Experience-dependent plasticity in the adult visual cortex is enhanced by locomotion, a process mediated by vasoactive intestinal peptide (VIP)-expressing interneurons. While VIP interneurons are known to signal through both Gamma-aminobutyric acid (GABA) and VIP peptide, the specific contributions of these pathways during different forms of plasticity remain unclear. Monocular deprivation (MD) in adult mice alters cortical responses, though more slowly and differently than during a critical period in early life. Here, we used two-photon calcium imaging in awake adult mice to dissect the roles of VIP and GABA release from VIP interneurons during adult MD and subsequent binocular recovery. We found comparable level of ocular dominance shifts after MD in mice deficient in either peptidergic or GABA signaling, but disrupting GABA signaling impaired recovery of binocular responses. We also showed that running preferentially enhances contralateral eye responses in binocular primary visual cortex. However, this eye-specific modulation of visual responses by running was altered during recovery from MD and was dependent on VIP signaling pathways. These findings highlight the GABA-mediated inhibition by VIP interneurons as a critical pathway for promoting visual restoration in the adult brain.

Cortical interneurons play an important role in mediating the juvenile critical period for ocular dominance plasticity in the mouse primary visual cortex. Previously, we showed that transplantation of cortical interneurons derived from the medial ganglionic eminence (MGE) opens a robust period of ocular dominance plasticity 33-35 days after transplantation into neonatal host visual cortex. The plasticity can be induced by transplanting either PV or SST MGE-derived cortical interneurons; it requires transplanted interneurons to express the vesicular GABAergic transporter; and it is manifested by changes to the host visual circuit. Here, we show that transplantation of MGE-derived cortical interneurons into the adult host visual cortex also opens a period of ocular dominance plasticity. The transplanted interneurons must be active to induce plasticity, and the neuronal activity and tuning of visually evoked responses in transplanted and host PV and SST interneurons are modulated by the locomotor state of the host. We also show that changes in activity over the period of plasticity induction are different between PV and SST interneurons but similar between host and transplanted interneurons of each type. The present findings demonstrate that the transplant-induced plasticity generated in adult visual cortex has many features in common with the role of these interneurons during the normal, juvenile critical period.

A challenge in sensory neuroscience is understanding how populations of neurons operate in concert to represent diverse stimuli. To meet this challenge, we have created "encoding manifolds" that reveal the overall responses of brain areas to diverse stimuli with the resolution of individual neurons and their response dynamics. Here we use encoding manifold to compare the population-level encoding of primary visual cortex (VISp) with five higher visual areas (VISam, VISal, VISpm, VISlm, and VISrl). We used data from the Allen Institute Visual Coding-Neuropixels dataset from the mouse. We show that the encoding manifold topology computed only from responses to grating stimuli is continuous, for V1 and for higher visual areas, with smooth coordinates spanning it that include orientation selectivity and firing-rate magnitude. Surprisingly, the manifolds for each visual area revealed novel relationships between how natural scenes are encoded relative to static gratings-a relationship that was conserved across visual areas. Namely, neurons preferring natural scenes preferred either low or high spatial frequency gratings, but not intermediate ones. Analyzing responses by cortical layer reveals a preference for gratings concentrated in layer 6, whereas preferences for natural scenes tended to be higher in layers 2/3 and 4. The results reveal how machine learning approaches can be used to organize and visualize the structure of sensory coding, thereby revealing novel relationships within and across brain areas and sensory stimuli.

Developmental myelination is a protracted process in the mammalian brain1. One theory for why oligodendrocytes mature so slowly posits that myelination may stabilize neuronal circuits and temper neuronal plasticity as animals age2-4. We tested this theory in the visual cortex, which has a well-defined critical period for experience-dependent neuronal plasticity5. During adolescence, visual experience modulated the rate of oligodendrocyte maturation in visual cortex. To determine whether oligodendrocyte maturation in turn regulates neuronal plasticity, we genetically blocked oligodendrocyte differentiation and myelination in adolescent mice. In adult mice lacking adolescent oligodendrogenesis, a brief period of monocular deprivation led to a significant decrease in visual cortex responses to the deprived eye, reminiscent of the plasticity normally restricted to adolescence. This enhanced functional plasticity was accompanied by a greater turnover of dendritic spines and coordinated reductions in spine size following deprivation. Furthermore, inhibitory synaptic transmission, which gates experience-dependent plasticity at the circuit level, was diminished in the absence of adolescent oligodendrogenesis. These results establish a critical role for oligodendrocytes in shaping the maturation and stabilization of cortical circuits and support the concept of developmental myelination acting as a functional brake on neuronal plasticity.

Type 2 diabetes mellitus (T2DM) is associated with impaired leptomeningeal collateral compensation and poor stroke outcome. Neutrophils tethering and rolling on endothelium after stroke can also independently reduce flow velocity. However, the chronology and topological changes in collateral circulation in T2DM is not yet defined. Here, we describe the spatial and temporal blood flow dynamics and vessel remodeling in pial arteries and veins and leukocyte-endothelial adhesion following middle cerebral artery (MCA) stroke using two-photon microscopy in awake control and T2DM mice. Relative to control mice prior to stroke, T2DM mice already exhibited smaller pial vessels with reduced flow velocity. Following stroke, T2DM mice displayed persistently reduced blood flow in pial arteries and veins, resulting in a poor recovery of downstream penetrating arterial flow and a sustained deficit in microvascular flow. There was also persistent increase of leukocyte adhesion to the endothelium of veins, coincided with elevated neutrophils infiltration into brain parenchyma in T2DM mice compared to control mice after stroke. Our data suggest that T2DM-induced increase in chronic inflammation may contribute to the remodeling of leptomeningeal collateral circulation and the observed hemodynamics deficiency that potentiates poor stroke outcome.

The retina and primary visual cortex (V1) both exhibit diverse neural populations sensitive to diverse visual features. Yet it remains unclear how neural populations in each area partition stimulus space to span these features. One possibility is that neural populations are organized into discrete groups of neurons, with each group signaling a particular constellation of features. Alternatively, neurons could be continuously distributed across feature-encoding space. To distinguish these possibilities, we presented a battery of visual stimuli to the mouse retina and V1 while measuring neural responses with multi-electrode arrays. Using machine learning approaches, we developed a manifold embedding technique that captures how neural populations partition feature space and how visual responses correlate with physiological and anatomical properties of individual neurons. We show that retinal populations discretely encode features, while V1 populations provide a more continuous representation. Applying the same analysis approach to convolutional neural networks that model visual processing, we demonstrate that they partition features much more similarly to the retina, indicating they are more like big retinas than little brains.

Cortical function critically depends on inhibitory/excitatory balance. Cortical inhibitory interneurons (cINs) are born in the ventral forebrain and migrate into cortex, where their numbers are adjusted by programmed cell death. Previously, we showed that loss of clustered gamma protocadherins (Pcdhγ), but not of genes in the alpha or beta clusters, increased dramatically cIN BAX-dependent cell death in mice. Here we show that the sole deletion of the Pcdhγc4 isoform, but not of the other 21 isoforms in the Pcdhγ gene cluster, increased cIN cell death in mice during the normal period of programmed cell death. Viral expression of the Pcdhγc4 isoform rescued transplanted cINs lacking Pcdhγ from cell death. We conclude that Pcdhγ, specifically Pcdhγc4, plays a critical role in regulating the survival of cINs during their normal period of cell death. This demonstrates a novel specificity in the role of Pcdhγ isoforms in cortical development.

When adult mice are repeatedly exposed to a particular visual stimulus for as little as 1 h per day for several days while their visual cortex (V1) is in the high-gain state produced by locomotion, that specific stimulus elicits much stronger responses in V1 neurons for the following several weeks, even when measured in anesthetized animals. Such stimulus-specific enhancement (SSE) is not seen if locomotion is prevented. The effect of locomotion on cortical responses is mediated by vasoactive intestinal peptide (VIP) positive interneurons, which can release both the peptide and the inhibitory neurotransmitter GABA. Previous studies have examined the role of VIP-ergic interneurons, but none have distinguished the individual roles of peptide from GABA release. Here, we used genetic ablation to determine which of those molecules secreted by VIP-ergic neurons is responsible for SSE. SSE was not impaired by VIP deletion but was prevented by compromising release of GABA from VIP cells. This finding suggests that SSE may result from Hebbian mechanisms that remain present in adult V1. NEW & NOTEWORTHY Many neurons package and release a peptide along with a conventional neurotransmitter. The conventional view is that such peptides exert late, slow effects on plasticity. We studied a form of cortical plasticity that depends on the activity of neurons that express both vasoactive intestinal peptide (VIP) and the inhibitory neurotransmitter GABA. GABA release accounted for their action on plasticity, with no effect of deleting the peptide on this phenomenon.

ABSTRACT Current hypotheses on the mechanisms underlying the development and plasticity of the ocular dominance system through competitive interactions between pathways serving the two eyes strongly suggest the involvement of neurotrophins and their high affinity receptors. In the cat, infusion of the tyrosine kinase B ligand (trkB), neurotrophin-4/5 (NT-4/5), abolishes ocular dominance plasticity that follows monocular deprivation (Gillespie et al., 2000), while tyrosine kinase A and C ligands (trkA and trkC) do not have this effect. One interpretation of this finding is that NT-4/5 causes overgrowth and sprouting of thalamocortical and/or corticocortical terminals, leading to promiscuous neuronal connections which override the experience-dependent fine tuning of connections based on correlated activity. The present study tested whether neurons in cortical regions infused with NT-4/5 showed anatomical changes compatible with this hypothesis. Cats at the peak of the critical period received chronic infusion NT-4/5 into visual cortical areas 17/18 via an osmotic minipump. Visual cortical neurons were labeled in fixed slices using the DiOlistics methods (Gan et al., 2000) and analyzed in confocal microsco-py. Infusion of NT-4/5 induced a significant increase of spine-like processes on primary dendrites and a distinctive sprouting of protuberances from neuronal somata in all layers. The increase of neuronal membrane was paralleled by an increase in density of the presynaptic marker synaptophysin in infused areas, suggesting an increase in the numbers of synapses. A contingent of these newly formed synapses may feed into inhibitory circuits, as suggested by an increase of GAD-65 immunostaining in NT-4/5 affected areas. These anatomical changes are consistent with the physiological changes in such animals, suggesting that excess trkB neurotrophin can stimulate the formation of promiscuous connections during the critical period.

We have previously shown that recovery of visual responses to a deprived eye dur-ing the critical period in mouse primary visual cortex requires phosphorylation of the TrkB receptor for BDNF [M. Kaneko, J. L. Hanover, P. M. England, M. P. Stryker, Nat. Neurosci.11, 497-504 (2008)]. We have now studied the temporal relationship between the production of mature BDNF and the recovery of visual responses under several different conditions. Visual cortical responses to an eye whose vision has been occluded for several days during the critical period and is then re-opened recover rapidly during binocular vision or much more slowly following reverse occlusion, when the previously intact fellow eye is occluded in a model of "patch therapy" for amblyopia. The time to recovery of visual responses differed by more than 18 h between these two procedures, but in each, the production of mature BDNF preceded the physiological recovery. These findings suggest that a spurt of BDNF production is permissive for the growth of connections serving the deprived eye to restore visual responses. Attenuation of recovery of deprived-eye responses by interference with TrkB receptor activation or reduction of BDNF production by interference with homeostatic synaptic scaling had effects consistent with this suggestion

Electrical stimulation is an effective tool for mapping and altering brain connectivity, with applications ranging from treating pharmacology-resistant neurological disorders to providing sensory feedback for neural prostheses. Paramount to the success of these applications is the ability to manipulate electrical currents to precisely control evoked neural activity patterns. However, little is known about stimulation-evoked responses in inhibitory neurons nor how stimulation-evoked activity patterns depend on ongoing neural activity. In this study, we used 2-photon imaging and cell-type specific labeling to measure single-cell responses of excitatory and inhibitory neurons to electrical stimuli in the visual cortex of awake mice. Our data revealed strong interactions between electrical stimulation and pre-stimulus activity of single neurons in awake animals and distinct recruitment and response patterns for excitatory and inhibitory neurons. This work demonstrates the importance of cell-type-specific labeling of neurons in future studies.

ABSTRACTWhen adult mice are repeatedly exposed to a particular visual stimulus for as little as one hour per day for several days while their visual cortex (V1) is in the high-gain state produced by locomotion, that specific stimulus elicits much stronger responses in V1 neurons for the following several weeks, even when measured in anesthetized animals. Such stimulus-specific enhancement (SSE) is not seen if locomotion is prevented. The effect of locomotion on cortical responses is mediated by vasoactive intestinal peptide (VIP) positive interneurons, which can release both the peptide and the inhibitory neurotransmitter GABA. Here we used genetic ablation to determine which of those molecules secreted by VIP-ergic neurons is responsible for SSE. SSE was not impaired by VIP deletion but was prevented by compromising release of GABA from VIP cells. This finding suggests that SSE may result from Hebbian mechanisms that remain present in adult V1.SIGNIFICANCEMany neurons package and release a peptide along with a conventional neurotransmitter. The conventional view is that such peptides exert late, slow effects on plasticity. We studied a form of cortical plasticity that depends on the activity of neurons that express both vasoactive intestinal peptide (VIP) and the inhibitory neurotransmitter GABA. GABA release accounted for their action on plasticity, with no effect of deleting the peptide on this phenomenon.

BACKGROUND:Developmental myelination is a protracted process in the mammalian brain. One theory for why oligodendrocytes mature so slowly posits that myelination may stabilize neuronal circuits and temper neuronal plasticity as animals age. We tested this hypothesis in the visual cortex, which has a well-defined critical period for experience-dependent neuronal plasticity. OBJECTIVES/METHODS:To prevent myelin progression, we conditionally deleted Myrf, a transcription factor necessary for oligodendrocyte maturation, from oligodendrocyte precursor cells (Myrf cKO) in adolescent mice. To induce experience-dependent plasticity, adult control and Myrf cKO mice were monocularly deprived by eyelid suture. Functional and structural neuronal plasticity in the visual cortex were assessed in vivo by intrinsic signal optical imaging and longitudinal two photon imaging of dendritic spines, respectively. RESULTS:During adolescence, visual experience modulated the rate of oligodendrocyte maturation in visual cortex. Myrf deletion from oligodendrocyte precursors during adolescence led to inhibition of oligodendrocyte maturation and myelination that persisted into adulthood. Following monocular deprivation, visual cortex activity in response to visual stimulation of the deprived eye remained stable in adult control mice, as expected for post-critical period animals. By contrast, visual cortex responses to the deprived eye decreased significantly following monocular deprivation in adult Myrf cKO mice, reminiscent of the plasticity observed in adolescent mice. Furthermore, visual cortex neurons in adult Myrf cKO mice had fewer dendritic spines and a higher level of spine turnover. Finally, monocular deprivation induced spatially coordinated spine size decreases in adult Myrf cKO, but not control, mice. CONCLUSIONS:These results demonstrate a critical role for oligodendrocytes in shaping the maturation and stabilization of cortical circuits and support the concept of myelin acting as a brake on neuronal plasticity during development.

Electrical stimulation has emerged as a powerful and precise treatment in which to modulate aberrant neural activity patterns common in neural dysfunction and disease; however, the physiological process involved in microstimulation is poorly understood, particularly regarding the contributions of inhibitory neurons to shaping stimulation-evoked activity. To address this issue, we used 2-photon imaging of transgenic mice to measure the widespread responses of inhibitory and excitatory neurons to electrical stimulation through a chronically-implanted cortical microelectrode. We found that increasing stimulation amplitude both raised the fraction of neurons that responded to a stimulus and increased the distance at which inhibitory and excitatory neurons were significantly modulated by stimulation; however, the lateral spread of inhibitory activity preceded that of excitatory activity. By 50 µ A, a significantly larger fraction of inhibitory neurons vs. excitatory neurons were modulated by stimulation. Increasing amplitude also shifted the temporal response properties of the population — towards longer-latency excitation close to the electrode tip and strong inhibition of more distant neurons. Animal behavior, specifically their locomotion patterns, strongly correlated with trial-to-trial variability in stimulation-evoked responses. We conclude that changing electrical stimulation amplitude can shift the balance of excitation to inhibition in the brain in a manner that interact with ongoing animal behavior.

Traumatic brain injury (TBI) is a leading cause of long-term neurological disability in the world and the strongest environmental risk factor for the development of dementia. Even mild TBI (resulting from concussive injuries) is associated with a greater than twofold increase in the risk of dementia onset. Little is known about the cellular mechanisms responsible for the progression of long-lasting cognitive deficits. The integrated stress response (ISR), a phylogenetically conserved pathway involved in the cellular response to stress, is activated after TBI, and inhibition of the ISR-even weeks after injury-can reverse behavioral and cognitive deficits. However, the cellular mechanisms by which ISR inhibition restores cognition are unknown. Here, we used longitudinal two-photon imaging in vivo after concussive injury in mice to study dendritic spine dynamics in the parietal cortex, a brain region involved in working memory. Concussive injury profoundly altered spine dynamics measured up to a month after injury. Strikingly, brief pharmacological treatment with the drug-like small-molecule ISR inhibitor ISRIB entirely reversed structural changes measured in the parietal cortex and the associated working memory deficits. Thus, both neural and cognitive consequences of concussive injury are mediated in part by activation of the ISR and can be corrected by its inhibition. These findings suggest that targeting ISR activation could serve as a promising approach to the clinical treatment of chronic cognitive deficits after TBI.

Visual perception in natural environments depends on the ability to focus on salient stimuli while ignoring distractions. This kind of selective visual attention is associated with gamma activity in the visual cortex. While the nucleus reticularis thalami (nRT) has been implicated in selective attention, its role in modulating gamma activity in the visual cortex remains unknown. Here, we show that somatostatin- (SST) but not parvalbumin-expressing (PV) neurons in the visual sector of the nRT preferentially project to the dorsal lateral geniculate nucleus (dLGN), and modulate visual information transmission and gamma activity in primary visual cortex (V1). These findings pinpoint the SST neurons in nRT as powerful modulators of the visual information encoding accuracy in V1 and represent a novel circuit through which the nRT can influence representation of visual information.

Studies of visual callosal connections proposed that bilateral projections from temporal retina promote the formation of callosal linkages between cortical loci that are retinotopically matched and non-mirror symmetric with respect to the brain midline. It is therefore possible for a spontaneously active retinal locus to simultaneously activate retinotopically corresponding loci in both cortices, leading to Hebbian-like stabilization of connections between them before the eyes open. Interhemispheric correlated activity could stem from single ganglion cells that send axon branches to both sides, or from closely located cells that project to one side or the other, but which fire in synchrony due to spontaneously generated retinal activity waves. We hypothesized that lack of retinal waves could induce callosal map anomalies similar to those produced by neonatal enucleation. We studied mice lacking retinal waves due to deficiency in the ß2 subunit of the nicotinic acetylcholine receptor. The organization of callosal projections revealed with small tracer injections was correlated with V1 maps made by imaging intrinsic optical responses to drifting stimuli. Consistent with studies showing that retinofugal and geniculocortical projections are less focused in ß2 -/- mice, we found that the overall callosal pattern in V1 is markedly broader in ß2 -/- mice than in wild type mice. However, the fine topography of the callosal map in ß2 -/- mice is similar to that in wild type and ß2 -/+ mice, indicating that lack of retinal waves is not sufficient for inducing the reversal in the callosal map caused by neonatal enucleation.

Cortical function critically depends on inhibitory/excitatory balance. Cortical inhibitory interneurons (cINs) are born in the ventral forebrain and migrate into cortex, where their numbers are adjusted by programmed cell death. Here, we show that loss of clustered gamma protocadherins (Pcdhg), but not of genes in the alpha or beta clusters, increased dramatically cIN BAX-dependent cell death in mice. Surprisingly, electrophysiological and morphological properties of Pcdhg-deficient and wild-type cINs during the period of cIN cell death were indistinguishable. Co-transplantation of wild-type with Pcdhg-deficient interneuron precursors further reduced mutant cIN survival, but the proportion of mutant and wild-type cells undergoing cell death was not affected by their density. Transplantation also allowed us to test for the contribution of Pcdhg isoforms to the regulation of cIN cell death. We conclude that Pcdhg, specifically Pcdhgc3, Pcdhgc4, and Pcdhgc5, play a critical role in regulating cIN survival during the endogenous period of programmed cIN death.