Hippocampal theta rhythm after serotonergic activation of the pedunculopontine tegmental nucleus in anesthetized rats.

The pedunculopontine tegmental nucleus (PPN), as a part of reticular formation activating system, is thought to be involved in the sleep/wake cycle regulation, and plays an important role in the generation and regulation of hippocampal rhythmical slow activity. The activity of PPN can be modulated by serotonergic system, mainly through multiple projections from raphe nuclei, which can influence PPN neurons through different classes of 5-HT receptors. In the present study, the effect of intra-PPN injection of two serotonin agonists: 8-OH-DPAT and 5-CT, on hippocampal formation EEG activity was examined in urethane-anesthetized rats. The study found that the microinjections induced prolonged spontaneous theta rhythm in both hippocampi with a short latency. The results obtained suggest that local inhibition of presumably cholinergic neurons in the PPN acts as a trigger for hippocampal theta activity.