Neurotrophin-3 upregulation associated with intravenous transplantation of bone marrow mononuclear cells induces axonal sprouting and motor functional recovery in the long term after neocortical ischaemia.

Bone marrow mononuclear cells (BMMCs) have been identified as a relevant therapeutic strategy for the treatment of several chronic diseases of the central nervous system. The aim of this work was to evaluate whether intravenous treatment with BMMCs facilitates the reconnection of lesioned cortico-cortical and cortico-striatal pathways, together with motor recovery, in injured adult Wistar rats using an experimental model of unilateral focal neocortical ischaemia. Animals with cerebral cortex ischaemia underwent neural tract tracing for axonal fibre analysis, differential expression analysis of genes involved in apoptosis and neuroplasticity by RT-qPCR, and motor performance assessment by the cylinder test. Quantitative and qualitative analyses of axonal fibres labelled by an anterograde neural tract tracer were performed. Ischaemic animals treated with BMMCs showed a significant increase in axonal sprouting in the ipsilateral neocortex and in the striatum contralateral to the injured cortical areas compared to untreated rodents. In BMMC-treated animals, there was a trend towards upregulation of the Neurotrophin-3 gene compared to the other genes, as well as modulation of apoptosis by BMMCs. On the 56th day after ischaemia, BMMC-treated animals showed significant improvement in motor performance compared to untreated rats. These results suggest that in the acute phase of ischaemia, Neurotrophin-3 is upregulated in response to the lesion itself. In the long run, therapy with BMMCs causes axonal sprouting, reconnection of damaged neuronal circuitry and a significant increase in motor performance.

Effects of bone marrow mononuclear cells on induction of axonal sprouting in cortico-cortical and cortico-striatal pathways in an animal model of cortical ablation.

OBJECTIVES: Many therapies have been proposed in order to investigate the mechanisms of neural repair associated with neurological diseases, including bone marrow mononuclear cells (BMMC) transplantation. However, there is evidence that some encephalic injuries are less responsive to neural repair, such as, for example, cortical ablation. On the other hand, some models of cortical ablation have shown functional recovery after BMMC transplantation. Thus, it is relevant to expand the knowledge of BMMC transplantation-induced neuroplasticity in animal models, considering a promising approach for the rehabilitation of patients with neurological diseases. Using an experimental model of cerebral cortex ablation in adult male Wistar rats, which is known to be poorly responsive to neuroplasticity, the aim of this study was to investigate the effects of BMMC on axonal sprouting in cortico-cortical and cortico-striatal pathways synaptic fields. An anterograde neurotracer was used to evaluate the distribution of axonal fibres. RESULTS: The results showed that BMMC were not able to significantly induce axonal sprouting in the evaluated synaptic fields. Our results reinforced the idea that cortical ablation may be less responsive to neuroplasticity and the beneficial effects of BMMC therapy depend on the particularities of a neural microenvironment intrinsic to a given cortical lesion.

Comparison among bone marrow mesenchymal stem and mononuclear cells to promote functional recovery after spinal cord injury in rabbits

Abstract Purpose: To investigate the efficacy of allogeneic mesenchymal stem-cells and autologous mononuclear cells to promote sensorimotor recovery and tissue rescue. Methods: Female rabbits were submitted to the epidural balloon inflation method and the intravenous cells administrations were made after 8 hours or seven days after injury induction. Sensorimotor evaluation of the hindlimbs was performed, and the euthanasia was made thirty days after the treatment. Spinal cords were stained with hematoxylin and eosin. Results: Both therapies given 8 hours after the injury promoted the sensorimotor recovery after a week. Only the group treated after a week with mononuclear cells showed no significant recovery at post-injury day 14. In the days 21 and 28, all treatments promoted significant recovery. Histopathological analysis showed no difference among the experimental groups. Our results showed that both bone marrow-derived cell types promoted significant sensorimotor recovery after injury, and the treatment made at least a week after injury is efficient. Conclusion: The possibilities of therapy with bone marrow-derived cells are large, increasing the therapeutic arsenal for the treatment of spinal cord injury.

Comparison among bone marrow mesenchymal stem and mononuclear cells to promote functional recovery after spinal cord injury in rabbits.

PURPOSE: To investigate the efficacy of allogeneic mesenchymal stem-cells and autologous mononuclear cells to promote sensorimotor recovery and tissue rescue. METHODS: Female rabbits were submitted to the epidural balloon inflation method and the intravenous cells administrations were made after 8 hours or seven days after injury induction. Sensorimotor evaluation of the hindlimbs was performed, and the euthanasia was made thirty days after the treatment. Spinal cords were stained with hematoxylin and eosin. RESULTS: Both therapies given 8 hours after the injury promoted the sensorimotor recovery after a week. Only the group treated after a week with mononuclear cells showed no significant recovery at post-injury day 14. In the days 21 and 28, all treatments promoted significant recovery. Histopathological analysis showed no difference among the experimental groups. Our results showed that both bone marrow-derived cell types promoted significant sensorimotor recovery after injury, and the treatment made at least a week after injury is efficient. CONCLUSION: The possibilities of therapy with bone marrow-derived cells are large, increasing the therapeutic arsenal for the treatment of spinal cord injury.

Effect of the treatment of focal brain ablation in rat with bone marrow mesenchymal stromal cells on sensorimotor recovery and cytokine production.

Increased efficacy of the multipotent mesenchymal stromal cells (MSCs) for the treatment of CNS injuries has been shown when they are administrated within a collagen scaffold, an environment in three dimensions (3D), when compared to the cultivation over a plastic surface (2D). We evaluated the MSC therapeutic effect in the 2D and 3D conditions using the model of focal cortical ablation. Male Wistar rats were submitted to the ablation by aspiration. Intravenous injection (IV) of MSC cultured in 2D, and the intralesional administration (IL) of MSC cultured in 2D or 3D were tested. Administrations were made 24h after ablation. Unskilled and skilled forelimb movements were evaluated by sensorimotor tests. The level of cytokines was measured two days after ablation in the 2D IV groups. Only the MSC 3D IL promoted recovery of the skilled movements. MSC 2D IV promoted recovery of the unskilled movements in all tests, and the MSC 3D IL promoted it only in the adhesive test. MSC 2D IL was unable to promote any recovery. DAPI-stained MSC was found in the perilesional parenchyma at the third post-ablation day after 2D and 3D IL. A significant reduction in the levels of cytokines by the MSC 2D IV was observed in the plasma. Our study strengthens the evidences of the MSC as a prospective therapeutic approach for the CNS injuries.

Effect of age and gender on recovery after stroke in rats treated with bone marrow mononuclear cells.

Stroke is a disease of the elderly. However, most of the preclinical studies about the treatment of stroke with bone marrow-derived cells have used young animals. Here, it was assessed whether the sensorimotor recovery promoted by the treatment of the brain ischemia with the bone marrow mononuclear cells (BMMCs) is influenced by age and/or gender. Unilateral cortical ischemia by thermocoagulation was made in the primary motor and sensorimotor cortices in young and middle-aged rats of both genders. Twenty four hours after ischemia, animals received intravenous injection of BMMCs or vehicle. Each combination of age and gender received BMMCs from donor with the same combination. Survival rate and ischemic lesion size were quantified. Sensorimotor recovery was evaluated by the cylinder and adhesive tests. The results showed that the treatment with BMMCs resulted in sensorimotor recovery of both young and middle-aged ischemic rats. No important effect of gender was found, but age was a significant factor. Middle-aged animals had increased mortality and lesion sizes. In the adhesive test, middle-aged animals had lower BMMCs-induced sensorimotor recovery. The results suggest that the treatment of stroke with the BMMCs should be beneficial for males and females in the elderly.

Increase in medial frontal cortex ERK activation following the induction of apomorphine sensitization.

Repeated high dose injections of the direct acting D1/D2 agonist apomorphine (APO) induces context specific behavioral sensitization. We assessed the effects of 2.0 mg/kg APO on open-field locomotor responses of rats over a 30 min period following either single or five daily APO injections. Acute injections increased locomotor activity, which was markedly increased in rats given 5 daily APO injections. This progressive increase in locomotion during the repeated APO treatments is indicative of behavioral sensitization. Immediately following the open-field test for the acute and the fifth apomorphine injection, the animals were euthanized and their brain tissue was prepared for immunohistochemistry. ERK immunoreactive nuclei in the medial prefrontal cortex (PFC), nucleus accumbens (NAcc), amygdala (AMYG) and lateral hypothalamus (LH) were quantified. The acute apomorphine injections increased ERK in all brain areas as compared to vehicle. Following the fifth apomorphine injection, ERK significantly increased in the PFC, decreased in the amygdala but was unchanged in the LH and NAcc. The selective increase in ERK activity in the PFC associated with behavioral sensitization, points to a possible pivotal role of the dopamine projection to the medial frontal cortex in the mediation of neural plasticity, considered to underlie the sensitization processes induced by dopaminergic drugs.

Therapeutic potential of treatment with the flavonoid rutin after cortical focal ischemia in rats.

Flavonoids have known anti-inflammatory and antioxidative actions, and they have been described as neuroprotective and able to reduce damage in CNS diseases. We evaluated the action of the flavonoid rutin in an animal model of focal cortical ischemia induced by unilateral thermocoagulation of superficial blood vessels of motor (M1) and somatosensory (S1) primary cortices. Ischemic rats were submitted to daily injections (i.p.) for five days, starting immediately after induction of ischemia. We tested two doses: 50mg/kg or 100mg/kg of body weight. Sensorimotor tests were used to evaluate functional recovery. Bioavailability in plasma was done by chromatographic analysis. The effect of treatment in lesion volume and neurodegeneration was evaluated 48 h and 72 h after ischemia, respectively. We observed significant sensorimotor recovery induced by rutin, and the dose of 50mg/kg had more pronounced effect. Thus, this dose was used in further analyses. Plasma availability of rutin was detected from 2h to at least 8h after ischemia. The treatment did not result in reduction of lesion volume but reduced the number of degenerated neurons at the periphery of the lesion. The results suggest rutin as an efficient drug to treat brain ischemia since it was able to promote significant recovery of sensorimotor loss, which was correlated to the reduction of neurodegeneration in the periphery of cortical injury. Increasing studies with rutin and other flavonoids might give support for further clinical trials with these drugs.

Does treatment with bone marrow mononuclear cells recover skilled motor function after focal cortical ischemia? Analysis with a forelimb skilled motor task in rats.

Previous studies have shown sensorimotor recovery by treatment with bone marrow mononuclear cells (BMMCs) after focal brain ischemia. However, sensorimotor tests commonly used are designed to examine motor patterns that do not involve skill or training. We evaluated whether BMMCs treatment was able to recover forelimb skilled movements. Reaching chamber/pellet retrieval (RCPR) task was used, in which animals had to learn to grasp a single food pellet and lead it to its mouth. We also evaluated therapeutic effect of this training on unskilled sensorimotor function. Adult male Wistar rats suffered unilateral cortical ischemia by thermocoagulation in motor and somesthetic primary areas. A day later, they received i.v. injection of 3×10(7) BMMCs or vehicle (saline), forming four experimental groups: BMMCs+RCPR; saline+RCPR; BMMCs and saline. Cylinder and adhesive tests were applied in all experimental groups, and all behavioral tests were performed before and along post-ischemic weeks after induction of ischemia. Results from RCPR task showed no significant difference between BMMCs+RCPR and saline+RCPR groups. In cylinder test, BMMCs-treated groups showed significant recovery, but no significant effect of RCPR training was observed. In adhesive test, BMMCs treatment promoted significant recovery. Synergistic effect was found since only together they were able to accelerate recovery. The results showed that BMMCs treatment promoted increased recovery of unsophisticated sensorimotor function, but not of skilled forepaw movements. Thus, BMMCs might not be able to recover all aspects of sensorimotor functions, although further studies are still needed to investigate this treatment in ischemic lesions with different locations and extensions.

Comparative study between bone marrow mononuclear fraction and mesenchymal stem cells treatment in sensorimotor recovery after focal cortical ablation in rats.

BACKGROUND: Different models of cortical lesion lead to different effects on plasticity of connections and loss of function. In opposition to ischemia, cortical lesion made by ablation does not induce significant adaptive plasticity of corticocortical and corticostriatal projections and leads to functional alterations other than those observed after ischemia. We have demonstrated sensorimotor recovery after treatment with bone marrow-derived mesenchymal stem cells (MSCs) or bone marrow mononuclear cells (BMMCs) in a model of focal cortical ischemia. Here, we extended this analysis evaluating the effect of these cells on sensorimotor recovery after focal cortical ablation, reproducing the same size and location of previous ischemic lesion. FINDINGS: Focal cerebral aspiration of the six cortical layers in left frontoparietal cortex was performed in male Wistar rats. One day later, MSCs or BMMCs were administrated (i.v.) in the ablated animals. Vehicle was administrated in the control group. Sensorimotor tests were performed before and after injury followed by i.v. injection. The monitoring of functional recovery was performed weekly during three post-ablation months. The results showed significant sensorimotor recovery with both treatments, whereas control groups had no recovery. Moreover, both cell types induced the same level of recovery. CONCLUSIONS: Bone marrow cells showed therapeutic efficacy in a model of brain injury known to promote low structural plasticity. Thus, the results support the idea of BMMCs as better candidates to treat acute CNS injuries than MSCs, since they have the same therapeutic potential, but its obtainment for autologous transplantation has been shown to be faster and easier.

Bone marrow mononuclear cells and mannose receptor expression in focal cortical ischemia.

The use of bone marrow mononuclear cells (BMMCs) has been shown as a putative efficient therapy for stroke. However, the mechanisms of therapeutic action are not yet completely known. Mannose receptor (MR) is a subgroup of the C-type lectin receptor superfamily involved in innate immune response in several tissues. Although known primarily for its immune function, MR also has important roles in cell migration, cell debris clearance and tissue remodeling during inflammation and wound healing. Here we analyzed MR expression in brains of rats one week after induction of unilateral focal cortical ischemia by thermocoagulation in blood vessels of sensorimotor cortex. Additionally, we evaluated possible changes in such expression in cortices of rats subjected to ischemia plus treatment with BMMCs. Our results showed high expression of MR in an unknown GFAP(+) cell type and in phagocytic macrophages/microglia within the lesion boundary zone whereas in the non-injured (contralateral) cortical parenchyma, low levels of MR expression were observed. Moreover, therapy with BMMCs induced overexpression of MR in ipsilateral (injured) cortex. Previous studies from our group have shown functional recovery and decreased neurodegeneration in BMMC-treated rats in the same model of focal cortical ischemia. Thus, we suggest that ischemic injury induces large increase in MR expression as part of a mechanism for clearance of damage-associated molecular patterns (DAMPs). In addition, induction of MR overexpression by BMMCs might increase the efficiency of clearance, being one of the protective mechanisms of these cells.

Therapeutic window for treatment of cortical ischemia with bone marrow-derived cells in rats.

The beneficial effect of treatment with bone marrow mononuclear cells (BMMCs) was evaluated in different therapeutic windows in a rat model of focal ischemia induced by thermocoagulation of the blood vessels in the left motor, somestesic, and sensorimotor cortices. We also compared the therapeutic benefits between BMMCs and bone marrow-derived mesenchymal stem cells (MSCs). BMMCs and MSCs were obtained from donor rats and injected into the jugular vein after ischemia. BMMCs-treated animals received approximately 3x10(7) cells at post-ischemic days (PIDs) 1, 7, 14, or 30. MSCs-treated animals received approximately 3x10(6) cells at PIDs 1 and 30. Control animals received only the vehicle. The animals were then evaluated for functional sensorimotor recovery weekly with behavioral tests (cylinder test and adhesive test). Significant recovery of sensorimotor function was only observed in the cylinder test in animals treated with BMMCs at PIDs 1 and 7. Similar effects were also observed in the animals treated with MSCs 1 day after ischemia, but not in animals treated with MSCs 30 days after ischemia. Significant decrease in glial scarring did not seem to be a mechanism of action of BMMCs, since treatment with BMMCs did not change the level of expression of GFAP, indicating no significant change in the astrocytic scar in the periphery of the ischemic lesion. These results suggest that BMMCs might be an efficient treatment protocol for stroke only in the acute/subacute phase of the disease, and its efficiency in inducing functional recovery is similar to that of MSCs.

Terapia celular no acidente vascular cerebral / Cell therapy in strokes

O AVC é o recordista em número de óbitos e a maior causa de incapacidade no Brasil. Apesar das inúmeras pesquisas realizadas ao longo dos últimos anos não há terapias farmacológicas adequadas para este quadro e, neste cenário, as terapias celulares vêm sendo consideradas como alternativas terapêuticas para diminuir as perdas funcionais decorrentes do AVC. Nesta revisão comentaremos os resultados de diversos estudos pré-clinicos e de alguns clínicos que utilizaram diferentes tipos de células-tronco em AVC.

Stroke is the leading cause of death and incapacity in Brazil. Over the last few years, numerous preclinical and clinical studies have been carried out, however to date, none of the drugs tested in these studies were effective in patients. The emerging field of stem cell research has raised hope of therapy to ameliorate the functional loss after strokes. In this review we will discuss the results of several preclinical studies and clinical trials using different types of stem cells in the treatment of strokes.

A critical survey on nitric oxide synthase expression and nitric oxide function in the retinotectal system.

The ongoing research on the roles of the gas nitric oxide (NO) in the nervous system has demonstrated its involvement in neurotransmission, synaptic plasticity, learning, excitotoxicity, neurodegenerative diseases and regulation of the cerebral blood flow. Thus, this molecule has been currently considered an important neuromodulator in CNS. Studies carried out in the visual system, particularly in the retinotectal component, have contributed to this current concept about NO. In the present work, we reviewed critically current data about nitric oxide synthase (NOS) expression in the superior colliculus/optic tectum, as well as the roles of NO in the formation of the retinotopic map and in synaptic plasticity. Several vertebrate species have been used in studies about the NOS expression in the retinotectal system and most of the available results are in agreement with the involvement of NO in the developmental refinement of the retinotectal projections, and its role as a neuromodulator of synaptic function during the processing of visual information. However, the few studies about the functional linkage between NOS expression/NO synthesis and retinotectal topographic refinement/tectal synaptic plasticity are not conclusive and/or sometimes inconsistent, indicating that more experimental data are necessary to improve the understanding about NO functions in this visual subsystem. Predictive models for the involvement of NO as a retrograde messenger in the developmental retinotectal refinement are discussed.

Visually-induced NGFI-A protein expression in the calbindin-, parvalbumin- and nitric oxide synthase-neuronal populations of the rat superior colliculus.

The expression of the immediate early gene NGFI-A in the nervous system is induced by sensory stimulation and seems to be related to long-term synaptic plasticity. We have used double-labeling immunohistochemistry to identify calbindin (CB)(+), parvalbumin (PV)(+) and neuronal nitric oxide synthase (nNOS)(+) neurons that also expressed the protein encoded by this immediate early gene after light-exposure on in the superficial layers of the rat superior colliculus (sSC). The majority of the NGFI-A(+) cells were not double-labeled for the tested markers. In the stratum zonale+stratum griseum superficiale (SZ/SGS), only 17.8%, 8.0% and 12.1% of NGFI-A(+) cells were also labeled for CB, PV or nNOS, respectively. In the stratum opticum (SO), only 10.5% of the NGFI-A(+) cells were also CB(+). Furthermore, only a small subset of each population expressed the NGFI-A protein after light-exposure. In the SZ/SGS, 35.7% of the CB(+), 32.1% of the PV(+) and 26.6% of the nNOS(+) neurons also expressed the NGFI-A. In the SO, 31.7% of the CB(+) neurons also expressed the NGFI-A. The proportional distribution of the nNOS(+)/NGFI-A(+) neurons throughout the SZ/SGS layers showed a slight but significant rostro-caudal gradient. No significant difference was observed for the other markers, indicating homogeneous activation of these populations throughout the retinotopic map. Our results suggest that the visually-driven NGFI-A expression is not restricted to a specific population of the sSC and that visual processing in this structure, as assessed by the expression of this candidate-plasticity protein, involves the activation of subsets of ascending and non-ascending projection neurons.

Induction of the candidate-plasticity NGFI-A protein in the adult rat superior colliculus after visual stimulation.

In this work, we studied the visually driven expression of the plasticity-related transcription factor NFGI-A in the superficial layers of the rat superior colliculus (sSC) using immunohistochemistry. After dark adaptation, NGFI-A expression was completely down-regulated, indicating this protein is not constitutively expressed in the sSC. Light stimulation for 10 min after dark adaptation was insufficient to induce detectable levels of this protein. But after 30 min of light stimulation, few NGFI-A+ cells were observed in the superficial layers, indicating that the minimal time of stimulation that is sufficient to induce this protein is sometime between 10 and 30 min. The number of NGFI-A+ cells increased progressively, reaching a peak after 90 min. This peak is not reached if animals are returned to darkness after 30 min of stimulation, when a presumable peak in NGFI-A mRNA is reached. Light stimulation of animals in which the retinocollicular or corticocollicular projections were removed revealed that NGFI-A expression is mainly driven by retinal contralateral projections. Removal of corticocollicular projections did not cause any change in the NGFI-A expression in the ipsilateral sSC, in relation to the contralateral (control) sSC, suggesting that this pathway has a minor influence. Our results showed that NGFI-A protein expression in the sSC is entirely dependent on visual stimulation and suggests that the sSC visual circuitry is an interesting model for studies about the involvement of this transcription factor in synaptic plasticity.

A quantitative study of the neuronal nitric oxide synthase expression in the superficial layers of the adult rat superior colliculus after perinatal enucleation.

The level of expression of the neuronal nitric oxide synthase (nNOS) in the retinorecipient layers of rat superior colliculus (SC) was investigated in adult rats after neonatal enucleation using two biochemical methods: (1) measurement of the in vitro specific-activity of NOS by the conversion of [3H]-arginine to [3H]-citrulline and (2) immunochemical analysis by western blotting and densitometry of immunoreactive bands using antibodies that recognise the three prominent isoforms of nNOS, alpha, beta and gamma. A total of 20 Lister rats were used in this study. We have shown that the deprivation of the retinocollicular projections at early postnatal ages induces no significant change in the specific-activity of nNOS. We also have shown that the deafferentation does not significantly influence either the total amount of nNOS in the SC superficial layers or the relative contribution ratio of nNOS isoforms. In conclusion, the expression and activity of nNOS in the SC retinorecipient layers was shown not to be dependent on the presence of retinal afferents during development.

Removal of the cortical projections alters expression of NOS in the different cell types of the superficial layers of the superior colliculus in rats

Nitric oxide has several biological roles and nitric oxide synthase (NOS) is expressed in the nervous system, and co-localizes with NADPH-diaphorase. The superficial layers of the superior colliculus (SC), which receive retinal and cortical inputs, present NADPH-d staining in a sub-population of neurons that include all cell types. We have previously shown, by NADPH-diaphorase, that eye enucleation alters the intracellular distribution of NOS. Here, we studied the effect of cortical ablation on NOS expression by neurons in collicular superficial layers. Our results show that cortical ablation alters the proportion of different NOS-positive cell types, but not the intracellular distribution of the enzyme

Removal of the cortical projections alters expression of NOS in the different cell types of the superficial layers of the superior colliculus in rats.

Nitric oxide has several biological roles and nitric oxide synthase (NOS) is expressed in the nervous system, and co-localizes with NADPH-diaphorase. The superficial layers of the superior colliculus (SC), which receive retinal and cortical inputs, present NADPH-d staining in a sub-population of neurons that include all cell types. We have previously shown, by NADPH-diaphorase, that eye enucleation alters the intracellular distribution of NOS. Here, we studied the effect of cortical ablation on NOS expression by neurons in collicular superficial layers. Our results show that cortical ablation alters the proportion of different NOS-positive cell types, but not the intracellular distribution of the enzyme.

Patters of nitric oxide synthase expression in the developing superior colliculus

Nitric oxide (NO) is synthesized in cells of both the central and peripheral nervous system and has been implicated in several forms of synaptic plasticity. The enzyme that produces NO, nitric oxide synthase (NOS), can be visualized in the brain by the reduced nicotinamide adenine dinucleotide phosphate diaphorase histochemistry technique (NADPH-d). We have used NADPH-d activity to detect the presence of NOS-positive cells in the developing rat superior colliculus. Our results showed that NOS is present in cells and neuropil in the developing and adult rat superior colliculus. The first NOS-positive cells appeared at postnatal day 7 and were weakly stained. The number and intensity of the NOS-positive cells increased progressively during the following days reaching a maximum at postnatal day 15. By the end of the third postnatal week, both the number and intensity of stained cells showed an adult-like pattern. The NOS-positive cells showed a Golgi-like mosphology and we have found that all cell types present in the superior colliculus express the enzyme. The expression of NOS by tectal cells parallels the functional development of the retino-collicular and cortico-tectal projections and suggest that nitric oxide synthase-positive cells might be involved in this process. In this review we highlighted some of the recent descriptions of the expression of NOS in the mammalian visual system with emphasis in the superior colliculus and correlate these findings with several developmental events taking place in this structure.