Intravitreal Interleukin-2 modifies retinal excitatory circuits and retinocollicular innervation.

Interleukin-2 is a classical immune cytokine whose neural functions have received little attention. Its levels have been found to be increased in some neuropathologies, such as Alzheimer's disease, multiple sclerosis and uveitis. Mechanistically, it has been demonstrated the role of IL-2 in regulating glutamate and acetylcholine transmission, thus being relevant for CNS physiology. In fact, our previous work showed that an acute intravitreal IL-2 injection during retinotectal development promoted contralateral eye axonal plasticity in the superior colliculus, but the involved mechanisms were not explored. So, our present study aimed to investigate the effect of increased intravitreal IL-2 levels on the retinal glutamatergic and cholinergic signalling required for retinotectal normal development. We showed through HRP neuronal tracing that intravitreal IL-2 also induces ipsilateral eye axonal sprouting. Protein level and/or immunolocalization analysis in the retina confirmed IL-2 pathway activation by increased expression of phospho-STAT-3, coupled to transient (24h) reduced levels of Egr1, PSD-95 and nicotinic acetylcholine receptor ß2 subunit, suggesting reduced neural activity and synaptic sites. Also, AChE activity and GluN2B and GluA2 contents were reduced within 96h after IL-2 treatment. Therefore, IL-2-induced retinotectal plasticity might be driven by changes in cholinergic and glutamatergic pathways of the retina.

Short-term hypoxia induces bidirectional pathological long-term plasticity of neurotransmission in visual retinocollicular pathway.

Using the paired patch-clamp technique, we studied the effects of short-term hypoxia on retinocollicular synaptic transmission in an originally-developed coculture of dissociated retinal cells and superficial superior colliculus (SSC) neurons. Pharmacologically isolated N-methyl-D-aspartate receptor (NMDA)-, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA)- and gamma-aminobutyric acid receptor (GABAA)-mediated postsynaptic currents (PSCs) were evoked in SSC neurons by generation action potentials in presynaptic retinal ganglion cells. Spontaneous and miniature PSCs were recorded in SSC neurons in the absence of presynaptic stimulation. Short-term (up to 5 min) hypoxia induced long-term potentiation of NMDA transmission, long-term depression of GABAA neurotransmission and temporary suppression of AMPA transmission. Also, we observed hypoxia-induced reduction of voltage-dependent magnesium blockade of evoked NMDA response. Evoked, spontaneous and miniature postsynaptic currents were analyzed in terms of a binomial model. This analysis revealed that hypoxia acts mainly presynaptically on excitatory neurotransmission and both pre‒ and postsynaptically on inhibitory retinocollicular transmission. Thus, we showed for the first time hypoxia-induced bidirectional long-term plasticity of the retinocollicular synaptic transmission. The results obtained reflect the electrophysiological basis of hypoxia-involved pathological lesion of the retinocollicular pathway.

Activity dependent mechanisms of visual map formation--from retinal waves to molecular regulators.

The refinement of neural connections requires activity-dependent mechanisms in addition to the genetic program initially establishing wiring diagrams. The well-understood organization of the visual system makes it an accessible model for analyzing the contribution of activity in the formation of connectivity. Prior to visual experience, patterned spontaneous activity in the form of retinal waves has an important role for the establishment of eye-specific and retinotopic maps by acting on the refinement of axon arborization. In the present review, which focuses on experimental data obtained in mice and ferrets, we highlight the features of retinal activity that are important for visual map formation and question whether synaptic release and Hebbian based competition rules apply to this system. Recent evidence using genetic tools that allowed the manipulation of different features of neural activity have clarified the controversy on whether activity is instructive or permissive for visual map formation. Furthermore, current evidence strongly suggests that different mechanisms are at play for different types of axons (ipsilateral vs. contralateral), maps (eye-specific vs. retinotopic) or targets. Many molecules that either modulate activity or are modulated by activity are important in the formation of the visual map, such as adenylate cyclase 1, serotonin, or molecules from the immune system. Finally, new players in the game include retrograde messengers signaling from the target cell to the retinal axons as well as microglia that could help to eliminate inappropriate synapses.

Multiple EphB receptors mediate dorsal-ventral retinotopic mapping via similar bi-functional responses to ephrin-B1.

The projection from the retina to the superior colliculus in mice is organized in a retinotopic map that develops through the formation and guidance of interstitial branches extended by retinal ganglion cell axons. Bidirectional branch guidance along the lateral-medial collicular axis is critical to mapping the dorsal-ventral retinal axis. EphB receptor tyrosine kinases expressed in an overall low to high dorsal-ventral retinal gradient have been implicated in this mapping in response to the graded low to high lateral-medial expression of a ligand, ephrin-B1, in the superior colliculus. However, the relative contributions of EphBs and ephrin-B1 are not well understood. We examined EphB1, EphB2, and EphB3 mutant mice and find that each has ectopic arborizations of retinal axon branches lateral to their appropriate termination zone, with no qualitative differences in aberrant mapping, suggesting a similar role for each EphB. However, the frequency of cases with map defects progressively rises in compound EphB mutants coincident with the number of EphB null alleles from one to five of the six total alleles indicating that EphB level is critical. We analyzed branch extension in vitro and find that dorsal branches, with low EphB levels, exhibit a negative response to ephrin-B1, whereas ventral branches, with high EphB levels, exhibit a positive response to ephrin-B1. Using EphB mutant retina, we show that both of these differential branch extension responses are dependent on EphB level. Our findings show a bifunctional action of ephrin-B1 regulated by EphB levels that can account for the bidirectional extension of interstitial branches required to establish a retinotopic map.

Protein kinase C mediates hypoxia-induced long-term potentiation of NMDA neurotransmission in the visual retinocollicular pathway.

The identification of processes and mechanisms underlying the early stage of hypoxic injury of the retinocollicular pathway may be beneficial for the future prevention and treatment of navigation, orientation, and visual attention impairments. Previously, we have demonstrated that short-term hypoxia led to long-term potentiation (LTP) of NMDA neurotransmission in the background of long-term depression of GABAA retinocollicular transmission. Here, we sought to obtain insight into the mechanisms of hypoxia-induced LTP of NMDA retinocollicular neurotransmission and the role of the protein kinase C (PKC) signaling pathway in it. To investigate these, we recorded pharmacologically isolated NMDA transmission in cocultivated pairs of rat retinal ganglion cells and superficial superior colliculus neurons under normoxic and hypoxic conditions, using the paired patch-clamp technique and method of fast local superfusion. We tested the involvement of the PKC by adding the potent and selective inhibitor chelerythrine chloride (ChC, 5 µM). We observed that hypoxia-induced LTP of NMDA neurotransmission is associated with the shortening of current kinetics. We also found that the PKC signaling pathway mediates hypoxia-induced LTP and associated shortening of NMDA currents. The ChC completely blocked the induction of LTP by hypoxia and associated kinetic changes. Contrary effects of ChC were observed with already induced LTP. ChC led to the reversal of LTP to the initial synaptic strength but the current kinetics remain irreversibly shortened. Our results show that ChC is a promising agent for the prevention and treatment of hypoxic injuries of NMDA retinocollicular neurotransmission and provide necessary electrophysiological basics for further research.

NMDA Receptor Expression by Retinal Ganglion Cells Is Not Required for Retinofugal Map Formation nor Eye-Specific Segregation in the Mouse.

Retinal ganglion cells (RGCs) project topographically to the superior colliculus (SC) and dorsal lateral geniculate nucleus (dLGN). Spontaneous activity plays a critical role in retinotopic mapping in both regions; however, the molecular mechanisms underlying activity-dependent refinement remain unclear. Previous pharmacologic studies implicate NMDA receptors (NMDARs) in the establishment of retinotopy. In other brain regions, NMDARs are expressed on both the presynaptic and postsynaptic side of the synapse, and recent work suggests that presynaptic and postsynaptic NMDARs play distinct roles in retinotectal developmental dynamics. To directly test the role of NMDARs expressed by RGCs in retinofugal map formation, we took a conditional genetic knock-out approach to delete the obligate GluN1 subunit of NMDARs in RGCs. Here, we demonstrate reduced GluN1 expression in the retina of Chrnb3-Cre;GluN1flox/flox (pre-cKO) mice without altered expression in the SC. Anatomical tracing experiments revealed no significant changes in termination zone size in the SC and dLGN of pre-cKO mice, suggesting NMDAR function in RGCs is not an absolute requirement for topographic refinement. Further, we observed no change in the eye-specific organization of retinal inputs to the SC nor dLGN. To verify that NMDA induces activity in RGC terminals, we restricted GCaMP5 expression to RGCs and confirmed induction of calcium transients in RGC terminals. Together, these findings demonstrate that NMDARs expressed by RGCs are not required for retinofugal topographic map formation nor eye-specific segregation in the mouse.

Wiring subcortical image-forming centers: Topography, laminar targeting, and map alignment.

Efficient sensory processing is a complex and important function for species survival. As such, sensory circuits are highly organized to facilitate rapid detection of salient stimuli and initiate motor responses. For decades, the retina's projections to image-forming centers have served as useful models to elucidate the mechanisms by which such exquisite circuitry is wired. In this chapter, we review the roles of molecular cues, neuronal activity, and axon-axon competition in the development of topographically ordered retinal ganglion cell (RGC) projections to the superior colliculus (SC) and dorsal lateral geniculate nucleus (dLGN). Further, we discuss our current state of understanding regarding the laminar-specific targeting of subclasses of RGCs in the SC and its homolog, the optic tectum (OT). Finally, we cover recent studies examining the alignment of projections from primary visual cortex with RGCs that monitor the same region of space in the SC.

Rapid and coarse face detection: With a lack of evidence for a nasal-temporal asymmetry.

Humans have structures dedicated to the processing of faces, which include cortical components (e.g., areas in occipital and temporal lobes) and subcortical components (e.g., superior colliculus and amygdala). Although faces are processed more quickly than stimuli from other categories, there is a lack of consensus regarding whether subcortical structures are responsible for rapid face processing. In order to probe this, we exploited the asymmetry in the strength of projections to subcortical structures between the nasal and temporal hemiretina. Participants detected faces from unrecognizable control stimuli and performed the same task for houses. In Experiments 1 and 3, at the fastest reaction times, participants detected faces more accurately than houses. However, there was no benefit of presenting to the subcortical pathway. In Experiment 2, we probed the coarseness of the rapid pathway, making the foil stimuli more similar to faces and houses. This eliminated the rapid detection advantage, suggesting that rapid face processing is limited to coarse representations. In Experiment 4, we sought to determine whether the natural difference between spatial frequencies of faces and houses were driving the effects seen in Experiments 1 and 3. We spatially filtered the faces and houses so that they were matched. Better rapid detection was again found for faces relative to houses, but we found no benefit of preferentially presenting to the subcortical pathway. Taken together, the results of our experiments suggest a coarse rapid detection mechanism, which was not dependent on spatial frequency, with no advantage for presenting preferentially to subcortical structures.

Quantitative assessment of computational models for retinotopic map formation.

Molecular and activity-based cues acting together are thought to guide retinal axons to their terminal sites in vertebrate optic tectum or superior colliculus (SC) to form an ordered map of connections. The details of mechanisms involved, and the degree to which they might interact, are still not well understood. We have developed a framework within which existing computational models can be assessed in an unbiased and quantitative manner against a set of experimental data curated from the mouse retinocollicular system. Our framework facilitates comparison between models, testing new models against known phenotypes and simulating new phenotypes in existing models. We have used this framework to assess four representative models that combine Eph/ephrin gradients and/or activity-based mechanisms and competition. Two of the models were updated from their original form to fit into our framework. The models were tested against five different phenotypes: wild type, Isl2-EphA3(ki/ki), Isl2-EphA3(ki/+), ephrin-A2,A3,A5 triple knock-out (TKO), and Math5(-/-) (Atoh7). Two models successfully reproduced the extent of the Math5(-/-) anteromedial projection, but only one of those could account for the collapse point in Isl2-EphA3(ki/+). The models needed a weak anteroposterior gradient in the SC to reproduce the residual order in the ephrin-A2,A3,A5 TKO phenotype, suggesting either an incomplete knock-out or the presence of another guidance molecule. Our article demonstrates the importance of testing retinotopic models against as full a range of phenotypes as possible, and we have made available MATLAB software, we wrote to facilitate this process.

Optimal axonal and dendritic branching strategies during the development of neural circuitry.

In developing brain, axons and dendrites are capable of connecting to each other with high precision. Imaging of axonal and dendritic dynamics in vivo shows that the majority of axonal and dendritic branches are formed 'in error', only to be retracted later. The functional significance of the overproduction of branches is not clear. Here we show that branching of both axons and dendrites can accelerate finding appropriate synaptic targets during the development of neuronal circuitry. We suggest that branching rules implemented by axons and dendrites minimize the number of erroneous branches. We find that optimal branching rules are different for axons and dendrites in agreement with experimentally observed branch dynamics. Thus, our studies suggest that the developing neural system employs a set of sophisticated computational strategies that facilitate the formation of required circuitry in the fastest and most frugal way.