A subset of clones in the chick telencephalon arranged in rostrocaudal arrays

Background Different areas of the vertebrate central nervous system appear to follow different rules during development for determining the position of sibling cells. For example, in the chick hindbrain, clones are frequently confined to a single functional unit that derives from a single rhombomere. In contrast, clones in the mammalian cerebral cortex often cross functional boundaries because of the extensive migration of sibling cells in orthogonal directions. We have investigated whether the pattern of clonal distribution in the chick telencephalon is similar to that of the hindbrain or to the more functionally analogous mammalian cerebral cortex. Progenitor cells in the chick telencephalon were marked using a retroviral library encoding alkaline phosphatase and over 10⁵ distinct molecular tags. Patterns of dispersion were detected using alkaline phosphatase histochemistry, followed by the recovery and sequencing of the molecular tag. We also analyzed the phenotypes of cells that occurred within the clones.Results A subset of progenitors gave rise to clones that were found in rostrocaudal arrays resembling tubes. Arrays were restricted in the mediolateral and dorsoventral planes but could span up to 4 mm in the rostrocaudal direction. They were found throughout the telencephalon and a single clone often spanned more than one telencephalic nucleus. Rostrocaudal clones comprised 60% of clones containing five or more cells and contained many different types of neurons, astrocytes, oligodendrocytes, or various combinations of these cell types.Conclusions Telencephalic progenitors are multipotent, producing progeny that become distinct cell types. Clonally related cells can migrate rostrocaudally within domains that are restrained in the mediolateral and dorsoventral directions. A subset of rostrocaudal clones resemble those seen in the mammalian cerebral cortex, with respect to the crossing of functional boundaries, but all rostrocaudal clones differ from the cerebral cortical clones in the pattern of spread of sibling cells, with the rostrocaudal clones being more constrained in the mediolateral and dorsoventral directions. A role for lineage in the patterning of the chick forebrain is supported by these observations. In addition, these data suggest a role for cues within the telencephalic marginal zone that serve to guide clones in their rostrocaudal migration.     

No effect of social group composition or size on hippocampal formation morphology and neurogenesis in mountain chickadees (Poecile gambeli)

Brain plasticity and adult neurogenesis may play a role in many ecologically important processes including mate recognition, song learning and production, and spatial memory processing. In a number of species, both physical and social environments appear to influence attributes (e.g., volume, neuron number, and neurogenesis) of particular brain regions. The hippocampus in particular is well known to be especially sensitive to such changes. Although social grouping in many taxa includes the formation of male and female pairs, most studies of the relationship between social environment and the hippocampus have typically considered only solitary animals and those living in same‐sex groups. Thus, the aim of this study was to compare the volume of the hippocampal formation, the total number of hippocampal neurons, and the number of immature neurons in the hippocampus (as determined by doublecortin expression) in mountain chickadees (Poecile gambeli) housed in groups of males and females, male–female pairs, same sex pairs of either males or females, and as solitary individuals. The different groups were visually and physically, but not acoustically, isolated from each other. We found no significant differences between any of our groups in hippocampal volume, the total number of hippocampal neurons, or the number of immature neurons. Our results thus provided no support to the hypothesis that social group composition and/or size have an effect on hippocampal morphology and neurogenesis. © 2010 Wiley Periodicals, Inc. Develop Neurobiol 70:538–547, 2010     

Decreasing temperature shifts hippocampal function from memory formation to modulation of hibernation bout duration in Syrian hamsters

Previous studies in hibernating species have characterized two forms of neural plasticity in the hippocampus, long-term potentiation (LTP) and its reversal, depotentiation, but not de novo long-term depression (LTD), which is also associated with memory formation. Studies have also shown that histamine injected into the hippocampus prolonged hibernation bout duration. However, spillover into the ventricles may have affected brain stem regions, not the hippocampus. Here, we tested the hypothesis that decreased brain temperature shifts the major function of the hippocampus in the Syrian hamster (Mesocricetus auratus) from one of memory formation (via LTP, depotentiation, and de novo LTD) to increasing hibernation bout duration. We found reduced evoked responses in hippocampal CA1 pyramidal neurons following low-frequency stimulation in young (<30 days old) and adult (>60 days old) hamsters, indicating that de novo LTD was generated in hippocampal slices from both pups and adults at temperatures >20°C. However, at temperatures below 20°C, synchronization of neural assemblies (a requirement for LTD generation) was markedly degraded, implying that de novo LTD cannot be generated in hibernating hamsters. Nonetheless, even at temperatures below 16°C, pyramidal neurons could still generate action potentials that may traverse a neural pathway, suppressing the ascending arousal system (ARS). In addition, histamine increased the excitability of these pyramidal cells. Taken together, these findings are consistent with the hypothesis that hippocampal circuits remain operational at low brain temperatures in Syrian hamsters and suppress the ARS to prolong bout duration, even though memory formation is muted at these low temperatures.     

Hippocampal glucocorticoid receptor expression in the tree shrew: Regulation by psychosocial conflict

Summary 1. This study was conducted to determine whether chronic psychosocial conflict alters the expression of glucocorticoid receptor (GR) mRNA in the hippocampus of male tree shrews (Tupaia belangeri).2. To generate probes for thein situ hybridization, the tree shrew GR gene was partly cloned. There was a 90% homology between the deduced amino acid sequence of the cloned tree shrew GR and that of the corresponding human GR sequence.³⁵S-Labeled riboprobes which had been transcribed from the tree shrew GR clone hybridized to pyramidal neurons in all subregions of the tree shrew hippocampal formation and to granule neurons in the dentate gyrus.3. Afterin situ hybridization, the expression of GR mRNA was semiquantitatively determined by counting silver grains over single neurons of the hippocampal formation of psychosocially stressed tree shrews and control animals. After 12 days of social conflict, the number of silver grains in the CA1 and CA3 pyramidal neurons of stressed animals was significantly lower than in controls. No statistically significant differences in mRNA expression were observed in the pyramidal neurons of the subiculum and in the granule neurons of the dentate gyrus.4. The present results suggest that psychosocial stress leads to a site-specific down-regulation of hippocampal GR via modification of mRNA expression.     

Sex and seasonal differences in hippocampal volume and neurogenesis in brood‐parasitic brown‐headed cowbirds (Molothrus ater)

Brown‐headed cowbirds (Molothrus ater) are one of few species in which females show more complex space use than males. Female cowbirds search for, revisit, and parasitize host nests and, in a previous study, outperformed males on an open field spatial search task. Previous research reported a female‐biased sex difference in the volume of the hippocampus, a region of the brain involved in spatial memory. Neurons produced by adult neurogenesis may be involved in the formation of new memories and replace older neurons that could cause interference in memory. We tested for sex and seasonal differences in hippocampal volume and neurogenesis of brood‐parasitic brown‐headed cowbirds and the closely related non‐brood‐parasitic red‐winged blackbird (Agelaius phoeniceus) to determine whether there were differences in the hippocampus that reflected space use in the wild. Females had a larger hippocampus than males in both species, but hippocampal neurogenesis, measured by doublecortin immunoreactivity (DCX+), was greater in female than in male cowbirds in the absence of any sex difference in blackbirds, supporting the hypothesis of hippocampal specialization in female cowbirds. Cowbirds of both sexes had a larger hippocampus with greater hippocampal DCX+ than blackbirds. Hippocampus volume remained stable between breeding conditions, but DCX+ was greater post‐breeding, indicating that old memories may be lost through hippocampal reorganization following breeding. Our results support, in part, the hypothesis that the hippocampus of cowbirds is specialized for brood parasitism. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1275–1290, 2016     

Heparan sulfates isolated from adult neural progenitor cells can direct phenotypic maturation

Multipotent progenitor stem cells that generate both neurons and glia are components of the hippocampus, subventricular zone and olfactory system of adult mammalian nervous system. The lineage choices any stem cell makes are known to be greatly dependent on the constitution of the extracellular matrix to which they are exposed during their development. Here, the adult rat hippocampus was used as a source of cells for clonal culture in order to investigate the effects of the extracellular glycosaminoglycan heparan sulfate (HS). Neurospheres were readily generated from adult tissue and could be used as a source of cells for further experiments. HS species that promote the actions of fibroblast growth factor-2 (FGF2) for embryonic neural progenitors were found to inhibit the actions of this mitogen for adult progenitors. Only HS fractions that promoted the actions of FGF1 had mitogenic effects on these adult cells. The adult cells proved difficult to clone from single cells. However, when endogenous HS was purified from these cells and added back at high concentration to single cells, the clones were capable of generating plentiful neuronal and glial progeny. The adult hippocampal progenitor (AHP) HS is composed of 32 kDa chains bearing 3 sulfated domains. A proportion of primary osteoblast stem cells exposed to the hippocampal HS adopt neuronal phenotypes. Hence, there appears to be a combination of HS-binding extracellular molecules that predispose cells to particular lineages.     

Melanin concentrating hormone induces hippocampal acetylcholine release via the medial septum in rats

Among various actions of melanin concentrating hormone (MCH), its memory function has been focused in animal studies. Although MCH neurons project to various areas in the brain, one main target site of MCH is hippocampal formation for memory consolidation. Recent immunohistochemical study shows that MCH neurons directly project to the hippocampal formation and may indirectly affect the hippocampus through the medial septum nucleus (MS). It has been reported that sleep is necessary for memory and that hippocampal acetylcholine (ACh) release is indispensable for memory consolidation. However, there is no report how MCH actually influences the hippocampal ACh effluxes in accordance with the sleep–wake cycle changes. Thus, we investigated the modulatory function of intracerebroventricular (icv) injection of MCH on the sleep–wake cycle and ACh release using microdialysis techniques. Icv injection of MCH significantly increased the rapid eye movement (REM) and non-REM episode time and the hippocampal, not cortical, ACh effluxes. There was a significant correlation between REM episode time and hippocampal ACh effluxes, but not between REM episode time and cortical ACh effluxes. Microinjection of MCH into the MS increased the hippocampal ACh effluxes with no influence on the REM episode time. It appears that the effect sites of icv MCH for prolongation of REM episode time may be other neuronal areas than the cholinergic neurons in the MS. We conclude that MCH actually increases the hippocampal ACh release at least in part through the MS in rats.     

New neurons generated from running are broadly recruited into neuronal activation associated with three different hippocampus-involved tasks

Running increases the formation of new neurons in the adult rodent hippocampus. However, the function of new neurons generated from running is currently unknown. One hypothesis is that new neurons from running contribute to enhanced cognitive function by increasing plasticity in the adult hippocampus. An alternative hypothesis is that new neurons generated from running incorporate into experience-specific hippocampal networks that only become active during running. The purpose of this experiment was to determine if new neurons generated from running are selectively activated by running, or can become recruited into granule cell activity occurring during performance on other behavioral tasks that engage the hippocampus. Therefore, the activation of new 5-6 week neurons was detected using BrdU, NeuN, and Zif268 triple-label immunohistochemistry in cohorts of female running and sedentary adult C57BL/6J mice following participation in one of three different tasks: the Morris water maze, novel environment exploration, or wheel running. Results showed that running and sedentary mice displayed a nearly equivalent proportion of new neurons that expressed Zif268 following each task. Since running approximately doubled the number of new neurons, the results demonstrated that running mice had a greater number of new neurons recruited into the Zif268 induction in the granule cell layer following each task than sedentary mice. The results suggest that new neurons incorporated into hippocampal circuitry from running are not just activated by wheel running itself, but rather become broadly recruited into granule cell layer activity during distinct behavioral experiences. ? 2012 Wiley Periodicals, Inc.     

Dual Coding with STDP in a Spiking Recurrent Neural Network Model of the Hippocampus

The firing rate of single neurons in the mammalian hippocampus has been demonstrated to encode for a range of spatial and non-spatial stimuli. It has also been demonstrated that phase of firing, with respect to the theta oscillation that dominates the hippocampal EEG during stereotype learning behaviour, correlates with an animal''s spatial location. These findings have led to the hypothesis that the hippocampus operates using a dual (rate and temporal) coding system. To investigate the phenomenon of dual coding in the hippocampus, we examine a spiking recurrent network model with theta coded neural dynamics and an STDP rule that mediates rate-coded Hebbian learning when pre- and post-synaptic firing is stochastic. We demonstrate that this plasticity rule can generate both symmetric and asymmetric connections between neurons that fire at concurrent or successive theta phase, respectively, and subsequently produce both pattern completion and sequence prediction from partial cues. This unifies previously disparate auto- and hetero-associative network models of hippocampal function and provides them with a firmer basis in modern neurobiology. Furthermore, the encoding and reactivation of activity in mutually exciting Hebbian cell assemblies demonstrated here is believed to represent a fundamental mechanism of cognitive processing in the brain.     

Synaptic plasticity, memory and the hippocampus: a neural network approach to causality

Two facts about the hippocampus have been common currency among neuroscientists for several decades. First, lesions of the hippocampus in humans prevent the acquisition of new episodic memories; second, activity-dependent synaptic plasticity is a prominent feature of hippocampal synapses. Given this background, the hypothesis that hippocampus-dependent memory is mediated, at least in part, by hippocampal synaptic plasticity has seemed as cogent in theory as it has been difficult to prove in practice. Here we argue that the recent development of transgenic molecular devices will encourage a shift from mechanistic investigations of synaptic plasticity in single neurons towards an analysis of how networks of neurons encode and represent memory, and we suggest ways in which this might be achieved. In the process, the hypothesis that synaptic plasticity is necessary and sufficient for information storage in the brain may finally be validated.     

Sleepy dialogues between cortex and hippocampus: who talks to whom?

During NREM sleep, neocortical neurons undergo near-synchronous transitions, every second or so, between UP states, during which they are depolarized and fire actively, and DOWN states, during which they are hyperpolarized and completely silent. In this issue of Neuron, Isomura et al. report that slow oscillations of membrane potential occur near-synchronously not only in neocortex but also in entorhinal cortex and subiculum. Within the hippocampus proper, pyramidal neurons lack the bistability of UP and DOWN states, but their firing is strongly modulated by cortical activity during the UP state. Intriguingly, many hippocampal neurons fire during the cortical DOWN state. Thus, during sleep UP states, the cortex can talk to the hippocampus, but it is unclear whether the hippocampus talks back.     

The Interesting Interplay Between Interneurons and Adult Hippocampal Neurogenesis

Adult neurogenesis is a unique form of plasticity found in the hippocampus, a brain region key to learning and memory formation. While many external stimuli are known to modulate the generation of new neurons in the hippocampus, little is known about the local circuitry mechanisms that regulate the process of adult neurogenesis. The neurogenic niche in the hippocampus is highly complex and consists of a heterogeneous population of cells including interneurons. Because interneurons are already highly integrated into the hippocampal circuitry, they are in a prime position to influence the proliferation, survival, and maturation of adult-generated cells in the dentate gyrus. Here, we review the current state of our understanding on the interplay between interneurons and adult hippocampal neurogenesis. We focus on activity- and signaling-dependent mechanisms, as well as research on human diseases that could provide better insight into how interneurons in general might add to our comprehension of the regulation and function of adult hippocampal neurogenesis.     

Neuroprotective effects of lactation against kainic acid treatment in the dorsal hippocampus of the rat

Marked hippocampal changes in response to excitatory amino acid agonists occur during pregnancy (e.g. decreased frequency in spontaneous recurrent seizures in rats with KA lesions of the hippocampus) and lactation (e.g. reduced c-Fos expression in response to N-methyl-d,l-aspartic acid but not to kainic acid). In this study, the possibility that lactation protects against the excitotoxic damage induced by KA in hippocampal areas was explored. We compared cell damage induced 24 h after a single systemic administration of KA (5 or 7.5 mg/kg bw) in regions CA1, CA3, and CA4 of the dorsal hippocampus of rats in the final week of lactation to that in diestrus phase. To determine cellular damage in a rostro-caudal segment of the dorsal hippocampus, we used NISSL and Fluorojade staining, immunohistochemistry for active caspase-3 and TUNEL, and we observed that the KA treatment provoked a significant loss of neurons in diestrus rats, principally in the pyramidal cells of CA1 region. In contrast, in lactating rats, pyramidal neurons from CA1, CA3, and CA4 in the dorsal hippocampus were significantly protected against KA-induced neuronal damage, indicating that lactation may be a natural model of neuroprotection.     

Segmental lineage restrictions in the chick embryo spinal cord depend on the adjacent somites.

We have investigated whether the developing spinal cord is intrinsically segmented in its rostrocaudal (anteroposterior) axis by mapping the spread of clones derived from single labelled cells within the neural tube of the chick embryo. A single cell in the ventrolateral neural tube of the trunk was marked in situ with the fluorescent tracer lysinated rhodamine dextran (LRD) and its descendants located after two days of further incubation. We find that clones derived from cells labelled before overt segmentation of the adjacent mesoderm do not respect any boundaries within the neural tube. Those derived from cells marked after mesodermal segmentation, however, never cross an invisible boundary aligned with the middle of each somite, and tend to be elongated along the mediolateral axis of the neural tube. When the somite pattern is surgically disturbed, neighbouring clones derived from neuroectodermal cells labelled after somite formation behave like clones derived from younger cells: they no longer respect any boundaries, and are not elongated mediolaterally. These results indicate that periodic lineage restrictions do exist in the developing spinal cord of the chick embryo, but their maintenance requires the presence of the adjacent somite mesoderm.     

Regional health and function in the hippocampus: Evolutionary compromises for a critical brain region

The hippocampus is especially vulnerable to damage caused by metabolic dysregulation. However distinct sub-regions within the hippocampus differ by their relative susceptibility to such damage. Region CA1 pyramidal neurons are most sensitive to metabolic perturbations while region CA3 pyramidal neurons show more resistance, and these unique profiles of susceptibility are but one example that differentiates CA1/CA3 neurons. We present here a hypothesis that inextricably links the unique biochemistries of learning and memory in region CA1, to that of cell survival signaling, and in so doing, suggest an explanation for region CA1 susceptibility to metabolic dysfunction. Further, we propose a signaling mechanism to explain how both pathways can be simultaneously regulated. Critical to this process is the protein phosphatase PHLPP1. Finally we discuss the implications of this hypothesis and the inherent challenges it poses for treatment of neurological disorders resulting in reduced hippocampal function by increased neuron death.     

Induction of excitatory and inhibitory presynaptic differentiation by GluD1

The δ subfamily of ionotropic glutamate receptor subunits consists of GluD1 and GluD2. GluD2, which is selectively expressed in cerebellar Purkinje neurons, has been shown to contribute to the formation of synapses between granule neurons and Purkinje neurons through interaction with Cbln1 (cerebellin precursor protein1) and presynaptic Neurexin. On the other hand, the synaptogenic activity of GluD1, which is expressed not in the cerebellum but in the hippocampus, remains to be characterized. Here, we report that GluD1 expressed in non-neuronal HEK cells, induced presynaptic differentiation of granule neurons through its N-terminal domain in co-cultures with cerebellar neurons, similarly to GluD2. We also show that GluD1 rescued the defect of synapse formation in GluD2-knockout Purkinje neurons, indicating the functional similarity of GluD1 and GluD2. In contrast, GluD1 expression alone did not induce presynaptic differentiation in co-cultures of HEK cells with hippocampal neurons. However, when Cbln1 was exogenously added to the culture medium, GluD1 induced presynaptic differentiation of not only glutamatergic presynaptic terminals but also GABAergic ones. Cbln1 is not expressed in hippocampal neurons but is expressed in entorhinal cortical neurons projecting to the hippocampus. In co-cultures of HEK cells expressing GluD1 and entorhinal cortical neurons, both glutamatergic and GABAergic presynaptic terminals were formed on the HEK cells without exogenous application of Cbln1. These results suggest that GluD1 might contribute to the formation of specific synapses in the hippocampus such as those formed by the projecting neurons of the entorhinal cortex.     

Dominance-related changes in spatial memory are associated with changes in hippocampal cell proliferation rates in mountain chickadees

It is well established that spatial memory is dependent on the hippocampus in both mammals and birds. As memory capacity can fluctuate on a temporal basis, it is important to understand the mechanisms mediating such changes. It is known that early memory-dependent experiences in young animals result in hippocampal enlargement and in increased neurogenesis, including cell proliferation and neuron survival. It is less clear, however, whether temporal changes in spatial memory are also associated with changes in hippocampal anatomy and cell proliferation in fully grown and experienced adult animals. In a previous study, we experimentally demonstrated that socially subordinate mountain chickadees (Poecile gambeli) showed inferior spatial memory performance compared to their dominant group mates, in the absence of significant differences in baseline corticosterone levels. Here we investigated whether these differences in memory between dominant and subordinate birds were associated with changes in the hippocampus. Following memory tests, chickadees were injected with 5-bromo-2'-deoxyuridine to label dividing cells and sacrificed 2 days after the injections. We found no significant differences in volume or the total number of neurons in the hippocampal formation between dominant and subordinate chickadees, but subordinate birds had significantly lower cell proliferation rates in the ventricular zone adjacent to both the hippocampus and mesopallium compared to the dominants. Individuals, which performed better on spatial memory tests tended to have higher levels of cell proliferation. These results suggest that social status can affect cell proliferation rates in the ventricular zone and support the hypothesis that neurogenesis might be involved in memory function in adult animals.     

Dominance‐related changes in spatial memory are associated with changes in hippocampal cell proliferation rates in mountain chickadees

It is well established that spatial memory is dependent on the hippocampus in both mammals and birds. As memory capacity can fluctuate on a temporal basis, it is important to understand the mechanisms mediating such changes. It is known that early memory‐dependent experiences in young animals result in hippocampal enlargement and in increased neurogenesis, including cell proliferation and neuron survival. It is less clear, however, whether temporal changes in spatial memory are also associated with changes in hippocampal anatomy and cell proliferation in fully grown and experienced adult animals. In a previous study, we experimentally demonstrated that socially subordinate mountain chickadees (Poecile gambeli) showed inferior spatial memory performance compared to their dominant group mates, in the absence of significant differences in baseline corticosterone levels. Here we investigated whether these differences in memory between dominant and subordinate birds were associated with changes in the hippocampus. Following memory tests, chickadees were injected with 5‐bromo‐2′‐deoxyuridine to label dividing cells and sacrificed 2 days after the injections. We found no significant differences in volume or the total number of neurons in the hippocampal formation between dominant and subordinate chickadees, but subordinate birds had significantly lower cell proliferation rates in the ventricular zone adjacent to both the hippocampus and mesopallium compared to the dominants. Individuals, which performed better on spatial memory tests tended to have higher levels of cell proliferation. These results suggest that social status can affect cell proliferation rates in the ventricular zone and support the hypothesis that neurogenesis might be involved in memory function in adult animals. © 2004 Wiley Periodicals, Inc. J Neurobiol, 2005     

Age-related decline in the tyrosine hydroxylase-immunoreactive innervation of the amygdala and dentate gyrus in mice

Numbers of catecholaminergic neurons are known to decline with aging. Whether projections of these neurons to the forebrain are similarly affected is not known. High densities of tyrosine hydroxylase-immunoreactive (TH-ir) fibers are found in the hippocampal formation (CA1-3, dentate gyrus) and in the amygdala of normal adult mice. We report here that densities of TH-ir fibers in the amygdala and hippocampus in aged mice (21-26 months) decrease dramatically and in a subregion-specific fashion. There is a reduction of 35% in the dentate gyrus, while hippocampal regions CA1 through CA3 are almost entirely spared. In the amygdala the lateral, basolateral, basomedial, and central nucleus were affected, with fiber reduction ranging from 19% to 34%. These results indicate that the age-related decline of TH-ir catecholaminergic cell bodies in the substantia nigra and the ventral tegmental area induces substantial losses of TH-ir fibers in the amygdala and dentate gyrus, but not in other areas of the hippocampal formation. This suggests that region-specific factors may be implicated in the regulation of maintenance vs. degeneration of TH-ir fibers during aging.     

Interaction between endogenous nitric oxide and carbon monoxide in the pathogenesis of recurrent febrile seizures

The aim of the study was to investigate the interaction between nitric oxygenase (NOS)/nitric oxide (NO) and heme oxygenase (HO)/carbon monoxide (CO) system in the pathogenesis of recurrent febrile seizures (FS). On a rat model of recurrent FS, the ultrastructure of hippocampal neurons was observed under electron microscopy, and expression of neuronal NOS (nNOS) in hippocampus and NO formation in plasma were examined after treatment with ZnPP-IX, an HO-1 inhibitor. In the ultrastructure of hippocampal neurons, the expression of HO-1 in hippocampus and CO formation in plasma were examined after treatment with L-NAME, a NOS inhibitor. We found that hippocampal neurons were injured after recurrent FS. The gene and protein expression of nNOS and HO-1 increased markedly in hippocampus in FS rats, while CO formation in plasma increased markedly and the concentration of NO in plasma increased slightly. ZnPP-IX could worsen the neuronal damage of recurrent FS rats. However, it further increased the expression of nNOS and endogenous production of NO obviously. L-NAME alleviated the neuronal damage of recurrent FS rats, but decreased the expression of HO-1 and CO formation. The results of this study suggested that endogenous NOS/NO and HO/CO systems might interact with each other and therefore play an important regulating role in recurrent FS brain damage.