Synchronous neuronal activity in the hippocampus (theta rhythm) can be elicited in urethanized rats with sensory stimulation as well as with electrical or pharmacological stimulation of different nuclei of the brainstem. It is known that two of these nuclei, the nucleus pontis oralis (RPO) and the pedunculopontine nucleus (PPN), play an important role in theta regulation, however, it is still unclear which of them is essential for expression of theta in the hippocampus. In the present study we investigated the effect of temporal inactivation of PPN on the hippocampal theta rhythm induced by electrical stimulation applied to RPO. The experiments were performed on 5 male Wistar rats in deep urethane anesthesia with its level monitored on the basis of breathing rate. Animals were implanted with bilateral recording electrodes into the dorsal hippocampus and stimulation electrode into the RPO. Hippocampal EEG was recorded during repeated electrical stimulation of RPO in control conditions and also following intra-PPN administration of procaine. In all animals electrical stimulation of the RPO (200 - 300 mA, 30 s) induced episodes of robust hippocampal theta rhythm in both hippocampi which lasted for the whole period of the electrical stimulation (30 s) with no latency. After temporal inactivation of the PPN by direct procaine microinjection (20% solution/0.5 μl), electrical stimulations of the RPO were not able to induce synchronous activity in the hippocampus. Neuronal activity within the RPO and PPN nuclei changes during sleep/wake cycle including paradoxical sleep, of which hippocampal theta rhythm is an important indicator. Regular theta rhythm in the hippocampus is also present during urethane anesthesia which was applied in our experiments. Our results indicate that undisturbed neuronal activity within the PPN is crucial for evoking hippocampal theta rhythm with electrical stimulation of RPO, which suggests superior role of the PPN.
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