Bipolar Cell Dendrites Research Articles

Cytochemical correlates of the sleep-wake interface: concerted expression of brain-derived nitric oxide synthase (bNOS) and the nicotinic acetylcholine receptor (nAChR) in a columnoid organization of the primate prefrontal cortex

Nitric oxide (NO) was recently proposed to be involved in the sleep-wake cycle and cortical spreading depression. As a structural correlate of these functions, we found that bNOS IR was expressed by three cell types in the prefrontal cortex, viz. bipolar, multipolar, and stellate cells. Dendrites of bipolar cells established bundles resulting in a columnoid organization; in addition, the monoclonal antibody mAb 35 which labels subunits alpha1, alpha3 and alpha5 of nAChR, also visualized apical axons proceeding alongside the columnoids. In contrast, alpha-bungarotoxin which labels the alpha7-subunit of nAChR, visualized only perikarya of interneurons from where the apical axons arose. In the prefrontal cortex of monkeys which were anesthetized for 6-24 hours, only traces of the columnoid organization were found, while perikaryal bNOS and nAChR were invariably expressed. It is suggested that interactions between NO and presynaptically released ACh might be involved in cortical functions such as the sleep/wake cycle.

Expression of connexin36 in cone pedicles and OFF-cone bipolar cells of the mouse retina.

Transgenic technology, immunocytochemistry, electrophysiology, intracellular injection techniques, and reverse transcription PCR were combined to study the expression of neuronal connexin36 (Cx36) in the outer plexiform layer of the mouse retina. Transgenic animals expressed either a fusion protein of full-length Cx36 with enhanced green fluorescent protein (EGFP) attached at the C terminus or exon 2 of Cx36 was replaced bybeta-galactosidase (beta-gal). In the outer nuclear layer,beta-gal-positive cell bodies, which were confined to the most distal region close to the outer limiting membrane, displayed immunoreactivity against S-cone opsin. Cx36-EGFP puncta colocalized with cone pedicles, which were visualized by intracellular injection. In reverse transcriptase PCR experiments, Cx36 mRNA was never detected in samples of rods harvested from the outer nuclear layer. These results strongly suggest expression of Cx36 in cones but not in rods. In vertical sections, Cx36 expression in the vitreal part of the outer plexiform layer was characterized by a patchy distribution. Immunocytochemistry with antibodies against the neurokinin-3 receptor and the potassium channel HCN4 (hyperpolarization-activated cyclic nucleotide-gated potassium channel) displayed clusters of the Cx36 label on the dendrites of OFF-cone bipolar cells. In horizontal sections, these clusters of Cx36 appeared as round or oval-shaped groups of individual puncta, and they were always aligned with the base of cone pedicles. Double-labeling experiments and single-cell reverse transcriptase PCR ruled out expression of Cx36 in horizontal cells and rod bipolar cells. At light microscopic resolution, we found close association of Cx36-EGFP with the AMPA-type glutamate receptor subunit GluR1 but not with GluR2-GluR4, the kainate receptor subunit GluR5, or the metabotropic glutamate receptor mGluR6.

Regressive and reactive changes in the connectivity patterns of rod and cone pathways of P23H transgenic rat retina

We have used the P23H line 1 homozygous albino rat to study how progressive photoreceptor degeneration affects rod and cone relay pathways. We examined P23H retinas at different stages of degeneration by confocal microscopy of immunostained sections and electroretinogram (ERG) recordings. By 21 days of age in the P23H rat retina, there is already substantial loss of rods and reduction in rod bipolar dendrites along with reduction of metabotropic glutamate receptor 6 (mGluR6) and rod-associated bassoon staining. The cone pathway is relatively unaffected. By 150 days, when rods are absent from much of the retina, some rod bipolars remain and dendrites of rod and cone bipolar cells form synaptic complexes associated with cones and horizontal cell processes. These complexes include foci of mGluR6 and bassoon staining; they develop further by 270 days of age. Over the course of degeneration, beginning at 21 days, bipolar axon terminals atrophy and the inner retina undergoes further changes including a reduced and disorganized AII amacrine cell population and thinning of the inner plexiform layer. Electroretinogram (ERG) results at 23 days show reductions in a-wave amplitude, in rod and cone-associated b-waves (using a double flash paradigm) and in the amplitude of oscillatory potentials (OPs). By 38 days, rod scotopic a-wave responses and OPs are lost. B-wave amplitudes decline until 150 days, at which point they are purely cone-driven and remain stable up to 250 days. The results show that during the course of photoreceptor loss in the P23H rat, there are progressive degenerative changes, particularly in the rod relay pathway, and these are reflected in the changing ERG response patterns. Later reactive changes involving condensation of cone terminals and neurotransmitter receptors associated with rod and cone bipolar dendrites and with horizontal cell processes suggest that at this stage, there are likely to be complex changes in the relay of sensory information through the retina.

Group I Metabotropic Glutamate Receptors Bind to Protein Phosphatase 1C: MAPPING AND MODELING OF INTERACTING SEQUENCES

The modulation of neurotransmitter receptors by kinases and phosphatases represents a key mechanism in controlling synaptic signal transduction. However, molecular determinants involved in the specific targeting and interactions of these enzymes are largely unknown. Here, we identified both catalytic gamma-isoforms of protein phosphatase 1C (PP1gamma1 and PP1gamma2) as binding partners of the group I metabotropic glutamate receptors type 1a, 5a, and 5b in yeast cells and pull-down assays, using recombinant and native protein preparations. The tissue distribution of interacting proteins was compared, and protein phosphatase 1C was detected in dendrites of retinal bipolar cells expressing the respective interacting glutamate receptors. We mapped interacting domains within binding partners and identified five amino acids in the intracellular C termini of the metabotropic glutamate receptors type 1a, 5a, 5b, and 7b being both necessary and sufficient to bind protein phosphatase 1C. Furthermore, we show a dose-dependent competition of these C termini in binding the enzyme. Based on our data, we investigated the structure of the identified amino acids bound to protein phosphatase 1C by homology-based molecular modeling. In summary, these results provide a molecular description of the interaction between protein phosphatase 1C and metabotropic glutamate receptors and thereby increase our understanding of glutamatergic signal transduction.

Ectopic synaptogenesis during retinal degeneration in the royal college of surgeons rat

Rod photoreceptor-specific mutations cause ectopic synapses to form between cone photoreceptor terminals and rod bipolar cell dendrites in degenerating retinas of rhodopsin transgenic (P347L) pigs and retinal degeneration mice. Since the mutations occur in rod photoreceptor-specific genes in these two models, it is not known if ectopic synaptogenesis occurs specifically due to some rod photoreceptor cell-autonomous properties of a mutation or as a general consequence of photoreceptor degeneration. In the Royal College of Surgeons (RCS) rat, a mutation in the receptor tyrosine kinase gene, Mertk, causes failure of the retinal pigment epithelial (RPE) cells to phagocytose shed photoreceptor outer segments; subsequently, both rod and cone photoreceptors die. The non-phagocytic phenotype of the RCS rat is RPE cell-autonomous and the photoreceptors degenerate secondarily. Here we show that in 35-day-old RCS rats, where a majority of rod and cone photoreceptors remained, rod bipolar cell dendrites had abnormal (flat-contact type) synaptic contacts with rod and cone terminals. Demonstration of ectopic synapses in the RCS rat suggested that ectopic synaptogenesis could occur as a result of photoreceptor degeneration, even when the rods and cones were developmentally normal. This further supported the hypothesis that ectopic synaptogenesis may be a common step in the disease progression of different forms of retinal degeneration that include photoreceptor death as a feature, such as retinitis pigmentosa.

Immunohistochemical analysis of the outer plexiform layer in the nob mouse shows no abnormalities.

In the nob mouse, a mutation in nyctalopin results in a loss of signal transmission from photoreceptors to depolarizing bipolar cells (DBCs). We used immunohistochemical techniques to assess the expression pattern of proteins found at either the photoreceptor terminal or bipolar cell dendrites within the outer plexiform layer. We labeled normal and nob retinas with antibodies against mGluR6, PKC, G0alpha, bassoon, PSD-95, the alpha1F subunit of voltage-gated calcium channels, trkB, and dystrophin. All labeling patterns in nob and normal retinas were comparable to those previously reported in mouse retina. Our results indicate that the absence of nyctalopin does not disrupt the expression pattern of other proteins known to be required for synaptic transmission.

Functional modifications in rod bipolar cells in a mouse model of retinitis pigmentosa

The rd mouse has been widely used as an animal model of retinitis pigmentosa. In this model, a mutation of rod-specific phosphodiesterase leads to a loss of rods during the early period of postnatal life. Morphological modifications at the level of the outer plexiform layer have been shown (Proc. Nat. Acad. Sci. USA 97 (2000) 11020) in bipolar and horizontal cells. However, very little is known about the functional changes suffered by these cells postsynaptic to the degenerated rods. In the present work we have studied the neurotransmitter-induced currents in rod bipolar cells from the rd mouse retina. Currents induced by glutamate and GABA were studied by the patch clamp-whole cell technique, on rod bipolar cells enzymatically dissociated from the rd mouse retina. Data from rd animals were compared with non-dystrophic NMRI mice. GABA (30–100 μM) and glutamate (100 μM) were applied from a puff pipette in the near proximity of rod bipolar cell dendrites, clamped at physiological membrane potentials, and their evoked currents were studied. In rod bipolar cells from non-dystrophic mouse, puff application of glutamate induced an outward current. This current was increased twofold in absence of extracellular calcium (nominally 0 calcium). In rod bipolar cells from adult rd mouse, currents induced by glutamate were absent. Two types of GABA mediated currents were isolated in rod bipolar cells both in control and rd mouse retinas. The currents mediated by GABA C receptors were observed exclusively at the axon terminal, while the currents mediated by the GABA A receptors were observed upon GABA application to the bipolar cell dendrites. The currents mediated by GABA A receptors in rod bipolar cells from rd mouse were larger than those from control animals. We conclude that after the degeneration of rod photoreceptors in rd mouse, rod bipolar cells lost their glutamate (rod-neurotransmitter) input while they increase their response to GABA (horizontal cell-neurotransmitter). In our opinion, this work describes for the first time the changes in neurotransmitter sensitivity that affect rod bipolar cells after photoreceptor degeneration of the mouse retina.

Control of intracellular chloride concentration and GABA response polarity in rat retinal ON bipolar cells.

GABAergic modulation of retinal bipolar cells plays a crucial role in early visual processing. It helps to form centre-surround receptive fields which filter the visual signal spatially at the bipolar cell dendrites in the outer retina, and it produces temporal filtering at the bipolar cell synaptic terminals in the inner retina. The observed chloride transporter distribution in ON bipolar cells has been predicted to produce an intracellular chloride concentration, [Cl(-)](i), that is significantly higher in the dendrites than in the synaptic terminals. This would allow dendritic GABA-gated Cl(-) channels to generate the depolarization needed for forming the lateral inhibitory surround of the cell's receptive field, while synaptic terminal GABA-gated Cl(-) channels generate the hyperpolarization needed for temporal shaping of the light response. In contrast to this idea, we show here that in ON bipolar cells [Cl(-)](i) is only slightly higher in the dendrites than in the synaptic terminals, and that GABA-gated channels in the dendrites may generate a hyperpolarization rather than a depolarization. We also show that [Cl(-)](i) is controlled by movement of Cl(-) through ion channels in addition to transporters, that changes of [K(+)](o) alter [Cl(-)](i) and that voltage-dependent equilibration of [Cl(-)](i) in bipolar cells will produce a time-dependent adaptation of GABAergic modulation with a time constant of 8 s after illumination-evoked changes of membrane potential. Time-dependent adaptation of [Cl(-)](i) to voltage changes in retinal bipolar cells may add a previously unsuspected layer of temporal processing to signals as they pass through the retina.

Glycinergic synaptic transmission to bullfrog retinal bipolar cells is input-specific

Glycinergic inhibitory postsynaptic currents (IPSCs) focally elicited at the dendrites and axon terminals were recorded from bipolar cells in the bullfrog retinal slice, using the whole-cell clamp technique. IPSCs driven by input from interplexiform cells at bipolar cell dendrites ( ipc-IPSCs) had a much slower decay time constant (25.2±7.8 ms) than IPSCs driven by input from amacrine cells at bipolar cell axon terminals ( ac-IPSCs) (14.7±5.5 ms). Furthermore, peak-scaled non-stationary noise analysis revealed that the weighted mean single-channel conductance of the glycine receptors underlying bipolar cell dendritic ipc-IPSCs (20.8±6.6 pS) was significantly larger than that of those underlying bipolar cell axon terminal ac-IPSCs (12.9±2.9 pS). These results demonstrate that glycinergic synaptic transmission with different properties at bipolar cell dendrites and axon terminals differentially mediates intraretinal centrofugal signal transfer from the inner retina to the outer retina provided by interplexiform cells and lateral inhibition offered by amacrine cells in the inner retina.

Morphological and functional abnormalities in the inner retina of the rd/rd mouse.

We investigated the effects of photoreceptor degeneration on the anatomy and physiology of inner retinal neurons in a mouse model of retinitis pigmentosa, the retinal degeneration (rd) mutant mouse. Although there is a general assumption that the inner retinal cells do not suffer from photoreceptor death, we confirmed major changes both accompanying and after this process. Changes include sprouting of horizontal cells, lack of development of dendrites of rod bipolar cells, and progressive atrophy of dendrites in cone bipolar cells. Electrophysiological recordings demonstrate a selective impairment of second-order neurons that is not predictable on the basis of a pure photoreceptor dysfunction. Our data point out the necessity to prove integrity of the inner retina before attempting restoring visual function through photoreceptor intervention. This is even more important when considering that although intervention can be performed before the onset of any symptoms in animals carrying inherited retinopathies, this is obviously not true for human subjects.

Morphological and Functional Abnormalities in the Inner Retina of the rd/rd Mouse

We investigated the effects of photoreceptor degeneration on the anatomy and physiology of inner retinal neurons in a mouse model of retinitis pigmentosa, the retinal degeneration (rd) mutant mouse. Although there is a general assumption that the inner retinal cells do not suffer from photoreceptor death, we confirmed major changes both accompanying and after this process. Changes include sprouting of horizontal cells, lack of development of dendrites of rod bipolar cells, and progressive atrophy of dendrites in cone bipolar cells. Electrophysiological recordings demonstrate a selective impairment of second-order neurons that is not predictable on the basis of a pure photoreceptor dysfunction. Our data point out the necessity to prove integrity of the inner retina before attempting restoring visual function through photoreceptor intervention. This is even more important when considering that although intervention can be performed before the onset of any symptoms in animals carrying inherited retinopathies, this is obviously not true for human subjects.

Cellular localization of the vesicular inhibitory amino acid transporter in the mouse and human retina.

Horizontal cells are classically thought to mediate lateral inhibition by gamma-aminobutyric acid (GABA)-transporter mediated release. In the mammalian retina, however, GABA uptake and cloned GABA transporter were not detected in horizontal cells. Furthermore, the vesicular inhibitory amino acid transporter (VIAAT or VGAT) that loads GABA and glycine into synaptic vesicles was reported recently to be expressed in horizontal cells. To further assess synaptic transmission in mammalian horizontal cells, we examined the subcellular distribution of VIAAT in mouse and human retina by confocal microscopy with specific cell markers. VIAAT was observed in the mouse outer plexiform layer as punctate structures that localized in calbindin-positive horizontal cells. These structures were in close apposition with synaptophysin-, PSD-95-, dystrophin-, and bassoon-immunopositive photoreceptor terminals, suggesting that VIAAT is localized in horizontal cell tips at photoreceptor terminals. VIAAT-positive puncta were also in apposition to lectin-labeled cone terminals or dendrites of PKCalpha-immunopositive rod bipolar cells, indicating that VIAAT is expressed in horizontal cell tips at both rod and cone terminals. By contrast, only a very few puncta were observed in the human outer plexiform layer, whereas the inner plexiform layer remained labeled as in the mouse retina. When using adult human retinal cells in culture, horizontal cells identified by parvalbumin immunostaining were found to contain VIAAT, either at their terminals or throughout the entire cell similarly as in syntaxin-immunopositive cells. These differences between human retinal tissue and cultured cells were attributed to VIAAT degradation in postmortem retinal tissue. VIAAT localization in mouse and human horizontal cells further support the role of inhibitory transmitters in lateral inhibition at the photoreceptor terminals.

Localization of the AMPA subunit GluR2 in the outer plexiform layer of goldfish retina.

L-glutamate, the photoreceptor neurotransmitter, depolarizes horizontal cells and OFF bipolar cells by ionotropic AMPA-glutamate receptors. The AMPA-receptor subunit (GluR4) is localized to dendrites of OFF bipolar cells in goldfish retina. Here, we used immunohistochemical techniques to identify AMPA-receptor subunits on horizontal cell dendrites. A monoclonal antibody against rat GluR2, with high sequence homology to the recently cloned goldfish GluR2a receptor, was used for light- and electron-microscopical immunocytochemistry. Light- and dark-adapted retinas were analyzed, with no major difference in results. GluR2-immunoreactivity (IR) was restricted to a narrow band in the outer plexiform layer, in which it appeared as bright dome-shaped structures amidst numerous puncta. At the ultrastructural level, GluR2-IR was found in horizontal cell dendrites that invaginated cones and rods. Dendrites of OFF bipolar cells were not labeled. GluR2-IR was present mostly in horizontal cell dendrites that were the lateral elements of the triad, rather than in dendrites that were the central elements. In light-adapted retinas, GluR2-IR was found in many horizontal cell spinules. GluR2-IR was observed, on occasion, in a mixed rod/cone (Mb) ON bipolar cell process that innervated rod spherules. Verification of the Mb ON bipolar cell was made by protein kinase C and metabotropic mGluR1alpha immunolabeling. The presence of GluR2-IR in lateral elements suggests that lateral horizontal cell dendrites are postsynaptic to cones rather than only sites of feedback inhibition. All horizontal cell types express the GluR2 subunit, uniquely differentiating themselves from OFF bipolar cells that express the GluR4 subunit. This differentiation most likely has a major influence on the glutamate pharmacology and response kinetics of these cell types to glutamate.

Voltage-Dependent Sodium Channels Are Expressed in Nonspiking Retinal Bipolar Neurons

Retinal bipolar neurons transmit visual information by means of graded synaptic potentials that spread to the synaptic terminal without sodium-dependent action potentials. Although action potentials are not involved, voltage-dependent sodium channels may enhance subthreshold depolarizing potentials in the dendrites and soma of bipolar cells, as they do in other CNS neurons. We report here that voltage-dependent sodium currents are observed in a subset of bipolar neurons from goldfish retina. Single-cell reverse transcriptase-PCR identified four different sodium channel alpha subunits in goldfish bipolar cells, putatively corresponding to the mammalian voltage-gated sodium channels Na(v)1.1, Na(v)1.2, Na(v)1.3, and Na(v)1.6. The amount of sodium current was largest in cells with smaller synaptic terminals, which probably represent cone bipolar cells. Localization of sodium channel immunoreactivity in goldfish retina confirmed the expression of voltage-gated sodium channels in cone bipolar cells of both ON and OFF types. Both immunocytochemical and physiological evidence suggests that the sodium channels are localized to the soma and dendrites where they may play a role in transmission of synaptic signals, particularly in the long, thin dendrites of cone bipolar cells.

Selective synaptic distribution of AMPA and kainate receptor subunits in the outer plexiform layer of the carp retina.

The subunit composition of ionotropic glutamate receptors (GluRs) is extremely diverse and responsible for the diversity of postsynaptic responses to the release of glutamate, which is the major excitatory neurotransmitter in the retina. To understand the functional consequences of this diversity, it is necessary to reveal the synaptic localization and subunit composition of GluRs. We have used immuno light and electron microscopy to localize AMPA and kainate (GluR1, GluR2/3, GluR4, GluR5-7) subunits in identified carp retinal neurons contributing to the outer plexiform layer. GluR1 could not be detected within the outer plexiform layer. Rod and cone horizontal cells all express only GluR2/3 at the tips of their invaginating dendrites. These receptors are also inserted into the membrane of spinules, light-dependent protrusions of the horizontal cell dendrites, flanking the synaptic ribbon of the cone synapse. Bipolar cells express GluR2/3, GluR4, and GluR5-7 at their terminal dendrites invaginating cone pedicles and rod spherules. Colocalization data suggest that each subunit is expressed by a distinct bipolar cell type. The majority of bipolar cells expressing these receptors seem to be of the functional OFF-type; however, in a few instances, GluR2/3 could also be detected on dendrites of bipolar cells that, based on their localization within the cone synaptic complex, appeared to be of the functional ON-type. The spatial arrangement of the different subunits within the cavity of the cone pedicle appeared not to be random: GluR2/3 was found predominantly at the apex of the cavity, GluR4 at its base and GluR5-7 dispersed between the two.

The Synaptic Architecture of AMPA Receptors at the Cone Pedicle of the Primate Retina

Cone pedicles, the output synapses of cone photoreceptors, transfer the light signal onto the dendrites of bipolar and horizontal cells. Cone pedicles contain between 20 and 45 ribbon synapses (triads) which are the release sites for glutamate, the cone transmitter. Several hundred postsynaptic dendrites contact individual cone pedicles, and we studied the glutamate receptors expressed and clustered at these contacts, particularly the AMPA receptor subunits. Using immunocytochemistry and confocal imaging we were able to resolve individual triads within the cone pedicles by light microscopy. We studied their differences in L/M- and S-cones, and we counted the number of triads per pedicle across the retina. The presynaptic matrix protein bassoon, the synapse-associated membrane protein P84, and peanut agglutinin were used to specifically label synaptic ribbons, invaginating dendrites of horizontal cells and invaginating dendrites of ON-cone bipolar cells, respectively. Pre- and post-embedding immunocytochemistry and electron microscopy were used to localize the AMPA receptor subunits at the cone pedicle base. They were aggregated at three different postsynaptic sites: at horizontal cell invaginating contacts, at bipolar cell flat contacts, and at desmosome-like junctions underneath the cone pedicles. We also performed double-labeling experiments with the triad-specific markers and the antibodies against the AMPA receptor subunits. AMPA receptors were preferentially expressed by horizontal cells, and to a lesser extent by OFF-cone bipolar cells. We did not observe any cone-selective expression of AMPA receptor subunits postsynaptic to L/M- or S-cones, suggesting AMPA receptors are not the key to understanding trichromatic signaling in the primate retina.

Immunocytochemical localization of kainate-selective glutamate receptor subunits GluR5, GluR6, and GluR7 in the cat retina

Localizations of the kainate-selective glutamate receptor subunits GluR5, 6, and 7 were studied in the cat retina by light and electron microscopic immunocytochemistry. GluR5 immunoreactivity was observed in the cell bodies and dendrites of numerous cone bipolar cells and ganglion cells. The labeled cone bipolar cells make basal or flat contacts with cone pedicles in the outer plexiform layer, leading to their identification as OFF-center bipolar cells. Reaction product within the inner plexiform layer was observed in processes of ganglion cells at their sites of input from cone bipolar cells. Staining for GluR6 was localized to A- and B-type horizontal cells, numerous amacrine cells, and an occasional cone bipolar cell. The larger ganglion cells were also immunoreactive. As with other GluR molecules, labeling was usually confined to one of the two postsynaptic elements at a cone bipolar dyad contact. Immunoreactivity for GluR7 was very limited and was seen only in a few amacrine and displaced amacrine cells. Findings of this study are consistent with a major role for kainate receptors in mediating OFF pathways in the outer retina with participation in both OFF and ON pathways in the inner retina.

Heterogeneous distribution of AMPA glutamate receptor subunits at the photoreceptor synapses of rodent retina

In the retina the segregation of different aspects of visual information starts at the first synapse in signal transfer from the photoreceptors to the second-order neurons, via the neurotransmitter glutamate. We examined the distribution of the four AMPA glutamate receptor subunits GluR1-GluR4 at the photoreceptor synapses in mouse and rat retinae by light and immunoelectron microscopy and serial section reconstructions. On the dendrites of OFF-cone bipolar cells, which make flat, noninvaginating contacts postsynaptic at cone synaptic terminals, the subunits GluR1 and GluR2 were predominantly found. Horizontal cell processes postsynaptic at both rod and cone synaptic terminals preferentially expressed the subunits GluR2, GluR2/3 and GluR4. An intriguing finding was the presence of GluR2/3 and GluR4 subunits on dendrites of putative rod bipolar cells, which are thought to signal through the sign-inverting metabotropic glutamate receptor 6, mGluR6. Furthermore, at the rod terminals, horizontal cell processes and rod bipolar cell dendrites showed labelling for the AMPA receptor subunits at the ribbon synaptic site or perisynaptically at their site of invagination into the rod terminal. The wide distribution of AMPA receptor subunits at the photoreceptor synapses suggests that AMPA receptors play an important role in visual signal transfer from the photoreceptors to their postsynaptic partners.

The cone pedicle, a complex synapse in the retina.

Cone pedicles, the synaptic terminals of cone photoreceptors, are connected in the macaque monkey retina to several hundred postsynaptic dendrites. Using light and electron microscopy, we found underneath each cone pedicle a laminated distribution of dendritic processes of bipolar and horizontal cells. Superimposed were three strata of glutamate receptor (GluR) aggregates, including a novel layer of glutamate receptors clustered at desmosome-like junctions. They are, most likely, postsynaptic densities on horizontal cell dendrites. GABA(A) and GABA(C) receptors are aggregated on bipolar cell dendrites in a narrow band underneath the cone pedicle. Glutamate released from cone pedicles and GABA released from horizontal cell dendrites act not only through direct synaptic contacts but also (more so) through diffusion to the appropriate receptors.

Impaired behavioral suppression by light in metabotropic glutamate receptor subtype 6-deficient mice

The metabotropic glutamate receptor subtype 6 is localized on the dendrites of ON bipolar cells in mammalian retina, and is responsible for synaptic transmission from photoreceptors to ON bipolar cells. We have previously provided electrophysiological evidence that metabotropic glutmate receptor subtype 6-deficient mice have an impairment in the ON visual pathway. In this study, we compared, between metabotropic glutamate receptor subtype 6-deficient ( n=9) and wild-type mice ( n=7), their daily wheel-running activity in constant dark and light–dark cycle environments. There was no difference in their free-running rhythmicity in a constant dark environment nor in their ability to entrain their active/rest phase to the phase-shifted light–dark cycle environment, indicating that the circadian system in mutant mice was functioning normally. However, the wheel-running activity was suppressed immediately after light onset of the light–dark cycle in wild-type mice (suppressive effect), whereas that of mutant mice was prolonged for several hours in spite of light onset (very weak suppressive effect). The suppression of activity in wild-type mice is a “masking effect” of the endogenous circadian rhythm in response to light stimuli. The results indicate that the failure of mutant mice to suppress their activity upon light onset is not due to abnormality in their circadian system, but to their lack of response to light stimuli. This study clearly demonstrates that the dysfunction of the ON visual pathway in metabotropic glutamate receptor subtype 6-deficient mice impairs their behavioral responsiveness to light and yet preserves their circadian system.