The Role of Retinal Connexins Cx36 and Horizontal Cell Coupling in Emmetropization in Guinea Pigs

Purpose

The purpose of this study was to determine whether retinal gap junctions (GJs) via connexin 36 (Cx36, mediating coupling of many retinal cell types) and horizontal cell (HC-HC) coupling, are involved in emmetropization.

Methods

Guinea pigs (3 weeks old) were monocularly form deprived (FD) or raised without FD (in normal visual [NV] environment) for 2 days or 4 weeks; alternatively, they wore a -4 D lens (hyperopic defocus [HD]) or 0 D lens for 2 days or 1 week. FD and NV eyes received daily subconjunctival injections of a nonspecific GJ-uncoupling agent, 18-β-Glycyrrhetinic Acid (18-β-GA). The amounts of total Cx36 and of phosphorylated Cx36 (P-Cx36; activated state that increases cell-cell coupling), in the inner and outer plexiform layers (IPLs and OPLs), were evaluated by quantitative immunofluorescence (IF), and HC-HC coupling was evaluated by cut-loading with neurobiotin.

Results

FD per se (excluding effect of light-attenuation) increased HC-HC coupling in OPL, whereas HD did not affect it. HD for 2 days or 1 week had no significant effect on retinal content of Cx36 or P-Cx36. FD for 4 weeks decreased the total amounts of Cx36 and P-Cx36, and the P-Cx36/Cx36 ratio, in the IPL. Subconjunctival 18-β-GA induced myopia in NV eyes and increased the myopic shifts in FD eyes, while reducing the amounts of Cx36 and P-Cx36 in both the IPL and OPL.

Conclusions

These results suggest that cell-cell coupling via GJs containing Cx36 (particularly those in the IPL) plays a role in emmetropization and form deprivation myopia (FDM) in mammals. Although both FD and 18-β-GA induced myopia, they had opposite effects on HC-HC coupling. These findings suggest that HC-HC coupling in the OPL might not play a significant role in emmetropization and myopia development.

Mefloquine effects on ventral tegmental area dopamine and GABA neuron inhibition: a physiologic role for connexin-36 GAP junctions

Connexin-36 (Cx36) gap junctions (GJs) appear to be involved in the synchronization of GABA interneurons in many brain areas. We have previously identified a population of Cx36-connected ventral tegmental area (VTA) GABA neurons that may regulate mesolimbic dopamine (DA) neurotransmission, a system implicated in reward from both natural behaviors and drugs of abuse. The aim of this study was to determine the effect mefloquine (MFQ) has on midbrain DA and GABA neuron inhibition, and the role Cx36 GJs play in regulating midbrain VTA DA neuron activity in mice. In brain slices from adolescent wild-type (WT) mice the Cx36-selective GJ blocker mefloquine (MFQ, 25 μM) increased VTA DA neuron sIPSC frequency sixfold, and mIPSC frequency threefold. However, in Cx36 KO mice, MFQ only increased sIPSC and mIPSC frequency threefold. The nonselective GJ blocker carbenoxolone (CBX, 100 μM) increased DA neuron sIPSC frequency twofold in WT mice, did not affect Cx36 KO mouse sIPSCs, and did not affect mIPSCs in WT or Cx36 KO mice. Interestingly, MFQ had no effect on VTA GABA neuron sIPSC frequency. We also examined MFQ effects on VTA DA neuron firing rate and current-evoked spiking in WT and Cx36 KO mice, and found that MFQ decreased WT DA neuron firing rate and current-evoked spiking, but did not alter these measures in Cx36 KO mice. Taken together these findings suggest that blocking Cx36 GJs increases VTA DA neuron inhibition, and that GJs play in key role in regulating inhibition of VTA DA neurons. Synapse, 2011. © 2011 Wiley-Liss, Inc.

Electrical synapses in retinal ON cone bipolar cells: subtype-specific expression of connexins

Retinal bipolar cells are known to form a complex, interconnecting network through electrical synapses that are either heterologous (with amacrine cells) or homologous (with other bipolar cells). These electrical synapses can be functionally as important as chemical synapses because their distinct properties provide a different character for the network. Much less is known, however, about electrical synapses in retinal bipolar cells than about chemical synapses. Here we report the molecular basis for electrical synapses in retinal bipolar cells, particularly ON cone bipolar cells. We have found variable connexin 36 (cx36) expression in different types of ON cone bipolar cells: cx36 message was found in some, but not all, ON cone bipolar cells (4 of 14 cells). In one specific type of ON cone bipolar cell (BPGus-GFP), however, cx36 was detected in 17 of 19 cells. Moreover, we have located cx36 puncta at the axonal terminals of BPGus-GFP cells, and we have found that these BPGus-GFP-associated cx36 puncta always colocalized with AII amacrine cell processes. Molecular and immunocytochemical evidence obtained in this study also shows that connexin 45 (cx45) is not present in BPGus-GFP cells. Taken together, our results suggest that connexins are expressed in bipolar cells in a neuronal subtype-specific manner and that cx36/cx36 gap junctions form the heterologous electrical synapses between AII amacrine cells and BPGus-GFP cells. Our findings imply that visual information can be differently processed by distinct subtypes of ON cone bipolar cells via electrical synapses.

Different functional types of bipolar cells use different gap-junctional proteins

Rod signals are transmitted to ON retinal ganglion cells by means of gap junctions between AII amacrine cells and ON bipolars. The AII amacrine cells are known to express connexin36 (Cx36), but previous studies of Cx36 in ON cone bipolars have been ambiguous. Here, we studied bipolar cells in a transgenic mouse line that expresses high levels of green fluorescent protein (GFP) in one type of ON cone bipolar cell. We found strong Cx36 immunostaining in the axon terminals of the GFP-labeled type 357 bipolar cells in both vertical sections and whole mounts of the retina. This finding was confirmed by single-cell immunostaining and single-cell reverse transcription-PCR (RT-PCR). As reported previously (Maxeiner et al., 2005), Cx45 was found in some ON bipolar cells, but RT-PCR showed Cx36 and not Cx45 to be expressed by the type 357 bipolar cells. Some of the remaining GFP-negative bipolar cells expressed Cx45 but not Cx36. It appears that different types of ON cone bipolar cells express different connexins at their gap junctions with AII amacrine cells.

Visual transmission deficits in mice with targeted disruption of the gap junction gene connexin36

In the mammalian retina, rods feed into the cone pathway through electrotonic coupling, and recent histological data suggest the involvement of connexin36 (Cx36) in this pathway. We therefore generated Cx36 null mice and monitored the functional consequences of this deficiency on early visual transmission. The homozygous mutant mice had a normally developed retina and showed no changes in the cellular organization of the rod pathway. In contrast, the functional coupling between AII amacrine cells and bipolar cells was impaired. Recordings of electroretinograms revealed a significant decrease of the scotopic b-wave in mutant animals and an increased cone threshold that is compatible with a distorted, gap junctional transmission between AII amacrine cells and cone bipolar cells. Recordings of visual evoked potentials showed extended latency in mutant mice but unaffected ON and OFF components. Our results demonstrate that Cx36-containing gap junctions are essential for normal synaptic transmission within the rod pathway.

Functional expression of the murine connexin 36 gene coding for a neuron-specific gap junctional protein

The mouse connexin 36 (Cx36) gene was mapped on chromosome 2 and an identical transcriptional start site was determined in brain and retina on exon I. Rabbit polyclonal antibodies to the presumptive cytoplasmic loop of the Cx36 protein recognized in immunohistochemical analyses Cx36 expression in the retina, olfactory bulb, hippocampus, inferior olive and cerebellum. In olivary neurons strong punctate labeling at dendritic cell contacts and weaker labeling in the cytoplasm of dendrites were shown by immuno electron microscopy. After expression of mouse Cx36 cDNA in human HeLa cells, neurobiotin transfer was increased 1.8-fold and electrical conductance at least 15-fold compared to untransfected HeLa cells. No Lucifer Yellow transfer was detected in either untransfected or Cx36 transfected HeLa cells. Single Cx36 channels in transfected HeLa cells showed a unitary conductance of 14.3 + or - 0. 8 pS. The sensitivity of Cx36 channels to transjunctional voltage was low in both HeLa-Cx36 cells and Xenopus oocytes expressing mouse Cx36. No increased transfer of neurobiotin was detected in heterotypic gap junctions formed by Cx36 and 9 other connexins expressed in HeLa cells. Our results suggest that Cx36 channels function as electrical synapses for transmission of electrical and metabolic signals between neurons in the central nervous system.

Functional properties of channels formed by the neuronal gap junction protein connexin36

The expression and functional properties of connexin36 (Cx36) were examined in two communication-deficient cell lines (N2A-neuroblastoma and PC-12 cells) transfected with Cx36 and in hippocampal neurons that express the connexin endogenously. Transfected cells expressed the expected 2.9 kb Cx36 transcript and Cx36 immunoreactivity, whereas nontransfected cells were devoid of Cx36. The relationship between steady-state junctional conductance (g(j)) and transjunctional voltage was well described by a two-state Boltzmann equation. The half-inactivation voltage (V(0)), the ratio of minimal to maximal g(j) (g(min)/g(max)), and the equivalent gating charge were +/- 75 mV, 0.55, and 1.75, respectively, indicating that Cx36 exhibits very low voltage sensitivity. Conductance of single Cx36 channels measured with patch pipettes containing 130 mM CsCl was 10-15 pS (n = 15 cell pairs); despite this low unitary conductance, Cx36 channels were permeable to the dye Lucifer yellow. Hippocampal neurons expressed Cx36 both in vivo and in culture. The electrophysiological properties of channels in cultured hippocampal neurons were similar to those of the channels expressed by the transfected cell lines, and the neuronal channels were similarly permeable to Lucifer yellow. The unique combination of weak voltage sensitivity, small unitary conductance, and permeation by anions as large as second messenger molecules endows Cx36 gap junction channels with properties well suited for mediating flexible electrical and biochemical interactions between neurons.