Neurons in rat hippocampus that synthesize nitric oxide

Mild hypoxia-induced structural and functional changes of the hippocampal network

Hypoxia causes structural and functional changes in several brain regions, including the oxygen-concentration-sensitive hippocampus. We investigated the consequences of mild short-term hypoxia on rat hippocampus in vivo. The hypoxic group was treated with 16% O2 for 1 h, and the control group with 21% O2. Using a combination of Gallyas silver impregnation histochemistry revealing damaged neurons and interneuron-specific immunohistochemistry, we found that somatostatin-expressing inhibitory neurons in the hilus were injured. We used 32-channel silicon probe arrays to record network oscillations and unit activity from the hippocampal layers under anaesthesia. There were no changes in the frequency power of slow, theta, beta, or gamma bands, but we found a significant increase in the frequency of slow oscillation (2.1-2.2 Hz) at 16% O2 compared to 21% O2. In the hilus region, the firing frequency of unidentified interneurons decreased. In the CA3 region, the firing frequency of some unidentified interneurons decreased while the activity of other interneurons increased. The activity of pyramidal cells increased both in the CA1 and CA3 regions. In addition, the regularity of CA1, CA3 pyramidal cells' and CA3 type II and hilar interneuron activity has significantly changed in hypoxic conditions. In summary, a low O2 environment caused profound changes in the state of hippocampal excitatory and inhibitory neurons and network activity, indicating potential changes in information processing caused by mild short-term hypoxia.

Decreased Hippocampal Neurogenesis in Aged Male Wistar Rats Is Not Associated with Memory Acquisition in a Water Maze

Brain aging is associated with a progressive decrease in learning abilities, memory, attention, decision making, and sensory perception. Age-related cognitive disturbances may be related to a decrease in the functional capacities of the hippocampus. This brain region is essential for learning and memory, and the lifelong neurogenesis occurring in the subgranular zone of the dentate gyrus may be a key event mediating the mnemonic functions of the hippocampus. In the present study, we investigated whether age-related changes in hippocampal neurogenesis are associated with learning and memory disturbances. Four- and 24-month-old rats were trained to find a hidden platform in a water maze. Though the older group showed higher latency to search the platform as compared to the younger group, both groups learned the task. However, the density of proliferating (PCNA-positive), differentiating (Dcx-positive), and new neurons (pre-labeled BrdU-positive) was significantly lower in the hippocampus of aged rats as compared to young ones. This inhibition of neurogenesis could be related to increased local production of nitric oxide since the density of neurons expressing neuronal NO-synthase was higher in the aged hippocampus. Thus, we can suggest that an age-related decrease in neurogenesis is not directly associated with place learning in aged rats.

Alterations in the intrinsic discharge activity of CA1 pyramidal neurons associated with possible changes in the NADPH diaphorase activity in a rat model of autism induced by prenatal exposure to valproic acid

Autism spectrum disorder is a neurodevelopmental disorder characterized by sensory abnormalities, social skills impairment and cognitive deficits. Although recent evidence indicated that induction of autism-like behavior in animal models causes abnormal neuronal excitability, the impact of autism on neuronal properties is still an important issue. Thus, new findings at the cellular level may shed light on the pathophysiology of autism and may help to find effective treatment strategies. Here, we investigated the behavioral, electrophysiological and histochemical impacts of prenatal exposure to valproic acid (VPA) in rats. Findings revealed that VPA exposure caused a significant increase in the hot plate response latency. The novel object recognition ability was also impaired in VPA-exposed rats. Along with these behavioral alterations, neurons from VPA-exposed animals exhibited altered excitability features in response to depolarizing current injections relative to control neurons. In the VPA-exposed group, these changes consisted of a significant increase in the amplitude, evoked firing frequency and the steady-state standard deviation of spike timing of action potentials (APs). Moreover, the half-width, the AHP amplitude and the decay time constant of APs were significantly decreased in this group. These changes in the evoked electrophysiological properties were accompanied by intrinsic hyperexcitability and lower spike-frequency adaptation and also a significant increase in the number of NADPH-diaphorase stained neurons in the hippocampal CA1 area of the VPA-exposed rats. Taken together, findings demonstrate that abnormal nociception and recognition memory is associated with alterations in the neuronal responsiveness and nitrergic system in a rat model of autism-like.

The Role of 5-HT1A Receptors and Neuronal Nitric Oxide Synthase in a Seizur Induced Kindling Model in Rats

BACKGROUND AND OBJECTIVE

Dentate gyrus (DG) has a high density of 5-HT1A receptors. It has neural nitric oxide synthase (nNOS), which is involved in neural excitability. The purpose of this study was to investigate the role of 5-HT1A receptors and nNOS of DG in perforant path kindling model of epilepsy.

MATERIAL AND METHODS

To achieve this purpose, a receptor antagonist (WAY100635, 0.1 mg/kg, intracerebroventricular, i.c.v) and neuronal nitric oxide synthase inhibitor (7-NI, 15 mg/kg, intraperitoneal, i.p.) were injected during kindling aquisition. Adult male Wistar rats (280 ± 20 g) were used in this study Animals were kindled through the daily administration of brief electrical stimulations (10 stimulations per day) to the perforant pathway. Field potential recordings were performed for 20 min in DG beforehand. Additionally, glial fibrillary acidic protein (GFAP) expression rate in the DG was determined using immunohistochemistry as a highly specific marker for glia.

RESULTS

WAY100635 (0.1 mg/kg) significantly attenuated the kindling threshold compared to the kindled + vehicle group (P < 0.001). The co-administration of WAY100635 with 7-NI, exerted a significant anticonvulsive effect. Furthermore, the slope of field Excitatory Post Synaptic Potentials (fEPSP) at the end of 10 days in the kindled + 7-NI + WAY100635 group was significantly lower than in the kindled + vehicle group (P < 0.001). Furthermore, immunohistochemistry showed that the density of GAFP⁺ cells in the kindled + 7-NI + WAY100635 group was significantly higher than in the kindled + vehicle group (P < 0.001).

CONCLUSION

Our data demonstrate that antagonists of 5-HT1A receptors have proconvulsive effects and that astrocyte cells are involved in this process, while nNOS has an inhibitory effect on neuronal excitability.

Ictal onset sites and γ-aminobutyric acidergic neuron loss in epileptic pilocarpine-treated rats

OBJECTIVE

The present study tested whether ictal onset sites are regions of more severe interneuron loss in epileptic pilocarpine-treated rats, a model of human temporal lobe epilepsy.

METHODS

Local field potential recordings were evaluated to identify ictal onset sites. Electrode sites were visualized in Nissl-stained sections. Adjacent sections were processed with proximity ligation in situ hybridization for glutamic acid decarboxylase 2 (Gad2). Gad2 neuron profile numbers at ictal onset sites were compared to contralateral regions. Other sections were processed with immunocytochemistry for reelin or nitric oxide synthase (NOS), which labeled major subtypes of granule cell layer-associated interneurons. Stereology was used to estimate numbers of reelin and NOS granule cell layer-associated interneurons per hippocampus.

RESULTS

Ictal onset sites varied between and within rats but were mostly in the ventral hippocampus and were frequently bilateral. There was no conclusive evidence of more severe Gad2 neuron profile loss at sites of earliest seizure activity compared to contralateral regions. Numbers of granule cell layer-associated NOS neurons were reduced in the ventral hippocampus.

SIGNIFICANCE

In epileptic pilocarpine-treated rats, ictal onset sites were mostly in the ventral hippocampus, where there was loss of granule cell layer-associated NOS interneurons. These findings suggest the hypothesis that loss of granule cell layer-associated NOS interneurons in the ventral hippocampus is a mechanism of temporal lobe epilepsy.

Characterization of NADPH Diaphorase- and Doublecortin-Positive Neurons in the Lizard Hippocampal Formation

The lizard cortex has remarkable similarities with the mammalian hippocampus. Both regions process memories, have similar cytoarchitectural properties, and are important neurogenic foci in adults. Lizards show striking levels of widespread neurogenesis in adulthood and can regenerate entire cortical areas after injury. Nitric oxide (NO) is an important regulatory factor of mammalian neurogenesis and hippocampal function. However, little is known about its role in nonmammalian neurogenesis. Here, we analyzed the distribution, morphology, and dendritic complexity (Neurolucida reconstructions) of NO-producing neurons through NADPH diaphorase (NADPHd) activity, and how they compare with the distribution of doublecortin-positive (DCX+) neurons in the hippocampal formation of the neotropical lizard Tropidurus hispidus. NADPHd-positive (NADPHd+) neurons in the dorsomedial cortex (DMC; putatively homologous to mammalian CA3) were more numerous and complex than the ones in the medial cortex (MC; putatively homologous to the dentate gyrus). We found that NADPHd+ DMC neurons send long projections into the MC. Interestingly, in the MC, NADPHd+ neurons existed in 2 patterns: small somata with low intensity of staining in the outer layer and large somata with high intensity of staining in the deep layer, a pattern similar to the mammalian cortex. Additionally, NADPHd+ neurons were absent in the granular cell layer of the MC. In contrast, DCX+ neurons were scarce in the DMC but highly numerous in the MC, particularly in the granular cell layer. We hypothesize that NO-producing neurons in the DMC provide important input to proliferating/migrating neurons in the highly neurogenic MC.

Immunohistochemical investigation of the internal structure of the mouse subiculum

The subiculum is the output component of the hippocampal formation and holds a key position in the neural circuitry of memory. Previous studies have demonstrated the subiculum's connectivity to other brain areas in detail; however, little is known regarding its internal structure. We investigated the cytoarchitecture of the temporal and mid-septotemporal parts of the subiculum using immunohistochemistry. The border between the CA1 region and subiculum was determined by both cytoarchitecture and zinc transporter 3 (ZnT3)-immunoreactivity (IR), whereas the border between the subiculum and presubiculum (PreS) was partially indicated by glutamate receptor 1 (GluR1)-IR. The subiculum was divided into proximal and distal subfields based on cytoarchitecture and immunohistochemistry for calbindin (CB), nitric oxide synthase (NOS) and Purkinje cell protein 4 (PCP4). The proximal subiculum (defined here as subiculum 2) was composed of five layers: the molecular layer (layer 1), the medium-sized pyramidal cell layer (layer 2) that contained NOS- and PCP4-positive neurons, the large pyramidal cell layer (layer 3) characterized by the accumulation of ZnT3- (more proximally) and vesicular glutamate transporter 2-positive (more distally) boutons, layer 4 containing polymorphic cells, and the deepest layer 5 composed of PCP4-positive cells with long apical dendrites that reached layer 1. The distal subiculum (subiculum 1) consisting of smaller neurons did not show these features. Quantitative analyses of the size and numerical density of somata substantiated this delineation. Both the proximal-distal division and five-layered structure in the subiculum 2 were confirmed throughout the temporal two-thirds of the subiculum. These findings will provide a new structural basis for hippocampal investigations.

Neurotransmitter-mediated control of neurogenesis in the adult vertebrate brain

It was long thought that no new neurons are added to the adult brain. Similarly, neurotransmitter signaling was primarily associated with communication between differentiated neurons. Both of these ideas have been challenged, and a crosstalk between neurogenesis and neurotransmitter signaling is beginning to emerge. In this Review, we discuss neurotransmitter signaling as it functions at the intersection of stem cell research and regenerative medicine, exploring how it may regulate the formation of new functional neurons and outlining interactions with other signaling pathways. We consider evolutionary and cross-species comparative aspects, and integrate available results in the context of normal physiological versus pathological conditions. We also discuss the potential role of neurotransmitters in brain size regulation and implications for cell replacement therapies.

The ventral hippocampus NMDA receptor/nitric oxide/guanylate cyclase pathway modulates cardiovascular responses in rats

The hippocampus is a limbic structure that is involved in the expression of defensive reactions and autonomic changes in rats. The injection of L-glutamate (L-glu) into the ventral hippocampus (VH) decreases blood pressure and heart rate in anesthetized rats. Activation of NMDA receptors in the VH increases the production of nitric oxide (NO), leading to guanylate cyclase activation. The hypothesis of the present study was that a local NMDA receptor-NO-guanylate cyclase interaction mediates the cardiovascular effects of microinjection of L-glu into the VH. Microinjection of increasing doses of L-glu (30, 60 and 200 nmol/200 nL) into the VH of conscious rats caused dose-related pressor and tachycardiac responses. The cardiovascular effects of L-glu were abolished by local pretreatment with: the glutamate receptor antagonist AP-7 (0.4 nmol); the selective neuronal NO synthase (nNOS) inhibitor N(ω)-Propyl-L-arginine (0.04 nmol); the NO scavenger C-PTIO (2 nmol) or the guanylate cyclase inhibitor 1H-[1,2,4] oxadiazolol [4,3-a]quinoxalin-1-one (2 nmol). Moreover, these cardiovascular responses were blocked by intravenous pretreatment with: the ganglionic blocker mecamylamine (2mg/Kg); the nonselective β-adrenergic receptor antagonist propranolol (2mg/Kg); the β1-adrenergic receptor selective antagonist atenolol (1mg/kg). However, pretreatment with the selective α1-adrenergic receptor antagonist prazosin (0,5mg/kg) caused only a small reduction in the pressor response, without affecting the L-glu evoked tachycardia. In conclusion, our results suggest that cardiovascular responses caused by L-glu microinjection into the VH are mediated by NMDA glutamate receptors and involve local nNOS and guanylate cyclase activation. Moreover, these cardiovascular responses are mainly mediated by cardiac sympathetic nervous system activation, with a small involvement of the vascular sympathetic nervous system.

Prenatal stress affects network properties of rat hippocampal neurons

BACKGROUND

Long-term effects of stress during pregnancy on brain and behavior have been analyzed extensively in recent years. One major problem with these studies is the inability to separate between the net effects of the prenatal stress (PS) and the effects of the stressed mother and siblings on the newborn animals.

METHODS

To address these issues, we studied morphological and electrophysiological properties of neurons in dissociated cultures of the hippocampus taken from newborn PS rats. We complemented these studies with experiments on behaving rats and recordings from slices taken from PS rats and their control rats.

RESULTS

While the density of cultured neurons was not different between PS and control rats, there were fewer glutamic acid decarboxylase-positive neurons in the former cultures. Additionally, cells taken from PS pups developed more extensive dendrites than control animals. These differences were correlated with a higher rate of synchronous activity in the PS cultures and a lower rate of spontaneous miniature inhibitory postsynaptic current activity. There were no differences in the excitatory synaptic currents or the passive and active properties of the recorded neurons in the two groups. Young PS rats were more motile in open field and elevated plus maze than control rats, and they learned faster to navigate in a water maze. Slices taken from hippocampus of PS rats expressed less paired-pulse inhibition than slices from control rats.

CONCLUSIONS

These results indicate that PS affects network properties of hippocampal neurons, by reducing gamma-aminobutyric acidergic inhibition.

Distribution and neurochemical features of neuronal nitric oxide synthase-expressing interneurons in the rat dentate gyrus

Neuronal nitric oxide synthase (nNOS)-expressing interneurons are abundant in the dentate gyrus (DG) of rodents. In the present study, we immunohistochemically analyzed nNOS-positive cells in the rat DG by focusing on their GABAergicity, laminar distribution, and co-localization with calcium-binding proteins and neuropeptides. Experiments were conducted in adult male Sprague Dawley rats. Within the DG, nNOS-positive cells were found to reside in all three layers of DG; percentages of distribution in the molecular layer, granule cell layer and the hilus are 25.4%, 9.4% and 65.2%, respectively. Almost every nNOS-positive cell expressed glutamic acid decarboxylase 67 (GAD67) or glutamic acid decarboxylase 65 (GAD65). In the molecular layer, nearly two-thirds of GAD67-positive cells expressed nNOS. Percentages of nNOS-positive interneurons that expressed cholecystokinin, vasoactive intestinal polypeptide, parvalbumin, somatostatin, neuropeptide Y, and calretinin were approximately 0.8%, 1.8%, 9.2%, 10.3%, 13.8%, and 24.4%, respectively. In the molecular layer, the number of nNOS-positive cells far exceeded the sum total of cells positive for both nNOS and any of the above mentioned calcium-binding proteins or neuropeptides, indicating that a large proportion of nNOS-positive interneurons seldom express calcium-binding proteins or neuropeptides in this area. We conclude that nNOS expressing cells are an important neurochemically defined type of GABAergic interneuron in the rat DG showing a specific laminar-dependent distribution and expressing calcium-binding proteins and neuropeptides at different frequencies. In the molecular layer, most nNOS-positive interneurons do not express calcium-binding proteins or neuropeptides; they could be the missing pieces in the GABAergic interneuron jigsaw puzzle of this DG layer.

Neurons of the dentate molecular layer in the rabbit hippocampus

The molecular layer of the dentate gyrus appears as the main entrance gate for information into the hippocampus, i.e., where the perforant path axons from the entorhinal cortex synapse onto the spines and dendrites of granule cells. A few dispersed neuronal somata appear intermingled in between and probably control the flow of information in this area. In rabbits, the number of neurons in the molecular layer increases in the first week of postnatal life and then stabilizes to appear permanent and heterogeneous over the individuals' life span, including old animals. By means of Golgi impregnations, NADPH histochemistry, immunocytochemical stainings and intracellular labelings (lucifer yellow and biocytin injections), eight neuronal morphological types have been detected in the molecular layer of developing adult and old rabbits. Six of them appear as interneurons displaying smooth dendrites and GABA immunoreactivity: those here called as globoid, vertical, small horizontal, large horizontal, inverted pyramidal and polymorphic. Additionally there are two GABA negative types: the sarmentous and ectopic granular neurons. The distribution of the somata and dendritic trees of these neurons shows preferences for a definite sublayer of the molecular layer: small horizontal, sarmentous and inverted pyramidal neurons are preferably found in the outer third of the molecular layer; vertical, globoid and polymorph neurons locate the intermediate third, while large horizontal and ectopic granular neurons occupy the inner third or the juxtagranular molecular layer. Our results reveal substantial differences in the morphology and electrophysiological behaviour between each neuronal archetype in the dentate molecular layer, allowing us to propose a new classification for this neural population.

Real-time measurement of murine hippocampus NO levels in response to cerebral ischemia/reperfusion

Nitric oxide has been implicated as a mediator of synaptic transmission and a pathological factor in stroke/reperfusion. The purpose of this study was to detect the change of nitric oxide concentration in rat hippocampus during global cerebral ischemia and reperfusion in vivo and to reveal effects of different nitric oxide synthases. In the present study, the real-time record of nitric oxide levels in rat hippocampus was obtained by using a nitric oxide sensor during global cerebral ischemia and the initial stage of reperfusion. We also observed the effects of two inhibitors of nitric oxide synthases on nitric oxide concentration. The two inhibitors were administrated intravenously at the onset of reperfusion and 1 h later. The change of the nitric oxide concentration in the initial stage of reperfusion was 0.768 ± 0.029 μM. 7-Nitroindazole (7-NI7-NI , inhibitor of nNOS) had a strong inhibitive effect on nitric oxide synthesis at both time points, while 1400 W1400 W dihydrochloride (1400 W, inhibitor of iNOSiNOS ) had no significant effect on nitric oxide synthesis. The results showed that during the initial stage of reperfusion, nitric oxide biosynthesis was mainly an nNOS-dependent process.

The dynamic detection of NO during stroke and reperfusion in vivo

PURPOSE

Nitric oxide (NO) has been implicated as a mediator of synaptic transmission and a pathological factor in stroke/reperfusion. The purpose of this study was to detect the change of NO concentration in rat hippocampus during global cerebral ischemia and reperfusion in vivo and to reveal effects of different NO synthases (NOS).

METHOD

In the present study, the real-time record of NO levels in rat hippocampus was obtained by using a NO sensor during the global cerebral ischemia and the initial stage of reperfusion. The effects of two inhibitors of NOS on NO concentration were also observed. The two inhibitors were respectively administrated intravenously at the onset of reperfusion and 1 hour later.

RESULTS

The change of the NO concentration in the initial stage of reperfusion was 0.768 +/- 0.029 microM. 7-nitroindazole (7-NI, inhibitor of nNOS) had a strong inhibitive effect on NO synthesis at both time points, while 1400W dihydrochloride (1400W, inhibitor of iNOS) had no significant effect on the NO synthesis.

CONCLUSIONS

The in vivo detection revealed the real dynamic change of NO concentration, which is much more reliable than the in vitro method. The results showed that, during the initial stage of reperfusion, NO biosynthesis was mainly in an nNOS-dependent manner. Thus, the toxicity of NO in this process had a close relationship with the activity of nNOS but not iNOS.

Role of ventral hippocampal nitric oxide/cGMP pathway in anxiety-related behaviors in rats submitted to the elevated T-maze

The L-arginine/nitric oxide (NO)/cGMP pathways have been implicated in the control of a variety of physiological mechanisms and are believed to participate in the modulation of anxiety in the CNS. The aim of this study was to investigate the effects of N(G)-nitro-L-arginine-methyl-ester (L-NAME), a non-selective inhibitor of NO synthase (NOS); 7-nitroindazole (7-NI), a preferential inhibitor of neuronal NOS; and sodium nitroprusside (SNP), an NO donor, administered into the ventral hippocampus (VH) of rats submitted to the elevated T-maze (ETM). The ETM, an animal model derived from the elevated plus-maze, allows the measurement of two defensive behavioral responses in the same rat: inhibitory avoidance and escape. Results showed that L-NAME and 7-NI impaired the acquisition of inhibitory avoidance and prolonged escape latency in the ETM, suggesting an anxiolytic-like and panicolytic-like effect, respectively. SNP facilitated the acquisition of inhibitory avoidance without interfering with escape performance, suggesting an anxiogenic-like effect. Treatment with methylene blue did not alter per se any of the behavioral responses measured in the ETM, but blocked the effect promoted by SNP. Thus, altogether these results suggest that NO in the VH is critically involved in the modulation of defensive behavior of rats exposed to the ETM.

Endogenous nitric oxide is a key promoting factor for initiation of seizure-like events in hippocampal and entorhinal cortex slices

Nitric oxide (NO) modulates synaptic transmission, and its level is elevated during epileptic activity in animal models of epilepsy. However, the role of NO for development and maintenance of epileptic activity is controversial. We studied this aspect in rat organotypic hippocampal slice cultures and acute hippocampal-entorhinal cortex slices from wild-type and neuronal NO synthase (nNOS) knock-out mice combining electrophysiological and fluorescence imaging techniques. Slice cultures contained nNOS-positive neurons and an elaborated network of nNOS-positive fibers. Lowering of extracellular Mg(2+) concentration led to development of epileptiform activity and increased NO formation as revealed by NO-selective probes, 4-amino-5-methylamino-2',7'-difluorofluorescein and 1,2-diaminoanthraquinone sulfate. NO deprivation by NOS inhibitors and NO scavengers caused depression of both EPSCs and IPSCs and prevented initiation of seizure-like events (SLEs) in 75% of slice cultures and 100% of hippocampal-entorhinal cortex slices. This effect was independent of the guanylyl cyclase/cGMP pathway. Suppression of SLE initiation in acute slices from mice was achieved by both the broad-spectrum NOS inhibitor N-methyl-L-arginine acetate and the nNOS-selective inhibitor 7-nitroindazole, whereas inhibition of inducible NOS by aminoguanidine was ineffective, suggesting that nNOS activity was crucial for SLE initiation. Additional evidence was obtained from knock-out animals because SLEs developed in a significantly lower percentage of slices from nNOS(-/-) mice and showed different characteristics, such as prolongation of onset latency and higher variability of SLE intervals. We conclude that enhancement of synaptic transmission by NO under epileptic conditions represents a positive feedback mechanism for the initiation of seizure-like events.

A classification of NOergic neurons in the inferior colliculus of rat according to co-existence with classical amino acid transmitters

Since the localization of nitric oxide synthase (NOS) can be identified by enzyme histochemistry for NADPH-diaphorse (NADPH-d), this method has been used widely for mapping NOS-containing (presumably NOergic) neurons in the central nervous system. So far several studies suggest that NADPH-d is present in distinct neuronal populations in the inferior colliculus (IC), a major processing center for both the ascending and descending auditory pathway, and NO may play an important role in audition. On one hand, there is evidence from several lines of research that the IC makes extensive use of the neuroactive amino acids, in particular the inhibitory transmitter g-aminobutyric acid (GABA) and the excitatory amino acid glutamate (GLU). However, lacking is a description of the distribution of NOergic neurons to which traditional neurotransmitters may be linked. The present research utilized NADPH-d enzyme histochemistry in combination with immunocytochemistry to determine if NO may colocalize with either or both GABA and glutamate in distinct subpopulations of IC neurons. The NADPH-d positive neurons were predominantly found in two main subdivisions of the IC: the external cortex (ECIC) and the dorsal cortex (DCIC). The large numbers of these NADPH-d positive neurons appeared immunostained for GLU while only a small number, seemed to belong to the small cells (somatic area < 100 microm2) similarity to stellate cells group was positive for GABA throughout the cortex of the IC. Owing to no coexistence between GABA and GLU in the same NADPH-d positive neuron in the pairs of adjacent sections of the IC by the mirror-image technique, the present results consequently support that NOergic neurons could be subdivided into at least three distinct populations with a large proportion of about 77% being GLUergic, much lower frequency of about 11% being GABAergic and the remaining 12% expressing non-GABA and non-GLU. In summary, the existence of two functionally distinct populations of NO/GABAergic and NO/GLUergic neurons in the NOergic neurons of IC suggest that at least two differential pattern of GLU-mediated excitatory NO transmission and GABA-mediated inhibitory NO transmission are involved in the networking of auditory communication in the cortex of IC.

Anxiolytic effects induced by inhibition of the nitric oxide-cGMP pathway in the rat dorsal hippocampus

RATIONALE

Conflicting results have been reported regarding the role of the nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) pathway in the hippocampus on anxiety modulation.

OBJECTIVES

To investigate the effects of intrahippocampal injections of drugs that modify the NO-cGMP pathway in rats submitted to two animal models that are sensitive to anxiolytic drugs, the elevated plus-maze and the Vogel punished licking test.

MATERIALS AND METHODS

Male Wistar rats with cannulae aimed at the dentate gyrus of the dorsal hippocampus received microinjections of the NO synthase (NOS) inhibitors N (G)-nitro-L: -arginine methyl ester (LNAME, 15-300 nmol/0.2 microl), N (G)-nitro-L: -arginine (LNOARG, 50-300 nmol/0.2 microl), 7-nitroindazole (7NI, 10-100 nmol/0.2 microl), or the soluble guanylate cyclase inhibitor 1H-oxadiazolo-quinoxalin-1 one (ODQ, 10-100 nmol/0.2 microl), and were submitted to the elevated plus-maze. In a second group, the animals received 7NI, LNAME, or ODQ and were submitted to the Vogel punished licking test. To control for drug-induced changes in locomotor behavior, the animals were submitted to an open arena or to the Rota-rod test.

RESULTS

All drugs increased the exploration of the open arms of the elevated plus-maze. They also increased the number of punished licks in the Vogel test, indicating an anxiolytic effect. The anxiolytic effect of LNAME was prevented by previous treatment with L: -arginine (300 nmol/0.2 microl). Except for the lower dose of LNAME (15 nmol), administration of the NOS inhibitors or ODQ did not change exploratory activity in the open field nor cause any gross locomotor impairment in the Rota-rod test.

CONCLUSION

The results suggest that NO plays an anxiogenic role in the dentate gyrus of the dorsal hippocampus.

Splice-isoform specific immunolocalization of neuronal nitric oxide synthase in mouse and rat brain reveals that the PDZ-complex-building nNOSalpha beta-finger is largely exposed to antibodies

Knock out mice deficient for the splice-isoform alphaalpha of neuronal nitric oxide synthase (nNOSalphaalpha) display residual nitric oxide synthase activity and immunosignal. To attribute this signal to the two minor neuronal nitric oxide synthase splice variants, betabeta and gammagamma, we generated isoform-specific anti-peptide antibodies against the nNOSalphaalpha specific betabeta-finger motif involved in PDZ domain scaffolding and the nNOSbetabeta specific N-terminus. The nNOSalphaalpha betabeta-finger-specific antibody clearly recognized the 160-kDa band of recombinant nNOSalphaalpha on Western blots. Using immunocytochemistry, this antibody displayed, in rats and wild-type mice, a labeling pattern similar to but not identical with that obtained using a commercial pan-nNOS antibody. This similarity indicates that the majority of immunocytochemically detectable nNOS is not likely to be complexed with PDZ-domain proteins via the betabeta-finger motif. This conclusion was confirmed by the inhibition of PSD-95/nNOS interaction by the nNOSalphaalpha betabeta-finger antibody in pull-down assays. By contrast, nNOSalphaalpha betabeta-finger labeling was clearly reduced in hippocampal and cortical neuropil areas enriched in NMDA receptor complex containing spine synapses. In nNOSalphaalpha knock out mice, nNOSalphaalpha was not detectable, whereas the pan-nNOS antibody showed a distinct labeling of cell bodies throughout the brain, most likely reflecting betabeta/gammagamma-isoforms in these cells. The nNOSbetabeta antibody clearly detected bacterial expressed nNOSbetabeta fusion protein and nNOSbetabeta in overexpressing HEK cells by Western blotting. Immunocytochemically, individual cell bodies in striatum, cerebral cortex, and in some brain stem nuclei were labeled in knock out but not in wild-type mice, indicating an upregulation of nNOSbetabeta in nNOSalphaalpha deficient animals.

Interneurons of the dentate gyrus: an overview of cell types, terminal fields and neurochemical identity

Interneurons of the dentate gyrus are a diverse group of neurons that use GABA as their primary neurotransmitter. Morphological studies of these neurons have been challenging since no single neuroanatomical method provides a complete view of these interneurons. However, through the integration of findings obtained from multiple methods, an interesting picture of this complex group of neurons is emerging, and this review focuses on studies in rats and mice. In situ hybridization of mRNAs for the two isoforms of the GABA synthesizing enzyme, glutamate decarboxylase (GAD65 and GAD67), demonstrates the abundance of GABA neurons in the dentate gyrus and their high concentration in the hilus and along the base of the granule cell layer. Likewise, immunohistochemical studies, particularly of GAD65, demonstrate the rich fields of GABA terminals not only around the somata of granule cells but also in the dendritic regions of the molecular layer. This broad group of GABA neurons and their terminals can be subdivided according to their morphological characteristics, including the distribution of their axonal plexus, and their neurochemical identity. Intracellular labeling of single interneurons has been instrumental in demonstrating the extensiveness of their axonal plexus and the relatively specific spatial distribution of their axonal fields. These findings have led to the broad classification of interneurons into those that terminate primarily at perisomatic regions and those that innervate the dendrites of granule cells. The interneurons also can be classified according to their neuropeptide and calcium-binding protein content. These and other molecules contribute to the rich diversity of dentate interneurons and may provide opportunities for selectively regulating specific groups of GABA neurons in the dentate gyrus in order to enhance their function or protect vulnerable neurons from damage.

Behavioural and expressional phenotyping of nitric oxide synthase-I knockdown animals

The gaseous messenger nitric oxide (NO) has been implicated in a wide range of behaviors, including aggression, anxiety, depression, and cognitive functioning. To further elucidate the physiological role of NO and its down-stream mechanisms, we conducted behavioral and expressional phenotyping of mice lacking the neuronal isoform of nitric oxide synthase (NOS-I), the major source of NO in the central nervous system. No differences were observed in activity-related parameters; in contrast to the a priori hypothesis, derived from pharmacological treatments, depression-related tests (Forced Swim Test, Learned Helplessness) also yielded no significantly different results. A subtle anxiolytic phenotype however was present, with knockdown mice displaying a higher open arm time as compared to their respective wildtypes, yet all other investigated anxiety-related parameters were unchanged. The most prominent feature however was gender-independent cognitive impairment in spatial learning and memory, as assessed by the Water Maze test and an automatized holeboard paradigm. No significant dysregulation of monoamine transporters was evidenced by qRT PCR. To further examine the underlying molecular mechanisms, the transcriptome of knockdown animals was thus examined in the hippocampus, striatum and cerebellum by microarray analysis. A set of >120 differentially expressed genes was identified, whereat the hippocampus and the striatum showed similar expressional profiles as compared to the cerebellum in hierarchical clustering. Among the most significantly up-regulated genes were Peroxiredoxon 3, Atonal homologue 1, Kcnj1, Kcnj8, CCAAT/enhancer binding protein (C/EBP), alpha, 3 genes involved in GABA(B) signalling and, intriguingly, the glucocorticoid receptor GR. While GABAergic genes might underlie reduced anxiety, dysregulation of the glucocorticoid receptor can well contribute to a blunted stress response as found in NOS1 knockdown mice. Furthermore, by CREB inhibition, glucocorticoid receptor upregulation could at least partially explain cognitive deficits in these animals. Taken together, NOS1 knockdown mice display a characteristic behavioural profile consisting of reduced anxiety and impaired learning and memory, paralleled by differential expression of the glucocorticoid receptor and GABAergic genes. Further research has to assess the value of these mice as animal models e.g. for Alzheimer's disease or attention deficit disorder, in order to clarify a possible pathophysiological role of NO therein.

Nitric oxide synthase-containing neurons in the amygdaloid nuclear complex of the rat

The nitric oxide-producing neurons in the rat amygdala (Am) were studied, using reduced nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry. Almost all nuclei of the Am contained NADPHd-positive neurons and fibers, but the somatodendritic morphology and the intensity of staining of different subpopulations varied. The strongly stained neurons displayed labeling of the perikaryon and the dendritic tree with Golgi impregnation-like quality, whilst the dendrites of the lightly stained neurons were less successfully followed. Many strongly positive neurons were located in the external capsule and within the intraamygdaloid fiber bundles. A large number of small, strongly stained cells was present in the amygdalostriatal transition area. In the Am proper, a condensation of deeply stained cells occurred in the lateral amygdaloid nucleus. In the basolateral nucleus, the strongly NADPHd-positive neurons were few, and were located mainly along the lateral border of the nucleus. These cells clearly differed from the large, pyramidal, and efferent cells. The basomedial nucleus contained numerous positive cells but most of them were only lightly labeled. A moderate number of strongly stained neurons appeared in the medial division of the central nucleus, and a larger accumulation of strongly positive cells was present in the lateral and the capsular divisions. The medial amygdaloid nucleus contained numerous moderately stained neurons and displayed the strongest diffuse neuropil staining in Am. In the nucleus of the lateral olfactory tract, the first layer contained only NADPHd-stained axons, in the second layer, there were numerous moderately stained cells, and in the third layer, a few but deeply stained neurons. From the cortical nuclei, the most appreciable number of stained neurons was seen in the anterior cortical nucleus. The anterior amygdaloid area contained numerous NADPHd-positive neurons; in its dorsal part the majority of cells were only moderately stained, whereas in the ventral part the neurons were very strongly stained. The intercalated amygdaloid nucleus lacked NADPHd-positive neurons but an appreciable plexus of fine, tortuous axons was present. In the intra-amygdaloid part of the bed nucleus of the stria terminalis (st) some lightly stained cells were seen but along the entire course of st strongly stained neurons were observed. Some Am nuclei, and especially the central lateral nucleus and the intercalated nucleus, display considerable species differences when compared with the primate Am. The age-related changes of the nitrergic Am neurons, as well as their involvement in neurodegenerative diseases is discussed.

NOS-positive local circuit neurons are exclusively axo-dendritic cells both in the neo- and archi-cortex of the rat brain

Neuronal nitric oxide synthase (nNOS)-containing neurons and axon terminals were examined in the rat somatosensory and temporal neocortex, in the CA3/a-c areas of Ammon's horn and in the hippocampal dentate gyrus. In these areas, only nonpyramidal neurons were labeled with the antibody against nNOS. Previous observations suggested that all nNOS-positive nonpyramidal cells are GABAergic local circuit neurons, which form exclusively symmetric synapses. In agreement with this, nNOS-positive axon terminals in the hippocampal formation formed symmetric synapses exclusively with dendritic shafts. In the neocortex, in contrast, in addition to the nNOS-positive axon terminals that formed synapses with unlabeled spiny and aspiny dendrites and with nNOS-positive aspiny dendrites, a small proportion of the nNOS-positive axon terminals formed symmetric synapses with dendritic spines. These results suggest that nNOS-positive local circuit neurons form a distinct group of axo-dendritic cells displaying slightly different domain specificity in the archi- and neocortex. However, nNOS-positive cells show no target selectivity, because they innervate principal cells and local circuit neurons. Afferents to the NOS-positive cells display neither domain nor target selectivity, because small unlabeled terminals formed synapses with both the soma or dendrites of nNOS-positive neurons and an adjacent unlabeled dendrite or spine in both the hippocampal formation and in neocortex.

Vascular and renal effects of dopamine during extracellular volume expansion: Role of nitric oxide pathway

OBJECTIVE

The aim of the study was to determine the possible role of NO-system activation in vascular and renal effects of the dopaminergic system and the probable interaction between both systems during acute volume expansion in rats.

DESIGN AND METHODS

Expanded (10% bw) and non-expanded anaesthetized male Wistar rats were treated with haloperidol, a DA receptor antagonist (3 mg/kg bw, ip). Mean arterial pressure, diuresis, natriuresis, renal plasma flow, glomerular filtration rate, nitrites and nitrates excretion (NOx) were determined. NADPH diaphorase activity was measured using a histochemistry technique in kidney, aorta and renal arteries. NOS activity in kidney and aorta from expanded and non-expanded animals was determined with L-[U14C]-arginine substrate, in basal conditions and after DA (1 microM) administration.

RESULTS

The hypotensive effect of L-arg and hypertension induced by L-NAME were not modified by haloperidol. This blocker reverted the increase in diuresis, natriuresis and RPF induced by L-arg in both groups. Dopaminergic blockade induced a decrease in NOx excretion and in NADPH-diaphorase activity in glomeruli, proximal tubule and medullar collecting duct and in endothelium and vascular smooth muscle of renal arteries. DA induced an increase in NOS activity in renal medulla and cortex in both groups, but no changes in the aorta were observed.

CONCLUSIONS

Our results suggest that renal DA would be associated with the renal response induced by NO during extracellular volume expansion. NO-system activation would be one of the mechanisms involved in renal DA activity during saline load, but NO appears not to be involved in DA vascular effects.

Role of nitric oxide pathway in hypotensive and renal effects of furosemide during extracellular volume expansion

OBJECTIVE

In previous studies we demonstrated that the administration of furosemide associated with L-arginine contributes to enhanced hypotension and induces greater water than electrolyte excretion, in both normal and expansion conditions. The aim of the present study was to elucidate the interaction between furosemide and the nitric oxide (NO) system in renal and vascular responses during extracellular volume expansion.

DESIGN AND METHODS

Expanded [10% body weight (bw)] and non-expanded anaesthetized male Wistar rats were treated with furosemide (7.5 mg/kg bw). Mean arterial pressure, nitrite and nitrate excretion (NOx) were determined. NADPH-diaphorase activity, a marker of nitric oxide synthase (NOS) activity, was measured histochemically in different segments of the nephron, aorta and renal arteries. NOS activity was determined using an L-[U14C]-arginine substrate in the kidney and aorta of expanded and non-expanded rats, in basal conditions and after furosemide (10 micromol/l).

RESULTS

The hypotensive effect of furosemide was enhanced when NO production was stimulated in expanded and non-expanded animals. The diuretic treatment induced a significant increase in NOx excretion, in NADPH-diaphorase activity in the thick ascending limb of Henle, renal arteries and aorta, and in NOS activity in aorta and kidney in both groups.

CONCLUSIONS

Our results suggest that the hypotensive effect of furosemide may be attributed to NO-mediated vasodilation. The enhanced NOS activity, observed in the renal artery of furosemide-treated rats, could explain the increased renal plasma flow induced by furosemide. In addition, NO-pathway stimulation in the kidney could be one of the mechanisms by which furosemide exerts its diuretic and natriuretic effects, in control and in expansion conditions.

Distribution of nitric oxide synthase immunoreactivity in the mouse brain

Nitric oxide (NO) is a gaseous intercellular messenger with a wide range of neural functions. NO is synthesized by activation of different isoforms of nitric oxide synthases (NOS). At present NOS immunoreactivity has been described in mouse brain in restricted and definite areas and no detailed mapping studies have yet been reported for NOS immunoreactivity. We have studied the distribution of neuronal NOS-containing neurons in the brain of three months male mice, using a specific commercial polyclonal antibody against the neuronal isoform of nitric oxide synthase (nNOS). Neuronal cell bodies exhibiting nNOS immunoreactivity were found in several distinct nuclei throughout the brain. The neurons that were positively stained exhibited different intensities of reaction. In some brain areas (i.e., cortex, striatum, tegmental nuclei) neurons were intensely stained in a Golgi-like fashion. In other regions, immunoreactive cells are moderately stained (i.e., magnocellular nucleus of the posterior commissure, amygdaloid nucleus, interpeduncular nucleus, lateral periaqueductal gray) or weakly stained (i.e., vascular organ of the lamina terminalis, hippocampus, inferior colliculus, reticular nucleus). In the mouse, the NO-producing system appears well developed and widely diffused. In particular, nNOS immunoreactive neurons seem chiefly present in several sensory pathways like all the nuclei of the olfactory system, as well as in many regions of the lymbic system. These data suggest a widespread role for the NO system in the mouse nervous system.

Effects of nitric oxide on dentate gyrus cell proliferation after seizures induced by pentylenetrazol in the adult rat brain

Epileptic seizures have been shown to increase the proliferation of granule cell precursors in the adult brain, but the underlying mechanisms remain largely unknown. This study examined the effect of nitric oxide (NO) on the proliferation of granule cell precursors in adult rats after pentylenetrazol (PTZ)-induced generalized clonic seizures. Using systemic bromodeoxyuridine (BrdU) to label dividing cells, we found that injection of the neuronal nitric oxide synthase (nNOS) inhibitor 7-nitroindazole (50 mg/kg i.p.) 10 min before PTZ significantly reduced the number of BrdU labeled cells in the dentate gyrus 3, 7, and 14 days after seizures (P < 0.05). Administration of the inducible NOS (iNOS) inhibitor aminoguanidine (100 mg/kg i.p.) also significantly inhibited the proliferation rate of neural precursor cells in the dentate gyrus at various time points after PTZ-induced seizures. Our findings suggest that epileptic seizures lead to increased cell proliferation in the adult rat dentate gyrus through NO-dependent mechanisms. Both the NO originating from nNOS and iNOS may be involved in brain repair after seizures.

The NOS/sGC pathway in the rat central nervous system: a microdialysis overview

It is now well established that nitric oxide is involved in a variety of physiopathological processes in the central nervous system, which mainly result from the interaction of this gaseous molecule with the heme group of soluble guanylyl cyclase and the elevation of intracellular cGMP in target neurons. During the last decade, several studies have monitored extracellular cGMP, by means of intracerebral microdialysis, to investigate in vivo the functioning and modulation of this neurochemical pathway under different experimental conditions and in various brain regions. In this review, we summarise some of the most relevant results obtained in this research field.

Exposure to interferon-γ during synaptogenesis increases inhibitory activity after a latent period in cultured rat hippocampal neurons

Certain disorders of the nervous system may have their origin in disturbances in the development of synaptic connections and network structure that may not become overt until later in life. As inflammatory cytokines can influence synaptic activity in neuronal cultures, we analysed whether cytokine exposure during synaptogenesis can lead to imbalances in a neuronal network. Short-term application of interferon-gamma (IFN-gamma), but not tumour necrosis factor-alpha, during peak synaptogenesis (but not before or after) in Sprague-Dawley rat hippocampal cultures, caused both a decrease in the frequency of spontaneous excitatory postsynaptic currents (EPSCs) and an increase in the frequency of spontaneous inhibitory postsynaptic currents (IPSCs). These effects were only detected in recordings made weeks later. This was not due to a depression of glutamatergic synapses or to a change in the relative number of neurons containing glutamic acid decarboxylase (GAD). There was an increase in the average amplitude of miniature IPSCs, and in GAD-expressing neurons the amplitude of miniature EPSCs were larger as well as the responses to glutamate. This indicates that IFN-gamma-treatment induced increased inhibition via postsynaptic changes. These effects of IFN-gamma treatment were not observed when neuronal nitric oxide synthase was inhibited. Our study therefore shows that exposure to IFN-gamma during a restricted period of development, which coincides with the peak of excitatory synaptogenesis, can cause progressive changes in synaptic activity in the network. Thus, cytokine exposure at a critical period of development may constitute a 'hit-and-run' mechanism for certain nervous system disorders that become manifest after a latency period.

Patterns of colocalization of neuronal nitric oxide synthase and somatostatin-like immunoreactivity in the mouse hippocampus: quantitative analysis with optical disector

In some brain regions, previous studies reported the frequent coexistence between neuronal nitric oxide synthase (nNOS) and somatostatin (SOM). In the hippocampus, nNOS and SOM were mainly expressed in GABAergic nonprincipal neurons. Here we estimated the immunocytochemical colocalization of nNOS and SOM in the mouse hippocampus using the optical disector. Both in the Ammon's horn and dentate gyrus, we encountered only a few nNOS-immunoreactive (IR)/SOM-like immunoreactive (LIR) neurons. They were mainly located in the stratum oriens of the Ammon's horn and in the dentate hilus. The nNOS-IR/SOM-LIR neurons usually showed characteristic large somata with thick dendrites, whereas the majority of nNOS-IR/SOM-negative neurons showed small somata with thin dendrites. Quantitative data revealed that the double-labeled cells represented only 4% and 7% of nNOS-IR neurons and SOM-LIR neurons, respectively, in the whole area of the hippocampus. We also found the laminar and dorsoventral differences in the degree of colocalization between nNOS and SOM. The percentages of nNOS-IR neurons containing SOM-like immunoreactivity were relatively high in the stratum oriens of the ventral CA1 region (24%), stratum lucidum of the dorsal CA3 region (29%) and dorsal dentate hilus (32%), but they were quite low in the other layers. On the other hand, the percentages of SOM-LIR neurons containing nNOS immunoreactivity were somewhat high in the stratum lucidum of the dorsal CA3 region (19%) and dorsal dentate hilus (28%), whereas they were very low in the other layers. Immunofluorescent triple labeling of axon terminals for nNOS, SOM and glutamic acid decarboxylase indicated that some nNOS-IR/SOM-LIR neurons might be dendritic inhibitory cells. The present results show the infrequent colocalization of nNOS and SOM in the mouse hippocampus, and also suggest that the double-labeled cells may be a particular subpopulation of hippocampal GABAergic nonprincipal neurons.

Hippocampal neurons in organotypic slice culture are highly resistant to damage by endogenous and exogenous nitric oxide

Nitric oxide (NO) has been proposed to mediate neurodegeneration arising from NMDA receptor activity, but the issue remains controversial. The hypothesis was re-examined using organotypic slice cultures of rat hippocampus, with steps being taken to avoid known artefacts. The NO-cGMP signalling pathway was well preserved in such cultures. Brief exposure to NMDA resulted in a concentration-dependent delayed neuronal death that could be nullified by administration of the NMDA antagonist MK801 (10 microm) given postexposure. Two inhibitors of NO synthesis failed to protect the slices, despite fully blocking NMDA-induced cGMP accumulation. By comparing NMDA-induced cGMP accumulation with that produced by an NO donor, toxic NMDA concentrations were estimated to produce only physiological NO concentrations (2 nm). In studies of the vulnerability of the slices to exogenous NO, it was found that continuous exposure to up to 4.5 microm NO failed to affect ATP levels (measured after 6 h) or cause damage during 24 h, whereas treatment with the respiratory inhibitors myxothiazol or cyanide caused ATP depletion and complete cell death within 24 h. An NO concentration of 10 microm was required for ATP depletion and cell death, presumably through respiratory inhibition. It is concluded that sustained activity of neuronal NO synthase in intact hippocampal tissue can generate only low nanomolar NO concentrations, which are unlikely to be toxic. At the same time, the tissue is remarkably resistant to exogenous NO at up to 1000-fold higher concentrations. Together, the results seriously question the proposed role of NO in NMDA receptor-mediated excitotoxicity.

Acute and delayed restraint stress-induced changes in nitric oxide producing neurons in limbic regions

RATIONALE

Microinjection into the dentate gyrus of the hippocampus of N(omega)-nitro-l-arginine methyl ester hydrochloride (l-NAME), a nitric oxide synthase (NOS) inhibitor, induces antinociceptive effect 5 days after a single restraint episode. The mechanisms of this stress-antinociceptive modulatory effect have not been investigated but may involve plastic changes in the hippocampal formation (HF).

OBJECTIVE

The objective of the present study was to investigate possible mechanisms of the stress-modulating effect on antinociception induced by NOS inhibition in the hippocampus. We analyzed the effects of restraint stress on neuronal NOS (nNOS) expression and nicotinamide adenine dinucleotide phosphate-diaphorase histochemical activity (NADPH-d) in the HF and related brain regions.

METHODS

Male Wistar rats (n=6-11/group) were submitted to a single (acute stress) or repeated (5 days) episodes of 2-h restraint. Control animals remained in their home cages being all animals daily handled during this period. In the fifth day, animals received unilateral microinjection of l-NAME (150 nmol/0.2 microl) or saline (control) into the dentate gyrus of the dorsal hippocampus (DG). Immediately before and after drug microinjection tail-flick reflex latency or hotplate licking reaction was measured. Animals were killed i. immediately; ii. 5 days after acute stress; or iii. after repeated stress. NADPH-d and nNOS expression were quantified in the HF, caudate-putamen, secondary somatosensorial, entorhinal and piriform cortices and amygdaloid complex.

RESULTS

Five days after one or five restraint episodes l-NAME microinjection into the DG elicited antinociceptive effect (analysis of variance [ANOVA], P<0.05). Acute restraint stress induced a significant increase in the density of neurons expressing NADPH-d and nNOS in the amygdaloid nuclei. nNOS expression increased also in the DG and piriform cortex. Five days after a single or repeated restraint stress there was an additional increase in NADPH-d- and nNOS-positive neurons in CA1, CA3, and entorhinal cortex. No changes were seen in non-limbic regions such as the caudate-putamen and secondary somatosensorial cortex.

CONCLUSION

The results confirm that the dorsal hippocampus participates in the modulation of stress consequences. They also show that a single stress episode causes acute changes in nitric oxide system in the amygdala complex and delayed modifications in the HF. The delayed (5 days) antinociceptive effect of NOS inhibition in the HF after a single restraint episode suggests that those latter modifications may have functional consequences. It remains to be tested if the acute amygdala and delayed hippocampal changes are causally related.

Effect of erythropoietin on nitric oxide production in the rat hippocampus using in vivo brain microdialysis

In order to investigate the role of erythropoietin (EPO) in the hippocampus, we studied the effect of EPO on nitric oxide (NO) production in the rat hippocampus using brain microdialysis. The dialysis probe was stereotaxically inserted into the rat hippocampus 24 h before the dialysis experiment. The perfusion fluid (Krebs-HEPES buffer, pH 7.4) was collected at 15-min intervals under freely moving conditions and NO metabolites (NOx) in the perfusate were immediately measured using a NOx-analyzing high performance liquid chromatography (HPLC)system. Following the collection of four fractions, 1 microl of EPO (10(-10) M, 10(-8) M and 10(-6) M) or vehicle (saline) was gently injected into the hippocampal tissue. The perfusion fluid was collected for 3 h after the injection. The NOx levels were unchanged by the injection of vehicle alone. After the injection of EPO, NOx levels gradually increased. The EPO-induced increase in NOx levels was significant at 10(-6) M EPO. The EPO-induced increases in NOx levels were eliminated in the presence of anti-EPO antibody. The increase in NOx levels induced by EPO was blunted by nicardipine, a Ca2+ channel blocker, but not by MK-801, an antagonist of N-methyl-D-aspartate (NMDA) receptors. These findings, taken together, suggest that EPO increased NO production in the rat hippocampus by activating voltage-gated Ca2+ channels, but not through NMDA receptors.

Species, strain and developmental variations in hippocampal neuronal and endothelial nitric oxide synthase clarify discrepancies in nitric oxide-dependent synaptic plasticity

Nitric oxide (NO) has been implicated in long-term potentiation (LTP) in pyramidal neurons in cellular area 1 (CA1) of the hippocampus. However, considerable confusion exists about the exact role of NO, and the contribution of the endothelial nitric oxide synthase (eNOS) and neuronal nitric oxide synthase (nNOS) isoforms of NO synthase to NO-dependent LTP (NO-LTP), with results often varying, depending on the organism and experimental paradigm used. Using immunohistochemistry and in situ hybridization, we contrast NO synthase expression and activity in rat, mouse, and human hippocampus. nNOS is prominently expressed in all CA1 pyramidal cells of C57B6 mice and humans, while in rats and SV129 mice, its levels are much lower and restricted to the caudal hippocampus. By contrast, eNOS is restricted to endothelial cells. We observe N-methyl-D-aspartate-dependent citrulline production in pyramidal cells of mouse hippocampus, which is absent in nNOS(Delta/Delta) animals. Finally, we observe robust nNOS expression in human CA1 pyramidal cells.The considerable axial, developmental, strain and species-dependent variations in nNOS expression in CA1 pyramidal neurons can explain much of the variation observed in reports of NO-dependent LTP. Moreover, our data suggest that NO produced by eNOS in endothelial cells may play a paracrine role in modulating LTP.

Nitric Oxide System and Protein Nitration are Modified by an Acute Hypobaric Hypoxia in the Adult Rat Hippocampus

Changes in the nitric oxide system of the hippocampus from rats submitted to hypobaric hypoxia were investigated. Adult rats were exposed to a simulated altitude of 8,325 m (27,000 ft) for 7 h and killed after 0 h, 1, 3, 5, 10 and 20 days of reoxygenation. The number of neuronal nitric oxide synthase immunoreactive neurons and their dendritic plexus, as well as neuronal nitric oxide synthase immunoblotting densitometry and calcium-dependent activity increased from 0 h to 3 days of reoxygenation. In addition, endothelial nitric oxide synthase immunoreactivity peaked after 7 h of hypobaric hypoxia. Nitrotyrosine immunoreactivity showed an increase in the pyramidal cells of CA2-CA3 and in glial cells surrounding the blood vessels after 0 h, 1 and 3 days of reoxygenation. Immunoblotting densitometry of 1 of the 2 nitrotyrosine-immunoreactive bands detected also increased after 0 h and 1 day of reoxygenation. Inducible nitric oxide synthase immunoreactivity was found only in some blood vessels after 0 h, 1 and 3 days of reoxygenation, but no changes in inducible nitric oxide synthase activity or immunoblotting were detected. We conclude that transient activation of the nitric oxide system constitutes a hippocampal response to hypobaric hypoxia.

Delayed stress-induced antinociceptive effect of nitric oxide synthase inhibition in the dentate gyrus of rats

Stimulation of the hippocampal formation can modulate nociceptive mechanisms, whereas painful stimuli can activate this structure. Stress exposure can produce plastic changes in the hippocampus. Nitric oxide (NO) is an important neuroregulatory agent present in the hippocampus. The objective of the present study was to investigate the effects of intrahippocampal administration of N(omega)-nitro-L-arginine methyl ester hydrochloride (L-NAME), an inhibitor of NO synthase (NOS), on nociceptive processes in stressed and nonstressed rats. Male Wistar rats (n=6-11/group) received unilateral microinjection of L-NAME (50-300 nmol/0.2 microl) into the dentate gyrus (DG) of the dorsal hippocampus. Immediately after the injection tail-flick reflex latency was measured. Stressed animals were submitted to 2 h of restraint and tested immediately or 1, 2, 5 or 10 days later. L-NAME failed to modify nociception in nonstressed rats. However, 5 days after, restraint L-NAME, at all doses tested, produced an antinociceptive effect (ANOVA, P<.05). The dose-response curve had an inverted U shape. L-NAME antinociceptive effect was antagonized by previous treatment with L-arginine (150 nmol/0.2 microl, P<.05). The results suggest that the modulation of nociceptive processes by NO in the dorsal hippocampus is dependent on previous stress exposure and on poststress interval.

Synaptic localization of nitric oxide synthase and soluble guanylyl cyclase in the hippocampus

Functional evidence suggests that nitric oxide released from CA1 pyramidal cells can act as a retrograde messenger to mediate hippocampal long-term potentiation, but the failure to find neuronal nitric oxide synthase (NOS-I) in the dendritic spines of these cells has cast doubt on this suggestion. We hypothesized that NOS-I may be in spines but in a form inaccessible to antibody when using standard histological fixation procedures. Supporting this hypothesis, we found that after a weak fixation protocol shown previously to enhance staining of synaptic proteins, CA1 pyramidal cells exhibit clear immunoreactivity for NOS-I. Confocal microscopy revealed that numerous dendritic spines in the stratum radiatum contained the NR2 subunit of the NMDA receptor and the adaptor protein postsynaptic density-95, and a subset of these spines also contained NOS-I. Quantitative studies showed that only approximately 8% of synaptic puncta (identified by synaptophysin staining) were associated with NOS-I, and approximately 9% contained the beta subunit of soluble guanylyl cyclase (sGC), a major target of NO. However, the majority of NOS-I-positive synaptic puncta was associated with sGC and vice versa. Postembedding immunogold electron microscopy showed that NOS-I concentrates just inside the postsynaptic plasma membrane of asymmetric axospinous synapses in the stratum radiatum of CA1, whereas sGCbeta concentrates just inside the presynaptic membrane. Together, these findings support the possibility that NO may act as a retrograde messenger to help mediate homosynaptic plasticity in a subpopulation of synapses in the stratum radiatum of CA1.

Phosphorylation of neuronal nitric oxide synthase at Ser847 by CaM-KII in the hippocampus of rat brain after transient forebrain ischemia

The authors previously demonstrated that Ca2+/calmodulin (CaM)-dependent protein kinase IIalpha (CaM-KIIalpha) can phosphorylate neuronal nitric oxide synthase (nNOS) at Ser847 and attenuate NOS activity in neuronal cells. In the present study, they established that forebrain ischemia causes an increase in the phosphorylation of nNOS at Ser847 in the hippocampus. This nNOS phosphorylation appeared to be catalyzed by CaM-KII: (1) it correlated with the autophosphorylation of CaM-KIIalpha; (2) it was blocked by the CaM-KII inhibitor, KN-93; and (3) nNOS and CaM-KIIalpha were found to coexist in the hippocampus. Examination of the spatial relation between nNOS and CaM-KIIalpha in the brain revealed coexistence in the hippocampus but not in the cortex during reperfusion, with a concomitant increase in autophosphorylation of CaM-KIIalpha. The phosphorylation of nNOS at Ser847 probably takes place in nonpyramidal hippocampal neurons, which increased after 30 minutes of reperfusion in the hippocampus, whereas no significant increase was detected in the cortex. An intraventricular injection of KN-93 significantly decreased the phosphorylation of nNOS in the hippocampus. These results point to CaM-KII as a protein kinase, which by its colocalization may attenuate the activity of nNOS through its Ser847 phosphorylation, and may thus contribute to promotion of tolerance to postischemic damage in hippocampal neurons.

Immunocytochemical characterization of hippocamposeptal projecting GABAergic nonprincipal neurons in the mouse brain: a retrograde labeling study

The neurochemical contents of hippocamposeptal projecting nonprincipal neurons were examined in the mouse brain by using retrograde labeling techniques. We used the immunofluorescent multiple labeling method with a confocal laser-scanning microscope. First of all, the hippocamposeptal projecting nonprincipal neurons were glutamic acid decarboxylase 67-immunoreactive (IR), i.e., these hippocamposeptal projecting nonprincipal neurons were immunocytochemically GABAergic in the mouse brain. Next, most (93.0%) of the hippocamposeptal projecting GABAergic neurons were somatostatin-like immunoreactive (SS-LIR). The SS-LIR hippocamposeptal projecting neurons were frequently found in the stratum oriens of the CA1 and CA3 regions, and were also occasionally found in the stratum radiatum, stratum lucidum, and stratum pyramidale of the CA3 region. They were also frequently found in the dentate hilus. On the other hand, at least 40.6% of SS-LIR neurons in the hippocampus projected to the medial septum. Next, 38.0% of hippocamposeptal projecting GABAergic neurons were calbindin D28K (CB)-IR. Although the distribution of the CB-IR hippocamposeptal projecting neurons was generally similar to that of the SS-LIR projecting neurons in Ammon's horn, they were never seen in the dentate hilus. At least 22.1% of CB-IR GABAergic neurons in the hippocampus projected to the medial septum. Furthermore, 5.8% of hippocamposeptal projecting GABAergic neurons were parvalbumin-IR, which were most always found in Ammon's horn. Finally, no hippocamposeptal projecting GABAergic neurons were neuronal nitric oxide synthase-IR nor calretinin-IR. These results indicate that the SS-LIR neurons play a crucial role in the hippocamposeptal projection of the mouse brain, and they are also assumed to be involved in the theta oscillation of the mouse hippocampus.

Nitric oxide signalling system in rat brain stem: immunocytochemical studies

Nitric oxide is a free radical, which is produced in several tissues of the body and is thought to be the first of a new class of neural messenger molecules and a retrograde modulator of synaptic transmission in the brain. Nitric oxide synthase (NOS) is the enzyme that produces nitric oxide from the substrate l-arginine. The pattern of the distribution of neuronal isoform of NOS was investigated in neurones and fibres in the brain stem using standard immunocytochemistry. In our results, NOS positive neurones and processes were seen in the spinal trigeminal nucleus, gracile nucleus, nucleus of the solitary tract, nucleus ambiguus, reticular nuclei and lateral to the pyramidal tract of the medulla. In the pons, heavily labelled NOS containing neurones were seen in the pedunuclopontine tegmental nucleus, ventral tegmental nucleus and in the laterodorsal tegmental nucleus. The localization of neuronal NOS expressing neurones suggests a widespread neuromodulatory role for the nitric oxide in the central nervous system of rat.

The nitric oxide synthase inhibitor NG-Nitro-L-arginine increases basal forebrain acetylcholine release during sleep and wakefulness

Cholinergic neurotransmission in the basal forebrain changes across the sleep/wake cycle, and considerable data show cortical activation by ACh originating from basal forebrain neurons. These findings have stimulated efforts to elucidate molecular modulators of ACh release within the basal forebrain. Basal forebrain cholinergic neurons contain nitric oxide synthase (NOS), the enzyme that produces the gaseous neuromodulator nitric oxide. This study tested the hypothesis that administration of an NOS inhibitor to the basal forebrain would alter basal forebrain ACh release, sleep, and respiratory rate. Seven cats were instrumented for recording sleep and wakefulness and for in vivo microdialysis and microinjection. Compared with Ringer's solution (control), microdialysis delivery of the NOS inhibitor N(G)-nitro-l-arginine (NLA; 10 mm) increased ACh release during wakefulness (33%), non-rapid eye movement (NREM) sleep (70%), and rapid eye movement (REM) sleep (16%). Mean +/- SEM ACh levels (pmol/10 min) during control and NLA dialysis, respectively, were 0.58 +/- 0.03 and 0.77 +/- 0.06 in wakefulness, 0.36 +/- 0.01 and 0.61 +/- 0.06 in NREM sleep, and 0.68 +/- 0.06 and 0.79 +/- 0.09 in REM sleep. Increases in ACh release were not evoked by dialysis delivery of the less active enantiomer N(G)-nitro-d-arginine. Dialysis administration of NLA did not alter respiratory rate. Sleep-dependent changes in basal forebrain ACh release were localized specifically to lateral basal forebrain regions and did not occur in medial basal forebrain sites. Microinjection of NLA into the lateral basal forebrain did not significantly alter the sleep/wake cycle. In contrast to NLA-induced depression of REM sleep and ACh release in the cat pons, the present results demonstrate that NLA increased ACh release in the cat basal forebrain and had no effect on sleep. The different effects of NLA on ACh release in the cat pons and cat basal forebrain may prove relevant for developing compounds that differentially alter cholinergic neurotransmission in specific brain regions.

Patterns of expression of calcium binding proteins and neuronal nitric oxide synthase in different populations of hippocampal GABAergic neurons in mice

We examined the expression of calcium binding proteins parvalbumin (PV), calretinin (CR), and calbindin D28K (CB), and neuronal nitric oxide synthase (nNOS) in gamma-aminobutyric acid (GABA)ergic neurons of the mouse hippocampus, with particular reference to areal and dorsoventral differences. First, we estimated the colocalization of the calcium binding proteins and nNOS. GABAergic neurons containing both PV and nNOS, i.e., PV-immunoreactive (-IR)/nNOS-IR neurons, were rare in Ammon's horn but frequent in the dentate gyrus (DG). CR-IR/nNOS-IR neurons and CB-IR/nNOS-IR neurons were frequent in Ammon's horn but rare in the DG. In the entire hippocampus, the percentage of CR-IR neurons containing nNOS was significantly higher at the ventral level (44.3%) than at the dorsal level (17.0%). The percentage of CB-IR neurons containing nNOS was also significantly higher at the ventral level (42.3%) than at the dorsal level (29.3%). Next, we estimated the numerical densities (NDs) of calcium binding protein-containing GABAergic neurons. The ND of PV-IR neurons was comparable at the dorsal (1.16 x 10(3)/mm(3)) and ventral levels (1.23 x 10(3)/mm(3)), respectively. The ND of CR-IR neurons was less at the dorsal level (0.52 x 10(3)/mm(3)) than at the ventral level (0.64 x 10(3)/mm(3)). The ND of CB-IR neurons was also less at the dorsal level (0.91 x 10(3)/mm(3)) than at the ventral level (1.57 x 10(3)/mm(3)). Overall, approximately half of the GABAergic neurons contained one of the three calcium binding proteins (45% at the dorsal level and 47% at the ventral level). These data establish a baseline for examining potential roles of GABAergic neurons in hippocampal network activity in mice.

Evidence for an involvement of nitric oxide and gamma aminobutyric acid in the anticonvulsant action of L-arginine on picrotoxin-induced convulsions in rats

Five, 30, and 60 min pretreatment of 1000 mg/kg and not 500 mg/kg of L-arginine inhibited convulsions induced by picrotoxin. The concentrations of nitric oxide (NO) and gamma aminobutyric acid (GABA) were increased in the brain 5, 30, and 60 min after administration of 1000 mg/kg and not 500 mg/kg of L-arginine. A much higher dose of L-arginine (2000 mg/kg), 30 min after administration, produced a lesser anticonvulsant and NO and GABA increasing actions as compared to that produced by 1000 mg/kg of L-arginine. The same dose of L-arginine, 60 min after administration, decreased the concentrations of both NO and GABA and increased the convulsion frequency of picrotoxin. An NO decreasing dose of nitric oxide synthase (NOS) inhibitor, N-nitro-L-arginine methyl ester (L-NAME) decreased brain GABA concentration and increased the convulsant action of picrotoxin. Further, L-NAME pretreatment prevented L-arginine (1000 mg/kg) from producing anticonvulsant and NO and GABA increasing effects. An interpretation of these results suggests that NO synthesized from systemically administered L-arginine inhibits convulsions by increasing the concentration of GABA in the brain. However, the effects of L-arginine are reversible, if it is administered at a higher dose (2000 mg/kg) 60 min prior to the test. It is concluded that L-arginine produces anticonvulsant or proconvulsant action depending upon the dose and time of its administration-related changes in the concentrations of NO and GABA in the brain.