Nitric oxide synthase immunoreactivity in the rat hippocampus after status epilepticus induced by perforant pathway stimulation

Nitric oxide impairs spatial learning and memory in a rat model of Alzheimer's disease via disturbance of glutamate response in the hippocampal dentate gyrus during spatial learning

Nitric oxide (NO)-dependent pathways may play a significant role in the decline of synaptic and cognitive functions in Alzheimer's disease (AD). However, whether NO in the hippocampal dentate gyrus (DG) is involved in the spatial learning and memory impairments of AD by affecting the glutamate (Glu) response during these processes is not well-understood. Here, we prepared an AD rat model by long-term i.p. of D-galactose into ovariectomized rats, and then the effects of L-NMMA (a NO synthase inhibitor) on Glu concentration and amplitude of field excitatory postsynaptic potential (fEPSP) were measured in the DG region during the Morris water maze (MWM) test in freely-moving rats. During the MWM test, compared with the sham group, the escape latency was increased in the place navigation trial, and the percentage of time spent in target quadrant and the number of platform crossings were decreased in the spatial probe trial, in addition, the increase of fEPSP amplitude in the DG was significantly attenuated in AD group rats. L-NMMA significantly attenuated the spatial learning and memory impairment in AD rats, and reversed the inhibitory effect of AD on increase of fEPSP amplitude in the DG during the MWM test. In sham group rats, the Glu level in the DG increased significantly during the MWM test, and this response was markedly enhanced in AD rats. Furthermore, the response of Glu in the DG during spatial learning was recovered by microinjection of L-NMMA into the DG. Our results suggest that NO in the DG impairs spatial learning and memory and related synaptic plasticity in AD rats, by disturbing the Glu response during spatial learning.

Short-Term Amygdala Low-Frequency Stimulation Does not Influence Hippocampal Interneuron Changes Observed in the Pilocarpine Model of Epilepsy

Temporal lobe epilepsy (TLE) is characterized by changes in interneuron numbers in the hippocampus. Deep brain stimulation (DBS) is an emerging tool to treat TLE seizures, although its mechanisms are not fully deciphered. We aimed to depict the effect of amygdala DBS on the density of the most common interneuron types in the CA1 hippocampal subfield in the lithium-pilocarpine model of epilepsy. Status epilepticus was induced in male Wistar rats. Eight weeks later, a stimulation electrode was implanted to the left basolateral amygdala of both pilocarpine-treated (Pilo, n = 14) and age-matched control rats (n = 12). Ten Pilo and 4 control animals received for 10 days 4 daily packages of 50 s 4 Hz regular stimulation trains. At the end of the stimulation period, interneurons were identified by immunolabeling for parvalbumin (PV), neuropeptide Y (NPY), and neuronal nitric oxide synthase (nNOS). Cell density was determined in the CA1 subfield of the hippocampus using confocal microscopy. We found that PV+ cell density was preserved in pilocarpine-treated rats, while the NPY+/nNOS+ cell density decreased significantly. The amygdala DBS did not significantly change the cell density in healthy or in epileptic animals. We conclude that DBS with low frequency applied for 10 days does not influence interneuron cell density changes in the hippocampus of epileptic rats.

Cannabinoids, TRPV and nitric oxide: the three ring circus of neuronal excitability

Endocannabinoid system is considered a relevant player in the regulation of neuronal excitability, since it contributes to maintaining the balance of the synaptic ionic milieu. Perturbations to bioelectric conductances have been implicated in the pathophysiological processes leading to hyperexcitability and epileptic seizures. Cannabinoid influence on neurosignalling is exerted on classic receptor-mediated mechanisms or on further molecular targets. Among these, transient receptor potential vanilloid (TRPV) are ionic channels modulated by cannabinoids that are involved in the transduction of a plethora of stimuli and trigger fundamental downstream pathways in the post-synaptic site. In this review, we aim at providing a brief summary of the most recent data about the cross-talk between cannabinoid system and TRPV channels, drawing attention on their role on neuronal hyperexcitability. Then, we aim to unveil a plausible point of interaction between these neural signalling systems taking into consideration nitric oxide, a gaseous molecule inducing profound modifications to neural performances. From this novel perspective, we struggle to propose innovative cellular mechanisms in the regulation of hyperexcitability phenomena, with the goal of exploring plausible CB-related mechanisms underpinning epileptic seizures.

Dysfunction of hippocampal interneurons in epilepsy

Gamma-amino-butyric acid (GABA)-containing interneurons are crucial to both development and function of the brain. Down-regulation of GABAergic inhibition may result in the generation of epileptiform activity. Loss, axonal sprouting, and dysfunction of interneurons are regarded as mechanisms involved in epileptogenesis. Recent evidence suggests that network connectivity and the properties of interneurons are responsible for excitatory-inhibitory neuronal circuits. The balance between excitation and inhibition in CA1 neuronal circuitry is considerably altered during epileptic changes. This review discusses interneuron diversity, the causes of interneuron dysfunction in epilepsy, and the possibility of using GABAergic neuronal progenitors for the treatment of epilepsy.

Ghrelin induced memory facilitation implicates nitric oxide synthase activation and decrease in the threshold to promote LTP in hippocampal dentate gyrus

Although the hypothalamus has been long considered the main ghrelin (Ghr) target organ mediating orexigenic effects, recently it has been shown that in-vivo Ghr hippocampus administration improves learning and memory in the inhibitory avoidance paradigm. However, the possible mechanisms underlying this memory facilitation effect have not been clarified. Given that the biochemical memory cascade into the hippocampus involves nitric oxide (NO) synthesis via NO synthase (NOS) activation, we investigated 1) if Ghr administration modulated NOS activity in the hippocampus; and 2) if hippocampal NOS inhibition influenced Ghr-induced memory facilitation, using a behavioral paradigm, biochemical determinations and an electrophysiological model. Our results showed that intra-hippocampal Ghr administration increased the NOS activity in a dose dependent manner, and reduced the threshold for LTP generation in dentate gyrus of rat hippocampus. Moreover, pre-administration of NG-nitro-l-arginine (l-NOArg) in the hippocampus partially prevented the Ghr-induced memory improvement, abolished the increase in NOS activity, and prevented the decreased threshold to generate LTP induced by Ghr. These findings suggest that activation of the NOS/NO pathway in hippocampus participates in the effects of Ghr on memory consolidation and is related with plastic properties of the hippocampal three-synaptic loop.

Modulatory effects of nitric oxide-active drugs on the anticonvulsant activity of lamotrigine in an experimental model of partial complex epilepsy in the rat

Background

The effects induced by administering the anticonvulsant lamotrigine, the preferential inhibitor of neuronal nitric oxide synthase 7-nitroindazole and the precursor of NO synthesis L-arginine, alone or in combination, on an experimental model of partial complex seizures (maximal dentate gyrus activation) were studied in urethane anaesthetized rats. The epileptic activity of the dentate gyrus was obtained through the repetitive stimulation of the angular bundle and maximal dentate gyrus activation latency, duration and post-stimulus afterdischarge duration were evaluated.

Results

Either Lamotrigine (10 mg kg^-1) or 7-nitroindazole (75 mg kg^-1) i.p. administration had an anticonvulsant effect, significantly reducing the number of animals responding to angular bundle stimulation. On the contrary, i.p. injection of L-arginine (1 g kg^-1) induced an aggravation of the epileptiform phenomena, demonstrated by the significant augmentation of the duration of both maximal dentate activation and afterdischarge. Furthermore, the injection of lamotrigine and 7-nitroindazole in combination significantly increased the anticonvulsant effects induced by the same drugs separately, either reducing the number of responding animals or decreasing both maximal dentate gyrus activation and afterdischarge durations. On the contrary, the combined treatment with L-arginine and lamotrigine did not modify the maximal dentate gyrus activation parameters suggesting an adversative effect of L-arginine-increased nitric oxide levels on the lamotrigine-induced anticonvulsant action.

Conclusion

The present results indicate that the nitrergic neurotransmission exerts a significant modulatory role in the control of the development of paroxystic phenomena in the maximal dentate gyrus activation model of epilepsy. Finally, our data suggest a functional relationship between the nitric oxide system and the anticonvulsant effect of lamotrigine which could be enhanced by reducing nitric oxide levels and, conversely, dampened by an increased nitrergic activity.

Involvement of nitric oxide-soluble guanylyl cyclase pathway in the control of maximal dentate gyrus activation in the rat

Nitric oxide/soluble Guanylyl cyclase (NO/sGC) pathway on the maximal dentate gyrus activation (MDA) was studied in rats. The cerebral NO levels were modified by administrating 7-Nitroindazole (7-NI), a selective inhibitor of neuronal NOS, and L-arginine, a precursor of the synthesis of NO. 1H-[1,2,4]Oxadiazole[4,3-a]quinoxalin-1-one (ODQ), a specific inhibitor of the NO-sGC pathway, was administered to study the involvement of cGMP pathway. The epileptic activity of the dentate gyrus was obtained through the repetitive stimulation of the angular bundle; MDA parameters studied were: onset time, MDA duration and post-stimulus afterdischarge (AD) duration. 7-NI caused an increase of MDA onset time and a decrease of MDA and AD duration. L-arginine, induced an aggravation of the epileptiform phenomena. ODQ induced modifications of MDA parameters as those caused by 7-NI. Our results indicate that the nitrergic neurotransmission exerts a modulatory role in the proneness to the epileptogenic phenomena through the activation of sGC metabolic pathway.

Nitric oxide synthase (NOS)-interacting protein interacts with neuronal NOS and regulates its distribution and activity

Mechanisms governing the activity of neuronal nitric oxide synthase (nNOS), the major source of nitric oxide (NO) in the nervous system, are not completely understood. We report here a protein-protein interaction between nNOS and NOSIP (nitric oxide synthase-interacting protein) in rat brain in vivo. NOSIP and nNOS are concentrated in neuronal synapses and demonstrate significant colocalization in various regions of the central and peripheral nervous systems. NOSIP produces a significant reduction in nNOS activity in a neuroepithelioma cell line stably expressing nNOS. Furthermore, overexpression of NOSIP in cultured primary neurons reduces the availability of nNOS in terminal dendrites. These results thus suggest that the interaction between NOSIP and nNOS is functionally involved in endogenous mechanisms regulating NO synthesis. Furthermore, we found that the subcellular distribution and expression levels of NOSIP are dynamically regulated by neuronal activity in vitro as well as in vivo, suggesting that NOSIP may contribute to a mechanism via which neuronal activity regulates the synaptic availability and activity of nNOS.

Changes in NOS protein expression and activity in the rat hippocampus, entorhinal and postrhinal cortices after unilateral electrolytic perirhinal cortex lesions

The integrity of the perirhinal cortex is critical for certain types of learning and memory. One important issue relating to the function of this region is its interaction with other brain areas that play a role in memory processing. This study investigates the time course of changes in activity and protein expression of nitric oxide synthase (NOS), which transforms L-arginine into nitric oxide (NO) and citrulline, in the hippocampus and the entorhinal and postrhinal cortices after unilateral electrolytic lesions of the perirhinal cortex. Electrolytic lesions of the perirhinal cortex resulted in long lasting changes in NOS activity and protein expression in the entorhinal and postrhinal cortices (< or = 2 weeks post-lesion). In contrast, there was a small and transient decrease in nNOS expression (with no change in NOS activity) in the dorsal portion of the hippocampus. iNOS was not expressed in any region examined at any time point. These findings provide the first evidence that electrolytic lesions of the perirhinal cortex can result in long-term neurochemical changes in its anatomically related structures. Given that NO has been implicated in neuroplasticity processes, the interpretation of memory impairments induced by electrolytic lesions of the perirhinal cortex (and possibly, therefore, other brain regions) need to be considered with regard to these findings.

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.

Mitochondrial dysfunction associated with neuronal death following status epilepticus in rat

Status epilepticus (SE) in humans and animal models results in significant cerebral damage and an increased risk of subsequent seizures, associated with a characteristic pattern of neuronal loss particularly affecting the hippocampus. Seizure related cell death is considered to be excitotoxic, but studies have been limited, concentrating on terminal events rather than initial mechanisms. We have studied the biochemical events in the first few days following SE. Self-sustaining limbic SE was induced in adult rats using perforant path stimulation, and animals were allowed to recover. Biochemical studies were performed at 16, 44 h and 8 days following SE, using spectrophotometric enzyme assays and HPLC on regional brain homogenates compared with those from sham-operated controls. Haematoxylin and eosin histology was also undertaken at each time point. Brain aconitase and alpha-ketoglutarate dehydrogenase (alphaKDH) activity were both significantly (P<0.05) reduced by approximately 20% in the first 16-44 h following status, but had returned to normal by 8 days. These enzymes are part of the tri-carboxylic acid (Krebbs) cycle in the mitochondrial matrix, and are known to be sensitive to free radical, especially peroxynitrite damage. There was a similar decrease in reduced glutathione levels. Histological studies confirmed evidence of acute neuronal damage up to 44 h, and neuronal loss by 8 days. This is the first in vivo demonstration of this pattern of mitochondrial dysfunction and loss of brain glutathione following SE. The pattern of abnormalities is consistent with reversible mechanisms being involved in excitotoxic cell damage. This, together with the timing of changes, suggests new avenues for therapeutic intervention.

Therapy of status epilepticus in adults and children

Clinical studies of the treatment of status epilepticus are extremely difficult to carry out, therefore a paucity of new clinical studies have been reported. Much of the progress regarding the therapy of status epilepticus has come from a better understanding of the epidemiology of status epilepticus and its consequences and from laboratory studies of experimental status. Status epilepticus has been used as an experimental tool to study epileptogenesis, but from such studies have come insights that can be applied to the therapy of status epilepticus itself. This review will focus on information from epidemiological, experimental, and clinical studies of status epilepticus, which may contribute to the improved treatment of this life-threatening disorder.

Inflammatory regulators in Parkinson's disease: iNOS, lipocortin-1, and cyclooxygenases-1 and -2

Degeneration of dopaminergic neurons and focal gliosis are pathological hallmarks of Parkinson's disease and although the brain is described as immune-privileged focal immune reactions surround failing nigral neurons. We examined the cellular distribution of pro- and anti-inflammatory molecules in human parkinsonian and neurologically normal substantia nigra and caudate-putamen postmortem. An up-regulation of nitric oxide synthase- and cyclo-oxygenase-1- and -2-containing amoeboid microglia was found in parkinsonian but not control nigra. Astroglia contained low levels of these molecules in both groups. Lipocortin-1-immunoreactive amoeboid microglia were present within the astrocytic envelope of neurons adjacent to or within glial scars in parkinsonian nigra only. Lipocortin-1 is known to have neuroprotective and anti-inflammatory properties. Up-regulation of nitric oxide synthase is generally associated with neurodestruction whereas prostaglandin synthesis may be either neurodestructive or protective. The balance of these molecules is likely to be decisive in determining neuronal survival or demise.