Endogenous serotonin inhibits epileptiform activity in rat hippocampal CA1 neurons via 5-hydroxytryptamine1A receptor activation

Sigma-1 receptor and seizures

Over the last decade, sigma-1 receptor (Sig1R) has been recognized as a valid target for the treatment of seizure disorders and seizure-related comorbidities. Clinical trials with Sig1R ligands are underway testing therapies for the treatment of drug-resistant seizures, developmental and epileptic encephalopathies, and photosensitive epilepsy. However, the direct molecular mechanism by which Sig1R modulates seizures and the balance between excitatory and inhibitory pathways has not been fully elucidated. This review article aims to summarize existing knowledge of Sig1R and its involvement in seizures by focusing on the evidence obtained from Sig1R knockout animals and the anti-seizure effects of Sig1R ligands. In addition, this review article includes a discussion of the advantages and disadvantages of the use of existing compounds and describes the challenges and future perspectives on the use of Sig1R as a target for the treatment of seizure disorders.

Serotonin Pretreatment Abolishes Sex-specific NMDA-induced Seizure Behavior in Developing Rats

Neuronal excitability and susceptibility to excitotoxic damage can be sex-specific, with neurons from males usually being more 'easily excitable' compared to neurons from females, especially during development. Increased excitability at an individual neuronal level can lead to the formation of hyperexcitable neuronal networks, which, consequently can make the brain more seizure prone. Both animal and clinical data suggest that males experience more frequent and severe seizures than do females. Serotonin (5-hydroxytryptamine; 5-HT) can mediate neuronal excitability and seizure behavior, often serving as an anticonvulsant. Importantly, 5-HT signaling during parts of the perinatal period is sexually dimorphic. Sex differences during development have been reported in both serotonin levels and receptor type (excitatory vs. inhibitory) expression in a manner that may leave the male brain more vulnerable to over-excitation. Thus, we aimed to determine if the anticonvulsant effects of 5-HT were sex- and/or age-dependent in juvenile animals. We report a baseline sex difference in N-methyl-d-aspartate (NMDA)-induced seizure behavior and hippocampal neuronal loss, with postnatal day (PND) 14 males exhibiting more severe seizure behavior compared to females. Pretreatment with the general 5-HT receptor agonist 5-methoxytryptamine (5-MT) abolishes baseline sex differences, providing an anticonvulsant effect for males only. These sex differences appear to be at least in part organized by testosterone, as females given neonatal androgen exhibit a seizure behavior profile in between that of males and females.

5-HT/GABA interaction in epilepsy

Epilepsy is a neurological condition characterized by synchronous neuronal oscillations (seizures) in the electroencephalogram. Seizures are classified in focal or generalized (depending on the brain territory interested during seizures), and in convulsive and/or not convulsive (depending on the presence or not of involuntary movements). The current pharmacological treatments are mainly based on GABA modulation although different neurotransmitters are also involved in epilepsy, including serotonin. However despite much extensive progress in the understanding of epilepsy mechanisms, still, a percentage of people with epilepsy are pharmaco-resistant calling for the need for new therapeutic targets. Here we review preclinical and human evidence showing that serotonin modulates epilepsy that this likely happens via a major modulation/interaction with GABA.

Multi-site dynamic recording for Aβ oligomers-induced Alzheimer's disease in vitro based on neuronal network chip

Alzheimer's disease (AD) is a chronic central neurodegenerative disease. The pathological features of AD are the extracellular deposition of senile plaques formed by amyloid-β oligomers (AβOs) and the intracellular accumulation of neurofibrillary tangles. However, due to the lack of effective method and experimental models to study the cognitive decline, communication at cell resolution and the implementation of interventions, the diagnosis and treatment on AD still progress slowly. In this paper, we established a pathological model of AD in vitro based on AβOs-induced hippocampal neuronal network chip for multi-site dynamic analysis of the neuronal electrical activity and network connection. The multiple characteristic parameters, including positive and negative spike intervals, firing rate and peak-to-peak values, were extracted through the analysis of spike signals, and two firing patterns from the interneurons and pyramidal neurons were recorded. The spatial firing patterns mapping and cross-correlation between channels were performed to validate the degeneration of neuronal network connectivity. Moreover, an electrical stimulation with frequency at 40 Hz was exerted to preliminarily explore the therapeutic effect on the pathological model of AD. This neuronal network chip enables the implementation of AD models in vitro for studying basic mechanisms of neurodegeneration within networks and for the parallel testing of various potential therapies. It can be a novel technique in the research of AD pathological model in vitro.

Do the accelerating actions of tianeptine and l-arginine on cortical spreading depression interact? An electrophysiological analysis in young and adult rats

In the rat, we previously demonstrated that serotonin-enhancing drugs impair cortical spreading depression (CSD) and that l-arginine (arginine) treatment enhances CSD. Here, we investigated the interaction between topical application of the serotonin uptake enhancer tianeptine and systemic arginine administration on CSD. From postnatal day 7-28, female Wistar rats (n=40) received by gavage 300mg/Kg/day arginine (n=20) or water (n=20). Half of the arginine- or water-treated rats underwent CSD recording at 30-40days of age (young), while the other half was recorded at 90-120days (adult). Following baseline recording (four episodes of CSD), we applied tianeptine solution (10mg/ml) to a rectangular portion of the intact dura mater for 10-min and then elicited CSD. This procedure was repeated three times. Compared to baseline values, CSD velocities and amplitudes following tianeptine application increased, and CSD duration decreased significantly (p<0.05) in both young and adult rats, regardless of treatment group. CSD acceleration caused by systemic treatment with arginine is in agreement with previous findings. Topical cortical application of tianeptine replicated the effect of systemic application, suggesting a cortically based mechanism for tianeptine's action. However, the absence of interaction between arginine and tianeptine treatments suggests that they probably act through separate mechanisms.

Third Trimester Equivalent Alcohol Exposure Reduces Modulation of Glutamatergic Synaptic Transmission by 5-HT1A Receptors in the Rat Hippocampal CA3 Region

Fetal alcohol exposure has been associated with many neuropsychiatric disorders that have been linked to altered serotonin (5-hydroxytryptamine; 5-HT) signaling, including depression and anxiety. During the first 2 weeks of postnatal life in rodents (equivalent to the third trimester of human pregnancy) 5-HT neurons undergo significant functional maturation and their axons reach target regions in the forebrain (e.g., cortex and hippocampus). The objective of this study was to identify the effects of third trimester ethanol (EtOH) exposure on hippocampal 5-HT signaling. Using EtOH vapor inhalation chambers, we exposed rat pups to EtOH for 4 h/day from postnatal day (P) 2 to P12. The average serum EtOH concentration in the pups was 0.13 ± 0.04 g/dl (legal intoxication limit in humans = 0.08 g/dl). We used brain slices to assess the modulatory actions of 5-HT on field excitatory postsynaptic potentials in the hippocampal CA3 region at P13-P15. Application of the GABA_A/glycine receptor antagonist, picrotoxin, caused broadening of field excitatory postsynaptic potentials (fEPSPs), an effect that was reversed by application of 5-HT in slices from air exposed rats. However, this effect of 5-HT was absent in EtOH exposed animals. In slices from naïve animals, application of a 5-HT_1A receptor antagonist blocked the effect of 5-HT on the fEPSPs recorded in presence of picrotoxin, suggesting that third trimester ethanol exposure acts by inhibiting the function of these receptors. Studies indicate that 5-HT_1A receptors play a critical role in the development of hippocampal circuits. Therefore, inhibition of these receptors by third trimester ethanol exposure could contribute to the pathophysiology of fetal alcohol spectrum disorders.

Common Mechanisms Underlying Epileptogenesis and the Comorbidities of Epilepsy

The importance of comorbidities in determining the quality of life of individuals with epilepsy and their families has received increasing attention in the past decade. Along with it has come a recognition that in some individuals, certain comorbidities may have preexisted, and may have contributed to their developing epilepsy. Many mechanisms are capable of interconnecting different dysfunctions that manifest as distinct disorders, often diagnosed and managed by different specialists. We review the human data from the perspective of epidemiology as well as insights gathered from neurodiagnostic and endocrine studies. Animal studies are reviewed to refine our mechanistic understanding of the connections, because they permit the narrowing of variables, which is not possible when studying humans.

The dilemma of treatments for epileptic patients with depression

Depression is a state of low mood and aversion to activity. It may occur due to existence of other mental or physical diseases or from the medications for those illnesses. It is one of the leading sources of disability. Among these physical diseases, epilepsy is widely recognized as one of the main causes of depression. Patients with epilepsy are at high risk of developing depressive symptoms, and the suicide rates in patients with epilepsy have been reported to be much higher than in the general population. However, due to fears of lowering seizure threshold and adverse drug interactions between antidepressants and antiepileptic drugs, physician are reluctant to place patients with epilepsy on antidepressant medication. As a result, the question has been raised that what the best managements should be used to treat epileptic patients with depression. In this review, the currently used medications for antidepressants and antiepileptic drugs were summarized by their working targets in order to establish appropriate pharmacological management of depression and epilepsy. Despite the complex relationship between epilepsy and depression, coadministration of antidepressants and AEDs can still be done safely and effectively under the conditions of good clinical management. The ideal antidepressants for people with epilepsy should be efficacious but with few adverse effects, which will not antagonize GABAergic mechanisms or interfere with plasma anticonvulsant concentrations.

Biomarkers of epileptogenesis: psychiatric comorbidities (?)

The last decade has witnessed a significant shift on our understanding of the relationship between psychiatric disorders and epilepsy. While traditionally psychiatric disorders were considered as a complication of the underlying seizure disorder, new epidemiologic data, supported by clinical and experimental research, have suggested the existence of a bidirectional relation between the two types of conditions: not only are patients with epilepsy at greater risk of experiencing a psychiatric disorder, but patients with primary psychiatric disorders are at greater risk of developing epilepsy. Do these data suggest that some of the pathogenic mechanisms operant in psychiatric comorbidities play a role in epileptogenesis? The aim of this article is to review the epidemiologic data that demonstrate that primary psychiatric disorders are more frequent in people who develop epilepsy, before the onset of the seizure disorder than among controls. The next question looks at the available data of pathogenic mechanisms of primary mood disorders and their potential for facilitating the development and/or exacerbation in the severity of epileptic seizures. Finally, we review data derived from experimental studies in animal models of depression and epilepsy that support a potential role of pathogenic mechanisms of mood disorders in the development of epileptic seizures and epileptogenesis. The data presented in this article do not yet establish conclusive evidence of a pathogenic role of psychiatric comorbidities in epileptogenesis, but raise important research questions that need to be investigated in experimental, clinical, and population-based epidemiologic research studies.

Serotonin 1A receptor inhibits the status epilepticus induced by lithium-pilocarpine in rats

Status epilepticus (SE) is a life-threatening neurological emergency associated with a high mortality rate. The serotonin 1A (5-HT1A) receptor is a possible target for the treatment of SE, but its role in animal models and the precise area of brain involved remain controversial. The hippocampus is a candidate site due to its key role in the development of SE and the existence of a high density of 5-HT1A receptors. Therefore, we investigated the effects of subcutaneous and intrahippocampal activation of 5-HT1A receptors in lithium-pilocarpine-induced SE, and tested whether the hippocampus is a true effector site. We developed SE in male Sprague-Dawley rats by giving lithium chloride (LiCl; 3 meq/kg, i.p.) 22-24 h prior to pilocarpine (25 mg/kg, i.p.), and found that 8-OH-DPAT, a 5-HT1A receptor agonist administered subcutaneously (s.c.) at 0.5 or 1.0 mg/kg 1 h before pilocarpine injection increased the latency to the first epileptiform spikes, the electrographic SE, and the behavioral generalized seizures (GS), while reducing the total EEG seizure time (P <0.01). The duration of GS was shortened only by 1.0 mg/kg 8-OH-DPAT s.c. (P <0.05). All these effects were inhibited by combined administration of WAY-100635 (1.0 mg/kg, s.c.) (P <0.05), an antagonist of the 5-HT1A receptor, but WAY-100635 alone and low doses of 8-OHDPAT (0.01 and 0.1 mg/kg) did not alter seizure activity. Furthermore, intrahippocampal 8-OH-DPAT only shortened the GS duration (P <0.05). These findings imply that the 5-HT1A receptor is a promising therapeutic target against the generation and propagation of SE, and hippocampal receptors are involved in reducing the seizure severity.

The role of different serotonin receptor subtypes in seizure susceptibility

5-Hydroxytryptamine (5-HT) has the most diverse set of receptors in comparison with any other neurotransmitter or hormone in the body. To date, seven families of 5-HT receptors have been characterized. A great number of studies have been published regarding the role of 5-HT and its receptors in seizures. However, with a few exceptions, the net effect of activating or inhibiting each 5-HT receptor subtype on the development or severity of seizures remains controversial. Additionally, the results of studies, which have used knockout animals to investigate the role of 5-HT receptors in seizures, have sometimes been contradictory to those which have used pharmacological tools. The present study aims to review the available data regarding the influence of each receptor subtype on seizure development and, when possible, reconcile between the apparently different results obtained in these studies.

Antidepressant therapy in epilepsy: can treating the comorbidities affect the underlying disorder?

There is a high incidence of psychiatric comorbidity in people with epilepsy (PWE), particularly depression. The manifold adverse consequences of comorbid depression have been more clearly mapped in recent years. Accordingly, considerable efforts have been made to improve detection and diagnosis, with the result that many PWE are treated with antidepressant drugs, medications with the potential to influence both epilepsy and depression. Exposure to older generations of antidepressants (notably tricyclic antidepressants and bupropion) can increase seizure frequency. However, a growing body of evidence suggests that newer ('second generation') antidepressants, such as selective serotonin reuptake inhibitors or serotonin-noradrenaline reuptake inhibitors, have markedly less effect on excitability and may lead to improvements in epilepsy severity. Although a great deal is known about how antidepressants affect excitability on short time scales in experimental models, little is known about the effects of chronic antidepressant exposure on the underlying processes subsumed under the term 'epileptogenesis': the progressive neurobiological processes by which the non-epileptic brain changes so that it generates spontaneous, recurrent seizures. This paper reviews the literature concerning the influences of antidepressants in PWE and in animal models. The second section describes neurobiological mechanisms implicated in both antidepressant actions and in epileptogenesis, highlighting potential substrates that may mediate any effects of antidepressants on the development and progression of epilepsy. Although much indirect evidence suggests the overall clinical effects of antidepressants on epilepsy itself are beneficial, there are reasons for caution and the need for further research, discussed in the concluding section.

Fluoxetine suppresses synaptically induced [Ca²⁺]i spikes and excitotoxicity in cultured rat hippocampal neurons

Fluoxetine is a widely used antidepressant with an action that is primarily attributed to the inhibition of serotonin re-uptake into the synaptic terminals of the central nervous system. Fluoxetine also has blocking effects on various ion channels, including Ca(2+) channels. It remains unclear, however, how fluoxetine may affect synaptically induced [Ca(2+)](i) spikes. We investigated the effects of fluoxetine on [Ca(2+)](i) spikes, along with the subsequent neurotoxicity that is synaptically evoked by lowering extracellular Mg(2+) in cultured rat hippocampal neurons. Fluoxetine inhibited the synaptically induced [Ca(2+)](i) spikes in p-chloroamphetamine-treated and non-treated neurons, in a concentration-dependent manner. However, other selective serotonin reuptake inhibitors, such as paroxetine and citalopram, did not significantly affect the spikes. Pretreatment with fluoxetine for 5 min inhibited [Ca(2+)](i) increases induced by glutamate, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, and N-methyl-d-aspartate. Fluoxetine also inhibited α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-induced currents. In addition, fluoxetine decreased the [Ca(2+)](i) responses induced by the metabotrophic glutamate receptor agonist (S)-3,5-dihydroxyphenylglycine or the ryanodine receptor agonist caffeine. Fluoxetine inhibited [Ca(2+)](i) responses induced by 20mM KCl. Fluoxetine decreased the release of FM1-43 induced by electric field stimulation. Furthermore, fluoxetine inhibited 0.1mM [Mg(2+)](o)-induced cell death. Collectively, our results suggest that fluoxetine suppresses the spikes and protects neurons against excitotoxicity, particularly in cultured rat hippocampal neurons, presumably due to both direct inhibition of presynaptic glutamate release and postsynaptic glutamate receptor-mediated [Ca(2+)](i) signaling. In addition to an indirect inhibitory effect via 5-HT levels, these data suggest a new, possibly direct inhibitory action of fluoxetine on synaptically induced [Ca(2+)](i) spikes and neuronal cell death.

Receptor targets for antidepressant therapy in bipolar disorder: an overview

The treatment of bipolar depression is one of the most challenging issues in contemporary psychiatry. Currently only quetiapine and the olanzapine-fluoxetine combination are officially approved by the FDA against this condition. The neurobiology of bipolar depression and the possible targets of bipolar antidepressant therapy remain relatively elusive. We performed a complete and systematic review to identify agents with definite positive or negative results concerning efficacy followed by a second systematic review to identify the pharmacodynamic properties of these agents. The comparison of properties suggests that the stronger predictors for antidepressant efficacy in bipolar depression were norepinephrine alpha-1, dopamine D1 and histamine antagonism, followed by 5-HT2A, muscarinic and dopamine D2 and D3 antagonism and eventually by norepinephrine reuptake inhibition and 5HT-1A agonism. Serotonin reuptake which constitutes the cornerstone in unipolar depression treatment does not seem to play a significant role for bipolar depression. Our exhaustive review is compatible with a complex model with multiple levels of interaction between the major neurotransmitter systems without a single target being either necessary or sufficient to elicit the antidepressant effect in bipolar depression.

Association of serotonin transporter promoter (5-HTTLPR) and intron 2 (VNTR-2) polymorphisms with treatment response in temporal lobe epilepsy

PURPOSE

Temporal lobe epilepsy (TLE) is the most common epilepsy and about 30% of patients have poorly controlled seizures. Neurobiology underlying responsiveness to medical treatment in TLE patients is unclear and there are currently no biological tests to predict course of the disease. Animal and human studies repeatedly suggested serotonergic dysfunction in subjects with TLE. We investigated association of serotonin transporter (5-HTT) gene polymorphisms with medical treatment response in patients with TLE.

METHODS

We analyzed 5-HTT gene linked polymorphic region (5-HTTLPR) in promoter and variable number of tandem repeats in the second intron of the 5-HTT gene (VNTR-2) in 101 consecutive subjects with TLE.

RESULTS

TLE patients with the combination of transcriptionally more efficient genotypes, i.e. 5-HTTLPR L/L and VNTR-2 12/12, had increased seizure refractoriness to antiepileptic medication therapy and shorter periods of seizure freedom, than subjects with other combinations of the 5-HTT genotypes. There were no other clinical or demographic differences among patient groups based on the 5-HTT genotypes.

CONCLUSION

Combination of the 5-HTT genotypes linked with higher 5-HTT gene expression was found to be associated with worse response to optimal drug therapy. Further studies should determine potential role of this 5-HTT genotype polymorphism in epileptogenesis.

Early life stress as an influence on limbic epilepsy: an hypothesis whose time has come?

The pathogenesis of mesial temporal lobe epilepsy (MTLE), the most prevalent form of refractory focal epilepsy in adults, is thought to begin in early life, even though seizures may not commence until adolescence or adulthood. Amongst the range of early life factors implicated in MTLE causation (febrile seizures, traumatic brain injury, etc.), stress may be one important contributor. Early life stress is an a priori agent deserving study because of the large amount of neuroscientific data showing enduring effects on structure and function in hippocampus and amygdala, the key structures involved in MTLE. An emerging body of evidence directly tests hypotheses concerning early life stress and limbic epilepsy: early life stressors, such as maternal separation, have been shown to aggravate epileptogenesis in both status epilepticus and kindling models of limbic epilepsy. In addition to elucidating its influence on limbic epileptogenesis itself, the study of early life stress has the potential to shed light on the psychiatric disorder that accompanies MTLE. For many years, psychiatric comorbidity was viewed as an effect of epilepsy, mediated psychologically and/or neurobiologically. An alternative – or complementary – perspective is that of shared causation. Early life stress, implicated in the pathogenesis of several psychiatric disorders, may be one such causal factor. This paper aims to critically review the body of experimental evidence linking early life stress and epilepsy; to discuss the direct studies examining early life stress effects in current models of limbic seizures/epilepsy; and to suggest priorities for future research.

Don't be afraid to treat depression in patients with epilepsy!

Major depression and related depressive disorders are highly prevalent in the general population and even more so in patients with epilepsy. Yet depression in these patients remains underdiagnosed and undertreated. This is particularly worrisome as depression has greater negative impact on quality of life than seizure frequency. Additionally, depression is associated with poorer seizure control, and the risk of suicide in patients with epilepsy is greatly increased. Reluctance to treat depression results from the traditional belief that antidepressants should be restricted in epilepsy because of a supposed decrease in seizure threshold. However, there is growing evidence that many antidepressants rather have anticonvulsant effects. Experimental studies show that in critical brain regions such as the frontal lobes and the limbic system enforced serotonergic circuits increase seizure threshold. Clinical data suggest that modern antidepressants may reduce seizure frequency in patients with pharmacoresistant epilepsy. Here we review the concept that selective reuptake inhibitors of serotonin (SSRIs) have a positive effect on the mood disorder as well as on epilepsy. When adhering to the usual precautions, treatment with SSRIs in patients with epilepsy and depression is safe and should not be withheld.

A pharmacological analysis of mice with a targeted disruption of the serotonin transporter

RATIONALE

Partial or complete ablation of serotonin transporter (SERT) expression in mice leads to altered responses to serotonin receptor agonists and other classes of drugs.

OBJECTIVES

In the current report, we review and integrate many of the major behavioral, physiological, and neurochemical findings in the current literature regarding pharmacological assessments made in SERT mutant mice.

RESULTS

The absence of normal responses to serotonin reuptake inhibiting (SRI) antidepressants in SERT knockout (-/-) mice demonstrates that actions on SERT are a critical principle mechanism of action of members of this class of antidepressants. Drugs transported by SERT, (+)-3,4-methylenedioxymethamphetamine (MDMA) and 1-methyl-4-(2'-aminophenyl)-1,2,3,6-tetrahydropyridine (2'-NH(2)-MPTP), are also inactive in SERT -/- mice. Temperature, locomotor, and electrophysiological responses to various serotonin receptor agonists, including 8-hydroxy-2-(di-n-propylamino)-tetraline (8-OH-DPAT), ipsapirone, and RU24969, are reduced in SERT -/- mice, despite comparatively lesser reductions in Htr1a and Htr1b binding sites, G-proteins, and other signaling molecules. SERT -/- mice exhibit an approximately 90% reduction in head twitches in response to the Htr2a/2c agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), associated with a profound reduction in arachidonic acid signaling, yet only modest changes in Htr2a and Htr2c binding sites. SERT -/- mice also exhibit altered behavioral responses to cocaine and ethanol, related to abnormal serotonin, and possibly dopamine and norepinephrine, homeostasis.

CONCLUSIONS

Together, these studies demonstrate a complex and varied array of modified drug responses after constitutive deletion of SERT and provide insight into the role of serotonin, and in particular, its transporter, in the modulation of complex behavior and in the pharmacological actions of therapeutic agents and drugs of abuse.

5-HT1A receptor binding in temporal lobe epilepsy patients with and without major depression

BACKGROUND

Major depressive disorder (MDD) is the most common comorbid psychiatric condition associated with temporal lobe epilepsy (TLE). Preclinical and clinical studies suggest that 5-HT(1A) receptors play a role in the pathophysiology of both TLE and MDD. There is preliminary evidence for an association between decreased 5-HT(1A) receptor binding in limbic brain areas and affective symptoms in TLE patients. The objective of this study was to compare 5-HT(1A) receptor binding between TLE patients with and without MDD. For the first time, 5-HT(1A) receptor binding was measured in a sample large enough to permit sensitive comparisons between TLE patients with and without comorbid MDD diagnosed by clinical and structured psychiatric interviews.

METHODS

Thirty-seven epilepsy patients with temporal lobe foci confirmed by ictal video-electroencephalogram (EEG) monitoring were recruited from the Clinical Epilepsy Section, National Institute of Neurological Disorders and Stroke. We performed interictal positron emission tomography scanning, with [(18)F]FCWAY, a fluorinated derivative of WAY100635, on a GE Medical Systems (Waukesha, Wisconsin) Advance scanner with continuous EEG monitoring. The 5-HT(1A) receptor binding was estimated by partial volume-corrected [(18)F]FCWAY V/f(1) values.

RESULTS

In addition to decreased 5-HT(1A) receptor binding in the epileptic focus itself, comorbid MDD was associated with a significantly more pronounced reduction in 5-HT(1A) receptor binding in TLE patients, extending into non-lesional limbic brain areas outside the epileptic focus. Focus side and the presence of mesial temporal sclerosis were not associated with the presence of comorbid depression.

CONCLUSIONS

Reductions in 5-HT(1A) receptor binding might help elucidate the neurobiological mechanisms underlying the TLE-MDD comorbidity.

Which clinical and experimental data link temporal lobe epilepsy with depression?

The association of temporal lobe epilepsy with depression and other neuropsychiatric disorders has been known since the early beginnings of neurology and psychiatry. However, only recently have in vivo and ex vivo techniques such as Positron Emission Tomography, Magnetic Resonance Imaging and Magnetic Resonance Spectroscopy in combination with refined animal models and behavioral tests made it possible to identify an emerging pattern of common pathophysiological mechanisms. We now have growing evidence that in both disorders altered interaction of serotonergic and noradrenergic neurons with glutamatergic systems is associated with abnormal neuronal circuits and hyperexcitability. Neuronal hyperexcitability can possibly evoke seizure activity as well as disturbed emotions. Moreover, decreased synaptic levels of neurotransmitters and high glucocorticoid levels influence intracellular signaling pathways such as cAMP, causing disturbances of brain-derived and other neurotrophic factors. These may be associated with hippocampal atrophy seen on Magnetic Resonance Imaging and memory impairment as well as altered fear processing and transient hypertrophy of the amygdala. Positron Emission Tomography studies additionally suggest hypometabolism of glucose in temporal and frontal lobes. Last, but not least, in temporal lobe epilepsy and depression astrocytes play a role that reaches far beyond their involvement in hippocampal sclerosis and ultimately, therapeutic regulation of glial-neuronal interactions may be a target for future research. All these mechanisms are strongly intertwined and probably bidirectional such that the structural and functional alterations from one disease increase the risk for developing the other. This review provides an integrative update of the most relevant experimental and clinical data on temporal lobe epilepsy and its association with depression.

Effect of 8-OH-DPAT on electrographic activity during the kainic acid-induced status epilepticus in rats

The effect of 8-OH-DPAT, a 5-HT1A receptor agonist, on electrographic activity during the kainic acid (KA)-induced status epilepticus (SE) was evaluated in male Wistar rats. Electrographic (EEG) recordings from the ventral hippocampus and the frontal cortex along with behavioral changes were evaluated in animals that received KA administration (10mg/kg, i.p.) 20 min after saline solution (control group) or 8-OH-DPAT (1mg/kg, s.c.) injection. Rats pretreated with 8-OH-DPAT presented augmented latency for wet dog shakes (71%), generalized seizures (54%) and behavioral SE (31%). 8-OH-DPAT delayed occurrence of the first KA-induced paroxystic spikes (70%), increased latency to the EEG SE (39%) and decreased spike frequency (35-43%) recorded from the frontal cortex, and increased the time necessary for the high voltage EEG activity synchronization of the hippocampus and the frontal cortex (125%). However, EEG ictal activity recorded in hippocampus was not modified after 8-OH-DPAT pretreatment. These results indicate that 8-OH-DPAT reduces the EEG activity associated with the KA-induced SE in the frontal cortex, but not the hippocampus, and suggest an inhibitory effect in the propagation of epileptic seizures during the KA-induced SE.

Changes in central 5-HT(1A) receptor binding in mesial temporal epilepsy measured by positron emission tomography with [(11)C]WAY100635

OBJECTIVE

The possible involvement of the brain 5-HT(1A) receptor in epilepsy has been indicated in animal seizure models. Recent in vivo neuroimaging studies demonstrated decreased 5-HT(1A) receptor binding in epilepsy. Using positron emission tomography (PET) with [(11)C]WAY100635, we investigated 5-HT(1A) receptor binding in patients with mesial temporal lobe epilepsy and aimed to clarify the involvement of the brain 5-HT(1A) receptor system in epilepsy.

METHOD

PET measurements with [(11)C]WAY100635 were performed on 23 healthy volunteers and 13 patients who were diagnosed with mesial temporal lobe epilepsy based on clinical symptoms and electroencephalogram (EEG) findings. They had non-lesional mesial temporal lobe epilepsy with unilateral EEG foci and no hippocampal atrophy on magnetic resonance imaging. The binding potential (BP) of [(11)C]WAY100635 was calculated by the reference tissue model method. Data were analyzed for each region of interest (ROI) and on a voxel-by-voxel basis by statistical parametric mapping (SPM) system.

RESULTS

ROI and voxel-based analyses consistently demonstrated that 5-HT(1A) receptor BP was significantly decreased in the temporal lobe, hippocampus and amygdala on the ipsilateral side of the EEG focus compared to controls. In addition, decreased 5-HT(1A) receptor BP was also observed on the contralateral side of the amygdala.

CONCLUSION

5-HT(1A) receptor binding in patients with mesial temporal lobe epilepsy decreased predominantly in the ipsilateral mesial temporal lobe structures but also in the contralateral side. The imaging of 5-HT(1A) receptor binding by PET detects functional changes of the limbic system in mesial temporal lobe epilepsy, proving to be a sensitive and useful method.

Behavioral effects of high frequency electrical stimulation of the hippocampus on electrical kindling in rats

Experiments were designed to evaluate the effects of high frequency electrical stimulation (HFS) applied in ventral hippocampus during the hippocampal kindling process, as well as on the expression of fully kindled seizures and the refractoriness for subsequent convulsions during their postictal period. Male Wistar rats, stereotactically implanted in both ventral hippocampus, received daily bilateral HFS (pulses of 60 micros width at 130 Hz at subthreshold current intensity) during 1h immediately after each kindling stimulation (1s train of 60 Hz biphasic square waves, each 1 ms) during 40 days or until the kindled state was achieved. Rats were classified as follows: (a) Responder animals, who required low current intensity for HFS (208+/-38.2 microA), did not show progress of the kindling process and remained in stages II and III seizures. (b) Nonresponders rats, in which the current intensity for HFS was higher (434.5+/-51.7 microA), developed the kindling process as the kindling control group. When HFS was applied before the kindling stimulation in fully kindled rats, animals presented a reduced expression of the fully kindled seizures (nonresponders animals) and an enhanced refractoriness for subsequent seizures during the postictal period (kindling control and nonresponder animals). There was no correlation between the area where the HFS was applied and the effects induced. It was concluded that HFS at 130 Hz in ventral hippocampus is able to modify the epileptogenesis induced by the hippocampal kindling process and the refractoriness to subsequent seizures during the postictal period in rats.

Fluoxetine inhibits cortical spreading depression in weaned and adult rats suckled under favorable and unfavorable lactation conditions

Wistar rats (n = 58) were injected subcutaneously during the lactation period with fluoxetine (5, 10, 20 or 40 mg/kg/day) and cortical spreading depression (SD) was recorded immediately after weaning (25-30 days of life). An additional group (10 mg/kg; n = 8) was SD-recorded at 60-70 days. As compared to the saline-injected (n = 24) or "ingenuous" (n = 16) controls, fluoxetine dose-dependently reduced (P < 0.05) SD-velocities in the young rats by 4, 6, 16 and 15%, respectively, and in adult rats by 13%. In another experiment (26 adult rats), topical cortical application of fluoxetine (5 and 10 mg/ml solutions over the intact dura-mater for 10 min; n = 12 and 14, respectively) dose-dependently reduced SD-velocity (7.6% and 43.3% maximal reductions; P < 0.05). SD-propagation was blocked in 4 out of the 14 W-rats topically treated with the highest fluoxetine concentration (10 mg/ml). This topical fluoxetine effect was reverted after flushing the treated region with saline. In additional, 58 early-malnourished rats, fluoxetine applied during the suckling period (10 mg/kg/day, s.c.) and topically (10 mg/ml) also reduced (P < 0.05) SD-velocities by 18 and 22% for the systemic treatment (young and adult animals, respectively) and by 22.4% for the topical one. The present fluoxetine action supports the hypothesis of an antagonistic serotoninergic influence on SD, as previously suggested in experiments using other serotoninergic drugs. Data also suggest that early malnutrition does not greatly affect fluoxetine effects on SD.

Serotonin inhibits the induction of NMDA receptor-dependent long-term potentiation in the rat primary visual cortex

An increase in serotonin [5-hydroxytryptamine (5-HT)] levels in the rat visual cortex is correlated with the developmental decrease in long-term potentiation (LTP), and 5-HT may play an important role in the closure of the critical period by regulating LTP. The effect of 5-HT on the induction of N-methyl-D-aspartate receptor (NMDAR)-dependent and metabotropic glutamate receptor (mGluR)-dependent LTP in visual cortex slices from young rats was investigated. The field potential in layer II/III was recorded by stimulating the underlying layer IV. NMDAR-dependent LTP was induced in slices from 3-week-old rats by theta-burst stimulation (TBS) but not in slices from 5-week-old rats. However, LTP was induced in 5-HT-depleted slices from 5-week-old rats by incubation with para-chloroamphetamine (10 microM, 2 h), a 5-HT-depleting agent. The reinstated LTP in 5-HT-depleted slices was inhibited by the application of D-aminopentanoate, an NMDAR antagonist (50 microM) and 5-HT (10 and 30 microM). In contrast, the induction of mGluR-dependent LTP by weak TBS in disinhibited slices with picrotoxin (1 microM) in the bath was not affected by 5-HT application. The coapplication of 5-HT1A and 5-HT2 receptor agonists inhibited the induction of NMDAR-dependent LTP in 5-HT-depleted slices. 5-HT levels in the visual cortex increased with age. Based on these findings, we conclude that NMDAR-dependent LTP is specifically inhibited by coactivation of 5-HT1A and 5-HT2 receptors with the increase in 5-HT levels in the rat visual cortex at the end of the critical period.

Involvement of the somatostatin-2 receptor in the anti-convulsant effect of angiotensin IV against pilocarpine-induced limbic seizures in rats

The anti-convulsant properties of angiotensin IV (Ang IV), an inhibitor of insulin-regulated aminopeptidase (IRAP) and somatostatin-14, a substrate of IRAP, were evaluated in the acute pilocarpine rat seizure model. Simultaneously, the neurochemical changes in the hippocampus were monitored using in vivo microdialysis. Intracerebroventricularly (i.c.v.) administered Ang IV or somatostatin-14 caused a significant increase in the hippocampal extracellular dopamine and serotonin levels and protected rats against pilocarpine-induced seizures. These effects of Ang IV were both blocked by concomitant i.c.v. administration of the somatostatin receptor-2 antagonist cyanamid 154806. These results reveal a possible role for dopamine and serotonin in the anti-convulsant effect of Ang IV and somatostatin-14. Our study suggests that the ability of Ang IV to inhibit pilocarpine-induced convulsions is dependent on somatostatin receptor-2 activation, and is possibly mediated via the inhibition of IRAP resulting in an elevated concentration of somatostatin-14 in the brain.

Zimelidine decreases seizure susceptibility in stressed mice

To further evaluate whether selective serotonin reuptake inhibitors (SSRIs) have pro- or anticonvulsant properties and whether these properties will be modified by stress, we studied the effect of zimelidine on the convulsions produced by picrotoxin, a GABA(A) receptor antagonist, in unstressed and swim stressed mice. Zimelidine potentiated the ability of swim stress to enhance the threshold doses of intravenously administered picrotoxin producing convulsant signs and death, without having an effect in unstressed mice. The anticonvulsant effect of zimelidine was counteracted with mianserin, the antagonist of 5-HT(2A/2C), and diminished with WAY-100635, a selective antagonist of 5-HT(1A) receptors. In stressed mice, WAY-100635 prevented the anticonvulsant effect of 8-OH-DPAT, a 5-HT(1A) receptor agonist. SB-269970 and ketanserin, the antagonists of 5-HT(7) and 5-HT(2A) receptors, respectively, failed to reduce the effect of zimelidine. The results suggest the involvement of 5-HT(2C) and 5-HT(1A) receptors in the anticonvulsant effects of zimelidine and possibly other SSRIs in stress.

Effect of lamotrigine on cerebral blood flow in patients with idiopathic generalised epilepsy

PURPOSE

The purpose of this study was to investigate the effects of the new anti-epileptic drug, lamotrigine, on cerebral blood flow by performing (99m)Tc-ethylcysteinate dimer (ECD) single-photon emission computed tomography (SPECT) before and after medication in patients with drug-naive idiopathic generalised epilepsy.

METHODS

Interictal (99m)Tc-ECD brain SPECT was performed before drug treatment started and then repeated after lamotrigine medication for 4-5 months in 30 patients with generalised epilepsy (M/F=14/16, 19.3+/-3.4 years). Seizure types were generalised tonic-clonic seizure in 23 patients and myoclonic seizures in seven. The mean lamotrigine dose used was 214.1+/-29.1 mg/day. For SPM analysis, all SPECT images were spatially normalised to the standard SPECT template and then smoothed using a 12-mm full-width at half-maximum Gaussian kernel. The paired t test was used to compare pre- and post-lamotrigine SPECT images.

RESULTS

SPM analysis of pre- and post-lamotrigine brain SPECT images showed decreased perfusion in bilateral dorsomedial nuclei of thalami, bilateral uncus, right amygdala, left subcallosal gyrus, right superior and inferior frontal gyri, right precentral gyrus, bilateral superior and inferior temporal gyri and brainstem (pons, medulla) after lamotrigine medication at a false discovery rate-corrected p<0.05. No brain region showed increased perfusion after lamotrigine administration.

CONCLUSION

Our study demonstrates for the first time the effect of lamotrigine on interictal cerebral perfusion in drug-naive idiopathic generalised epilepsy patients. In summary, lamotrigine medication was found to reduce perfusion in cortico-thalamo-limbic areas, the orbitofrontal cortex, and brainstem.

Stimulation of 5-HT 1A receptors increases the seizure threshold for picrotoxin in mice

To evaluate the possible role of 5-HT 1A and 5-HT 2A receptors in the anticonvulsant effect of swim stress, mice were pre-treated with agonists and antagonists of these receptors prior to exposure to stress and the intravenous infusion of picrotoxin. 8-OH-DPAT ((+/-)-8-hydroxy-2-(di-n-propylamino) tetralin) and WAY-100635 (a selective agonist and antagonist of 5-HT 1A receptors), DOI (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) and ketanserin (a 5-HT 2A/2C receptor agonist and antagonist) were used. Results demonstrated that 1 and 3 mg/kg of 8-OH-DPAT increased the doses of picrotoxin producing running/bouncing clonus, tonic hindlimb extension and death in stressed and unstressed mice, respectively. Pre-treatment with WAY (0.3 mg/kg) prevented the effect of 8-OH-DPAT (3 mg/kg). DOI (2.5 mg/kg) and ketanserin (1 mg/kg) failed to affect the seizure threshold for picrotoxin. The results show that stimulation of 5-HT 1A receptors exerts anticonvulsant actions in stressed and unstressed mice, while stimulation of 5-HT 2A/2C receptors does not interfere with the effect of stress on picrotoxin-induced convulsions.

In vivo modulatory action of extracellular glutamate on the anticonvulsant effects of hippocampal dopamine and serotonin

PURPOSE

Our recent work (Clinckers et al., J Neurochem 2004;89:834-43) demonstrated that intrahippocampal perfusion of 2 nM dopamine or serotonin via a microdialysis probe offered complete protection against focal pilocarpine-induced limbic seizures and did not influence basal extracellular hippocampal glutamate levels. Ten nanomolar dopamine or serotonin perfusion, however, worsened seizures and was accompanied by significant extracellular glutamate increases to approximately 200%. The significance of these glutamate elevations in seizure generation remains unclear. The present microdialysis study investigated the modulatory role of extracellular hippocampal glutamate levels in these monoaminergic protective and proconvulsant effects.

METHODS

A first group of male Wistar albino rats was perfused intrahippocampally for 240 min with 6.25 microM glutamate alone to increase extracellular levels by 200%. Other animals were perfused with anticonvulsant concentrations of monoamines throughout the experiments while receiving continuous coperfusions of 6.25 microM glutamate either before, during, and after (240 min) or only after (100 min) pilocarpine perfusion (40 min). Rats were scored for epileptic behavior, and the mean scores were compared with those of the control group. Microdialysates were analyzed for monoamine and glutamate content with microbore liquid chromatography.

RESULTS

No convulsions occurred during glutamate perfusion alone. When monoamines and glutamate were coperfused before pilocarpine administration, the anticonvulsant effect of the monoamines was lost. Glutamate addition after pilocarpine administration did not affect monoaminergic seizure protection.

CONCLUSIONS

These results indicate that extracellular glutamate increases per se do not necessarily induce seizures but that they can modulate the anticonvulsant effects exerted by hippocampal monoamines.

Anticonvulsant effects of acute and repeated fluoxetine treatment in unstressed and stressed mice

Comorbidity of epilepsy and depression is not rare. Stress can affect both depression and seizures. Therefore, it is important to know whether an antidepressant drug has pro- or anticonvulsant properties and whether these properties will be modified by stress. We tested the effects of the antidepressant drug fluoxetine on the seizure threshold for picrotoxin in unstressed and swim-stressed mice. The mice were, prior to exposure to swim stress and the intravenous infusion of picrotoxin (a non-competitive GABA(A) receptor antagonist), pretreated with fluoxetine (a selective serotonin reuptake inhibitor), either acutely or repeatedly (5 days), and the latency to the onset of two convulsant signs and death was registered. The convulsant signs were running/bouncing clonus and tonic hindlimb extension. As expected, swim stress enhanced the seizure threshold for picrotoxin. Fluoxetine (20 mg/kg ip) given acutely increased in unstressed and swim-stressed mice the dose of picrotoxin producing tonic hindlimb extension and in unstressed mice the dose of picrotoxin producing death. Neither 10 nor 20 mg/kg of fluoxetine affected doses of picrotoxin needed to produce running bouncing/clonus. Repeated treatment with fluoxetine (20 mg/kg ip) enhanced significantly in unstressed and swim-stressed mice doses of picrotoxin needed to produce tonic hindlimb extension and death, and in stressed mice also the dose of picrotoxin producing running/bouncing clonus. The results demonstrate that the antidepressant drug fluoxetine, given acutely or repeatedly, shows anticonvulsant properties against convulsions induced in unstressed and swim-stressed mice by antagonist of GABA(A) receptors, picrotoxin. Swim stress failed to modify the anticonvulsant properties of fluoxetine.

Anticonvulsant action of GBR-12909 and citalopram against acute experimentally induced limbic seizures

We recently showed that intrahippocampally administered dopamine and serotonin exert concentration-dependent non-protective, protective and proconvulsant effects against limbic seizures in rats. Anticonvulsant action was mediated via, respectively, hippocampal D2 and 5-HT1A receptor stimulation, while proconvulsant effects were associated with concomitant hippocampal glutamate increases. We here examined whether increases in endogenous hippocampal dopamine and serotonin exert similar actions. Initially, dose-response experiments were performed with intrahippocampal perfusions of GBR-12909 and citalopram, respectively, selective dopamine and serotonin re-uptake blockers. Based on their effects on monoaminergic release, a potential non-protective, protective and proconvulsant concentration was selected. The predicted non-protective GBR-12909 (10 microM) and citalopram (0.5 microM) concentrations failed to prevent pilocarpine-induced seizures. The predicted protective GBR-12909 (100 microM) and citalopram (1 microM) perfusions resulted in complete anticonvulsant action, again mediated by D2 and 5-HT1A receptors. Unexpectedly, at predicted proconvulsant concentrations complete anticonvulsant action was obtained and hippocampal Glu remained unaltered. This study shows that selective monoamine re-uptake blockers have important anticonvulsant properties. Based on the previously established anticonvulsant monoamine ranges, anticonvulsant threshold concentrations can be predicted for compounds with endogenous dopamine or serotonin promoting effects. Non-selective actions curtailing glutamatergic activity may further be responsible for the unexpected anticonvulsant effects at predicted proconvulsant concentrations.

The effect of fluoxetine in a model of chemically induced seizures--behavioral and immunocytochemical study

The aim of this study was to examine the effects of acute fluoxetine treatment on pentylenetetrazol-induced convulsions in order to shape a model of seizures associated with treatment with antidepressants in rats. Moreover, the putative role of the hippocampal formation in this respect was investigated with the help of c-fos immuncytochemistry to mark local neuronal activity. It was found that fluoxetine (10.0 mg/kg, i.p.) enhanced the proconvulsive effect of pentylenetetrazol (50.0 mg/kg, i.p.), and simultaneously inhibited pentylenetetrazol-stimulated c-Fos expression in some areas of the hippocampus. Fluoxetine pretreatment did not alter pentylenetetrazol brain concentration indicating that this phenomenon was not related to the pharmacokinetic interaction. It is suggested that inhibition by fluoxetine of some neuronal populations contributing to the local feedback mechanism controlling excessive epileptiform discharges within the hippocampus might lead to an increase in epileptic activity. The reported in the present paper fluoxetine versus pentylenetetrazol interaction may, therefore, serve as a model of seizures associated with treatment with antidepressants.

Statistical parametric mapping of 5-HT1A receptor binding in temporal lobe epilepsy with hippocampal ictal onset on intracranial EEG

Experimental data in animals show that 5-HT(1A) receptors are predominantly located in limbic areas and suggest that serotonin, via these receptors, mediates an antiepileptic and anticonvulsant effect. In this PET study, we used an antagonist of the 5-HT(1A) receptor, [(18)F]MPPF, to assess the extent of 5-HT(1A) receptor binding changes in a group of seven temporal lobe epilepsy (TLE) patients with hippocampal ictal onset demonstrated by intracerebral EEG recording. On the basis of MRI-measured hippocampal volumes (HV), patients were classified into "normal HV" or "hippocampal atrophy" (HA). Voxel-based analyses (SPM99) were performed to objectively assess the differences in [(18)F]MPPF binding potential (BP) between patients (taken as a group or as individuals) and a database of 48 controls subjects. In the full group of patients, a significant decreased BP was detected ipsilateral to the epileptogenic zone in the hippocampus, temporal pole, insula, and temporal neocortex. This result was confirmed in the subgroup of patients with HA. In patients with normal HV, the BP decrease was restricted to the temporal pole. TLE patients also demonstrated an increased BP in various regions contralateral to the epileptogenic zone. These data suggest that in TLE patients with hippocampal seizure onset, the decrease in 5-HT(1A) receptor binding partly reflects hippocampal neuronal loss, but is also observed in various regions involved in temporo-limbic epileptogenic networks that appeared normal on MRI. Further studies are warranted to evaluate the clinical usefulness of [(18)F]MPPF-PET as compared to other established PET tracers in drug resistant TLE.

Anticonvulsant action of hippocampal dopamine and serotonin is independently mediated by D2 and 5-HT1A receptors

The present microdialysis study evaluated the anticonvulsant activity of extracellular hippocampal dopamine (DA) and serotonin (5-HT) with concomitant assessment of the possible mutual interactions between these monoamines. The anticonvulsant effects of intrahippocampally applied DA and 5-HT concentrations were evaluated against pilocarpine-induced seizures in conscious rats. DA or 5-HT perfusions protected the rats from limbic seizures as long as extracellular DA or 5-HT concentrations ranged, respectively, between 70-400% and 80-350% increases compared with the baseline levels. Co-perfusion with the selective D(2) blocker remoxipride or the selective 5-HT(1A) blocker WAY-100635 clearly abolished all anticonvulsant effects. These anticonvulsant effects were mediated independently since no mutual 5-HT and DA interactions were observed as long as extracellular DA and 5-HT levels remained within these protective ranges. Simultaneous D(2) and 5-HT(1A) receptor blockade significantly aggravated pilocarpine-induced seizures. High extracellular DA (> 1000% increases) or 5-HT (> 900% increases) concentrations also worsened seizure outcome. The latter proconvulsive effects were associated with significant increases in extracellular glutamate (Glu) and mutual increases in extracellular monoamines. Our results suggest that, within a certain concentration range, DA and 5-HT contribute independently to the prevention of hippocampal epileptogenesis via, respectively, D(2) and 5-HT(1A) receptor activation.

Epilepsy and depression: imaging potential common factors

Patients with seizure disorders have an increased incidence of depression. This may be due in part to psychosocial factors; or side effects of antiepileptic drugs. However, there may be underlying physiologic mechanisms for the relationship. Neuroimaging studies, including structural magnetic resonance imaging, positron emission tomography measurements of cerebral glucose metabolism, and, more recently, imaging of serotonin 1A receptors, may provide additional data to explain overlapping clinical manifestations of epilepsy and depression.

Endogenous 5-HT, released by MDMA through serotonin transporter- and secretory vesicle-dependent mechanisms, reduces hippocampal excitatory synaptic transmission by preferential activation of 5-HT1B receptors located on CA1 pyramidal neurons

A multitude of different serotonin (5-HT) receptor types are expressed in the hippocampus, but the identity of receptors actually mediating the physiological response to endogenous 5-HT has not been determined. We combined pharmacologically induced release of 5-HT with patch-clamp recordings on disinhibited rat CA1 minislices to determine effects of endogenous 5-HT on the excitability of pyramidal neurons and synaptic transmission among them. We found that application of 5-HT releasers, 3,4-methylenedioxy-methamphetamine (MDMA) or p-methylthioamphetamine, at concentrations ranging from 2 to 50 microm, reduced the excitatory synaptic transmission between CA1 pyramidal neurons without altering their basal electrical properties. This effect of MDMA was blocked by the selective 5-HT1B antagonist GR 55562, was dependent on endogenous 5-HT content and was mediated by presynaptically located, pertussis-toxin sensitive mechanisms. We found no other MDMA effects in our preparation, which indicates that the release of endogenous 5-HT preferentially stimulates 5-HT1B receptors on CA1 pyramidal neurons. Therefore, 5-HT1B receptor activation may represent a predominant component of the physiological response to endogenous 5-HT in the CA1. The high sensitivity of the 5-HT1B receptor-mediated reduction of polysynaptic excitatory responses to the extracellular 5-HT level enabled us to study mechanisms of the 5-HT releasing action of MDMA. Block of the serotonin transporter (SERT) with citalopram slowed the time course and reduced overall 5-HT release by MDMA. Depletion of vesicular 5-HT, by inhibition of vesicular monoamine transporter type 2 with tetrabenazine prevented the release. Thus although the SERT reversal contributes, a direct vesicle-depleting action is essential for MDMA release of 5-HT.

Does Serotonin Play a Role in Epilepsy?

Studies in experimental models have suggested a potential role for serotonergic transmission in epilepsy, and interest in this research has been increased by the development of positron emission tomography (PET) ligands that can be used to study 5-hydroxytryptamine (5-HT) receptors and transporters. The serotonergic system is very complex. At least 13 distinct G protein-coupled 5-HT receptors and one ligand-gated ion channel receptor (5-HT(3)) are divided into seven distinct classes (5-HT(1) to 5-HT(7)) ((1)). The receptors vary widely in their distribution and effects, innervating vascular structures and gut smooth muscle as well as neuronal tissue. Several receptor subtypes may be relevant to epilepsy.

Pentylenetetrazol-kindling of seizures selectively decreases [3H]-citalopram binding in the CA-3 area of rat hippocampus

The present study was aimed at determining the changes in the 5-HT transporter activity, in different brain structures after pentylenetetrazol induced kindling of seizures. We examined [3H]-citalopram binding in the rat brain structures, and the neurodegenerative effects in the hippocampal formation using autoradiographic and immunohistochemical methods. A statistically significant and selective reduction in the binding of [3H]-citalopram was found in the CA3 field of the hippocampus (P=0.009), and a similar tendency, close to the significance level, in the dentate gyrus (P=0.05). This effect was accompanied by a loss of neurons and activation of microglia in the hippocampal formation. The present data suggest the important role for CA3- serotonergic innervation in pentylenetetrazol induced kindling of seizures.

Effects of fluoxetine and TFMPP on spontaneous seizures in rats with pilocarpine-induced epilepsy

PURPOSE

Fluoxetine is a selective serotonin [5-hydroxytryptamine (5-HT)] reuptake inhibitor (SSRI) commonly used to treat depression. Some uncontrolled clinical studies have reported that SSRIs increase seizures, but animal experiments with evoked-seizure models have suggested that SSRIs at therapeutic doses decrease seizure susceptibility. We tested the hypothesis that fluoxetine and trifluoromethylphenylpiperazine (TFMPP, a nonselective 5-HT-receptor agonist) reduce the frequency of spontaneous motor seizures in pilocarpine-treated rats.

METHODS

Fluoxetine (20 mg/kg) and TFMPP (5 mg/kg) were administered to rats with pilocarpine-induced epilepsy. Phenobarbital (PB; 10 mg/kg) was a positive control, and saline (i.e., 0.5 ml) controlled for the injection protocol. Each rat received each treatment (intraperitoneally) once per day for 5 consecutive days with 1 week between treatments. Rats were continuously video-monitored for the last 72 h of each treatment.

RESULTS

When compared with saline over the entire 72-h observation period, PB and fluoxetine treatment, but not TFMPP, reduced the spontaneous-seizure rate. Plots of magnitude of the drug effect as a function of seizure frequency after saline treatment revealed larger drug effects for fluoxetine and PB in the rats with the highest control seizure rate. When the data from the five rats with the highest seizure frequency in saline were analyzed for the first 6 h after treatment, TFMPP also significantly reduced seizure frequency.

CONCLUSIONS

Animal models with spontaneous seizures can be used to screen potential antiepileptic drugs, and fluoxetine and TFMPP reduce spontaneous seizures in the pilocarpine model of temporal lobe epilepsy.

Citalopram has an antagonistic action on cortical spreading depression in well-nourished and early-malnourished adult rats

Adult, well-nourished (W) and early-malnourished (M) male Wistar rats were injected intraperitoneally for 7 days with 20 mg/kg CIT and cortical spreading depression (CSD) was recorded for 4 h on the day following the treatment. M-animals presented lower body weights, as well as higher CSD velocities of propagation, than the W ones, as previously reported. Compared to saline-injected controls, rats treated with CIT for 7 days presented comparable body weights and lower mean CSD velocities, per hour of recording, the differences being significant at the second hour (3.29+/-0.31 versus 3.56+/-0.40 mm/min; P < 0.05). Topical, cortical application of CIT (1- and 5 mg/ml solutions over the intact dura-mater) reduced dose-dependently the CSD velocity (maximal reductions of 16.3 and 55.8% for the 1 and 5 mg/ml solutions, respectively; P < 0.05), as well as the amplitude of the CSD-slow potential change (58.2 and 88.3%). In three out of seven W-rats and in one out of seven M-rats, topical CIT (5 mg/ml) blocked CSD propagation. The effects were reverted by flushing the treated region with saline. In the M-groups, CIT affected CSD in the same manner as in the W ones. The results reinforce previous evidence for an antagonistic influence of the serotoninergic activity on CSD.

Comparative study of hippocampal neuronal loss and in vivo binding of 5-HT1a receptors in the KA model of limbic epilepsy in the rat

A high density of 5-HT1a receptors is present in pyramidal hippocampal cells. Mapping of these receptors may be performed in vivo using the tracer no-carrier-added 4-(18)F-fluoro-N-2-(1-(2-methoxyphenyl)-1-piperazinyl)ethyl-N-2-pyridinyl-benzamide (MPPF). We tested the hypothesis of a relationship between MPPF binding and post-epileptic neuronal loss in the hippocampus. The model of limbic epilepsy induced by kainic acid (KA) in the rat was used. Rats were sacrificed at various times (1 h-240 days) after systemic injection of 10 mg/kg KA. Determination of MPPF binding in the brain was combined with a quantification of neuronal loss using DNA labeling with propidium iodide and confocal microscopy. Hippocampal MPPF binding varied according to time elapsed from KA injection. An initial decrease from day 1 to day 6 post injection was followed by a relative increase between day 6 and day 30. This effect was observed in rats which showed hippocampal neuronal loss but also in one rat which did not. In KA treated rats, statistically significant relationship between MPPF binding and neuronal count was found during the acute period (rats sacrificed 1 h-day 6 after KA injection) and the chronic phase (rats sacrificed beyond day 60 after KA injection). The late relative increase of MPPF binding suggests an epilepsy-induced increase of 5-HT1a receptors in the hippocampus. This effect needs to be further characterized before considering PET determination of hippocampal MPPF binding as a method of post-epileptic neuronal loss assessment.