Retinoid therapy restores eye-specific cortical responses in adult mice with retinal degeneration.

Despite the recent emergence of multiple cellular and molecular strategies to restore vision in retinal disorders, it remains unclear to what extent central visual circuits can recover when retinal defects are corrected in adulthood. We addressed this question in an Lrat-/- mouse model of Leber congenital amaurosis (LCA) in which retinal light sensitivity and optomotor responses are partially restored by 9-cis-retinyl acetate administration in adulthood. Following treatment, two-photon calcium imaging revealed increases in the number and response amplitude of visually responsive neurons in the primary visual cortex (V1). In particular, retinoid treatment enhanced responses from the ipsilateral eye, restoring the normal balance of eye-specific responses in V1. Additionally, the treatment rescued the modulation of cortical responses by arousal. These findings illustrate the significant plasticity of the adult central visual system and underscore the therapeutic potential of retinoid administration for adults with retinal diseases.

Juvenile depletion of microglia reduces orientation but not high spatial frequency selectivity in mouse V1.

Microglia contain multiple mechanisms that shape the synaptic landscape during postnatal development. Whether the synaptic changes mediated by microglia reflect the developmental refinement of neuronal responses in sensory cortices, however, remains poorly understood. In postnatal life, the development of increased orientation and spatial frequency selectivity of neuronal responses in primary visual cortex (V1) supports the emergence of high visual acuity. Here, we used the colony-stimulating factor 1 receptor (CSF1R) inhibitor PLX5622 to rapidly and durably deplete microglia in mice during the juvenile period in which increased orientation and spatial frequency selectivity emerge. Excitatory and inhibitory tuning properties were measured simultaneously using multi-photon calcium imaging in layer II/III of mouse V1. We found that microglia depletion generally increased evoked activity which, in turn, reduced orientation selectivity. Surprisingly, microglia were not required for the emergence of high spatial frequency tuned responses. In addition, microglia depletion did not perturb cortical binocularity, suggesting normal depth processing. Together, our finding that orientation and high spatial frequency selectivity in V1 are differentially supported by microglia reveal that microglia are required normal sensory processing, albeit selectively.

Functional Differentiation of Mouse Visual Cortical Areas Depends upon Early Binocular Experience.

The mammalian visual cortex contains multiple retinotopically defined areas that process distinct features of the visual scene. Little is known about what guides the functional differentiation of visual cortical areas during development. Recent studies in mice have revealed that visual input from the two eyes provides spatiotemporally distinct signals to primary visual cortex (V1), such that contralateral eye-dominated V1 neurons respond to higher spatial frequencies than ipsilateral eye-dominated neurons. To test whether binocular visual input drives the differentiation of visual cortical areas, we used two-photon calcium imaging to characterize the effects of juvenile monocular deprivation (MD) on the responses of neurons in V1 and two higher visual areas, LM (lateromedial) and PM (posteromedial). In adult mice of either sex, we find that MD prevents the emergence of distinct spatiotemporal tuning in V1, LM, and PM. We also find that, within each of these areas, MD reorganizes the distinct spatiotemporal tuning properties driven by the two eyes. Moreover, we find a relationship between speed tuning and ocular dominance in all three areas that MD preferentially disrupts in V1, but not in LM or PM. Together, these results reveal that balanced binocular vision during development is essential for driving the functional differentiation of visual cortical areas. The higher visual areas of mouse visual cortex may provide a useful platform for investigating the experience-dependent mechanisms that set up the specialized processing within neocortical areas during postnatal development.SIGNIFICANCE STATEMENT Little is known about the factors guiding the emergence of functionally distinct areas in the brain. Using in vivo Ca2+ imaging, we recorded visually evoked activity from cells in V1 and higher visual areas LM (lateromedial) and PM (posteromedial) of mice. Neurons in these areas normally display distinct spatiotemporal tuning properties. We found that depriving one eye of normal input during development prevents the functional differentiation of visual areas. Deprivation did not disrupt the degree of speed tuning, a property thought to emerge in higher visual areas. Thus, some properties of visual cortical neurons are shaped by binocular experience, while others are resistant. Our study uncovers the fundamental role of binocular experience in the formation of distinct areas in visual cortex.

Long-term Monocular Deprivation during Juvenile Critical Period Disrupts Binocular Integration in Mouse Visual Thalamus.

Study of the neural deficits caused by mismatched binocular vision in early childhood has predominantly focused on circuits in the primary visual cortex (V1). Recent evidence has revealed that neurons in mouse dorsolateral geniculate nucleus (dLGN) can undergo rapid ocular dominance plasticity following monocular deprivation (MD). It remains unclear, however, whether the long-lasting deficits attributed to MD during the critical period originate in the thalamus. Using in vivo two-photon Ca2+ imaging of dLGN afferents in superficial layers of V1 in female and male mice, we demonstrate that 14 d MD during the critical period leads to a chronic loss of binocular dLGN inputs while sparing response strength and spatial acuity. Importantly, MD leads to profoundly mismatched visual tuning properties in remaining binocular dLGN afferents. Furthermore, MD impairs binocular modulation, reducing facilitation of responses of both binocular and monocular dLGN inputs during binocular viewing. As predicted by our findings in thalamic inputs, Ca2+ imaging from V1 neurons revealed spared spatial acuity but impaired binocularity in L4 neurons. V1 L2/3 neurons in contrast displayed deficits in both binocularity and spatial acuity. Our data demonstrate that critical-period MD produces long-lasting disruptions in binocular integration beginning in early binocular circuits in dLGN, whereas spatial acuity deficits first arise from circuits further downstream in V1. Our findings indicate that the development of normal binocular vision and spatial acuity depend upon experience-dependent refinement of distinct stages in the mammalian visual system.SIGNIFICANCE STATEMENT Abnormal binocular vision and reduced acuity are hallmarks of amblyopia, a disorder that affects 2%-5% of the population. It is widely thought that the neural deficits underlying amblyopia begin in the circuits of primary visual cortex. Using in vivo two-photon calcium imaging of thalamocortical axons in mice, we show that depriving one eye of input during a critical period in development chronically impairs binocular integration in thalamic inputs to primary visual cortex. In contrast, visual acuity is spared in thalamic inputs. These findings shed new light on the role for developmental mechanisms in the thalamus in establishing binocular vision and may have critical implications for amblyopia.

Feature-Specific Organization of Feedback Pathways in Mouse Visual Cortex.

Higher and lower cortical areas in the visual hierarchy are reciprocally connected [1]. Although much is known about how feedforward pathways shape receptive field properties of visual neurons, relatively little is known about the role of feedback pathways in visual processing. Feedback pathways are thought to carry top-down signals, including information about context (e.g., figure-ground segmentation and surround suppression) [2-5], and feedback has been demonstrated to sharpen orientation tuning of neurons in the primary visual cortex (V1) [6, 7]. However, the response characteristics of feedback neurons themselves and how feedback shapes V1 neurons' tuning for other features, such as spatial frequency (SF), remain largely unknown. Here, using a retrograde virus, targeted electrophysiological recordings, and optogenetic manipulations, we show that putatively feedback neurons in layer 5 (hereafter "L5 feedback") in higher visual areas, AL (anterolateral area) and PM (posteromedial area), display distinct visual properties in awake head-fixed mice. AL L5 feedback neurons prefer significantly lower SF (mean: 0.04 cycles per degree [cpd]) compared to PM L5 feedback neurons (0.15 cpd). Importantly, silencing AL L5 feedback reduced visual responses of V1 neurons preferring low SF (mean change in firing rate: -8.0%), whereas silencing PM L5 feedback suppressed responses of high-SF-preferring V1 neurons (-20.4%). These findings suggest that feedback connections from higher visual areas convey distinctly tuned visual inputs to V1 that serve to boost V1 neurons' responses to SF. Such like-to-like functional organization may represent an important feature of feedback pathways in sensory systems and in the nervous system in general.

Excitatory Inputs Determine Phase-Locking Strength and Spike-Timing of CA1 Stratum Oriens/Alveus Parvalbumin and Somatostatin Interneurons during Intrinsically Generated Hippocampal Theta Rhythm.

UNLABELLED: Theta oscillations are essential for learning and memory, and their generation requires GABAergic interneurons. To better understand how theta is generated, we explored how parvalbumin (PV) and somatostatin (SOM) interneurons in CA1 stratum oriens/alveus fire during hippocampal theta and investigated synaptic mechanisms underlying their behavior. Combining the use of transgenic mice to visually identify PV and SOM interneurons and the intact hippocampal preparation that can generate theta oscillations in vitro without cholinergic agonists, we performed simultaneous field and whole-cell recordings. We found that PV interneurons uniformly fire strongly phase-locked to theta, whereas SOM neurons are more heterogeneous with only a proportion of cells displaying tight phase-locking. Differences in phase-locking strength could be explained by disparity in excitatory inputs received; PV neurons received significantly larger EPSCs compared with SOM neurons, and the degree of phase-locking in SOM neurons was significantly correlated with the size of EPSCs. In contrast, IPSC amplitude did not differ between cell types. We determined that the local CA1 rhythm plays a more dominant role in driving CA1 interneuron firing than afferent inputs from the CA3. Last, we show that PV and strongly phase-locked SOM neurons fire near the peak of CA1 theta, under the tight control of excitatory inputs that arise at a specific phase of each theta cycle. These results reveal a fundamental mechanism of neuronal phase-locking and highlight an important role of excitation from the local network in governing firing behavior during rhythmic network states. SIGNIFICANCE STATEMENT: Rhythmic activity in the theta range (3-12 Hz) is important for proper functioning of the hippocampus, a brain area essential for learning and memory. To understand how theta rhythm is generated, we investigated how two types of inhibitory neurons, those that express parvalbumin and somatostatin, fire action potentials during theta in an in vitro preparation of the mouse hippocampus. We found that the amount of excitatory input they receive from the local network determines how closely their spikes follow the network theta rhythm. Our findings reveal an important role of local excitatory input in driving inhibitory neuron firing during rhythmic states and may have implications for diseases, such as epilepsy and Alzheimer's disease, which affect the hippocampus and related areas.

Network models provide insights into how oriens-lacunosum-moleculare and bistratified cell interactions influence the power of local hippocampal CA1 theta oscillations.

Hippocampal theta is a 4-12 Hz rhythm associated with episodic memory, and although it has been studied extensively, the cellular mechanisms underlying its generation are unclear. The complex interactions between different interneuron types, such as those between oriens-lacunosum-moleculare (OLM) interneurons and bistratified cells (BiCs), make their contribution to network rhythms difficult to determine experimentally. We created network models that are tied to experimental work at both cellular and network levels to explore how these interneuron interactions affect the power of local oscillations. Our cellular models were constrained with properties from patch clamp recordings in the CA1 region of an intact hippocampus preparation in vitro. Our network models are composed of three different types of interneurons: parvalbumin-positive (PV+) basket and axo-axonic cells (BC/AACs), PV+ BiCs, and somatostatin-positive OLM cells. Also included is a spatially extended pyramidal cell model to allow for a simplified local field potential representation, as well as experimentally-constrained, theta frequency synaptic inputs to the interneurons. The network size, connectivity, and synaptic properties were constrained with experimental data. To determine how the interactions between OLM cells and BiCs could affect local theta power, we explored how the number of OLM-BiC connections and connection strength affected local theta power. We found that our models operate in regimes that could be distinguished by whether OLM cells minimally or strongly affected the power of network theta oscillations due to balances that, respectively, allow compensatory effects or not. Inactivation of OLM cells could result in no change or even an increase in theta power. We predict that the dis-inhibitory effect of OLM cells to BiCs to pyramidal cell interactions plays a critical role in the resulting power of network theta oscillations. Overall, our network models reveal a dynamic interplay between different classes of interneurons in influencing local theta power.

Parvalbumin Interneurons of Hippocampus Tune Population Activity at Theta Frequency.

Hippocampal theta rhythm arises from a combination of recently described intrinsic theta oscillators and inputs from multiple brain areas. Interneurons expressing the markers parvalbumin (PV) and somatostatin (SOM) are leading candidates to participate in intrinsic rhythm generation and principal cell (PC) coordination in distal CA1 and subiculum. We tested their involvement by optogenetically activating and silencing PV or SOM interneurons in an intact hippocampus preparation that preserves intrinsic connections and oscillates spontaneously at theta frequencies. Despite evidence suggesting that SOM interneurons are crucial for theta, optogenetic manipulation of these interneurons modestly influenced theta rhythm. However, SOM interneurons were able to strongly modulate temporoammonic inputs. In contrast, activation of PV interneurons powerfully controlled PC network and rhythm generation optimally at 8 Hz, while continuously silencing them disrupted theta. Our results thus demonstrate a pivotal role of PV but not SOM interneurons for PC synchronization and the emergence of intrinsic hippocampal theta.

Simple, biologically-constrained CA1 pyramidal cell models using an intact, whole hippocampus context.

The hippocampus is a heavily studied brain structure due to its involvement in learning and memory. Detailed models of excitatory, pyramidal cells in hippocampus have been developed using a range of experimental data. These models have been used to help us understand, for example, the effects of synaptic integration and voltage gated channel densities and distributions on cellular responses. However, these cellular outputs need to be considered from the perspective of the networks in which they are embedded. Using modeling approaches, if cellular representations are too detailed, it quickly becomes computationally unwieldy to explore large network simulations. Thus, simple models are preferable, but at the same time they need to have a clear, experimental basis so as to allow physiologically based understandings to emerge. In this article, we describe the development of simple models of CA1 pyramidal cells, as derived in a well-defined experimental context of an intact, whole hippocampus preparation expressing population oscillations. These models are based on the intrinsic properties and frequency-current profiles of CA1 pyramidal cells, and can be used to build, fully examine, and analyze large networks.

Experimentally constrained CA1 fast-firing parvalbumin-positive interneuron network models exhibit sharp transitions into coherent high frequency rhythms.

The coupling of high frequency oscillations (HFOs; >100 Hz) and theta oscillations (3-12 Hz) in the CA1 region of rats increases during REM sleep, indicating that it may play a role in memory processing. However, it is unclear whether the CA1 region itself is capable of providing major contributions to the generation of HFOs, or if they are strictly driven through input projections. Parvalbumin-positive (PV+) interneurons may play an essential role in these oscillations due to their extensive connections with neighboring pyramidal cells, and their characteristic fast-spiking. Thus, we created mathematical network models to investigate the conditions under which networks of CA1 fast-spiking PV+ interneurons are capable of producing high frequency population rhythms. We used whole-cell patch clamp recordings of fast-spiking, PV+ cells in the CA1 region of an intact hippocampal preparation in vitro to derive cellular properties, from which we constrained an Izhikevich-type model. Novel, biologically constrained network models were constructed with these individual cell models, and we investigated networks across a range of experimentally determined excitatory inputs and inhibitory synaptic strengths. For each network, we determined network frequency and coherence. Network simulations produce coherent firing at high frequencies (>90 Hz) for parameter ranges in which PV-PV inhibitory synaptic conductances are necessarily small and external excitatory inputs are relatively large. Interestingly, our networks produce sharp transitions between random and coherent firing, and this sharpness is lost when connectivity is increased beyond biological estimates. Our work suggests that CA1 networks may be designed with mechanisms for quickly gating in and out of high frequency coherent population rhythms, which may be essential in the generation of nested theta/high frequency rhythms.

Glutamatergic neurons of the mouse medial septum and diagonal band of Broca synaptically drive hippocampal pyramidal cells: relevance for hippocampal theta rhythm.

Neurons of the medial septum and diagonal band of Broca (MS-DBB) provide an important input to the hippocampus and are critically involved in learning and memory. Although cholinergic and GABAergic MS-DBB neurons are known to modulate hippocampal activity, the role of recently described glutamatergic MS-DBB neurons is unknown. Here, we examined the electrophysiological properties of glutamatergic MS-DBB neurons and tested whether they provide a functional synaptic input to the hippocampus. To visualize the glutamatergic neurons, we used MS-DBB slices from transgenic mice in which the green fluorescent protein is expressed specifically by vesicular glutamate transporter 2-positive neurons and characterized their properties using whole-cell patch-clamp technique. For assessing the function of the glutamatergic projection, we used an in vitro septohippocampal preparation, electrically stimulated the fornix or chemically activated the MS-DBB using NMDA microinfusions and recorded postsynaptic responses in CA3 pyramidal cells. We found that glutamatergic MS-DBB neurons as a population display a highly heterogeneous set of firing patterns including fast-, cluster-, burst-, and slow-firing. Remarkably, a significant proportion exhibited fast-firing properties, prominent I(h), and rhythmic spontaneous firing at theta frequencies similar to those found in GABAergic MS-DBB neurons. Activation of the MS-DBB led to fast, AMPA receptor-mediated glutamatergic responses in CA3 pyramidal cells. These results describe for the first time the electrophysiological signatures of glutamatergic MS-DBB neurons, their rhythmic firing properties, and their capacity to drive hippocampal principal neurons. Our findings suggest that the glutamatergic septohippocampal pathway may play an important role in hippocampal theta oscillations and relevant cognitive functions.

Chronic exposure to nerve growth factor increases acetylcholine and glutamate release from cholinergic neurons of the rat medial septum and diagonal band of Broca via mechanisms mediated by p75NTR.

Basal forebrain neurons play an important role in memory and attention. In addition to cholinergic and GABAergic neurons, glutamatergic neurons and neurons that can corelease acetylcholine and glutamate have recently been described in the basal forebrain. Although it is well known that nerve growth factor (NGF) promotes synaptic function of cholinergic basal forebrain neurons, how NGF affects the newly identified basal forebrain neurons remains undetermined. Here, we examined the effects of NGF on synaptic transmission of medial septum and diagonal band of Broca (MS-DBB) neurons expressing different neurotransmitter phenotypes. We used MS-DBB neurons from 10- to 13-d-old rats, cultured on astrocytic microislands to promote the development of autaptic connections. Evoked and spontaneous postsynaptic currents were recorded, and neurotransmitters released were characterized pharmacologically. We found that chronic exposure to NGF significantly increased acetylcholine and glutamate release from cholinergic MS-DBB neurons, whereas glutamate and GABA transmission from noncholinergic MS-DBB neurons were not affected by NGF. Interestingly, the NGF-induced increase in neurotransmission was mediated by p75(NTR). These results demonstrate a previously unidentified role of NGF and its receptor p75(NTR); their interactions are crucial for cholinergic and glutamatergic transmission in the septohippocampal pathway.