Skin-type-dependent development of murine mechanosensory neurons.

Mechanosensory neurons innervating the skin underlie our sense of touch. Fast-conducting, rapidly adapting mechanoreceptors innervating glabrous (non-hairy) skin form Meissner corpuscles, while in hairy skin, they associate with hair follicles, forming longitudinal lanceolate endings. How mechanoreceptors develop axonal endings appropriate for their skin targets is unknown. We report that mechanoreceptor morphologies across different skin regions are indistinguishable during early development but diverge post-natally, in parallel with skin maturation. Neurons terminating along the glabrous and hairy skin border exhibit hybrid morphologies, forming both Meissner corpuscles and lanceolate endings. Additionally, molecular profiles of neonatal glabrous and hairy skin-innervating neurons largely overlap. In mouse mutants with ectopic glabrous skin, mechanosensory neurons form end-organs appropriate for the altered skin type. Finally, BMP5 and BMP7 are enriched in glabrous skin, and signaling through type I bone morphogenetic protein (BMP) receptors in neurons is critical for Meissner corpuscle morphology. Thus, mechanoreceptor morphogenesis is flexibly instructed by target tissues.

Neural bases for the genesis and CO2 therapy of periodic Cheyne-Stokes breathing in neonatal male connexin-36 knockout mice.

Periodic Cheyne-Stokes breathing (CSB) oscillating between apnea and crescendo-decrescendo hyperpnea is the most common central apnea. Currently, there is no proven therapy for CSB, probably because the fundamental pathophysiological question of how the respiratory center generates this form of breathing instability is still unresolved. Therefore, we aimed to determine the respiratory motor pattern of CSB resulting from the interaction of inspiratory and expiratory oscillators and identify the neural mechanism responsible for breathing regularization induced by the supplemental CO2 administration. Analysis of the inspiratory and expiratory motor pattern in a transgenic mouse model lacking connexin-36 electrical synapses, the neonatal (P14) Cx36 knockout male mouse, with a persistent CSB, revealed that the reconfigurations recurrent between apnea and hyperpnea and vice versa result from cyclical turn on/off of active expiration driven by the expiratory oscillator, which acts as a master pacemaker of respiration and entrains the inspiratory oscillator to restore ventilation. The results also showed that the suppression of CSB by supplemental 12% CO2 in inhaled air is due to the stabilization of coupling between expiratory and inspiratory oscillators, which causes the regularization of respiration. CSB rebooted after washout of CO2 excess when the inspiratory activity depressed again profoundly, indicating that the disability of the inspiratory oscillator to sustain ventilation is the triggering factor of CSB. Under these circumstances, the expiratory oscillator activated by the cyclic increase of CO2 behaves as an "anti-apnea" center generating the crescendo-decrescendo hyperpnea and periodic breathing. The neurogenic mechanism of CSB identified highlights the plasticity of the two-oscillator system in the neural control of respiration and provides a rationale base for CO2 therapy.

Genetic elimination of rod/cone coupling reveals the contribution of the secondary rod pathway to the retinal output.

In the retina, signals originating from rod and cone photoreceptors can reach retinal ganglion cells (RGCs)-the output neurons-through different pathways. However, little is known about the exact sensitivities and operating ranges of these pathways. Previously, we created rod- or cone-specific Cx36 knockout (KO) mouse lines. Both lines are deficient in rod/cone electrical coupling and therefore provide a way to selectively remove the secondary rod pathway. We measured the threshold of the primary rod pathway in RGCs of wild-type mice. Under pharmacological blockade of the primary rod pathway, the threshold was elevated. This secondary component was removed in the Cx36 KOs to unmask the threshold of the third rod pathway, still below cone threshold. In turn, the cone threshold was estimated by several independent methods. Our work defines the functionality of the secondary rod pathway and describes an additive contribution of the different pathways to the retinal output.

Respiratory disturbances and high risk of sudden death in the neonatal connexin-36 knockout mouse.

Neural circuits at the brainstem involved in the central generation of the motor patterns of respiration and cardiorespiratory chemoreflexes organize as cell assemblies connected by chemical and electrical synapses. However, the role played by the electrical connectivity mainly mediated by connexin36 (Cx36), which expression reaches peak value during the postnatal period, is still unknown. To address this issue, we analyzed here the respiratory phenotype of a mouse strain devoid constitutively of Cx36 at P14. Male Cx36-knockout mice at rest showed respiratory instability of variable degree, including a periodic Cheyne-Stokes breathing. Moreover, mice lacking Cx36 exhibited exacerbated chemoreflexes to normoxic and hypoxic hypercapnia characterized by a stronger inspiratory/expiratory coupling due to an increased sensitivity to CO2 . Deletion of Cx36 also impaired the generation of the recurrent episodes of transient bradycardia (ETBs) evoked during hypercapnic chemoreflexes; these EBTs constituted a powerful mechanism of cardiorespiratory coupling capable of improving alveolar gaseous exchange under hypoxic hypercapnia conditions. Approximately half of the homo- and heterozygous Cx36KO, but none WT, mice succumbed by respiratory arrest when submitted to hypoxia-hypercapnia, the principal exogenous stressor causing sudden infant death syndrome (SIDS). The early suppression of EBTs, which worsened arterial O2  saturation, and the generation of a paroxysmal generalized clonic-tonic activity, which provoked the transition from eupneic to gasping respiration, were the critical events causing sudden death in the Cx36KO mice. These results indicate that Cx36 expression plays a pivotal role in respiratory control, cardiorespiratory coordination, and protection against SIDS at the postnatal period.

Molecular and functional architecture of the mouse photoreceptor network.

Mouse photoreceptors are electrically coupled via gap junctions, but the relative importance of rod/rod, cone/cone, or rod/cone coupling is unknown. Furthermore, while connexin36 (Cx36) is expressed by cones, the identity of the rod connexin has been controversial. We report that FACS-sorted rods and cones both express Cx36 but no other connexins. We created rod- and cone-specific Cx36 knockout mice to dissect the photoreceptor network. In the wild type, Cx36 plaques at rod/cone contacts accounted for more than 95% of photoreceptor labeling and paired recordings showed the transjunctional conductance between rods and cones was ~300 pS. When Cx36 was eliminated on one side of the gap junction, in either conditional knockout, Cx36 labeling and rod/cone coupling were almost abolished. We could not detect direct rod/rod coupling, and cone/cone coupling was minor. Rod/cone coupling is so prevalent that indirect rod/cone/rod coupling via the network may account for previous reports of rod coupling.

Primary hyperparathyroidism versus familial hypocalciuric hypercalcemia: a challenging diagnostic evaluation in an adolescent female.

Primary hyperparathyroidism (PHPT) and familial hypocalciuric hypercalcemia (FHH) have significantly different treatment approaches, so physicians must be careful to differentiate these 2 diseases. Herein, we report a 14-year-old female who presented with symptomatic hypercalcemia (12 mg/dL; reference range, 9.2-10.7 mg/dL), elevated intact parathyroid hormone (iPTH) (236 pg/mL; reference range, 9-69 pg/mL), and vitamin D deficiency (6 ng/mL; reference range, ≥ 20 ng/mL). On numerous occasions, her 24-hour urine calcium was more than 4 mg/kg/day, consistent with PHPT, but her fractional excretion of calcium on 24-hour urine collection was consistently below 1%, in line with FHH. 99mTc-Sestamibi scan failed to detect any abnormalities. However, a 4-dimensional computed tomography scan of the neck revealed a right superior parathyroid adenoma which was excised with a focused parathyroidectomy. Although the patient's calcium and iPTH levels normalized, her nonspecific symptoms persisted. This case illustrates both the challenges of differentiating PHPT from FHH and the limitations of a first-line imaging tool in identifying a parathyroid adenoma.

Multiplexed peroxidase-based electron microscopy labeling enables simultaneous visualization of multiple cell types.

Electron microscopy (EM) is a powerful tool for circuit mapping, but identifying specific cell types in EM datasets remains a major challenge. Here we describe a technique enabling simultaneous visualization of multiple genetically identified neuronal populations so that synaptic interactions between them can be unequivocally defined. We present 15 adeno-associated virus constructs and 6 mouse reporter lines for multiplexed EM labeling in the mammalian nervous system. These reporters feature dAPEX2, which exhibits dramatically improved signal compared with previously described ascorbate peroxidases. By targeting this enhanced peroxidase to different subcellular compartments, multiple orthogonal reporters can be simultaneously visualized and distinguished under EM using a protocol compatible with existing EM pipelines. Proof-of-principle double and triple EM labeling experiments demonstrated synaptic connections between primary afferents, descending cortical inputs, and inhibitory interneurons in the spinal cord dorsal horn. Our multiplexed peroxidase-based EM labeling system should therefore greatly facilitate analysis of connectivity in the nervous system.

Gap Junctions Contribute to Differential Light Adaptation across Direction-Selective Retinal Ganglion Cells.

Direction-selective ganglion cells (DSGCs) deliver signals from the retina to multiple brain areas to indicate the presence and direction of motion. Delivering reliable signals in response to motion is critical across light levels. Here we determine how populations of DSGCs adapt to changes in light level, from moonlight to daylight. Using large-scale measurements of neural activity, we demonstrate that the population of DSGCs switches encoding strategies across light levels. Specifically, the direction tuning of superior (upward)-preferring ON-OFF DSGCs becomes broader at low light levels, whereas other DSGCs exhibit stable tuning. Using a conditional knockout of gap junctions, we show that this differential adaptation among superior-preferring ON-OFF DSGCs is caused by connexin36-mediated electrical coupling and differences in effective GABAergic inhibition. Furthermore, this adaptation strategy is beneficial for balancing motion detection and direction estimation at the lower signal-to-noise ratio encountered at night. These results provide insights into how light adaptation impacts motion encoding in the retina.

Diencephalic syndrome: a rare cause of failure to thrive.

Timely diagnosis of diencephalic syndrome is not often the case for patients presenting with failure to thrive (FTT) because of its rarity and lack of specific symptoms. Herein, we report two cases of diencephalic syndrome (2-year-old girl and 10-month-old boy) presenting with severe emaciation. Both patients had histories of poor weight gain for months despite having good appetites prior to diagnosis. Initial work-up did not reveal the diagnosis. Horizontal nystagmus was noted in both patients: by a neurologist in the first patient and by a family member in the second patient. MRI of the brain showed large suprasellar mass and pilocytic astrocytoma was confirmed by pathology in each case. The patients were started on appropriate chemotherapy with interval improvements in weight gain. These cases illustrate the importance of cranial imaging and consideration of diencephalic syndrome for children presenting with FTT despite normal or increased caloric intake.

Influences on teleconsultation project utilization rates: the role of dominant logic.

BACKGROUND: This research analyzes teleconsultation from both a mechanistic and complex adaptive system (CAS) dominant logic in order to further understand the influence of dominant logic on utilization rates of teleconsultation projects. In both dominant logics, the objective of teleconsultation projects is to increase access to and quality of healthcare delivery in a cost efficient manner. A mechanistic dominant logic perceives teleconsultation as closely resembling the traditional service delivery model, while a CAS dominant logic focuses on the system's emergent behavior of learning resulting from the relationships and interactions of participating healthcare providers. METHODS: Qualitative case studies of 17 teleconsultation projects that were part of four health sciences center (HSC) based telemedicine networks was utilized. Data were collected at two points in time approximately 10 years apart. Semi-structured interviews of 85 key informants (clinicians, administrators, and IT professionals) involved in teleconsultation projects were the primary data collection method. RESULTS: The findings indicated that the emergent behavior of effective and sustainable teleconsultation projects differed significantly from what was anticipated in a mechanistic dominant logic. Teleconsultation projects whose emergent behavior focused on continuous learning enabled remote site generalists to manage and treat more complex cases and healthcare problems on their own without having to refer to HSC specialists for assistance. In teleconsultation projects that continued to be effectively utilized, participant roles evolved and were expanded. Further, technology requirements for teleconsultation projects whose emergent behavior was learning did not need to be terribly sophisticated. CONCLUSIONS: When a teleconsultation project is designed with a mechanistic dominant logic, it is less likely to be sustained, whereas a teleconsultation project designed with a CAS dominant logic is more likely to be sustained. Consistent with a CAS dominant logic, teleconsultation projects that continued to be utilized involved participants taking on new roles and continuously learning. This continuous learning enabled remote site generalists to better handle the constantly changing nature of the problems faced. A CAS dominant logic provides a theoretical framework which explains why the teleconsultation literature about the role of technology, which is based on a mechanistic dominate logic, does not have adequate explanatory power.

Facilitating telemedicine project sustainability in medically underserved areas: a healthcare provider participant perspective.

BACKGROUND: Very few telemedicine projects in medically underserved areas have been sustained over time. This research furthers understanding of telemedicine service sustainability by examining teleconsultation projects from the perspective of healthcare providers. Drivers influencing healthcare providers' continued participation in teleconsultation projects and how projects can be designed to effectively and efficiently address these drivers is examined. METHODS: Case studies of fourteen teleconsultation projects that were part of two health sciences center (HSC) based telemedicine networks was utilized. Semi-structured interviews of 60 key informants (clinicians, administrators, and IT professionals) involved in teleconsultation projects were the primary data collection method. RESULTS: Two key drivers influenced providers' continued participation. First was severe time constraints. Second was remote site healthcare providers' (RSHCPs) sense of professional isolation. Two design steps to address these were identified. One involved implementing relatively simple technology and process solutions to make participation convenient. The more critical and difficult design step focused on designing teleconsultation projects for collaborative, active learning. This learning empowered participating RSHCPs by leveraging HSC specialists' expertise. CONCLUSIONS: In order to increase sustainability the fundamental purpose of teleconsultation projects needs to be re-conceptualized. Doing so requires HSC specialists and RSHCPs to assume new roles and highlights the importance of trust. By implementing these design steps, healthcare delivery in medically underserved areas can be positively impacted.

Segregated Foxc2, NFATc1 and Connexin expression at normal developing venous valves, and Connexin-specific differences in the valve phenotypes of Cx37, Cx43, and Cx47 knockout mice.

Venous valves (VVs) are critical for unidirectional blood flow from superficial and deep veins towards the heart. Congenital valve aplasia or agenesis may, in some cases, be a direct cause of vascular disease, motivating an understanding of the molecular mechanisms underlying the development and maintenance of VVs. Three gap junction proteins (Connexins), Cx37, Cx43, and Cx47, are specifically expressed at VVs in a highly polarized fashion. VVs are absent from adult mice lacking Cx37; however it is not known if Cx37 is required for the initial formation of valves. In addition, the requirement of Cx43 and Cx47 for VV development has not been studied. Here, we provide a detailed description of Cx37, Cx43, and Cx47 expression during mouse vein development and show by gene knockout that each Cx is necessary for normal valve development. The valve phenotypes in the knockout lines exhibit Cx-specific differences, however, including whether peripheral or central VVs are affected by gene inactivation. In addition, we show that a Cx47 null mutation impairs peripheral VV development but does not affect lymphatic valve formation, a finding of significance for understanding how some CX47 mutations cause inherited lymphedema in humans. Finally, we demonstrate a striking segregation of Foxc2 and NFATc1 transcription factor expression between the downstream and upstream faces, respectively, of developing VV leaflets and show that this segregation is closely associated with the highly polarized expression of Cx37, Cx43, and Cx47. The partition of Foxc2 and NFATc1 expression at VV leaflets makes it unlikely that these factors directly cooperate during the leaflet elongation stage of VV development.

Current issues in patient safety in surgery: a review.

Current surgical safety guidelines and checklists are generic and are not specifically tailored to address patient issues and risk factors in surgical subspecialties. Patient safety in surgical subspecialties should be templated on general patient safety guidelines from other areas of medicine and mental health but include and develop specific processes dedicated for the care of the surgical patients. Safety redundant systems must be in place to decrease errors in surgery. Therefore, different surgical subspecialties should develop a specific curriculum in patient safety addressing training in academic centers and application of these guidelines in all practices. Clearly, redundant safety systems must be in place to decrease errors in surgery, in analogy to safety measures in other high-risk industries. Specific surgical subspecialties are encouraged to develop a specific patient safety curriculum that address training in academic centers and applicability to daily practice, with the goal of keeping our surgical patients safe in all disciplines. The present review article is designed to outline patient safety practices that should be adapted and followed to fit particular specialties.

Gap junction-mediated death of retinal neurons is connexin and insult specific: a potential target for neuroprotection.

Secondary cell death via gap junctions (GJs) plays a role in the propagation of neuronal loss under a number of degenerative disorders. Here, we examined the role of GJs in neuronal death in the retina, which has arguably the most diverse expression of GJs in the CNS. Initially, we induced apoptotic death by injecting single retinal ganglion cells and glia with cytochrome C and found that this resulted in the loss of neighboring cells to which they were coupled via GJs. We next found that pharmacological blockade of GJs eradicated nearly all amacrine cell loss and reduced retinal ganglion cell loss by ∼70% after induction of either excitotoxic or ischemic insult conditions. These data indicate that the GJ-mediated secondary cell death was responsible for the death of most cells. Whereas genetic deletion of the GJ subunit Cx36 increased cell survivability by ∼50% under excitotoxic condition, cell loss in Cx45 knock-out mouse retinas was similar to that seen in wild-type mice. In contrast, ablation of Cx45 reduced neuronal loss by ∼50% under ischemic insult, but ablation of Cx36 offered no protection. Immunolabeling of the connexins showed differential changes in protein expression consistent with their differing roles in propagating death signals under the two insults. These data indicate that secondary cell death is mediated by different cohorts of GJs dependent on the connexins they express and the type of initial insult. Our results suggest that targeting specific connexins offers a novel therapeutic strategy to reduce progressive cell loss under different neurodegenerative conditions.

Inhibition of connexin 36 hemichannels by glucose contributes to the stimulation of insulin secretion.

The existence of functional connexin36 (Cx36) hemichannels in ß-cells was investigated in pancreatic islets of rat and wild-type (Cx36(+/+)), monoallelic (Cx36(+/-)), and biallelic (Cx36(-/-)) knockout mice. Hemichannel opening by KCl depolarization was studied by measuring ATP release and changes of intracellular ATP (ADP). Cx36(+/+) islets lost ATP after depolarization with 70 mM KCl at 5 mM glucose; ATP loss was prevented by 8 and 20 mM glucose or 50 µM mefloquine (connexin inhibitor). ATP content was higher in Cx36(-/-) than Cx36(+/+) islets and was not decreased by KCl depolarization; Cx36(+/-) islets showed values between that of control and homozygous islets. Five minimolar extracellular ATP increased ATP content and ATP/ADP ratio and induced a biphasic insulin secretion in depolarized Cx36(+/+) and Cx36(+/-) but not Cx36(-/-) islets. Cx36 hemichannels expressed in oocytes opened upon depolarization of membrane potential, and their activation was inhibited by mefloquine and glucose (IC50 ∼8 mM). It is postulated that glucose-induced inhibition of Cx36 hemichannels in islet ß-cells might avoid depolarization-induced ATP loss, allowing an optimum increase of the ATP/ADP ratio by sugar metabolism and a biphasic stimulation of insulin secretion. Gradual suppression of glucose-induced insulin release in Cx36(+/-) and Cx36(-/-) islets confirms that Cx36 gap junction channels are necessary for a full secretory stimulation and might account for the glucose intolerance observed in mice with defective Cx36 expression. Mefloquine targeting of Cx36 on both gap junctions and hemichannels also suppresses glucose-stimulated secretion. By contrast, glucose stimulation of insulin secretion requires Cx36 hemichannels' closure but keeping gap junction channels opened.

Thyroid disease in children: part 1: State-of-the-art imaging in pediatric hypothyroidism.

Hypothyroidism, defined as inadequate production of thyroid hormone, can be secondary to various underlying abnormalities in the pediatric population. Most frequently, hypothyroidism is related to structural abnormalities of the gland (dysgenesis), particularly in the neonatal population. However, other etiologies including intrinsic biochemical (dyshormonogenesis) and autoimmune abnormalities, as well as other rare causes, must be considered. Imaging is required to differentiate among the various etiologies of hypothyroidism and can be helpful in guiding therapy. This review aims to present an organized approach to hypothyroidism in the pediatric population, and assist the imager in guiding patient care.

A novel, highly sensitive method for assessing gap junctional coupling.

To assess gap junctional intercellular communication we have developed a tracer-based methodology which is both highly sensitive and potentially adaptable for in vivo measurements. We found that injection of serotonin revealed significantly more intercellular communication than that injection of the most permeant synthetic tracer currently in use, neurobiotin. Furthermore, mechanical tracer loading steps can be replaced by transfection with human serotonin transporter and the inclusion of serotonin in the medium. Tracer and transporter are detected using immunocytochemical techniques and the presence of cells that are tracer-positive but transporter-negative indicates junctional communication. Tracer loading in vivo using transgenesis, electroporation or viral transduction to direct expression of transporter should be more easily accomplished than with mechanical loading methods.

Gap junctions are essential for generating the correlated spike activity of neighboring retinal ganglion cells.

Neurons throughout the brain show spike activity that is temporally correlated to that expressed by their neighbors, yet the generating mechanism(s) remains unclear. In the retina, ganglion cells (GCs) show robust, concerted spiking that shapes the information transmitted to central targets. Here we report the synaptic circuits responsible for generating the different types of concerted spiking of GC neighbors in the mouse retina. The most precise concerted spiking was generated by reciprocal electrical coupling of GC neighbors via gap junctions, whereas indirect electrical coupling to a common cohort of amacrine cells generated the correlated activity with medium precision. In contrast, the correlated spiking with the lowest temporal precision was produced by shared synaptic inputs carrying photoreceptor noise. Overall, our results demonstrate that different synaptic circuits generate the discrete types of GC correlated activity. Moreover, our findings expand our understanding of the roles of gap junctions in the retina, showing that they are essential for generating all forms of concerted GC activity transmitted to central brain targets.

Survey on amacrine cells coupling to retrograde-identified ganglion cells in the mouse retina.

PURPOSE: Retinal amacrine cells (ACs) may make inhibitory chemical synapses and potentially excitatory gap junctions on ganglion cells (GCs). The total number and subtypes of ACs coupled to the entire GC population were investigated in wild-type and three lines of transgenic mice. METHODS: GCs and GC-coupled ACs were identified by the previously established LY-NB (Lucifer yellow-Neurobiotin) retrograde double-labeling technique, in conjunction with specific antibodies and confocal microscopy. RESULTS: GC-coupled ACs (NB-positive and LY-negative) comprised nearly 11% of displaced ACs and 4% of conventional ACs in wild-type mice, and were 9% and 4% of displaced ACs in Cx45(-/-) and Cx36/45(-/-) mice, respectively. Their somas were small in Cx36/45(-/-) mice, but variable in other strains. They were mostly γ-aminobutyric acid (GABA)-immunoreactive (IR) and located in the GC layer. They comprised only a small portion in the AC subpopulations, including GABA-IR, glycine-IR, calretinin-IR, 5-HT-accumulating, and ON-type choline acetyltransferase (ChAT) ACs in wild-type and ChAT transgenic mice (ChAT- tdTomato). In the distal 80% of the inner plexiform layer (IPL), dense GC dendrites coexisted with rich glycine-IR and GABA-IR. In the inner 20% of the IPL, sparse GC dendrites presented with a major GABA band and sparse glycine-IR. CONCLUSIONS: Various subtypes of ACs may couple to GCs. ACs of the same immunoreactivity may either couple or not couple to GCs. Cx36 and Cx45 dominate GC-AC coupling except for small ACs. The overall potency of GC-AC coupling is moderate, especially in the proximal 20% of the IPL, where inhibitory chemical signals are dominated by GABA ACs.

Rod, M-cone and M/S-cone inputs to hyperpolarizing bipolar cells in the mouse retina.

Bipolar cells are the central neurons of the retina that convey visual signals from rod and cone photoreceptors in the outer retina to higher-order neurons in the inner retina and the brain. Early anatomical studies have suggested that there are four types of cone hyperpolarizing (OFF) bipolar cells (HBCs) in the mouse retina, but no light responses have been systematically examined. By analysing light-evoked cation and chloride currents (I(C) and I(Cl)) from over 50 morphologically identified HBCs in the dark-adapted wildtype and connexin36 knockout (Cx36(-/-)) mouse retinas, we identified three types of HBCs, each with distinct light responses and morphological characteristics. The HBC(R/MC)s with axon terminals ramifying between 0% and 30% of the inner plexiform layer (IPL) receive mixed inputs from rods and M-cones, the HBC(MC)s with axon terminals ramifying between 10% and 50% of the IPL receive inputs primarily from M-cones, and the HBC(M/SC)s with axon terminals ramifying between 25% and 50% of IPL receive inputs primarily from cones with mixed M- and S-cone pigments. Moreover, we found that HBC(R/MC)s in the Cx36(-/-) mice exhibit light responses very similar to the wildtype HBC(R/MC)s, suggesting that the mixed rod-cone inputs are not mediated by connexin36-dependent rod-cone coupling, but rather by direct synaptic contacts from rods and M-cones. This study constitutes the first systematic investigation that correlates light response characteristics and axonal morphology of HBCs in dark-adapted mouse retina, and contributes to recently emerging evidence that revises the traditional view that mammalian HBCs only contact cone photoreceptors.