Conventional Synapse Research Articles

Tonic Signaling Leads to Off-Target Activation of T Cells Engineered with Chimeric Antigen Receptors That Is Not Seen in T Cells Engineered with T Cell Antigen Coupler (TAC) Receptors

Chimeric antigen receptors (CARs) are powerful tools that enable MHC-independent activation of T cells. Recent reports have indicated that constitutive, low-level (tonic) signaling by CARs can impair the utility of the engineered T cells. The single-chain antibody (scFv) binding domain was one of the features determined to promote tonic signaling. We have recently developed a novel chimeric receptor, known as the T cell antigen coupler (TAC), that is less prone to tonic signaling than second-generation CARs. The TAC consists of a scFv-based antigen binding domain, a CD3-binding domain that couples the TAC to endogenous T cell receptor (TCR), and a transmembrane and cytoplasmic coreceptor (CD4) domain. In contrast to CARs, this design enables TAC-T cells to signal through the endogenous TCR, which we propose provides a fidelity to natural T cell signal regulation. Interestingly, we have recently reported that CAR-T cells have a greater propensity for off-target activation than TAC-T cells, suggesting a safety advantage to TAC-T cells (Helsen et al., Nat. Comm., 2019). Further characterization of the differences between CAR- and TAC-T cell signal initiation and activation is required to understand how their design affects sensitivity, specificity and regulation of T cell activation. Examination of the activation requirements for BCMA-specific CAR-T cells and TAC-T cells confirmed that TAC-T cells are reliant upon the endogenous TCR for T cell activation whereas CAR-T cells are TCR-independent. TRAC knock-out CAR-T cells retained potent effector function at levels similar to CAR-T cells with intact TCR expression, whereas TRAC knock-out TAC T-cells showed significant impairment in effector function. Consistent with TCR-dependence, the immunological synapse produced by TAC-T cells displays all the hallmarks of a conventional immunological synapse, whereas CAR-T cells form unconventional synapses. Unlike TAC-T cells, immunological synapses formed by CAR-T cells display non-uniform central supramolecular activation clusters, disperse Lck distribution, a lack of an LFA-1 associated adhesion ring (Figure), as well as more disperse delivery of perforin to the cell interface. CAR-T cells also formed synapses faster than TAC-T cells. This suggests that while TAC T-cells are beholden to the requirement of organized, mature synapse formation, CAR T-cells can rapidly form less structurally organized synapses. Transcriptional profiling of CAR-T cells in the absence of antigen stimulation revealed a basal activation status associated with upregulation of Nur77, a transcription factor that is downstream of TCR activation. Transcriptional profiling of TAC-T cells failed to reveal evidence of TCR signaling in the absence of stimulation. Further evaluation of CAR- and TAC- T cells in the absence of stimulation revealed elevated levels of CD69, PD-1 and LAG-3 in CAR-T cells compared with TAC-T cells, as well as higher expression of IL-2, IFNγ, and TNF in CAR-T cells. Interestingly, the level of tonic signaling was dependent on the antigen-binding scFV, as otherwise identical BCMA-specific CAR- and TAC-T cells displayed different levels of CD69, PD-1 and LAG-3 depending on the identity of the BCMA-specific scFv. Despite different levels of basal activation, both CAR- and TAC-T cells displayed comparable activation kinetics as measured by upregulation of CD69 and Ki-67, as well as proliferation. However, the elevated level of basal activation rendered the CAR-T cells more easily activated by a cross-reactive off-target antigen that failed to stimulate TAC-T cells carrying the same binding domain. These data suggest that the TAC receptor offers a valuable alternate platform to CAR-T cells. The antigen-binding scFv domain has a direct impact on tonic signaling and basal activation in CAR-T cells. Conversely, TAC-T cells are less susceptible to basal activation and this works suggests that the TAC receptor can deploy scFv binding domains that are not suitable for CARs. This work was supported by Triumvira Immunologics and Genome Canada. Figure 1 Disclosures Bramson: McMaster University: Current Employment, Patents & Royalties; Triumvira Immunologics: Current Employment, Current equity holder in private company, Research Funding.

Multilayer Neural Network with Synapse Based on Two Successive Memristors

Introduction: Synapse based on two successive memristors builds the synaptic weights of the artificial neural network for training three-bit parity problem and five-character recognition. Methods: The proposed memristor synapse circuit creates positive weights in the range [0;1], and maps it to range [-1;1] to program both the positive and negative weights. The proposed scheme achieves the same accuracy rate as the conventional bridge synapse schemes which consist of four memristors. Results and Conclusion: However, proposed synapse circuit decreases 50% the number of memristors and 76.88% power consumption compared to the conventional bridge memristor synapse.

Improved Conductance Linearity and Conductance Ratio of 1T2R Synapse Device for Neuromorphic Systems

We report on a 1-transisor/2-resistor (1T2R) synapse device with improved conductance linearity and conductance ratio under an identical pulse condition for hardware neural networks with high pattern-recognition accuracy. Utilizing an additional series-connected resistor, the conductance linearity of a synapse device was significantly improved owing to the reduced initial voltage drop on an resistive RAM (RRAM) device during depression conditions. Moreover, to maximize the conductance ratio of a synapse device, we utilized a steep subthreshold region of an MOSFET by a parallel connection of an RRAM and a transistor. A small change in voltage on the RRAM directly controlled the gate bias of the MOSFET, which causes a large change in the drain current. Compared with a conventional RRAM synapse device, the 1T2R synapse device shows an improved conductance linearity and conductance ratio ( > ×100). Finally, we confirmed an excellent classification accuracy by using a neural network simulation based on a multilayer perceptron.

Bigger, Stronger, Faster: Chimeric Antigen Receptor T Cells Are Olympic Killers

Chimeric antigen receptor T cells (CAR T) re-directed to CD19, have induced remarkable responses in clinical trials for patients with B-cell malignancies. Patients have responded to therapy with a CAR T dose which is a fraction of the pre-existing tumor burden. Explanations for this observation include studies which show the proliferative potential of the CAR T cells (Kalos M et al Sci Transl Med 2011), as well as our recent study which showed that individual CAR T cells can serial kill tumor cells (Davenport AJ et al CIR 2015, Figure 6). Using our dual antigen receptor model (OT-I T cell receptor and 2nd generation HER-2 CAR in the same T cell, termed CAR.OTI cells), we also observed a reproducible and significantly shorter time from CAR- vs TCR-mediated activation to detachment from dying tumor cells (Davenport AJ et al CIR 2015, Figure 4D)suggesting that CAR-mediated individual killing events are actually faster. To explore how this may occur, we examined the immune synapse structure at 20 minutes of CAR.OTI CTL co-culture with tumor cells expressing the cognate antigen for either the CAR or TCR. At this timepoint, CAR.OTI CTL, activated via the TCR, formed a conventional bull's eye immune synapse with accumulation of LCK and actin clearance (Figure 1). Surprisingly, CAR.OTI CTL activated via their CAR had an immune synapse with no or diffuse LCK and small actin rings (Figure 1). At the same timepoint, CAR-activated CAR.OTI CTL conjugates with tumor cells were characterized by a microtubule organizing center (MTOC) distant from the immune synapse LCK accumulation. In contrast TCR activated CAR.OTI CTL conjugates consistently had the MTOC proximal to the LCK accumulation (Figure 2A). Despite this, CAR-mediated CAR.OTI CTL killing of tumor targets was inhibited by a protein kinase C zeta inhibitor and is, therefore, MTOC dependent (Figure 2B). The MTOC circumnavigates the activated CTL nucleus and moves to the immune synapse, bringing cytotoxic granules with it. Using time lapse live video (TLLV) microscopy, we compared CAR.OTI CTL cytotoxic granule movement when the CAR.OTI were activated via the TCR vs CAR. We showed that following CAR vs TCR activation, CAR.OTI cytotoxic granules moved with a significantly higher velocity, and had a shorter time lapse to reach the immune synapse following activation (Ca2+ flux), and a significantly shorter time to detachment from the dying tumor cell (Figure 2C).We then re-examined immune synapse formation at an earlier timepoint, to explore whether the data from Figures 1-2 could be explained by a more rapid CTL response following CAR-mediated signaling. In contrast to our observations at 20 minutes, at five minutes we showed CAR-activated CAR.OTI CTL formed conjugates with tumor targets and the immune synapse showed distinct LCK accumulation and actin clearance (Figure 3A). Finally, we explored CAR.OTI CTL signaling and showed that CAR-mediated activation induced a significantly lower number of Ca2+ fluxes, however each Ca2+ flux amplitude was not different (Figure 3B). We also examined changes in proximal (phospho-LCK, pLCK) and distal (phospho-ERK, pERK) signals in CAR- versus TCR- activated CAR.OTI cells, and showed that CAR-mediated activation induced more rapid proximal and distal activation signals (Figure 3C). In conclusion, this study showed that compared to activation by TCR ligation, T cells respond to CAR ligation with faster phospho-protein signaling, Ca2+ flux, formation of an immune synapse and a more rapid movement of the MTOC and delivery of the cytotoxic granules to kill the tumor cells. Furthermore, LFA-1 did not accumulate at the immune synapse following CAR activation, therefore, reduced adhesion may facilitate the observed rapid detachment from the dying tumor cell, and enable the CAR T to rapidly move onto the next tumor target for 'bigger, stronger, faster' killing. [Display omitted] [Display omitted] [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Astrocytes Modulate Local Field Potential Rhythm?

Astrocytes Modulate Local Field Potential Rhythm?

Macrophage infection via selective capture of HIV-1-infected CD4+ T cells.

SummaryMacrophages contribute to HIV-1 pathogenesis by forming a viral reservoir and mediating neurological disorders. Cell-free HIV-1 infection of macrophages is inefficient, in part due to low plasma membrane expression of viral entry receptors. We find that macrophages selectively capture and engulf HIV-1-infected CD4+ T cells leading to efficient macrophage infection. Infected T cells, both healthy and dead or dying, were taken up through viral envelope glycoprotein-receptor-independent interactions, implying a mechanism distinct from conventional virological synapse formation. Macrophages infected by this cell-to-cell route were highly permissive for both CCR5-using macrophage-tropic and otherwise weakly macrophage-tropic transmitted/founder viruses but restrictive for nonmacrophage-tropic CXCR4-using virus. These results have implications for establishment of the macrophage reservoir and HIV-1 dissemination in vivo.

The SNARE Proteins SNAP25 and Synaptobrevin Are Involved in Endocytosis at Hippocampal Synapses

SNAP25, an essential component of the soluble NSF (N-ethylmaleimide-sensitive factor) attachment protein receptor (SNARE) complex that mediates exocytosis, is not considered to play a role in endocytosis, which couples to exocytosis by retrieving a similar amount of exocytosed vesicles. By knocking down SNAP25 and imaging slow endocytosis at a conventional synapse, the rat cultured hippocampal synapse, we found that SNAP25 is involved in slow, clathrin-dependent endocytosis. With similar techniques, we found that not only SNAP25, but also synaptobrevin is involved in slow endocytosis. These results provide the first evidence showing the dual role of SNAP25 and synaptobrevin in both exocytosis and slow endocytosis at conventional synapses. Such a dual role may contribute to mediate the coupling between exocytosis and clathrin-dependent endocytosis at conventional synapses, a mechanism critical for the maintenance of synaptic transmission and the normal structure of nerve terminals.

The Cytomatrix Protein Bassoon Contributes to Fast Transmission at Conventional and Ribbon Synapses

The presynaptic active zone contains a complex web of proteins involved in synaptic transmission. In this issue of Neuron, two articles show evidence that one of these proteins, Bassoon, coordinates multiple functions in a conventional and ribbon-type synapse.

Membrane nanotubes facilitate long-distance interactions between natural killer cells and target cells

Membrane nanotubes are membranous tethers that physically link cell bodies over long distances. Here, we present evidence that nanotubes allow human natural killer (NK) cells to interact functionally with target cells over long distances. Nanotubes were formed when NK cells contacted target cells and moved apart. The frequency of nanotube formation was dependent on the number of receptor/ligand interactions and increased on NK cell activation. Most importantly, NK cell nanotubes contained a submicron scale junction where proteins accumulated, including DAP10, the signaling adaptor that associates with the activating receptor NKG2D, and MHC class I chain-related protein A (MICA), a cognate ligand for NKG2D, as occurs at close intercellular synapses between NK cells and target cells. Quantitative live-cell fluorescence imaging suggested that MICA accumulated at small nanotube synapses in sufficient numbers to trigger cell activation. In addition, tyrosine-phosphorylated proteins and Vav-1 accumulated at such junctions. Functionally, nanotubes could aid the lysis of distant target cells either directly or by moving target cells along the nanotube path into close contact for lysis via a conventional immune synapse. Target cells moving along the nanotube path were commonly polarized such that their uropods faced the direction of movement. This is the opposite polarization than for normal cell migration, implying that nanotubes can specifically drive target cell movement. Finally, target cells that remained connected to an NK cell by a nanotube were frequently lysed, whereas removing the nanotube using a micromanipulator reduced lysis of these target cells.

Advances in Design and Application of Spiking Neural Networks

This paper presents new findings in the design and application of biologically plausible neural networks based on spiking neuron models, which represent a more plausible model of real biological neurons where time is considered as an important feature for information encoding and processing in the brain. The design approach consists of an evolutionary strategy based supervised training algorithm, newly developed by the authors, and the use of different biologically plausible neuronal models. A dynamic synapse (DS) based neuron model, a biologically more detailed model, and the spike response model (SRM) are investigated in order to demonstrate the efficacy of the proposed approach and to further our understanding of the computing capabilities of the nervous system. Unlike the conventional synapse, represented as a static entity with a fixed weight, employed in conventional and SRM-based neural networks, a DS is weightless and its strength changes upon the arrival of incoming input spikes. Therefore its efficacy depends on the temporal structure of the impinging spike trains. In the proposed approach, the training of the network free parameters is achieved using an evolutionary strategy where, instead of binary encoding, real values are used to encode the static and DS parameters which underlie the learning process. The results show that spiking neural networks based on both types of synapse are capable of learning non-linearly separable data by means of spatio-temporal encoding. Furthermore, a comparison of the obtained performance with classical neural networks (multi-layer perceptrons) is presented.

BIOPHYSICAL MECHANISMS AND ESSENTIAL TOPOGRAPHY OF DIRECTIONALLY SELECTIVE SUBUNITS IN RABBIT'S RETINA

We commemorate the 40th anniversary of the classical study undertaken by Barlow-Levick with a new challenge: to show how direction selectivity in the dendritic plexus of starburst amacrine cells is being computed. In the rabbit retina, although the cellular locus of direction selectivity is known to occur predominantly in the dendrites of starburst amacrine cells, the biophysical mechanism by which this takes place and its essential topography are yet to be specified with precision. A cotransmission model, involving a conjoint release of excitation/inhibition (i.e., a bisynaptic relay of endogenous ACh and GABA) from the distal varicosities of individual starburst amacrines, will be non-diphasic when the vesicular release of Ach and the non-vesicular, carrier-mediated release of GABA by transporters in the anterograde direction are preferentially suppressed by a negative feedback mechanism involving autoreceptors. Such biophysical mechanisms, including the asymmetric distribution of chloride cotransporters, explain somatofugal motion bias in starburst amacrine cells leading to autonomous functioning "subunits" that underlie the formation of directional selectivity. However, the functional independence of starburst amacrine cell "subunits" is partly a question of their network organization. The topography of directionally selective "subunits" resides in the plexus of crisscrossing dendrites of juxtaposed starburst amacrines, consisting of (i) serial synapses of three or more starburst amacrines and a ON-OFF directionally selective ganglion cell; (ii) a synaptic couplet between two starburst amacrines; and (iii) a conventional synapse between a starburst amacrine and a ON-OFF directionally selective ganglion cell. Cholinergic and GABAergic monosynaptic interactions between starburst amacrine cells, including glutamatergic interactions with cone bipolar cells, are involved in the primary circuit underlying directional selectivity. Furthermore, the secondary circuit underlying directional selectivity, consists of starburst amacrine cells and cone bipolar cells arranged in a "push-pull" configuration, interacting synaptically onto ON-OFF directionally selective ganglion cells.

Glia Get a GRIP on Neurons

The migration of cerebellar granule cells during brain development requires stimulation of their N -methyl-D-aspartate (NMDA) receptors by glutamate and either D-serine or glycine. They are guided by radial glia cells, although this intercellular relationship does not appear to require conventional synapse formation. Kim et al. report that production of D-serine by serine racemase in Bergmann glia cells constitutes a chemokinetic signaling mechanism that controls granule cell migration in the cerebellum. D-serine is generated in glia after activation of the AMPA receptors by glutamate. Using a two-hybrid screen, the authors determined that serine racemase binds to a PDZ domain of the glutamate receptor interacting protein (GRIP). GRIP associates with activated AMPA receptors in glia. Endogenous GRIP, AMPA receptor, and serine racemase were detected in cultures of primary glia. Stimulation of the AMPA receptors in cultured glia increased the secretion of D-serine, whereas treatment with an AMPA receptor antagonist blocked basal production of D-serine. Overexpression of the GRIP PDZ domain in either cultured glia or in cerebellar glia cells of postnatal mice increased D-serine production, even in the absence of AMPA receptor activation. Treatment of mice cerebellar slices with either an inhibitor of serine racemase or an enzyme that degrades D-serine reduced granule cell migration by 60%. The authors suggest that activation of AMPA receptors on Bergmann glia by glutamate results in the association of GRIP with serine racemase. The enzyme then generates D-serine that is released to act, in concert with glutamate, on NMDA receptors on granule cells. Thus, rather than relying on conventional synaptic communication, Bergmann glia regulate neuronal migration by releasing D-serine as a neuromodulator. P. M. Kim, H. Aizawa, P. S. Kim, A. S. Huang, S. R. Wickramasinghe, A. H. Kashani, R. K. Barrow, R. L. Huganir, A. Ghosh, S. H. Snyder, Serine racemase: Activation by glutamate neurotransmission via glutamate receptor interacting protein and mediation of neuronal migration. Proc. Natl. Acad. Sci. U.S.A. 102 , 2105-2110 (2005). [Abstract] [Full Text]

Synaptic distribution of ionotropic glutamate receptors in the inner plexiform layer of the primate retina.

The distribution and synaptic clustering of glutamate receptors (GluRs) were studied in the inner plexiform layer (IPL) of the macaque monkey retina by using subunit specific antisera. A punctate immunofluorescence pattern was observed in the IPL for all subunits tested, and electron microscopy confirmed that the immunoreactive puncta represent clustering of receptors at sites postsynaptic to the bipolar cell ribbon synapses (dyads). Usually only one of the two postsynaptic processes at the dyads expressed a given subunit. Immunoreactive GluR2, GluR2/3, and GluR4 puncta were found at high density throughout the IPL and are probably expressed at every dyad. The GluR1 subunit was expressed at lower density. The N-methyl-D-aspartate (NMDA) receptor subunits NR2A and NR1C2' were restricted to synapses localized in two broad bands in the center of the IPL. They were often colocalized with GluR2/3 and GluR4 subunits. The orphan receptor subunits delta 1/2 predominated in three horizontal bands. The kainate receptor subunits GluR6/7 were clustered in large postsynaptic densities adjacent to bipolar cell axon terminals but lacking a synaptic ribbon on the presynaptic side. This might represent a conventional synapse made by a bipolar axon terminal. The results suggest that GluR2/3 and GluR4, together with NMDA receptors, are preferentially expressed on ganglion cell dendrites, whereas kainate receptors and the delta 1/2 subunits are mostly localized on amacrine cell processes.

Dendrodendritic inhibition through reversal of dopamine transport.

Synapses in the central nervous system are usually defined by presynaptic exocytotic release sites and postsynaptic differentiations. We report here a demonstration of dendrodendritic inhibition that does not engage a conventional synapse. Using amperometric and patch-clamp recordings in rat brain slices of the substantia nigra, we found that blockade of the dopamine transporter abolished the dendritic release of dopamine and the resulting self-inhibition. These findings demonstrate that dendrodendritic autoinhibition entails the carrier-mediated release of dopamine rather than conventional exocytosis. This suggests that some widely used antidepressants that inhibit the dopamine transporter may benefit patients in the early stages of Parkinson's disease.

Glycine‐receptor immunoreactivity in retinal bipolar cells is postsynaptic to glycinergic and GABAergic amacrine cell synapses

Glycinergic innervation of the synaptic terminals of mixed rod-cone bipolar cells in the goldfish retina was investigated by electron microscopical immunocytochemistry with presynaptic and postsynaptic markers for glycinergic neurons: a monoclonal antibody (mAb 7A) against the 93 kDa subunit of the strychnine-sensitive glycine receptor and polyclonal antisera against a glycine/BSA conjugate. Conventional "glycinergic" synaptic contacts, made by amacrine cell processes, accounted for 7-10% of the input to the bipolar cell terminals, whether determined by glycine receptor immunoreactivity (GlyR-IR) or glycine-IR. In addition to the conventional synapses, the large bipolar cell terminals in the proximal inner plexiform layer (type Mb) gave rise to spinules (spine-like protrusions) that invaginated into presynaptic amacrine cell processes. Although 85% of the spinules were GlyR-IR, no spinules were postsynaptic to glycine-IR processes; yet 86% of the spinules were postsynaptic to GAD-IR processes, suggesting that the GlyR-IR spinules were postsynaptic to GABAergic terminals. Furthermore, a single amacrine cell process could make two synapses with an Mb terminal: a GlyR-IR contact onto a spinule and a conventional synapse that was not GlyR-IR. We suggest that glycinergic innervation of bipolar cell terminals involves conventional glycinergic synapses as well as an unconventional situation in which GABA and glycine may interact in as yet undetermined manner, perhaps by potentiation.

Histological and electron microscopic milestones in the development of the retina of a marsupial wallaby, Macropus eugenii.

Retinal differentiation in the pouch young of the wallaby Macropus eugenii was characterized microscopically and morphometrically. Mitosis occurs until early in the second month in the central retina, and until early in the fourth month, peripherally. Separation of the neuroblast layer by the outer plexiform layer did not immediately halt cell division. The retinal surface continued to expand well past the time of cessation of proliferation. Cell death in the ganglion cell layer continued through the fourth month centrally and to nearly five months in the periphery. The major period of cell death was coincident with the segregation of retinal afferents and the refinement of topography in the superior colliculus and dorsal lateral geniculate nucleus. Beginning in the third month retinal thickness, measured between the outer limiting membrane and nerve fiber layer, declined equally in peripheral and central regions. At all stages the combined thicknesses of the outer and inner nuclear layer in the retinal periphery was greater than that in the center. Together with a late thickening of the inner plexiform layer, the data are consistent with the suggestion that expansion of peripheral non-ganglion cell elements may play a role in development of center to periphery differences in ganglion cell distributions. Retinal differentiation of the wallaby follows the pattern of most mammals. The onset of development of key milestones for the acquisition of retinal function occurred in the sequence: conventional synapse formation prior to ribbon synapse formation in the inner plexiform layer, and photoreceptor outer segment differentiation prior to terminal triad synapse formation.

Structure of the synaptic membranes in the inner plexiform layer of the retina: a freeze-fracture study in monkeys and rabbits.

The internal structure of the synaptic membranes in the inner plexiform layer (IPL) of the retina of monkeys and rabbits was studied with the freeze-fracturing technique. In ribbon synapses, the presynaptic active zone is characterized by an aggregate of P-face particles, images of synaptic vesicle exocytosis, and forming coated vesicles which occupy distinct, contiguous membrane domains from apex to base of the synaptic ridge. The postsynaptic membrane contains a prominent aggregate of homogeneous particles which remain associated with the E-face. In the presynaptic membrane of conventional synapses, images of synaptic vesicle exocytosis are intermingled with large P-face particles, whereas forming coated vesicles surround the active zone. Three types of internal organization characterize the postsynaptic membrane of conventional synapses. Usually, the postsynaptic membrane exhibits the same internal structure as the surrounding nonjunctional plasmalemma. A second, less common type of conventional synapse contains a loose aggregate of heterogeneous particles which remain associated with the P-face. Finally, synapses were exceptionally found which are macular in shape and contain an aggregate of E-free particles within the postsynaptic membrane. The freeze-fracture evidence suggests that the axonal endings of bipolar cells--or at least some of them--make excitatory synapses, whereas the vast majority of amacrine cell dendrites make inhibitory synapses. Additional specializations of the cell surface in the IPL include gap junctions, puncta adhaerentia, subsurface cisterns, and cell corner aggregates.

Observations on the synaptic organization of the retina of the albino rat: a light and electron microscopic study.

AbstractAn electron microscopic study of the retina of the albino rat, with particular emphasis on the synaptic organization of the inner and outer plexiform layers, has been correlated with specimens impregnated with a modified Golgi technique.The central element of the photoreceptor “triad” in the outer plexiform layer is a bipolar cell dendrite.Two types of synaptic contacts were observed in the inner plexiform layer, the “dyad” ribbon synapse and the conventional synapse. The postsynaptic elements of the “dyad” consisted of an amacrine process and a ganglion cell dendrite. Conventional synapses were made by amacrine processes which were usually presynaptic to bipolar terminals. Reciprocal synapses between processes making ribbon synapses and those making conventional synapses were seen.Golgi technique revealed the presence of two types of bipolar cells, three types of amacrine cells, and one type each of horizontal and ganglion cell.These findings are discussed in relation to reported receptive field organization.

Synaptic Specialization in the Cristae Ampullaris of the Frog

The neurohumoral synapses of the Type II receptor cells of the frog's cristae are of two types: the sphere synapse and the modified conventional synapse. Within the receptor cell, the sphere synapse (analogous to the ribbon synapse) is recognized by the prominent electron dense synaptic sphere which is surrounded by synaptic vesicles (Fig. 1). Immediately below the synaptic sphere, three arciform electron dense structures are present as localized thickenings of the receptor's plasma membrane. In the region of the arciform structures, the plasma membrane of the receptor cell presents a scalloped appearance. The plasma membrane of the connecting neuron also shows a similar scalloped contour opposite the arciform structures, as well as an increase in thickness. The receptor and neuronal plasma membranes are separated by a synaptic gap which varies from synapse to synapse, but remains relatively constant in any given synapse. Several mitochondria are located within the neuron near the sphere synapse, and synaptic vesicles may or may not be present in the neuron. Figure 1 is an example where numerous synaptic vesicles are present in the neuron.

Synaptic relationships in the plexiform layers of carp retina.

The synaptic contacts made by carp retinal neurons were studied with electron microscopic techniques. Three kinds of contacts are described: (1) a conventional synapse in which an accumulation of agranular vesicles is found on the presynaptic side along with membrane densification of both pre- and postsynaptic elements; (2) a ribbon synapse in which a presynaptic ribbon surrounded by a halo of agranular vesicles faces two postsynaptic elements; and (3) close apposition of plasma membranes without any vesicle accumulation or membrane densification.