Prolonged Attenuation of Amygdla Kindled Seizures by Local Delivery of Large Molecular Weight Therapeutic Agents by Convection-Enhanced Delivery in Rats

Many patients with temporal lobe epilepsy (TLE) respond inadequately to antiepileptic drugs, necessitating resective surgery as a last resort. An alternative less invasive approach is site-specific delivery of therapeutic agents into the epileptic focus. However, conventional intraparenchymal injection produces localized tissue damage and there is poor control over the drug distribution. Convection-enhanced delivery (CED) is an alternative infusion technique that allows for the nondestructive and well-controlled localized delivery of solutes, including high molecular weight agents (e.g., peptides, proteins, and even viral vectors). The present study examined whether long-term changes in amygdala-kindled seizure susceptibility can be produced by a one-time CED infusion into the amygdala. Agents selected for study were presynaptic neurotoxins capable of producing long-term attenuation of neurotransmitter release either by inhibiting N-type calcium channels (i.e., ω-conotoxins GVIA and MVIIA) or cleaving intracellular proteins involved in exocytosis (i.e., botulinum toxins type A and B). A conventional, small-molecule antiepileptic drug, carbamazepine, was used for comparison.

Each rat was implanted with a combination infusion cannula and stimulation electrode into the right basolateral amygdala. Daily stimulations were carried out until the animals were fully kindled (stage 5 seizures for at least five consecutive days). Then, the rats received infusions of vehicle alone or ω-conotoxins (5-500 pmol), botulinum toxins (7-67 fmol), or carbamazepine (500 nmol) through the cannula whose tip was adjacent to the stimulation site. Each dose was delivered in a volume of 5 μL at a rate of 0.25 μL/min. Electrophysiological (afterdischarge threshold and duration) and behavioral (seizure stage and duration) measures of amygdala-kindled seizures were recorded daily for up to 64 days after the infusion.

There were no changes in any of the kindling parameters in rats that had received vehicle infusions. In contrast, infusions of ω-conotoxins and botulinum toxins resulted in a dose- and time-dependent increase in the afterdischarge threshold and a decrease in seizure stage and duration, indicting an inhibitory effect on kindled seizure expression. The protective effects of ω-conotoxins reached a maximum at 48 h postinfusion and then gradually dissipated within the next five days; in contrast, effects of botulinum toxins lasted for several weeks. Infusions of each toxin appeared well tolerated and devoid of any immediate or long-term side effects. In marked contrast to the long-lasting effects produced by ω-conotoxins and botulinum toxins, attenuation of seizures produced by carbamazepine lasted only a few hours after the infusion.

These results indicate that local CED-mediated delivery of high molecular weight presynaptic toxins can produce long-lasting anticonvulsant effects that persist for weeks. Whether this approach will be useful in the treatment of human epilepsy or other brain disorders requires further investigation.