Development of abnormal lamination and binocular segregation in the retinotectal pathways of the rat

Signaling pathways modulated by monocular enucleation in the superior colliculus of juvenile rats

Monocular eye enucleation (ME) is a classical paradigm to induce neural plasticity in retinal ganglion cells (RGCs) axons from the intact eye, especially when performed within the critical period of visual system development. However, the precise mechanisms underlying the axonal sprouting and synaptogenesis seen in this model remain poorly understood. In the present work, we investigated the temporal alterations in phosphorylation of three kinases related to axonal growth and synaptogenesis-GSK3β (an important repressor of axonal outgrowth), AKT, and ERK-in superior colliculus of rats submitted to ME during early postnatal development. Western blotting analysis showed an increase in pGSK3β, the inactive form of this enzyme, 24 and 48 hr after ME. Accordingly, an increase in pERK levels was detected 24 hr after ME, indicating that phosphorylation of these enzymes might be related to axonal reorganization induced by ME. Interestingly, AKT phosphorylation was increased just 1 week after ME, suggesting it may be involved in the stabilization of newly formed synapses, rising from the axonal reorganization of remaining eye. A better understanding of how signaling pathways are modulated in a model of intense axonal sprouting can highlight possible therapeutic targets in RGCs injuries in adult individuals, where axonal regrowth is nearly absent.

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 β2 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.

In vivo effect of acute exposure to interleukin-6 on the developing visual system

Interleukin-6 (IL-6) is involved in different processes of the central nervous system. Our aims were to investigate the effect of IL-6 on retinotectal topography and on different signaling pathways. Rats were submitted to an intravitreous injection of either IL-6 (50 ng/ml) or PBS (vehicle) at postnatal day 10 (PND10). At PND11 or PND14, different groups were processed for western blot, histochemistry or immunofluorescence analysis. IL-6 treatment leads to an increase in pSTAT-3 levels in the retina and a disruption in the retinotectal topographic map, suggesting that a transient increase in interleukin-6 levels may impact neural circuitry development.

Rapid plasticity of intact axons following a lesion to the visual pathways during early brain development is triggered by microglial activation

Lesions in the central nervous system (CNS) can often induce structural reorganization within intact circuits of the brain. Several studies show advances in the understanding of mechanisms of brain plasticity and the role of the immune system activation. Microglia, a myeloid derived cell population colonizes the CNS during early phases of embryonic development. In the present study, we evaluated the role of microglial activation in the sprouting of intact axons following lesions of the visual pathways. We evaluated the temporal course of microglial activation in the superior colliculus following a contralateral monocular enucleation (ME) and the possible involvement of microglial cells in the plastic reorganization of the intact, uncrossed, retinotectal pathway from the remaining eye. Lister Hooded rats were enucleated at PND 10 and submitted to systemic treatment with inhibitors of microglial activation: cyclosporine A and minocycline. The use of neuroanatomical tracers allowed us to evaluate the time course of structural axonal plasticity. Immunofluorescence and western blot techniques were used to observe the expression of microglial marker, Iba-1 and the morphology of microglial cells. Following a ME, Iba-1 immunoreactivity showed a progressive increase of microglial activation in the contralateral SC at 24 h, peaking at 72 h after the lesion. Treatment with inhibitors of microglial activation blocked both the structural plasticity of intact uncrossed retinotectal axons and microglial activation as seen by the decrease of Iba-1 immunoreactivity. The local blockade of TNF-α with a neutralizing antibody was also able to block axonal plasticity of the intact eye following a ME. The data support the hypothesis that microglial activation is a necessary step for the regulation of neuroplasticity induced by lesions during early brain development.

The fine tuning of retinocollicular topography depends on reelin signaling during early postnatal development of the rat visual system

During postnatal development, neural circuits are extremely dynamic and develop precise connection patterns that emerge as a result of the elimination of synaptic terminals, a process instructed by molecular cues and patterns of electrical activity. In the rodent visual system, this process begins during the first postnatal week and proceeds during the second and third postnatal weeks as spontaneous retinal activity and finally use-dependent fine tuning takes place. Reelin is a large extracellular matrix glycoprotein able to affect several steps of brain development, from neuronal migration to the maturation of dendritic spines and use-dependent synaptic development. In the present study, we investigated the role of reelin on the topographical refinement of primary sensory connections studying the development of retinal ganglion cell axon terminals in the rat superior colliculus. We found that reelin levels in the visual layers of the superior colliculus are the highest between the second and third postnatal weeks. Blocking reelin signaling with a neutralizing antibody (CR-50) from PND 7 to PND 14 induced a non-specific sprouting of ipsilateral retinocollicular axons outside their typical distribution of discrete patches of axon terminals. Also we found that reelin blockade resulted in reduced levels of phospho-GAP43, increased GluN1 and GluN2B-NMDA subunits and decreased levels of GAD65 content in the visual layers of the superior colliculus. The results suggest that reelin signaling is associated with the maturation of excitatory and inhibitory synaptic machinery influencing the development and fine tuning of topographically organized neural circuits during postnatal development.

Intravitreous Injection of Interleukin-6 Leads to a Sprouting in the Retinotectal Pathway at Different Stages of Development

OBJECTIVE

The development of retinotectal pathways form precise topographical maps is usually completed by the third postnatal week. Cytokines participate in the development and plasticity of the nervous system. We have previously shown that in vivo treatment with interleukin 2 disrupts the retinocollicular topographical order in early stages of development. Therefore, we decided to study the effect of a single intravitreous injection of IL-6 upon retinotectal circuitry in neonates and juvenile rats.

MATERIALS AND METHODS

Lister Hooded rats received an intravitreous injection of IL-6 (50 ng/ml) or vehicle (PBS) at either postnatal day (PND)10 or PND30 and the ipsilateral retinotectal pathway was evaluated 4 or 8 days later, respectively.

RESULTS

Our data showed that, at different stages of development, a single IL-6 intravitreous treatment did not produce an inflammatory response and increased retinal axon innervation throughout the visual layers of the superior colliculus.

CONCLUSIONS

Taken together, our data provide the first evidence that a single intravitreous injection with IL-6 leads to sprouting in the subcortical visual connections and suggest that small changes in IL-6 levels might be sufficient to impair the correct neuronal circuitry fine-tuning during brain development.

Nitric oxide in the nervous system: biochemical, developmental, and neurobiological aspects

Nitric oxide (NO) is a very reactive molecule, and its short half-life would make it virtually invisible until its discovery. NO activates soluble guanylyl cyclase (sGC), increasing 3',5'-cyclic guanosine monophosphate levels to activate PKGs. Although NO triggers several phosphorylation cascades due to its ability to react with Fe II in heme-containing proteins such as sGC, it also promotes a selective posttranslational modification in cysteine residues by S-nitrosylation, impacting on protein function, stability, and allocation. In the central nervous system (CNS), NO synthesis usually requires a functional coupling of nitric oxide synthase I (NOS I) and proteins such as NMDA receptors or carboxyl-terminal PDZ ligand of NOS (CAPON), which is critical for specificity and triggering of selected pathways. NO also modulates CREB (cAMP-responsive element-binding protein), ERK, AKT, and Src, with important implications for nerve cell survival and differentiation. Differences in the regulation of neuronal death or survival by NO may be explained by several mechanisms involving localization of NOS isoforms, amount of NO being produced or protein sets being modulated. A number of studies show that NO regulates neurotransmitter release and different aspects of synaptic dynamics, such as differentiation of synaptic specializations, microtubule dynamics, architecture of synaptic protein organization, and modulation of synaptic efficacy. NO has also been associated with synaptogenesis or synapse elimination, and it is required for long-term synaptic modifications taking place in axons or dendrites. In spite of tremendous advances in the knowledge of NO biological effects, a full description of its role in the CNS is far from being completely elucidated.

Intravitreous interleukin-2 treatment and inflammation modulates glial cells activation and uncrossed retinotectal development

Interleukin-2 (IL-2) plays regulatory functions both in immune and nervous system. However, in the visual system, little is known about the cellular types which respond to IL-2 and its effects. Herein, we investigated the influence of IL-2 in the development of central visual pathways. Lister Hooded rats were submitted to multiple (at postnatal days [PND]7/10/13) or single (at PND10) intravitreous injections of phosphate-buffered saline (PBS) (vehicle), zymosan, or IL-2. IL-2 receptor α subunit was detected in the whole postnatal retina. Chronic treatment with either PBS or IL-2 increases retinal glial fibrillary acidic protein (GFAP) expression, induces intravitreous inflammation revealed by the presence of macrophages, and results in a slight rearrangement of retinotectal axons. Acute zymosan treatment disrupts retinotectal axons distribution, confirming the influence of inflammation on retinotectal pathway reordering. Furthermore, acute IL-2 treatment increases GFAP expression in the retina without inflammation and produces a robust sprouting of the intact uncrossed retinotectal pathway. No difference was observed in glial cells activity in superior colliculus. Taken together, these data suggest that inflammation and interleukin-2 modulate retinal ganglion cells development and the distribution of their axons within central targets.

Purinergic modulation in the development of the rat uncrossed retinotectal pathway

Adenosine is a neuromodulator implicated in nervous system development and plasticity and its effects are mediated by inhibitory (A(1), A(3)) and excitatory (A(2a), A(2b)) receptors. The role of adenosine in the synaptic activity depends mainly on a balanced activation of A(1) and A(2a) receptors which are activated by various ranges of adenosine concentrations. Herein, we investigated the expression of A(1) and A(2a) receptors and also the accumulation of cAMP in the superior colliculus at different stages of development. Furthermore, we examined the effects of an acute in vivo blockade of adenosine deaminase during the critical period when the elimination of misplaced axons/terminals takes place with a simultaneous fine tuning of terminal arbors into appropriate terminal zones. Lister Hooded rats ranging from postnatal days (PND) 0-70 were used for ontogeny studies. Our results indicate that A(1) expression in the visual layers of the superior colliculus is higher until PND 28, while A(2a) expression increases after PND 28 in a complementary developmental pattern. Accordingly, the incubation of collicular slices with 5'-N-ethylcarboxamido-adenosine, a non-specific adenosine receptor agonist, showed a significant reduction in cAMP accumulation at PND 14 and an increase in adults. For the anatomical studies, the uncrossed retinotectal projections were traced after the intraocular injection of horseradish peroxidase. One group received daily injections of an adenosine deaminase inhibitor (erythro-9(2-hydroxy-3-nonyl adenine), 10 mg/kg i.p.) between PND 10 and 13, while control groups were treated with vehicle injections (NaCl 0.9%, i.p.). We found that a short-term blockade of adenosine deaminase during the second postnatal week induced an expansion of retinotectal terminal fields in the rostrocaudal axis of the tectum. Taken together, the results suggest that a balance of purinergic A(1) and A(2a) receptors through cAMP signaling plays a pivotal role during the development of topographic order in the retinotectal pathway.

Matrix metalloproteinase-9 is involved in the development and plasticity of retinotectal projections in rats

OBJECTIVE

During postnatal development, retinotectal projections undergo a process of misplaced axon elimination, leading to a topographical matching between the retinal surface and the superior colliculus. Matrix metalloproteinases (MMPs) have been implicated in the development and plasticity of the nervous system. We studied the expression and role of MMPs during normal development of retinotectal projections and after monocular enucleation-induced plasticity.

MATERIAL AND METHODS

Lister hooded rats at different postnatal ages received subpial ethylene vinyl acetate 40W implants to deliver an MMP inhibitor or vehicle to the superior colliculus. Animals received intraocular injections of horseradish peroxidase for anterograde tracing of ipsilateral projections. For immunoblotting and zymography, colliculi were removed without fixation.

RESULTS

We observed the highest MMP activity in the first postnatal week, with decreasing activity thereafter. Monocular enucleation at postnatal day 10 yielded a rapid increase in MMP activity, 24 h following denervation of the contralateral colliculus. Importantly, inhibition of MMP activity in vivo induced a marked delay of axonal clustering along the medial aspect of colliculus.

CONCLUSIONS

Our data indicate that MMPs are crucial in retinotectal development concurring to the fine tuning of topographical order and synaptic specificity of these connections.

Blockade of arachidonic acid pathway induces sprouting in the adult but not in the neonatal uncrossed retinotectal projection

The uncrossed retinotectal projection of rats undergoes extensive axonal elimination and subsequent growth of axonal arbors in topographically appropriate territories within the first two/three postnatal weeks. Nitric oxide has been implicated in development and stabilization of synapses in the retinotectal pathway since blockade of nitric oxide synthesis disrupts the normal pattern of retinal innervation in subcortical nuclei. The present work investigated the role of arachidonic acid pathway in the development and maintenance of ipsilateral retinotectal axons. We also investigated the role of this retrograde messenger in the modulation of plasticity that follows retinal lesions in the opposite eye. Pigmented rats received systemic treatment with quinacrine, a phospholipase A2 inhibitor, indomethacin, a cyclooxygenase inhibitor, nordihydroguaiaretic acid, a 5-lipoxygenase inhibitor or vehicle during 4-8 days at various postnatal ages. Rats given a unilateral temporal retinal lesion were treated with either quinacrine or vehicle during the same period. For anterograde tracing of ipsilateral retinal projections, animals received intraocular injections of horseradish peroxidase. Before the third postnatal week no difference was observed in the laminar or topographic organization of the ipsilateral retinotectal projection between vehicle and treated rats in either normal or lesion conditions. After the third postnatal week, however, systemic blockade of phospholipase A2 or 5-lipoxygenase, but not cyclooxygenase induced sprouting of uncrossed axons throughout the collicular visual layers in unoperated rats. In retinal lesion groups, phospholipase A2 blockade increased the sprouting of uncrossed intact axons to the collicular surface in the same period. The results suggest that arachidonic acid or lipoxygenase metabolites play a role in the maintenance of the retinotectal synapses after the critical period and that the blockade of the arachidonic acid pathway induces reactive sprouting of retinal axons late in development.

Rapid and long-term plasticity in the neonatal and adult retinotectal pathways following a retinal lesion

The uncrossed retinotectal projection restricts its terminal fields to the ventral boundary of the visual layers at the rostral tectum during early post natal development. During this critical period, temporal retinal lesions in one eye induce laminar rearrangements in the uncrossed pathway of the intact eye toward the collicular surface previously occupied, almost exclusively, by the crossed retinal axon population. We have compared, using anterograde tracing techniques, the time course and magnitude of the axonal sprouting resulting from retinal lesions in neonates and adults. Early retinal lesions (within the first two post natal weeks) induced extensive and rapid plasticity of the ipsilateral projection 48 h after the lesions. On the third post natal week, similar retinal lesions induced a small reorganization of the intact eye's uncrossed projection within a 3-week survival time. Nevertheless, giving the animals a long-term survival, resulted in an increased plastic capability, suggesting that even after the critical period, intact retinal axons can respond efficiently to injury. The results suggest two phases of axonal reorganization within this subcortical pathway: a rapid plasticity within the critical period and a slow, but continuous plasticity in adulthood.

Acute blockade of nitric oxide synthesis induces disorganization and amplifies lesion-induced plasticity in the rat retinotectal projection

In the rat visual system, the uncrossed retinotectal projection undergoes a topographical refinement within the first two postnatal weeks. We have studied the role of nitric oxide (NO), a retrograde messenger which couples pre- and postsynaptic activation, in the development of the uncrossed retinotectal projection and in the plasticity of this pathway as a result of a restricted retinal lesion in the opposite eye. During development, maximal nitric oxide synthase (NOS) activity was observed in homogenates of tectal tissue at postnatal day 5 (PND 5), followed by a two-step decrease at the end of the topographical fine tuning period (PND 21) and the adult stage (PND 42). We also tested the effects of an acute in vivo blockade of NOS during the development of both animals that had not been operated on, and lesioned animals. Animals ranging from PND 4 to PND 42 were treated either with the NOS inhibitor, L-nitro-arginine (Narg 50 mg/kg ip.) or vehicle (NaCl 0.9%) during 4 days (from PND 4-7 or PND 9-12) or 8 days (from PND 20-27 or PND 34-41). Reduction of NOS activity induced sprouting of the ipsilateral pathway up to the second postnatal week in the animals that had not been operated on. Rats that had been operated on, however, showed an amplification of the lesion-induced plasticity up to the fourth postnatal week under NOS blockade. The data suggest that NO plays a role in the stabilization of retinotectal synapses during the critical period of topographic refinement, and indicate that an acute blockade of retrograde signals enables plastic rearrangements in the visual system within this time window.

Fluoxetine-induced plasticity in the rodent visual system

We studied the effect of fluoxetine, a selective serotonin reuptake inhibitor, in the development and lesion-induced plasticity of retinotectal axons in pigmented rats. Neonatal rats received a daily injection of either fluoxetine or vehicle from postnatal day 1 (PND 1) to PND 10 or from PND 14 to PND 28 (fluoxetine, 7.5 and 10.0 mg/kg, respectively). In the latter group, some animals received a single lesion at the temporal periphery of the left retina at PND 21. Unoperated animals were use as the control. At the end of the treatment, the animals received an intraocular injection of horseradish peroxidase (HRP) in the right (intact) eye to trace the uncrossed retinotectal pathway. Chronic fluoxetine treatment, induced, in unoperated rats, an expansion of the retinal terminal fields along the rostro-caudal axis of the tectum both in the PND 10 and PND 28 groups. Following a retinal lesion in the left eye at PND 21, the vehicle-treated group showed a small reorganization of the intact uncrossed projection. In this group only a few terminals were labeled invading the denervated tectal surface one-week after the lesion. Fluoxetine-treated animals on the other hand, showed a great amplification of plasticity with a conspicuous sprouting of the uncrossed retinal axons into denervated areas. The data suggest that fluoxetine induces extensive axonal rearrangements in neonatal and juvenile central nervous system and amplifies neuroplasticity following retinal lesions late in development.

Retinal input induces three firing patterns in neurons of the superficial superior colliculus of neonatal rats

By using an in vitro isolated brain stem preparation, we recorded extracellular responses to electrical stimulation of the optic tract (OT) from 71 neurons in the superficial superior colliculus (SC) of neonatal rats (P1-13). At postnatal day 1 (P1), all tested neurons (n = 10) already received excitatory input from the retina. Sixty-nine (97%) superficial SC neurons of neonatal rats showed three response patterns to OT stimulation, which depended on stimulus intensity. A weak stimulus evoked only one spike that was caused by activation of non-N-methyl-D-aspartate (NMDA) glutamate receptors. A moderate stimulus elicited a short train (<250 ms) of spikes, which was induced by activation of both NMDA and non-NMDA receptors. A strong stimulus gave rise to a long train (>300 ms) of spikes, which was associated with additional activation of L-type high-threshold calcium channels. The long train firing pattern could also be induced either by temporal summation of retinal inputs or by blocking gamma-aminobutyric acid-A receptors. Because retinal ganglion cells show synchronous bursting activity before eye opening at P14, the retinotectal inputs appear to be sufficient to activate L-type calcium channels in the absence of pattern vision. Therefore activation of L-type calcium channels is likely to be an important source for calcium influx into SC neurons in neonatal rats.