Jozsef Vigh

Prolonged reciprocal signaling via NMDA and GABA receptors at a retinal ribbon synapse.

AMPA and GABAA receptors mediate most of the fast signaling in the CNS. However, the retina must, in addition, also convey slow and sustained signals. Given that AMPA and GABAA receptors desensitize quickly in the continuous presence of agonist, how are sustained excitatory and inhibitory signals transmitted reliably across retinal synapses? Reciprocal synapses between bipolar and amacrine cells in the retina are thought to play a fundamental role in tuning the bipolar cell output to the dynamic range of ganglion cells. Here, we report that glutamate release from goldfish bipolar cell terminals activates first AMPA receptors, followed by fast and transient GABAA-mediated feedback. Subsequently, prolonged NMDA receptor activation triggers GABAA and a slow, sustained GABAC-mediated reciprocal inhibition. The synaptic delay of the NMDA/GABAC-mediated feedback showed stronger dependence on the depolarization of the bipolar cell terminal than the fast AMPA/GABAA-mediated response. Although the initial depolarization mediated by AMPA receptors was important to prime the NMDA action, NMDA receptors could trigger feedback by themselves in most of the bipolar terminals tested. This AMPA-independent feedback (delay ≈ 10 ms) was eliminated in 2 mm external Mg2+ and reduced in some terminals, but not eliminated, by TTX. NMDA receptors on amacrine cells with depolarized resting membrane potentials therefore can mediate the late reciprocal feedback triggered by continuous glutamate release. Our findings suggest that the characteristics of NMDA receptors (high agonist affinity, slow desensitization, and activation/deactivation kinetics) are well suited to match the properties of GABAC receptors, which thus provide part of the prolonged inhibition to bipolar cell terminals.

Alterations of conditioned taste aversion after microiontophoretically applied neurotoxins in the medial prefrontal cortex of the rat

The prefrontal cortex (PFC) has been reported to be essential in neural control of feeding. In the present study, we aimed to provide a complex characterization of behavioral consequences of PFC microlesions in CFY rats. Kainic acid (KA) was microiontophoretically applied into the mediodorsal division of PFC to damage intrinsic neurons, whereas in another group of rats, 6-hydroxydopamine (6-OHDA) was microiontophoretized into the same region to destroy catecholaminergic (CA) projection fiber terminals. Body weights, food and fluid intake of both lesioned and (sham-operated or intact) control animals were daily measured. Effects of intracellular dehydration and water deprivation were also studied. Open field activity, stereotyped behaviors, and orientation towards visual and somesthetic stimuli were pre- and postoperatively tested. To examine hypothesized consequences of mPFC microlesions on central taste information processing, the acquisition and retention of saccharine conditioned taste aversion (CTA) were studied. No major changes were recorded in body weights, food and water consumption. Dehydration or deprivation similarly increased water intake in all animals. Scores of open field activity and stereotyped behaviors in the 6-OHDA group were significantly higher than those of the other groups. As the main findings of the present studies, both KA and 6-OHDA lesioned rats displayed significant deficits in CTA acquisition and retention tests. These results suggest that the medial PFC has a substantial role in both the formation and the retrieval of CTA. Furthermore, the present findings also indicate the general significance of prefrontal CA mechanisms in the organization of goal-directed, adaptive behaviors.

Light-Evoked Lateral GABAergic Inhibition at Single Bipolar Cell Synaptic Terminals Is Driven by Distinct Retinal Microcircuits

Inhibitory amacrine cells (ACs) filter visual signals crossing the retina by modulating the excitatory, glutamatergic output of bipolar cells (BCs) on multiple temporal and spatial scales. Reciprocal feedback from ACs provides focal inhibition that is temporally locked to the activity of presynaptic BC activity, whereas lateral feedback originates from ACs excited by distant BCs. These distinct feedback mechanisms permit temporal and spatial computation at BC terminals. Here, we used a unique preparation to study light-evoked IPSCs recorded from axotomized terminals of ON-type mixed rod/cone BCs (Mb) in goldfish retinal slices. In this preparation, light-evoked IPSCs could only reach axotomized BC terminals via the lateral feedback pathway, allowing us to study lateral feedback in the absence of overlapping reciprocal feedback components. We found that light evokes ON and OFF lateral IPSCs (L-IPSCs) in Mb terminals having different temporal patterns and conveyed via distinct retinal pathways. The relative contribution of rods versus cones to ON and OFF L-IPSCs was light intensity dependent. ACs presynaptic to Mb BC terminals received inputs via AMPA/KA- and NMDA-type receptors in both the ON and OFF pathways, and used TTX-sensitive sodium channels to boost signal transfer along their processes. ON and OFF L-IPSCs, like reciprocal feedback IPSCs, were mediated by both GABAA and GABAC receptors. However, our results suggest that lateral and reciprocal feedback do not cross-depress each other, and are therefore mediated by distinct populations of ACs. These findings demonstrate that retinal inhibitory circuits are highly specialized to modulate BC output at different light intensities.

Long-term plasticity mediated by mGluR1 at a retinal reciprocal synapse.

The flow of information across the retina is controlled by reciprocal synapses between bipolar cell terminals and amacrine cells. However, the synaptic delays and properties of plasticity at these synapses are not known. Here we report that glutamate release from goldfish Mb-type bipolar cell terminals can trigger fast (delay of 2-3 ms) and transient GABA(A) IPSCs and a much slower and more sustained GABA(C) feedback. Synaptically released glutamate activated mGluR1 receptors on amacrine cells and, depending on the strength of presynaptic activity, potentiated subsequent feedback. This poststimulus enhancement of GABAergic feedback lasted for up to 10 min. This form of mGluR1-mediated long-term synaptic plasticity may provide retinal reciprocal synapses with adaptive capabilities.

Membrane properties of an unusual intrinsically oscillating, wide‐field teleost retinal amacrine cell

In the retina, amacrine cells modulate the transfer of information from bipolar to ganglion cells. The nature of the modulation depends on the synaptic input and the membrane properties of the cells. In the retina of white bass, we identified a class of bistratified, wide‐field amacrine cell characterized by immunopositive labelling for GABA and calmodulin. In isolation, the cells presented resting membrane potentials averaging ‐69 mV although some cells settled at more depolarized values (‐30 mV). Injection of depolarizing current pulses induced oscillatory membrane responses. When elicited from depolarized cells, the oscillations were short‐lived (< 40 ms). For the most part, the oscillatory potentials of hyperpolarized cells remained unattenuated throughout the depolarizing pulse. The frequency of the oscillations increased logarithmically with mean membrane potential, ranging from 74 to 140 Hz. Cells exhibiting depolarized membrane potentials oscillated at twice that rate. When the membrane potential of these cells was hyperpolarized to ‐70 mV, the oscillations became unattenuated and slowed. We found the cells expressed voltage‐gated sodium, potassium and calcium currents and calcium‐dependent potassium currents. We demonstrate that the oscillatory potentials arose as a result of the interplay between calcium and potassium currents. The cells responded to local application of GABA and glycine, both of which modulate the oscillatory potentials. Glutamate and its analogues depolarized the cell and induced oscillatory potentials. Our results indicate that oscillatory responses of a type of wide‐field amacrine cell are an intrinsic feature of the cell and not due to circuit properties.

Intracellular calcium release resulting from mGluR1 receptor activation modulates GABAA currents in wide-field retinal amacrine cells: a study with caffeine.

The modulatory action of calcium (Ca2+) released from intracellular stores on GABAA receptor‐mediated current was investigated in wide‐field amacrine cells isolated from the teleost, Morone chrysops, retina. Caffeine, ryanodine or inositol 1,4,5‐triphosphate (IP3) markedly inhibited the GABAA current by elevating [Ca2+]i. The inhibition resulted from the activation of a Ca2+→ Ca2+/calmodulin → calcineurin cascade. Long (>12 s) exposure to glutamate mimicked the caffeine effect, i.e. it inhibited the GABAA current by elevating [Ca2+]i through mGluR1 receptor activation and consequent IP3 generation. This pathway provides a ‘timed’ disinhibitory mechanism to potentiate excitatory postsynaptic potentials in wide‐field amacrine cells. It occurs as a result of the suppression of GABA‐mediated conductances as a function of the duration of presynaptic excitatory input activity. This is much like some forms of long‐term potentiation in the central nervous system. In a local retinal circuit this will selectively accentuate particular excitatory inputs to the wide‐field amacrine cell. Similar to other neural systems, we suggest that activity‐dependent postsynaptic disinhibition is an important feature of the signal processing in the inner retina.

Amacrine cells of the anuran retina: Morphology, chemical neuroanatomy, and physiology

Amacrine cells are third‐order retinal interneurons, projecting their processes into the inner plexiform layer. Historically, they were not considered as neurons first. By the middle of the 20th century, their neuronal nature was confirmed, and their enermous diversity established. Amacrine cells have been most succesfully subdivided into morphological categories based on two parameters: diameter of the dendritic field and ramification pattern in the inner plexiform layer. Works combining anatomy, physiology, and neurochemistry are scarce and in the case of the anuran retina, the situation is even worse. Correlation between morphology, neurochemistry, and physiology is little studied. Here we try to build up a database and pinpoint some of the missing data. Obtaining those could help to better understand retinal function. Sporadic attempts did not make it possible to develop a comprehensive catalog of morphologically distinct amacrine cell types in the anuran retina. The number of morphologically identified amacrine cells currently stands at 16. The list of neurochemically identified distinct cell types can be given as follows: five types GABA‐contaning cell types with secondary markers and at least one without; two glycinergic cell types and one interplexiform cell where glycine colocalizes with somatostatin; one dopaminergic amacrine cell and also a variant of this with interplexiform morphology; two types of serotoninergic cells; three NADPHdiaphorase‐positive cells, one substance P‐positive cell type without identified second marker; one CCK‐positive cell type without identified second marker and the calbindin positive cells (at least one but potentially more types). This adds up to 19 cell types, out of which two are interplexiform in character. This is more than that could be identified by purely morphological means. Out of Cajals original 13 amacrine cell types described in the frog retina, 5 parallel unequivocally with neurons defined by neurochemistry. Three others have one close match each, but their exact identity is uncertain. The remaining amacrine cells have more than one potential matches. At the same time, on one hand the amacrine cell named two‐layered by Cajal so far has no match among the neurochemically identified amacrine cells. On the other hand, the interplexiform subtype of the dopaminergic cell, the somatostatin‐containing glycinergic interplexiform cell, the starburst cell, and the bistratified neuropeptide Y‐immunoreactive cell have no match among Cajals cells. All in all, the number of known amacrine and interplexiform cells now stands at at least 21 in the anuran retina. Physiological characterization of amacrine cells shows that their general features seem to be rather similar to those described in tiger salamander retina. In Xenopus retina, morphologically and physiologically identified amacrine cells responded to light stimulation most frequently with ON‐OFF characteristics. Immunhistochemical identification of the recorded and dye injected cells showed that amacrine cells of the “same physiological type” might have different morphology. In other words, amacrine cells with different morphology can respond similarly to illumination. Even so, small differences between almost identical responses may reflect that the cell they stem from indeed belongs to different cell types. Microsc. Res. Tech. 50:373–383, 2000.

Short-Term Depression at the Reciprocal Synapses between a Retinal Bipolar Cell Terminal and Amacrine Cells

Visual adaptation is thought to occur partly at retinal synapses that are subject to plastic changes. However, the locus and properties of this plasticity are not well known. Here, we studied short-term plasticity at the reciprocal synapse between bipolar cell terminals and amacrine cells in goldfish retinal slices. Depolarization of a single bipolar cell terminal for 100 ms triggers the release of glutamate onto amacrine cell processes, which in turn leads to GABAergic feedback from amacrine cells onto the same terminal. We find that this release of GABA undergoes paired-pulse depression (PPD) that recovers in <1 min (single exponential time constant, τ ≅ 12 s). This disynaptic PPD is independent of mGluR-mediated plasticity and depletion of glutamatergic synaptic vesicle pools, because exocytosis assayed via capacitance jumps (ΔCm) recovered completely after 10 s (τ ≅ 2 s). Fast application of GABA (10 mm) onto outside-out patches excised from bipolar cell terminals showed that the recovery of GABAA receptors from desensitization depends on the duration of the application [fast recovery (<2 s) for short applications; slow (τ ≅ 12 s) for prolonged applications]. We thus blocked GABAA receptors and retested the GABAergic response mediated by nondesensitizing GABAC receptors to two rapid glutamate puffs onto the bipolar cell terminal. These responses consistently displayed PPD. Furthermore, blocking AMPA-receptor desensitization with cyclothiazide, or evoking GABA release with NMDA receptors, did not reduce PPD. We thus suggest that depletion of synaptic vesicle pools in GABAergic amacrine cells is a major contributor to PPD.

L-type calcium channels mediate transmitter release in isolated, wide-field retinal amacrine cells.

Transmitter release in neurons is triggered by intracellular Ca2+ increase via the opening of voltage-gated Ca2+ channels. Here we investigated the voltage-gated Ca2+ channels in wide-field amacrine cells (WFACs) isolated from the white-bass retina that are functionally coupled to transmitter release. We monitored transmitter release through the measurement of the membrane capacitance (Cm). We found that 500-ms long depolarizations of WFACs from -70 mV to 0 mV elicited about a 6% transient increase in the Cm or membrane surface area. This Cm jump could be eliminated either by intracellular perfusion with 10 mM BAPTA or by extracellular application of 4 mM cobalt. WFACs possess N-type and L-type voltage-gated Ca2+ channels. Depolarization-evoked Cm increases were unaffected by the specific N-type channel blocker omega-conotoxin GVIA, but they were markedly reduced by the L-type blocker diltiazem, suggesting a role for the L-type channel in synaptic transmission. Further supporting this notion, in WFACs the synaptic protein syntaxin always colocalized with the pore-forming subunit of the retinal specific L-type channels (Cav1.4 or alpha1F), but never with that of the N-type channels (Cav2.2 or alpha1B ).

Gastrin-releasing peptide microinjected into the amygdala inhibits feeding

Bombesin (BN)-like peptides including gastrin-releasing peptide (GRP) are known to inhibit feeding. In the amygdaloid body BN receptors have been found in moderate to high densities. The central part of the amygdala (ACE) is essentially involved in the regulation of feeding and body weight. In the present experiments GRP was injected into the ACE and liquid food intake, general behavioural activity, as well as core temperature, were examined in male CFY rats. Food intake was measured every 5 min for 30 min and at the 40th and the 60th min following GRP or vehicle microinjections. Bilateral application of 50, 100 or 150 ng GRP resulted in transient inhibition of food intake while bilateral injection of 25 or 300 ng GRP did not modify feeding. Effect of GRP was eliminated by prior application of BN receptor antagonist [Leu(13)-psi(CH(2)NH)-Leu(14)]BN. After GRP or vehicle treatments animals were video-monitored and food intake, the first meal latency (FML), intermeal intervals (IMI), the time spent feeding (FT), grooming, resting and exploration were analysed at 5-min intervals for 30 min. However, FML did not change after GRP, the first IMI increased and intake, FT and intake/FT significantly decreased during the first 5 min. Duration of resting gradually increased after GRP and animals spent less time with exploration after GRP treatment than after vehicle injection. These differences were significant during the 25-30-min period. In body temperature, no significant changes were observed. Our results show that GRP in the ACE inhibits feeding and that GRP may decrease the efficiency of eating and may act as a satiety signal.