Altered expression of NPY-Y1 receptors in kainic acid induced epilepsy in rats

Reduced Cholecystokinin-Expressing Interneuron Input Contributes to Disinhibition of the Hippocampal CA2 Region in a Mouse Model of Temporal Lobe Epilepsy

A significant proportion of temporal lobe epilepsy (TLE) patients experience drug-resistant seizures associated with mesial temporal sclerosis, in which there is extensive cell loss in the hippocampal CA1 and CA3 subfields, with a relative sparing of dentate gyrus granule cells and CA2 pyramidal neurons (PNs). A role for CA2 in seizure generation was suggested based on findings of a reduction in CA2 synaptic inhibition (Williamson and Spencer, 1994) and the presence of interictal-like spike activity in CA2 in resected hippocampal tissue from TLE patients (Wittner et al., 2009). We recently found that in the pilocarpine-induced status epilepticus (PILO-SE) mouse model of TLE there was an increase in CA2 intrinsic excitability associated with a loss of CA2 synaptic inhibition. Furthermore, chemogenetic silencing of CA2 significantly reduced seizure frequency, consistent with a role of CA2 in promoting seizure generation and/or propagation (Whitebirch et al., 2022). In the present study, we explored the cellular basis of this inhibitory deficit using immunohistochemical and electrophysiological approaches in PILO-SE male and female mice. We report a widespread decrease in the density of pro-cholecystokinin-immunopositive (CCK+) interneurons and a functional impairment of CCK+ interneuron-mediated inhibition of CA2 PNs. We also found a disruption in the perisomatic perineuronal net in the CA2 stratum pyramidale. Such pathologic alterations may contribute to an enhanced excitation of CA2 PNs and CA2-dependent seizure activity in the PILO-SE mouse model.SIGNIFICANCE STATEMENT Impaired synaptic inhibition in hippocampal circuits has been identified as a key feature that contributes to the emergence and propagation of seizure activity in human patients and animal models of temporal lobe epilepsy (TLE). Among the hippocampal subfields, the CA2 region is particularly resilient to seizure-associated neurodegeneration and has been suggested to play a key role in seizure activity in TLE. Here we report that perisomatic inhibition of CA2 pyramidal neurons mediated by cholecystokinin-expressing interneurons is selectively reduced in acute hippocampal slices from epileptic mice. Parvalbumin-expressing interneurons, in contrast, appear relatively conserved in epileptic mice. These findings advance our understanding of the cellular mechanisms underlying inhibitory disruption in hippocampal circuits in a mouse model of spontaneous recurring seizures.

Seizure-induced overexpression of NPY induces epileptic tolerance in a mouse model of spontaneous recurrent seizures

Epileptic seizures result in pronounced over-expression of neuropeptide Y (NPY). In vivo and in vitro studies revealed that NPY exerts potent anticonvulsive actions through presynaptic Y2 receptors by suppressing glutamate release from principal neurons. We now investigated whether seizure-induced over-expression of NPY contributes to epileptic tolerance induced by preceding seizures. We used a previously established animal model based on selective inhibition of GABA release from parvalbumin (PV)-containing interneurons in the subiculum in mice. The animals present spontaneous recurrent seizures (SRS) and clusters of interictal spikes (IS). The frequency of SRS declined after five to six weeks, indicating development of seizure tolerance. In interneurons of the subiculum and sector CA1, SRS induced over-expression of NPY that persisted there for a prolonged time despite of a later decrease in SRS frequency. In contrast to NPY, somatostatin was not overexpressed in the respective axon terminals. Contrary to interneurons, NPY was only transiently expressed in mossy fibers. To demonstrate a protective function of endogenous, over-expressed NPY, we injected the selective NPY-Y2 receptor antagonist JNJ 5207787 simultaneously challenging the mice by a low dose of pentylenetetrazol (PTZ, 30 or 40 mg/kg, i.p.). In control mice, neither PTZ nor PTZ plus JNJ 5207787 induced convulsions. In mice with silenced GABA/PV neurons, PTZ alone only modestly enhanced EEG activity. When we injected JNJ 5207787 together with PTZ (either dose) the number of seizures, however, became significantly increased. In addition, in the epileptic mice CB1 receptor immunoreactivity was reduced in terminal areas of basket cells pointing to reduced presynaptic inhibition of GABA release from these neurons. Our experiments demonstrate that SRS result in overexpression of NPY in hippocampal interneurons. NPY overexpression persists for several weeks and may be related to later decreasing SRS frequency. Injection of the Y2 receptor antagonist JNJ 5207787 prevents this protective action of NPY only when release of the peptide is triggered by injection of PTZ and induces pronounced convulsions. Thus, over-expressed NPY released “on demand” by seizures may help terminating acute seizures and may prevent from recurrent epileptic activity.

NPY and Gene Therapy for Epilepsy: How, When,... and Y

Neuropeptide Y (NPY) is a neuropeptide abundantly expressed in the mammalian central and peripheral nervous system. NPY is a pleiotropic molecule, which influences cell proliferation, cardiovascular and metabolic function, pain and neuronal excitability. In the central nervous system, NPY acts as a neuromodulator, affecting pathways that range from cellular (excitability, neurogenesis) to circuit level (food intake, stress response, pain perception). NPY has a broad repertoire of receptor subtypes, each activating specific signaling pathways in different tissues and cellular sub-regions. In the context of epilepsy, NPY is thought to act as an endogenous anticonvulsant that performs its action through Y2 and Y5 receptors. In fact, its overexpression in the brain with the aid of viral vectors can suppress seizures in animal models of epilepsy. Therefore, NPY-based gene therapy may represent a novel approach for the treatment of epilepsy patients, particularly for pharmaco-resistant and genetic forms of the disease. Nonetheless, considering all the aforementioned aspects of NPY signaling, the study of possible NPY applications as a therapeutic molecule is not devoid of critical aspects. The present review will summarize data related to NPY biology, focusing on its anti-epileptic effects, with a critical appraisal of key elements that could be exploited to improve the already existing NPY-based gene therapy approaches for epilepsy.

Effects of Huazhuo Jiedu Shugan Decoction on Cognitive and Emotional Disorders in a Rat Model of Epilepsy: Possible Involvement of AC-cAMP-CREB Signaling and NPY Expression

Background

Huazhuo Jiedu Shugan decoction (HJSD), a traditional Chinese medicine (TCM), has been used to treat epileptic seizures for many years. Some ingredients in these herbs have been demonstrated to be effective for the treatment of brain damage caused by epilepsy.

Aim of the Study

The object of the study is to determine the effects of HJSD on cognitive and emotional disorders in a rat model of epilepsy.

Materials and Methods

After a predetermined time period, rats were intraperitoneally injected with pentylenetetrazol and observed in different phases of convulsions. The cognitive and emotional changes in the epileptic rats were assessed using behavioral and immunohistochemical tests.

Results

Compared with the epilepsy group, the seizure grade was reduced and seizure latency was prolonged following HJSD-H treatment (P < 0.01). Compared with the control group, the epilepsy group displayed marked worse performance on the animal behavior tests (P < 0.05) and the HJSD-H group displayed improved behavioral performance (P < 0.05). After HJSD-H treatment, the expression of adenylate cyclase (AC), cyclic adenosine monophosphate (cAMP), cAMP-response element binding protein (CREB), and neuropeptide Y (NPY) immunoreactive cells markedly increased in the hippocampus, compared with that of the epilepsy group (P < 0.05).

Conclusions

The current results demonstrate that HJSD treatment in epileptic rats markedly inhibits epileptic seizures and improves cognitive and emotional disorders, which may be related to the regulation of AC-cAMP-CREB signaling and NPY expression in the hippocampus. The effects of the HJSD treatment may provide a foundation for the use of HJSD as a prescription medicinal herb in the TCM for the treatment of epilepsy.

Interactions Between Epilepsy and Plasticity

Undoubtedly, one of the most interesting topics in the field of neuroscience is the ability of the central nervous system to respond to different stimuli (normal or pathological) by modifying its structure and function, either transiently or permanently, by generating neural cells and new connections in a process known as neuroplasticity. According to the large amount of evidence reported in the literature, many stimuli, such as environmental pressures, changes in the internal dynamic steady state of the organism and even injuries or illnesses (e.g., epilepsy) may induce neuroplasticity. Epilepsy and neuroplasticity seem to be closely related, as the two processes could positively affect one another. Thus, in this review, we analysed some neuroplastic changes triggered in the hippocampus in response to seizure-induced neuronal damage and how these changes could lead to the establishment of temporal lobe epilepsy, the most common type of focal human epilepsy.

Preserved Function of Afferent Parvalbumin-Positive Perisomatic Inhibitory Synapses of Dentate Granule Cells in Rapidly Kindled Mice

Parvalbumin- (PV-) containing basket cells constitute perisomatic GABAergic inhibitory interneurons innervating principal cells at perisomatic area, a strategic location that allows them to efficiently control the output and synchronize oscillatory activity at gamma frequency (30–90 Hz) oscillations. This oscillatory activity can convert into higher frequency epileptiform activity, and therefore could play an important role in the generation of seizures. However, the role of endogenous modulators of seizure activity, such as Neuropeptide Y (NPY), has not been fully explored in at PV input and output synapses. Here, using selective optogenetic activation of PV cells in the hippocampus, we show that seizures, induced by rapid kindling (RK) stimulations, enhance gamma-aminobutyric acid (GABA) release from PV cells onto dentate gyrus (DG) granule cells (GC). However, PV-GC synapses did not differ between controls and kindled animals in terms of GABA release probability, short-term plasticity and sensitivity to NPY. Kinetics of gamma-aminobutyric acid A (GABA-A) mediated currents in postsynaptic GC were also unaffected. When challenged by repetitive high-frequency optogenetic stimulations, PV synapses in kindled animals responded with enhanced GABA release onto GC. These results unveil a mechanism that might possibly contribute to the generation of abnormal synchrony and maintenance of epileptic seizures.

Effective G-protein coupling of Y2 receptors along axonal fiber tracts and its relevance for epilepsy

Neuropeptide Y (NPY)-Y2 receptors are G-protein coupled receptors and, upon activation, induce opening of potassium channels or closing of calcium channels. They are generally presynaptically located. Depending on the neuron in which they are expressed they mediate inhibition of release of NPY and of the neuron's classical transmitter GABA, glutamate or noradrenaline, respectively. Here we provide evidence that Y2 receptor binding is inhibited dose-dependently by GTPγS along Schaffer collaterals, the stria terminalis and the fimbria indicating that Y2 receptors are functionally coupled to G-proteins along these fiber tracts. Double immune fluorescence revealed coexistence of Y2-immunoreactivity with β-tubulin, a marker for axons in the stria terminalis, but not with synaptophysin labeling presynaptic terminals, supporting the localization of Y2 receptors along axonal tracts. After kainic acid-induced seizures in rats, GTPγS-induced inhibition of Y2 receptor binding is facilitated in the Schaffer collaterals but not in the stria terminalis. Our data indicate that Y2 receptors are not only located at nerve terminals but also along fiber tracts and are there functionally coupled to G-proteins.

The role of Neuropeptide Y in fear conditioning and extinction

While anxiety disorders are the brain disorders with the highest prevalence and constitute a major burden for society, a considerable number of affected people are still treated insufficiently. Thus, in an attempt to identify potential new anxiolytic drug targets, neuropeptides have gained considerable attention in recent years. Compared to classical neurotransmitters they often have a regionally restricted distribution and may bind to several distinct receptor subtypes. Neuropeptide Y (NPY) is a highly conserved neuropeptide that is specifically concentrated in limbic brain areas and signals via at least 5 different G-protein-coupled receptors. It is involved in a variety of physiological processes including the modulation of emotional-affective behaviors. An anxiolytic and stress-reducing property of NPY is supported by many preclinical studies. Whether NPY may also interact with processing of learned fear and fear extinction is comparatively unknown. However, this has considerable relevance since pathological, inappropriate and generalized fear expression and impaired fear extinction are hallmarks of human post-traumatic stress disorder and a major reason for its treatment-resistance. Recent evidence from different laboratories emphasizes a fear-reducing role of NPY, predominantly mediated by exogenous NPY acting on Y1 receptors. Since a reduction of fear expression was also observed in Y1 receptor knockout mice, other Y receptors may be equally important. By acting on Y2 receptors, NPY promotes fear extinction and generates a long-term suppression of fear, two important preconditions that could support cognitive behavioral therapies in human patients. A similar effect has been demonstrated for the closely related pancreatic polypeptide (PP) when acting on Y4 receptors. Preliminary evidence suggests that NPY modulates fear in particular by activation of Y1 and Y2 receptors in the basolateral and central amygdala, respectively. In the basolateral amygdala, NPY signaling activates inhibitory G protein-coupled inwardly-rectifying potassium channels or suppresses hyperpolarization-induced I(h) currents in a Y1 receptor-dependent fashion, favoring a general suppression of neuronal activity. A more complex situation has been described for the central extended amygdala, where NPY reduces the frequency of inhibitory and excitatory postsynaptic currents. In particular the inhibition of long-range central amygdala output neurons may result in a Y2 receptor-dependent suppression of fear. The role of NPY in processes of learned fear and fear extinction is, however, only beginning to emerge, and multiple questions regarding the relevance of endogenous NPY and different receptor subtypes remain elusive. Y2 receptors may be of particular interest for future studies, since they are the most prominent Y receptor subtype in the human brain and thus among the most promising therapeutic drug targets when translating preclinical evidence to potential new therapies for human anxiety disorders.

Thyroid hormones: Possible roles in epilepsy pathology

Thyroid hormones (THs) L-thyroxine and L-triiodothyronine, primarily known as metabolism regulators, are tyrosine-derived hormones produced by the thyroid gland. They play an essential role in normal central nervous system development and physiological function. By binding to nuclear receptors and modulating gene expression, THs influence neuronal migration, differentiation, myelination, synaptogenesis and neurogenesis in developing and adult brains. Any uncorrected THs supply deficiency in early life may result in irreversible neurological and motor deficits. The development and function of GABAergic neurons as well as glutamatergic transmission are also affected by THs. Though the underlying molecular mechanisms still remain unknown, the effects of THs on inhibitory and excitatory neurons may affect brain seizure activity. The enduring predisposition of the brain to generate epileptic seizures leads to a complex chronic brain disorder known as epilepsy. Pathologically, epilepsy may be accompanied by mitochondrial dysfunction, oxidative stress and eventually dysregulation of excitatory glutamatergic and inhibitory GABAergic neurotransmission. Based on the latest evidence on the association between THs and epilepsy, we hypothesize that THs abnormalities may contribute to the pathogenesis of epilepsy. We also review gender differences and the presumed underlying mechanisms through which TH abnormalities may affect epilepsy here.

Neuropeptide Y-stimulated [(35) S]GTPγs functional binding is reduced in the hippocampus after kainate-induced seizures in mice

Kainate-induced seizures constitute a model of temporal lobe epilepsy where prominent changes are observed in the hippocampal neuropeptide Y (NPY) system. However, little is known about the functional state and signal transduction of the NPY receptor population resulting from kainate exposure. Thus, in this study, we explored functional NPY receptor activity in the mouse hippocampus and neocortex after kainate-induced seizures using NPY-stimulated [(35) S]GTPγS binding. Moreover, we also studied levels of [(125) I]-peptide YY (PYY) binding and NPY, Y1, Y2, and Y5 receptor mRNA in these kainate-treated mice. Functional NPY binding was unchanged up to 12 h post-kainate, but decreased significantly in all hippocampal regions after 24 h and 1 week. Similarly, a decrease in [(125) I]-PYY binding was found in the dentate gyrus (DG) 1 week post-kainate. However, at 2 h, 6 h, and 12 h, [(125) I]-PYY binding was increased in all regions, and in the CA1 also at 24 h post-kainate. NPY mRNA levels were prominently increased in hippocampal regions, reaching maximum at 12 and 24 h. Y1 and Y5 mRNA levels were lowered in the DG at 24 and 2 h, respectively, while Y2 mRNA levels were elevated at 24 h in the DG and CA3. This study confirms rat kainate studies by showing pronounced adaptive changes in the mouse hippocampus both with regard to NPY synthesis and NPY receptor synthesis and binding, which may contribute to regulating neuronal seizure susceptibility after kainate. However, the potential seizure-suppressant effects of increased NPY gene expression at late time points post-kainate could be attenuated by the novel finding of reduced NPY-receptor G-protein activation.

Neuropeptides as targets for the development of anticonvulsant drugs

Epilepsy is a common neurological disorder characterized by recurrent seizures. These seizures are due to abnormal excessive and synchronous neuronal activity in the brain caused by a disruption of the delicate balance between excitation and inhibition. Neuropeptides can contribute to such misbalance by modulating the effect of classical excitatory and inhibitory neurotransmitters. In this review, we discuss 21 different neuropeptides that have been linked to seizure disorders. These neuropeptides show an aberrant expression and/or release in animal seizure models and/or epilepsy patients. Many of these endogenous peptides, like adrenocorticotropic hormone, angiotensin, cholecystokinin, cortistatin, dynorphin, galanin, ghrelin, neuropeptide Y, neurotensin, somatostatin, and thyrotropin-releasing hormone, are able to suppress seizures in the brain. Other neuropeptides, such as arginine-vasopressine peptide, corticotropin-releasing hormone, enkephalin, β-endorphin, pituitary adenylate cyclase-activating polypeptide, and tachykinins have proconvulsive properties. For oxytocin and melanin-concentrating hormone both pro- and anticonvulsive effects have been reported, and this seems to be dose or time dependent. All these neuropeptides and their receptors are interesting targets for the development of new antiepileptic drugs. Other neuropeptides such as nesfatin-1 and vasoactive intestinal peptide have been less studied in this field; however, as nesfatin-1 levels change over the course of epilepsy, this can be considered as an interesting marker to diagnose patients who have suffered a recent epileptic seizure.

Altered expression of neuropeptide Y, Y1 and Y2 receptors, but not Y5 receptor, within hippocampus and temporal lobe cortex of tremor rats

As an endogenous inhibitor of glutamate-mediated synaptic transmission in mammalian central nervous system, neuropeptide Y (NPY) plays a crucial role in regulating homeostasis of neuron excitability. Loss of balance between excitatory and inhibitory neurotransmission is thought to be a chief mechanism of epileptogenesis. The abnormal expression of NPY and its receptors observed following seizures have been demonstrated to be related to the production of epilepsy. The tremor rat (TRM) is a hereditary epileptic animal model. So far, there is no report concerning whether NPY and its receptors may be involved in TRM pathogenesis. In this study, we focused on the expression of NPY and its three receptor subtypes: Y1R, Y2R and Y5R in the TRM brain. We first found the expression of NPY in TRM hippocampus and temporal lobe cortex was increased compared with control (Wistar) rats. The mRNA and protein expression of Y1R was down-regulated in hippocampus but up-regulated in temporal lobe cortex, whereas Y2R expression was significantly increased in both areas. There was no significant change of Y5R expression in either area. The immunohistochemistry data showed that Y1R, Y2R, Y5R were present throughout CA1, CA3, dentate gyrus (DG) and the entorhinal cortex which is included in the temporal lobe cortex of TRM. In conclusion, our results showed the altered expression of NPY, Y1R and Y2R but not Y5R in hippocampus and temporal lobe cortex of TRM brain. This abnormal expression may be associated with the generation of epileptiform activity and provide a candidate target for treatment of genetic epilepsy.

Neuropeptides in epilepsy

Neuropeptides play an important role in modulating seizures and epilepsy. Unlike neurotransmitters which operate on a millisecond time-scale, neuropeptides have longer half lives; this leads to modulation of neuronal and network activity over prolonged periods, so contributing to setting the seizure threshold. Most neuropeptides are stored in large dense vesicles and co-localize with inhibitory interneurons. They are released upon high frequency stimulation making them attractive targets for modulation of seizures, during which high frequency discharges occur. Numerous neuropeptides have been implicated in epilepsy; one, ACTH, is already used in clinical practice to suppress seizures. Here, we concentrate on neuropeptides that have a direct effect on seizures, and for which therapeutic interventions are being developed. We have thus reviewed the abundant reports that support a role for neuropeptide Y (NPY), galanin, ghrelin, somatostatin and dynorphin in suppressing seizures and epileptogenesis, and for tachykinins having pro-epileptic effects. Most in vitro and in vivo studies are performed in hippocampal tissue in which receptor expression is usually high, making translation to other brain areas less clear. We highlight recent therapeutic strategies to treat epilepsy with neuropeptides, which are based on viral vector technology, and outline how such interventions need to be refined in order to address human disease.

Regulators of synaptic transmission: roles in the pathogenesis and treatment of epilepsy

Synaptic transmission is the communication between a presynaptic and a postsynaptic neuron, and the subsequent processing of the signal. These processes are complex and highly regulated, reflecting their importance in normal brain functioning and homeostasis. Sustaining synaptic transmission depends on the continuing cycle of synaptic vesicle formation, release, and endocytosis, which requires proteins such as dynamin, syndapin, synapsin, and synaptic vesicle protein 2A. Synaptic transmission is regulated by diverse mechanisms, including presynaptic modulators of synaptic vesicle formation and release, postsynaptic receptors and signaling, and modulators of neurotransmission. Neurotransmitters released presynaptically can bind to their postsynaptic receptors, the inhibitory γ-aminobutyric acid (GABA)ergic receptors or the excitatory glutamate receptors. Once released, glutamate activates a variety of postsynaptic receptors including α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), N-methyl-D-aspartate (NMDA), kainate, and metabotropic receptors. The activation of the receptors triggers downstream signaling cascades generating a vast array of effects, which can be modulated by a numerous auxiliary regulatory subunits. Moreover, different neuropeptides such as neuropeptide Y, brain-derived neurotrophic factor (BDNF), somatostatin, ghrelin, and galanin, act as regulators of diverse synaptic functions and along with the classic neurotransmitters. Abnormalities in the regulation of synaptic transmission play a critical role in the pathogenesis of numerous brain diseases, including epilepsy. This review focuses on the different mechanisms involved in the regulation of synaptic transmission, which may play a role in the pathogenesis of epilepsy: the presynaptic modulators of synaptic vesicle formation and release, postsynaptic receptors, and modulators of neurotransmission, including the mechanism by which drugs can modulate the frequency and severity of epileptic seizures.

Altered profile of basket cell afferent synapses in hyper-excitable dentate gyrus revealed by optogenetic and two-pathway stimulations

Cholecystokinin (CCK-) positive basket cells form a distinct class of inhibitory GABAergic interneurons, proposed to act as fine-tuning devices of hippocampal gamma-frequency (30-90 Hz) oscillations, which can convert into higher frequency seizure activity. Therefore, CCK-basket cells may play an important role in regulation of hyper-excitability and seizures in the hippocampus. In normal conditions, the endogenous excitability regulator neuropeptide Y (NPY) has been shown to modulate afferent inputs onto dentate gyrus CCK-basket cells, providing a possible novel mechanism for excitability control in the hippocampus. Using GAD65-GFP mice for CCK-basket cell identification, and whole-cell patch-clamp recordings, we explored whether the effect of NPY on afferent synapses to CCK-basket cells is modified in the hyper-excitable dentate gyrus. To induce a hyper-excitable state, recurrent seizures were evoked by electrical stimulation of the hippocampus using the well-characterized rapid kindling protocol. The frequency of spontaneous and miniature excitatory and inhibitory post-synaptic currents recorded in CCK-basket cells was decreased by NPY. The excitatory post-synaptic currents evoked in CCK-basket cells by optogenetic activation of principal neurons were also decreased in amplitude. Interestingly, we observed an increased proportion of spontaneous inhibitory post-synaptic currents with slower rise times, indicating that NPY may inhibit gamma aminobutyric acid release preferentially in peri-somatic synapses. These findings indicate that increased levels and release of NPY observed after seizures can modulate afferent inputs to CCK-basket cells, and therefore alter their impact on the oscillatory network activity and excitability in the hippocampus.

Focal administration of neuropeptide Y into the S2 somatosensory cortex maximally suppresses absence seizures in a genetic rat model

PURPOSE

Neuropeptide Y (NPY) is an inhibitory neurotransmitter that suppresses focal and generalized seizures in animal models. In this study, we investigated the sites within the thalamocortical circuit that NPY acts to suppress seizures in genetic absence epilepsy rats from Strasbourg (GAERS).

METHODS

In conscious freely moving GAERS, NPY was administered via intracerebral microcannulae implanted bilaterally into one of the following regions: primary somatosensory cortex (S1), secondary somatosensory cortex (S2), the primary motor cortex (M1), caudal nucleus reticular thalamus (nRT), or ventrobasal thalamus (VB). Animals received vehicle and up to three doses of NPY, in a randomized order. Electroencephalography (EEG) recordings were carried out for 30 min prior to injection and 90 min after the injection of NPY or vehicle.

KEY FINDINGS

Focal microinjections of NPY into the S2 cortex suppressed seizures in a dose-dependent manner, with the response being significantly different at the highest dose (1.5 mm) compared to vehicle for total time in seizures postinjection (7.2 ± 3.0% of saline, p < 0.01) and average number of seizures (9.4 ± 4.9% of saline, p < 0.05). In contrast NPY microinjections into the VB resulted in an aggravation of seizures.

SIGNIFICANCE

NPY produces contrasting effects on absence-like seizures in GAERS depending on the site of injection within the thalamocortical circuit. The S2 is the site at which NPY most potently acts to suppress absence-like seizures in GAERS, whereas seizure-aggravating effects are seen in the VB. These results provide further evidence to support the proposition that these electroclinically "generalized" seizures are being driven by a topographically restricted region within the somatosensory cortex.

Neuroprotective effect of neuropeptide Y against β-amyloid 25-35 toxicity in SH-SY5Y neuroblastoma cells is associated with increased neurotrophin production

BACKGROUND

In the central nervous system, several neuropeptides are believed to be involved in the pathophysiology of Alzheimer's disease (AD). Among them, neuropeptide Y (NPY) is a small peptide widely distributed throughout the brain, where it serves as a neurotransmitter and/or a modulator of several neuroendocrine functions. More recently, NPY has generated interest because of its role in neuroprotection against excitotoxicity and modulation of neurogenesis. Interestingly, these effects are also influenced by neurotrophins, critical molecules for the function and survival of neurons that degenerate in AD.

OBJECTIVE

Our purpose was to investigate whether NPY might be a neuroprotective agent in AD and whether neurotrophins are involved in NPY-induced neuroprotection.

METHODS

To test this hypothesis, we exposed the SH-SY5Y neuroblastoma cell line to toxic concentrations of β-amyloid (Aβ) peptide fragment 25-35 (Aβ(25-35)) and measured cell survival and neurotrophin expression before and after a preincubation with NPY in the growth medium.

RESULTS

Our results demonstrated that preincubation with NPY prevented cell loss due to the toxic effect of Aβ(25-35). Moreover, while intracellular production of nerve growth factor and brain-derived neurotrophic factor were reduced by Aβ, NPY restored or even increased neurotrophin protein and mRNA in SH-SY5Y cells.

CONCLUSION

In conclusion, this study demonstrates that NPY increases the survival of SH-SY5Y neuroblastoma cells and counteracts the toxic effect of Aβ. In addition, NPY restores the neurotrophin levels in these cells. Although preliminary, these observations might be useful to understand the pathology of Alzheimer's and/or develop new therapeutic strategies.

In vitro effects of neuropeptide Y in rat neocortical and hippocampal tissue

Neuropeptide Y (NPY) network effects in hippocampus and frontal cortex and its impact on epileptiform neocortical discharges were investigated in rat juvenile brain slices. NPY (1 μM) reduced amplitudes of paired pulse stimulation in hippocampal brain tissue (p<0.05) whereas NPY (1 nM-2 μM) had no effect in neocortex. Late stage epileptiform activity in the neocortex was unaffected by NPY (1 μM). Our results point to a region dependent effect of NPY with a high impact on hippocampal and minimal impact on neocortical networks.

Adeno-associated virus-mediated expression and constitutive secretion of NPY or NPY13-36 suppresses seizure activity in vivo

Neuropeptide Y (NPY) is a 36-amino-acid peptide that attenuates seizure activity following direct infusion or adeno-associated virus (AAV)-mediated expression in the central nervous system. However, NPY activates all NPY receptor subtypes, potentially causing unwanted side effects. NPY13-36 is a C-terminal peptide fragment of NPY that primarily activates the NPY Y2 receptor, thought to mediate the antiseizure activity. Therefore, we investigated if recombinant adeno-associated virus-mediated expression and constitutive secretion of NPY or NPY13-36 could alter limbic seizure sensitivity. Rats received bilateral piriform cortex infusions of AAV vectors that express and constitutively secrete full-length NPY (AAV-FIB-NPY) or NPY13-36 (AAV-FIB-NPY13-36). Control rats received no infusion, as we have previously shown that vectors expressing and secreting reporter genes like GFP (AAV-FIB-EGFP), as well as vectors expressing peptides that lack secretion sequences (AAV-GAL) have no effect on seizures. One week later, all animals received kainic acid (10 mg kg(-1), intraperitoneally), and the latencies to wet dog shakes and limbic seizure behaviors were determined. Although both control and vector-treated rats developed wet dog shake behaviors with similar latencies, the latencies to class III and class IV limbic seizures were significantly prolonged in both NPY- and NPY13-36-treated groups. Thus, AAV-mediated expression and constitutive secretion of NPY and NPY13-36 is effective in attenuating limbic seizures, and provides a platform for delivering therapeutic peptide fragments with increased receptor selectivity.

Infusion of neuropeptide Y into CA3 region of hippocampus produces antidepressant-like effect via Y1 receptor

A couple of papers indicate that patients with depression show a decrease in serum neuropeptide Y (NPY). To study the role of NPY in depression, we examined the effects of infusion of NPY into the hippocampus of learned helplessness (LH) rats (an animal model of depression). Infusion of NPY into the cerebral ventricle of LH rats showed antidepressant-like effects. Infusion of NPY into the CA3 region, but not the dentate gyrus (DG), produced antidepressant-like effects in the LH paradigm. Infusion of NPY did not affect locomotor activity or aversive learning ability. Coadministration of BIBO3304 (a Y1 receptor antagonist) with NPY to the CA3 region blocked the antidepressant-like effects of NPY, whereas coadministration of NPY with BIIE0246 (a Y2 receptor antagonist) to the CA3 region failed to block antidepressant-like effects. Furthermore, infusions of [Leu(31) Pro(34)]PYY (a Y1 and Y5 receptor agonist) alone and BIIE0246 alone into the CA3 region produced the antidepressant-like effects in LH rats. These results suggest that infusion of NPY into the CA3 region of hippocampus of LH rats produces antidepressant-like activity through Y1 receptors and attenuating effects through Y2 receptors.

Neuropeptide Y suppresses absence seizures in a genetic rat model primarily through effects on Y2 receptors

Neuropeptide Y (NPY) potently suppresses absence seizures in a model of genetic generalized epilepsy, genetic absence epilepsy rats of Strasbourg (GAERS). Here we investigated the Y-receptor subtype(s) on which NPY exerts this anti-absence effect. A dual in vivo approach was used: the cumulative duration of seizures was quantified in adult male GAERS in 90-min electroencephalogram recordings following intracerebroventricular (i.c.v.) injection of: (i) subtype-selective agonists of Y1 ([Leu31Pro34]NPY, 2.5 nmol), Y2 (Ac[Leu(28,31)]NPY24-36, 3 nmol), Y5 receptors [hPP1(-17),Ala31,Aib32]NPY, 4 nmol), NPY (3 nmol) or vehicle; and following (ii) i.c.v. injection of antagonists of Y1 (BIBP3226, 20 nmol), Y2 (BIIE0246, 20 nmol) and Y5 (NPY5RA972, 20 nmol) receptors or vehicle, followed by NPY (3 nmol). Injection of the Y1- and Y5-selective agonists resulted in significantly less mean seizure suppression (37.4% and 53.9%, respectively) than NPY (83.2%; P < 0.05), while the Y2 agonist had similar effects to NPY (62.3% suppression, P = 0.57). Food intake was not increased following injection of the Y2 agonist, while significant increases in food intake were seen following NPY and the other Y-subtype agonists. Compared with vehicle, NPY injection suppressed seizures following the Y1 and Y5 antagonists (45.3% and 80.1%, respectively, P < 0.05), but not following the Y2 antagonist (5.1% suppression, P = 0.46). We conclude that NPY Y2 receptors are more important than Y1 and Y5 receptors in mediating the effect of NPY to suppress absence seizures in a genetic rat model. Y2 receptor agonists may represent targets for novel drugs against genetic generalized epilepsies without resulting in appetite stimulation.

Unaltered neuropeptide Y (NPY)-stimulated [35S]GTPgammaS binding suggests a net increase in NPY signalling after repeated electroconvulsive seizures in mice

Although electroconvulsive seizures (ECS) are widely used as a treatment for severe depression, the working mechanism of ECS remains unclear. Repeated ECS causes anticonvulsant effects that have been proposed to underlie the therapeutic effect of ECS, and neuropeptide Y (NPY) is a potential candidate for mediating this anticonvulsant effect. Repeated ECS results in prominent increases in NPY synthesis. In contrast, NPY-sensitive receptor binding is decreased, so it is unclear whether ECS causes a net increase in NPY signalling. Agonist-stimulated [35S]GTPgammaS binding is a method for detecting functional activation of G-protein-coupled receptors. The present study in mice examined the effects of daily ECS for 14 days on NPY-stimulated [35S]GTPgammaS functional binding and compared this with gene expression of NPY and NPY receptors as well as [125I]peptide YY (PYY) binding in hippocampus of the same animals. Significant increases in NPY mRNA and concomitant reductions in NPY-sensitive binding were found in the dentate gyrus, hippocampal CA1, and neocortex of ECS treated mice, which is consistent with previous rat data. These changes remained significant 1 week after repeated ECS. Significant increases in NPY Y1, Y2, and Y5 mRNA were found in the dentate gyrus after ECS. Surprisingly, unaltered levels of functional NPY receptor binding accompanied the decreased NPY-sensitive binding. This suggests that mechanisms coupling NPY receptor stimulation to G-protein activation could be augmented after repeated ECS. Thus increased synthesis of NPY after repeated ECS should result in a net increase in NPY signalling in spite of reduced levels of NPY-sensitive binding.

Neuropeptide Y in the dentate gyrus

Neuropeptide Y (NPY) is contained in at least four types of GABAergic interneurons in the dentate gyrus, many of which also contain somatostatin and give rise to the dense NPY innervation of the dentate outer molecular layer. In humans but not rats, minute amounts of NPY are also normally expressed in dentate granule cells, while seizure activity in rats induces robust NPY expression in granule cells. Y1 and Y2 receptors are the most abundant NPY receptors expressed in the dentate gyrus. Y1 receptors are postsynaptic receptors, primarily located on granule cell dendrites in the molecular layer and some interneurons, while Y2 receptors are presynaptic receptors mediating inhibition of glutamate release, and potentially that of NPY and GABA depending on their presynaptic localization, and may also be expressed on some hilar interneurons. In humans, monkeys and mice, Y2 receptors are also present on mossy fibers, but not in most rat species, though functional evidence suggests their presence. Hilar interneurons containing NPY degenerate in temporal lobe epilepsy and in Alzheimer's disease and reduced levels of NPY in dentate hilus are associated with depression. By activating Y1 receptors, NPY also exerts powerful neuroproliferative effects on subgranular zone progenitor cells, increasing the number of newly born granule cells in the adult dentate gyrus. Functionally, NPY exerts anticonvulsive actions mediated by Y2 receptors at mossy fiber terminals, but there are no presynaptic responses to NPY at perforant path inputs to dentate granule cells in rats or mice. NPY also has potentially complicated actions on NPY-containing interneurons. Elevated expression of NPY in mossy fibers of the rat, sprouting of NPY interneurons in the human dentate, and over-expression of Y2 receptors in mossy fibers indicate an anticonvulsive role of endogenous NPY in epilepsy. However, the physiological role of NPY in the healthy dentate gyrus remains unclear.

Physiology and gene regulation of the brain NPY Y1 receptor

Neuropeptide Y (NPY) is one of the most prominent and abundant neuropeptides in the mammalian brain where it interacts with a family of G-protein coupled receptors, including the Y(1) receptor subtype (Y(1)R). NPY-Y(1)R signalling plays a prominent role in the regulation of several behavioural and physiological functions including feeding behaviour and energy balance, sexual hormone secretion, stress response, emotional behaviour, neuronal excitability and ethanol drinking. Y(1)R expression is regulated by neuronal activity and peripheral hormones. The Y(1)R gene has been isolated from rodents and humans and it contains multiple regulatory elements that may participate in the regulation of its expression. Y(1)R expression in the hypothalamus is modulated by changes in energetic balance induced by a wide variety of conditions (fasting, pregnancy, hyperglycaemic challenge, hypophagia, diet induced obesity). Estrogens up-regulate responsiveness to NPY to stimulate preovulatory GnRH and gonadotropin surges by increasing Y(1)R gene expression both in the hypothalamus and the pituitary. Y(1)R expression is modulated by different kinds of brain insults, such as stress and seizure activity, and alteration in its expression may contribute to antidepressant action. Chronic modulation of GABA(A) receptor function by benzodiazepines or neuroactive steroids also affects Y(1)R expression in the amygdala, suggesting that a functional interaction between the GABA(A) receptor and Y(1)R mediated signalling may contribute to the regulation of emotional behaviour. In this paper, we review the state of the art concerning Y(1)R function and gene expression, including our personal contribution to many of the subjects mentioned above.

MTH1, an oxidized purine nucleoside triphosphatase, suppresses the accumulation of oxidative damage of nucleic acids in the hippocampal microglia during kainate-induced excitotoxicity

Enhanced oxidative stress has been implicated in the excitotoxicity of the CNS, and 8-oxo-7,8-dihydro-guanine (8-oxoG), a major type of oxidative damage in nucleic acids, was reported to be accumulated in the rat hippocampus after kainate administration. We herein showed that the 8-oxoG levels in mitochondrial DNA and cellular RNA increased significantly in the CA3 subregion of the mouse hippocampus 6-12 h after kainate administration but returned to basal levels within a few days. Laser-scanning confocal microscopy revealed the 8-oxoG accumulation in mitochondrial DNA to be remarkable in CA3 microglia, whereas that in nuclear DNA or cellular RNA was also detected in the CA3 pyramidal cells and astrocytes. 8-oxoG accumulation in cellular DNA or RNA should be suppressed by MutT homolog 1 (MTH1) with 8-oxo-dGTPase (8-oxo-7,8-dihydro-2'-deoxyguanosine triphosphatase) activity and 8-oxoG-DNA glycosylase 1 (OGG1) with 8-oxoG DNA glycosylase activity. We thus examined the expression level of MTH1 and OGG1 in the mouse hippocampus after kainate administration. The Mth1 mRNA level decreased soon after kainate administration and then quickly recovered beyond the basal level, and a continuously increased MTH1 protein level was observed, whereas the Ogg1 mRNA level remained constant. MTH1-null and wild-type mice exhibited a similar degree of CA3 neuron loss after kainate administration; however, the 8-oxoG levels that accumulated in mitochondrial DNA and cellular RNA in the CA3 microglia significantly increased in the MTH1-null mice in comparison with wild-type mice, thus demonstrating that MTH1 efficiently suppresses the accumulation of 8-oxoG in both cellular DNA and RNA in the hippocampus, especially in microglia, caused by excitotoxicity.

Large-scale expression study of human mesial temporal lobe epilepsy: evidence for dysregulation of the neurotransmission and complement systems in the entorhinal cortex

Human mesial temporal lobe epilepsies (MTLE) are the most frequent form of partial epilepsies and display frequent pharmacoresistance. The molecular alterations underlying human MTLE remain poorly understood. A two-step transcriptional analysis consisting in cDNA microarray experiments followed by quantitative RT-PCR validations was performed. Because the entorhinal cortex (EC) plays an important role in the pathophysiology of the MTLE and usually discloses no detectable or little cell loss, resected EC and each corresponding lateral temporal neocortex (LTC) of MTLE patients were used as the source of disease-associated and control RNAs, respectively. Six genes encoding (i) a serotonin receptor (HTR2A) and a neuropeptide Y receptor type 1 (NPY1R), (ii) a protein (FHL2) associating with the KCNE1 (minK) potassium channel subunit and with presenilin-2 and (iii) three immune system-related proteins (C3, HLA-DR-gamma and CD99), were found consistently downregulated or upregulated in the EC of MTLE patients as compared with non-epileptic autopsy controls. Quantitative western blot analyses confirmed decreased expression of NPY1R in all eight MTLE patients tested. Immunohistochemistry experiments revealed the existence of a perivascular infiltration of C3 positive leucocytes and/or detected membrane attack complexes on a subset of neurons, within the EC of nine out of eleven MTLE patients. To summarize, a large-scale microarray expression study on the EC of MTLE patients led to the identification of six candidate genes for human MTLE pathophysiology. Altered expression of NPY1R and C3 was also demonstrated at the protein level. Overall, our data indicate that local dysregulation of the neurotransmission and complement systems in the EC is a frequent event in human MTLE.

Plasticity of neuropeptide Y in the dentate gyrus after seizures, and its relevance to seizure-induced neurogenesis

In summary, NPY is clearly an important peptide in the adult rat dentate gyrus because it has the potential to influence synaptic transmission and neurogenesis. It may even have other functions, as yet undiscovered, mediated by glia or vasculature. The remarkable plasticity of NPY puts it in a position to allow dentate gyrus function to be modified in a changing environment. The importance of this plasticity in the context of epilepsy cannot be emphasized enough. It could help explain a range of observations about epilepsy that currently is poorly understood. For example, rapid increases in NPY could mediate postictal depression, the period of depression that can last for several hours after generalized seizures. It may mediate the "priming effect," which is a reduction in seizure threshold following an initial period of seizures. Finally, it could contribute to the resistance of dentate granule cells to degeneration after seizures. However, despite the focus in this review on seizure-induced changes, the changes described here also appear to occur after other types of manipulations, which considerably broadens the scope of NPY's role in the brain.

The anti-epileptic actions of neuropeptide Y in the hippocampus are mediated by Y2 and not Y5 receptors

Neuropeptide Y (NPY) potently inhibits glutamate release and seizure activity in rodent hippocampus in vitro and in vivo, but the nature of the receptor(s) mediating this action is controversial. In hippocampal slices from rats and several wild-type mice, a Y2-preferring agonist mimicked, and the Y2-specific antagonist BIIE0246 blocked, the NPY-mediated inhibition both of glutamatergic transmission and of epileptiform discharges in two different slice models of temporal lobe epilepsy, stimulus train-induced bursting (STIB) and 0-Mg2+ bursting. Whereas Y5 receptor-preferring agonists had small but significant effects in vitro, they were blocked by BIIE0246, and a Y5 receptor-specific antagonist did not affect responses to any agonist tested in any preparation. In slices from mice, NPY was without effect on evoked potentials or in either of the two slice seizure models. In vivo, intrahippocampal injections of Y2- or Y5-preferring agonists inhibited seizures caused by intrahippocampal kainate, but again the Y5 agonist effects were insensitive to a Y5 antagonist. Neither Y2- nor Y5-preferring agonists affected kainate seizures in mice. A Y5-specific antagonist did not displace the binding of two different NPY ligands in WT or mice, whereas all NPY binding was eliminated in the mouse. Thus, we show that Y2 receptors alone mediate all the anti-excitatory actions of NPY seen in the hippocampus, whereas our findings do not support a role for Y5 receptors either in vitro or in vivo. The results suggest that agonists targeting the Y2 receptor may be useful anticonvulsants.

Differential suppression of seizures via Y2 and Y5 neuropeptide Y receptors

Neuropeptide Y (NPY) prominently inhibits epileptic seizures in different animal models. The NPY receptors mediating this effect remain controversial partially due to lack of highly selective agonists and antagonists. To circumvent this problem, we used various NPY receptor knockout mice with the same genetic background and explored anti-epileptic action of NPY in vitro and in vivo. In Y2 (Y2-/-) and Y5 (Y5-/-) receptor knockouts, NPY partially inhibited 0 Mg2+-induced epileptiform activity in hippocampal slices. In contrast, in double knockouts (Y2Y5-/-), NPY had no effect, suggesting that in the hippocampus in vitro both receptors mediate anti-epileptiform action of NPY in an additive manner. Systemic kainate induced more severe seizures in Y5-/- and Y2Y5-/-, but not in Y2-/- mice, as compared to wild-type mice. Moreover, kainate seizures were aggravated by administration of the Y5 antagonist L-152,804 in wild-type mice. In Y5-/- mice, hippocampal kindling progressed faster, and afterdischarge durations were longer in amygdala, but not in hippocampus, as compared to wild-type controls. Taken together, these data suggest that, in mice, both Y2 and Y5 receptors regulate hippocampal seizures in vitro, while activation of Y5 receptors in extra-hippocampal regions reduces generalized seizures in vivo.

Up-regulation of neuropeptide Y levels and modulation of glutamate release through neuropeptide Y receptors in the hippocampus of kainate-induced epileptic rats

Kainate-induced epilepsy has been shown to be associated with increased levels of neuropeptide Y (NPY) in the rat hippocampus. However, there is no information on how increased levels of this peptide might modulate excitation in kainate-induced epilepsy. In this work, we investigated the modulation of glutamate release by NPY receptors in hippocampal synaptosomes isolated from epileptic rats. In the acute phase of epilepsy, a transient decrease in the efficiency of NPY and selective NPY receptor agonists in inhibiting glutamate release was observed. Moreover, in the chronic epileptic hippocampus, a decrease in the efficiency of NPY and the Y(2) receptor agonist, NPY13-36, was also found. Simultaneously, we observed that the epileptic hippocampus expresses higher levels of NPY, which may account for an increased basal inhibition of glutamate release. Consistently, the blockade of Y(2) receptors increased KCl-evoked glutamate release, and there was an increase in Y(2) receptor mRNA levels 30 days after kainic acid injection, suggesting a basal effect of NPY through Y(2) receptors. Taken together, these results indicate that an increased function of the NPY modulatory system in the epileptic hippocampus may contribute to basal inhibition of glutamate release and control hyperexcitability.

Y1-receptors regulate the expression of Y2-receptors in distinct mouse forebrain areas

Y-receptor-knockout mice have become an important tool to elucidate specific physiological roles of individual Y-receptors. However, their phenotypes are not always confirmatory to results obtained by pharmacological investigations in vivo or in vitro. These discrepancies may, at least in part, be due to compensatory changes in the expression of remaining Y-receptor types. To determine whether deletion of individual Y-receptors results in altered mRNA expression and/or binding toward other Y-receptor types, we applied in-situ hybridization and radioligand-binding studies on brain slices of Npy1r-, Npy2r- or Npy5r-knockout mice. Significant changes were seen in Y1-receptor-deficient mice. Thus, Y2-receptor mRNA and (125)I-peptide YY(3-36) binding in the hippocampus proper were increased by up to 55% and 89%, respectively. Similar increases in (125)I-peptide YY(3-36) binding were observed in the caudo-dorsal extension of the lateral septum, an area heavily targeted by hippocampal projections and involved in Y1-receptor-regulated anxiety. Increased (125)I-peptide YY(3-36) binding and Y2-receptor mRNA levels were also observed in the medial amygdaloid nucleus. In contrast, (125)I-peptide YY(3-36) binding was reduced in the central amygdaloid nucleus. Y2-receptor mRNA in the intermediate part of the lateral septum was reduced by 42%. Only minimal changes were observed in Y2- or Y5-receptor-deficient mice. Our results demonstrate that compensatory changes in the expression of Y2-receptors occur in Y1-receptor-deficient mice. These adaptations are likely to contribute to changed physiological function. Thus, alterations in Y2-receptors have to be taken in account upon discussion of Y1-receptor function, especially in emotional aspects like anxiety and aggression, but also alcoholism.

Overexpression of NPY and Y2 receptors in epileptic brain tissue: an endogenous neuroprotective mechanism in temporal lobe epilepsy?

Recurrent epileptic seizures in the rat enhance the expression of neuropeptide Y (NPY) and its mRNA in various brain areas including the hippocampus, cerebral cortex and the amygdala. In the hippocampus, the most prominent expression of NPY is observed in mossy fibers and in GABAergic interneurons. At the same time, expression of Y2 receptors is also increased whereas Y1 receptors are reduced. Similar changes in Y1 and Y2 receptors were observed in the hippocampus of patients with temporal lobe epilepsy (TLE). In contrast to the rat, NPY expression is not enhanced in mossy fibers in TLE. In the same tissue, surviving NPY interneurons show marked axonal sprouting into areas innervated by mossy fibers (dentate hilus, stratum lucidum, inner molecular layer of the dentate gyrus). Stimulation of presynaptic Y2 receptors inhibits glutamate release, and exert an anticonvulsant action in experimental models. Y1 receptors mediate a weak excitatory component of NPY action. These findings suggest that changes in the NPY system induced by seizures represent an endogenous adaptive mechanism aimed at counteracting hyperexcitability underlying epileptic activity. This concept is strongly supported by evidence that genetically modified rats overexpressing the NPY gene are less susceptible to seizures while deletion of NPY or Y2 receptor genes results in increased susceptibility to seizures.

Neuropeptide Y and epilepsy: recent progress, prospects and controversies

Neuropeptide Y (NPY), a 36 amino-acid member of the pancreatic polypeptide family, has received considerable attention in recent years as an endogenous modulator of epileptic activity. Prominently expressed in brain regions involved in seizure generation and propagation, NPY can exert powerful effects on synaptic transmission. Here, we discuss the anti-epileptic actions of NPY and receptor subtypes responsible.

Induced down-regulation of neuropeptide Y-Y1 receptors delays initiation of kindling

Neuropeptide Y appears to modulate epileptic seizures differentially according to the receptor subtypes involved. In the hippocampus, neuropeptide Y expression and release are enhanced in different models of epileptogenesis. On the contrary, the expression of Y1 receptors is decreased and it has been shown that activation of these receptors has pro-convulsant effects. The aim of our study was to investigate the role of Y1 receptors during hippocampal kindling epileptogenesis using (i) knock-out mice lacking Y1 receptors and (ii) intrahippocampal infusion of Y1 antisense oligodeoxynucleotide in rats. Y1 knock-out mice showed similar susceptibility to seizure induction and presented no difference in kindling development as compared with their control littermates. Conversely, local hippocampal down-regulation of Y1 receptors during the first week of hippocampal kindling, induced by a local infusion of a Y1 antisense oligodeoxynucleotide, significantly increased seizure threshold intensity and decreased afterdischarge duration. A reverse effect was observed during the week following the infusion period, which was confirmed by a significant decrease in the number of hippocampal stimulations necessary to evoke generalized seizures. At the end of this second week, an up-regulation of Y1 receptors was observed in kindled rats infused with the antisense as compared with the mismatch-treated controls. Our results in the rat suggest that the down-regulation of Y1 receptors in the hippocampus participates in the control of the initiation of epileptogenesis. The lack of an effect of the deficiency of Y1 receptors in the control of kindling development in Y1 knock-out mice could be due to compensatory mechanisms.

Changes in NPY-mediated modulation of hippocampal [3H]D-aspartate outflow in the kindling model of epilepsy

The anticonvulsant effect of NPY may depend on Y(2) and/or Y(5) receptor-mediated inhibition of glutamate release in critical areas, such as the hippocampus. However, Y(2) and Y(5) receptor levels have been reported to increase and decrease, respectively, in the epileptic hippocampus, implicating that the profile of NPY effects may change accordingly. The aim of this study was to evaluate the differential effects of NPY on glutamate release in the normal and in the epileptic hippocampus. Thus, we pharmacologically characterized the effects of NPY on the release of [(3)H]D-aspartate, a valid marker of endogenous glutamate, from synaptosomes prepared from the whole hippocampus and from the three hippocampal subregions (dentate gyrus and CA1 and CA3 subfields) of control and kindled rats, killed 1 week after the last stimulus-evoked seizure. In the whole hippocampus, NPY does not significantly affect stimulus-evoked [(3)H]D-aspartate overflow. In synaptosomes prepared from control rats, NPY significantly inhibited 15 mM K(+)-evoked [(3)H]D-aspartate overflow only in the CA1 subfield (approx. -30%). Both Y(2) and Y(5) receptor antagonists (respectively, 1 microM BIIE0246 and 1 microM CGP71683A) prevented this effect, suggesting the involvement of both receptor types. In contrast, in synaptosomes prepared from kindled rats NPY significantly inhibited 15 mM K(+)-evoked [(3)H]D-aspartate overflow in the CA1 subfield and in the dentate gyrus (approx. -30%). Only the Y(2) (not the Y(5)) antagonist prevented these effects. These data indicate a critical role for the Y(2) receptor in the inhibitory control of glutamate release in the kindled hippocampus and, thus, suggest that the anticonvulsant effect of NPY in the epileptic brain is most likely Y(2), but not Y(5), receptor-mediated.

Differential changes in messenger RNA expressions and binding sites of neuropeptide Y Y1, Y2 and Y5 receptors in the hippocampus of an epileptic mutant rat: Noda epileptic rat

The anti-convulsive effects of neuropeptide Y have been suggested in several animal models of epilepsy. We have found the sustained increase of neuropeptide Y contents and the seizure-induced elevation of hippocampal messenger RNA in a novel spontaneous epileptic mutant rat: Noda epileptic rat. In the present study, we investigated the change of neuropeptide Y Y1 and Y2 receptor messenger RNA expressions and binding sites in the hippocampus following a spontaneous generalized tonic-clonic seizure of Noda epileptic rat. Furthermore, the binding sites of a more recently isolated receptor subtype, neuropeptide Y Y5 receptors, were also evaluated by receptor autoradiography. A marked elevation of neuropeptide Y immunoreactivity in the mossy fiber, and Y2-receptor up-regulation in the dentate gyrus were observed in the hippocampus of Noda epileptic rat, which coincided with the previous results of the other epileptic models. In contrast, Y1-receptor down-regulation was not found after a spontaneous seizure of Noda epileptic rat while this occurs in kindling and after kainic acid-induced seizures. [125I][Leu31, Pro34]peptide YY/BIBP 3226-insensitive (Y5 receptor) binding sites in CA1 stratum radiatum were significantly decreased following a spontaneous seizure of Noda epileptic rat. The present results suggest that a spontaneous seizure of Noda epileptic rat induces significant changes in neuropeptide Y-mediated transmission in the hippocampus via Y2 and Y5 receptors, but not Y1 receptors. Therefore, specific subset of neuropeptide Y receptor subtypes might be involved in the epileptogenesis of Noda epileptic rat.

Neuropeptide Y and its receptors as potential therapeutic drug targets

Neuropeptide Y (NPY) is a 36-amino-acid peptide that exhibits a large number of physiological activities in the central and peripheral nervous systems. NPY mediates its effects through the activation of six G-protein-coupled receptor subtypes named Y(1), Y(2), Y(3), Y(4), Y(5), and y(6). Evidence suggests that NPY is involved in the pathophysiology of several disorders, such as the control of food intake, metabolic disorders, anxiety, seizures, memory, circadian rhythm, drug addiction, pain, cardiovascular diseases, rhinitis, and endothelial cell dysfunctions. The synthesis of agonists and antagonists for these receptors could be useful to treat several of these diseases.

Extracellular levels of NPY in the dorsal hippocampus of freely moving rats are markedly elevated following a single electroconvulsive stimulation, irrespective of anticonvulsive Y1 receptor blockade

Neuropeptide Y (NPY) has been proposed to play a role in the pathophysiology of depression and also to act as an endogenous anticonvulsant. Repeated administration of electroconvulsive stimulations (ECS) has been shown to induce a long-term increase in hippocampal NPY neurotransmission, while the effects of single ECS are largely unexplored. In this study, we assessed extracellular levels of NPY in the dorsal hippocampus of freely moving rats following a single ECS. We also studied the effect of locally administered BIBP3226, a selective NPY Y1 receptor antagonist with reported anticonvulsant properties, on the duration of the ECS-induced seizure and NPY release in freely moving animals. Our data demonstrate that a single ECS increases extracellular NPY-like immunoreactivity (LI) levels in the dorsal hippocampus, reaching statistical significance 2h following the treatment. KCl transiently and calcium-dependently increased extracellular levels of NPY, suggesting that the measured NPY-LI is derived from functional neurons. Local BIBP3226 perfusion essentially abolished the ECS-induced seizure but had no effect on the basal NPY-LI outflow or on the ECS-induced rise in extracellular NPY levels. Our data are in line with the hypothesis that one mechanism of action of ECS is to release NPY in the hippocampus and suggest that the increase is in itself not associated with anticonvulsant activity but may represent other properties of NPY.

Seizure susceptibility and epileptogenesis are decreased in transgenic rats overexpressing neuropeptide Y

Functional studies in epileptic tissue indicate that neuropeptide Y and some of its peptide analogs potently inhibit seizure activity. We investigated seizure susceptibility in transgenic rats overexpressing the rat neuropeptide Y gene under the control of its natural promoter. Seizures were induced in adult transgenic male rats and their wild-type littermates by i.c.v. injection of 0.3 microg kainic acid or by electrical kindling of the dorsal hippocampus. Transgenic rats showed a significant reduction in the number and duration of electroencephalographic seizures induced by kainate by 30% and 55% respectively (P<0.05 and 0.01). Transgenic rats were also less susceptible to epileptogenesis than wild-type littermates as demonstrated by a 65% increase in the number of electrical stimuli required to induce stage 5 seizures (P<0.01). This phenotype was associated with a strong and specific expression of neuropeptide Y mRNA in area CA1, a brain area involved in the seizure network. We conclude that endogenous neuropeptide Y overexpression in the rat hippocampus is associated with inhibition of seizures and epileptogenesis suggesting that this system may be a valuable target for developing novel antiepileptic treatments.

Epilepsy as an example of neural plasticity

Epilepsy is a devastating disease affecting more than 1% of the population. Yet, if one considers the neurobiological substrates of this disease, what is revealed is an array of phenomenon that exemplify the remarkable capacity for the brain to change its basic structure and function, that is, neural plasticity. Some of these alterations are transient and merely impressive for their extent, or for their robust nature across animal models and human epilepsy. Others are notable for their persistence, often enduring for months or years. As an example, the dentate gyrus, and specifically the principal cell of the dentate gyrus, the granule cell, is highlighted. This area of the brain and this particular cell type, for reasons that are currently unclear, hold an uncanny capacity to change after seizures. For those interested in plasticity, it is suggested that perhaps the best examples for studying plasticity lie in the field of epilepsy.

Y5 receptors mediate neuropeptide Y actions at excitatory synapses in area CA3 of the mouse hippocampus

Neuropeptide Y (NPY) is a potent modulator of excitatory synaptic transmission and limbic seizures. NPY is abundantly expressed in the dentate gyrus and is thought to modulate hippocampal excitability via activation of presynaptic Y2 receptors (Y2R). Here we demonstrate that NPY, and commonly used Y2R-preferring (NPY(13-36)) and Y5 receptor (Y5R)-preferring ([D-Trp(32)]NPY and hPP) peptide agonists, evoke similar levels of inhibition at excitatory CA3 synapses in hippocampal slices from wild-type control mice (WT). In contrast, NPYergic inhibition of excitatory CA3 synaptic transmission is absent in mice lacking the Y5R subtype (Y5R KO). In both analyses of evoked population spike activity and spontaneous excitatory postsynaptic synaptic currents (EPSCs), NPY agonists induced powerful inhibitory effects in all hippocampal slices from WT mice, whereas these peptides had no effect in slices from Y5R KO mice. In slices from WT mice, NPY (and NPY receptor-preferring agonists) reduced the frequency of spontaneous EPSCs but had no effect on sEPSC amplitude, rise time, or decay time. Furthermore, NPYergic modulation of spontaneous EPSCs in WT mice was mimicked by bath application of a novel Y5R-selective peptide agonist ([cpp]hPP) but not the selective Y2R agonist ([ahx(5-24)]NPY). In situ hybridization was used to confirm the presence of NPY, Y2, and Y5 mRNA in the hippocampus of WT mice and the absence of Y5R in knockout mice. These results suggest that the Y5 receptor subtype, previously believed to mediate food intake, plays a critical role in modulation of hippocampal excitatory transmission at the hilar-to-CA3 synapse in the mouse.

Plasticity of Y1 and Y2 receptors and neuropeptide Y fibers in patients with temporal lobe epilepsy

Marked expression of neuropeptide Y (NPY) and its Y2 receptors in hippocampal mossy fibers has been reported in animal models of epilepsy. Because NPY can suppress glutamate release by activating presynaptic Y2 receptors, these changes have been proposed as an endogenous protective mechanism. Therefore, we investigated whether similar changes in the NPY system may also take place in human epilepsy. We investigated Y1 and Y2 receptor binding and NPY immunoreactivity in hippocampal specimens that were obtained at surgery from patients with temporal lobe epilepsy and in autopsy controls. Significant increases in Y2 receptor binding (by 43-48%) were observed in the dentate hilus, sectors CA1 to CA3, and subiculum of specimens with, but not in those without, hippocampal sclerosis. On the other hand, Y1 receptor binding was significantly reduced (by 62%) in the dentate molecular layer of sclerotic specimens. In the same patients, the total lengths of NPY immunoreactive (NPY-IR) fibers was markedly increased (by 115-958%) in the dentate molecular layer and hilus, in the stratum lucidum of CA3, and throughout sectors CA1 to CA3 and the subiculum, as compared with autopsies. In nonsclerotic specimens, increases in lengths of NPY-IR fibers were more moderate and statistically not significant. NPY mRNA was increased threefold in hilar interneurons of sclerotic and nonsclerotic specimens. It is suggested that abundant sprouting of NPY fibers, concomitant upregulation of Y2 receptors, and downregulation of Y1 receptors in the hippocampus of patients with Ammon's horn sclerosis may be endogenous anticonvulsant mechanisms.

Neuropeptide Y and epilepsy: varying effects according to seizure type and receptor activation

In vitro and in vivo experiments suggest antiepileptic properties for NPY. In this study, the pharmacology of these effects was examined and compared in different rat models of seizures. Agonists for Y(1), Y(2) and Y(5) receptors reduced seizure-like activity in hippocampal cultures. Intracerebral injection of NPY or Y(5) agonists reduced the expression of focal seizures produced by a single electrical stimulation of the hippocampus. Conversely, NPY agonists increased the duration of generalized convulsive seizures induced by pentylenetetrazol. These results suggest that NPY reduces seizures of hippocampal origin through activation of Y(5) receptors. They also point to probable modulatory effects of NPY in brain structures other than the hippocampus, involved in initiation, propagation or control of seizures.

Plastic changes in neuropeptide Y receptor subtypes in experimental models of limbic seizures

PURPOSE

Neuropetide Y (NPY)-mediated neurotransmission in the hippocampus is altered by limbic seizures. The functional consequences of this change are still unresolved and clearly depend on the type of NPY receptors involved. NPY Y2 and Y1 receptors are increased on mossy fiber terminals and decreased on granule cell dendrites after seizures, respectively. We investigated (a) whether seizures modify the NPY Y5 receptors in the hippocampus, and (b) the effect of an agonist at Y2/Y5 receptors and antagonists at Y1 receptors on acute and chronic seizure susceptibility.

METHODS

Limbic seizures were induced in rats by electrical stimulation of the dorsal hippocampus, leading to stage 5 kindled seizures, or by intrahippocampal or systemic injections of kainic acid. Pentylentetrazol was administered to epileptic rats to assess their enhanced susceptibility to seizures. NPY Y5 receptor protein was measured in hippocampal homogenates using a specific polyclonal antibody and quantitative Western blotting.

RESULTS

Y5 receptors (57-kD band) were transiently decreased (23 to 35%) in all hippocampal subregions 2 and 7 days, but not 2.5 hours, after seizures induced by systemic kainic acid. A minor band of 51 kD was reduced significantly in CA3 and dentate gyrus, although it was increased in CA1, 30 days after seizures, suggesting long-term posttranslational changes in this protein. NPY Y5 receptors were increased by 200% in total homogenate from the stimulated hippocampus 2 days but not 30 days after fully kindled seizures. Intracerebral injections of NPY 13-36 (Y2/Y5 receptor agonist) or BIBP 3225 and BIBO 3304 (selective Y1 receptor antagonists) decreased seizure susceptibility in rats.

CONCLUSIONS

These results indicate that NPY Y5 receptors change after limbic seizures and suggest that NPY receptors may provide novel target(s) for the treatment of epilepsy.

Decreased levels of neuropeptide Y(5) receptor binding sites in two experimental models of epilepsy

It has been suggested that the anticonvulsant effects of neuropeptide Y (NPY) could be mediated by the activation of Y(2) and/or Y(5) receptors. NPY Y(1) receptor levels are known to decrease and Y(2) to increase in rat models of epilepsy. By using an autoradiographic approach, we investigated whether epilepsy models (kainic acid and kindling) are also associated with changes in Y(5) receptors. Compared with naive controls, [125I][Leu(31), Pro(34)]PYY/BIBP3226-insensitive (Y(5)) binding sites in the hippocampus (strata oriens and radiatum of CA3 and CA1) and in the neocortex (superficial layers) were unchanged in sham-stimulated rats, but reduced by approximately 50% in kindled rats (seven days after the last stimulus evokes seizure), and further reduced (to approximately -90%) 1h after a kindled seizure. Additionally, Y(5) receptor binding sites in the hippocampus and in the neocortex were unchanged 6h after kainic acid injection, but were highly reduced at 12 and 24h. No changes in Y(5) binding levels were found in the dentate gyrus and the pyramidal cell layer of the hippocampus. The present data suggest that changes in Y(5) receptor levels occur in epilepsy models. These changes may play a role in seizure expression and/or in the maintenance of kindling hyperexcitability.

Electroconvulsive stimuli enhance both neuropeptide Y receptor Y1 and Y2 messenger RNA expression and levels of binding in the rat hippocampus

Repeated electroconvulsive stimulations and other seizure modalities produce an increase in neuropeptide Y synthesis and local release in the rat hippocampus, and perhaps as a consequence, a change in the concentration of neuropeptide Y binding sites in the same region. The aim of the present study was to determine possible changes in the expression of neuropeptide Y receptor subtypes affected by repeated stimulations in the hippocampus. Rats were exposed to 14 daily stimulations, and the brains were removed 24h after the last stimulation. For in vitro receptor autoradiography and in situ hybridisation histochemistry, the brains were frozen, sectioned, and levels of neuropeptide Y binding sites and messenger RNA expressions were determined quantitatively on sections from the same animals. In order to determine the contribution of different neuropeptide Y receptor subtypes, serial sections were incubated with either 125I-labelled peptide YY alone or the same radio-labelled peptide mixed with an excess of a number of displacing compounds with affinity for either neuropeptide Y receptor subtype Y1, Y2, or both. Binding studies revealed that the majority of peptide YY binding sites was represented by Y2, and that electroconvulsive stimulations reduced the binding capacity or the concentration of this receptor. A prominent reduction of Y1-preferring binding sites was determined in the dentate gyrus, and to a lesser extent in the CA1 and CA3 regions. Similarly, the treatment produced a significant reduction of Y2-preferring binding sites in the CA1 and CA3 region, but not in the granular cell layer of the dentate gyrus. Using semi-quantitative in situ hybridization, Y1 receptor messenger RNA level in the granular cell layer of the dentate increased by the stimulations. In the same region, Y2 receptor messenger RNA was expressed in low to undetectable amounts, but after the repeated stimulations, this transcript was found in moderate to high levels. These data suggest that the neuropeptide Yergic system in the dentate gyrus and the pyramidal cell layer are affected by the treatment, and that this includes both Y1 and Y2 receptor subtypes. Because levels of messenger RNA and binding are distinctly regulated, the turnover of both Y1 and Y2 molecules is strongly increased under electroconvulsive stimulations, suggesting that the intrahippocampal neuropeptide Yergic neurotransmission is also increased under the stimulations.

Association of neuropeptide Y Y1 receptors with glutamate-positive and NPY-positive neurons in rat hippocampal cultures

The hippocampus is particularly enriched with neuropeptide tyrosine (NPY) and NPY receptors including the Y1, Y2 and Y5 subtypes. We have previously reported on the enrichment of cultured rat hippocampal neurons in specific [125I][Leu31, Pro34]PYY/BIBP3226-sensitive (Y1) binding sites and Y1 receptor mRNAs [St-Pierre et al. (1998) Br. J. Pharmacol., 123, p183]. We have now identified which cell types express the Y1 receptor. The majority of Y1 receptors, visualized using either the radiolabeled probe [125I][Leu31,Pro34]PYY or two antibodies directed against distinct domains of the Y1 receptor, was expressed in neurons as revealed by neuron-specific enolase (NSE) immunostaining. One antibody was directed against the second extracelllular loop of the Y1 receptor (amino acids 185-203) whereas the second was directed against the intracellular C-terminal loop (amino acids 355-382). The labelling was evident over both perikarya and processes. Neurons labelled by the various Y1 receptor probes were mostly glutamate-positive as revealed by double immunostaining. Most interestingly, a number of NPY-positive cultured hippocampal neurons were also enriched with the Y1 receptor, suggesting that this subtype may act as an autoreceptor to regulate NPY release in the hippocampus. These results thus provide an anatomical basis for the modulation of glutamate and NPY release by the Y1 receptor in the hippocampus.

Chronic modulation of the GABA(A) receptor complex regulates Y1 receptor gene expression in the medial amygdala of transgenic mice

NPY exerts anxiolytic effects, which are mediated by activation of Y1 receptors in the amygdala. It has been shown that diazepam counteracts the anxiogenic effect of Y1 receptor antagonists, suggesting that NPYergic and GABAergic systems are coupled in the regulation of anxiety. We used a transgenic mouse model, expressing a mouse Y1 receptor-beta-galactosidase fusion gene (Y1R/LacZ), to study the effect of positive or negative modulators of GABA(A) receptors on Y1 receptor gene expression. Mice were treated for 14 days with diazepam (4 or 20 mg/kg), the anxiolytic beta-carboline-derivative abecarnil (0.3 or 6 mg/kg) and the anxiogenic beta-carboline FG7142 (20 mg/kg). Transgene expression was determined by quantitative analysis of beta-galactosidase histochemical staining in the medial amygdala and in the medial habenula as a control region. Chronic treatment with 20 mg/kg diazepam or 6 mg/kg abecarnil significantly increased, whereas FG 7142 decreased, transgene expression in the medial amygdala. A transient decrease in transgene expression was observed in the medial amygdala six hours after the acute treatment with 20 mg/kg FG 7142 but not with diazepam or abecarnil. No significant changes were observed in the medial habenula. These data suggest that modulation of GABA(A) receptor function may regulate Y1 receptor gene expression in medial amygdala.

Multiple receptors for neuropeptide Y in the hippocampus: putative roles in seizures and cognition

Neuropeptide Y (NPY) is widely distributed throughout the central nervous system (CNS) and is one of the most conserved peptides in evolution, suggesting an important role in the regulation of basic physiological functions, including learning and memory. In addition, experimental studies have suggested that NPY, together with its receptors, may have a direct implication in several pathological disorders, including epilepsy/seizure. NPY-like immunoreactivity and NPY receptors have been shown to be present throughout the brain, but is concentrated in the hippocampus. The hippocampal formation has been repeatedly implicated in the modulation of cognition, as well as the pathogenesis of seizure. This review will concentrate on the hippocampal distribution of NPY, its receptors and the putative role played by this peptide in seizure, together with the regulation of cognitive function associated with learning and memory.