Effects of serotonin on induced epileptiform activity in CA1 pyramidal neurons of genetically epilepsy prone rats

Tabernaemontana arborea and ibogaine induce paroxysmal EEG activity in freely moving mice: involvement of serotonin 5-HT1A receptors

Several Apocynaceae species, most notably Tabernanthe iboga, Voacanga africana and many Tabernaemontana species, produce ibogan-type alkaloids. Although a large amount of information exists about the Tabernaemontana genus, knowledge concerning chemistry and biological activity remains lacking for several species, especially related to their effects on the central nervous system (CNS). The aim of this study was to evaluate the effect of Tabernaemontana arborea Rose ex J.D.Sm. (T. arborea) hydroalcoholic extract (30, 56.2 and 100 mg/kg, i.p.) and two of its main alkaloids (ibogaine and voacangine, 30 mg/kg, i.p.) on electroencephalographic (EEG) activity alone and in the presence of the chemical convulsant agent pentylenetetrazole (PTZ, 85 mg/kg, i.p.) in mice. EEG spectral power analysis showed that T. arborea extract (56.2 and 100 mg/kg) and ibogaine (30 mg/kg, i.p.) promoted a significant increase in the relative power of the delta band and a significant reduction in alpha band values, denoting a CNS depressant effect. Voacangine (30 mg/kg, i.p.) provoked an EEG flattening pattern. The PTZ-induced seizures were not modified in the presence of T. arborea, ibogaine, or voacangine. However, sudden death was observed in mice treated with T. arborea extract at 100 mg/kg, i.p., combined with PTZ. Because T. arborea extract (100 mg/kg, i.p.) and ibogaine (30 mg/kg, i.p.), but not voacangine (30 mg/kg, i.p.), induced paroxysmal activity in the EEG, both were explored in the presence of a serotonin 5-HT_1A receptor antagonist (WAY100635, 1 mg/kg, i.p.). The antagonist abolished the paroxysmal activity provoked by T. arborea (100 mg/kg, i.p.) but not that observed with ibogaine, corroborating the participation of serotonin neurotransmission in the T. arborea effects. In conclusion, high doses of the T. arborea extract induced abnormal EEG activity due in part to the presence of ibogaine and involving serotonin 5-HT_1A receptor participation. Nevertheless, other possible constituents and mechanisms might participate in this complex excitatory activity that would be interesting to explore in future studies.

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.

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.

Synthesis, anticonvulsant activity and 5-HT1A, 5-HT2A receptor affinity of new N-[(4-arylpiperazin-1-yl)-alkyl] derivatives of 2-azaspiro[4.4]nonane and [4.5]decane-1,3-dione

The synthesis, physicochemical and pharmacological properties of new N-[(4-arylpiperazin-1-yl)-alkyl]-2-azaspiro[4.4]nonane- (8a-c, 10a-d) and [4.5]decane-1,3-dione (9a-c, 11a-d) derivatives were described. The antiepileptic effects of those compounds were examined by a maximal electroshock (MES) and a pentylenetetrazole (sc. PTZ) tests, and their neurotoxicity was determined using a rota-rod test. Compounds 8c, 9c, 10c, d, 11c, d with a CF(3) group at the 3-position of the 4-arylpiperazine fragment exhibited anti-seizure properties in the MES model; in contrast, their 2-CH(3) and 2-OCH(3) analogues were inactive in both the tests used. Moreover, since the investigated compounds belong to the class of long-chain arylpiperazines, their serotonin 5-HT(1A) and 5-HT(2A) receptor affinity was determined. The relationship between the length of alkylene spacer and 5-HT(1A)/5-HT(2A) receptor activity was observed. Compounds with an ethylene and a propylene bridge (10a-d and 11a-d) were 3-80-fold more potent (K(i) ranged from 3.1 to 94 nM for 5-HT(1A) and 32-465 nM for 5-HT(2A)) than their methylene analogues (8a-c and 9a-c; K(i) ranged from 81 to 370 nM for 5-HT(1A) and 126-1370 nM for 5-HT(2A)). The highest 5-HT(1A) receptor affinity was displayed by 2-OCH(3) and 3-CF(3) phenyl derivatives (10b, 11b: K(i)=6.8 and 5.7 nM, respectively, and 10c, 11c: K(i)=6.0 and 3.1 nM, respectively), while in the case of 5-HT(2A) receptor the highest affinity was observed for the 3-CF(3) phenyl derivatives 10c, d, 11c, d (K(i) ranged from 32 to 86 nM).

Lateral habenula and hippocampus: a complex interaction raphe cells-mediated

The study has shown an excitatory influence exerted by lateral habenula (LH) on hippocampal pyramidal cells. The modulatory influence is paradoxically serotonine-mediated; in fact all LH stimulation effects were abolished by intrahippocampal iontophoretic methysergide application. The data suggest the involvement of dorsal raphe nucleus. In fact, the dorsal raphe nucleus stimulation caused on hippocampus an expected inhibitory effect antagonized by intrahippocampal iontophoretic methysergide application. In the context of this neural structure we have highlighted a disinhibitory relation between two types of cells: slow serotonergic efferent neurones and fast GABAergic interneurones. The disinhibitory hypothesis is also supported by the following experimental tests performed on both slow and fast raphe cells: a) LH stimulation at low and high frequencies; b) iontophoretic administration of NMDA and GABA; c) LH stimulation during intraraphe iontophoretic injection of 2-APV (NMDA antagonist) and bicuculline (GABA antagonist).

Serotonin inhibits epileptiform discharge by activation of 5-HT1A receptors in CA1 pyramidal neurons

The anti-epileptiform effect of serotonin was characterized in cellular models of epilepsy using electrophysiological recording techniques. In the bicuculline model, both serotonin (20 microM) and its 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 10 microM) completely blocked the epileptiform discharge and caused membrane hyperpolarization and reduction in input resistance. These effects were completely antagonized by the 5-HT1A receptor antagonist N-t-butyl-3(4-[2-methoxyphenyl]piperazin-1-yl)-2-phenyl-propanamid e(WAY 100135) (10 microM). Epileptiform discharge induced by positive current injection was also blocked by serotonin. The presence of WAY 100135 renders serotonin ineffective in the same model. In the bicuculline model, epileptiform discharge blocked by serotonin reappeared and was also intensified when BaCl2 was added to the medium. To rule out the possibility of serotonin-induced hyperpolarization strengthening the inhibitory effect of endogenous Mg2+ on glutamate N-methyl-D-aspartic acid (NMDA) receptor we studied the antiepileptic effect of serotonin in the 0 Mg2+ model. Spontaneous activity and evoked bursts seen with the 0 Mg2+ model were completely blocked by serotonin. WAY 100135 completely antagonized serotonin effects in this model as well. This study provides evidence suggesting that in rat CA1 pyramidal neurons, serotonin can inhibit epileptiform activity in a variety of accepted epilepsy cellular models and that inhibition of epileptiform bursts by serotonin may be mediated by activation of the 5-HT1A receptor subtype.