On the apparent nonadhesive nature of axospinous dendritic synapses

Synaptosome preparations are rich in synaptic sacs with adherent fragments of postsynaptic membrane and it has been hypothesized that pre- and postsynaptic membranes are strongly adherent across the synaptic cleft. Scanning electron microscopy of tissue specimens subject to preliminary tearing reveals synaptic terminals on neuronal cell bodies and dendrites as well as pits in the plasma membrane from which other terminals have been avulsed. However, neither synaptic terminals nor membrane pits were seen on dendrite spines in our material. These observations support the notion of adhesion between pre- and postsynaptic membranes but suggest an absence of such adhesiveness between the elements of the axospinous synapse, fitting well with recent conceptions about spine lability and the rapid making and breaking of axospinous synapses.     

Expression of the neural cell adhesion molecule and polysialic acid during early mouse embryogenesis

The expression of the neural cell adhesion molecule (N-CAM) and α 2–8 linked polysialic acid (PSA), which is believed to be predominantly expressed on N-CAM, was investigated during early embryonic development of the mouse (embryonic days 7.5 to 10.0). By immunocytochemistry, in tissue sections, N-CAM and PSA were not detectable at embryonic day 7.5 but were expressed in the prominent body regions such as somites, unsegmented mesoderm, developing heart, and neuroectoderm at embryonic day 8.0 N-CAM and PSA immunoreactivities were always predominantly associated with the plasma membrane. No tissue could be detected which was positive for PSA but negative for N-CAM. In Western blot analysis of whole embryos, by contrast, only the lightly sialylated and PSA-negative 180 and 140 kD isoforms of N-CAM were present at embryonic day 8.0 and strong expression of PSA-bearing, heavily sialylated N-CAM was not detectable before embryonic day 10.0. In Western blot analysis of N-CAM immunoaffinity purified from whole embryos and digested with neuraminidase as well as in Northern blot analysis, the 120 kD isoform of N-CAM or its corresponding mRNA were not expressed in detectable amounts during the time period investigated. © 1994 Wiley-Liss, Inc.     

Plasticity of the parallel Fiber-Purkinje cell synapse by spine takeover and new synapse formation in the adult rat

Alteration in synaptic connectivity between Purkinje cell spines and parallel fibers of the cerebellum were studied following partial deafferentation of Purkinje cells in the adult rat. Transection of parallel fibers by two lesions placed at a 1 mm interval on the folial crest were used to produce degeneration of these afferents. Ultrastructural analysis of synapses on Purkinje cell spines revealed degeneration with vacating of postsynaptic sites within 6 h. Reactive synaptogenesis as takeover of Purkinje cell spines by formation of new synapses from remaining parallel fibers occurred even before degenerating parallel fibers had vacated postsynaptic sites. This was accompanied by a marked increase in the number of dual innervations by reactive parallel fibers within one day. Some vacated postsynaptic sites were lost as indicated by a reduction in the number of synapses and others may have been taken over by newly formed synapses on spines. In addition, new synapses formed between the shafts of Purkinje cell branchlets and parallel fibers. Sprouting of parallel fibers occurred as small extensions without tubules while Purkinje cell spines reacted by forming elongated and multiple heads which contacted different parallel fibers. After 5 days degenerating boutons were rarely found. Enlarged spine heads were each capped by a proportionally enlarged parallel fiber bouton and joined by an elongated synaptic junction to parallel fibers. Some parallel fiber boutons were greatly enlarged and capped numerous profiles of spines.

This study shows that formation of new pre- and postsynaptic sites takes precedence over reoccupation of original contacts and that multiple synapses on individual spines are being eliminated to give rise to single contacts with boutons. This elimination resulted in enlargement of synaptic contact areas between Purkinje cell spines and parallel fibers by taking over postsynaptic sites from some vacated and eliminated boutons.     

Ecto-ATPase activity in cerebellum: implication to the function of synaptic transmission

The involvement of ATP in synaptic transmission was examined in synapses on granule cells of the rat cerebellum using ecto-ATPase activity. Reaction product was found in a majority but not all synapses between axodendritic, axoaxonic, and dendrodendritic appositions of granule cells and was associated with extracellular surface of both pre- and postsynaptic membranes. Specificity of the detection was justified by using diethyl pyrocarbonate, specific inhibitor of ecto-ATPase activity. These observations provide direct morphological evidence in support of the view that ATP participates in synaptic transmission and indicate functional heterogeneity of synapses in cerebellum.     

Distinct localization of GABA(B) receptors relative to synaptic sites in the rat cerebellum and ventrobasal thalamus

Metabotropic gamma-aminobutyric acid receptors (GABA(B)Rs) are involved in modulation of synaptic transmission and activity of cerebellar and thalamic neurons. We used subtype-specific antibodies in pre- and postembedding immunohistochemistry combined with three-dimensional reconstruction of labelled profiles and quantification of immunoparticles to reveal the subcellular distribution of pre- and postsynaptic GABA(B)R1a/b and GABA(B)R2 in the rat cerebellum and ventrobasal thalamus. GABA(B)R1a/b and R2 were extensively colocalized in most brain regions including the cerebellum and thalamus. In the cerebellum, immunoreactivity for both subtypes was prevalent in the molecular layer. The most intense immunoreactivity was found in Purkinje cell spines with a high density of immunoparticles at extrasynaptic sites peaking at around 240 nm from glutamatergic synapses between spines and parallel fibre varicosities. This is in contrast to dendrites at sites around GABAergic synapses where sparse and random distribution was found for both subtypes. In addition, more than one-tenth of the synaptic membrane specialization of spine-parallel fibre synapses were labelled at pre- or postsynaptic sites. Weak immunolabelling for both subtypes was also seen in parallel fibres but only rarely in GABAergic axons. In the ventrobasal thalamus, immunolabelling for both receptor subtypes was intense over the dendritic field of thalamocortical cells. Electron microscopy demonstrated an extrasynaptic localization of GABA(B)R1a/b and R2 exclusively in postsynaptic elements. Quantitative analysis further revealed the density of GABA(B)R1a/b around GABAergic synapses was higher than glutamatergic synapses on thalamocortical cell dendrites. The distinct localization of GABA(B)Rs relative to synaptic sites in the cerebellum and ventrobasal thalamus suggests that GABA(B)Rs differentially regulate activity of different neuronal populations.     

Differences in membrane structure between excitatory and inhibitory synapses in the cerebellar cortex

Several synapses of known physiological action are antomically segregated in the cerebellar cortex and are readily identified in freeze-fracture preparations. Excitatory synapses, such as the parallel fiber-to-Purkinje spine synapse, climbing fiber-to-Purkinje spine synapse, and mossy or climbing fiber-to-granule cell dendrite synapse, were characterized by small aggregates of large particles on the cytoplasmic half of the presynaptic membrane, by a distinctly widened synaptic cleft, and by a large aggregate of particles on the external half of the postsynaptic membrane. Inhibitory synapses, such as the stellate cell axon-to-Purkinje dendrite synapse and the basket cell axon-to-Purkinje soma synapse, had no comparable specialization of either the pre- and postsynaptic membrane. The striking contrast in membrane structure at excitatory and inhibitory synaptic contacts presumably reflects differences in either the composition or organization of membrane proteins integral to synaptic function. Puncta adhaerentia between granule cell dendrites in cerebellar glomeruli were characterized by particles aggregated on the external half of both apposed membranes and were further differentiated from synaptic contacts by the smaller size of the particles. Protuberances on the external half of the presynaptic membrane were either small and coextensive with the synaptic contact or were larger and surrounded it; it is suggested that the small protuberances are synaptic vesicle sites whereas the large ones are coated vesicle sites.     

Molecular Association of the Neural Adhesion Molecules L1 and N-CAM in the Surface Membrane of Neuroblastoma Cells is Shown by Chemical Cross-linking

The two neural cell adhesion molecules L1 and N-CAM could be shown to be associated in the surface membrane of cultured neuroblastoma cells by chemical cross-linking with 3,3'-dithiobis(sulphosuccinimidyl-propionate) and subsequent immunopurification and precipitation using antibodies to L1 and N-CAM. Glycoproteins recognized in neuroblastoma cells by antibodies to mouse liver membranes were not chemically cross-linked to L1 or N-CAM. These observations suggest that a molecular association between the two molecules may be the basis for their functional cooperativity (Kadmon et al., 1990a,b, J. Cell Biol., 110, 193-208; 209-218).     

Differentiation-regulated loss of the polysialylated embryonic form and expression of the different polypeptides of the neural cell adhesion molecule by cultured oligodendrocytes and myelin

The expression of the neural cell adhesion molecule (N-CAM) on cultured murine oligodendrocytes, their precursors, and myelin was examined by indirect immunofluorescence, biosynthetic radiolabeling followed by immunoprecipitation and Western blot analysis, using antibodies specific for various forms of the molecule. In all culture systems studied, whether the oligodendrocytes were cultured as an enriched fraction containing precursor cells or in the presence of astrocytes and neurons, a similar differentiation-stage-related expression of N-CAM was seen. At early developmental stages many tetanus toxin receptor- and A2B5 antigen-positive putative oligodendrocyte precursors with bipolar morphology were seen and found to express N-CAM in its embryonic form. Of the 04 antigen-positive immature oligodendrocytes with few slender processes most expressed N-CAM, but few the embryonic form of N-CAM. The more mature 01 or 010 antigen-positive oligodendrocytes were found to express exclusively the adult form of N-CAM. Oligodendrocytes synthesized the 120 and 140 kD forms of N-CAM (N-CAM 120 and N-CAM 140), but not N-CAM 180, although with differentiation, N-CAM 120 predominated in oligodendrocytes and also in pure myelin. N-CAM 120 could be released from oligodendrocytes and myelin by phosphatidylinositol-specific phospholipase C, suggesting that in both oligodendrocytes and myelin N-CAM 120 is inserted into the membrane by covalent linkage to phosphatidylinositol.     

Accumulation of N-CAM 180 at Contact Sites Between Neuroblastoma Cells and Latex Beads Coated with Extracellular Matrix Molecules

Neuronal cells expressing neural cell adhesion molecule (N-CAM) accumulate the largest N-CAM component (N-CAM 180) at cell - cell contact sites. To test whether this accumulation is induced by interactions at the surface membrane, latex beads coated with several purified adhesion molecules or extracellular matrix (ECM) components were co-cultured with neuroblastoma cells. Beads coated with L1, N-CAM, the L2/HNK-1 carbohydrate epitope-carrying molecules from adult mouse brain or laminin from Engelbreth-Holm-Swarm (EHS) sarcoma did not induce an accumulation of N-CAM 180 or L1 at sites of contact suggesting that these molecules are not directly involved in N-CAM 180 accumulation or that their mobility is required for this process. Beads coated with ECM components of the PF-HR9 cell line induced accumulation of N-CAM 180 at sites of contact with neuroblastoma cells. Accumulation was seen at cell bodies of undifferentiated and differentiated neuroblastoma cells, as well as on neurites and growth cones of differentiated neuroblastoma cells. Accumulation of the neural adhesion molecule L1 was also seen, but less prominently and reproducibly. These observations suggest that molecules of the ECM can directly or indirectly, e.g. via molecules linked to N-CAM 180 on the cell surface, induce accumulation of N-CAM 180.     

Developmental expression of neural cell adhesion molecules in the mouse neocortex and olfactory bulb.

Polyclonal antibodies to N-CAM and L1 and monoclonal antibodies to epitopes of N-CAM (designated 12F11, 8A2, and 12F8) were used to investigate the spatial and temporal distribution of these neural cell adhesion molecules during the development of mouse cortex and olfactory bulb. The aim of the study was to correlate developmental events such as cell migration, dendritic and axonal outgrowth, and synaptogenesis with the appearance and disappearance of specific molecules involved in cell-cell interactions. Western transfer studies indicated that 12F8 antibody recognized polysialic acid found on embryonic N-CAM; 8A2 antibody primarily recognized the 140 kD component of N-CAM while the 12F11 antibody recognized the 180 and the 140 kD forms. The study demonstrates a high degree of cell surface molecular specialization of different compartments in developing neocortex and olfactory bulb. L1 is found on a variety of unmyelinated fiber tracts including thalamocortical fibers, olfactory nerve, and inner plexiform layer of the olfactory bulb. In contrast, N-CAM epitope recognized by 12F11 antibody is present on olfactory nerve fibers but appears later and is much weaker than L1 on thalamocortical fibers and is absent from the olfactory lobe inner plexiform layer. Dendritic regions are best labeled by 12F8 antibody; the epitope becomes faint in adult cortex but remains strongly expressed in olfactory bulb. This study reveals that widespread N-CAM expression in the central nervous system is constituted by a diversity of local expression of different molecular forms of N-CAM; their different anatomical distributions suggest they may each have unique roles.     

Differences in membrane structure between excitatory and inhibitory components of the reciprocal synapse in the olfactory bulb

The reciprocal dendro-dendritic synapse between granule and mitral or tufted dendrites in the external plexiform layer of the olfactory bulb consists of an excitatory mitral-to-granule synaptic contact and an adjacent inhibitory granule-to-mitral synaptic contact. The pre- and postsynaptic membranes of both synaptic contacts were identified in replicas of freeze-fractured external plexiform layer in rabbits, mice, and chinchillas. At the excitatory synaptic contact there is a prominent specialization in the postsynaptic memberane, represented by an aggregate of homogeneous particles associated with the external half of the membrane. In contrast, the postsynaptic membrane at the inhibitory granule-to-mitral synaptic contact lacks evident internal specializations, and the distribution of particles on both fracture faces resembles that at non-synaptic regions. Less marked differences in particle distribution characterized the cytoplasmic half of the presynaptic membranes. These differences probably reflect diversity in the nature or distribution of membrane proteins at excitatory and inhibitory synapses. Protuberances on the external half of the presynaptic membrane, possibly sites of vesicle interaction with the plasma membrane, surrounded but were not coextensive with both types of synaptic contact. A few gap junctions connected proximal dendrites of mitral or tufted cells with granule cell dendrites.     

Electron microscopic autoradiography of the uptake of [H]GABA in dispersed cell cultures of rat cerebellums. II. The development of gabaergic synapses

The formation of GABAergic synapses in dispersed cell cultures of the rat cerebellum was followed from 7 to 21 days in vitro (DIV). The majority of GABAergic synapses appeared between 10 and 14 DIV, and apparently no additional GABAergic synapses formed after 14 DIV.The first step in the development of a GABAergic synapse appeared to be the formation of a large diameter swelling in a GABAergic neuronal process. After the initial contact between the pre- and postsynaptic elements was established, both the number of synaptic vesicles and the thickness of the postsynaptic density increased, while the cross-sectional area of the presynaptic element decreased. The length of the postsynaptic density showed some increase, but no changes were noted in the synaptic cleft thickness, size of the synaptic vesicles or the shape of the synaptic vesicles.Our findings indicate that the formation of GABAergic synapses was not preceded by the formation of other types of junctions or preformed synaptic elements. In addition, the timing and the rate of formation of GABAergic synapses appears not to depend on contact with a single type of postsynaptic neuron, but rather to depend upon intrinsic properties of the development of the GABAergic neuron.     

Differential synaptic distribution of the scaffold proteins Cask and Caskin1 in the bovine retina

Scaffold proteins organize pre- and postsynaptic compartments and align pre- and postsynaptic events. Cask is a multi-domain scaffold protein essential for brain synaptic functions. Caskin1 is a recently discovered, brain-specific Cask-interacting multi-domain protein of unknown function. In the present study, we determined the localization of these scaffold proteins in the bovine retina. The retina contains tonically active ribbon synapses and conventional synapses. We found Cask highly enriched in virtually all retinal synapses. Cask was localized in close vicinity to the active zone protein RIM1/2 in ribbon and conventional synapses. Caskin1 is also enriched in retinal synapses but is present only in a subset of Cask-positive synapses. These findings suggest that Cask plays an important role in all retinal synapses. In contrast, Caskin1 appears to execute more specialized functions in distinct sets of retinal synapses, possibly for neuronal pathway formation and stabilization of distinct synaptic contacts.     

Subcellular localization of GABA(B) receptor subunits in rat visual cortex

Although studies in the visual cortex have found gamma-aminobutyric acid B (GABA(B)) receptor-mediated pre- and postsynaptic inhibitory effects on neurons, the subcellular localization of GABA(B) receptors in different types of cortical neurons and synapses has not been shown directly. To provide this information, we have used antibodies against the GABA(B) receptor (R)1a/b and GABA(B)R2 subunits and have studied the localization of immunoreactivities in rat visual cortex. Light microscopic analyses have shown that both subunits are expressed in cell bodies and dendrites of 65-92% of corticocortically projecting pyramidal neurons and in 92-100% of parvalbumin (PV)-, calretinin (CR)-, and somatostatin (SOM)-containing GABAergic neurons. Electron microscopic analyses of immunoperoxidase- and immunogold-labeled tissue revealed staining in the nucleus, cytoplasm and cell surface membranes with both antibodies. Colocalization of both subunits was observed in all of these structures. GABA(B)R1a/b and GABA(B)R2 were concentrated in excitatory and inhibitory synapses and in extrasynaptic membranes. In GABAergic synapses, GABA(B)R1a/b and GABA(B)R2 were more strongly expressed postsynaptically on pyramidal and nonpyramidal cells than presynaptically. In type 1 synapses GABA(B)R1a/b and GABA(B)R2 was found in pre- and postsynaptic membranes. The nuclear localization of GABA(B)R1 and GABA(B)R2 subunits suggests a novel role for neurotransmitter receptors in controlling gene expression. The synaptic colocalization of GABA(B)R1 and GABA(B)R2 indicates that subunits form heteromeric assemblies of the functional receptor in inhibitory and excitatory synapses. Subunit coexpression in GABAergic synapses that include PV-containing and PV-deficient terminals suggests that pre- and postsynaptic GABA(B) receptor activation is provided by several different types of interneurons. The coexpression of both subunits in excitatory synapses suggests a role for GABA(B) receptors in the regulation of glutamate release and raises the question how these receptors are activated in the absence of pre-or postsynaptic GABAergic synaptic inputs to excitatory synapses.     

Neurocytology of the cerebral ganglion of Fasciola hepatica (Platyhelminthes)

An ultrastructural study of the organization and fine structure of the nervous system of the parasitic flatworm Fasciola hepatica was undertaken. The brain consists of paired cerebral ganglia, located just posterior to the oral sucker, that are connected by a transverse commissure which crosses over the dorsal surface of the pharynx. The cell bodies of the cerebral ganglia are not organized into a clearly defined rind around the neuropile but are loosely scattered around and within the neuropile area. The neuropile consists of two morphologically distinct types of unmyelinated nerve processes. The small nerve processes, with smooth cell membranes, are less than 2 micron in diameter, whereas the "giant" processes are greater than 12 micron in diameter and have extensively invaginated cell membranes. Giant processes make up the bulk of the nerve fibers in the transverse commissure and longitudinal nerve cords. Four morphological types of vesicles are present in the small processes; small clear vesicles (which were found associated with synapses), spheroidal and ellipsoidal dense vesicles, and dense-core vesicles. Two types of synapses are found in the neuropile: simple synapses characterized by pre- and postsynaptic membrane densities, clusters of small clear vesicles, and a clear synaptic cleft; and wedge-shaped synapses or divergent diads each having one presynaptic process synapsing onto two postsynaptic processes. Synaptic contacts were observed only between small processes; no synapses were observed on the cell bodies or on giant processes.     

Evidence for autoimmune antibodies directed against embryonic neural cell adhesion molecules (N-CAM) in patients with group B meningitis

Human brain tissue shares alpha 2-8 linked polymers of neuraminic acid with the carbohydrates expressed on the capsule of group B Neisseria meningitidis bacteria (Finne et al. (1983) Lancet ii, 355-357; Finne (1985) Trends Biochem. Sci. 10, 129-132; Rougon et al. (1986) J. Cell. Biol. 103, 2429-2437). We report that sera from patients suffering from group B meningitis exhibited IgM antibodies directed against the embryonic, but not the adult, form of neural cell adhesion molecules (N-CAM). These sera also stained live ATt20 cells as well as neuron membranes in mouse embryonic brain cultures. We have demonstrated that such antibodies, directed against carbohydrate moieties of bacterial capsula, were able to lyse cells expressing embryonic N-CAM in a complement-dependent cytotoxic assay. These data infer (1) that humans are able to develop anti-MenB humoral responses, (2) that such responses could initiate autoimmune disorders or be potentially detrimental by interfering with processes mediated by N-CAM interactions, (3) that the development of a vaccine against group B meningitidis should be considered with caution.     

Neuroligin-3 is a neuronal adhesion protein at GABAergic and glutamatergic synapses

Synaptic adhesion molecules are thought to play a critical role in the formation, function and plasticity of neuronal networks. Neuroligins (NL1-4) are a family of presumptive postsynaptic cell adhesion molecules. NL1 and NL2 isoforms are concentrated at glutamatergic and GABAergic synapses, respectively, but the cellular expression and synaptic localization of the endogenous NL3 and NL4 isoforms are unknown. We generated a panel of NL isoform-specific antibodies and examined the expression, developmental regulation and synaptic specificity of NL3. We found that NL3 was enriched in brain, where NL3 protein levels increased during postnatal development, coinciding with the peak of synaptogenesis. Subcellular fractionation revealed a concentration of NL3 in synaptic plasma membranes and postsynaptic densities. In cultured hippocampal neurons, endogenous NL3 was highly expressed and was localized at both glutamatergic and GABAergic synapses. Clustering of NL3 in hippocampal neurons by neurexin-expressing cells resulted in coaggregation of NL3 with glutamatergic and GABAergic scaffolding proteins. Finally, individual synapses contained colocalized NL2 and NL3 proteins, and coimmunoprecipitation studies revealed the presence of NL1-NL3 and NL2-NL3 complexes in brain extracts. These findings suggest that rodent NL3 is a synaptic adhesion molecule that is a shared component of glutamatergic and GABAergic synapses.     

Cadherins and synaptic specificity.

Cadherins are a family of cell-cell adhesion molecules that regulate morphogenesis in a variety of organs during development. In this review, we summarize recent evidence that cadherins may be involved in synaptogenesis in the vertebrate central nervous system. The first cadherin identified in synapses was N-cadherin, which is a major glycoprotein in postsynaptic density preparations. Electron microscopic studies have shown that this molecule is present at the synaptic cleft, bordering the transmitter release zone. To date, several other cadherins have also been found in synaptic junctions. Some cadherins have been observed in distinct subsets of synapses. The homophilic binding properties of cadherins may provide a molecular basis for the adhesive interactions between opposing synaptic membranes, and cadherins may promote a stable locking-in of pre- and postsynaptic membranes. Thus, cadherins may play a role in the formation and maintenance of synapses. Cadherin expression in synapses has been studied during development, regeneration, and activity-dependent plasticity. Moreover, it has been shown that each cadherin is expressed in specific neural circuits. In this context, we discuss the possibility that the differential expression of cadherins in the nervous system provides an adhesive framework for synaptic specificity.     

Analysis of a multiple-contact synapse missing a normally obligatory postsynaptic neuron: an electron microscopic study in the compound eye of Musca domestica

In the compound eye of the adult female fly Musca domestica, photoreceptors form populations of multiple-contact output synapses, stereotypic in their architecture and in the identity of the four postsynaptic elements. Two postsynaptic elements, always originating from monopolar interneurons L1 and L2, lie side by side beneath the elongated presynaptic bar. Beneath each end of the bar, a further postsynaptic contact is located. These contacts most often are two processes either of amacrine cells or of epithelial glial cells. Monopolar cell L3 may be postsynaptic as well, with either an amacrine or a glial process completing the tetrad. To learn more about the factors determining connectivity and synaptic architecture, a three-dimensional reconstruction of serial electron microscopic sections was used to analyze a population of photoreceptor synapses at which one of the normally obligatory postsynaptic neurons, L1, was missing. In this abnormal case, the synapses make the normal four postsynaptic contacts in only 39% of the cases, otherwise making three (39%) or two (22%) contacts. Specificity of connectivity is preserved faithfully except that beta processes of T1 cells were postsynaptic at 2% of the synapses, where they do not normally contribute. In contrast to normal synapses, where amacrine and glial cell processes are mutual exclusive, such pairings could coexist in the aberrant synapse (27% of all synapses). All postsynaptic cells contributed the normal number of processes to a synaptic site, except for three synapses each with a supernumerary amacrine cell process. The postsynaptic cells therefore may be involved in regulating the number of their contacts made to a synaptic site.     

Rod bipolar cells and horizontal cells form displaced synaptic contacts with rods in the outer nuclear layer of the nob2 retina

The nob2 mouse carries a null mutation in the Cacna1f gene, which encodes the pore-forming subunit of the L-type calcium channel, Ca(v)1.4. The loss of the electroretinogram b-wave in these mice suggests a severe reduction in transmission between photoreceptors and second-order neurons in the retina and supports a central role for the Ca(v)1.4 calcium channel at photoreceptor ribbon synapses, to which it has been localized. Here we show that the loss of Ca(v)1.4 leads to the aberrant outgrowth of rod bipolar cell dendrites and horizontal cell processes into the outer nuclear layer (ONL) of the nob2 retina and to the formation of ectopic synaptic contacts with rod photoreceptors in the ONL. Ectopic contacts are predominantly between rods and rod bipolar cells, with horizontal cell processes also present at some sites. Ectopic contacts contain apposed pre- and postsynaptic specializations, albeit with malformed synaptic ribbons. Cone photoreceptor terminals do not participate in ectopic contacts in the ONL. During retinal development, ectopic contacts appear in the days after eye opening, appearing progressively farther into the ONL at later postnatal stages. Ectopic contacts develop at the tips of rod bipolar cell dendrites and are less frequently associated with the tips of horizontal cell processes, consistent with the adult phenotype. The relative occurrence of pre- and postsynaptic markers in the ONL during development suggests a mechanism for the formation of ectopic synaptic contacts that is driven by the retraction of rod photoreceptor terminals and neurite outgrowth by rod bipolar cell dendrites.