Serotonergic Modulation of Absence-Like Seizures in Groggy Rats: a Novel Rat Model of Absence Epilepsy

The Psychedelic N,N-Dipropyltryptamine Prevents Seizures in a Mouse Model of Fragile X Syndrome via a Mechanism that Appears Independent of Serotonin and Sigma1 Receptors

The serotonergic psychedelic psilocybin shows efficacy in treating neuropsychiatric disorders, though the mechanism(s) underlying its therapeutic effects remain unclear. We show that a similar psychedelic tryptamine, N,N-dipropyltryptamine (DPT), completely prevents audiogenic seizures (AGS) in an Fmr1 knockout mouse model of fragile X syndrome at a 10 mg/kg dose but not at lower doses (3 or 5.6 mg/kg). Despite showing in vitro that DPT is a serotonin 5-HT2A, 5-HT1B, and 5-HT1A receptor agonist (with that rank order of functional potency, determined with TRUPATH Gα/βγ biosensors), pretreatment with selective inhibitors of 5-HT2A/2C, 5-HT1B, or 5-HT1A receptors did not block DPT's antiepileptic effects; a pan-serotonin receptor antagonist was also ineffective. Because 5-HT1A receptor activation blocks AGS in Fmr1 knockout mice, we performed a dose-response experiment to evaluate DPT's engagement of 5-HT1A receptors in vivo. DPT elicited 5-HT1A-dependent effects only at doses greater than 10 mg/kg, further supporting that DPT's antiepileptic effects were not 5-HT1A-mediated. We also observed that the selective sigma1 receptor antagonist, NE-100, did not impact DPT's antiepileptic effects, suggesting DPT engagement of sigma1 receptors was not a crucial mechanism. Separately, we observed that DPT and NE-100 at high doses caused convulsions on their own that were qualitatively distinct from AGS. In conclusion, DPT dose-dependently blocked AGS in Fmr1 knockout mice, but neither serotonin nor sigma1 receptor antagonists prevented this action. Thus, DPT might have neurotherapeutic effects independent of its serotonergic psychedelic properties. However, DPT also caused seizures at high doses, showing that DPT has complex dose-dependent in vivo polypharmacology.

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.

Activity of the Lateral Hypothalamus during Genetically Determined Absence Seizures

(1) Background: Absence seizures (ASs) are sudden, transient lapses of consciousness associated with lack of voluntary movements and generalized 2.5–4 Hz spike-wave discharges (SWDs) in the EEG. In addition to the thalamocortical system, where these pathological oscillations are generated, multiple neuronal circuits have been involved in their modulation and associated comorbidities including the serotonergic system. Neuronal activity in one of the major synaptic input structures to the brainstem dorsal raphé nucleus (DRN), the lateral hypothalamus (LH), has not been characterized. (2) Methods: We used viral tract tracing and optogenetics combined with in vitro and in vivo electrophysiology to assess the involvement of the LH in absence epilepsy in a genetic rodent model. (3) Results: We found that a substantial fraction of LH neurons project to the DRN of which a minority is GABAergic. The LH to DRN projection can lead to monosynaptic iGluR mediated excitation in DRN 5-HT neurons. Neuronal activity in the LH is coupled to SWDs. (4) Conclusions: Our results indicate that a brain area involved in the regulation of autonomic functions and heavily innervating the RN is involved in ASs. The decreased activity of LH neurons during SWDs could lead to both a decreased excitation and disinhibition in the DRN. These results support a long-range subcortical regulation of serotonergic neuromodulation during ASs and further our understanding of the state-dependence of these seizures and some of their associated comorbidities.

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.

Interactions of antiepileptic drugs with drugs approved for the treatment of indications other than epilepsy

Introduction: Comorbidities of epilepsy may significantly interfere with its treatment as diseases in the general population are also encountered in epilepsy patients and some of them even more frequently (for instance, depression, anxiety, or heart disease). Obviously, some drugs approved for other than epilepsy indications can modify the anticonvulsant activity of antiepileptics. Areas covered: This review highlights the drug-drug interactions between antiepileptics and aminophylline, some antidepressant, antiarrhythmic (class I-IV), selected antihypertensive drugs and non-barbiturate injectable anesthetics (ketamine, propofol, etomidate, and alphaxalone). The data were reviewed mainly from experimental models of seizures. Whenever possible, clinical data were provided. PUBMED data base was the main search source.Expert opinion: Aminophylline generally reduced the protective activity of antiepileptics, which, to a certain degree, was consistent with scarce clinical data on methylxanthine derivatives and worse seizure control. The only antiarrhythmic with this profile of action was mexiletine when co-administered with VPA. Among antidepressants and non-barbiturate injectable anesthetics, trazodone, mianserin and etomidate or alphaxalone, respectively, negatively affected the anticonvulsant action of some antiepileptic drugs. Clinical data indicate that only amoxapine, bupropion, clomipramine and maprotiline should be used with caution. Possibly, drugs reducing the anticonvulsant potential of antiepileptics should be avoided in epilepsy patients.

Trazodone increases seizures in a genetic WAG/Rij rat model of absence epilepsy while decreasing them in penicillin-evoked focal seizure model

AIM

Psychiatric disorders, especially depression and anxiety, are among the most disabling comorbidities in patients with epilepsy, and they are difficult to treat because many antidepressants cause proconvulsive effects. Thus, it is important to identify the seizure risks associated with antidepressants. Trazodone is one of the most frequently prescribed selective serotonin reuptake inhibitor (SSRI) antidepressant drugs for the treatment of depression and anxiety. The aim of the present study was to evaluate the effects of trazodone on epileptiform activity in a penicillin-evoked focal seizure model in Wistar rats and in a genetic absence epilepsy model in Wistar Albino Glaxo/Rijswijk strain (WAG/Rij) rats.

METHODS

Trazodone at 5-, 10-, and 30-mg/kg doses was injected intraperitoneally in Wistar rats 30 min after penicillin injection, and spike frequency and amplitude of penicillin-induced epileptiform activity were evaluated. In a separate experimental model, the same trazodone doses were injected in WAG/Rij rats to elucidate their effects on number, duration, and amplitude of spike-and-wave discharges (SWDs) and on depression-anxiety like behavior. In both experimental groups, after trazodone injections recordings were made for 3 h. Depression-anxiety like behaviors in WAG/Rij rats were examined using forced swim test and open-field test.

RESULTS

Trazodone at 10- and 30-mg/kg doses significantly reduced the frequency of penicillin-induced epileptiform activity without changing the amplitude. Trazodone at a 5-mg/kg dose had no effect on either frequency or amplitude of epileptiform activity. Trazodone at all doses significantly increased number and duration of SWDs without changing the amplitude. In addition, all doses of trazodone decreased the number of squares crossed and duration of grooming in open-field test, and reduced swimming time activity and increased immobility time in forced swim test.

CONCLUSION

Our results suggest that depending on the dose used, trazodone had an anticonvulsant effect or no effect on penicillin-evoked focal seizure model, but all trazodone doses resulted in proconvulsant and depression-anxiety like behavior in WAG/Rij rats, which represent a genetic absence model of epilepsy.

Serotonin abnormalities in Dravet syndrome mice before and after the age of seizure onset

Dravet syndrome (DS) is a genetic form of severe epilepsy often associated with mutation of the SCN1A gene encoding the voltage gated sodium channel Nav1.1. Typically refractive to conventional therapy, serotonin neurotransmission may be an innovative target for treatment. To further understand the role of serotonin in this disorder, in this study we examined the state of the endogenous serotonin system in an Scn1a^+/- mouse model of DS. Examined at an age before seizures appear, we found the hypothermic effect of 5-HT_1A receptor agonist administration was attenuated. HPLC analysis of brain monoamine content revealed modestly reduced serotonin levels in tissue samples of the midbrain that included the dorsal raphe nucleus but no changes elsewhere in the brain. The reduced sensitivity to 5-HT_1A agonist administration seen at young ages reversed after the age of seizure development when mice showed an exaggerated hypothermic response. Likewise, adult DS mice showed a pronounced hypersensitivity to a 5-HT_2A/2C agonist. As adults however monoamine levels were not detectably altered. Thus there are alterations in the endogenous serotonin system that both precede and follow the appearance of seizure in DS mice, most strikingly in the response to agonist administration.

Pharmacokinetic/pharmacodynamic considerations for epilepsy - depression comorbidities

INTRODUCTION

Epilepsy may be frequently associated with psychiatric disorders and its co-existence with depression usually results in the reduced quality of life of patients with epilepsy. Also, the efficacy of antiepileptic treatment in depressed patients with epilepsy may be significantly reduced.

AREAS COVERED

Results of experimental studies indicate that antidepressants co-administered with antiepileptic drugs may either increase their anticonvulsant activity, remain neutral or decrease the protective action of antiepileptic drugs in models of seizures. Apart from purely pharmacodynamic interactions, pharmacokinetic mechanisms have been proven to contribute to the final outcome. We report on clinical data regarding the pharmacokinetic interactions of enzyme-inducing antiepileptic drugs with various antidepressants, whose plasma concentration may be significantly reduced. On the other hand, antidepressants (especially selective serotonin reuptake inhibitors) may influence the metabolism of antiepileptics, in many cases resulting in the elevation of plasma concentration of antiepileptic drugs.

EXPERT OPINION

The preclinical data may provide valuable clues on how to combine these two groups of drugs - antidepressant drugs neutral or potentiating the anticonvulsant action of antiepileptics are recommended in this regard. Avoidance of antidepressants clearly decreasing the convulsive threshold or decreasing the anticonvulsant efficacy of antiepileptic drugs (f.e. bupropion or mianserin) in patients with epilepsy is recommended.

Role for serotonin2A (5-HT2A) and 2C (5-HT2C) receptors in experimental absence seizures

Absence seizures (ASs) are the hallmark of childhood/juvenile absence epilepsy. Monotherapy with first-line anti-absence drugs only controls ASs in 50% of patients, indicating the need for novel therapeutic targets. Since serotonin family-2 receptors (5-HT₂Rs) are known to modulate neuronal activity in the cortico-thalamo-cortical loop, the main network involved in AS generation, we investigated the effect of selective 5-HT_2AR and 5-HT_2CR ligands on ASs in the Genetic Absence Epilepsy Rats from Strasbourg (GAERS), a well established polygenic rat model of these non-convulsive seizures. GAERS rats were implanted with fronto-parietal EEG electrodes under general anesthesia, and their ASs were later recorded under freely moving conditions before and after intraperitoneal administration of various 5-HT_2AR and 5-HT_2CR ligands. The 5-HT_2A agonist TCB-2 dose-dependently decreased the total time spent in ASs, an effect that was blocked by the selective 5-HT_2A antagonist MDL11,939. Both MDL11,939 and another selective 5-HT_2A antagonist (M100,907) increased the length of individual seizures when injected alone. The 5-HT_2C agonists lorcaserin and CP-809,101 dose-dependently suppressed ASs, an effect blocked by the selective 5-HT_2C antagonist SB 242984. In summary, 5-HT_2ARs and 5-HT_2CRs negatively control the expression of experimental ASs, indicating that selective agonists at these 5-HT₂R subtypes might be potential novel anti-absence drugs.

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5-HT2AR activation decreases absence seizures in GAERS.

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5-HT2CR activation decreases absence seizures in GAERS.

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5-HT2AR blockade increases absence seizures in GAERS.

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5-HT2CR blockade does not affect absence seizures in GAERS.

New therapeutic opportunities for 5-HT2C receptor ligands in neuropsychiatric disorders

The 5-HT2C receptor (R) displays a widespread distribution in the CNS and is involved in the action of 5-HT in all brain areas. Knowledge of its functional role in the CNS pathophysiology has been impaired for many years due to the lack of drugs capable of discriminating among 5-HT2R subtypes, and to a lesser extent to the 5-HT1B, 5-HT5, 5-HT6 and 5-HT7Rs. The situation has changed since the mid-90s due to the increased availability of new and selective synthesized compounds, the creation of 5-HT2C knock out mice, and the progress made in molecular biology. Many pharmacological classes of drugs including antipsychotics, antidepressants and anxiolytics display affinities toward 5-HT2CRs and new 5-HT2C ligands have been developed for various neuropsychiatric disorders. The 5-HT2CR is presumed to mediate tonic/constitutive and phasic controls on the activity of different central neurobiological networks. Preclinical data illustrate this complexity to a point that pharmaceutical companies developed either agonists or antagonists for the same disease. In order to better comprehend this complexity, this review will briefly describe the molecular pharmacology of 5-HT2CRs, as well as their cellular impacts in general, before addressing its central distribution in the mammalian brain. Thereafter, we review the preclinical efficacy of 5-HT2C ligands in numerous behavioral tests modeling human diseases, highlighting the multiple and competing actions of the 5-HT2CRs in neurobiological networks and monoaminergic systems. Notably, we will focus this evidence in the context of the physiopathology of psychiatric and neurological disorders including Parkinson's disease, levodopa-induced dyskinesia, and epilepsy.

Unbalanced Peptidergic Inhibition in Superficial Neocortex Underlies Spike and Wave Seizure Activity

Slow spike and wave discharges (0.5-4 Hz) are a feature of many epilepsies. They are linked to pathology of the thalamocortical axis and a thalamic mechanism has been elegantly described. Here we present evidence for a separate generator in local circuits of associational areas of neocortex manifest from a background, sleep-associated delta rhythm in rat. Loss of tonic neuromodulatory excitation, mediated by nicotinic acetylcholine or serotonin (5HT3A) receptors, of 5HT3-immunopositive interneurons caused an increase in amplitude and slowing of the delta rhythm until each period became the "wave" component of the spike and wave discharge. As with the normal delta rhythm, the wave of a spike and wave discharge originated in cortical layer 5. In contrast, the "spike" component of the spike and wave discharge originated from a relative failure of fast inhibition in layers 2/3-switching pyramidal cell action potential outputs from single, sparse spiking during delta rhythms to brief, intense burst spiking, phase-locked to the field spike. The mechanisms underlying this loss of superficial layer fast inhibition, and a concomitant increase in slow inhibition, appeared to be precipitated by a loss of neuropeptide Y (NPY)-mediated local circuit inhibition and a subsequent increase in vasoactive intestinal peptide (VIP)-mediated disinhibition. Blockade of NPY Y1 receptors was sufficient to generate spike and wave discharges, whereas blockade of VIP receptors almost completely abolished this form of epileptiform activity. These data suggest that aberrant, activity-dependent neuropeptide corelease can have catastrophic effects on neocortical dynamics.

Central serotonin-2A (5-HT2A) receptor dysfunction in depression and epilepsy: the missing link?

5-Hydroxytryptamine 2A receptors (5-HT_2A-Rs) are G-protein coupled receptors. In agreement with their location in the brain, they have been implicated not only in various central physiological functions including memory, sleep, nociception, eating and reward behaviors, but also in many neuropsychiatric disorders. Interestingly, a bidirectional link between depression and epilepsy is suspected since patients with depression and especially suicide attempters have an increased seizure risk, while a significant percentage of epileptic patients suffer from depression. Such epidemiological data led us to hypothesize that both pathologies may share common anatomical and neurobiological alteration of the 5-HT_2A signaling. After a brief presentation of the pharmacological properties of the 5-HT_2A-Rs, this review illustrates how these receptors may directly or indirectly control neuronal excitability in most networks involved in depression and epilepsy through interactions with the monoaminergic, GABAergic and glutamatergic neurotransmissions. It also synthetizes the preclinical and clinical evidence demonstrating the role of these receptors in antidepressant and antiepileptic responses.

Advances on genetic rat models of epilepsy

Considering the suitability of laboratory rats in epilepsy research, we and other groups have been developing genetic models of epilepsy in this species. After epileptic rats or seizure-susceptible rats were sporadically found in outbred stocks, the epileptic traits were usually genetically-fixed by selective breeding. So far, the absence seizure models GAERS and WAG/Rij, audiogenic seizure models GEPR-3 and GEPR-9, generalized tonic-clonic seizure models IER, NER and WER, and Canavan-disease related epileptic models TRM and SER have been established. Dissection of the genetic bases including causative genes in these epileptic rat models would be a significant step toward understanding epileptogenesis. N-ethyl-N-nitrosourea (ENU) mutagenesis provides a systematic approach which allowed us to develop two novel epileptic rat models: heat-induced seizure susceptible (Hiss) rats with an Scn1a missense mutation and autosomal dominant lateral temporal epilepsy (ADLTE) model rats with an Lgi1 missense mutation. In addition, we have established episodic ataxia type 1 (EA1) model rats with a Kcna1 missense mutation derived from the ENU-induced rat mutant stock, and identified a Cacna1a missense mutation in a N-Methyl-N-nitrosourea (MNU)-induced mutant rat strain GRY, resulting in the discovery of episodic ataxia type 2 (EA2) model rats. Thus, epileptic rat models have been established on the two paths: ‘phenotype to gene’ and ‘gene to phenotype’. In the near future, development of novel epileptic rat models will be extensively promoted by the use of sophisticated genome editing technologies.

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.

Neonatal tricyclic antidepressant clomipramine treatment reduces the spike-wave discharge activity of the adult WAG/Rij rat

Recently it was revealed that the absence-like epileptic activity of the WAG/Rij (Wistar Albino Glaxo/Rijswijk) rat is associated with depression-like behavioural symptoms. Whether these depressive-like symptoms are accompanying epileptic activity (manifested in spike-wave discharges, SWDs, in the EEG) or whether they are causative for each other are open questions. Neonatally administered tricyclic antidepressant clomipramine is a well characterized animal model of major depression. It evokes behavioural symptoms of depression and changes sleep pattern in normal adult rats. We investigated whether in the WAG/Rij rat the neonatally administered clomipramine would aggravate the depression-like behavioural symptoms and the SWD activity. Male WAG/Rij pups from postnatal day 8 (PD8) to PD21 were treated with clomipramine (20mg/kg) or saline (control animals) twice daily intraperitoneally (i.p.). In the 8 months old rats, sleep parameters and sucrose solution intake (as hedonic index) as well as the SWD activity were measured. While the neonatal clomipramine treatment significantly increased the rapid eye movement sleep (REM) amount and decreased the sucrose preference score, it surprisingly attenuated the adult (8 months old) SWD activity. We concluded that neonatal clomipramine treatment produced aggravation of depression-like symptoms while decreased the SWD activity in the adult (8 months old) WAG/Rij rat.