Chemoconvulsant Test

Brief Description

Assessment of the effect of an investigational compound against the chemoconvulsants, bicuculline and picrotoxin.

Introduction

Excitatory and inhibitory neurotransmission plays a critical role in mediating normal neuronal signaling, and an imbalance between these two pathways can contribute to the onset of seizures, and ultimately epileptogenesis. Chemically disrupting this finely tuned balance can artificially induce a seizure. Thus, in addition to the intravenous Metrazol seizure threshold test (i.v. Met, Test 15) and the subcutaneous pentylenetetrazol seizure susceptibility test (s.c. Met), the ETSP employs additional chemoconvulsant tests wherein a seizure is induced by the GABAA antagonist, bicuculline, and the GABAA chloride-channel blocker, picrotoxin. Bicuculline appears to induce seizures by disrupting normal inhibitory neurotransmission (i.e., weakening synaptic inhibition) through GABA receptor disruption [1, 2]. Additionally, picrotoxin acts on GABAA receptors through a noncompetitive antagonism of the chloride channel, stabilizing the GABAA receptor-coupled Cl-channel in a non-conducting state and thus reducing GABAergic inhibitory tone [3]. For these reasons, inhibitory neurotransmission plays a critical role in seizure onset and susceptibility. Bicuculline and picrotoxin, through their ability to disrupt the GABAergic pathway provides the ETSP with two additional acute seizure tests; i.e., the s.c. bicuculline (BIC) or s.c. picrotoxin (PIC) tests that can be used for evaluating the anticonvulsant potential of investigational ASDs.

Methods

The test compound is given at or below the TD50 dose determined in Test 4 and tested at the s.c. Met time of peak effect (TPE). This test measures the ability of the test substance to prevent a clonic seizure produced by the s.c. injection of either BIC (2.7 mg/kg) or PIC (2.5 mg/kg). Following the administration of BIC, CF1 mice are placed in isolation cages and observed for 30 min for the presence or absence of a seizure; those receiving PIC are observed for 45 min because of the slower absorption of this convulsant. BIC- and PIC-induced seizures typically consist of an episode of clonic spasms of the fore- and hind limbs, jaws and vibrissae. BIC-induced clonic seizures are generally followed by tonic extension of the hind limbs and death.

Results

For effective compounds, activity will be quantitated in groups of 8 animals and the ED50 and 95% confidence interval of the test compound will be determined by probit analysis. The duration of protection can also be determined at the request of the COR.

Discussion

At present, no single ETSP screen can solely predict the clinical potential of an investigational compound. Thus, the ETSP utilizes a battery of tests to characterize the anticonvulsant potential of an investigational compound. Bicuculline and picrotoxin are two widely-used chemoconvulsant models of seizure susceptibility and seizure threshold. These compounds produce generalized clonic seizures as a result of their selective disruption of normal GABA-mediated synaptic inhibition [1-3]. The results obtained with the investigational ASDs are often compared with those obtained with a number of clinical available ASDs, which are summarized in the table below. A review of the data summarized in this table shows some interesting trends. For example, only three of the ASDs are active, at non-toxic doses, in both the s.c. Bic and s.c. Pic tests; i.e., ethosuximide, phenobarbital, and valproic acid. Ezogabine was active in both tests; albeit at doses that produced behavioral impairment on the rotorod. Levetiracetam was only active in the s.c. Bic test; whereas carbamazepine and tiagabine were only active in the s.c. Pic tests. In contrast, gabapentin, lacosamide, lamotrigine, and topiramate were all inactive in both tests. Thus, these tests provide another mechanism to differentiate the antiseizure activity of investigational ASDs from that of marketed ASDs. Whether those compounds that are effective in other ETSP screens are also found to elevate seizure threshold in the subcutaneous bicuculline and picrotoxin tests may provide therapeutic efficacy in the clinical management of epilepsy but will necessarily have to await the results of any ongoing or planned clinical trials in the patient with epilepsy.