Abstracts

Rationale: 5-HT₆ receptor antagonists have been shown to improve memory in wild-type, aged, and amnestic rodents, as well as patients with Alzheimer's disease (AD). 5-HT₆ receptor antagonists also have exhibited anticonvulsant effects in the rat maximal electroshock test. The mechanisms that underlie the procognitive and anticonvulsant effects of 5-HT₆ receptor antagonists remain unclear, and their effects in multiple seizure models are unreported. We hypothesized that 5-HT₆ receptor activation enhances inhibitory synaptic transmission in the dentate gyrus (DG), and that 5-HT₆ receptor antagonists are paradoxically anticonvulsant in rodents. Methods: For minimal clonic seizure testing, mice were stimulated at a previously determined strength that caused seizures in half of the male C57Bl6 or CF1 mice (6.5 or 6.8 mA, respectively, at 60 Hz for 0.2 sec.). To test the effects of 5-HT₆ receptor activation on synaptic transmission, coronal brain slices containing dorsal hippocampus were prepared from naïve rats. Field potential recordings and patch-clamp electrophysiology were used to test the effects of a 5-HT₆ receptor agonist WAY-208466 (1µM) on baseline field excitatory postsynaptic potentials (fEPSPs), evoked and spontaneous inhibitory postsynaptic currents (eIPSCs, sIPSCs). Drug effects on seizure thresholds and neurotransmission were compared with the Fisher exact test, Student's t-test or one-way ANOVA. Results: The 5-HT₆ receptor antagonist SB-399885 (10mg/kg, 30 min, i.p.) was proconvulsant in C57Bl6 mice (vehicle: 11/18 seized; drug: 16/16 seized). SB-399885 and another antagonist SB-271046 (10mg/kg, 30 min, i.p.) had no effect on seizures in CF1 mice. Additionally, the 5-HT₆ receptor agonist WAY-208466 significantly decreased the amplitude of fEPSPs in the inner molecular layer of the DG (90±4.5%, N=12, p<0.05) in brain slices from naïve rats. WAY-208466 also increased the amplitude of eIPSC (151.8±13.2%, N=9, p<0.01) onto DG granule cells (GCs) without affecting paired-pulse plasticity; the effect on amplitude was lost when fast glutamatergic synaptic transmission was blocked pharmacologically. Additionally, WAY-208466 had no effect on the frequency or amplitude of pharmacologically isolated sIPSCs measured in DG GCs, but did increase the amplitude of sIPSCs >100pA (drug: 171.8±3.7 pA; baseline: 135.6±1.40 pA, N=8, p<0.001). Conclusions: Contrary to earlier reports, the 5-HT₆ receptor antagonist SB-399885 exhibited proconvulsant effects in the minimal clonic seizure test in C57BL6 mice; future studies will test the pharmacokinetic-, pharmacodynamic-, and species-dependence of this effect. The 5-HT₆ receptor agonist WAY-208466 attenuated the amplitude of fEPSPs and increased the amplitude of eIPSCs onto DG GCs. The latter effect was abolished when fast glutamatergic neurotransmission was blocked pharmacologically despite an increase in sIPSCs >100pA. Together, these results suggest that 5-HT₆ receptor antagonists, which are in clinical trials in AD, have complex effects on the balance of excitation and inhibition in the DG and may affect seizure liability in AD patients.