Double-staining of horizontal and amacrine cells by intracellular injection with lucifer yellow and biocytin in carp retina

Adaptation-dependent synchronous activity contributes to receptive field size change of bullfrog retinal ganglion cell

Nearby retinal ganglion cells of similar functional subtype have a tendency to discharge spikes in synchrony. The synchronized activity is involved in encoding some aspects of visual input. On the other hand, neurons always continuously adjust their activities in adaptation to some features of visual stimulation, including mean ambient light, contrast level, etc. Previous studies on adaptation were primarily focused on single neuronal activity, however, it is also intriguing to investigate the adaptation process in population neuronal activities. In the present study, by using multi-electrode recording system, we simultaneously recorded spike discharges from a group of dimming detectors (OFF-sustained type ganglion cells) in bullfrog retina. The changes in receptive field properties and synchronization strength during contrast adaptation were analyzed. It was found that, when perfused using normal Ringer's solution, single neuronal receptive field size was reduced during contrast adaptation, which was accompanied by weakening in synchronization strength between adjacent neurons' activities. When dopamine (1 µM) was applied, the adaptation-related receptive field area shrinkage and synchronization weakening were both eliminated. The activation of D1 receptor was involved in the adaptation-related modulation of synchronization and receptive field. Our results thus suggest that the size of single neuron's receptive field is positively related to the strength of its synchronized activity with its neighboring neurons, and the dopaminergic pathway is responsible for the modulation of receptive field property and synchronous activity of the ganglion cells during the adaptation process.

Suppression of electrical synapses between retinal amacrine cells of goldfish by intracellular cyclic-AMP

Retinal amacrine cells of the same class in cyprinid fish are homotypically connected by gap junctions. The permeability of their gap junctions examined by the diffusion of Neurobiotin into neighboring amacrine cells under application of dopamine or cyclic nucleotides to elucidate whether electrical synapses between the cells are regulated by internal messengers. Neurobiotin injected intracellularly into amacrine cells in isolated retinas of goldfish, and passage currents through the electrical synapses investigated by dual whole-patch clamp recordings under similar application of their ligands. Control conditions led us to observe large passage currents between connected cells and adequate transjunctional conductance between the cells (2.02±0.82nS). Experimental results show that high level of intracellular cyclic AMP within examined cells block transfer of Neurobiotin and suppress electrical synapses between the neighboring cells. Transjunctional conductance between examined cells reduced to 0.23nS. However, dopamine, 8-bromo-cyclic AMP or high elevation of intracellular cyclic GMP leaves gap junction channels of the cells permeable to Neurobiotin as in the control level. Under application of dopamine (1.25±0.06nS), 8-bromo-cyclic AMP (1.79±0.51nS) or intracellular cyclic GMP (0.98±0.23nS), the transjunctional conductance also remains as in the control level. These results demonstrate that channel opening of gap junctions between cyprinid retinal amacrine cells is regulated by high level of intracellular cyclic AMP.

Intracellular cyclic-amp suppresses the permeability of gap junctions between retinal amacrine cells

Gap junctions are intercellular channels composed of subunit protein connexin and subserve electrotonic transmission between connected neurons. Retinal amacrine cells, as well as horizontal cells of the same class, are homologously connected by gap junctions. The gap junctions between these neurons extend their receptive fields, and may increase the inhibitory postsynaptic effects in the retina. In the present study, we investigated whether gap junctions between the neurons are modulated by internal messengers. The permeability of gap junctions was examined by the diffusion of intracellularly injected biotinylated tracers, biocytin or Neurobiotin, into neighboring cells since gap junctions are permeable to these molecules freely. 4% Lucifer Yellow and 6% biocytin or Neurobiotin were injected intracellularly into horizontal cells and amacrine cells in isolated retinas of carp and goldfish and Japanese dace following electrophysiological identification. In the control condition, the tracer spread into many neighboring cells from the recorded cells. Superfusion of retinas with dopamine (100 microM) suppressed diffusion of the tracer into the neighboring horizontal cells, but not in the case of amacrine cells. Intracellular injection of cyclic AMP (300 mM) completely blocked diffusion of the tracer into neighboring horizontal cells and amacrine cells. However, superfusion of retinas with 8-bromo-cyclic AMP (2 mM), membrane permeable cyclic AMP analog, permitted the tracer to diffuse into the neighboring horizontal cells or amacrine cells. Intracellular injection of cyclic GMP (300 mM) blocked the diffusion between neighboring horizontal cells, but did not suppress the diffusion between amacrine cells. These results show that the permeability of gap junctions between amacrine cells is regulated by high concentration of intracellular cyclic AMP level, but not for intracellular cyclic GMP or applied dopamine or extracellularly applied low concentrations of intracellular cyclic AMP level. The present study suggests that these laterally oriented inhibitory interneurons, horizontal cells and amacrine cells, express different connexins which may be differentially regulated by intercellular messengers.

Tracer coupling between fish rod horizontal cells: modulation by light and dopamine but not the retinal circadian clock

Horizontal cells are second order neurons that receive direct synaptic input from photoreceptors. In teleosts horizontal cells can be divided into two categories, cone-connected and rod-connected. Although the anatomy and physiology of fish cone horizontal cells have been extensively investigated, less is known about rod horizontal cells. This study was undertaken to determine whether light and/or the circadian clock regulate gap junctional coupling between goldfish rod horizontal cells. We used fine-tipped, microelectrode intracellular recording to monitor rod horizontal cells under various visual stimulation conditions, and tracer (biocytin) iontophoresis to visualize their morphology and evaluate the extent of coupling. Under dark-adapted conditions, rod horizontal cells were extensively coupled to cells of like-type (homologous coupling) with an average of approximately 120 cells coupled. Under these conditions, no differences were observed between day, night, the subjective day, and subjective night. In addition, under dark-adapted conditions, application of the dopamine D2-like agonist quinpirole (1 microM), the D2-like antagonist spiperone (10 microM), or the D1-like antagonist SCH23390 (10 microM) had no effect on rod horizontal cell tracer coupling. In contrast, the extent of tracer coupling was reduced by approximately 90% following repetitive light (photopic range) stimulation of the retina or application of the D1-agonist SKF38393 (10 microM) during the subjective day and night. We conclude that similarly to cone horizontal cells, rod horizontal cells are extensively coupled to one another in darkness and that the extent of coupling is dramatically reduced by bright light stimulation or dopamine D1-receptor activation. However, in contrast to cone horizontal cells whose light responses are under the control of the retinal clock, the light responses of rod horizontal cells under dark-adapted conditions were similar during the day, night, subjective day, and subjective night thus demonstrating that they are not under the influence of the circadian clock.

Connexin 35/36 is phosphorylated at regulatory sites in the retina

Connexin 35/36 is the most widespread neuronal gap junction protein in the retina and central nervous system. Electrical and/or tracer coupling in a number of neuronal circuits that express this connexin are regulated by light adaptation. In many cases, the regulation of coupling depends on signaling pathways that activate protein kinases such as PKA, and Cx35 has been shown to be regulated by PKA phosphorylation in cell culture systems. To examine whether phosphorylation might regulate Cx35/36 in the retina we developed phospho-specific polyclonal antibodies against the two regulatory phosphorylation sites of Cx35 and examined the phosphorylation state of this connexin in the retina. Western blot analysis with hybrid bass retinal membrane preparations showed Cx35 to be phosphorylated at both the Ser110 and Ser276 sites, and this labeling was eliminated by alkaline phosphatase digestion. The homologous sites of mouse and rabbit Cx36 were also phosphorylated in retinal membrane preparations. Quantitative confocal immunofluorescence analysis showed gap junctions identified with a monoclonal anti-Cx35 antibody to have variable levels of phosphorylation at both the Ser110 and Ser276 sites. Unusual gap junctions that could be identified by their large size (up to 32 microm2) and location in the IPL showed a prominent shift in phosphorylation state from heavily phosphorylated in nighttime, dark-adapted retina to weakly phosphorylated in daytime, light-adapted retina. Both Ser110 and Ser276 sites showed significant changes in this manner. Under both lighting conditions, other gap junctions varied from non-phosphorylated to heavily phosphorylated. We predict that changes in the phosphorylation states of these sites correlate with changes in the degree of coupling through Cx35/36 gap junctions. This leads to the conclusion that connexin phosphorylation mediates changes in coupling in some retinal networks. However, these changes are not global and likely occur in a cell type-specific or possibly a gap junction-specific manner.

STRUCTURAL AND FUNCTIONAL PROPERTIES OF HOMOLOGOUS ELECTRICAL SYNAPSES BETWEEN RETINAL AMACRINE CELLS

Retinal amacrine cells regulate activities of retinal ganglion cells, the output neurons to higher visual centers, through cellular mechanism of lateral inhibition in the inner plexiform layer (IPL). Electrical properties of gap junction networks between amacrine cells in the IPL were investigated using combined techniques of intracellular recordings, Lucifer yellow and Neurobiotin injection, dual patch-clamp recordings and high voltage electron microscopy in isolated retinas of cyprinid fish. Six types of gap-junctionally connected amacrine cells were classified after their light-evoked responses to light flashes were recorded. Among them, gap junction networks of three types of amacrine cells were studied with structure-function correlation analysis. Cellular morphology of intercellular connections between three homologous cell classes was characterized. The interconnections between laterally extending dendrites in the IPL were localized at dendritic tip terminals. Three types of cells presented the dendrodendritic connections of tip-contact manner in the homologous cell population. High voltage as well as conventional electron microscopy revealed gap junctions between the dendritic tips of Neurobiotin-coupled cells. Receptive field properties of these amacrine cells were examined, displacing a slit of light along the distance from recording sites in the dorsal intermediate region of the retina. Receptive field size, space length constant, response latency and conduction velocity were measured. Spatial and temporal properties of receptive fields were symmetric along horizontally expanding dendrites in the dorsal retina. Simultaneous dual patch-clamp recordings revealed that the lateral gap junction connections between homologous amacrine cells expressed bidirectional electrical synapses passing Na(+) spikes. These results demonstrate that bidirectional electrical transmission in gap junction networks of these amacrine cells is symmetric along the lateral gap junction connections between horizontally extending dendrites. Lateral inhibition regulated by amacrine cells in the IPL appears to be associated with the directional extension of the dendrites and the orientation of dendrodendritic gap junctions.

Fundamental GABAergic amacrine cell circuitries in the retina: nested feedback, concatenated inhibition, and axosomatic synapses

Presynaptic gamma-aminobutyrate-immunoreactive (GABA+) profiles were mapped in the cyprinid retina with overlay microscopy: a fusion of electron and optical imaging affording high-contrast ultrastructural and immunocytochemical visualization. GABA+ synapses, deriving primarily from amacrine cells (ACs), compose 92% of conventional synapses and 98% of the input to bipolar cells (BCs) in the inner plexiform layer. GABA+ AC synapses, the sign-inverting elements of signal processing, are deployed in micronetworks and distinctive synaptic source/target topologies. Nested feedback micronetworks are formed by three types of links (BC --> AC, reciprocal BC <-- AC, and AC --> AC synapses) arranged as nested BC<--> [AC --> AC] loops. Circuits using nested feedback can possess better temporal performance than those using simple reciprocal feedback loops. Concatenated GABA+ micronetworks of AC --> AC and AC --> AC --> AC chains are common and must be key elements for lateral spatial, temporal, and spectral signal processing. Concatenated inhibitions may represent exceptionally stable, low-gain, sign-conserving devices for receptive field construction. Some chain elements are GABA immunonegative (GABA-) and are, thus, likely glycinergic synapses. GABA+ synaptic baskets target the somas of certain GABA+ and GABA- cells, resembling cortical axosomatic synapses. Finally, all myelinated intraretinal profiles are GABA+, suggesting that some efferent systems are sources of GABAergic inhibition in the cyprinid retina and may comprise all axosomatic synapses. These micronetworks are likely the fundamental elements for receptive field shaping in the inner plexiform layer, although few receptive field models incorporate them as functional components. Conversely, simple feedback and feedforward synapses may often be chimeras: the result of an incomplete view of synaptic topology.

Normal differentiation of cultured lens cells after inhibition of gap junction-mediated intercellular communication

The cells of the vertebrate lens are linked to each other by gap junctions, clusters of intercellular channels that mediate the direct transfer of low-molecular-weight substances between the cytosols of adjoining cells. Although gap junctions are detectable in the unspecialized epithelial cells that comprise the anterior face of the organ, both their number and size are greatly increased in the secondary fiber cells that differentiate from them at the lens equator. In other organs, gap junctions have been shown to play an important role in tissue development and differentiation. It has been proposed, although not experimentally tested, that this may be true in the lens as well. To investigate the function of gap junctions in the development of the lens, we have examined the effect of the gap junction blocker 18beta-glycyrrhetinic acid (betaGA) on the differentiation of primary cultures (both dissociated cell-derived monolayers and central epithelium explants) of embryonic chick lens epithelial cells. We found that betaGA greatly reduced gap junction-mediated intercellular transfer of Lucifer yellow and biocytin throughout the 8-day culture period. betaGA did not, however, affect the differentiation of these cells into MP28-expressing secondary fibers. Furthermore, inhibition of gap junctions had no apparent effect on either of the two other types of intercellular (adherens and tight) junctions present in the lens. We conclude that the high level of gap junctional intercellular communication characteristic of the lens equator in vivo is not required for secondary fiber formation as assayed in culture. Up-regulation of gap junctions is therefore likely to be a consequence rather than a cause of lens fiber differentiation and may primarily play a role in lens physiology.

Neurobiology of retinal dopamine in relation to degenerative states of the tissue

Neurobiology of retinal dopamine is reviewed and discussed in relation to degenerative states of the tissue. The Introduction deals with the basic physiological actions of dopamine on the different neurons in vertebrate retinae with an emphasis upon mammals. The intimate relationship between the dopamine and melatonin systems is also covered. Recent advances in the molecular biology of dopamine receptors is reviewed in some detail. As degenerative states of the retina, three examples are highlighted: Parkinson's disease; ageing; and retinal dystrophy (retinitis pigmentosa). As visual functions controlled, at least in part, by dopamine, absolute sensitivity, spatial contrast sensitivity, temporal (including flicker) sensitivity and colour vision are reviewed. Possible cellular and synaptic bases of the visual dysfunctions observed during retinal degenerations are discussed in relation to dopaminergic control. It is concluded that impairment of the dopamine system during retinal degenerations could give rise to many of the visual abnormalities observed. In particular, the involvement of dopamine in controlling the coupling of horizontal and amacrine cell lateral systems appears to be central to the visual defects seen.

Pseudo-immunolabelling with the avidin-biotin-peroxidase complex (ABC) due to the presence of endogenous biotin in retinal Müller cells of goldfish and salamander

Immunodetection techniques are dependent on enzyme-protein conjugates for the visualisation of antigen-antibody complexes. One of the most widely used is the avidin-biotin-peroxidase complex (ABC) method. The present study demonstrates that direct treatment of goldfish and salamander retinal sections with ABC, followed by an incubation with the chromogenic substrate 3,3-diaminobenzidine tetrahydrochloride (DAB) and H2O2, manifested a punctate staining pattern across the neural retinae, presumably through binding of avidin to endogenous biotin. Incubation with a primary antiserum against biotin followed by immunoprocessing with the peroxidase--anti-peroxidase (PAP) method showed a pattern similar to the punctuate framework as detected with solo ABC-treated sections. Moreover, the ABC-DAB/H2O2 mediated pattern corresponded to the spatial orientation of Müller cells as identified by GFAP immunostaining. These findings indicate the presence of endogenous biotin in Müller cells and calls for caution in the application of the ABC method in immunotechniques in retinal research.

A comparison of receptive-field and tracer-coupling size of amacrine and ganglion cells in the rabbit retina

Recent studies have shown that amacrine and ganglion cells in the mammalian retina are extensively coupled as revealed by the intercellular movement of the biotinylated tracers biocytin and Neurobiotin. These demonstrations of tracer coupling suggest that electrical networks formed by proximal neurons (i.e. amacrine and ganglion cells) may underlie the lateral propagation of signals across the inner retina. We studied this question by comparing the receptive-field size, dendritic-field size, and extent of tracer coupling of amacrine and ganglion cells in the dark-adapted, superfused, isolated retina eyecup of the rabbit. Our results indicate that while the center-receptive fields of proximal neurons are approximately 15% larger than their corresponding dendritic diameters, this slight difference can be explained by factors other than electrical coupling such as tissue shrinkage associated with histological processing. However, the extent of tracer coupling of amacrine and ganglion cells was, on average, about twice the size of the corresponding receptive fields. Thus, the receptive field of an individual proximal neuron matched far more closely to its dendritic diameter than to the size of the tracer-coupled network of cells to which it belonged. The exception to this rule was the AII amacrine cells for which center-receptive fields were 2-3 times the size of their dendritic diameters but matched closely to the size of the tracer-coupled arrays. Thus, with the exception of AII cells, our data indicate that tracer coupling between proximal neurons is not associated with an enlargement of their receptive fields. Our results, then, provide no evidence for electrical coupling or, at least, indicate that extensive lateral spread of visual signals does not occur in the proximal mammalian retina.

Processing of color- and noncolor-coded signals in the gourami retina. II. Amacrine cells

The same set of stimuli and analytic methods that was used to study the dynamics of horizontal cells () was applied to a study of the response dynamics and signal processing in amacrine cells in the retina of the kissing gourami, Helostoma rudolfi. The retina contains two major classes of amacrine cells that could be identified from their morphology: C and N amacrine cells. C amacrine cells had a two-layered dendritic field, whereas N cells had a monolayered dendritic field. Both types of amacrine cell were tracer-coupled but coupling was more extensive in the N amacrine cells. Responses from C amacrine cells lacked a DC component and had a small linear component that was <10% in terms of mean square error (MSE); the second-order component often accounted for >50% of the modulation response. The C amacrine cells did not show any characteristic color coding under any stimulus condition. Most responses of N cells to a pulsatile stimulus consisted of a series of depolarizing transient potentials and steady illumination did not generate any DC potential in these cells. The response to a white-noise modulated input was composed of well-defined first- and second-order components and, possibly, higher-order components. The response evoked by a red or green white-noise-modulated stimulus given alone was not color coded. Modulated red illumination in the presence of a green illumination elicited a color-coded response from >70% of N amacrine cells. Color information was carried not only by the polarity but also by the dynamics of the first-order component. No convincing evidence was obtained to indicate that the second-order component might be involved in color processing. Some N amacrine cells produced a well-defined (second-order) interaction kernel to show that the temporal sequence of red and green stimuli was a parameter to be considered. In a complex cell such as an amacrine cell, responses evoked by a pulsatile stimulus given in darkness and by modulation of a mean luminance could be very different in terms of their characteristics. It was not always possible to predict the response evoked by one stimulus from observing the cell's response to another stimulus. This is because, in N cells, a flash-evoked (nonsteady state) response is composed largely of nonlinear components whereas a modulation (steady state) response is composed of linear as well as nonlinear components.

Tracer coupling pattern of amacrine and ganglion cells in the rabbit retina

We examined the tracer coupling pattern of more than 15 morphological types of amacrine and ganglion cells in the rabbit retina. Individual cells were injected intracellularly with the biotinylated tracer Neurobiotin, which was then allowed to diffuse across gap junctions to label neighboring neurons. We found that homologous and/or heterologous tracer coupling was common for most proximal neurons. In fact, the starburst amacrine cell was the only amacrine cell type that showed no evidence of coupling. The remaining types of amacrine cell were coupled exclusively to other amacrines, either homologously or, more often, through a combination of homologous and heterologous junctions. In only one case did we visualize labeled ganglion cells following injection of Neurobiotin into an amacrine cell. In contrast, injection of Neurobiotin into ganglion cells almost always resulted in the labeling of amacrine cells. Taken together, these results suggest a directionality to the movement of tracer across gap junctions connecting amacrine and ganglion cells. We found that the coupling pattern for a given morphological type of cell was generally stereotypic and consistent across retinas. The notable exceptions to this finding were alpha ganglion cells and cells with morphology corresponding to that of on-off direction selective ganglion cells. In both cases, individual cells showed either extensive coupling to both amacrine and ganglion cells or no coupling at all. A notable finding was that, in every case, the neighboring cells within a tracer-coupled array were always within one gap junction of the injected neuron. Furthermore, in many cases, the array formed by the somata of tracer-coupled cells was almost perfectly coincident with the dendritic arbor of the injected cell. Thus, our results indicate that whereas coupling is extensive within the proximal retina, individual cells partake in coupled networks that are stereotypic and highly circumscribed.

Tracer coupling among regenerated amacrine cells in the retina of the goldfish

This study sought to characterize the tracer coupling of regenerated amacrine cells in the retina of the goldfish and assess the integration of regenerated neurons into existing retinal circuits. Regeneration of new neurons from injury-induced progenitors was stimulated by surgically excising a small rectangular piece of retina. Several months after regeneration was complete, intracellular injections of Neurobiotin, a gap junction-permeant tracer, were made into single regenerated amacrine cells or nonregenerated (extant) amacrine cells lying outside the regenerated patch. Two groups of amacrine cells were injected: those that in normal retina are tracer coupled and a single type (the radiate amacrine cell) that is not. The data show that regenerated amacrine cells are tracer coupled to each other and to their homologous counterparts outside the patch of regenerated retina. Regenerated radiate cells possess morphologically abnormal dendrites, but these processes can extend out of regenerated retina into surrounding normal retina. Similarly, the dendrites of extant radiate cells, severed by the original lesion, can regenerate into the patch of regenerated retina. These results indicate that in the goldfish retina the cell-specific junctional circuitry present in normal retina is re-created in the regenerated retina, and suggest that regenerated neurons are functionally integrated into the existing retina.

A comparison of receptive field and tracer coupling size of horizontal cells in the rabbit retina

The large receptive fields of retinal horizontal cells are thought to reflect extensive electrical coupling via gap junctions. It was shown recently that the biotinylated tracers, biocytin and Neurobiotin, provide remarkable images of coupling between many types of retinal neuron, including horizontal cells. Further, these demonstrations of tracer coupling between horizontal cells rivaled the size of their receptive fields, suggesting that the pattern of tracer coupling may provide some index of the extent of electrical coupling. We studied this question by comparing the receptive field and tracer coupling size of dark-adapted horizontal cells recorded in the superfused, isolated retina-eyecup of the rabbit. Both the edge-to-edge receptive field and space constants (lambda) were computed for each cell using a long, narrow slit of light displaced across the retinal surface. Cells were subsequently labeled by iontophoretic injection of Neurobiotin. The axonless A-type horizontal cells showed extensive, homologous tracer coupling in groups greater than 1000 covering distances averaging about 2 mm. The axon-bearing B-type horizontal cells were less extensively tracer coupled, showing homologous coupling of the somatic endings in groups of about 100 cells spanning approximately 400 microns and a separate homologous coupling of the axon terminal endings covering only about 275 microns. Moreover, we observed a remarkable, linear relationship between the size of the receptive fields of each of the three horizontal cell endings and the magnitude of their tracer coupling. Our findings suggest that the extent of tracer coupling provides a strong, linear index of the magnitude of electrical current flow, as derived from receptive-field measures, across groups of coupled horizontal cells. These data thus provide the first direct evidence that the receptive-field size of horizontal cells is related to the extent of their coupling via gap junctions.

Synaptic input from CA3 pyramidal cells to dentate basket cells in rat hippocampus

1. Excitatory inputs from CA3 pyramidal cells to dentate basket cells were examined using the whole-cell recording technique in neonatal (10-16 days) rat hippocampal slices to characterize this unexpected feedback pathway. 2. Minimal electrical stimulation of the CA3 pyramidal layer evoked in basket cells short latency (5.2 +/- 0.4 ms) glutamate receptor-mediated excitatory postsynaptic currents (EPSCs) with fast rise times (at -70 mV, 0.9 +/- 0.2 ms), fast decay time constants (3.6 +/- 0.6 ms), and small amplitudes (-14 +/- 3.4 pA). Minimal electrical stimulation evoked monosynaptic EPSCs in only 48 +/- 9.2% of the trials suggesting that the CA3 pyramidal cell to basket cell pathway was unreliable. 3. CA3 pyramidal cell layer stimulation did not antidromically or synaptically activate granule cells but did evoke polysynaptic IPSCs in granule cells, suggesting that the net effect of CA3 pyramidal cell firing on the dentate gyrus was granule cell inhibition. 4. Stimulation of the CA3 pyramidal cell layer evoked both monosynaptic and polysynaptic EPSCs in basket cells, which were eliminated by a knife lesion separating CA3 from the dentate gyrus. The latencies of the EPSCs evoked in 0.6 mM extracellular calcium were the same as the earliest latencies of EPSCs in 1.5 mM calcium, suggesting that those EPSCs were monosynaptic. The polysynaptic input was more prominent in the presence of 10 microM bicuculline, implying that inhibitory GABAergic circuits normally limit this feedback from CA3 to basket cells. 5. In recordings from 103 pairs of CA3 pyramidal cells and dentate basket cells from 11 slices, two polysynaptic connections were found that were active only when the presynaptic CA3 pyramidal neuron fired in bursts. No monosynaptic connections between CA3 pyramidal cells and basket cells were identified indicating that connections between the two cell types may be sparse. 6. Raising the external potassium concentration from 3.5 to 8.5 mM, which elicited burst firing in CA3 pyramidal cells, resulted in a barrage of EPSCs and action potentials in basket cells. In contrast, granule cells neither fired action potentials nor exhibited increased EPSC frequency in elevated potassium but instead received a higher frequency of bicuculline-sensitive IPSCs, consistent with interneuron firing. The CA3 pyramidal cell to basket cell monosynaptic pathway exhibited paired-pulse facilitation as manifested by an increased probability of release, which supports the idea that basket cells were better activated by short trains of action potentials than by single inputs.(ABSTRACT TRUNCATED AT 400 WORDS)

Structure--function relationships in gap junctions

Gap junctions are metabolic and electrotonic pathways between cells and provide direct cooperation within and between cellular nets. They are among the cellular structures most frequently investigated. This chapter primarily addresses aspects of the assembly of the gap junction channel, considering the insertion of the protein into the membrane, the importance of phosphorylation of the gap junction proteins for coupling modulation, and the formation of whole channels from two hemichannels. Interactions of gap junctions with the subplasmalemmal cytoplasm on the one side and with tight junctions on the other side are closely considered. Furthermore, reviewing the significance and alterations of gap junctions during development and oncogenesis, respectively, including the role of adhesion molecules, takes up a major part of the chapter. Finally, the literature on gap junctions in the central nervous system, especially between astrocytes in the brain cortex and horizontal cells in the retina, is summarized and new aspects on their structure-function relationship included.