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Synaptic transmission from horizontal cells to cones is impaired by loss of connexin hemichannels.

Klaassen LJ, Sun Z, Steijaert MN, Bolte P, Fahrenfort I, Sjoerdsma T, Klooster J, Claassen Y, Shields CR, Ten Eikelder HM, Janssen-Bienhold U, Zoidl G, McMahon DG, Kamermans M - PLoS Biol. (2011)

Bottom Line: A reduction of feedback was also found when horizontal cells were pharmacologically hyperpolarized but was absent when they were pharmacologically depolarized.A model for feedback, in which the number of connexin hemichannels is reduced to about 40%, fully predicts the specific asymmetric modification of feedback.To our knowledge, this is the first successful genetic interference in the feedback pathway from horizontal cells to cones.

View Article: PubMed Central - PubMed

Affiliation: Research Unit Retinal Signal Processing, The Netherlands Institute for Neuroscience, Amsterdam, The Netherlands.

ABSTRACT
In the vertebrate retina, horizontal cells generate the inhibitory surround of bipolar cells, an essential step in contrast enhancement. For the last decades, the mechanism involved in this inhibitory synaptic pathway has been a major controversy in retinal research. One hypothesis suggests that connexin hemichannels mediate this negative feedback signal; another suggests that feedback is mediated by protons. Mutant zebrafish were generated that lack connexin 55.5 hemichannels in horizontal cells. Whole cell voltage clamp recordings were made from isolated horizontal cells and cones in flat mount retinas. Light-induced feedback from horizontal cells to cones was reduced in mutants. A reduction of feedback was also found when horizontal cells were pharmacologically hyperpolarized but was absent when they were pharmacologically depolarized. Hemichannel currents in isolated horizontal cells showed a similar behavior. The hyperpolarization-induced hemichannel current was strongly reduced in the mutants while the depolarization-induced hemichannel current was not. Intracellular recordings were made from horizontal cells. Consistent with impaired feedback in the mutant, spectral opponent responses in horizontal cells were diminished in these animals. A behavioral assay revealed a lower contrast-sensitivity, illustrating the role of the horizontal cell to cone feedback pathway in contrast enhancement. Model simulations showed that the observed modifications of feedback can be accounted for by an ephaptic mechanism. A model for feedback, in which the number of connexin hemichannels is reduced to about 40%, fully predicts the specific asymmetric modification of feedback. To our knowledge, this is the first successful genetic interference in the feedback pathway from horizontal cells to cones. It provides direct evidence for an unconventional role of connexin hemichannels in the inhibitory synapse between horizontal cells and cones. This is an important step in resolving a long-standing debate about the unusual form of (ephaptic) synaptic transmission between horizontal cells and cones in the vertebrate retina.

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Feedback measured in cones is reduced in the mutant zebrafish.(A) Light-induced feedback responses in wild-type (left) and mutant (right) zebrafish cones. Horizontal cells were hyperpolarized by full-field 1 s flashes of light while the recorded cone was saturated with a 20 µm spot of bright light. (B) IV relation of a cone ICa in control (green), 50 µM DNQX (red), and 30 µM KA (black) in wild-type (Bi, left) and mutant (Bii, left). Averaged DNQX- (red) and KA- (black) induced currents in wild-type (Bi, right) and mutants (Bii, right). The DNQX-induced current is significantly reduced in the mutant compared to the current in wild-type.
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pbio-1001107-g006: Feedback measured in cones is reduced in the mutant zebrafish.(A) Light-induced feedback responses in wild-type (left) and mutant (right) zebrafish cones. Horizontal cells were hyperpolarized by full-field 1 s flashes of light while the recorded cone was saturated with a 20 µm spot of bright light. (B) IV relation of a cone ICa in control (green), 50 µM DNQX (red), and 30 µM KA (black) in wild-type (Bi, left) and mutant (Bii, left). Averaged DNQX- (red) and KA- (black) induced currents in wild-type (Bi, right) and mutants (Bii, right). The DNQX-induced current is significantly reduced in the mutant compared to the current in wild-type.

Mentions: Next, we studied feedback responses in cones. Light-induced feedback responses were measured first. Cones were saturated with a 20 µm spot of light and 500 ms full-field stimuli were applied in addition. Such a protocol induces an inward current which is due to negative feedback from horizontal cells to cones [1],[3]. Figure 6A shows feedback-induced responses of wild-type (black) and mutant cones (red). On average the feedback-induced inward current in mutants was smaller than in wild-type cones (wild-type: −3.37±0.72 pA, n = 9; mutant 0.41±0.81 pA, n = 6; p = 0.004). The mutant feedback-induced currents did not differ significantly from zero (p = 0.635) while they did in the wild-type (p = 0.002).


Synaptic transmission from horizontal cells to cones is impaired by loss of connexin hemichannels.

Klaassen LJ, Sun Z, Steijaert MN, Bolte P, Fahrenfort I, Sjoerdsma T, Klooster J, Claassen Y, Shields CR, Ten Eikelder HM, Janssen-Bienhold U, Zoidl G, McMahon DG, Kamermans M - PLoS Biol. (2011)

Feedback measured in cones is reduced in the mutant zebrafish.(A) Light-induced feedback responses in wild-type (left) and mutant (right) zebrafish cones. Horizontal cells were hyperpolarized by full-field 1 s flashes of light while the recorded cone was saturated with a 20 µm spot of bright light. (B) IV relation of a cone ICa in control (green), 50 µM DNQX (red), and 30 µM KA (black) in wild-type (Bi, left) and mutant (Bii, left). Averaged DNQX- (red) and KA- (black) induced currents in wild-type (Bi, right) and mutants (Bii, right). The DNQX-induced current is significantly reduced in the mutant compared to the current in wild-type.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3139627&req=5

pbio-1001107-g006: Feedback measured in cones is reduced in the mutant zebrafish.(A) Light-induced feedback responses in wild-type (left) and mutant (right) zebrafish cones. Horizontal cells were hyperpolarized by full-field 1 s flashes of light while the recorded cone was saturated with a 20 µm spot of bright light. (B) IV relation of a cone ICa in control (green), 50 µM DNQX (red), and 30 µM KA (black) in wild-type (Bi, left) and mutant (Bii, left). Averaged DNQX- (red) and KA- (black) induced currents in wild-type (Bi, right) and mutants (Bii, right). The DNQX-induced current is significantly reduced in the mutant compared to the current in wild-type.
Mentions: Next, we studied feedback responses in cones. Light-induced feedback responses were measured first. Cones were saturated with a 20 µm spot of light and 500 ms full-field stimuli were applied in addition. Such a protocol induces an inward current which is due to negative feedback from horizontal cells to cones [1],[3]. Figure 6A shows feedback-induced responses of wild-type (black) and mutant cones (red). On average the feedback-induced inward current in mutants was smaller than in wild-type cones (wild-type: −3.37±0.72 pA, n = 9; mutant 0.41±0.81 pA, n = 6; p = 0.004). The mutant feedback-induced currents did not differ significantly from zero (p = 0.635) while they did in the wild-type (p = 0.002).

Bottom Line: A reduction of feedback was also found when horizontal cells were pharmacologically hyperpolarized but was absent when they were pharmacologically depolarized.A model for feedback, in which the number of connexin hemichannels is reduced to about 40%, fully predicts the specific asymmetric modification of feedback.To our knowledge, this is the first successful genetic interference in the feedback pathway from horizontal cells to cones.

View Article: PubMed Central - PubMed

Affiliation: Research Unit Retinal Signal Processing, The Netherlands Institute for Neuroscience, Amsterdam, The Netherlands.

ABSTRACT
In the vertebrate retina, horizontal cells generate the inhibitory surround of bipolar cells, an essential step in contrast enhancement. For the last decades, the mechanism involved in this inhibitory synaptic pathway has been a major controversy in retinal research. One hypothesis suggests that connexin hemichannels mediate this negative feedback signal; another suggests that feedback is mediated by protons. Mutant zebrafish were generated that lack connexin 55.5 hemichannels in horizontal cells. Whole cell voltage clamp recordings were made from isolated horizontal cells and cones in flat mount retinas. Light-induced feedback from horizontal cells to cones was reduced in mutants. A reduction of feedback was also found when horizontal cells were pharmacologically hyperpolarized but was absent when they were pharmacologically depolarized. Hemichannel currents in isolated horizontal cells showed a similar behavior. The hyperpolarization-induced hemichannel current was strongly reduced in the mutants while the depolarization-induced hemichannel current was not. Intracellular recordings were made from horizontal cells. Consistent with impaired feedback in the mutant, spectral opponent responses in horizontal cells were diminished in these animals. A behavioral assay revealed a lower contrast-sensitivity, illustrating the role of the horizontal cell to cone feedback pathway in contrast enhancement. Model simulations showed that the observed modifications of feedback can be accounted for by an ephaptic mechanism. A model for feedback, in which the number of connexin hemichannels is reduced to about 40%, fully predicts the specific asymmetric modification of feedback. To our knowledge, this is the first successful genetic interference in the feedback pathway from horizontal cells to cones. It provides direct evidence for an unconventional role of connexin hemichannels in the inhibitory synapse between horizontal cells and cones. This is an important step in resolving a long-standing debate about the unusual form of (ephaptic) synaptic transmission between horizontal cells and cones in the vertebrate retina.

Show MeSH
Related in: MedlinePlus