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The cerebral cortex is vital for the perception and processing of sensory stimuli. In the somatosensory axis, information is received by two distinct regions, the primary (S1) and secondary (S2) somatosensory cortices. Top-down circuits stemming from S1 can modulate mechanical and cooling but not heat stimuli such that circuit inhibition causes blunted mechanical and cooling perception. Using optogenetics and chemogenetics, we find that in contrast to S1, an inhibition of S2 output increases mechanical and heat, but not cooling sensitivity. Combining 2-photon anatomical reconstruction with chemogenetic inhibition of specific S2 circuits, we discover that S2 projections to the secondary motor cortex (M2) govern mechanical and thermal sensitivity without affecting motor or cognitive function. This suggests that while S2, like S1, encodes specific sensory information, that S2 operates through quite distinct neural substrates to modulate responsiveness to particular somatosensory stimuli and that somatosensory cortical encoding occurs in a largely parallel fashion.

Retinal ganglion cell (RGC) types relay parallel streams of visual feature information. We hypothesized that neuromodulators might efficiently control which visual information streams reach the cortex by selectively gating transmission from specific RGC axons in the thalamus. Using fiber photometry recordings, we found that optogenetic stimulation of serotonergic axons in primary visual thalamus of awake mice suppressed ongoing and visually evoked calcium activity and glutamate release from RGC boutons. Two-photon calcium imaging revealed that serotonin axon stimulation suppressed RGC boutons that responded strongly to global changes in luminance more than those responding only to local visual stimuli, while the converse was true for suppression induced by increases in arousal. Converging evidence suggests that differential expression of the 5-HT1B receptor on RGC presynaptic terminals, but not differential density of nearby serotonin axons, may contribute to the selective serotonergic gating of specific visual information streams before they can activate thalamocortical neurons.

Visual information is encoded in distinct retinal ganglion cell (RGC) types in the eye tuned to specific features of the visual space. These streams of information project to the visual thalamus, the first station of the image-forming pathway. In the mouse, this connection between RGCs and thalamocortical neurons, the retinogeniculate synapse, has become a powerful experimental model for understanding how circuits in the thalamus are constructed to process these incoming lines of information. Using modern molecular and genetic tools, recent studies have suggested a more complex circuit organization than was previously understood. In this review, we summarize the current understanding of the structural and functional organization of the retinogeniculate synapse in the mouse. We discuss a framework by which a seemingly complex circuit can effectively integrate and parse information to downstream stations of the visual pathway. Finally, we review how activity and visual experience can sculpt this exquisite connectivity.

PURPOSE. In humans, loss-of-function mutations in the gene encoding Chordin-like 1 (CHRDL1) cause X-linked megalocornea (MGC1), characterized by bilateral corneal enlargement, decreased cornea thickness, and increased anterior chamber depth (ACD). We sought to determine whether Chrdl1 knockout (KO) mice would recapitulate the ocular findings found in patients with MGC1. METHODS. We generated mice with a Chrdl1 knockout allele and confirmed that male Chrdl1 hemizygous KO mice do not express Chrdl1 mRNA. We examined the eyes of male mice that were hemizygous for either the wild-type (WT) or KO allele and measured corneal diameter, corneal area, corneal thickness, endothelial cell density, ACD, tear volume, and intraocular pressure. We also harvested retinas and counted retinal ganglion cell numbers. Eye segregation pattern in the dorsal lateral geniculate nucleus were also compared between male Chrdl1 KO and WT mice. RESULTS. Male Chrdl1 KO mice do not have larger cornea diameters than WT mice. KO mice have significantly thicker central corneas (116.5 ± 3.9 vs 100.9 ± 4.2 μm, p = 0.013) and smaller ACD (325.7 ± 5.7 vs 405.6 ± 6.3 μm, p < 0.001) than WT mice, which is the converse of what occurs in patients who lack CHRDL1. Retinal- thalamic projections and other ocular measurements did not significantly differ between KO and WT mice. CONCLUSIONS. Male Chrdl1 KO mice do not have the same anterior chamber abnormalities seen in humans with CHRDL1 mutations. Therefore, Chrdl1 KO mice do not recapitulate the human MGC1 phenotype. Nevertheless, Chrdl1 plays a role during mouse ocular development, since corneas in KO mice differ from those in WT mice.

In the mouse visual system, multiple types of retinal ganglion cells (RGCs) each encode distinct features of the visual space. A clear understanding of how this information is parsed in their downstream target, the dorsal lateral geniculate nucleus (dLGN), remains elusive. Here, we characterized retinogeniculate connectivity in Cart-IRES2-Cre-D and BD-CreER2 mice, which labels subsets of on-off direction-selective ganglion cells (ooDSGCs) tuned to the vertical directions and to only ventral motion, respectively. Our immunohistochemical, electrophysiological, and optogenetic experiments reveal that only a small fraction (15%) of thalamocortical (TC) neurons in the dLGN receives primary retinal drive from these subtypes of ooDSGCs. The majority of the functionally identifiable ooDSGC inputs in the dLGN are weak and converge together with inputs from other RGC types. Yet our modeling indicates that this mixing is not random: BD-CreER

SummaryHow sensory information is processed by the brain can depend on behavioral state. In the visual thalamus and cortex, arousal/locomotion is associated with changes in the magnitude of responses to visual stimuli. Here, we asked whether such modulation of visual responses might already occur at an earlier stage in this visual pathway. We measured neural activity of retinal axons using wide-field and two-photon calcium imaging in awake mouse thalamus across arousal states associated with different pupil sizes. Surprisingly, visual responses to drifting gratings in retinal axonal boutons were robustly modulated by arousal level, in a manner that varied across stimulus dimensions and across functionally distinct subsets of boutons. At low and intermediate spatial frequencies, the majority of boutons were suppressed by arousal. In contrast, at high spatial frequencies, the proportions of boutons showing enhancement or suppression were more similar, particularly for boutons tuned to regions of visual space ahead of the mouse. Arousal-related modulation also varied with a bouton’s sensitivity to luminance changes and direction of motion, with greater response suppression in boutons tuned to luminance decrements vs. increments, and in boutons preferring motion along directions or axes of optic flow. Together, our results suggest that differential filtering of distinct visual information channels by arousal state occurs at very early stages of visual processing, before the information is transmitted to neurons in visual thalamus. Such early filtering may provide an efficient means of optimizing central visual processing and perception of state-relevant visual stimuli.

Current models of somatosensory perception emphasize transmission from primary sensory neurons to the spinal cord and on to the brain1–4. Mental influence on perception is largely assumed to be acting locally within the brain. We have now examined if there is top-down control of sensory inflow through the spinal cord directly by the cortex. Although traditionally viewed as a primary motor pathway5, a subset of corticospinal neurons (CSNs) originating in the S1/S2 somatosensory cortex directly innervate the spinal dorsal horn via corticospinal tract (CST) axons. We show here that either reduction in somatosensory CSN activity or transection of the CST selectively impairs behavioral responses to light touch without altering responses to noxious stimuli. Moreover, such CSN manipulation greatly attenuates tactile allodynia in a peripheral neuropathic pain model. Tactile stimulation activates somatosensory CSNs and their corticospinal projections facilitate light touch-evoked activity of cholecystokinin (CCK+) interneurons in the deep dorsal horn. This represents a touch-driven feed forward spinal-cortical-spinal sensitization loop, which is important for the recruitment of spinal nociceptive neurons under tactile allodynia. These results reveal direct cortical modulation of normal and pathological tactile sensory processing in the spinal cord and open up opportunities for new treatments for neuropathic pain.

The retinogeniculate synapse transmits information from retinal ganglion cells (RGC) in the eye to thalamocortical relay neurons in the visual thalamus, the dorsal lateral geniculate nucleus (dLGN). Studies in mice have identified genetic markers for distinct classes of RGCs encoding different features of the visual space, facilitating the dissection of RGC subtype-specific physiology and anatomy. In this study, we examine the morphological properties of axon arbors of the BD-RGC class of ON-OFF direction selective cells that, by definition, exhibit a stereotypic dendritic arbor and termination pattern in the retina. We find that axon arbors from the same class of RGCs exhibit variations in their structure based on their target region of the dLGN. Our findings suggest that target regions may influence the morphologic and synaptic properties of their afferent inputs.

Since the discovery of ocular dominance plasticity, neuroscientists have understood that changes in visual experience during a discrete developmental time, the critical period, trigger robust changes in the visual cortex. State-of-the-art tools used to probe connectivity with cell-type-specific resolution have expanded the understanding of circuit changes underlying experience-dependent plasticity. Here, we review the visual circuitry of the mouse, describing projections from retina to thalamus, between thalamus and cortex, and within cortex. We discuss how visual circuit development leads to precise connectivity and identify synaptic loci, which can be altered by activity or experience. Plasticity extends to visual features beyond ocular dominance, involving subcortical and cortical regions, and connections between cortical inhibitory interneurons. Experience-dependent plasticity contributes to the alignment of networks spanning retina to thalamus to cortex. Disruption of this plasticity may underlie aberrant sensory processing in some neurodevelopmental disorders.

According to the prevailing view of neural development, sensory pathways develop sequentially in a feedforward manner, whereby each local microcircuit refines and stabilizes before directing the wiring of its downstream target. In the visual system, retinal circuits are thought to mature first and direct refinement in the thalamus, after which cortical circuits refine with experience-dependent plasticity. In contrast, we now show that feedback from cortex to thalamus critically regulates refinement of the retinogeniculate projection during a discrete window in development, beginning at postnatal day 20 in mice. Disrupting cortical activity during this window, pharmacologically or chemogenetically, increases the number of retinal ganglion cells innervating each thalamic relay neuron. These results suggest that primary sensory structures develop through the concurrent and interdependent remodeling of subcortical and cortical circuits in response to sensory experience, rather than through a simple feedforward process. Our findings also highlight an unexpected function for the corticothalamic projection.

Summary: Mutations in a number of chromatin modifiers are associated with human neurological disorders. KDM5C, a histone H3 lysine 4 di- and tri-methyl (H3K4me2/3)-specific demethylase, is frequently mutated in X-linked intellectual disability (XLID) patients. Here, we report that disruption of the mouse Kdm5c gene recapitulates adaptive and cognitive abnormalities observed in XLID, including impaired social behavior, memory deficits, and aggression. Kdm5c-knockout brains exhibit abnormal dendritic arborization, spine anomalies, and altered transcriptomes. In neurons, Kdm5c is recruited to promoters that harbor CpG islands decorated with high levels of H3K4me3, where it fine-tunes H3K4me3 levels. Kdm5c predominantly represses these genes, which include members of key pathways that regulate the development and function of neuronal circuitries. In summary, our mouse behavioral data strongly suggest that KDM5C mutations are causal to XLID. Furthermore, our findings suggest that loss of KDM5C function may impact gene expression in multiple regulatory pathways relevant to the clinical phenotypes. : In this study, Iwase et al. characterize Kdm5c-knockout mice to model an important class of intellectual disability. Kdm5c-knockout mice show limited learning, heightened aggression, and dendritic spine defects. Kdm5c is a histone demethylase, and the authors identify altered transcriptional profiles in Kdm5c-knockout brains and investigate the molecular changes in neurons.

The brain can flexibly filter out sensory information in a manner that depends on behavioral state. In the visual thalamus and cortex, arousal and locomotion are associated with changes in the magnitude of responses to visual stimuli. Here, we asked whether such modulation of visual responses might already occur at an earlier stage in this visual pathway. We measured neural activity of retinal axons using wide-field and two-photon calcium imaging in awake mouse thalamus across arousal states associated with different pupil sizes. Surprisingly, visual responses to drifting gratings in retinal axonal boutons were robustly modulated by arousal level in a manner that varied across stimulus dimensions and across functionally distinct subsets of boutons. At low and intermediate spatial frequencies, the majority of boutons were suppressed by arousal. In contrast, at high spatial frequencies, boutons tuned to regions of visual space ahead of the mouse showed enhancement of responses. Arousal-related modulation also varied with a bouton's preference for luminance changes and direction or axis of motion, with greater response suppression in boutons tuned to luminance decrements versus increments, and in boutons preferring motion along directions or axes of optic flow. Together, our results suggest that differential modulation of distinct visual information channels by arousal state occurs at very early stages of visual processing, before the information is transmitted to neurons in visual thalamus. Such early filtering may provide an efficient means of optimizing central visual processing and perception across behavioral contexts.

Immature neural circuits undergo synaptic refinement, in which activity-dependent competition between synapses results in pruning of inappropriate connections and maintenance of appropriate ones. A longstanding question is how neuronal activity eliminates specific synapses based on their strength. The technical challenges of in vivo studies have made it difficult to identify a molecular link between decreased activity and synapse elimination. We developed an organotypic coculture model of the mouse retinogeniculate system that facilitates real-time imaging and elucidation of molecular mechanisms underlying the removal of less active synapses during synaptic competition. Using this model we show for the first time that complement component C1q is necessary for activity-dependent synaptic competition and preferentially localizes to less active, competing presynaptic inputs. In conjunction with classic in vivo and ex vivo models, this coculture model is a new tool to reveal molecular pathways underlying CNS circuit refinement.

It is widely appreciated that neuronal activity contributes to the development of brain representations of the external world. In the visual system, in particular, it is well known that activity cooperates with molecular cues to establish the topographic organization of visual maps on a macroscopic scale [1,2] (Huberman et al., 2008; Cang and Feldheim, 2013), mapping axons in a retinotopic and eye-specific manner. In recent years, significant progress has been made in elucidating the role of activity in driving the finer-scale circuit refinement that shapes the receptive fields of individual cells. In this review, we focus on these recent breakthroughs-primarily in mice, but also in other mammals where noted.

The thalamocortical (TC) relay neuron of the dorsoLateral Geniculate Nucleus (dLGN) has borne its imprecise label for many decades in spite of strong evidence that its role in visual processing transcends the implied simplicity of the term “relay”. The retinogeniculate synapse is the site of communication between a retinal ganglion cell and a TC neuron of the dLGN. Activation of retinal fibers in the optic tract causes reliable, rapid, and robust postsynaptic potentials that drive postsynaptics spikes in a TC neuron. Cortical and subcortical modulatory systems have been known for decades to regulate retinogeniculate transmission. The dynamic properties that the retinogeniculate synapse itself exhibits during and after developmental refinement further enrich the role of the dLGN in the transmission of the retinal signal. Here we consider the structural and functional substrates for retinogeniculate synaptic transmission and plasticity, and reflect on how the complexity of the retinogeniculate synapse imparts a novel dynamic and influential capacity to subcortical processing of visual information.

Recent studies have demonstrated that vision influences the functional remodeling of the mouse retinogeniculate synapse, the connection between retinal ganglion cells and thalamic relay neurons in the dorsal lateral geniculate nucleus (LGN). Initially, each relay neuron receives a large number of weak retinal inputs. Over a 2- to 3-wk developmental window, the majority of these inputs are eliminated, and the remaining inputs are strengthened. This period of refinement is followed by a critical period when visual experience changes the strength and connectivity of the retinogeniculate synapse. Visual deprivation of mice by dark rearing from postnatal day (P)20 results in a dramatic weakening of synaptic strength and recruitment of additional inputs. In the present study we asked whether experience-dependent plasticity at the retinogeniculate synapse represents a homeostatic response to changing visual environment. We found that visual experience starting at P20 following visual deprivation from birth results in weakening of existing retinal inputs onto relay neurons without significant changes in input number, consistent with homeostatic synaptic scaling of retinal inputs. On the other hand, the recruitment of new inputs to the retinogeniculate synapse requires previous visual experience prior to the critical period. Taken together, these findings suggest that diverse forms of homeostatic plasticity drive experience-dependent remodeling at the retinogeniculate synapse.

Microglia regulate synaptic circuit remodeling and phagocytose synaptic material in the healthy brain; however, the mechanisms directing microglia to engulf specific synapses and avoid others remain unknown. Here, we demonstrate that an innate immune signaling pathway protects synapses from inappropriate removal. The expression patterns of “don’t eat me” signal CD47 and its receptor, SIRPα, correlated with peak pruning in the developing retinogeniculate system, and mice lacking these proteins exhibited increased microglial engulfment of retinogeniculate inputs and reduced synapse numbers in the dorsal lateral geniculate nucleus. CD47-deficient mice also displayed increased functional pruning as measured by electrophysiology. Additionally, CD47 was found to be required for neuronal activity-mediated changes in engulfment, as microglia in CD47 knockout mice failed to display preferential engulfment of less active inputs. Together, these results demonstrate that CD47-SIRPα signaling prevents excess microglial phagocytosis, and show that molecular brakes can be regulated by activity to protect specific inputs.

Summary Sensory experience influences the establishment of neural connectivity through molecular mechanisms that remain unclear. Here, we employ single-nucleus RNA sequencing to investigate the contribution of sensory-driven gene expression to synaptic refinement in the dorsal lateral geniculate nucleus of the thalamus, a region of the brain that processes visual information. We find that visual experience induces the expression of the cytokine receptor Fn14 in excitatory thalamocortical neurons. By combining electrophysiological and structural techniques, we show that Fn14 is dispensable for early phases of refinement mediated by spontaneous activity but that Fn14 is essential for refinement during a later, experience-dependent period of development. Refinement deficits in mice lacking Fn14 are associated with functionally weaker and structurally smaller retinogeniculate inputs, indicating that Fn14 mediates both functional and anatomical rearrangements in response to sensory experience. These findings identify Fn14 as a molecular link between sensory-driven gene expression and vision-sensitive refinement in the brain.

In the mammalian visual system, sensory experience is widely thought to sculpt cortical circuits during a precise critical period. In contrast, subcortical regions, such as the thalamus, were thought to develop at earlier ages in a vision-independent manner. Recent studies at the retinogeniculate synapse, however, have demonstrated an influence of vision on the formation of synaptic circuits in the thalamus. In mice, dark rearing from birth does not alter normal developmental maturation of the connection between retina and thalamus. However, deprivation 20 d after birth [postnatal day 20 (p20)] resulted in dramatic weakening of synaptic strength and an increase in the number of retinal inputs that innervate a thalamic relay neuron. Here, by quantifying changes in synaptic strength and connectivity in response to different time windows of deprivation, we find that several days of vision after eye opening is necessary for triggering experience-dependent plasticity. Shorter periods of visual experience do not permit similar experience-dependent synaptic reorganization. Furthermore, changes in connectivity are rapidly reversible simply by restoring normal vision. However, similar plasticity did not occur when shifting the onset of deprivation to p25. Although synapses still weakened, recruitment of additional retinal inputs no longer occurred. Therefore, synaptic circuits in the visual thalamus are unexpectedly malleable during a late developmental period, after the time when normal synapse elimination and pruning has occurred. This thalamic sensitive period overlaps temporally with experience-dependent changes in the cortex, suggesting that subcortical plasticity may influence cortical responses to sensory experience.

Several psychiatric disorders are associated with white matter defects, suggesting that oligodendrocyte (OL) abnormalities underlie some aspects of these diseases. Neuregulin 1 (NRG1) and its receptor, erbB4, are genetically linked with susceptibility to schizophrenia and bipolar disorder. In vitro studies suggest that NRG1-erbB signaling is important for OL development. To test whether erbB signaling contributes to psychiatric disorders by regulating the structure or function of OLs, we analyzed transgenic mice in which erbB signaling is blocked in OLs in vivo . Here we show that loss of erbB signaling leads to changes in OL number and morphology, reduced myelin thickness, and slower conduction velocity in CNS axons. Furthermore, these transgenic mice have increased levels of dopamine receptors and transporters and behavioral alterations consistent with neuropsychiatric disorders. These results indicate that defects in white matter can cause alterations in dopaminergic function and behavior relevant to neuropsychiatric disorders.

SummaryWe report the results of a genetic screen to identify molecules important for synapse formation and/or maintenance. siRNAs were used to decrease the expression of candidate genes in neurons, and synapse development was assessed. We surveyed 22 cadherin family members and demonstrated distinct roles for cadherin-11 and cadherin-13 in synapse development. Our screen also revealed roles for the class 4 Semaphorins Sema4B and Sema4D in the development of glutamatergic and/or GABAergic synapses. We found that Sema4D affects the formation of GABAergic, but not glutamatergic, synapses. Our screen also identified the activity-regulated small GTPase Rem2 as a regulator of synapse development. A known calcium channel modulator, Rem2 may function as part of a homeostatic mechanism that controls synapse number. These experiments establish the feasibility of RNAi screens to characterize the mechanisms that control mammalian neuronal development and to identify components of the genetic program that regulate synapse formation and/or maintenance.

Although a number of repair strategies have been shown to promote axon outgrowth following neuronal injury in the mammalian central nervous system, it remains unclear whether regenerated axons establish functional synapses and support behavior. Here, in both juvenile and adult mice, we show that either PTEN and SOCS3 co-deletion, or co-overexpression of osteopontin (OPN)/insulin-like growth factor 1 (IGF1)/ciliary neurotrophic factor (CNTF), induces regrowth of retinal axons and formation of functional synapses in the superior colliculus (SC), but not significant recovery of visual function. Further analyses suggest that regenerated axons fail to conduct action potentials from the eye to the SC due to lack of myelination. Consistent with this idea, administration of voltage-gated potassium channel blockers restores conduction and results in increased visual acuity. Thus, enhancing both regeneration and conduction effectively improves function after optic nerve injury.

SummarySensory experience and spontaneous activity play important roles in development of sensory circuits; however, their relative contributions are unclear. Here, we test the role of different forms of activity on remodeling of the mouse retinogeniculate synapse. We found that the bulk of maturation occurs without patterned sensory activity over 4 days spanning eye opening. During this early developmental period, blockade of spontaneous retinal activity by tetrodotoxin, but not visual deprivation, retarded synaptic strengthening and inhibited pruning of excess retinal afferents. Later in development, synaptic remodeling becomes sensitive to changes in visually evoked activity, but only if there has been previous visual experience. Synaptic strengthening and pruning were disrupted by visual deprivation following 1 week of vision, but not by chronic deprivation from birth. Thus, spontaneous activity is necessary to drive the bulk of synaptic refinement around the time of eye opening, while sensory experience is important for the subsequent maintenance of connections.

The relay of visual information converging in the lateral geniculate nucleus (LGN) en route to the visual cortex is modulated by projections from brainstem nuclei. The release of serotonin, one mediator of these effects, has been shown to act at a presynaptic site to inhibit neurotransmitter release at the retinogeniculate synapse, the connection between retinal ganglion cells and thalamocortical relay neurons in the LGN. To understand how serotonergic inhibition of synaptic transmission influences the transfer of information at this synapse, we examined the EPSCs and firing responses of relay neurons to 5-carboxytryptamine (5-CT), a 5-HT1receptor agonist that preferentially activates the presynaptic over postsynaptic modulatory effects of serotonin. Bath application of 5-CT inhibits synaptic strength, relieves synaptic depression, and reduces the total synaptic charge transferred at the retinogeniculate synapse in mouse LGN brain slices. In contrast, 5-CT does not significantly alter the membrane potential response of relay neurons to trains of intracellular current injections. Here we show that presynaptic serotonergic modulation results in a frequency-dependent inhibition of relay neuron firing. At low-frequency stimulation, 5-CT markedly reduces charge transfer at the retinogeniculate synapse, thus inhibiting relay neuron firing. However, inhibition of firing by 5-CT is diminished during high-frequency stimulation, because relief from synaptic depression partially offsets the reduction in charge transfer. Thus, presynaptic serotonergic inhibition plays a powerful role in modulating the frequency range of visual information transmitted via the retinogeniculate synapse such that high-frequency inputs are more reliably transmitted than low-frequency inputs.

How is the picture of the visual scene that the eye encodes represented by neural circuits in the brain? In this issue of Cell, Morgan et al. address this question by forming an ultrastructural "connectome" of the mouse's visual thalamus that depicts individual retinal afferents and every contact these form with target relay cells.

Neurotrophins are key regulators of neuronal survival and function. Here we show that TrkB, the receptor for brain-derived neurotrophic factor (BDNF), is located at parallel fiber to Purkinje cell (PF/PC) synapses of the cerebellum. To determine the effects of TrkB receptor activation on synapse formation and function, we examined the parallel fiber to Purkinje cell synapses of mice with a targeted deletion of the BDNF gene. Although Purkinje cell dendrites are abnormal in BDNF −/− mice, PF/PC synapses are still able to form. Immunohistochemical analysis of mutant animals revealed the formation of numerous PF/PC synapses with the appropriate apposition of presynaptic and postsynaptic proteins. These synapses are functional, and no differences were detected in the waveform of evoked EPSCs, the amplitude of spontaneous mini-EPSCs, or the response to prolonged 10 Hz stimulus trains. However, paired-pulse facilitation, a form of short-term plasticity, is significantly decreased in BDNF −/− mice. Detailed ultrastructural analysis of the presynaptic terminals demonstrated that this change in synaptic function is accompanied by an increase in the total number of synaptic vesicles in mutant mice and a decrease in the proportion of vesicles that are docked. These data suggest that BDNF regulates both the mechanisms that underlie short-term synaptic plasticity and the steady-state relationship between different vesicle pools within the terminal.

Precise connectivity between retinal ganglion cells (RGCs) and thalamocortical (TC) relay neurons is thought to be essential for the transmission of visual information. Consistent with this view, electrophysiological measurements have previously estimated that 1–3 retinal ganglion cells (RGCs) converge onto a mouse geniculate TC neuron. Recent advances in connectomics and rabies tracing have yielded much higher estimates of retinogeniculate convergence, although not all identified contacts may be functional. Here we use optogenetics and a computational simulation to determine the number of functionally relevant retinogeniculate inputs onto TC neurons in mice. We find an average of 10 RGCs converging onto a mature TC neuron, in contrast to >30 inputs before developmental refinement. However, only 30% of retinogeniculate inputs exceed the threshold for dominating postsynaptic activity. These results signify a greater role for the thalamus in visual processing and provide a functional perspective of anatomical connectivity data.

Neural circuits in sensory pathways develop through a general strategy of overproduction of synapses followed by activity-driven pruning to fine-tune connectivity for optimal function. The early visual pathway, consisting of the retina → visual thalamus → primary visual cortex, has served for decades as a powerful model system for probing the mechanisms and logic of this process. In addition to these feedforward projections, the early visual pathway also includes a substantial feedback component in the form of corticothalamic projections from the deepest layer of primary visual cortex. The role of this feedback in visual processing has been studied extensively in mature animals, yet historically, its role in development has received comparatively little attention. Recent technological advances allowing for selective manipulation of neural activity in development led to the uncovering of a role for feedback in guiding the refinement of the forward projection from retina to visual thalamus. Here we discuss the implications of feedback exerting influence on the development of sensory pathways. We propose several possible advantages to constructing neural circuits with top-down regulation, and discuss the potential significance of this finding for certain neurologic disorders.

The retinogeniculate synapse, the connection between retinal ganglion cells (RGC) and thalamic relay neurons, undergoes robust changes in connectivity over development. This process of synapse elimination and strengthening of remaining inputs is thought to require synapse specificity. Here we show that glutamate spillover and asynchronous release are prominent features of retinogeniculate synaptic transmission during this period. The immature excitatory postsynaptic currents exhibit a slow decay time course that is sensitive to low-affinity glutamate receptor antagonists and extracellular calcium concentrations, consistent with glutamate spillover. Furthermore, we uncover and characterize a novel, purely spillover-mediated AMPA receptor current from immature relay neurons. The isolation of this current strongly supports the presence of spillover between boutons of different RGCs. In addition, fluorescence measurements of presynaptic calcium transients suggest that prolonged residual calcium contributes to both glutamate spillover and asynchronous release. These data indicate that, during development, far more RGCs contribute to relay neuron firing than would be expected based on predictions from anatomy alone.

To achieve its precise neural connectivity, the developing mammalian nervous system undergoes extensive activity-dependent synapse remodelling. Recently, microglial cells have been shown to be responsible for a portion of synaptic pruning, but the remaining mechanisms remain unknown. Here we report a new role for astrocytes in actively engulfing central nervous system synapses. This process helps to mediate synapse elimination, requires the MEGF10 and MERTK phagocytic pathways, and is strongly dependent on neuronal activity. Developing mice deficient in both astrocyte pathways fail to refine their retinogeniculate connections normally and retain excess functional synapses. Finally, we show that in the adult mouse brain, astrocytes continuously engulf both excitatory and inhibitory synapses. These studies reveal a novel role for astrocytes in mediating synapse elimination in the developing and adult brain, identify MEGF10 and MERTK as critical proteins in the synapse remodelling underlying neural circuit refinement, and have important implications for understanding learning and memory as well as neurological disease processes.

Visual circuits mature and are refined by sensory experience. However, significant gaps remain in our understanding how deprivation influences the development of visual acuity in mice. Here, we perform a longitudinal study assessing the effects of chronic deprivation on the development of the mouse subcortical and cortical visual circuits using a combination of behavioral optomotor testing,in vivovisual evoked responses (VEP) and single-unit cortical recordings. As previously reported, orientation tuning was degraded and onset of ocular dominance plasticity was delayed and remained open in chronically deprived mice. Surprisingly, we found that the development of optomotor threshold and VEP acuity can occur in an experience-independent manner, although at a significantly slower rate. Moreover, monocular deprivation elicited amblyopia only during a discrete period of development in the dark. The rate of recovery of optomotor threshold upon exposure of deprived mice to light confirmed a maturational transition regardless of visual input. Together our results revealed a dissociable developmental trajectory for visual receptive-field properties in dark-reared mice suggesting a differential role for spontaneous activity within thalamocortical and intracortical circuits.

SummaryDuring development, neurons are constantly refining their connections in response to changes in activity. Experience-dependent plasticity is a key form of synaptic plasticity, involving changes in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) accumulation at synapses. Here, we report a critical role for the AMPAR auxiliary subunit stargazin in this plasticity. We show that stargazin is functional at the retinogeniculate synapse and that in the absence of stargazin, the refinement of the retinogeniculate synapse is specifically disrupted during the experience-dependent phase. Importantly, we found that stargazin expression and phosphorylation increased with visual deprivation and led to reduced AMPAR rectification at the retinogeniculate synapse. To test whether stargazin plays a role in homeostatic plasticity, we turned to cultured neurons and found that stargazin phosphorylation is essential for synaptic scaling. Overall, our data reveal an important role for stargazin in regulating AMPAR abundance and composition at glutamatergic synapses during homeostatic and experience-dependent plasticity.

Over the first few postnatal weeks, extensive remodeling occurs at the developing murine retinogeniculate synapse, the connection between retinal ganglion cells (RGCs) and the visual thalamus. Although numerous studies have described the role of activity in the refinement of this connection, little is known about the mechanisms that regulate glutamate concentration at and around the synapse over development. Here we show that interactions between glutamate transporters and metabotropic glutamate receptors (mGluRs) dynamically control the peak and time course of the excitatory postsynaptic current (EPSC) at the immature synapse. Inhibiting glutamate transporters by bath application of TBOA (dl- threo-β-benzyloxyaspartic acid) prolonged the decay kinetics of both α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) and N-methyl-d-aspartate receptor (NMDAR) currents at all ages. Moreover, at the immature synapse, TBOA-induced increases in glutamate concentration led to the activation of group II/III mGluRs and a subsequent reduction in neurotransmitter release at RGC terminals. Inhibition of this negative-feedback mechanism resulted in a small but significant increase in peak NMDAR EPSCs during basal stimulation and a substantial increase in the peak with coapplication of TBOA. Activation of mGluRs also shaped the synaptic response during high-frequency trains of stimulation that mimic spontaneous RGC activity. At the mature synapse, however, the group II mGluRs and the group III mGluR7-mediated response are downregulated. Our results suggest that transporters reduce spillover of glutamate, shielding NMDARs and mGluRs from the neurotransmitter. Furthermore, mechanisms of glutamate clearance and release interact dynamically to control the glutamate transient at the developing retinogeniculate synapse.

We have analyzed the gating kinetics of T-type Ca channels in 3T3 fibroblasts. Our results show that channel closing, inactivation, and recovery from inactivation each include a voltage-independent step which becomes rate limiting at extreme potentials. The data require a cyclic model with a minimum of two closed, one open, and two inactivated states. Such a model can produce good fits to our data even if the transitions between closed states are the only voltage-dependent steps in the activating pathway leading from closed to inactivated states. Our analysis suggests that the channel inactivation step, as well as the direct opening and closing transitions, are not intrinsically voltage sensitive. Single-channel recordings are consistent with this scheme. As expected, each channel produces a single burst per opening and then inactivates. Comparison of the kinetics of T-type Ca current in fibroblasts and neuronal cells reveals significant differences which suggest that different subtypes of T-type Ca channels are expressed differentially in a tissue specific manner.

Visual system circuitry, a canonical model system for the study of experience-dependent development, matures before and following the onset of vision. Sensory experience or deprivation during an early critical period results in substantial plasticity and is a crucial factor in establishing the mature circuitry. In adulthood, plasticity has been thought to be reduced or absent. However, recent studies point to the potential for change in neuronal circuits within the mature brain, raising the possibility that aberrant circuit function can be corrected. In this review, we will discuss recent exciting findings in the field of experience-dependent plasticity that advance our understanding of mechanisms underlying the activation, expression, and closure of critical periods in the visual system.

How the developing brain specifies precise neural connectivity has long interested neuroscientists. Because of the immense number of cells and synapses in the CNS, it seems unlikely that each individual cell’s identity and connectivity is intrinsically or genetically specified. Both neural activity and molecular cues provide possible mechanisms by which network properties of the developing brain can emerge from relative disorder. Emphasizing the visual system as a model for synaptic development, we review the role of various factors in synaptic maturation, including sensory activity. Furthermore, we elucidate why the mouse visual system could prove advantageous for investigation of mechanisms governing circuit development.

Modulatory projections from brainstem nuclei and intrinsic thalamic interneurons play a significant role in modifying sensory information as it is relayed from the thalamus to the cortex. In the lateral geniculate nucleus (LGN), neurotransmitters released from these modulatory inputs can affect the intrinsic conductances of thalamocortical relay neurons, thus altering their firing properties. Here, we show that in addition to postsynaptic effects, neuromodulators such as serotonin (5-HT) and GABA can act presynaptically to regulate neurotransmitter release at the synapse between retinal ganglion cells (RGCs) and relay neurons in the LGN, the retinogeniculate synapse. Activation of 5HT(1) and GABA(B) receptors significantly decreased EPSC amplitude. This inhibition was accompanied by a decrease in the extent of paired-pulse depression, suggesting that it is presynaptic in origin. In addition, fluorometric calcium measurements from retinal axon terminals labeled with Calcium Green-1 dextran revealed that 5HT(1) and GABA(B) receptor agonists decreased presynaptic calcium influx. Taken together, our data indicate that serotonin and GABA can act presynaptically to decrease calcium influx at the retinogeniculate synapse and modify transmission of visual information in the LGN.

The formation of precise synaptic connections in the developing central nervous system involves activity-dependent control of synaptic strength. Correlated activity between the afferent input and postsynaptic neuron strengthens connections through long-term potentiation (LTP), while uncorrelated synaptic inputs are selectively diminished. In this issue of Neuron, Tao et al. report that LTP in young animals has less synaptic specificity than in older animals, indicating that the properties of LTP change during development.

Fast neurotransmitter release is driven by high calcium (10–100 μm) near open channels (Calocal), followed by a much smaller (m), longer-lasting residual calcium (Cares). The most prominent component of release, phasic release, lasts several milliseconds and is thought to be triggered by Calocal. A transient tail of release then continues over the next 20 msec at 1–10% of peak rates. This transient component of release, which we refer to as TR, is poorly understood, and there is conflicting evidence regarding the role of Calocaland Caresin its generation. We used optical methods to monitor Caresand whole-cell voltage-clamp recordings to study TR at synapses between granule cells and stellate cells in rat cerebellar slices. After stimulation the probability of release is elevated greatly, peaking at 500 μsec and then slowly declining to prestimulus levels after tens of milliseconds. After speeding the decay of Careslevels with EGTA, release is confined to a 3 msec interval, and TR is eliminated. Thus, we find that Caresaccounts for a transient tail of release on the millisecond time scale that helps to shape the average synaptic current and accounts for at least 20% of the synaptic charge in the 20 msec interval after stimulation. Cares-dependent TR is likely to contribute significantly to fast synaptic transmission under physiological conditions, particularly during high-frequency bursts that elevate Cares.

Mamba snake dendrotoxins have been used extensively in biochemical and physiological studies of K+ channels of the brain. Their known targets of inhibition have been limited to the family of voltage-gated K+ channels. We report the isolation of a dendrotoxin inhibitor of ROMK1, a channel belonging to the inward rectifier family of K+ channels. The inhibitory activity, fractionated to purity with FPLC and HPLC, is identical to a previously identified delta-dendrotoxin. To verify that delta-dendrotoxin blocks ROMK1 channels, a cDNA encoding the toxin was synthesized and recombinant toxin expressed in Escherichia coli. Electrophysiological recordings reveal that recombinant delta-dendrotoxin has a half-maximal inhibition constant (Kd) of 150 nM when applied to ROMK1 channels expressed in Xenopus laevis oocytes. That the delta-dendrotoxin binding site exists on separate K+ channel classes is shown by its high affinity for two of the voltage-gated family of channels, Kv1.1 (Kd < 0.1 nM) and Kv1.6 (Kd = 23 nM). Single amino acid substitutions in ROMK1 indicate that delta-dendrotoxin binds to the pore region of ROMK1 even though it does not completely block conduction through the pore. These results suggest that dendrotoxins inhibit K+ channels by recognizing the structurally conserved pore region of these channels.

Increases in cAMP have been shown previously to enhance the strength of the granule cell to Purkinje cell synapse. We have examined the mechanisms underlying this enhancement in rat cerebellar brain slices. Elevation of cAMP levels by forskolin increased synaptic currents in a dose-dependent manner. Fluorometric calcium measurements revealed that forskolin did not affect presynaptic calcium influx or resting calcium levels. The waveform of the presynaptic volley was also unaltered, indicating that changes in the presynaptic action potential did not contribute to synaptic enhancement. However, forskolin enhanced the frequency but not the size of spontaneous miniature EPSCs. There was a one-to-one correspondence between increases of spontaneous and evoked neurotransmitter release. These results suggest that forskolin increases release at this synapse via presynaptic mechanisms that do not alter calcium influx. The effect of forskolin on paired-pulse facilitation was examined to assess the relative contributions of changes in the probability of release (p) and changes in the number of functional release sites (n) to this form of enhancement. These experiments suggest that although small changes inncannot be excluded, most of the enhancement arises from increases inp.

Na+ currents recorded from Xenopus oocytes expressing the Na+ channel alpha subunit alone inactivate with two exponential components. The slow component predominates in monomeric channels, while co-expression with the beta 1 subunit favors the fast component. Macropatch recordings show that the relative rates of these components are much greater than previously estimated from two-electrode measurements (approximately 30-fold vs approximately 5-fold). A re-assessment of steady-state inactivation, h infinity (V), shows that there is no depolarized shift of the slow component, provided a sufficiently long prepulse duration and repetition interval are used to achieve steady-state entry and recovery from inactivation, respectively. Deletion mutagenesis of the beta 1 subunit was used to define which regions of the subunit are required to modulate inactivation kinetics. The carboxy tail, comprising the entire predicted intracellular domain, can be deleted without a loss of activity; whereas small deletions in the extracellular amino domain or the signal peptide totally disrupt function.