The Role of the Adrenal-Gut-Brain Axis on Comorbid Depressive Disorder Development in Diabetes.

Diabetic patients are more affected by depression than non-diabetics, and this is related to greater treatment resistance and associated with poorer outcomes. This increase in the prevalence of depression in diabetics is also related to hyperglycemia and hypercortisolism. In diabetics, the hyperactivity of the HPA axis occurs in parallel to gut dysbiosis, weakness of the intestinal permeability barrier, and high bacterial-product translocation into the bloodstream. Diabetes also induces an increase in the permeability of the blood-brain barrier (BBB) and Toll-like receptor 4 (TLR4) expression in the hippocampus. Furthermore, lipopolysaccharide (LPS)-induced depression behaviors and neuroinflammation are exacerbated in diabetic mice. In this context, we propose here that hypercortisolism, in association with gut dysbiosis, leads to an exacerbation of hippocampal neuroinflammation, glutamatergic transmission, and neuronal apoptosis, leading to the development and aggravation of depression and to resistance to treatment of this mood disorder in diabetic patients.

Retinotectal plasticity induced by monocular enucleation during the critical period is dependent of A2a adenosine receptor: A possible role of astrocytes.

The retinotectal topography of rats develops within the first three postnatal weeks during the critical period. Previous studies have shown that monocular enucleation results in plasticity of the intact retinotectal pathway in a time-dependent manner. Glial fibrillary acidic protein (GFAP), an astrocyte marker, is up-regulated after central nervous system injury. Adenosine is a neuromodulator involved in the development and plasticity of the visual system acting through the inhibitory A1 and excitatory A2a receptor activities. Herein, we examined whether adenosine receptors and astrocytes are crucial for monocular enucleation (ME)-induced plasticity. We also investigate whether A2a blockade alters retinotectal plasticity in an astrocyte-dependent manner. Lister Hooded rats were submitted to monocular enucleation at postnatal day 10 (PND10) or PND21 and, after different survival times, were processed for immunohistochemistry or western blotting assays. Another group underwent subpial implantation of ELVAX containing vehicle (DMSO) or SCH58261 (1 µM - an A2a receptor antagonist), simultaneously with ME at PND10. After a 72 h survival, GFAP content and the retinotectal plasticity were evaluated. Our data show that monocular enucleation leads to an upregulation in GFAP expression in the contralateral superior colliculus. At PND10, a slight increase in GFAP labeling was observed at 72 h post-enucleation, while at PND21 GFAP increase was detected in the deafferented superior colliculus after 1 to 3 weeks of survival. The content of adenosine receptors also varies in the contralateral target after ME. A transient increase in A1 receptors is observed in the early periods of plasticity, while A2a receptors are upregulated later. Interestingly, the local blockade of A2a receptors abolished the increase in GFAP and the retinotectal reorganization induced by monocular enucleation during the critical period. Taken together these results suggest a correlation between astrocytes and A2a adenosine receptors in the subcortical visual plasticity.

Enteric glial cell reactivity in colonic layers and mucosal modulation in a mouse model of Parkinson's disease induced by 6-hydroxydopamine.

Enteric glial cells (EGCs) constitute the majority of the neural population of the enteric nervous system and are found in all layers of the gastrointestinal tract. It is active in enteric functions such as immunomodulation, participating in inflammation and intestinal epithelial barrier (IEB) regulation. Both EGCs and IEB have been described as altered in Parkinson's disease (PD). Using an animal model of PD induced by 6-hydroxydopamine (6-OHDA), we investigated the effect of ongoing neurodegeneration on EGCs and inflammatory response during short periods after model induction. C57Bl/6 male mice were unilaterally injected with 6-OHDA in the striatum. Compared to the control group, 6-OHDA animals showed decreased relative water content in their feces from 1 w after model induction. Moreover, at 1 and 2 w post-induction, groups showed histopathological changes indicative of intestinal inflammation. We identified an increase in IBA1 and GFAP levels in the intestinal mucosa. At an earlier survival of 48 h, we detected an increase in GFAP in the neuromuscular layer, suggesting that it was a primary event for the upregulation of GDNF, TNF-α, and occludin in the intestinal mucosa observed after 1 w. Within 2 w, we identified a decrease in the expression of occludin barrier proteins. Thus, EGCs modulation may be an early enteric signal induced by parkinsonian neurodegeneration, followed by inflammatory and dysmotility signs besides IEB modification.

Differential expression of glutamatergic receptor subunits in the hippocampus in carioca high- and low-conditioned freezing rats.

Anxiety is an emotional state that affects the quality of human life. Several neurotransmitters are involved in the regulation of anxiety, including glutamate. The major actions of glutamate are mediated by N-methyl-d-aspartate receptors (NMDARs) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). The present study performed a behavioral and neurochemical analysis of Carioca High-conditioned Freezing (CHF) and Carioca Low-conditioned Freezing (CLF) rats compared with control rats. We evaluated thermal nociception, anxiety-like behavior, depressive-like behavior, spatial memory, habituation memory, and the content and localization of different glutamatergic receptor subunits and postsynaptic density-95 (PSD-95), a postsynaptic protein. The CHF group exhibited an anxious-like phenotype, impairments in habituation and spatial memory, and a depressive-like phenotype compared with the control group. In the ventral hippocampus, an increase in the PSD-95, GluN1 and GluA1 subunits and a decrease in the GluN2A subunit of glutamatergic receptors. The CLF group exhibited a less anxious-like phenotype, hyperlocomotion and habituation impairments. Also, CLF animals, presented, in the ventral hippocampus, an increase in the PSD-95, GluN1 and GluA2 subunits and a decrease in the GluN2B subunit. These results suggest that the differential composition of NMDAR and AMPAR subunits may be related to the modulation of different phenotypes in CHF and CLF rats, which may help identify new targets for therapeutic interventions for anxiety disorders and other comorbidities.

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-GSK3ß (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 pGSK3ß, 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.

NMDA-induced nitric oxide generation and CREB activation in central nervous system is dependent on eukaryotic elongation factor 2 kinase.

The NMDA receptor is crucial to several functions in CNS physiology and some of its effects are mediated by promoting nitric oxide production from L-arginine and activation of signaling pathways and the transcription factor CREB. Our previous work demonstrated in retinal cells that increasing intracellular free L-arginine levels directly correlates to nitric oxide (NO) generation and can be promoted by protein synthesis inhibition and increase of free L-arginine concentration. Eukaryotic elongation factor 2 kinase (eEF2K), a calcium/calmodulin-dependent kinase, is also known to be activated by NMDA receptors leading to protein synthesis inhibition. Here we explored how does eEF2K participate in NMDA-induced NO signaling. We found that when this enzyme is inhibited, NMDA loses its ability to promote NO synthesis. On the other hand, when NO synthesis is increased by protein synthesis inhibition with cycloheximide or addition of exogenous L-arginine, eEF2K has no participation, showcasing a specific link between this enzyme and NMDA-induced NO signaling. We have previously shown that inhibition of the canonical NO signaling pathway (guanylyl cyclase/cGMP/cGK) blocks CREB activation by glutamate in retinal cells. Interestingly, pharmacological inhibition of eEF2K fully prevents CREB activation by NMDA, once again demonstrating the importance of eEF2K in NMDA receptor signaling. In summary, we demonstrated here a new role for eEF2K, directly controlling NMDA-dependent nitrergic signaling and modulating L-arginine availability in neurons, which can potentially be a new target for the study of physiological and pathological processes involving NMDA receptors in the central nervous system.

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.

Interleukin 4 Leads to Sustained Phosphorylation of the STAT6 and ERK Pathways in the Retina and Disrupts Subcortical Visual Circuitry in Rodents.

OBJECTIVE: Interleukin 4 (IL-4) is an anti-inflammatory cytokine related to different aspects of central nervous system development such as survival, proliferation, and differentiation, among others. Our goals were to investigate the effect of intravitreous treatment with IL-4 on the activation of downstream signaling pathways in the retina and the distribution of retinal axons within the superior colliculus (SC). MATERIAL AND METHODS: Lister hooded rats were submitted to an intravitreous injection of either IL-4 (5 U/µL) or PBS (vehicle) at postnatal day 10 (PND10). At PND11 or PND14, retinas were processed for Western blot or immunohistochemistry. At PND13, a group of animals received an intraocular injection of an anterograde tracer in the left (untreated) eye in order to label the uncrossed retinotectal axons. RESULTS: Our data revealed that intravitreous treatment with IL-4 at PND10 leads to a decrease in GFAP content and a sustained increase in the phosphorylation of STAT6 and ERK levels in the retina. IL-4 also increases retinal axonal arbors within the SC, compared to control groups. CONCLUSIONS: These data suggest that a single in vivo treatment with IL-4 during the early stages of development modulates signaling pathways in the retina, resulting in altered binocular subcortical visual connectivity.

Inflammatory demyelination alters subcortical visual circuits.

BACKGROUND: Multiple sclerosis (MS) is an inflammatory demyelinating disease classically associated with axonal damage and loss; more recently, however, synaptic changes have been recognized as additional contributing factors. An anatomical area commonly affected in MS is the visual pathway; yet, changes other than those associated with inflammatory demyelination of the optic nerve, i.e., optic neuritis, have not been described in detail. METHODS: Adult mice were subjected to a diet containing cuprizone to mimic certain aspects of inflammatory demyelination as seen in MS. Demyelination and inflammation were assessed by real-time polymerase chain reaction and immunohistochemistry. Synaptic changes associated with inflammatory demyelination in the dorsal lateral geniculate nucleus (dLGN) were determined by immunohistochemistry, Western blot analysis, and electrophysiological field potential recordings. RESULTS: In the cuprizone model, demyelination was observed in retinorecipient regions of the subcortical visual system, in particular the dLGN, where it was found accompanied by microglia activation and astrogliosis. In contrast, anterior parts of the pathway, i.e., the optic nerve and tract, appeared largely unaffected. Under the inflammatory demyelinating conditions, as seen in the dLGN of cuprizone-treated mice, there was an overall decrease in excitatory synaptic inputs from retinal ganglion cells. At the same time, the number of synaptic complexes arising from gamma-aminobutyric acid (GABA)-generating inhibitory neurons was found increased, as were the synapses that contain the N-methyl-D-aspartate receptor (NMDAR) subunit GluN2B and converge onto inhibitory neurons. These synaptic changes were functionally found associated with a shift toward an overall increase in network inhibition. CONCLUSIONS: Using the cuprizone model of inflammatory demyelination, our data reveal a novel form of synaptic (mal)adaption in the CNS that is characterized by a shift of the excitation/inhibition balance toward inhibitory network activity associated with an increase in GABAergic inhibitory synapses and a possible increase in excitatory input onto inhibitory interneurons. In addition, our data recognize the cuprizone model as a suitable tool in which to assess the effects of inflammatory demyelination on subcortical retinorecipient regions of the visual system, such as the dLGN, in the absence of overt optic neuritis.

Serotonin transporter immunoreactivity is modulated during development and after fluoxetine treatment in the rodent visual system.

The serotonin transporter (5-HTT) regulates serotonin homeostasis and has been used as a target for different drugs in depression treatment. Although the serotonergic system has received a lot of attention, little is known about the effects of these drugs over serotonin transporters. In this work, we investigated the expression pattern of 5-HTT during development of the visual system and the influence of fluoxetine on different signaling pathways. Our data showed that the expression of 5-HTT has a gradual increase from postnatal day 0 until 42 and decrease afterwards. Moreover, chronic fluoxetine treatment both in childhood and adolescence induces down regulation of 5-HTT expression and phosphorylation of ERK and AKT signaling pathways. Together these data suggest that the levels of 5-HTT protein could be important for the development of the central nervous system and suggest that the ERK and AKT are involved in the molecular pathways of antidepressants drugs, acting in concert to improve serotonergic signaling.

Monocular denervation of visual nuclei modulates APP processing and sAPPα production: A possible role on neural plasticity.

Amyloid precursor protein (APP) is essential to physiological processes such as synapse formation and neural plasticity. Sequential proteolysis of APP by beta- and gamma-secretases generates amyloid-beta peptide (Aß), the main component of senile plaques in Alzheimer Disease. Alternative APP cleavage by alpha-secretase occurs within Aß domain, releasing soluble α-APP (sAPPα), a neurotrophic fragment. Among other functions, sAPPα is important to synaptogenesis, neural survival and axonal growth. APP and sAPPα levels are increased in models of neuroplasticity, which suggests an important role for APP and its metabolites, especially sAPPα, in the rearranging brain. In this work we analyzed the effects of monocular enucleation (ME), a classical model of lesion-induced plasticity, upon APP content, processing and also in secretases levels. Besides, we addressed whether α-secretase activity is crucial for retinotectal remodeling after ME. Our results showed that ME induced a transient reduction in total APP content. We also detected an increase in α-secretase expression and in sAPP production concomitant with a reduction in Aß and ß-secretase contents. These data suggest that ME facilitates APP processing by the non-amyloidogenic pathway, increasing sAPPα levels. Indeed, the pharmacological inhibition of α-secretase activity reduced the axonal sprouting of ipsilateral retinocollicular projections from the intact eye after ME, suggesting that sAPPα is necessary for synaptic structural rearrangement. Understanding how APP processing is regulated under lesion conditions may provide new insights into APP physiological role on neural plasticity.

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.

TNF-α mediates PKR-dependent memory impairment and brain IRS-1 inhibition induced by Alzheimer's ß-amyloid oligomers in mice and monkeys.

Alzheimer's disease (AD) and type 2 diabetes appear to share similar pathogenic mechanisms. dsRNA-dependent protein kinase (PKR) underlies peripheral insulin resistance in metabolic disorders. PKR phosphorylates eukaryotic translation initiation factor 2α (eIF2α-P), and AD brains exhibit elevated phospho-PKR and eIF2α-P levels. Whether and how PKR and eIF2α-P participate in defective brain insulin signaling and cognitive impairment in AD are unknown. We report that ß-amyloid oligomers, AD-associated toxins, activate PKR in a tumor necrosis factor α (TNF-α)-dependent manner, resulting in eIF2α-P, neuronal insulin receptor substrate (IRS-1) inhibition, synapse loss, and memory impairment. Brain phospho-PKR and eIF2α-P were elevated in AD animal models, including monkeys given intracerebroventricular oligomer infusions. Oligomers failed to trigger eIF2α-P and cognitive impairment in PKR(-/-) and TNFR1(-/-) mice. Bolstering insulin signaling rescued phospho-PKR and eIF2α-P. Results reveal pathogenic mechanisms shared by AD and diabetes and establish that proinflammatory signaling mediates oligomer-induced IRS-1 inhibition and PKR-dependent synapse and memory loss.

Nutritional restriction of omega-3 fatty acids alters topographical fine tuning and leads to a delay in the critical period in the rodent visual system.

The development and maturation of sensory systems depends on the correct pattern of connections which occurs during a critical period when axonal elimination and synaptic plasticity are involved in the formation of topographical maps. Among the mechanisms involved in synaptic stabilization, essential fatty acids (EFAs), available only through diet, appear as precursors of signaling molecules involved in modulation of gene expression and neurotransmitter release. Omega-3 fatty acids, such as docosahexaenoic acid (DHA), are considered EFAs and are accumulated in the brain during fetal period and neonatal development. In this study, we demonstrated the effect of omega-3/DHA nutritional restriction in the long-term stabilization of connections in the visual system. Female rats were fed 5 weeks before mating with either a control (soy oil) or a restricted (coconut oil) diet. Litters were fed until postnatal day 13 (PND13), PND28 or PND42 with the same diets when they received an intraocular injection of HRP. Another group received a single retinal lesion at the temporal periphery at PND21. Omega-3 restriction induced an increase in the optical density in the superficial layers of the SC, as a result of axonal sprouting outside the main terminal zones. This effect was observed throughout the SGS, including the ventral and intermediate sub-layers at PND13 and also at PND28 and PND42. The quantification of optical densities strongly suggests a delay in axonal elimination in the omega3(-) groups. The supplementation with fish oil (DHA) was able to completely reverse the abnormal expansion of the retinocollicular projection. The same pattern of expanded terminal fields was also observed in the ipsilateral retinogeniculate pathway. The critical period window was studied in lesion experiments in either control or omega-3/DHA restricted groups. DHA restriction induced an increased sprouting of intact, ipsilateral axons at the deafferented region of the superior colliculus compared to the control group, revealing an abnormal extension of the critical period. Finally, in omega-3 restricted group we observed in the collicular visual layers normal levels of GAP-43 with decreased levels of its phosphorylated form, p-GAP-43, consistent with a reduction in synaptic stabilization. The data indicate, therefore, that chronic dietary restriction of omega-3 results in a reduction in DHA levels which delays axonal elimination and critical period closure, interfering with the maintenance of terminal fields in the visual system.

Expression of GAP-43 during development and after monocular enucleation in the rat superior colliculus.

The retinotectal projection of rodents presents a precise retinotopic organization that develops, from diffuse connections, from the day of birth to post-natal day 10. Previous data had demonstrated that these projections undergo reorganization after retinal lesions, nerve crush and monocular enucleation. The axonal growth seems to be directly related to growth-associated protein-43 (GAP-43) expression, a protein predominantly located in growth cones, which is regulated throughout development. GAP-43 is presented both under non-phosphorylated and phosphorylated (pGAP-43) forms. The phosphorylated form, has been associated to axon growth via polymerization of F-actin, and synaptic enhancement through neurotransmitter release facilitation. Herein we investigated the spatio-temporal expression of GAP-43 in the rat superior colliculus during normal development and after monocular enucleation in different stages of development. Lister Hooded rats ranging from post-natal day 0 to 70 were used for ontogeny studies. Another group of animals were submitted to monocular enucleation at post-natal day 10 (PND10) or PND21. After different survival-times, the animals were sacrificed and the brains processed for either immunohistochemistry or western blotting analysis. Our data show that GAP-43 is expressed in retinotectal axons in early stages of development but remains present in adulthood. Moreover, monocular enucleation leads to an increase in pGAP-43 expression in the deafferented colliculus. Taken together these results suggest a role for pGAP-43 in retinotectal morphological plasticity observed both during normal development and after monocular enucleation.

Nutritional tryptophan restriction impairs plasticity of retinotectal axons during the critical period.

The use-dependent specification of neural circuits occurs during post-natal development with a conspicuous influence of environmental factors, such as malnutrition that interferes with the major steps of brain maturation. Serotonin (5-HT), derived exclusively from the essential aminoacid tryptophan, is involved in mechanisms of development and use-dependent plasticity of the central nervous system. We studied the effects of the nutritional restriction of tryptophan in the plasticity of uncrossed retinotectal axons following a retinal lesion to the contralateral retina during the critical period in pigmented rats. Litters were fed through their mothers with a low tryptophan content diet, based on corn and gelatin, a complemented diet with standard tryptophan requirements for rodents or standard laboratory diet. The results suggest a marked reduction in the plasticity of intact axons into denervated territories in the tryptophan restricted group in comparison to control groups. Tryptophan complementation between PND10-21 completely restored retinotectal plasticity. However, the re-introduction of tryptophan after the end of the critical period (between PND28-P41) did not restore the sprouting ability of uncrossed axons suggesting a time-dependent effect to the reversion of plasticity deficits. Tryptophan-restricted animals showed a reduced activity of matrix metalloproteinase-9 and altered expressions of phosphorylated forms of ERK1/2 and AKT. Our results demonstrate the influence of this essential aminoacid as a modulator of neural plasticity during the critical period through the reduction of serotonin content which alters plasticity-related signaling pathways and matrix degradation.

Interleukin-4 blocks thapsigargin-induced cell death in rat rod photoreceptors: involvement of cAMP/PKA pathway.

Although the photoreceptors cell death is the main cause of some retinopathies diseases, the mechanisms involved in this process are poorly understood. The neuroprotective effects of interleukin-4 (IL-4) have been shown in several tissues, including retina. We demonstrate that treatment of rat retinal explants with IL-4 completely inhibited the thapsigargin-induced rod photoreceptor cell death after 24 hr in culture. We also showed that IL-4 receptor alpha subunit (IL-4Ralpha) is abundantly present in retina. Colocalization of IL-4Ralpha and rhodopsin indicate a direct effect of this cytokine in rod photoreceptor cells. Moreover, IL-4 increased the intracellular levels of cAMP in 7.4-fold, indicating that the neuroprotective effect of this cytokine was completely blocked by RpcAMP, an inhibitor of protein kinase (PKA). Our data demonstrate, for the first time, the neuroprotective effect of IL-4 through cAMP/PKA pathway in thapsigargin-induced photoreceptor cell death.

Nutritional tryptophan restriction and the role of serotonin in development and plasticity of central visual connections.

Tryptophan is an essential amino acid and metabolic precursor of serotonin. Serotonin is both a classical neurotransmitter and a signaling molecule that plays crucial roles in the development of neural circuits and plasticity. The specification of neural circuits in rodents occurs during the postnatal period with conspicuous influence of environmental factors including the nutritional status. Sensory, motor and cognitive systems develop during a critical period, a time window that is crucial to the use-dependent organization of neuronal circuits. This review presents recent experimental findings that disclose some mechanism of tryptophan- and serotonin-dependent plasticity in the developing and adult brain.