Brainstem serotonin neurons selectively gate retinal information flow to thalamus

Dynamic neural activity alterations and associated neurotransmitter profiles in children with monocular amblyopia

PURPOSE

To investigate the alterations in dynamic neural activity in the visual cortex in children with monocular amblyopia and its correlation with neurotransmitter profiles.

METHODS

A total of 61 participants with amblyopia (thirty right eye amblyopia, thirty-one left eye amblyopia) and 30 individuals with normal vision were recruited. The dynamic functional changes in brain areas, dynamic amplitude of low‑frequency fluctuations (dALFF) serves as the primary index. Spatial correlation between dALFF changes and neurotransmitter imaging was also examined.

RESULTS

Abnormal dALFF changes in the left and right lingual gyrus, the left and right middle occipital gyrus, the right calcarine fissure and surrounding cortex, and the left cuneus. The dALFF variation in lingual and middle occipital gyrus are negatively correlated with visual acuity and best-corrected visual acuity. Furthermore, cross-modal correlations displayed that there was a significant correlation between intergroup functional effect map and neurotransmitter densities.

CONCLUSION

The results of this study reveal dynamic alterations in neural activity within the visual brain regions, along with corresponding neurotransmitter levels, providing insights into the neural basis of monocular amblyopia.

Transcriptomics of Various Diseases Reveals the Core Role of Immune System Pathways in Retinal Damage Repair and Nerve Regeneration

Retinal ganglion cells (RGCs) are the only neuronal bridges connecting retinal inputs to the brain's visual processing centers, enabling visual perception. The axon of RGCs forms the optic nerve, which transmits visual information to the visual cortex. Damage to RGCs and their axons results in irreversible visual impairment. Acute retinal damage is commonly induced by conditions such as optic nerve compression, glaucoma, and optic neuritis, for which effective clinical treatments are currently unavailable. Therefore, understanding the response of RGCs and their axons to injury is crucial for the development of potential treatments. This study utilizes multiple models including optic nerve crush (ONC), acute intraocular pressure (IOP) elevation, and local lipopolysaccharide (LPS) injection into the optic nerve to mimic eye diseases. Three days post-surgery, mice underwent retinal isolation followed by bulk-RNA sequencing to analyze differential gene expression among models. Using thresholds of |Log2 fold change (FC)|> 2 and p-value < 0.05, the significant gene expression changes observed in each model were as follows: ONC (upregulated, 456; downregulated, 84), IOP (upregulated, 1946; downregulated, 655), and LPS (upregulated, 219; downregulated, 94). Gene ontology (GO) analysis of the upregulated genes unexpectedly revealed that immune system pathways were the primary shared targets across all three models. In contrast, the downregulated genes exhibited model-specific enrichment: synaptic components and functions in IOP, neurogenesis and neuronal development in ONC, and inflammation and antioxidant in LPS. These findings were further confirmed by Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. This suggests that managing immune activation is essential for treating acute retinal injury, and therapeutic strategies should address model-specific targets as well. Notably, 39 genes intersected across the models, and the protein-protein interaction (PPI) network identified Ccl5 as a key hub gene, underscoring its critical role in the pathophysiology of all three diseases.

Serotonin regulates in a cell-type specific manner light-evoked response and synaptic activity in mouse retinal ganglion cells

BACKGROUND

Serotonin (5-HT) is known to be synthesized and accumulated in the vertebrate retina through the 5-HT transporter, SERT. While manipulation of the serotonergic system has been shown to impact visual processing, the role of 5-HT and SERT as modulators of retinal synaptic function remains poorly understood.

RESULTS

Using mouse retinal slices, we show that acute application of 5-HT produces a cell-type specific reduction in light-evoked excitatory responses (L-EPSC) in ON-OFF retinal ganglion cells (RGCs), but not in ON RGCs. Similarly, increasing 5-HT tone by acute application of citalopram, a selective 5-HT reuptake inhibitor, also reduces L-EPSC in ON-OFF RGCs while not affecting ON RGCs. Importantly, citalopram-mediated reduction of L-EPSC was absent in ON-OFF RGCs recorded from SERT null retina, highlighting the role of SERT in regulating light-evoked responses in RGCs. The effects of both exogenous and endogenous 5-HT on L-EPSC in ON-OFF RGCs are likely due to a presynaptic reduction in excitatory synaptic strength as 5-HT and citalopram reduced the frequency but not the amplitude of spontaneous excitatory currents (sEPSCs) in ON-OFF RGCs. Moreover, 5-HT and citalopram had no effect on currents elicited by the direct activation of postsynaptic receptors in RGCs by brief application of glutamate in the inner retina.

CONCLUSIONS

Altogether these findings indicate that 5-HT modulates excitatory inputs onto RGCs in a cell-type specific manner and highlight that in the adult mouse retina, 5-HT-mediated effects onto RGCs are tightly controlled by the 5-HT transporter SERT.

Simultaneous, real-time tracking of many neuromodulatory signals with Multiplexed Optical Recording of Sensors on a micro-Endoscope

Dozens of extracellular molecules jointly impact a given neuron, yet we lack methods to simultaneously record many such signals in real time. We developed a probe to track ten or more neuropeptides and neuromodulators using spatial multiplexing of genetically encoded fluorescent sensors. Cultured cells expressing one sensor at a time are immobilized at the front of a gradient refractive index (GRIN) lens for 3D two-photon imaging in vitro and in vivo . The sensor identity and detection sensitivity of each cell are determined via robotic dipping of the probe into wells containing various ligands and concentrations. Using this probe, we detected stimulation-evoked release of multiple neuromodulators in acute brain slices. We also tracked endogenous and drug-evoked changes in cerebrospinal fluid composition in the awake mouse lateral ventricle, which triggered downstream activation of the choroid plexus epithelium. Our approach offers a first step towards quantitative, real-time, high-dimensional tracking of brain fluid composition.

Visual activity enhances neuronal excitability in thalamic relay neurons

Amblyopia, a highly prevalent loss of visual acuity, is classically thought to result from cortical plasticity. The dorsal lateral geniculate nucleus (dLGN) has long been held to act as a passive relay for visual information, but recent findings suggest a largely underestimated functional plasticity in the dLGN. However, the cellular mechanisms supporting this plasticity have not yet been explored. We show here that monocular deprivation (MD), an experimental model of amblyopia, reduces the intrinsic excitability of dLGN cells. Furthermore, dLGN neurons exhibit long-term potentiation of their intrinsic excitability (LTP-IE) when suprathreshold afferent retinal inputs are stimulated at 40 hertz or when spikes are induced with current injection. LTP-IE is observed after eye opening, requires calcium influx, is expressed through the down-regulation of Kv1 channels, and is altered following MD. In conclusion, our study provides the first evidence for intrinsic plasticity in dLGN neurons induced by natural stimuli.

Coordination of distinct sources of excitatory inputs enhances motion selectivity in the mouse visual thalamus

Multiple sources innervate the visual thalamus to influence image-forming vision prior to the cortex, yet it remains unclear how non-retinal and retinal input coordinate to shape thalamic visual selectivity. Using dual-color two-photon calcium imaging in the thalamus of awake mice, we observed similar coarse-scale retinotopic organization between axons of superior colliculus neurons and retinal ganglion cells, both providing strong converging excitatory input to thalamic neurons. At a fine scale of ∼10 µm, collicular boutons often shared visual feature preferences with nearby retinal boutons. Inhibiting collicular input significantly suppressed visual responses in thalamic neurons and specifically reduced motion selectivity in neurons preferring nasal-to-temporal motion. The reduction in motion selectivity could be the result of silencing sharply tuned direction-selective colliculogeniculate input. These findings suggest that the thalamus is not merely a relay but selectively integrates inputs from multiple regions to build stimulus selectivity and shape the information transmitted to the cortex.

HIGHLIGHTS

Chronic dual-color calcium imaging reveals diverse visual tuning of collicular axonal boutons.Nearby collicular and retinal boutons often share feature preferences at ∼10 µm scaleSilencing of collicular input suppresses visual responses in the majority of thalamic neurons.Silencing of collicular input reduces motion selectivity in thalamic neurons.

Long-term optical imaging of the spinal cord in awake behaving mice

Advances in optical imaging and fluorescent biosensors enable study of the spatiotemporal and long-term neural dynamics in the brain of awake animals. However, methodological difficulties and fibrosis limit similar advances in the spinal cord. Here, to overcome these obstacles, we combined in vivo application of fluoropolymer membranes that inhibit fibrosis, a redesigned implantable spinal imaging chamber and improved motion correction methods that together permit imaging of the spinal cord in awake behaving mice, for months to over a year. We demonstrated a robust ability to monitor axons, identified a spinal cord somatotopic map, performed months-long imaging in freely moving mice, conducted Ca2+ imaging of neural dynamics in behaving mice responding to pain-provoking stimuli and observed persistent microglial changes after nerve injury. The ability to couple in vivo imaging and behavior at the spinal cord level will drive insights not previously possible at a key location for somatosensory transmission to the brain.

Neurobehavioral meaning of pupil size

Pupil size is a widely used metric of brain state. It is one of the few signals originating from the brain that can be readily monitored with low-cost devices in basic science, clinical, and home settings. It is, therefore, important to investigate and generate well-defined theories related to specific interpretations of this metric. What exactly does it tell us about the brain? Pupils constrict in response to light and dilate during darkness, but the brain also controls pupil size irrespective of luminosity. Pupil size fluctuations resulting from ongoing "brain states" are used as a metric of arousal, but what is pupil-linked arousal and how should it be interpreted in neural, cognitive, and computational terms? Here, we discuss some recent findings related to these issues. We identify open questions and propose how to answer them through a combination of well-defined tasks, neurocomputational models, and neurophysiological probing of the interconnected loops of causes and consequences of pupil size.

Spiking activity in the visual thalamus is coupled to pupil dynamics across temporal scales

The processing of sensory information, even at early stages, is influenced by the internal state of the animal. Internal states, such as arousal, are often characterized by relating neural activity to a single "level" of arousal, defined by a behavioral indicator such as pupil size. In this study, we expand the understanding of arousal-related modulations in sensory systems by uncovering multiple timescales of pupil dynamics and their relationship to neural activity. Specifically, we observed a robust coupling between spiking activity in the mouse dorsolateral geniculate nucleus (dLGN) of the thalamus and pupil dynamics across timescales spanning a few seconds to several minutes. Throughout all these timescales, 2 distinct spiking modes-individual tonic spikes and tightly clustered bursts of spikes-preferred opposite phases of pupil dynamics. This multi-scale coupling reveals modulations distinct from those captured by pupil size per se, locomotion, and eye movements. Furthermore, coupling persisted even during viewing of a naturalistic movie, where it contributed to differences in the encoding of visual information. We conclude that dLGN spiking activity is under the simultaneous influence of multiple arousal-related processes associated with pupil dynamics occurring over a broad range of timescales.

Activity-induced MeCP2 phosphorylation regulates retinogeniculate synapse refinement

Mutations in MECP2 give rise to Rett syndrome (RTT), an X-linked neurodevelopmental disorder that results in broad cognitive impairments in females. While the exact etiology of RTT symptoms remains unknown, one possible explanation for its clinical presentation is that loss of MECP2 causes miswiring of neural circuits due to defects in the brain's capacity to respond to changes in neuronal activity and sensory experience. Here, we show that MeCP2 is phosphorylated at four residues in the mouse brain (S86, S274, T308, and S421) in response to neuronal activity, and we generate a quadruple knock-in (QKI) mouse line in which all four activity-dependent sites are mutated to alanines to prevent phosphorylation. QKI mice do not display overt RTT phenotypes or detectable gene expression changes in two brain regions. However, electrophysiological recordings from the retinogeniculate synapse of QKI mice reveal that while synapse elimination is initially normal at P14, it is significantly compromised at P20. Notably, this phenotype is distinct from the synapse refinement defect previously reported for Mecp2 null mice, where synapses initially refine but then regress after the third postnatal week. We thus propose a model in which activity-induced phosphorylation of MeCP2 is critical for the proper timing of retinogeniculate synapse maturation specifically during the early postnatal period.

Long-term optical imaging of the spinal cord in awake, behaving animals

Advances in optical imaging approaches and fluorescent biosensors have enabled an understanding of the spatiotemporal and long-term neural dynamics in the brain of awake animals. However, methodological difficulties and the persistence of post-laminectomy fibrosis have greatly limited similar advances in the spinal cord. To overcome these technical obstacles, we combined in vivo application of fluoropolymer membranes that inhibit fibrosis; a redesigned, cost-effective implantable spinal imaging chamber; and improved motion correction methods that together permit imaging of the spinal cord in awake, behaving mice, for months to over a year. We also demonstrate a robust ability to monitor axons, identify a spinal cord somatotopic map, conduct Ca2+ imaging of neural dynamics in behaving animals responding to pain-provoking stimuli, and observe persistent microglial changes after nerve injury. The ability to couple neural activity and behavior at the spinal cord level will drive insights not previously possible at a key location for somatosensory transmission to the brain.

Serotonin has an eye for detail

In this issue of Neuron, Reggiani et al.¹ show that serotonin and arousal suppress retinal inputs to the thalamus with opposing feature sensitivity, providing an elegant means for neuromodulation to selectively filter early visual processing.