Abstracts

Rationale: In vivo studies have shown that several rhythms can be found in hippocampus. Hippocampus receives cholinergic projection from medial septum and diagonal band, and acetylcholine has critical role on hippocampal rhythms in rat. Hippocampus is also known as a site that tends to be a focus of seizure. Interestingly, it is reported that theta rhythm has anticonvulsant effect in rat. We confirmed whether similar effect occurs in rat hippocampal slices. Especially we focus on the relationship of acetylcholine receptor activation and anticonvulsant effect. Methods: Extracellular recording by glass electrode (2M NaCl; 1-2 Mohm) was carried out in stratum.pyramidale in CA3 region using Wistar rat hippocampal slices. The study was carried out in compliance with the Guide for the Care and Use of Laboratory Animals at the Graduate School of Life Science and Systems Engineering of Kyushu Institute of Technology. Results: Carbachol, an acetylcholine receptor agonist, induces some different oscillations in hippocampal slices depending on the concentration. High concentration of carbachol (30-100μM) produced short episodes of discharges. As the mean frequency of discharge was 15.5 +- 0.3 (mean+-s.e.m.; n=30) at 30μM, we called the oscillation beta oscillation. Carbachol (30μM)was applied to the several models of epileptiform discharges. First, SR95531 or picrotoxin induced the epileptiform discharges in hippocampal slices with the inhibition of GABAA receptors. Application of carbachol on the epileptiform discharges induced beta oscillation and suppressed them. Next, we applied carbachol to high potassium-induced epileptiform discharges. The application abolished high the epileptiform discharges. At this case, however, carbachol did not induce beta oscillation. Therefore, we thought that the activation of acetylcholine receptors itself may play an important roles on the suppression of epileptiform discharges. Hence, in the present study, the effect of low concentration of carbachol was tested in picrotoxin- induced epileptiform discharges. Carbachol at low concentration (1-5μM) increased the frequency of epileptiform discharges and decreased the amplitude in a concentration-dependent manner. These changes in frequency and in amplitude would reflect the desynchronization of the epileptiform discharrges. Carbachol decreased population excitatory postsynaptic potential (pEPSP) in a concentration-dependent manner as shown in our previous study. In addition, picorotoxin-induced epileptiform discharges were not inhibited by the co-application of both carbachol and atoropine muscarinic acetylcholine receptor antagonist. Conclusions: Therefore these results suggest the following mechanism. Activation of muscarinic acetylcholine receptors reduces EPSP and it will desynchronize the population of pyramidal neurons in CA3 region. The desynchronization will induce the suppression effects on the epileptiform discharges induced in hippocampal slices.