AMPA receptors in epilepsy and as targets for antiepileptic drugs.

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are key mediators of seizure spread in the nervous system and represent promising targets for antiepileptic drugs. There is emerging evidence that AMPA receptors may play a role in epileptogenesis and in seizure-induced brain damage. This evidence suggests that AMPA receptor antagonists could have broad utility in epilepsy therapy. Regional, developmental, and disease-associated variations in AMPA receptors produced by differential expression of AMPA receptor subunits and variations in posttranscriptional processing, including alternative splicing and pre-mRNA editing, provide a diversity of functionally distinct AMPA receptor isoforms that allow opportunities for selective drug targeting. Four types of AMPA receptor antagonist are discussed in this chapter: (a) competitive AMPA recognition site antagonists, including those of the quinoxalinedione and newer nonquinoxalinedione classes, (b) 2,3-benzodiazepine noncompetitive (allosteric) antagonists, (c) desensitization enhancing antagonists, exemplified by SCN-, and (d) antagonists of Ca(2+)-permeable AMPA receptors, including polyamine amide arthropod toxins and their synthetic analogues. Competitive and noncompetitive AMPA receptor antagonists are broad-spectrum anticonvulsants in animal seizure models. Their effectiveness and safety for humans remain to be determined. There is evidence that these antagonists can potentiate the antiseizure activity of N-methyl-D-aspartate (NMDA) receptor antagonists and conventional antiepileptic drugs. This evidence suggests that the preferred use of AMPA receptor antagonists may be in combination therapies. Agents that enhance desensitization may have advantages in comparison with other AMPA receptor antagonists to the extent that they preferentially block high-frequency synaptic signaling and avoid depressing AMPA receptors on interneurons, which would lead to disinhibition and enhanced excitability. Evidence has accumulated that Ca(2+)-permeable AMPA receptors (those lacking the edited GluR2 subunit) may play a role in epileptogenesis and the brain damage occurring with prolonged seizures. Because Ca(2+)-permeable AMPA receptors are predominately expressed in gamma-aminobutyric acid (GABA)ergic interneurons, it is hypothesized that some forms of epilepsy might be caused by reduced GABA inhibition resulting from Ca(2+)-permeable AMPA receptor-mediated excitotoxic death of interneurons. It is further proposed that drugs that selectively target Ca(2+)-permeable AMPA receptors might have antiepileptogenic and neuroprotective properties. Certain polyamine toxins and their analogues are channel-blocking AMPA receptor antagonists that selectively inhibit Ca(2+)-permeable AMPA receptors. These substances might give clues to the development of such antagonists.