Key points• The mouse retina has become a key model for research on the function and dysfunction of the early stages of vision, but its photoreceptors have proven difficult to access with whole-cell recording techniques.• We have optimized the mouse retinal slice preparation to the degree required for studying photoreceptors with a reasonably high yield.• We find that single photon processing in rods is considerably more efficient than previously thought, implying a more favourable transmission at the rod-rod bipolar cell synapse.• Cones were recorded much more frequently than their numeric proportion of ∼3% allowing us to obtain direct functional evidence suggestive of rod-cone coupling in the mouse.• This study opens the way for further investigations into mammalian photoreceptor function by exploiting the powerful molecular genetic approaches available in the mouse. AbstractResearch on photoreceptors has led to important insights into how light signals are detected and processed in the outer retina. Most information about photoreceptor function, however, comes from lower vertebrates. The large majority of mammalian studies are based on suction pipette recordings of outer segment currents, a technique that doesn't allow examination of phenomena occurring downstream of phototransduction. Only a small number of whole-cell recordings have been made, mainly in the macaque. Due to the growing importance of the mouse in vision research, we have optimized a retinal slice preparation that allows the reliable collection of perforated-patch recordings from light responding rods and cones. Unexpectedly, the frequency of cone recordings was much higher than their numeric proportion of ∼3%. This allowed us to obtain direct functional evidence suggestive of rod-cone coupling in the mouse. Moreover, rods had considerably larger single photon responses than previously published for mammals (3.44 mV, SD 1.37, n = 19 at 24 • C; 2.46 mV, SD 1.08, n = 10 at 36 • C), and a relatively high signal/noise ratio (6.4, SD 1.8 at 24• C; 6.8, SD 2.8 at 36• C). Both findings imply a more favourable transmission at the rod-rod bipolar cell synapse. Accordingly, relatively few photoisomerizations were sufficient to elicit a half-maximal response (6.7, SD 2.7, n = 5 at 24• C; 10.6, SD 1.7, n = 3 at 36 • C), leading to a narrow linear response range. Our study demonstrates new features of mammalian photoreceptors and opens the way for further investigations into photoreceptor function using retinas from mutant mouse models.

Rod and cone photoreceptors are coupled by gap junctions (GJs), relatively large channels able to mediate both electrical and molecular communication. Despite their critical location in our visual system and evidence that they are dynamically gated for dark/light adaptation, the full impact that rod–cone GJs can have on cone function is not known. We recorded the photovoltage of mouse cones and found that the initial level of rod input increased spontaneously after obtaining intracellular access. This process allowed us to explore the underlying coupling capacity to rods, revealing that fully coupled cones acquire a striking rod-like phenotype. Calcium, a candidate mediator of the coupling process, does not appear to be involved on the cone side of the junctional channels. Our findings show that the anatomical substrate is adequate for rod–cone coupling to play an important role in vision and, possibly, in biochemical signaling among photoreceptors.DOI: http://dx.doi.org/10.7554/eLife.01386.001

Vertebrates acquired dim-light vision when an ancestral cone evolved into the rod photoreceptor at an unknown stage preceding the last common ancestor of extant jawed vertebrates (∼420 million years ago Ma). The jawless lampreys provide a unique opportunity to constrain the timing of this advance, as their line diverged ∼505 Ma and later displayed high-morphological stability. We recorded with patch electrodes the inner segment photovoltages and with suction electrodes the outer segment photocurrents of Lampetra fluviatilis retinal photoreceptors. Several key functional features of jawed vertebrate rods are present in their phylogenetically homologous photoreceptors in lamprey: crucially, the efficient amplification of the effect of single photons, measured by multiple parameters, and the flow of rod signals into cones. These results make convergent evolution in the jawless and jawed vertebrate lines unlikely and indicate an early origin of rods, implying strong selective pressure toward dim-light vision in Cambrian ecosystems.DOI: http://dx.doi.org/10.7554/eLife.07166.001

Rod-cone gap junctions mediate the so-called "secondary rod pathway", one of three routes that convey rod photoreceptor signals across the retina. Connexin 36 (Cx36) is expressed at these gap junctions, but an unidentified connexin protein also seems to be expressed. Cx36 knockout mice have been used extensively in the quest to dissect the roles in vision of all three pathways, with the assumption, never directly tested, that rod-cone electrical coupling is abolished by deletion of this connexin isoform. We previously showed that when wild type mouse cones couple to rods, their apparent dynamic range is extended toward lower light intensities, with the appearance of large responses to dim flashes (up to several mV) originating in rods. Here we recorded from the cones of Cx36del[LacZ]/del[LacZ] mice and found that dim flashes of the same intensity evoked at most small sub-millivolt responses. Moreover, these residual responses originated in the cones themselves, since: (i) their spectral preference matched that of the recorded cone and not of rods, (ii) their time-to-peak was shorter than in coupled wild type cones, (iii) a pharmacological block of gap junctions did not reduce their amplitude. Taken together, our data show that rod signals are indeed absent in the cones of Cx36 knockout mice. This study is the first direct demonstration that Cx36 is crucial for the assembly of functional rod-cone gap junctional channels, implying that its genetic deletion is a reliable experimental approach to eliminate rod-cone coupling.