Two different anticonvulsant actions of tiagabine in developing rats

Padsevonil suppresses seizures without inducing cell death in neonatal rats

BACKGROUND

Padsevonil (PSL) is a rationally designed anti-seizure medication (ASM) which has overlapping mechanisms of action with the two most common ASMs used for neonatal seizures, phenobarbital (PB) and levetiracetam (LEV). Here we evaluated the anti-seizure properties of PSL across the neonatal and adolescent period in rats in the pentlyenetetrazole (PTZ) induced seizures model.

METHODS

Postnatal day (P)7, P14 and P21 Sprague-Dawley rat pups were pre-treated with PSL (1-30 mg/kg), and assessed for seizure latency and severity 30 min later following injection of PTZ. A separate cohort of P7 pups were treated with neonatal ASMs and euthanized 24 h later (on P8) to assess induction of cell death, a feature common to many ASMs when given to P7 rodents. This effect has been extensively reported with PB, but not with LEV. Cell death was assessed by PathoGreen staining.

RESULTS

PSL suppressed PTZ-evoked seizures across multiple age groups, particularly at higher doses, without producing increased cell death compared to vehicle. The effects of PSL were particularly notable at suppressing tonic-clonic seizure manifestations (82% of P7 and 100% of P14 and P21 animals were protected from tonic-clonic seizures with the 30 mg/kg dose).

CONCLUSIONS

PSL displayed dose-dependent anti-seizure effects in immature rodents in the PTZ model of seizures in immature rats. While many ASMs, including PB, induce cell death in neonatal rats, PSL does not. This suggests that PSL may offer therapeutic benefit and a favorable safety profile for the treatment of neonatal seizures.

Cenobamate suppresses seizures without inducing cell death in neonatal rats

GABA modulators such as phenobarbital (PB) and sodium channel blockers such as phenytoin (PHT) have long been the mainstay of pharmacotherapy for the epilepsies. In the context of neonatal seizures, both PB and PHT display incomplete clinical efficacy. Moreover, in animal models, neonatal exposure to these medications result in neurodegeneration raising concerns about safety. Cenobamate, a more recently approved medication, displays unique pharmacology as it is both a positive allosteric modulator of GABA-A receptors, and a voltage-gated sodium channel blocker. While cenobamate is approved for adult use, its efficacy and safety profile against neonatal seizures is poorly understood. To address this gap, we assessed the efficacy and safety of cenobamate in immature rodents. Postnatal day (P)7 rat pups were pretreated with cenobamate and challenged with the chemoconvulsant pentylenetetrazole (PTZ) to screen for anti-seizure effects. In a separate experiment, P7 rats were treated with cenobamate, and brains were processed to assess induction of cell death. Cenobamate displays dose-dependent anti-seizure efficacy in neonatal rats. Unlike PHB and PHT, it does not induce neurotoxicity in P7 rats. Thus, cenobamate may be effective at treating neonatal seizures while avoiding unwanted neurotoxic side effects such as cell death.

Anticonvulsant action of allopregnanolone in immature rats

Anticonvulsant activity of allopregnanolone, a neurosteroid allosterically modulating GABA(A) receptor was tested in a model of motor seizures elicited by pentetrazol in immature rats. Rats 7, 12, 18, 25 or 90 days old were pretreated with allopregnanolone in doses from 5 to 40 mg/kg i.p. and 15 min later pentetrazol was injected subcutaneously in a dose of 100 mg/kg. Rats were observed in isolation for 30 min. Allopregnanolone dose-dependently suppressed both generalized tonic-clonic and minimal clonic seizures with the highest efficacy in 12-day-old rats. Anticonvulsant action was least expressed in adult animals. Duration of anticonvulsant action tested after a dose of 20 mg/kg in 12- and 90-day-old rats demonstrated markedly longer effects in young rats. Allopregnanolone compromised motor performance of rats but duration of this unwanted effect in 12-day-old rats was shorter than duration of anticonvulsant action. This difference can be important for possible clinical use of neurosteroids.

Mechanisms of action for the commonly used antiepileptic drugs: relevance to antiepileptic drug-associated neurobehavioral adverse effects

Antiepileptic drugs exert their anticonvulsant effects by interfering with brain processes that involve structures that are also involved in learning, memory, and emotional behavior. Thus, modulation of ion channels, neurotransmitters, second messengers, and other processes by antiepileptic drugs, although helpful in controlling seizures, can interfere with normal brain function in undesired ways. The specific mechanism(s) of action of an antiepileptic drug can increase the risk for particular types of adverse events. In this review, we examine the cognitive and behavioral effects of antiepileptic drugs in animal models. Although animal studies, in many respects, do not mimic clinical experience, the data suggest a connection between certain mechanisms of antiepileptic action and the occurrence of cognitive adverse effects. Specifically, antiepileptic drugs with traditional gamma-aminobutyric acid (GABA)ergic mechanisms have the most detrimental effects on cognitive function, possibly because they impair attention. Conversely, drugs with the predominant effects at Na+ channels appear to have minimal impact on cognition. Levetiracetam, with its nonconventional GABAergic and Ca2+ channel effects, has shown positive cognitive effects in animal studies. Antiglutamatergic drugs have the potential to be a double-edged sword: they can interfere with consolidation of learning and memory but can also provide neuroprotection in addition to their antiseizure effects.

GABA-B receptor activation in the rat globus pallidus potently suppresses pentylenetetrazol-induced tonic seizures

To determine the involvement of the globus pallidus in mediating epilepsy, the effects of microinjection of a GABA uptake blocker (tiagabine), a benzodiazepine agonist (zolpidem) and a GABA-B receptor agonist (baclofen) on pentylenetetrazol (PTZ)-induced tonic seizure were examined in adult rats. Administration of PTZ induced tonic seizures in all control animals, accompanied with a 100% mortality rate. Pretreatment with bilateral intrapallidal microinjection of tiagabine (1 mM) suppressed the incidence of tonic seizures to 67.7% and reduced the mortality rate to 16.7%. Furthermore, the latency to tonic seizures was 1,275 +/- 277 s, which was significantly longer than that of the corresponding control animals (319 +/- 225 s). On the other hand, administration of zolpidem (1 mM) was without significant effects on the mortality rate, the incidence and latency of the tonic seizure. In sharp contrast, microinjection of baclofen (1 mM) completely suppressed PTZ-induced tonic seizures and reduced the mortality rate to 0%. This effect was largely abolished by co-injection of the GABA-B receptor antagonist CGP55845. To elucidate the direct cellular action of baclofen, the excitability and membrane potential of globus pallidus neurons were studied by cell-attached and whole-cell patch-clamp recordings in the brain slice. Bath application of baclofen (50 microM) significantly reduced the firing of these neurons, and was often accompanied by a clear membrane hyperpolarization, in a CGP55845-sensitive manner. These data suggest that activation of GABA-B receptors in globus pallidus could significantly modulate PTZ-induced tonic seizures.