A population of gap junction-coupled neurons drives recurrent network activity in a developing visual circuit

Intrinsic temporal tuning of neurons in the optic tectum is shaped by multisensory experience

For a biological neural network to be functional, its neurons need to be connected with synapses of appropriate strength, and each neuron needs to appropriately respond to its synaptic inputs. This second aspect of network tuning is maintained by intrinsic plasticity; yet it is often considered secondary to changes in connectivity and mostly limited to adjustments of overall excitability of each neuron. Here we argue that even nonoscillatory neurons can be tuned to inputs of different temporal dynamics and that they can routinely adjust this tuning to match the statistics of their synaptic activation. Using the dynamic clamp technique, we show that, in the tectum of Xenopus tadpole, neurons become selective for faster inputs when animals are exposed to fast visual stimuli but remain responsive to longer inputs in animals exposed to slower, looming, or multisensory stimulation. We also report a homeostatic cotuning between synaptic and intrinsic temporal properties of individual tectal cells. These results expand our understanding of intrinsic plasticity in the brain and suggest that there may exist an additional dimension of network tuning that has been so far overlooked.NEW & NOTEWORTHY We use dynamic clamp to show that individual neurons in the tectum of Xenopus tadpoles are selectively tuned to either shorter (more synchronous) or longer (less synchronous) synaptic inputs. We also demonstrate that this intrinsic temporal tuning is strongly shaped by sensory experiences. This new phenomenon, which is likely to be mediated by changes in sodium channel inactivation, is bound to have important consequences for signal processing and the development of local recurrent connections.

Presenilin Regulates Retinotectal Synapse Formation through EphB2 Receptor Processing

As the catalytic component of γ-secretase, presenilin (PS) has long been studied in the context of Alzheimer's disease through cleaving the amyloid precursor protein. PS/γ-secretase, however, also cleaves a multitude of single-pass transmembrane proteins that are important during development, including Notch, the netrin receptor DCC, cadherins, drebrin-A, and the EphB2 receptor. Because transgenic PS-KO mice do not survive to birth, studies of this molecule during later embryonic or early postnatal stages of development have been carried out using cell cultures or conditional knock-out mice, respectively. As a result, the function of PS in synapse formation had not been well-addressed. Here, we study the role of PS in the developing Xenopus tadpole retinotectal circuit, an in-vivo model that allows for protein expression to be manipulated specifically during the peak of synapse formation between retinal ganglion cells and tectal neurons. We found that inhibiting PS in the postsynaptic tectal neurons impaired tadpole visual avoidance behavior. Whole cell recordings indicated weaker retinotectal synaptic transmission which was characterized by significant reductions in both NMDA receptor (NMDAR)- and AMPA receptor (AMPAR)-mediated currents. We also found that expression of the C-tail fragment of the EphB2 receptor, which is normally cleaved by PS/γ-secretase and which has been shown to upregulate NMDARs at the synapse, rescued the reduced NMDAR-mediated responses. Our data determine that normal PS function is important for proper formation and strengthening of retinotectal synapses through cleaving the EphB2 receptor.

Emergence of Selectivity to Looming Stimuli in a Spiking Network Model of the Optic Tectum

The neural circuits in the optic tectum of Xenopus tadpoles are selectively responsive to looming visual stimuli that resemble objects approaching the animal at a collision trajectory. This selectivity is required for adaptive collision avoidance behavior in this species, but its underlying mechanisms are not known. In particular, it is still unclear how the balance between the recurrent spontaneous network activity and the newly arriving sensory flow is set in this structure, and to what degree this balance is important for collision detection. Also, despite the clear indication for the presence of strong recurrent excitation and spontaneous activity, the exact topology of recurrent feedback circuits in the tectum remains elusive. In this study we take advantage of recently published detailed cell-level data from tadpole tectum to build an informed computational model of it, and investigate whether dynamic activation in excitatory recurrent retinotopic networks may on its own underlie collision detection. We consider several possible recurrent connectivity configurations and compare their performance for collision detection under different levels of spontaneous neural activity. We show that even in the absence of inhibition, a retinotopic network of quickly inactivating spiking neurons is naturally selective for looming stimuli, but this selectivity is not robust to neuronal noise, and is sensitive to the balance between direct and recurrent inputs. We also describe how homeostatic modulation of intrinsic properties of individual tectal cells can change selectivity thresholds in this network, and qualitatively verify our predictions in a behavioral experiment in freely swimming tadpoles.

Fragile X mental retardation protein knockdown in the developing Xenopus tadpole optic tectum results in enhanced feedforward inhibition and behavioral deficits

BACKGROUND

Fragile X Syndrome is the leading monogenetic cause of autism and most common form of intellectual disability. Previous studies have implicated changes in dendritic spine architecture as the primary result of loss of Fragile X Mental Retardation Protein (FMRP), but recent work has shown that neural proliferation is decreased and cell death is increased with either loss of FMRP or overexpression of FMRP. The purpose of this study was to investigate the effects of loss of FMRP on behavior and cellular activity.

METHODS

We knocked down FMRP expression using morpholino oligos in the optic tectum of Xenopus laevis tadpoles and performed a series of behavioral and electrophysiological assays. We investigated visually guided collision avoidance, schooling, and seizure propensity. Using single cell electrophysiology, we assessed intrinsic excitability and synaptic connectivity of tectal neurons.

RESULTS

We found that FMRP knockdown results in decreased swimming speed, reduced schooling behavior and decreased seizure severity. In single cells, we found increased inhibition relative to excitation in response to sensory input.

CONCLUSIONS

Our results indicate that the electrophysiological development of single cells in the absence of FMRP is largely unaffected despite the large neural proliferation defect. The changes in behavior are consistent with an increase in inhibition, which could be due to either changes in cell number or altered inhibitory drive, and indicate that FMRP can play a significant role in neural development much earlier than previously thought.

Early development and function of the Xenopus tadpole retinotectal circuit

The retinotectal circuit is the major component of the amphibian visual system. It is comprised of the retinal ganglion cells (RGCs) in the eye, which project their axons to the optic tectum and form synapses onto postsynaptic tectal neurons. The retinotectal circuit is relatively simple, and develops quickly: Xenopus tadpoles begin displaying retinotectal-dependent visual avoidance behaviors by approximately 7-8 days post-fertilization, early larval stage. In this review we first provide a summary of the dynamic development of the retinotectal circuit, including the microcircuitry formed by local tectal-tectal connections within the tectum. Second, we discuss the basic visual avoidance behavior generated specifically by this circuit, and how this behavior is being used as an assay to test visual system function.