Abstracts

Rationale: Seletracetam (SEL) is a pyrrolidone derivative, with a 10-fold greater affinity for synaptic vesicle protein 2A (SV2A) compared to the structurally related drug levetiracetam (LEV). Previous experiments demonstrated that LEV reduces synaptic transmission in a time- and stimulation frequency-dependent fashion. This effect of LEV appears to be mediated by synaptic vesicle recycling. To determine whether this LEV effect on synaptic transmission was mediated through SV2A binding, we investigated whether SEL replicated the LEV effect, but at lower concentrations. Methods: We measured extracellular evoked responses in the CA1 region in 500 ?m rat hippocampal slices after paired pulse stimulation or train stimulation of up to 20 pulses, applied at 5-40 Hz. Three different treatments were used to manipulate synaptic activity and vesicular endocytosis of SEL: 1.) a single cut across CA2 to reduce activity in CA3 synaptic terminals; 2.) addition of 100 M 4-Aminopyridine (4AP) into the incubating solution to increase vesicular release; and 3.) delivery of 1Hz train stimulation (600 stimuli) to slices. A 1Hz stimulation train (600 stimuli) was used to unload SEL after removing it form the extracellular bath.Results: When slices were briefly exposed to SEL (1, 3, 10, and 30 M) for 30 minutes, we found no alteration on synaptic responses by paired pulse or train stimulation. When the slices were pre-incubated in SEL (>3 M; n =10, see figure) for 3 hours, no change was found for paired pulse facilitation, but there was a significant, frequency and concentration dependent, reduction in late synaptic potentials in response to 20 - 40 Hz stimulus trains. When spontaneous CA3 activity was decreased by cutting across CA2, the effect of SEL was significantly reduced even after 3 hours. Conversely, when we increased synaptic activity by adding 4AP into the incubation solution or delivering 600 stimuli (1Hz) to the slices, the synaptic responses were significantly decreased after a 30 min incubation period. The effect of a 3 hour SEL incubation on synaptic responses could be eliminated by unloading (600 stimuli at 1 Hz) in the absence of SEL.Conclusions: Our data suggest that SEL resembles LEV in its effect on synaptic transmission, but is more potent in decreasing synaptic transmission than LEV. These results are consistent with our hypothesis that these drugs alter synaptic transmission by binding SV2A and that their access to SV2A is dependent upon ongoing synaptic transmission.