Pharmacological Characterization of EEG Spikes and Seizures Induced by a Specific Calcium‐Permeable AMPA Receptor Antagonist, 1‐Naphthylacetyl Spermine (1‐NA‐Spm)
Authors: Takazawa Akira , Yamazaki Osamu , Kinoshita Masakazu , Ishida Nobuya , Kato Nobumasa , Yarnauchi Toshio 
Published: August 2005

Purpose,: It is known that AMPA receptor has a crucial role in seimre generation and epileptogcnesis. Joro spider toxin (JSTX) sclectively blocks the calciurn‐permeable AMPA receptor a s a channel blocker. We have reported that a synthetic JSTX analoguc, 1 ‐NA‐Spin, suppresses amygdaloid and hippocampal kindled seizures in a dosedependent manner. This analogue induces EEG spikes both on the kindled animals and on the niiivc animals. The spike‐inducing effect of I ‐NA‐Spm is not directly related to the anticonvulsant effects of this drug, because time‐course of the inhibitory effects on the kindled seiurcs is different from that of the spike‐inducing effect. The drug soinctiines produces myoclonus and generalized seizures coinciding with the spike induction. To elucidate the mechanism underlying these proconvulsant cffects, participants of the AMPA, NMDA, and GABA transmitter systems to the I ‐NA‐Spin induced effects were tested.

Methods: Adult male Wistar rats were implanted with an injection cannula into the lelt ventricle and recording clectrodes in the right hensory‐motor cortex, amygdala, and CA3 of the hippocampus under pentobarbital anesthesia 1 week before the experiment. On the experiment day, NBQX (2,3‐dihydroy‐6‐nitro‐7‐sulfaiiioylbeiizo(F) quinoxalinc, 40 mg/kg, i.p.). a competitive antagonist of all the AMPA receptor subtypes, CPP (3‐((+/‐)‐2‐carboxypiperain‐4‐yl)‐propyl‐l‐phosphonic acid, 10 mg/kg, i.p.). a competitive antagonist of NMDA receptor, or midazolam (5 mg/kg, i.p.), 21 short‐acting benzodiazepine agonist was administered 30 minutes, 90 minutes, or IS minutes before I‐NA‐Spin (40 pg) infusion into the ventricle, respectively. The seizure severity is quantified by the seizure behavioral scale as scale I: EEG spikes without any motor manifestation, scale 2: myoclonic jerks, scale 3: spontaneous motor seizures, and scale 4: motor seizure status.

Results: I‐NA‐Spin induced EEG spikes in all animals tested, which initially appeared in the CA3 within a few minutes after the administration then propagated into the amygdala and the motor cortcx. The induced EEG spikes reached a peak 30 minutes after the drug administration and lasted for 2 hours. The myoclonic jerks coincided with the EEG spikes in all 1‐NA‐Spin treated animals. The value of seimre rating scale of I‐NA‐Spm treated animals was 2.40.2 (mean and S.E.,N = 8). The pretreatment of NBQX almost completely suppressed the myoclonus and pirtly inhibitcd the EEG spikes (I.3i0.2, N =9). The midazolam pretreatment also suppressed both spikes and seizure behaviors (I,820.3, N =4), but the inhibitory effects were relatively weak and short lasting compared to NBQX. The pretrcatmcnt of CPP, however, aggravated not only the EEG spikes but the seizures induced by I‐NA‐Spm (3320.5, N=4). The induced seizures progressed to the motor seizure status in ii half of the CPP pretreated animals.

Conclusions: The proconvulsant effects of 1 ‐NA‐Spm should be due to a block of the calcium‐permeable AMPA receptor, because the pretreatment of NBQX that blocks both the calcium‐permeable and the calcium‐impermeable AMPA receptors remarkably inhibited these proconvulsant effects. It is reported that the calcium‐permeable AMPA receptor mainly exists on GABAergic interneurons and thew neiirons receive excitatory inputs through the calciuin‐perincable AMPA and the NMDA receptor systems. The present results suggest that I ‐NA‐Spin decreases the excitatory input LO the interneurons and induces GABAergic disinhibition that causes the proconvulsant eflects. The hypothesis is possibly supported by the result that thc GABAcrgic augmentation by midazolam inhibited the proconvulhant efrccts of the drug. This could be also supported by the fact that the block of NMDA system, which is another important excitatory input to GABAergic interncurons, exaccrhatcd the I ‐NA‐Spm induced EEG spikes and seizures, whereas NMDA antagonism usually suppresses seizure induction in other seizure models.