Ontogenic profile of seizures evoked by the beta-carboline DMCM (methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate) in rats

Abstract

The beta-carboline, methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate (DMCM), is a potent chemoconvulsant. While it has been utilized in adult rodents, it has not been previously examined for effects across postnatal development. DMCM is a negative allosteric modulator of benzodiazepine-sensitive GABAA receptors, receptor subtypes that are particularly enriched in limbic brain regions. This raises the possibility that DMCM may be particularly effective at evoking forebrain seizures, which is a challenge in neonatal animals due to the relative immaturity of the forebrain seizure network. The ability to selectively evoke forebrain seizures is desirable when screening for drugs to use in temporal lobe epilepsy, which is characterized by seizures within the forebrain (limbic) network. To determine the profile of DMCM action across development, we examined the dose-dependent ability of DMCM to induce seizures in rats at P7, P10, P13, P14, P21 and in adulthood. We found that the highest sensitivity to DMCM occurred in P10, P13, and P14 rats. The lowest sensitivity occurred in P21 rats. Neonatal (P7) and adult (P60+) rats displayed moderate sensitivity. With moderate (0.2–0.4mg/kg) doses of DMCM, we were able to reliably evoke limbic motor seizures without tonic-clonic components in animals as young as P7. These data support the utility of DMCM in assessing seizure threshold during development and raise the possibility for future exploration of DMCM as an agent to screen anticonvulsant drugs during the postnatal period.

Profile of anticonvulsant action of levetiracetam, tiagabine and phenobarbital against seizures evoked by DMCM (methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate) in neonatal rats

“…In the pentylenetetrazole model, “minimal” (primarily clonic) seizures are rarely observed in young animals (i.e., before P18; Forcelli et al, 2012b; Kubova and Mares, 1991). With DMCM, clonic seizures can be evoked reliably in animals as young as P7 (Kulick et al, 2014). Thus, suppression of the tonic-clonic component of seizures evoked by pentylenetetrazole may unmask less severe seizure manifestations.…”

“…DMCM was administered intraperitoneally at a dose of 300 µg/kg. The dose of DMCM was selected on the basis of our previous report establishing a dose-response for DMCM in neonatal rats (Kulick et al, 2014). LEV [ (S)-2-(2-oxopyrrolidin-1-yl)butanamide], as Keppra oral solution (UCB Pharma) was diluted from the stock concentration of 100 mg/ml in normal saline, and administered at doses of 0 (vehicle), 10, 50, or 100 mg/kg.…”

“…Pup seizures were rated using a 5-point scoring system modified from Kubová and Mares (1991) as previously described (Forcelli et al, 2013, 2012b; Kulick et al, 2014): 0 = no change in behavior; 0.5 = scratching, chewing, tremors, wet dog shakes; 1 = 0.5+myoclonic jerks; 2 = unilateral clonus (threshold limbic motor seizure), chewing/shuffling, Straub tail; 3 = bilateral facial and forelimb clonus (FFC); 4 = clonic seizure with loss of righting; 4.5 = clonic seizure with running/bouncing (swimming-like behavior); 5 = tonic-clonic and tonic extensor seizure. Latency to seizure onset was defined as the time elapsed between drug injection and the first behavioral seizure response.…”

“…This compound is a member of the beta-carboline family of compounds and acts as negative allosteric modulator at the benzodiazepine-binding site on the GABA A receptor. We have recently documented the profile of DMCM-evoked seizures in developing rat pups (Kulick et al, 2014). Because DMCM offers a wider separation between the doses needed to evoke partial and generalized seizures than do other chemoconvulsants (e.g., pentylenetetrazole), we hypothesized that it would be an ideal model to screen drugs with preferential efficacy against partial seizures.…”

“…LEV and TGB are without effect against seizures in several common screening models (e.g., the maximal electroshock test [MES], maximal pentylenetetrazole seizures), instead showing preferential efficacy against models of partial seizures. We have recently described a method for reliably evoking partial seizures in neonatal animals by systemic administration of the chemoconvulsant, DMCM (Kulick et al, 2014). DMCM is a negative allosteric modulator of GABA A receptors, and offers a wide separation between doses required to evoke complex partial as compared to tonic-clonic seizures.…”