How do horizontal cells 'talk' to cone photoreceptors? Different levels of complexity at the cone-horizontal cell synapse

The first synapse of the retina plays a fundamental role in the visual system. Due to its importance, it is critical that it encodes information from the outside world with the greatest accuracy and precision possible. Cone photoreceptor axon terminals contain many individual synaptic sites, each represented by a presynaptic structure called a 'ribbon'. These synapses are both highly sophisticated and conserved. Each ribbon relays the light signal to one ON cone bipolar cell and several OFF cone bipolar cells, while two dendritic processes from a GABAergic interneuron, the horizontal cell, modulate the cone output via parallel feedback mechanisms. The presence of these three partners within a single synapse has raised numerous questions, and its anatomical and functional complexity is still only partially understood. However, the understanding of this synapse has recently evolved, as a consequence of progress in understanding dendritic signal processing and its role in facilitating global versus local signalling. Indeed, for the downstream retinal network, dendritic processing in horizontal cells may be essential, as they must support important functional operations such as contrast enhancement, which requires spatial averaging of the photoreceptor array, while at the same time preserving accurate spatial information. Here, we review recent progress made towards a better understanding of the cone synapse, with an emphasis on horizontal cell function, and discuss why such complexity might be necessary for early visual processing.

Comparison of the Diagnostic Utility of 4D-DSA with Conventional 2D- and 3D-DSA in the Diagnosis of Cerebrovascular Abnormalities

BACKGROUND AND PURPOSE

4D-DSA is a time-resolved technique that allows viewing of a contrast bolus at any time and from any desired viewing angle. Our hypothesis was that the information content in a 4D-DSA reconstruction was essentially equivalent to that in a combination of 2D acquisitions and a 3D-DSA reconstruction.

MATERIALS AND METHODS

Twenty-six consecutive patients who had both 2D- and 3D-DSA acquisitions were included in the study. The angiography report was used to obtain diagnoses and characteristics of abnormalities. Diagnoses included AVM/AVFs, aneurysms, stenosis, and healthy individuals. 4D-DSA reconstructions were independently reviewed by 3 experienced observers who had no part in the clinical care. Using an electronic evaluation form, these observers recorded their assessments based only on the 4D reconstructions. The clinical evaluations were then compared with the 4D evaluations for diagnosis and lesion characteristics.

RESULTS

Results showed both interrater and interclass agreements (κ = 0.813 and 0.858). Comparing the 4D diagnosis with the clinical diagnosis for the 3 observers yielded κ values of 0.906, 0.912, and 0.906. The κ values for agreement among the 3 observers for the type of abnormality were 0.949, 0.845, and 0.895. There was complete agreement on the presence of an abnormality between the clinical and 4D-DSA in 23/26 cases. In 2 cases, there were conflicting opinions.

CONCLUSIONS

In this study, the information content of 4D-DSA reconstructions was largely equivalent to that of the combined 2D/3D studies. The availability of 4D-DSA should reduce the requirement for 2D-DSA acquisitions.

Enhanced vascular biocompatibility of decellularized xeno-/allogeneic matrices in a rodent model

Glutaraldehyde preservation is the gold standard for cardiovascular biological prosthesis. However, secondary calcifications and the absence of tissue growth remain major limitations. Our study assessed in vitro and in vivo the biocompatibility of human (fascia lata, pericardium) and porcine tissues (pericardium, peritoneum) treated with a physicochemical procedure for decellularization and non-conventional pathogens inactivation. Biopsies were performed before and after treatment to assess decellularization (HE/Dapi staining/DNA quantification/MHC I/alpha gal immunostaining) and mechanical integrity. Forty-five rats received an abdominal aortic patch of native cryopreserved tissues (n = 20), treated tissues (n = 20) or glutaraldehyde-preserved bovine pericardium (GBP, control, n = 5). Grafts were explanted at 4 weeks and processed for HE/von Kossa staining and immunohistochemistries for lymphocytes (CD3)/macrophages (CD68) histomorphometry. 95% of decellularization was obtained for all tissues except for fascia lata (75%). Mechanical properties were slightly altered. In the in vivo model, a significant increase of CD3 and CD68 infiltrations was found in native and control implants in comparison with decellularized tissues (p < 0.05). Calcifications were found in 3 controls. Decellularized tissues were recolonized. GBP showed the most inflammatory response. This physicochemical treatment improves the biocompatibility of selected xeno/allogeneic tissues in comparison with their respective native cryopreserved tissues and with GBP. Incomplete decellularization is associated with a significantly higher inflammatory response. Our treatment is a promising tool in the field of tissue decellularization and tissue banking.

Inhibition decorrelates visual feature representations in the inner retina

The retina extracts visual features for transmission to the brain. Different types of bipolar cell split the photoreceptor input into parallel channels and provide the excitatory drive for downstream visual circuits. Anatomically and genetically, mouse bipolar cell types have been described at great detail, but a similarly deep understanding of their functional diversity is lacking. By imaging light-driven glutamate release from more than 13,000 bipolar cell axon terminals in the intact retina, we here show that bipolar cell functional diversity is generated by the interplay of dendritic excitatory inputs and axonal inhibitory inputs. The resultant centre and surround components of bipolar cell receptive fields interact to decorrelate bipolar cell output in the spatial and temporal domain. Our findings highlight the importance of inhibitory circuits in generating functionally diverse excitatory pathways and suggest that decorrelation of parallel visual pathways begins already at the second synapse of the mouse visual system.

Connectivity map of bipolar cells and photoreceptors in the mouse retina

In the mouse retina, three different types of photoreceptors provide input to 14 bipolar cell (BC) types. Classically, most BC types are thought to contact all cones within their dendritic field; ON-BCs would contact cones exclusively via so-called invaginating synapses, while OFF-BCs would form basal synapses. By mining publically available electron microscopy data, we discovered interesting violations of these rules of outer retinal connectivity: ON-BC type X contacted only ~20% of the cones in its dendritic field and made mostly atypical non-invaginating contacts. Types 5T, 5O and 8 also contacted fewer cones than expected. In addition, we found that rod BCs received input from cones, providing anatomical evidence that rod and cone pathways are interconnected in both directions. This suggests that the organization of the outer plexiform layer is more complex than classically thought.

Calcium dynamics change in degenerating cone photoreceptors

Cone photoreceptors (cones) are essential for high-resolution daylight vision and colour perception. Loss of cones in hereditary retinal diseases has a dramatic impact on human vision. The mechanisms underlying cone death are poorly understood, and consequently, there are no treatments available. Previous studies suggest a central role for calcium (Ca²⁺) homeostasis deficits in photoreceptor degeneration; however, direct evidence for this is scarce and physiological measurements of Ca²⁺in degenerating mammalian cones are lacking.Here, we took advantage of the transgenic HR2.1:TN-XL mouse line that expresses a genetically encoded Ca²⁺biosensor exclusively in cones. We cross-bred this line with mouse models for primary ("cone photoreceptor function loss-1", cpfl1) and secondary ("retinal degeneration-1", rd1) cone degeneration, respectively, and assessed resting Ca²⁺levels and light-evoked Ca²⁺responses in cones using two-photon imaging. We found that Ca²⁺dynamics were altered in cpfl1 cones, showing higher noise and variable Ca²⁺levels, with significantly wider distribution than for wild-type and rd1 cones. Unexpectedly, up to 21% of cpfl1 cones still displayed light-evoked Ca²⁺responses, which were larger and slower than wild-type responses. In contrast, genetically intact rd1 cones were characterized by lower noise and complete lack of visual function.Our study demonstrates alterations in cone Ca²⁺dynamics in both primary and secondary cone degeneration. Our results are consistent with the view that higher (fluctuating) cone Ca²⁺levels are involved in photoreceptor cell death in primary (cpfl1) but not in secondary (rd1) cone degeneration. These findings may guide the future development of therapies targeting photoreceptor Ca²⁺homeostasis.

Controlled synthesis of PVP-based carbon-supported Ru nanoparticles: synthesis approaches, characterization, capping agent removal and catalytic behavior

PVP-capped Ru nanoparticles were synthesized, immobilized on several carbon supports and tested in galactose hydrogenation.

Multiple Independent Oscillatory Networks in the Degenerating Retina

During neuronal degenerative diseases, microcircuits undergo severe structural alterations, leading to remodeling of synaptic connectivity. This can be particularly well observed in the retina, where photoreceptor degeneration triggers rewiring of connections in the retina’s first synaptic layer (e.g., Strettoi et al., 2003; Haq et al., 2014), while the synaptic organization of inner retinal circuits appears to be little affected (O’Brien et al., 2014; Figures 1A,B). Remodeling of (outer) retinal circuits and diminishing light-driven activity due to the loss of functional photoreceptors lead to spontaneous activity that can be observed at different retinal levels (Figure 1C), including the retinal ganglion cells, which display rhythmic spiking activity in the degenerative retina (Margolis et al., 2008; Stasheff, 2008; Menzler and Zeck, 2011; Stasheff et al., 2011). Two networks have been suggested to drive the oscillatory activity in the degenerating retina: a network of remnant cone photoreceptors, rod bipolar cells (RBCs) and horizontal cells in the outer retina (Haq et al., 2014), and the AII amacrine cell-cone bipolar cell network in the inner retina (Borowska et al., 2011). Notably, spontaneous rhythmic activity in the inner retinal network can be triggered in the absence of synaptic remodeling in the outer retina, for example, in the healthy retina after photo-bleaching (Menzler et al., 2014). In addition, the two networks show remarkable differences in their dominant oscillation frequency range as well as in the types and numbers of involved cells (Menzler and Zeck, 2011; Haq et al., 2014). Taken together this suggests that the two networks are self-sustained and can be active independently from each other. However, it is not known if and how they modulate each other. In this mini review, we will discuss: (i) commonalities and differences between these two oscillatory networks as well as possible interaction pathways; (ii) how multiple self-sustained networks may hamper visual restoration strategies employing, for example, microelectronic implants, optogenetics or stem cells, and briefly; and (iii) how the finding of diverse (independent) networks in the degenerative retina may relate to other parts of the neurodegenerative central nervous system.

Restoration of synaptic function in sight for degenerative retinal disease

Synaptic disorganization is a prominent feature of many neurological diseases of the CNS, including Parkinson's disease, intellectual development disorders, and autism. Although synaptic plasticity is critical for learning and memory, it is unclear whether this innate property helps restore synaptic function in disease once the primary cause of disease is abrogated. An answer to this question may come from a recent investigation in X-linked retinoschisis, a currently untreatable retinopathy. In this issue of the JCI, Ou, Vijayasarathy, and colleagues showed progressive disorganization of key functional elements of the synapse between photoreceptors and ON-bipolar cells in a retinoschisin-deficient mouse model. Moreover, they demonstrated that adeno-associated virus-mediated (AAV-mediated) delivery of the retinoschisin gene restores structure and function to the photoreceptor to ON-bipolar cell synapse in mouse models, even in adults at advanced stages of the disease. The results of this study hold promise that AAV-based supplemental gene therapy will benefit patients with X-linked retinoschisis in a forthcoming clinical trial.

Impact of perioperative chemotherapy on survival in patients with advanced primary urethral cancer: results of the international collaboration on primary urethral carcinoma

BACKGROUND

To investigate the impact of perioperative chemo(radio)therapy in advanced primary urethral carcinoma (PUC).

PATIENTS AND METHODS

A series of 124 patients (86 men, 38 women) were diagnosed with and underwent surgery for PUC in 10 referral centers between 1993 and 2012. Kaplan-Meier analysis with log-rank testing was used to investigate the impact of perioperative chemo(radio)therapy on overall survival (OS). The median follow-up was 21 months (mean: 32 months; interquartile range: 5-48).

RESULTS

Neoadjuvant chemotherapy (NAC), neoadjuvant chemoradiotherapy (N-CRT) plus adjuvant chemotherapy (ACH), and ACH was delivered in 12 (31%), 6 (15%) and 21 (54%) of these patients, respectively. Receipt of NAC/N-CRT was associated with clinically node-positive disease (cN+; P = 0.033) and lower utilization of cystectomy at surgery (P = 0.015). The objective response rate to NAC and N-CRT was 25% and 33%, respectively. The 3-year OS for patients with objective response to neoadjuvant treatment (complete/partial response) was 100% and 58.3% for those with stable or progressive disease (P = 0.30). Of the 26 patients staged ≥cT3 and/or cN+ disease, 16 (62%) received perioperative chemo(radio)therapy and 10 upfront surgery without perioperative chemotherapy (38%). The 3-year OS for this locally advanced subset of patients (≥cT3 and/or cN+) who received NAC (N = 5), N-CRT (N = 3), surgery-only (N = 10) and surgery plus ACH (N = 8) was 100%, 100%, 50% and 20%, respectively (P = 0.016). Among these 26 patients, receipt of neoadjuvant treatment was significantly associated with improved 3-year relapse-free survival (RFS) (P = 0.022) and OS (P = 0.022). Proximal tumor location correlated with inferior 3-year RFS and OS (P = 0.056/0.005).

CONCLUSION

In this series, patients who received NAC/N-CRT for cT3 and/or cN+ PUC appeared to demonstrate improved survival compared with those who underwent upfront surgery with or without ACH.

Imaging Ca2+ dynamics in cone photoreceptor axon terminals of the mouse retina

Retinal cone photoreceptors (cones) serve daylight vision and are the basis of color discrimination. They are subject to degeneration, often leading to blindness in many retinal diseases. Calcium (Ca²⁺), a key second messenger in photoreceptor signaling and metabolism, has been proposed to be indirectly linked with photoreceptor degeneration in various animal models. Systematically studying these aspects of cone physiology and pathophysiology has been hampered by the difficulties of electrically recording from these small cells, in particular in the mouse where the retina is dominated by rod photoreceptors. To circumvent this issue, we established a two-photon Ca²⁺ imaging protocol using a transgenic mouse line that expresses the genetically encoded Ca²⁺ biosensor TN-XL exclusively in cones and can be crossbred with mouse models for photoreceptor degeneration. The protocol described here involves preparing vertical sections (“slices”) of retinas from mice and optical imaging of light stimulus-evoked changes in cone Ca²⁺ level. The protocol also allows “in-slice measurement” of absolute Ca²⁺ concentrations; as the recordings can be followed by calibration. This protocol enables studies into functional cone properties and is expected to contribute to the understanding of cone Ca²⁺ signaling as well as the potential involvement of Ca²⁺ in photoreceptor death and retinal degeneration.

Morphology and connectivity of the small bistratified A8 amacrine cell in the mouse retina

Amacrine cells comprise ∼ 30 morphological types in the mammalian retina. The synaptic connectivity and function of a few γ-aminobutyric acid (GABA)ergic wide-field amacrine cells have recently been studied; however, with the exception of the rod pathway-specific AII amacrine cell, the connectivity of glycinergic small-field amacrine cells has not been investigated in the mouse retina. Here, we studied the morphology and connectivity pattern of the small-field A8 amacrine cell. A8 cells in mouse retina are bistratified with lobular processes in the ON sublamina and arboreal dendrites in the OFF sublamina of the inner plexiform layer. The distinct bistratified morphology was first visible at postnatal day 8, reaching the adult shape at P13, around eye opening. The connectivity of A8 cells to bipolar cells and ganglion cells was studied by double and triple immunolabeling experiments by using various cell markers combined with synaptic markers. Our data suggest that A8 amacrine cells receive glutamatergic input from both OFF and ON cone bipolar cells. Furthermore, A8 cells are coupled to ON cone bipolar cells by gap junctions, and provide inhibitory input via glycine receptor (GlyR) subunit α1 to OFF cone bipolar cells and to ON A-type ganglion cells. Measurements of spontaneous glycinergic postsynaptic currents and GlyR immunolabeling revealed that A8 cells express GlyRs containing the α2 subunit. The results show that the bistratified A8 cell makes very similar synaptic contacts with cone bipolar cells as the rod pathway-specific AII amacrine cell. However, unlike AII cells, A8 amacrine cells provide glycinergic input to ON A-type ganglion cells.

Attenuation of blood flow pulsatility along the Atlas slope: a physiologic property of the distal vertebral artery?

BACKGROUND AND PURPOSE

Physiologic and pathologic arterial tortuosity may attenuate blood flow pulsatility. The aim of this prospective study was to assess a potential effect of the curved V3 segment (Atlas slope) of the vertebral artery on arterial flow pulsatility. The pulsatility index and resistance index were used to assess blood flow pulsatility.

MATERIALS AND METHODS

Twenty-one healthy volunteers (17 men, 4 women; mean age, 32 years) were examined with a 3T MR imaging system. Blood velocities were measured at 2 locations below (I and II) and at 1 location above the V3 segment (III) of the vertebral artery by using a high-resolution 2D-phase-contrast sequence with multidirectional velocity-encoding.

RESULTS

Pulsatility and resistance indices decreased along all measurement locations from proximal to distal. The pulsatility index decreased significantly from location II to III and from I to II. However, the decrease was more pronounced along the Atlas slope than in the straight-vessel section below. The decrease of the resistance index was highly significant along the Atlas slope (location II to III). The decrease from location I to II was small and not significant.

CONCLUSIONS

The pronounced decrease in pulsatility and resistance indices along the interindividually uniformly bent V3 segment compared with a straight segment of the vertebral artery indicates a physiologic attenuating effect of the Atlas slope on arterial flow pulsatility. A similar effect has been described for the carotid siphon. A physiologic reduction of pulsatility in brain-supplying arteries would be in accordance with several recent publications reporting a correlation of increased arterial flow pulsatility with leukoencephalopathy and lacunar stroke.

First Report of Rose rosette virus Associated with Rose Rosette Disease Infecting Knockout Roses in Florida

Roses are one of the most popular flowering shrubs in the United States, with a total wholesale value of US$194 million. Among the major states, Florida is the fourth largest producer of roses with a total value exceeding US$20 million (4). In Florida, the roses have become especially popular in recent years with the introduction of Knock Out and other shrub roses. Virus-like symptoms including witches'-broom, excessive thorns, abnormal red discoloration of shoots and foliages, distorted leaves, and deformed buds and flowers were initially observed on Knock Out roses in a commercial nursery in Quincy, FL, in November 2013. Fifteen plants out of ~250,000 plants showed these characteristic symptoms. Total RNA extracts (RNeasy Plant Mini Kit, Qiagen, Valencia, CA) from eight symptomatic and two non-symptomatic rose samples were subjected to reverse-transcription (RT) assays using SuperScript III Reverse transcriptase (Invitrogen, Life Technologies, NY) and random hexamer primers. The cDNA synthesized was then subjected to PCR assay using Platinum Taq DNA polymerase (Invitrogen, Life Technologies) and using Rose rosette virus (RRV) specific primers RRV-F and RRV-R (1), targeting the core region of the RNA1 genome of the virus. The RT-PCR assays using the specific primers produced amplicons of 375 bp, only in the symptomatic leaf samples. The obtained amplicons were PCR purified and sequenced directly (GenBank Accession Nos. KF990370 to KF990377). BLAST analysis of these sequences revealed a higher identity of 99% with the RRV (HQ871942) in the NCBI database. Pairwise comparison of the eight RRV sequences exhibited 99 to 100% identity among themselves. These results revealed the association of RRV with the symptomatic rose plants. Eight symptomatic and two non-symptomatic rose plant samples were tested for RRV using blot hybridization assay, utilizing a digoxigenin-labeled DNA probe of 511 bp, targeting the RNA1 genome of the RRV. All eight symptomatic rose plants showed a positive reaction to the RRV-specific probes, confirming the presence of RRV in the samples, while the non-symptomatic and the buffer control did not produce any reactions. Even though the virus is reported to spread by an eriophyid mite Phyllocoptes fructiphilus, thorough examination of the infected samples showed absence of the vector. The samples were also tested using RT-PCR for the presence of Rose cryptic virus (RCV) and Blackberry chlorotic ringspot virus (BCRV) using specific primers (2,3). The samples tested negative for the RCV and BCRV. This is the first report of occurrence of RRV on rose in Florida. Considering the economic importance of the rose plants and the highly destructive nature of RRV, this report underscores the need for immediate effective quarantine and management of the virus for protecting the economically important rose industry in Florida. References: (1) A. G. Laney et al. J. Gen. Virol. 92:1727, 2011. (2) S. Sabanadzovic and N. Abou Ghanem-Sabanadzovic. J. Plant Pathol. 90:287, 2008. (3) I. E. Tzanetakis et al. Plant Pathol. 55:568, 2006. (4) USDA. 2007 Census of Agriculture 3:25, Washington, DC, 2010.

Synaptic remodeling generates synchronous oscillations in the degenerated outer mouse retina

During neuronal degenerative diseases, neuronal microcircuits undergo severe structural alterations, leading to remodeling of synaptic connectivity. The functional consequences of such remodeling are mostly unknown. For instance, in mutant rd1 mouse retina, a common model for Retinitis Pigmentosa, rod bipolar cells (RBCs) establish contacts with remnant cone photoreceptors (cones) as a consequence of rod photoreceptor cell death and the resulting lack of presynaptic input. To assess the functional connectivity in the remodeled, light-insensitive outer rd1 retina, we recorded spontaneous population activity in retinal wholemounts using Ca(2+) imaging and identified the participating cell types. Focusing on cones, RBCs and horizontal cells (HCs), we found that these cell types display spontaneous oscillatory activity and form synchronously active clusters. Overall activity was modulated by GABAergic inhibition from interneurons such as HCs and/or possibly interplexiform cells. Many of the activity clusters comprised both cones and RBCs. Opposite to what is expected from the intact (wild-type) cone-ON bipolar cell pathway, cone and RBC activity was positively correlated and, at least partially, mediated by glutamate transporters expressed on RBCs. Deletion of gap junctional coupling between cones reduced the number of clusters, indicating that electrical cone coupling plays a crucial role for generating the observed synchronized oscillations. In conclusion, degeneration-induced synaptic remodeling of the rd1 retina results in a complex self-sustained outer retinal oscillatory network, that complements (and potentially modulates) the recently described inner retinal oscillatory network consisting of amacrine, bipolar and ganglion cells.

Expectation mismatch: differences between self-generated and cue-induced expectations

Expectation of upcoming stimuli and tasks can lead to improved performance, if the anticipated situation occurs, while expectation mismatch can lead to less efficient processing. Researchers have used methodological approaches that rely on either self-generated expectations (predictions) or cue-induced expectations to investigate expectation mismatch effects. Differentiating these two types of expectations for different contents of expectation such as stimuli, responses, task sets and conflict level, we review evidence suggesting that self-generated expectations lead to larger facilitating effects and conflict effects on the behavioral and neural level - as compared to cue-based expectations. On a methodological level, we suggest that self-generated as compared to cue-induced expectations allow for a higher amount of experimental control in many experimental designs on expectation effects. On a theoretical level, we argue for qualitative differences in how cues vs. self-generated expectations influence performance. While self-generated expectations might generally involve representing the expected event in the focus of attention in working memory, cues might only lead to such representations under supportive circumstances (i.e., cue of high validity and attended).

Levetiracetam rectal administration in healthy dogs

BACKGROUND

Levetiracetam is used to manage status epilepticus (SE) and cluster seizures (CS) in humans. The drug might be absorbed after rectal administration and could offer a practical adjunct to rectal administration of diazepam in managing SE and CS.

HYPOTHESIS

Levetiracetam is rapidly absorbed after rectal administration in dogs and maintains target serum concentrations for at least 9 hours.

ANIMALS

Six healthy privately owned dogs between 2 and 6 years of age and weighing 10-20 kg.

METHODS

Levetiracetam (40 mg/kg) was administered rectally and blood samples were obtained immediately before (time zero) and at 10, 20, 40, 60, 90, 180, 360, and 540 minutes after drug administration. Dogs were observed for signs of adverse effects over a 24-hour period after drug administration.

RESULTS

CLEV at 10 minutes was 15.3 ± 5.5 μg/mL (mean, SD) with concentrations in the target range (5-40 μg/mL) for all dogs throughout the sampling period. Cmax (36.0 ± 10.7 μg/mL) and Tmax (103 ± 31 minutes) values were calculated and 2 disparate groups were appreciated. Dogs with feces in the rectum at the time of drug administration had lower mean Cmax values (26.7 ± 3.4 μg/mL) compared with those without (45.2 ± 4.4 μg/mL). Mild sedation was observed between 60 and 90 minutes without other adverse effects noted.

CONCLUSIONS AND CLINICAL IMPORTANCE

This study supports the use of rectally administered levetiracetam in future studies of clinical effectiveness in the management of epileptic dogs.

The neural correlates of perceptual load induced attentional selection: an fMRI study

The neural correlates of perceptual load induced attentional selection were investigated in an functional magnetic resonance imaging (fMRI) experiment in which attentional selection was manipulated through the variation of perceptual load in target search. Participants searched for a vertically or horizontally oriented bar among heterogeneously (the high load condition) or homogeneously (the low load condition) oriented distractor bars in the central display, which was flanked by a vertical or horizontal bar presented at the left or the right periphery. The search reaction times were longer when the central display was of high load than of low load, and were longer when the flanker was incongruent than congruent with the target. Importantly, the flanker congruency effect was manifested only in the low load condition, not in the high load condition, indicating that the perceptual load in target search determined whether the task-irrelevant flanker was processed. Imaging analyses revealed a set of fronto-parietal regions having higher activations in the high than in the low load condition. Anterior cingulate cortex (ACC) was more activated for the incongruent than for the congruent trials. Moreover, ACC and bilateral anterior insula were sensitive to the interaction between perceptual load and flanker congruency such that the activation differences between the incongruent and congruent conditions were significant in the low, but not in the high load condition. These results are consistent with the claim that ACC and bilateral anterior insula may exert executive control by selectively biasing processing in favor of task-relevant information and this biasing depends on the resources currently available to the control system.

Developmental regulation and activity-dependent maintenance of GABAergic presynaptic inhibition onto rod bipolar cell axonal terminals

Presynaptic inhibition onto axons regulates neuronal output, but how such inhibitory synapses develop and are maintained in vivo remains unclear. Axon terminals of glutamatergic retinal rod bipolar cells (RBCs) receive GABAA and GABAC receptor-mediated synaptic inhibition. We found that perturbing GABAergic or glutamatergic neurotransmission does not prevent GABAergic synaptogenesis onto RBC axons. But, GABA release is necessary for maintaining axonal GABA receptors. This activity-dependent process is receptor subtype specific: GABAC receptors are maintained, whereas GABAA receptors containing α1, but not α3, subunits decrease over time in mice with deficient GABA synthesis. GABAA receptor distribution on RBC axons is unaffected in GABAC receptor knockout mice. Thus, GABAA and GABAC receptor maintenance are regulated separately. Although immature RBCs elevate their glutamate release when GABA synthesis is impaired, homeostatic mechanisms ensure that the RBC output operates within its normal range after eye opening, perhaps to regain proper visual processing within the scotopic pathway.

OFF bipolar cells express distinct types of dendritic glutamate receptors in the mouse retina

Parallel representations of the visual world are already established at the very first synapse of the visual system. Cone photoreceptors, which hyperpolarize in response to light, forward the visual signal onto distinct types of ON and OFF cone bipolar cells (BCs). In the case of OFF BCs, the glutamatergic cone input is integrated by ionotropic glutamate receptors, giving rise to a sign-preserving mode of synaptic transmission. The combination of glutamate receptor (GluR) subunits, i.e. AMPA or kainate subunits, importantly contributes to shaping the OFF bipolar cells' distinct response properties. The mouse is one of the few mammals in which the (most likely) complete set of (five) retinal OFF BC types is identified. However, it is not clear which GluR subtypes are expressed by the different mouse OFF BC types. We addressed this question by combining immunolabeling, electrical whole-cell recordings and pharmacology, and present evidence that the different types of OFF BCs express distinct types of glutamate receptors: Type 1 BCs exclusively expressed AMPA receptors, whereas type 2 and type 3a BCs expressed kainate receptors of different subunit compositions. Additionally, we found that two OFF BC types (3b and 4) very likely express both AMPA and kainate receptors but, interestingly, the two receptor subunits were not co-localized at the same dendritic site. The complex, BC type-specific expression pattern of GluRs we describe here supports their essential role in establishing parallel pathways at the first synapse of the mouse visual system.