Enhanced visual experience rehabilitates the injured brain in Xenopus tadpoles in an NMDAR-dependent manner.

Traumatic brain injuries introduce functional and structural circuit deficits that must be repaired for an organism to regain function. We developed an injury model in which Xenopus laevis tadpoles are given a penetrating stab wound that damages the optic tectal circuit and impairs visuomotor behavior. In tadpoles, as in other systems, injury induces neurogenesis. The newly generated neurons are thought to integrate into the existing circuit; however, whether they integrate via the same mechanisms that govern normal neuronal maturation during development is not understood. Development of the functional visuomotor circuit in Xenopus is driven by sensory activity. We hypothesized that enhanced visual experience would improve recovery from injury by facilitating integration of newly generated neurons into the tectal circuit. We labeled newly generated neurons in the injured tectum by green fluorescent protein expression and examined their circuit integration using electrophysiology and in vivo imaging. Providing animals with brief bouts of enhanced visual experience starting 24 h after injury increased synaptogenesis and circuit integration of new neurons and facilitated behavioral recovery. To investigate mechanisms of neuronal integration and behavioral recovery after injury, we interfered with N-methyl-d-aspartate (NMDA) receptor function. Ifenprodil, which blocks GluN2B-containing NMDA receptors, impaired dendritic arbor elaboration. GluN2B blockade inhibited functional integration of neurons generated in response to injury and prevented behavioral recovery. Furthermore, tectal GluN2B knockdown blocked the beneficial effects of enhanced visual experience on functional plasticity and behavioral recovery. We conclude that visual experience-mediated rehabilitation of the injured tectal circuit occurs by GluN2B-containing NMDA receptor-dependent integration of newly generated neurons. NEW & NOTEWORTHY Recovery from brain injury is difficult in most systems. The study of regenerative animal models that are capable of injury repair can provide insight into cellular and circuit mechanisms underlying repair. Using Xenopus tadpoles, we show enhanced sensory experience rehabilitates the injured visual circuit and that this experience-dependent recovery depends on N-methyl-d-aspartate receptor function. Understanding the mechanisms of rehabilitation in this system may facilitate recovery in brain regions and systems where repair is currently impossible.

Direct intertectal inputs are an integral component of the bilateral sensorimotor circuit for behavior in Xenopus tadpoles.

The circuit controlling visually guided behavior in nonmammalian vertebrates, such as Xenopus tadpoles, includes retinal projections to the contralateral optic tectum, where visual information is processed, and tectal motor outputs projecting ipsilaterally to hindbrain and spinal cord. Tadpoles have an intertectal commissure whose function is unknown, but it might transfer information between the tectal lobes. Differences in visual experience between the two eyes have profound effects on the development and function of visual circuits in animals with binocular vision, but the effects on animals with fully crossed retinal projections are not clear. We tested the effect of monocular visual experience on the visuomotor circuit in Xenopus tadpoles. We show that cutting the intertectal commissure or providing visual experience to one eye (monocular visual experience) is sufficient to disrupt tectally mediated visual avoidance behavior. Monocular visual experience induces asymmetry in tectal circuit activity across the midline. Repeated exposure to monocular visual experience drives maturation of the stimulated retinotectal synapses, seen as increased AMPA-to-NMDA ratios, induces synaptic plasticity in intertectal synaptic connections, and induces bilaterally asymmetric changes in the tectal excitation-to-inhibition ratio (E/I). We show that unilateral expression of peptides that interfere with AMPA or GABAA receptor trafficking alters E/I in the transfected tectum and is sufficient to degrade visuomotor behavior. Our study demonstrates that monocular visual experience in animals with fully crossed visual systems produces asymmetric circuit function across the midline and degrades visuomotor behavior. The data further suggest that intertectal inputs are an integral component of a bilateral visuomotor circuit critical for behavior. NEW & NOTEWORTHY The developing optic tectum of Xenopus tadpoles represents a unique circuit in which laterally positioned eyes provide sensory input to a circuit that is transiently monocular, but which will be binocular in the animal's adulthood. We challenge the idea that the two lobes of tadpole optic tectum function independently by testing the requirement of interhemispheric communication and demonstrate that unbalanced sensory input can induce structural and functional plasticity in the tectum sufficient to disrupt function.

Experience-dependent plasticity of excitatory and inhibitory intertectal inputs in Xenopus tadpoles.

Communication between optic tecta/superior colliculi is thought to be required for sensorimotor behaviors by comparing inputs across the midline, however the development of and the role of visual experience in the function and plasticity of intertectal connections are unclear. We combined neuronal tracing, in vivo time-lapse imaging, and electrophysiology to characterize the structural and functional development of intertectal axons and synapses in Xenopus tadpole optic tectum. We find that intertectal connections are established early during optic tectal circuit development. We determined the neurotransmitter identity of intertectal neurons using both rabies virus-mediated tracing combined with post-hoc immunohistochemistry, and electrophysiology. Excitatory and inhibitory intertectal neuronal somata are similarly distributed throughout the tectum. Excitatory and inhibitory intertectal axons are structurally similar and elaborate broadly in the contralateral tectum. We demonstrate that intertectal and retinotectal axons converge onto tectal neurons by recording postsynaptic currents after stimulating intertectal and retinotectal inputs. Cutting the intertectal commissure removes synaptic responses to contralateral tectal stimulation. In vivo time-lapse imaging demonstrated that visual experience drives plasticity in intertectal bouton size and dynamics. Finally, visual experience coordinately drives the maturation of excitatory and inhibitory intertectal inputs by increasing AMPA- and GABA-receptor mediated currents, comparable to experience-dependent maturation of retinotectal inputs. These data indicate that visual experience regulates plasticity of excitatory and inhibitory intertectal inputs, maintaining the excitatory: inhibitory ratio of intertectal input. These studies place intertectal inputs as key players in tectal circuit development and suggest that they may play a role in sensory information processing critical to sensorimotor behaviors.

NMDA receptor subunit composition controls synaptogenesis and synapse stabilization.

During early postnatal development in the rat hippocampus, synaptogenesis occurs in parallel with a developmental switch in the subunit composition of NMDA receptors from NR2B to NR2A. It is unclear how this switch affects the process of synaptogenesis, synapse maturation, and synapse stabilization. We investigated the role of NR2 subunits in synaptogenesis during the period in which expression and synaptic incorporation of the NR2A protein begins through the time when it reaches adult levels. We found that early expression of NR2A in organotypic hippocampal slices reduces the number of synapses and the volume and dynamics of spines. In contrast, overexpression of NR2B does not affect the normal number and growth of synapses; however, it does increase spine motility, adding and retracting spines at a higher rate. The C terminus of NR2B, and specifically its ability to bind CaMKII, is sufficient to allow proper synapse formation and maturation. Conversely, the C terminus of NR2A was sufficient to stop the development of synapse number and spine growth. Our results indicate that the ratio of synaptic NR2B over NR2A controls spine motility and synaptogenesis, and suggest a structural role for the intracellular C terminus of NR2 in recruiting the signaling and scaffolding molecules necessary for proper synaptogenesis.

Dynamic regulation of NMDA receptor transmission.

N-methyl-D-aspartate receptors (NMDARs) are critical for establishing, maintaining, and modifying glutamatergic synapses in an activity-dependent manner. The subunit composition, synaptic expression, and some of the properties of NMDARs are regulated by synaptic activity, affecting processes like synaptic plasticity. NMDAR transmission is dynamic, and we were interested in studying the effect of acute low or null synaptic activity on NMDA receptors and its implications for synaptic plasticity. Periods of no stimulation or low-frequency stimulation increased NMDAR transmission. Changes became stable after periods of 20 min of low or no stimulation. These changes in transmission have a postsynaptic origin and are explained by incorporation of GluN2B-containing receptors to synapses. Importantly, periods of low or no stimulation facilitate long-term potentiation induction. Moreover, recovery after a weak preconditioning stimulus that normally blocks subsequent potentiation is facilitated by a nonstimulation period. Thus synaptic activity dynamically regulates the level of NMDAR transmission adapting constantly the threshold for plasticity.