Intra-hippocampal injection of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) inhibits partial and generalized seizures induced by kindling stimulation in cats

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

BACKGROUND AND OBJECTIVE

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

MATERIAL AND METHODS

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

RESULTS

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

CONCLUSION

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

Serotonin and sudden unexpected death in epilepsy

Epilepsy is a highly prevalent disease characterized by recurrent, spontaneous seizures. Approximately one-third of epilepsy patients will not achieve seizure freedom with medical management and become refractory to conventional treatments. These patients are at greatest risk for sudden unexpected death in epilepsy (SUDEP). The exact etiology of SUDEP is unknown, but a combination of respiratory, cardiac, neuronal electrographic dysfunction, and arousal impairment is thought to underlie SUDEP. Serotonin (5-HT) is involved in regulation of breathing, sleep/wake states, arousal, and seizure modulation and has been implicated in the pathophysiology of SUDEP. This review explores the current state of understanding of the relationship between 5-HT, epilepsy, and respiratory and autonomic control processes relevant to SUDEP in epilepsy patients and in animal models.

Do certain signal transduction mechanisms explain the comorbidity of epilepsy and mood disorders?

It is well known that mood disorders are highly prevalent in patients with epilepsy. Although several studies have aimed to characterize alterations in different types of receptors associated with both disturbances, there is a lack of studies focused on identifying the causes of this comorbidity. Here, we described some changes at the biochemical level involving serotonin, dopamine, and γ-aminobutyric acid (GABA) receptors as well as signal transduction mechanisms that may explain the coexistence of both epilepsy and mood disorders. Finally, the identification of common pathophysiological mechanisms associated with receptor-receptor interaction (heterodimers) could allow designing new strategies for treatment of patients with epilepsy and comorbid mood disorders.

Serotonin neurones have anti-convulsant effects and reduce seizure-induced mortality

Sudden unexpected death in epilepsy (SUDEP) is the leading cause of death in patients with refractory epilepsy. Defects in central control of breathing are important contributors to the pathophysiology of SUDEP, and serotonin (5-HT) system dysfunction may be involved. Here we examined the effect of 5-HT neurone elimination or 5-HT reduction on seizure risk and seizure-induced mortality. Adult Lmx1b(f/f/p) mice, which lack >99% of 5-HT neurones in the CNS, and littermate controls (Lmx1b(f/f)) were subjected to acute seizure induction by maximal electroshock (MES) or pilocarpine, variably including electroencephalography, electrocardiography, plethysmography, mechanical ventilation or pharmacological therapy. Lmx1b(f/f/p) mice had a lower seizure threshold and increased seizure-induced mortality. Breathing ceased during most seizures without recovery, whereas cardiac activity persisted for up to 9 min before terminal arrest. The mortality rate of mice of both genotypes was reduced by mechanical ventilation during the seizure or 5-HT2A receptor agonist pretreatment. The selective serotonin reuptake inhibitor citalopram reduced mortality of Lmx1b(f/f) but not of Lmx1b(f/f/p) mice. In C57BL/6N mice, reduction of 5-HT synthesis with para-chlorophenylalanine increased MES-induced seizure severity but not mortality. We conclude that 5-HT neurones raise seizure threshold and decrease seizure-related mortality. Death ensued from respiratory failure, followed by terminal asystole. Given that SUDEP often occurs in association with generalised seizures, some mechanisms causing death in our model might be shared with those leading to SUDEP. This model may help determine the relationship between seizures, 5-HT system dysfunction, breathing and death, which may lead to novel ways to prevent SUDEP.

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.

Should antidepressant drugs of the selective serotonin reuptake inhibitor family be tested as antiepileptic drugs?

For a long time, there has been a misconception that all antidepressant drugs have proconvulsant effects. Yet, antidepressants of the selective serotonin reuptake inhibitor (SSRI) family not only have been shown to be safe when used in patients with epilepsy (PWE) but also have been found to possess antiepileptic properties in animal models of epilepsy. In humans randomized to SSRIs vs. placebo for the treatment of major depressive episodes, the incidence of epileptic seizures was significantly lower among those treated with the antidepressants. These data raise the question of whether there is enough evidence that would support a randomized placebo-controlled trial to test antiepileptic effect of SSRIs in PWE. This article reviews the preclinical and clinical data to address this question.

Evaluation of anticonvulsant and nootropic effect of ondansetron in mice

The role of serotonin receptors have been implicated in various types of experimentally induced seizures. Ondansetron is a highly selective 5-hydroxytryptamine 3 (5-HT3) receptor antagonist used as antiemetic agent for chemotherapy-, and radiotherapy-induced nausea and vomiting. The present study was carried out to examine the effect of ondansetron on electroshock, pentylenetetrazole (PTZ)-induced seizures and cognitive functions in mice. Ondansetron was administered intraperitoneally (i.p.) at doses of 0.5, 1.0 and 2.0 mg/kg (single dose) to observe its effect on the increasing current electroshock seizure (ICES) test and PTZ-induced seizure test. In addition, a chronic study (21 days) was also performed to assess the effects of ondansetron on electroshock-induced convulsions and cognitive functions. The effect on cognition was assessed by elevated plus maze and passive avoidance paradigms. Phenytoin (25 mg/kg, i.p.) was used as a standard anticonvulsant drug and piracetam (200 mg/kg) was administered as a standard nootropic drug. The results were compared with an acute study, wherein it was found that the administration of ondansetron (1.0 and 2.0 mg/kg) significantly raised the seizure-threshold current as compared to control group in the ICES test. Similar results were observed after chronic administration of ondansetron. In PTZ test, ondansetron in all the three tested doses failed to show protective effect against PTZ-induced seizure test. Administration of ondansetron for 21 days significantly decreased the transfer latency (TL) and prolonged the step-down latency (SDL). The results of present study suggest the anticonvulsant and memory-enhancing effect of ondansetron in mice.

Neuromodulation in Epilepsy

Neuromodulation strategies have been proposed to treat a variety of neurological disorders, including medication-resistant epilepsy. Electrical stimulation of both central and peripheral nervous systems has emerged as a possible alternative for patients who are not deemed to be good candidates for resective procedures. In addition to well-established treatments such as vagus nerve stimulation, epilepsy centers around the world are investigating the safety and efficacy of neurostimulation at different brain targets, including the hippocampus, thalamus, and subthalamic nucleus. Also promising are the preliminary results of responsive neuromodulation studies, which involve the delivery of stimulation to the brain in response to detected epileptiform or preepileptiform activity. In addition to electrical stimulation, novel therapeutic methods that may open new horizons in the management of epilepsy include transcranial magnetic stimulation, focal drug delivery, cellular transplantation, and gene therapy. We review the current strategies and future applications of neuromodulation in epilepsy.

Preclinical activity profile of α-lactoalbumin, a whey protein rich in tryptophan, in rodent models of seizures and epilepsy

PURPOSE

To evaluate the potential anticonvulsant activity of α-lactalbumin (ALAC), a whey protein rich in tryptophan (TRP) relative to other large neutral amino acids (LNAAs), in rodent models of seizures and epilepsy.

METHODS

The effects of ALAC administered per os were evaluated by standard protocols against audiogenic seizures in Genetic Epilepsy Prone Rats (GEPR-9 rats), maximal electroshock (MES)-induced seizures in rats, pilocarpine-induced seizures in mice, spontaneous chronic seizures in mice exposed to pilocarpine-induced status epilepticus (SE), and absence seizures in WAG/Rij rats. In some models, carbamazepine (CBZ) was included as an active control. Plasma TRP/LNAAs ratios were measured by GC-MS.

RESULTS

Single doses of ALAC up to 500 or 6000 mg/kg were devoid of anticonvulsant activity in all models tested. Conversely, 5- and 12-day treatment with ALAC (250-1000 mg/kg/day) in GEPR rats reduced dose-dependently seizure scores and prolonged latency to clonus onset, with full persistence of the effect for up to 12h. ALAC (125-500 mg/kg/day for 15 days) protected against seizures induced by 250 mg/kg pilocarpine, but was less effective against higher pilocarpine doses. Similarly to CBZ, ALAC (125-500 mg/kg/day for 15 days) was also effective against spontaneous seizures in the post-pilocarpine SE model. ALAC (up to 6000 mg/kg/day for 12 days) did not prevent MES-induced seizures, although it reduced the duration of tonic extension at doses between 250 and 1000 mg/kg/day. Absence seizures in WAG/Rij rats were not significantly affected by ALAC. Plasma TRP/LNAAS ratios increased 2- to 3-fold after dosing with ALAC (250 mg/kg/day) for 7 and 14 days, respectively.

CONCLUSIONS

ALAC exerts significant protective activity against seizures in animal models, the effect being especially prominent against audiogenic seizures in GEPR-9 rats, seizures induced by low-dose pilocarpine in mice, and spontaneous seizures in mice exposed to pilocarpine-induced SE. This action is likely to be mediated by increased availability of TRP in the brain, with a consequent increase in 5-HT mediated transmission.

Intraventricular and intracerebral delivery of anti-epileptic drugs in the kindling model

A means to avoid the pharmacokinetic problems affecting the anti-epileptic drugs may be their direct intracerebroventricular (ICV) or intracerebral delivery. This approach may achieve a greater drug concentration at the epileptogenic area while minimizing it in other brain or systemic areas, and thus it could be an interesting therapeutic alternative in drug-resistant epilepsies. The objective of this article is to review a series of experiments, ranging from actute ICV injection to continuous intracerebral infusion of anti-epileptic drugs or grafting of neurotransmitter producing cells, in experimental models, especially in the kindling model of epilepsy in the rat. Acute ICV injection of phenytoin, phenobarbital or carbamacepine is able to diminish the intensity of kindling seizures, but it is also associated with a high neurologic toxicity, especially phenobarbital. Continuous ICV infusion of anti-epileptic drugs can effectively control seizures, but neurologic toxicity is not improved compared with systemic delivery. However, systemic toxicity may be improved, as in the case of valproic acid, whose continuous ICV infusion results in very low plasmatic or hepatic drug concentrations. Continuous intracerebral infusion at the epileptogenic area was studied as an alternative to minimize neurologic toxicity. Thus, intra-amygdalar infusion of gamma-aminobutyric acid (GABA) controls seizures with minimal neurotoxicity in amygdala-kindled rats. Similarly, continuous infusion of GABA into the dorsomedian nucleus of the thalamus improves seizure spread, while not affecting the local epileptogenic activity at the amygdala. Grafting of GABA releasing cells may reduce kindling parameter severity without behavioral side effects. We may conclude that ICV or intracerebral delivery of anti-epileptic drugs or neurotransmitters may be a useful technique to modulate epilepsy.

Autoradiography reveals selective changes in serotonin binding in neocortex of patients with temporal lobe epilepsy

The main goal of the present study was to evaluate binding to serotonin in the neocortex surrounding the epileptic focus of patients with mesial temporal lobe epilepsy (MTLE). Binding to 5-HT, 5-HT(1A), 5-HT(4), 5-HT(7) receptors and serotonin transporter (5-HTT) in T1-T2 gyri of 15 patients with MTLE and their correlations with clinical data, neuronal count and volume were determined. Autopsy material acquired from subjects without epilepsy (n=6) was used as control. The neocortex from MTLE patients demonstrated decreased cell count in layers III-IV (21%). No significant changes were detected on the neuronal volume. Autoradiography experiments showed the following results: reduced 5-HT and 5-HT(1A) binding in layers I-II (24% and 92%, respectively); enhanced 5-HT(4) binding in layers V-VI (32%); no significant changes in 5-HT(7) binding; reduced 5-HTT binding in all layers (I-II, 90.3%; III-IV, 90.3%, V-VI, 86.9%). Significant correlations were found between binding to 5-HT(4) and 5-HT(7) receptors and age of seizure onset, duration of epilepsy and duration of antiepileptic treatment. The present results support an impaired serotoninergic transmission in the neocortex surrounding the epileptic focus of patients with MTLE, a situation that could be involved in the initiation and propagation of seizure activity.

Localization of the serotonergic terminal fields modulating seizures in the genetically epilepsy-prone rat

Serotonin (5-HT) has been shown to exert antiepileptic effects in a variety of generalized convulsive seizure models, particularly the genetically epilepsy-prone rat (GEPR). The present study was designed to identify the region/site(s) where 5-HT exerts anticonvulsant effects in the GEPR-9, a model in which sound-evoked generalized tonic-clonic seizures (GTCS) are highly sensitive to manipulations in 5-HT concentration. Because the 5-HT reuptake inhibitor, fluoxetine, was known to exert anticonvulsant effects in GEPR-9s via a 5-HT-dependent mechanism, we utilized selective regional 5-HT depletion in combination with systemic fluoxetine administration to find the site where a 5-HT deficit would prevent the anticonvulsant action of fluoxetine. Widespread destruction of serotonergic terminal fields or regionally specific terminal field destruction was achieved using intracerebroventricular and more target specific infusions of 5,7-dihydroxytryptamine. The capacity of fluoxetine to suppress seizures in GEPR-9s following a loss of 5-HT was then examined. The present findings show the anticonvulsant action of fluoxetine is markedly attenuated following the loss of midbrain 5-HT, particularly in the region of the superior colliculus, while forebrain and spinal cord 5-HT do not appear to play a role in the action of fluoxetine. The importance of the deep layers of the SC was confirmed by demonstrating that direct microinfusion of fluoxetine into the SC can suppress seizures in rats pretreated with the 5-HT(1A) receptor antagonist pindolol.

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.

Seizure activity selectively reduces 5-HT1A receptor immunoreactivity in CA1 interneurons in the hippocampus of seizure-prone gerbils

Since the correlation between the serotonin (5-hydroxytryptamine, 5-HT) system and seizure activity remains to be clarified, we investigated the 5-HT system in the hippocampus of seizure-resistant (SR) and seizure-sensitive (SS) gerbils. There was no difference of the 5-HT system in the hippocampi of young animals (predisposed and juvenile gerbils) in both SR and SS gerbils. 5-HT immunoreactivity in the dorsal raphe nucleus and the median raphe nucleus was also similarly detected in both animal groups. As compared to SR adult gerbils, only 5-HT1A receptor immunoreactivity was selectively reduced in CA1 interneurons within SS adult gerbils. (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (a 5-HT1A receptor agonist, 1 and 2 mg/kg) markedly reduced paired-pulse inhibition in the CA1 region of SS adult gerbils only. These findings suggest that the selective reduction in 5-HT1A receptor expression on CA1 interneurons of SS adult gerbil may not be developmental defects, but be an acquired compensatory change induced by repeated seizure activity.

Serotonin and epilepsy

In recent years, there has been increasing evidence that serotonergic neurotransmission modulates a wide variety of experimentally induced seizures. Generally, agents that elevate extracellular serotonin (5-HT) levels, such as 5-hydroxytryptophan and serotonin reuptake blockers, inhibit both focal and generalized seizures, although exceptions have been described, too. Conversely, depletion of brain 5-HT lowers the threshold to audiogenically, chemically and electrically evoked convulsions. Furthermore, it has been shown that several anti-epileptic drugs increase endogenous extracellular 5-HT concentration. 5-HT receptors are expressed in almost all networks involved in epilepsies. Currently, the role of at least 5-HT(1A), 5-HT(2C), 5-HT(3) and 5-HT(7) receptor subtypes in epileptogenesis and/or propagation has been described. Mutant mice lacking 5-HT(1A) or 5-HT(2C) receptors show increased seizure activity and/or lower threshold. In general, hyperpolarization of glutamatergic neurons by 5-HT(1A) receptors and depolarization of GABAergic neurons by 5-HT(2C) receptors as well as antagonists of 5-HT(3) and 5-HT(7) receptors decrease the excitability in most, but not all, networks involved in epilepsies. Imaging data and analysis of resected tissue of epileptic patients, and studies in animal models all provide evidence that endogenous 5-HT, the activity of its receptors, and pharmaceuticals with serotonin agonist and/or antagonist properties play a significant role in the pathogenesis of epilepsies.

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.

Anticonvulsant effect of the selective 5-HT1B receptor agonist CP 94253 in mice

The effect of the selective 5-hydroxytryptamine1B (5-HT1B) receptor agonist 5-propoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-pyrrolo[3,2-b]pyridine (CP 94253) and the 5-HT1A/1B/1D receptor agonist 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridyl)-1H-indole (RU 24969) in maximal electroshock- and pentylenetetrazol-induced seizures in mice was examined. CP 94253 (10-40 mg/kg) afforded no protection against maximal electroshock-evoked convulsions, but produced anticonvulsant action in the pentylenetetrazol-induced seizures (ED50 = 29 mg/kg). The anticonvulsant effect of CP 94253 was abolished by the selective 5-HT1B receptor antagonist N-[3-(2-dimethylamino)ethoxy-4-methoxyphenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-(1,1'-biphenyl)-4-carboxamide (SB 216641; 20 mg/kg) but it was maintained following the p-chlorophenylalanine (p-CPA; 3 x 300 mg/kg)-induced 5-HT depletion. Interestingly, CP 94253 potentiated the anticonvulsant activity of diazepam in the pentylenetetrazol test; on the other hand, the benzodiazepine receptor antagonist, flumazenil (10 mg/kg), did not modify the anticonvulsant effect of CP 94253. RU 24969 (5 mg/kg) evoked no effect in the maximal electroshock model, but it produced anticonvulsant activity in the pentylenetetrazol assay, the latter effect being attenuated by the selective 5-HT1A receptor antagonist N-(2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl)-N-(2-pyridyl)-cyclohexanecarboxamide (WAY 100635; 0.3 mg/kg) and SB 216641 (10-20 mg/kg). The obtained results suggest that CP 94253 exerts anticonvulsant activity on pentylenetetrazol-induced seizures in mice, as a consequence of stimulation of 5-HT1B receptors (probably located postsynaptically and/or as heteroreceptors); the antiseizure activity of RU 24969 seems to depend on the stimulation of both 5-HT1A and 5-HT1B receptors.

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.

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.

Postsynaptic 5-HT1B receptors modulate electroshock-induced generalised seizures in rats

1. Although an important regulatory role for serotonin (5-HT) in seizure activation and propagation is well established, relatively little is known of the function of specific 5-HT receptor subtypes on seizure modulation. 2. The aim of the present study was to investigate the role of 5-HT(1A, 1B and 1D) receptors in modulating generalised seizures in the rat maximal electroshock seizure threshold (MEST) test. 3. The mixed 5-HT receptor agonists SKF 99101 (5-20 mg kg(-1) i.p.) and RU 24969 (1-5 mg kg(-1) i.p.), 0.5 h pretest, both produced marked dose-related increases in seizure threshold. These agents share high affinity for 5-HT(1A, 1B and 1D) receptors. 4. Antiseizure effects induced by submaximal doses of these agonists were maintained following p-chlorophenylalanine (150 mg kg(-1) i.p. x 3 days)-induced 5-HT depletion. 5. The anticonvulsant action of both SKF 99101 (15 mg kg(-1) i.p.) and RU 24969 (2.5 mg kg(-1) i.p.) was dose-dependently abolished by the selective 5-HT1B receptor antagonist SB-224289 (0.1-3 mg kg(-1) p.o., 3 h pretest) but was unaffected by the selective 5-HT1A receptor antagonist WAY 100635 (0.01-0.3 mg kg(-1) s.c., 1 h pretest). This indicates that 5-HT1B receptors are primarily involved in mediating the anticonvulsant properties of these agents. 6. In addition, the ability of the 5-HT(1B/1D) receptor antagonist GR 127935 (0.3-3 mg kg(-1) s.c., 60 min pretest) to dose-dependently inhibit SKF 99101-induced elevation of seizure threshold also suggests possible downstream involvement of 5-HT1D receptors in the action of this agonist, although confirmation awaits the identification of a selective 5-HT1D receptor antagonist. 7. Overall, these data demonstrate that stimulation of postsynaptic 5-HT1B receptors inhibits electroshock-induced seizure spread in rats.

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.

Focal treatment for refractory epilepsy: hope for the future?

Despite advances in anti-epileptic drug therapy and epilepsy surgery in recent years, intractable epilepsy remains a large clinical problem. Surgical resection, which can have an excellent outcome, is appropriate for only a minority of patients in whom an identifiable focus in non-eloquent brain can be identified. Systemic drug delivery is inevitably limited by the potential for unwanted side effects, due to actions both outside the CNS and in non-epileptic brain regions. Thus for a substantial number of patients novel treatment approaches are urgently needed. Both focal drug delivery and neuronal stem cell grafting have been evaluated in a variety of experimental epilepsy models in recent years, targeting either the seizure focus or key propagation pathways. The literature in this field is critically reviewed and considered in a clinical context. Studies in both areas are hampered by the limitations of available animal models, and by uncertainties in discerning which changes in the epileptic brain directly promote seizures, and which are compensatory. However, in many cases promising, though short-term, results have been obtained. Before such studies could be considered in humans further investigations that include long-term seizure and behavioural outcomes, in clinically relevant experimental models, are required. However, the current literature does provide proof in principle for a focal treatment approach, which may offer hope for many currently intractable patients for whom drug developments and surgical advances have proved disappointing.

MAOA knockout mice are more susceptible to seizures but show reduced epileptogenesis

The role of elevated neuroactive amine exposure during embryonic and early postnatal development on seizure threshold and epileptogenesis was examined using both electrical and pentylenetetrazol (PTZ) kindling in monoamine oxidase A knockout (MAO(A) KO) mice and their wildtype, parental strain (C3H). In the first experiment permanent bilateral electrodes were implanted in the amygdala of both C3H and MAO(A) KO mice. The mice had their afterdischarge threshold determined and then seizures were kindled daily for a total of 20 days. We observed that the MAO(A) KO mice had lower afterdischarge thresholds and less severe seizures compared to the C3H mice. In the second experiment, seizures were elicited in experimentally naive mice using 50mg/kg of PTZ once daily for 7 days. We observed that the MAO(A) KO mice had shorter latencies to the onset of the first seizure, shorter total duration of seizures and fewer seizures per day. Overall the results of both experiments suggest that MAO(A) KO mice have an increased susceptibility to seizures, but are more resistant to epileptogenesis. We conclude that the high levels of neuroactive amines in the MAO(A) KO mice reorganize the brain to make the mice more susceptible to seizures but the remaining high levels of serotonin and norepinephrine likely inhibit epileptogenesis.

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.

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.

Determination of effects of antiepileptic drugs on SNAREs-mediated hippocampal monoamine release using in vivo microdialysis

1. To elucidate possible mechanisms underlying the effects of carbamazepine (CBZ), valproate (VPA) and zonisamide (ZNS) on neurotransmitter exocytosis, the interaction between these three antiepileptic drugs (AEDs) and botulinum toxins (BoNTs) on basal, Ca(2+)- and K(+)-evoked release of dopamine (DA) and serotonin (5-HT) were determined by microdialysis in the hippocampus of freely moving rats. 2. Basal release of monoamine was decreased by pre-microinjection of the syntaxin inhibitor, BoNT/C, but only weakly affected by the synaptobrevin inhibitor, BoNT/B. Ca(2+)-evoked release was inhibited by BoNT/C selectively. K(+)-evoked release was reduced by BoNT/B predominantly and BoNT/C weakly. 3. Perfusion with low and high concentrations of CBZ and ZNS increased and decreased basal monoamine release, respectively. Perfusion with VPA increased basal 5-HT release concentration-dependently, whereas basal DA release was affected by VPA biphasic concentration-dependently, similar to CBZ and ZNS. This stimulatory action of AEDs on basal release was inhibited by BoNT/C predominantly. 4. Ca(2+)-evoked monoamine release was increased by low concentrations of CBZ, ZNS and VPA, but decreased by high concentrations. These effects of the AEDs on Ca(2+)-evoked release were inhibited by BoNT/C, but not by BoNT/B. 5. K(+)-evoked monoamine release was reduced by AEDs concentration-dependently. The inhibitory effect of these three AEDs on K(+)-evoked release was inhibited by BoNT/B, but not by BoNT/C. 6. These findings suggest that the therapeutic-relevant concentration of CBZ, VPA and ZNS affects exocytosis of DA and 5-HT, the enhancement of syntaxin-mediated monoamine release during resting stage, and the inhibition of synaptobrevin-mediated release during depolarizing stage.

Pharmacological discrimination between effects of carbamazepine on hippocampal basal, Ca(2+)- and K(+)-evoked serotonin release

To elucidate mechanisms of hippocampal serotonin release and possible mechanisms of clinical action of carbamazepine (CBZ), we determined interaction between antagonists of N-type (omega-conotoxin GVIA:GVIA), P-type (omega-agatoxin IVA:IVA) Ca(2+) channels, Na(+) channel (tetrodotoxin: TTX) and CBZ on hippocampal basal, Ca(2+)- and K(+)-evoked serotonin releases, using microdialysis in freely moving rats. Basal release was reduced by TTX, GVIA and IVA (GVIA>IVA). Ca(2+)-evoked release was reduced by GVIA but unaffected by TTX and IVA. K(+)-evoked release was reduced by TTX, GVIA and IVA (GVIA<IVA). TTX inhibited actions of IVA and GVIA on respective basal and K(+)-evoked releases, without affecting Ca(2+)-evoked release. Perfusion with 100 microM CBZ (estimated-concentration in hippocampal tissue: 19+/-2 microM) enhanced basal and Ca(2+)-evoked releases, but reduced K(+)-evoked release, whereas 1000 microM CBZ (estimated-concentration in hippocampal tissue: 188+/-16 microM) reduced three types of releases. Under condition of pretreatment with 100 and 1000 microM CBZ, TTX unaffected basal and K(+)-evoked releases. Under condition of pretreatment with 100 microM CBZ, IVA and GVIA unaffected basal and K(+)-evoked releases, respectively, but GVIA reduced basal, Ca(2+)-evoked releases and IVA also reduced K(+)-evoked release. Under condition of pretreatment with 1000 microM CBZ, GVIA unaffected three types of releases, and IVA unaffected basal release but reduced K(+)-evoked release. These findings contribute towards the possible mechanisms of concentration-dependent antiepileptic action of CBZ, which possibly inhibits Na(+) channel related neurotransmitter release mechanisms during K(+)-evoked stage, and simultaneously enhances N-type Ca(2+) channel related basal serotonin release at the resting stage.

The effect of the acute administration of various selective 5-HT receptor antagonists on focal hippocampal seizures in freely-moving rats

In this study, we assessed the effects of the acute administration of various 5-HT receptor antagonists on hippocampal partial seizures generated by low-frequency electrical stimulation in male Wistar rats. The seizure threshold and severity were determined by measuring the pulse number threshold and primary and secondary afterdischarges, respectively, and the latency of secondary discharge was also determined. The administration of either the selective 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazineyl]ethyl]-N-(pyridinyl)-c yclohe xanecarboximimde 3 HCl (WAY 100635, 0.1-1 mg/kg i.p.), the selective 5-HT(3) receptor antagonist granisetron (0.3-3 mg/kg i.p.), the selective 5-HT(2A) receptor antagonist R-(+)-a-(2, 3-dimethoxyphenyl)-1-[2-(4-fluorophenyl) ethyl]-4-piperidine-methanol (MDL 100907, 0.3-3 mg/kg i.p.) or the 5-HT(2B,C) receptor antagonist antagonist N-(1-methyl-5-indolyl)-N'-(3-pyridyl) urea HCl (SKB 200646A, 5-50 mg/kg i.p.) did not alter the pulse number threshold compared to vehicle-treated animals. However, the acute administration of WAY 100635 (0.3 mg/kg) and M100907 (1 mg/kg) significantly increased, whereas granisetron (1 mg/kg) decreased, the primary afterdischarge duration compared to vehicle-treated animals. The latency of secondary after discharge was significantly decreased by WAY 100635 (1 mg/kg) and granisetron (3 mg/kg) compared to vehicle-treated animals. These results suggest that in this model, the antagonism of 5-HT(1A), 5-HT(2A), 5-HT(3) or 5-HT(2B,C) receptors do not lower or raise seizure threshold. However, the antagonism of 5-HT(1A) receptors may increase or augment seizure severity.

Pindolol potentiates the effect of fluoxetine on hippocampal seizures in rats

Recent studies have shown that behavioral and neurochemical changes induced by selective serotonin (5-HT) reuptake inhibitors such as fluoxetine are potentiated by coadministration of a 5-HT1A receptor antagonist. The present study assessed the effects of concomitant administration of fluoxetine and a 5-HT1A receptor antagonist, pindolol, on focal hippocampal (HIP) seizures elicited by electrical stimulation in rats. A 10 mg/kg dose of fluoxetine, which was ineffective by itself, produced a significant increase in the afterdischarge threshold of HIP seizures when combined with pindolol at 10 mg/kg. The inhibitory effect of this combination was eliminated by pretreatment with parachlorophenylalanine, a depletor of brain 5-HT. These findings suggest that treatments designed to increase 5-HT neurotransmission inhibit the generation of HIP seizures. A combination of fluoxetine with a 5-HT1A receptor antagonist could thus be therapeutically useful for the treatment of depressive symptoms in patients with epilepsy.

Biphasic effects of zonisamide on serotonergic system in rat hippocampus

To study the mechanisms of antiepileptic action of zonisamide (ZNS), we determined the effects of ZNS on extracellular, total levels and re-uptake activity of serotonin (5-HT) in rat striatum and hippocampus. After acute administrations, plasma ZNS concentrations associated with anticonvulsive action (effective concentrations) increased the total levels of 5-HT, its metabolise (5-hydroxyindoleacetic acid: 5-HIAA) and precursor (5-hydroxytryptophan: 5-HTP). After chronic administration of ZNS, effective plasma concentrations also increased the extracellular and total levels of 5-HT, 5-HIAA, and 5-HTP. On the other hand, after both acute and chronic administrations of ZNS, a supra-effective ZNS concentration either decreased or did not affect the total levels of these substances. Therefore, the stimulatory effects of ZNS on the 5-HT system were reduced by an increase in ZNS concentration to supra-effective concentrations. ZNS concentrations of 30-1000 microM did not affect hippocampal 5HT re uptake activity in vitro. These results suggest that ZNS has biphasic effects on the 5-HT system, in that effective concentrations of ZNS enhance and supra-effective concentrations of ZNS reduce the function of the 5-HT system. These biphasic effects of ZNS on the 5-HT system may be involved in the mechanisms of action of the antiepileptic and psychotropic effects, and side effects of ZNS.

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

The modulatory effects of endogenous serotonin on the synaptic transmission and epileptiform activity were studied in the rat hippocampus with the use of extracellular and intracellular recording techniques. Field excitatory postsynaptic potential was reversibly depressed by serotonin in a concentration-dependent manner. Intracellular recordings revealed that serotonin-mediated synaptic depression was unaffected by extracellular Ba2+ or intracellular application of Cs+ while the postsynaptic hyperpolarizing effect was completely blocked. Epileptiform activity induced by picrotoxin (50 microM), a GABA(A) receptor antagonist, was also dose-dependently suppressed by serotonin. The antiepileptic effect was mimicked by 5-hydroxytryptamine1A agonist and was blocked by 5-hydroxytryptamine1A antagonists. 5-Hydroxytryptamine2 antagonist had no effect on the modulation. Similarly, fluoxetine, a selective serotonin re-uptake blocker, potently inhibited the epileptiform activity and this effect was blocked by 5-hydroxytryptamine1A receptor antagonist. Depletion of endogenous serotonin by pretreating the slices with p-chloroamphetamine completely prevented the antiepileptic action of fluoxetine, without modifying the action of serotonin in the same cells. These results suggest that the antiepileptic action of fluoxetine is due to an enhancement of endogenous serotonin which in turn is mediated by 5-hydroxytryptamine1A receptor. Endogenous serotonin transmission in the hippocampus is therefore capable of limiting the development and propagation of seizure activity.

Sensitization and kindling phenomena in mood, anxiety, and obsessive-compulsive disorders: the role of serotonergic mechanisms in illness progression

A number of untreated or inadequately treated psychiatric illnesses often demonstrate syndrome progression manifested by either increasing frequency, severity, or spontaneity of episodes. Behavioral sensitization to psychomotor stimulants (and its cross sensitization to stress) and electrophysiological kindling provide two very different models for conceptualizing physiological and behavioral abnormalities that progress in severity in response to the same inducing stimulation over time. These models are highly indirect, and the behaviors induced and specific pharmacologic interventions do not directly parallel those in many of these psychiatric syndromes. Nonetheless, these preclinical models help us conceptualize potential mechanisms involved in syndrome progression based on experience-dependent modifications of the genome at the level of transcriptional regulation. In both preclinical models, agents that are effective in the earlier developmental phase of sensitization or kindling are not necessarily effective in amelioration of the full-blown syndromes, and vice versa. Thus these models also suggest a variety of intervention principles that can be directly tested in the clinic, such as differential efficacy of treatment as a function of stage of evolution of the given syndrome. Although serotonergic mechanisms do not appear central to the basic phenomena of sensitization and kindling, they appear capable of modulating their development and severity. As such, it becomes of considerable importance to assess whether serotonergic mechanisms that have been implicated in acute treatment of mood and anxiety syndromes are also involved in the longitudinal course and prevention of syndrome progression or occurrence. Identification of the more precise molecular mechanisms involved might provide a target for new therapeutic approaches to these recurrent and potentially disabling major psychiatric illnesses.

Effects of carbamazepine on hippocampal serotonergic system

To establish the mechanism of action of the antiepileptic and psychotropic effects of carbamazepine (CBZ), its effects on serotonin (5-HT) transmission, metabolism and re-uptake activity in the rat hippocampus were studied. After acute and chronic administrations of 25 mg/kg CBZ, the plasma concentration of CBZ was found to be within the therapeutic range, whereas both acute and chronic administrations of 50 and 100 mg/kg CBZ resulted in a supratherapeutic plasma concentration. Acute administration of the therapeutic dose of CBZ resulted in an increase in the hippocampal extracellular and total level of 5-HT, its metabolite, 5-hydroxydoleacetic acid (5-HIAA) and its precursor, 5-hydroxytryptophan (5-HTP). The acute administration of 50 mg/kg CBZ resulted in an increase in the hippocampal levels of extracellular 5-HT and 5-HIAA as well as in the total levels of 5-HTP, whereas hippocampal levels of extracellular 5-HTP, total 5-HT and 5-HIAA remained unaffected. CBZ at a dose of 100 mg/kg decreased the levels of all of these substances. After chronic administration, 25 mg/kg/day CBZ increased hippocampal total levels of 5-HT, 5-HTP and 5-HIAA, whereas 100 mg/kg/day CBZ decreased all of these total levels. CBZ at a dose of 50 mg/kg/day decreased total levels of 5-HT, however neither total levels of 5-HIAA nor 5-HTP were affected. Both therapeutic and supratherapeutic plasma concentrations of CBZ inhibited 5-HTP accumulation, and did not affect 5-HT re-uptake activity in vitro. These results suggest that a therapeutic concentration of CBZ enhances 5-HT turnover and transmission, whereas a supratherapeutic concentration of CBZ inhibits 5-HT turnover and transmission without affecting 5-HT re-uptake activity. These effects of CBZ on serotonergic systems may be, at least partially, involved in the mechanisms of action of CBZ.

Effect of acute administration of various 5-HT receptor agonists on focal hippocampal seizures in freely moving rats

In this study, we assessed the effects of the acute administration of various 5-HT receptor agonists on hippocampal partial seizures generated by low-frequency electrical stimulation in male Wistar rats. The seizure threshold and severity were determined by measuring the pulse number threshold and primary and secondary afterdischarges and the latency of secondary discharge was also determined. The administration (0.1-1 mg/kg, i.p.) of either the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-aminopropyl)tetralin (8-OH-DPAT), or the selective 5-HT3 receptor agonist, 4-amino-(6-chloro-2-pyridyl)-1-piperidine (SR 57227A, 0.3-3 mg/kg, i.p.), did not alter any of the seizure parameters compared to those in vehicle-treated animals. Similarly, the administration of 0.3 and 1 mg/kg, i.p., of the 5-HT2A,C receptor agonist, (+/-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI), did not alter any of the seizure parameters, whereas 3 mg/kg significantly decreased the latency of the secondary afterdischarge compared to that in vehicle-treated animals. The selective serotonin reuptake inhibitor, (+/-)-fluoxetine (2 mg/kg, i.p.), significantly increased the pulse number threshold and decreased the primary afterdischarge duration compared to those in vehicle-treated animals. In contrast, higher doses (6 or 20 mg/kg, i.p.) of fluoxetine did not significantly alter any of the seizure parameters measured. These results suggest that, in this model, stimulation of 5-HT1A, 5-HT2A,C and 5-HT3 receptors does not alter seizure threshold or severity and that the blockade of 5-HT uptake produced by a low dose of fluoxetine appears to increase seizure threshold and decrease seizure severity.

Comparison of the effects of serotonin in the hippocampus and the entorhinal cortex

Among the molecular, cellular, and systemic events that have been proposed to modulate the function of the hippocampus and the entorhinal cortex (EC), one of the most frequently cited possibilities is the activation of the serotonergic system. Neurons in the hippocampus and in the EC receive a strong serotonergic projection from the raphe nuclei and express serotonin (5-HT) receptors at high density. Here we review the various effects of 5-HT on intrinsic and synaptic properties of neurons in the hippocampus and the EC. Although similar membrane-potential changes following 5-HT application have been reported for neurons of the entorhinal cortex and the hippocampus, the effects of serotonin on synaptic transmission are contrary in both areas. Serotonin mainly depresses fast and slow inhibition of the principal output cells of the hippocampus, whereas it selectively suppresses the excitation in the entorhinal cortex. On the basis of these data, we discuss the possible role of serotonin under physiological and pathophysiological circumstances.

Effects of the 5-HT3 receptor agonist 1-(m-chlorophenyl)-biguanide in the rat kindling model of epilepsy

This study assessed the action of the serotonin3 (5-HT3) receptor agonist, 1-(m-chlorophenyl)-biguanide (m-CPBG), against both kindled seizures and kindling development from the rat amygdala (AM). The intracerebroventricular (i.c.v.) administration of 40 microg m-CPBG significantly increased the duration of afterdischarge and bilateral forelimb clonus of generalized kindled seizures. In addition, daily i.c.v. treatment with m-CPBG at the same dose prior to each electrical stimulation to the AM significantly facilitated behavioral and electrographic seizure development and reduced the number of stimulations needed to elicit generalized seizures. The present results indicate that m-CPBG increases the duration of fully kindled seizures and facilitates the developmental seizure process, suggesting an excitatory role of 5-HT3 receptors in the kindling model of epilepsy.

Role of serotonin receptor subtypes in the development of amygdaloid kindling in rats

The present study was conducted to identify serotonin (5-HT) receptor subtypes involved in the development of amygdala (AM) kindling. We used 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A agonist, and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a 5-HT2 agonist, both of which were injected subcutaneously 15 min prior to each daily electrical stimulation to the rat AM. Treatment with 8-OH-DPAT (1 mg/kg) slightly suppressed behavioral and electrographic seizure development during the course of kindling. In contrast, DOI (1 mg/kg) strongly facilitated kindling development and reduced the number of stimulations needed to produce generalized seizures. These facilitatory effects of DOI were completely blocked by pretreatment with a 5-HT2 antagonist ketanserin. The present results suggest that the activation of 5-HT1A receptors can retard the development of AM kindling, whereas 5-HT2 receptors play a facilitatory role in this developmental seizure process.

Serotonin blocks different patterns of low Mg2+-induced epileptiform activity in rat entorhinal cortex, but not hippocampus

Low Mg2+-induced epileptiform activity in the entorhinal cortex is characterized by an initial expression of seizure-like events followed by late recurrent discharges. Both these forms of activity as well as the transition between them were blocked by serotonin. In contrast, serotonin had little effect upon the epileptiform activity in areas CA3 and CA1 of the hippocampus. Both forms of epileptiform activity in the entorhinal cortex are sensitive to N-methyl-D-aspartate receptor antagonists and it is shown here that serotonin blocked both types of epileptiform activity through an effective concentration-dependent reduction of N-methyl-D-aspartate receptor-mediated excitatory postsynaptic potentials in deep layer entorhinal cortex cells. Serotonin also prolonged or even prevented the transition between the two types of epileptiform activity and we suggest that this may be through activation of the Na+/K+-ATPase. The resistance of epileptiform activity in CA1 and CA3 to serotonin was most likely related to the inability of serotonin to reduce Schaffer collateral-evoked excitatory postsynaptic potentials. Given the strong serotonergic inputs to both the hippocampus and entorhinal cortex, the differential sensitivity of the two regions to serotonin suggests functional differences. In addition since the late recurrent discharges in the entorhinal cortex are resistant to all clinically used anticonvulsants, serotonin may open new avenues for the development of novel anticonvulsant compounds.

Stimulation of 5-HT1A receptors in the dorsal hippocampus and inhibition of limbic seizures induced by kainic acid in rats

1. We studied whether the stimulation of 5-HT1A receptors by 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a specific 5-HT1A receptor agonist, reduced electroencephalographic (EEG) seizures induced by intrahippocampal injection of 0.04 microgram in 0.5 microliter of the glutamate analogue kainic acid in freely-moving rats. 2. Pretreatment with 8-OH-DPAT 15 min earlier at the same site as kainic acid injection, caused a dose-dependent decrease of kainic acid-induced seizure activity. One and 10 micrograms significantly reduced the total time spent in seizures by 72% on average and the total number of seizures by 58% (P < 0.01) and 43% (P < 0.05) respectively. The latency to onset of the first seizure was increased 2.8 times (P < 0.01) only after 1 microgram 8-OH-DPAT; 0.1 microgram was ineffective on all seizure parameters. 3. Systemic administration of 25, 100 and 1000 micrograms kg-1 8-OH-DPAT significantly reduced the total number of seizures and the total time in seizures induced by intrahippocampal kainic acid by 52% and 74% on average. The latency to onset of the first seizure was delayed 1.8 times by 100 and 1000 micrograms kg-1 (P < 0.05). 4. The anticonvulsant action of 8-OH-DPAT given intrahippocampally or systemically was significantly blocked by 5 micrograms, but not 1 microgram WAY 100635, a selective 5-HT1A receptor antagonist, administered in the hippocampus before the agonist. 5. These results indicate that postsynaptic 5-HT1A receptors in the hippocampus mediate the anticonvulsant action of 8-OH-DPAT and that their stimulation has an inhibitory role in the generation of limbic seizures.

Abnormalities in 5-HT1A and 5-HT1B receptor binding in severe-seizure genetically epilepsy-prone rats (GEPR-9s)

The present study was designed to determine whether abnormalities in serotonin receptor binding co-exist with the presynaptic serotonergic deficits that have previously been identified in the genetically epilepsy-prone rat (GEPR) brain. In vitro binding of [3H]8-OH-DPAT (0.16-10.3 nM) to 5-HT1A receptor sites was found to be decreased in the hippocampus of severe seizure GEPRs (GEPR-9s) when compared to nonepileptic control rats, while no difference in [3H]8-OH-DPAT binding was observed in the GEPR-9 corpora quadrigemina or midbrain tegmentum. The decreased binding of [3H]8-OH-DPAT to hippocampal membranes was due to a decrease in Bmax (P < 0.001), rather than to a change in the Kd. Conversely, in vitro binding of [125I]cyanopindolol (2-400 pM) to 5-HT1B receptor sites was increased in the GEPR-9 hippocampus, corpora quadrigemina and midbrain tegmentum when compared to nonepileptic control rats. The increased binding of [125I]cyanopindolol in all three regions resulted from an increase in the Bmax (P < 0.05), rather than a change in the Kd. These finding suggest that in addition to the innate reduction in 5-HT presynaptic markers, GEPR-9s also exhibit abnormalities in the density of 5-HT1A and 5-HT1B receptors in some regions of the brain. Inasmuch as serotonin acts to attenuate audiogenic seizures in GEPRs, these abnormalities in 5-HT receptor binding may contribute to the seizure susceptibility exhibited by these animals.

Prolonged but not acute fluoxetine administration produces its inhibitory effect on hippocampal seizures in rats

This study assessed the effects of acute as well as long-term administration of fluoxetine, a selective serotonin (5-HT) reuptake inhibitor with anti-depressant properties, on hippocampal (HIP) seizures elicited by electrical stimulation in rats. The fluoxetine effect on HIP seizures was also assessed following long-term treatment with gepirone, a 5-HT1A receptor agonist. Acute single administration of fluoxetine (1, 10 mg/kg; IP) was found to produce no significant effect on HIP seizure activity. Following daily IP administration of fluoxetine (10 mg/kg per day) or gepirone (10 mg/kg per day) for 21 days, animals were given a 7-day drug-free period and then challenged with an acute dose of 10 mg/kg fluoxetine. These treatment regimens resulted in a significantly increased afterdischarge threshold of HIP seizures in response to acute fluoxetine administration. The inhibitory effect of fluoxetine, however, was not present 4 weeks after long-term treatment with either fluoxetine or gepirone. The present results indicate that long-term treatment with these compounds enhances the antiepileptic effect of subsequent fluoxetine administration on the generation of HIP seizures. This effect is possibly related to the well-demonstrated evidence that fluoxetine and gepirone, on long-term treatment, facilitate net 5-HT neurotransmission through desensitization of presynaptic 5-HT autoreceptors.

Adaptive changes in 5-HT1A receptor-mediated hippocampal inhibition in the alert rat produced by repeated 8-OH-DPAT treatment

1. The effect of acute and repeated treatment with 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A receptor ligand, on excitatory amino acid-mediated synaptic transmission was examined in the stratum radiatum CA1 region of the dorsal hippocampus of alert, gently restrained, rats. 2. Acute administration of 8-OH-DPAT transiently reduced the amplitude of the field excitatory postsynaptic potential (e.p.s.p.) in a dose-dependent (25-75 micrograms kg-1, i.p.) manner. This effect was blocked by the postsynaptic 5-HT1A receptor antagonist, MDL 73005EF (2 and 4 mg kg-1, i.p.). 3. 8-OH-DPAT (25 micrograms kg-1, i.p.) administered daily for 7 days produced a gradual reduction in the 24 h pre-injection baseline field e.p.s.p. amplitude. The reduction reached its lowest level after 7-8 days and was transiently reversed by acute injection of MDL 73005EF (2 mg kg-1, i.p.) on day 8. The field e.p.s.p. baseline amplitude recovered fully 5-8 days after cessation of drug treatment. 4. 8-OH-DPAT (25 micrograms kg-1, i.p.) administered daily for 7 days produced a marked reduction in acute response to 8-OH-DPAT (25 and 50 micrograms kg-1, i.p.) which did not recover until between day 36 and day 80 of the study. 5. It was concluded that repeated treatment with 8-OH-DPAT produced adaptive changes which resulted in a reduction in the dynamic range of 5-HT1A receptor-mediated transmission in the hippocampus.