made with both dopaminergic and non-dopaminergic neurons. Complementary electrophysiological data reveal that short-latency visual responses in the SNc are abolished by ipsilateral lesions of the SC and increased by local collicular stimulation. These results show that the tectonigral projection is ideally located to relay short-latency visual information to dopamine-containing regions of the ventral midbrain. We conclude that it is within this afferent sensory circuitry that the critical perceptual discriminations that identify stimuli as both unpredicted and biologically salient are made.

The first thalamocortical synapses are made in the cortical plate in the developing visual cortex of the wallaby (Macropus eugenii)

The time course of development and laminar distribution of thalamocortical synapses in the visual cortex of the marsupial mammal the wallaby (Macropus eugenii) has been studied by electron microscopy from the time of afferent ingrowth to the appearance of layer 4, the main target for thalamic axons. Axons were labeled from the thalamus by a fluorescent carbocyanine dye in fixed tissue or by transneuronal transport of horseradish peroxidase conjugated to wheat germ agglutinin from the eye. Thalamic axons first reached the cortex 2 weeks after birth and grew into the developing cortical plate without a waiting period in the subplate. The first thalamocortical synapses were detected 2 weeks later solely throughout the loosely packed zone of the cortical plate, where layer 6 cells previously have been shown to reside. As the thickness of the cortex increased with age, thalamocortical synapses were increasingly prevalent in the loosely packed zone of the cortical plate. With the appearance of layer 4, thalamocortical synapses were found there as well as in the marginal zone and layer 6. There was no evidence for an early population of thalamocortical synapses in the subplate. The first synapses made by thalamic axons were in a region containing layer 6 cells, one of their normal targets in the mature cortex.

The protocadherin Flamingo is required for axon target selection in the Drosophila visual system

Photoreceptor neurons (R cells) in the Drosophila visual system elaborate a precise map of visual space in the brain. The eye contains some 750 identical modules called ommatidia, each containing eight photoreceptor cells (R1-R8). Cells R1-R6 synapse in the lamina; R7 and R8 extend through the lamina and terminate in the underlying medulla. In a screen for visual behavior mutants, we identified alleles of flamingo (fmi) that disrupt the precise maps elaborated by these neurons. These mutant R1-R6 neurons select spatially inappropriate targets in the lamina. During target selection, Flamingo protein is dynamically expressed in R1-R6 growth cones. Loss of fmi function in R cells also disrupts the local pattern of synaptic terminals in the medulla, and Flamingo is transiently expressed in R8 axons as they enter the target region. We propose that Flamingo-mediated interactions between R-cell growth cones within the target field regulate target selection.

Flamingo regulates R8 axon-axon and axon-target interactions in the Drosophila visual system

Photoreceptors (R cells) in the Drosophila retina connect to targets in three distinct layers of the optic lobe of the brain: R1-R6 connect to the lamina, and R7 and R8 connect to distinct layers in the medulla. In each of these layers, R axon termini are arranged in evenly spaced topographic arrays. In a genetic screen for mutants with abnormal R cell connectivity, we recovered mutations in flamingo (fmi). fmi encodes a seven-transmembrane cadherin, previously shown to function in planar cell polarity and in dendritic patterning. Here, we show that fmi has two specific functions in R8 axon targeting: it facilitates competitive interactions between adjacent R8 axons to ensure their correct spacing, and it promotes the formation of stable connections between R8 axons and their target cells in the medulla. The former suggests a general role for Fmi in establishing nonoverlapping dendritic and axonal target fields. The latter, together with the finding that N-Cadherin has an analogous role in R7 axon-target interactions, points to a cadherin-based system for target layer specificity in the Drosophila visual system.