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.

Neurochemical comparison of synaptic arrangements of parvocellular, magnocellular, and koniocellular geniculate pathways in owl monkey (Aotus trivirgatus) visual cortex

As in other primates, the lateral geniculate nucleus (LGN) of owl monkeys contains three anatomically and physiologically distinct relay cell classes, the magnocellular (M), parvocellular (P), and koniocellular (K) cells. M and P LGN cells send axons to the upper and lower tiers of layer IV, and K cells send axons to the cytochrome oxidase (CO) blobs of layer III and to layer I of primary visual cortex (V1). Our objective was to compare the synaptic arrangements made by these axon classes. M, P, and K axons were labeled in adult owl monkeys by means of injections of wheat germ agglutinin-horseradish peroxidase into the appropriate LGN layers. The neurochemical content of both pre- and postsynaptic profiles were identified by postembedding immunocytochemistry for gamma-aminobutyric acid (GABA) and glutamate. Our key finding is that the synaptic arrangements made by M, P, and K axons in owl monkey exhibit more similarities than differences. They are exclusively presynaptic, contain glutamate and form asymmetric synapses mainly with glutamate-positive dendritic spines. The majority of the remaining axons synapse with glutamatergic dendritic shafts. There are also differences between LGN pathways. M and P terminals are significantly larger and more likely to make multiple synapses than K axons, although M and P axons do not differ from each other in either of these characteristics. Of interest, a larger percentage of M and K axons than P axons make synapses with GABAergic dendritic shafts. Cells directly postsynaptic to M and K axons are known to exhibit orientation selectivity and, in some cases, direction selectivity. Cells postsynaptic to P axons do not show these properties, but instead tend to reflect their LGN inputs more faithfully; therefore, it is possible that these physiologic differences seen in the cortical cells postsynaptic to different LGN pathways reflect the differential involvement of inhibitory circuits.

Serotonin immunoreactivity in the retinal projecting isthmo-optic nucleus and evidence of brainstem raphe connections in the pigeon

Serotonin (5-HT) immunoreactive (-ir) profiles within the isthmo-optic nucleus (ION) of the centrifugal visual system (CVS) were studied in the pigeon using light microscopic immunohistofluorescent and electron microscopic immunocytochemical pre-embedding techniques. The brainstem origin of the 5-HT input upon the ION was determined by combining 5-HT immunohistofluorescence (FITC) and retrograde transneuronal tracing after intraocular injection of Rhodamine beta-isothiocyanate. The light microscopic results showed that 5-HT endings were mainly localised within the neuropillar zones of the ventral ION. The 5-HT-ir cell bodies, belonging to a lateral extension of the dorsal raphe system, were observed within the same region as the centrifugal ectopic neurons (EN) underlying the ION and some displayed dendritic processes which penetrated the nucleus. Double-labeled neurons, representing 5-HT-ir afferents to the ION, were identified only within the n. linearis caudalis region of the ventral raphe. The electron microscopic results confirmed the presence of 5-HT-ir dendritic processes within the ventral part of the nucleus and showed that they were contacted by axon terminals belonging to intrinsic interneurons. The functional organisation of the ION and the possible contribution of serotonergic raphe afferents and efferents are discussed in relation to present hypotheses linking the avian CVS to mechanisms of visual attention.

Mesencephalic and diencephalic afferent connections to the thalamic nucleus rotundus in the lizard, Psammodromus algirus

The present work is an analysis of the afferent projections to the thalamic nucleus rotundus in a lizard, both at the light- and electron-microscopic level, using biotinylated dextran amine (BDA) as a neuroanatomical tracer. This study has confirmed previously reported afferent projections to nucleus rotundus in reptiles and has also identified a number of new cellular aggregates projecting to this dorsal thalamic nucleus. After BDA injections into nucleus rotundus, retrogradely labelled neurons were observed consistently within the following neuronal groups in the midbrain and the diencephalon: (i) the stratum griseum centrale of the optic tectum; (ii) the nucleus subpretectalis in the pretectum; (iii) the nucleus ansa lenticularis posterior, the posterior nucleus of the ventral supraoptic commissure, and the posteroventral nucleus, in the dorsal thalamus and (iv) the lateral suprachiasmatic nucleus and part of the reticular complex in the ventral thalamus. Tectal axons entering nucleus rotundus were fine and varicose and formed exclusively asymmetric synaptic contacts, mainly on small dendritic profiles. Rotundal neurons had symmetric synapses made by large boutons probably of nontectal origin. After comparing our results with those in other reptiles, birds and mammals, we propose that the sauropsidian nucleus rotundus forms part of a visual tectofugal pathway that conveys mesencephalic visual information to the striatum and dorsal ventricular ridge, and is similar to the mammalian colliculo-posterior/intralaminar-striatoamygdaloid pathway, the function of which may be to participate in visually guided behaviour.

GABAergic pretectal terminals contact GABAergic interneurons in the cat dorsal lateral geniculate nucleus

Anterograde tracing techniques combined with postembedding immunocytochemical staining were used to determine the gamma amino butyric acid (GABA) content of pretectogeniculate (PT-LGN) terminals and their postsynaptic targets. The results provide evidence that PT-LGN terminals are GABAergic and that they contact GABAergic interneurons. These results corroborate previous anatomical studies and support the idea that the PT-LGN projection functions to disinhibit thalamocortical cells in the dorsal lateral geniculate nucleus.

Connection from cortical area V2 to MT in macaque monkey

The extrastriate visual area of the macaque monkey called MT or V5, receives its input from multiple sources. We have previously examined the synaptic connections made by V1 cells that project to MT (Anderson et al., 1998). Here, we provide a similar analysis of the projection from V2 to MT. The major target of the V2 projection in MT is layer 4, where it forms clusters of asymmetric (excitatory) synapses. Unlike the V1 projection, it also forms synapses in layers 1 and 2 and does not form synapses in layer 6. The most frequently encountered targets of boutons labeled from V2 were spines (67% in layer 4; 82% in layer 2/3). Unusually, only 5/12 boutons examined in layer 1 actually formed synapses. Unlike the V1 projection, multisynaptic boutons were rare (mean, 1.1 synapses per bouton vs. 1.7 for the V1 projection). Like the V1 projection, the input to MT from any point in V2 is sparse (contributing approximately 4-6% of the asymmetric synapses in the densest clusters in layer 4). The synapses of the V2 projection were similar in size to those of the V1 projection (0.1 microm(2) vs. 0.09 microm(2)) and both formed more complex postsynaptic densities on spines than on dendritic shafts. The clear differences between the V1 and V2 projection to MT indicate that their functions are complementary rather than completely overlapping.

Serotonergic innervation of the isthmo-optic nucleus of the pigeon centrifugal visual system. An immunocytochemical electron microscopic study

The ultrastructural features of serotonergic fibers, terminals and synaptic contacts were studied with the pre-embedding immunocytochemical method in the isthmo-optic nucleus of the pigeon centrifugal visual system. The 5-HT immunoreactive (-ir) profiles were diffusely distributed and their density was low. The labeled axons were thin and unmyelinated (mean diameter=0.21+/-0.03 microm) though a few larger myelinated axons were observed (mean diameter=0.51+/-0.07 microm). The 5-HT-ir terminals or varicosities were small (diameter=0.71+/-0.54 microm) and contained small agranular synaptic vesicles (diameter=28.5+/-6.9 nm) and large granular vesicles (diameter=102.2+/-19.5 nm). The latter only constituted approximately 1% of the total profiles containing synaptic vesicles in the isthmo-optic nucleus. In single thin sections, only 5% of the 5-HT-ir varicosities exhibited an active asymmetrical zone synapsing upon dendritic profiles of centrifugal visual neurons. Calculations indicated that 17% of these 5-HT-ir varicosities were actually engaged in junctional synaptic relationships, whereas the remaining (83%) were nonjunctional. The data suggest that, within the isthmo-optic nucleus, 5-HT acts both at synaptic junctions (wiring transmission) and at a distance via the extracellular space (volume transmission). These 5-HT afferents could thus modulate the activity of the retinopetal neurons and visual information processing.

Subcellular localization of neuronal nitric oxide synthase in the superficial gray layer of the rat superior colliculus

The superficial layers of the rat superior colliculus (sSC) receive innervation from retina and include nitric oxide synthase (NOS)-immunoreactive neurons. We used electron microscopic immunocytochemistry to assess the subcellular localization of neuronal NOS (nNOS) in the sSC. nNOS immunoreactivity was detected on the external membrane of mitochondria, endoplasmic reticulum, in pre- and postsynaptic profiles and also diffusely distributed in the cytosol. Postsynaptic labeled regions were often associated with presumptive retinal unlabeled terminals. Microtubules also appeared intensely labeled. These results show that NOS immunoreactive neurons may be innervated by retinal terminals and suggest an association of nNOS with cytoskeletal elements.

Competition in the development of nerve connections: a review of models

The establishment and refinement of neural circuits involve both the formation of new connections and the elimination of already existing connections. Elimination of connections occurs, for example, in the development of mononeural innervation of muscle fibres and in the formation of ocular dominance columns in the visual cortex. The process that leads to the elimination of connections is often referred to as axonal or synaptic competition. Although the notion of competition is commonly used, the process is not well understood-with respect to, for example, the type of competition, what axons and synapses are competing for, and the role of electrical activity. This article reviews the types of competition that have been distinguished and the models of competition that have been proposed. Models of both the neuromuscular system and the visual system are described. For each of these models, the assumptions on which it is based, its mathematical structure, and the extent to which it is supported by the experimental data are evaluated. Special attention is given to the different modelling approaches and the role of electrical activity in competition.

Expression of presynaptic proteins is closely correlated with the chronotopic pattern of axons in the retinotectal system of the chick

Newly synthesized presynaptic integral membrane proteins in neurons are transported in precursor vesicles from the site of protein biosynthesis in the cell body by fast axonal flow to the presynaptic terminal. We followed the path that presynaptic proteins travel on the way to their central targets of the highly ordered primary visual pathway of the chick and analyzed the developmental changes in the expression of synaptic vesicle protein 2 (SV2), synaptotagmin, and syntaxin. Immunofluorescences revealed that: (1) the onset of protein expression in the retinal ganglion cells occurs in a central to peripheral developmental pattern from embryonic day 4 (E4) onward; (2) the proteins were found first in the inner and later in the outer plexiform layer of the retina; and (3) they were redistributed from the photoreceptor inner segments and cell bodies to the terminals in the outer plexiform layer. From E4 onward, immunopositive axons for SV2, synaptotagmin, and syntaxin were found in the optic nerve, disappearing after E9 for SV2 and synaptotagmin. The optic tract was stained for SV2 and synaptotagmin between E7 and E12, for syntaxin until the posthatching period. Finally, immunoreactivities for the investigated proteins were present at the surface of the tectum from E8 onward, when first retinal axons arrived there. The present study revealed that SV2 and synaptotagmin, but not syntaxin, are, expressed in a transient wave that follows the advancement of optic axons and the proteins towards the optic tectum.

Cytochemical evidence that acetylcholine is a neurotransmitter of neurons that make excitatory and inhibitory outputs in the locust ocellar visual system

Three different cytochemical methods were used to detect acetylcholine in large, second-order neurons of locust ocelli (L-neurons). The first method used polyclonal antibodies raised against choline cleaved from acetylcholine and then conjugated with native protein, and this revealed strong staining for acetylcholine in axons whose number, size, and location indicated that they were of L-neurons. A corresponding staining pattern was found using the second method with a polyclonal antiserum against choline acetyltransferase (ChAT). The third method was the histochemical detection at the electron microscope level of acetylcholinesterase, the enzyme responsible for the breakdown of acetylcholine. We found that this enzyme is located in synaptic clefts of L-neurons in both of the brain regions where L-neurons are known to make excitatory and inhibitory output synapses. Acetylcholinesterase was confined to synaptic sites, which is consistent with a role in synaptic transmission at these synapses. Taken together, the findings suggest that L-neurons use acetylcholine as a neurotransmitter.

GAL4/UAS-WGA system as a powerful tool for tracing Drosophila transsynaptic neural pathways

Visualization of specific transsynaptic neural pathways is an indispensable technique for understanding the relationship between structure and function in the nervous system. Here, we demonstrate the application of the wheat germ agglutinin (WGA) transgene technique for tracing transsynaptic neural pathways in Drosophila. The intracellular localization of WGA was examined by immunoelectron microscopy. WGA signals were detected in granule-like structures in both the outer photoreceptor cells expressing WGA and the second-order laminar neurons. Misexpression of tetanus toxin (TNT), which inactivates N-synaptobrevin, in the outer photoreceptor cells resulted in the elimination of on/off transients in electroretinogram (ERG) recordings and in a great reduction in WGA transfer into laminar neurons, suggesting that anterograde WGA transsynaptic transfer is dependent mainly on synaptic transmission. Retrograde WGA transfer was also detected upon its forced expression in muscle cells. WGA primarily expressed in muscle cells was taken up by motoneuron axons and transported to their cell bodies in the ventral nerve cord, suggesting that WGA can trace motoneuronal pathways in combination with the muscle-specific GAL4 driver. Thus, the GAL4/UAS-WGA system should facilitate the dissection of the Drosophila neural circuit formation and/or synaptic activity in various regions and at various developmental stages.

Myelin debris clearance during Wallerian degeneration in the goldfish visual system

We have examined the clearance of myelin debris from the visual pathways of the goldfish during Wallerian degeneration. Both the rate and pattern of myelin disappearance from the optic nerve and tract were determined using immunohistochemistry on frozen sections, as well as plastic sections and electron microscopy. Animals with and without regenerating optic axons were examined in order to determine whether the axons play a role in myelin clearance. We found that myelin is cleared at different rates along the visual paths. Thus, virtually all myelin debris is gone in the optic tract and distal optic nerve stump by 4 weeks after surgery, while in the cranial nerve segment, myelin clearance is still incomplete at 6 weeks postoperative. These temporal and spatial patterns of myelin clearance are the same in animals with and without regenerating axons, thus indicating that growing axons do not influence this process. Finally, ultrastructural observations revealed that both astrocytes and microglia participate in phagocytosing myelin debris in the optic nerve, while in the tract, the vast majority of debris is removed by microglia alone. These data are discussed with regard to possible mechanisms controlling the differential expression of myelin clearance.

In vivo calcium accumulation in presynaptic and postsynaptic dendrites of visual interneurons

In this comparative in vivo study of dendritic calcium accumulation, we describe the time course and spatial integration properties of two classes of visual interneurons in the lobula plate of the blowfly. Calcium accumulation was measured during visual motion stimulation, ensuring synaptic activation of the neurons within their natural spatial and temporal operating range. The compared cell classes, centrifugal horizontal (CH) and horizontal system (HS) cells, are known to receive retinotopic input of similar direction selectivity, but to differ in morphology, biophysics, presence of dendrodendritic synapses, and computational task. 1) The time course of motion-induced calcium accumulation was highly invariant with respect to stimulus parameters such as pattern contrast and size. In HS cells, the rise of [Ca(2+)](i) can be described by a single exponential with a time constant of 5-6 s. The initial rise of [Ca(2+)](i) in CH cells was much faster (tau approximately 1 s). The decay time constant in both cell classes was estimated to be at least 3.5 times longer than the corresponding rise time constant. 2) The voltage-[Ca(2+)](i) relationship was best described by an expansive nonlinearity in HS cells and an approximately linear relationship in CH cells. 3) Both cell classes displayed a size-dependent saturation nonlinearity of the calcium accumulation. Although in CH cells calcium saturation was indistinguishable from saturation of the membrane potential, saturation of the two response parameters differed in HS cells. 4) There was spatial overlap of the calcium signal in response to nonoverlapping visual stimuli. Both the area and the amplitude of the overlap profile was larger in CH cells than in HS cells. Thus calcium accumulation in CH cells is spatially blurred to a greater extent than in HS cells. 5) The described differences between the two cell classes may reflect the following computational tasks of these neurons: CH cells relay retinotopic information within the lobula plate via dendritic synapses with pronounced spatial low-pass filtering. HS cells are output neurons of the lobula plate, in which the slow, local calcium accumulation may be suitable for local modulatory functions.

Growth-associated protein 43 is located in type I corticothalamic terminals in the cat visual thalamus

Growth-associated protein 43 (GAP 43) is a presynaptic protein that has been proposed to be involved in synaptic plasticity. To determine the location of GAP 43 within the synaptic circuitry of the thalamus, immunocytochemical staining for GAP 43 was examined in a relay nucleus, the dorsal lateral geniculate nucleus (dLGN), and two association nuclei, the pulvinar nucleus and the lateral subdivision of the lateral posterior (LP) nucleus. In the dLGN, moderate neuropil staining was seen in the A laminae, and denser staining was found in the interlaminar zones and the C laminae. Uniform dense staining of the neuropil was found in both the pulvinar and LP nuclei. At the ultrastructural level, the GAP 43 staining was restricted to small-diameter myelinated axons, thin unmyelinated fibers, and small terminals that contained densely packed round vesicles (RS profiles) and made asymmetric synaptic contacts with small-caliber dendrites in the extraglomerular neuropil. The distribution of immunocytochemical label within the visual thalamus suggests that GAP 43 is confined to type I corticothalamic terminals and axons that originate from extrastriate cortical areas. These results also suggest that in both relay and association nuclei GAP 43 may be used to augment the cortical control of thalamic activity. In addition, these results underscore the distinction between the small type I corticothalamic terminals, which appear to contain GAP 43 throughout the visual thalamus, and the large type II corticothalamic terminals that, like the type II retinal terminals in the dLGN, do not contain GAP 43.

Intrinsic determinants of retinal axon collateralization and arborization patterns

Permanent, novel retinal projections to the principal thalamic somatosensory (ventrobasal) or auditory (medial geniculate) nuclei can be produced in adult hamsters if the superior colliculus is ablated bilaterally and the somatosensory and auditory lemniscal axons are transected unilaterally on the day of birth. We studied the development of those novel projections by labeling retinal axons with the fluorescent tracer 1,1'-dioctadecyl-3,3,3', 3'-tetramethylindocarbocyanine perchlorate to examine the relative roles of intrinsic factors and axon-target interactions in the specification of retinal axon connections. Our principal findings are as follows: (1) In hamsters operated on the day of birth to produce the novel retinal projections, retinal ganglion cell axons projecting to the ventrobasal or medial geniculate nuclei develop in three morphologically distinct stages, i.e., elongation, collateralization, and arborization, as do retinal axons projecting to the dorsal lateral geniculate nucleus, the principal thalamic visual nucleus, in normal hamsters. (2) In both the ventrobasal and medial geniculate nuclei of operated hamsters, as in the dorsal lateral geniculate nucleus of normal hamsters, collateral branches were initially formed by retinal ganglion cell axons in both the superficial and internal components of the optic tract and only collaterals from the superficial component formed permanent projections. (3) The retinofugal axon terminal arbors in the ventrobasal and medial geniculate nuclei of mature, operated hamsters resemble the same three morphologic classes that are observed in the lateral geniculate nucleus of normal hamsters, although their absolute size appears to be altered. These data suggest that both superficial and internal optic tract axons can produce thalamic collaterals during development but that only superficial optic tract axons can permanently retain thalamic collaterals. Furthermore, the same morphologic types of retinofugal axons appear to contribute to normal and surgically induced retinal projections.

Synaptic and neurochemical features of calcitonin gene-related peptide containing neurons in the rat accessory optic nuclei

Within the rodent visual system, calcitonin gene-related peptide (CGRP) is selectively expressed in neurons in the accessory optic nuclei (AON), including the dorsal terminal nucleus (DTN), lateral terminal nucleus (LTN) and medial terminal nucleus (MTN). To determine whether CGRP-immunoreactive neurons are involved in visual circuitry, electron microscopic preparations were analyzed from normal rats and rats with optic nerve transections. A co-localization analysis was also made because CGRP-labeled neurons had features of GABAergic neurons. Thus, sections were prepared for light microscopy to determine whether CGRP-containing neurons also had glutamate decarboxylase (GAD) and other markers for GABAergic neurons, such as calcium binding proteins: calbindin (CB), calretinin (CR) and parvalbumin (PV). Electron microscopy of the DTN and LTN showed CGRP-labeled somata and dendrites that were postsynaptic to axon terminals forming asymmetric synapses. Many of these axon terminals degenerated following optic nerve transection indicating that retinal ganglion cells form synapses with CGRP-labeled neurons in the AON. In the DTN, LTN and MTN, CGRP-labeled axon terminals formed symmetric synapses with unlabeled somata as well as dendritic shafts and spines. Consistent with this type of synapse being GABAergic were the co-localization data showing that about 90% of the CGRP-labeled neurons co-localized GAD in the AON. Many CGRP-labeled neurons showed immunostaining for CR (40%) whereas only a few had labeling for CB (5%). No CGRP-labeled neurons had PV. These data show that CGRP-containing neurons receive direct retinal input and represent a subpopulation of GABAergic neurons which differentially co-express calcium-binding proteins.

Ultrastructural features of non-commissural GABAergic neurons in the medial vestibular nucleus of the monkey

The ultrastructural characteristics of non-degenerating GABAergic neurons in rostrolateral medial vestibular nucleus were identified in monkeys following midline transection of vestibular commissural fibers. In the previous papers, we reported that most degenerated cells and terminals in this tissue were located in rostrolateral medial vestibular nucleus, and that many of these neurons were GABA-immunoreactive. In the present study, we examined the ultrastructural features of the remaining neuronal elements in this medial vestibular nucleus region, in order to identify and characterize the GABAergic cells that are not directly involved in the vestibular commissural pathway related to the velocity storage mechanism. Such cells are primarily small, with centrally-placed nuclei. Axosomatic synapses are concentrated on polar regions of the somata. The proximal dendrites of GABAergic cells are surrounded by boutons, although distal dendrites receive only occasional synaptic contacts. Two types of non-degenerated GABAergic boutons are distinguished. Type A terminals are large, with very densely-packed spherical synaptic vesicles and clusters of large, irregularly-shaped mitochondria with wide matrix spaces. Such boutons form symmetric synapses, primarily with small GABAergic and non-GABAergic dendrites. Type B terminals are smaller and contain a moderate density of round/pleomorphic vesicles, numerous small round or tubular mitochondria, cisterns and vacuoles. These boutons serve both pre- and postsynaptic roles in symmetric contacts with non-GABAergic axon terminals. On the basis of ultrastructural observations of immunostained tissue, we conclude that at least two types of GABAergic neurons are present in the rostrolateral portion of the monkey medial vestibular nucleus: neurons related to the velocity storage pathway, and a class of vestibular interneurons. A multiplicity of GABAergic bouton types are also observed, and categorized on the basis of subcellular morphology. We hypothesize that "Type A" boutons correspond to Purkinje cell afferents in rostrolateral medial vestibular nucleus, "Type B" terminals represent the axons of GABAergic medial vestibular nucleus interneurons, and "Type C" boutons take origin from vestibular commissural neurons of the velocity storage pathway.

Location of muscarinic type 2 receptors within the synaptic circuitry of the cat visual thalamus

A cholinergic projection from the parabrachial region (PBR) of the brainstem to the visual thalamus has been studied in great detail during the past 20 years. A number of physiological studies have demonstrated that this projection causes a dramatic change in thalamic activity during the transition from sleep to wakefulness. Additionally, the PBR may mediate more subtle changes in thalamic activity as attentional levels fluctuate during the waking state. The synaptic circuitry underlying these events has been identified in the cat thalamus. However, there is currently no anatomical information regarding the distribution of cholinergic receptors in relation to this circuitry. To begin to understand how the PBR projection modulates thalamic activity, we used immunocytochemical techniques to examine the distribution of muscarinic type 2 (M2) receptors in the visual thalamus of the cat. The distribution of M2 receptors correlates well with previous reports of the distribution of cholinergic terminals in the visual thalamus. At the light microscopic level, dense M2 staining was seen in the neuropil of the dorsal lateral geniculate nucleus (dLGN) and pulvinar nucleus and in somata and proximal dendrites of cells in the thalamic reticular nucleus (TRN). In the dLGN and pulvinar nucleus, we quantitatively analyzed the distribution of M2 receptors using electron microscopy. Postembedding immunocytochemistry for gamma aminobutyric acid (GABA) was used to determine whether M2 receptors are present on interneurons or thalamocortical cells. In particular, we examined the distribution of M2 receptors with respect to the known sites of PBR terminations. The dendrites of both thalamocortical cells and interneurons were stained for the M2 receptors in both the glomerular and extraglomerular neuropil. However, the densest staining was found in glomerular GABAergic profiles that displayed the morphology associated with interneuron dendritic terminals (F2 profiles). Our data suggest that M2 receptors play an important role both in blocking thalamic spindle oscillations and in increasing the efficacy of signal transmission during increased attentional states.

Synaptogenesis in retino-receptive layers of the superior colliculus of the opossum Didelphis marsupialis

The maturation of the neuropil and synapse formation were examined in the retino-receptive layers of the superior colliculus (SCr-r) in the opossum from a period prior to the onset of arborization of retinocollicular fibers (postnatal day 22 - P22), at 44% of the coecal period (CP), to the end of the fast phase of optic fiber myelination and weaning time (P81 - 118% CP). Development of the SCr-r neuropil follows a protracted time course and can be divided into three broad stages, which are characterized by (I) Large extracellular spaces, numerous growth cones that participate rarely in synaptic junctions, vesicles-poor immature synapses (P22-P30), (II) Synapses of varied morphology with abundant synaptic vesicles, and small terminals with dark mitochondria and round synaptic vesicles (RSD terminals) synapsing mostly onto dendritic shafts, flat-vesicles (F) terminals (P40-P56), (III) Sequential appearance of retinal (R) and pleomorphic-vesicles (P) terminals and of RSD terminals synapsing onto spine or spine-like processes, appearance of glomerulus-like synaptic arrays (synaptic islets) (P61-P81). The advancement of synaptogenesis in SCr-r from stage I to II and from stage II to III correlates closely with the differentiation of astrocytes and oligodendrocytes, respectively.

The role of pattern vision in the development of cortico-cortical connections

The development of cortico-cortical connections was studied in kittens deprived of vision by binocular eyelid suture during the formation of axonal arbors and synaptogenesis, i.e. between the second postnatal week and the end of the third postnatal month. Axons originating in area 17 and terminating either in ipsilateral or contralateral visual areas were visualized with biocytin. In ipsilateral areas 17 and 18, distinct clusters of branches begin to form, distally from the injection, during the second half of the first postnatal month, independently of pattern vision. More proximal clusters differentiate during the second postnatal month, and this seems to involve elimination of exuberant axonal branches. In kittens deprived of vision for 3 or more months, beginning before natural eye opening, the distal clusters regress and the proximal ones fail to differentiate. In extrastriate areas, distinct clusters of branches have segregated by the end of the second postnatal month, independently of visual experience; however, in kittens deprived of vision for 2 or more months, one of the clusters was selectively eliminated. In contralateral areas 17 and 18, we found stunted terminal arbors in kittens continuously deprived of vision. This was already noticeable at the end of the first postnatal month. Apparently, in the absence of pattern vision, most axons undergo only limited growth and do not form their characteristic terminal columns. Many of these axons are subsequently eliminated. In contrast, 8 days of vision beginning at natural eye opening and followed by visual deprivation caused a nearly normal development of intrahemispheric and interhemispheric connections. In conclusion, pattern vision appears to validate connections at early stages of their development; this validation is necessary for their further growth and differentiation that can then continue autonomously.

The network-selective actions of quinoxalines on the neurocircuitry operations of the rabbit retina

We examined the contribution of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxalole-4-propionic acid (AMPA)/kainate (KA) receptors to the light-responses of rabbit retinal neurons. In the outer retina, bath application of the AMPA/KA receptor antagonists 6,7-dinitro-quinoxaline-2,3-dione (DNQX) and 2,3,dihydroxy-6-nitro-7-sulfamoyl-benzo-f-quinoxaline (NBQX) blocked the light-responses of horizontal cells. Application of quinoxalines enhanced ON-bipolar cell light-responses, and was associated with a hyperpolarization of their resting potentials. In the inner retina, application of both AMPA/KA and NMDA antagonists to AII amacrine-like cells only partially blocked their light-responses. Their residual responses may reflect electrical coupling to neighboring ON-center cone bipolar cells, and can inhibit OFF-center ganglion cells. ON-sustained ganglion cells were highly sensitive to the quinoxalines, which reduced their light-evoked firing, while the firing of ON-transient cells remained as NMDA-mediated light-responses. Quinoxalines had differential effects on the firing rates of ON- and OFF-center ganglion cells: ON-cells were reduced, while OFF-cells were increased. In contrast, firing rates of ON-OFF ganglion cells were not excited by NBQX, and showed a recovered light-response mediated by NMDA receptors. The receptive field surround was lost in ganglion cells. For comparison, NMDA antagonists had only moderate effects on all ganglion cell light-responses. Our results indicate that NMDA and AMPA/KA receptors both contribute to ganglion cell light-responses. However, AMPA/KA receptors also significantly effect the light-response of neurons presynaptic to retinal ganglion cells, altering the observed pharmacology at the ganglion cell level.