ABSTRACT. The age-related changes in two types of theta rhythms recorded from the hippocampus in young (4 months-old), mature (12-13 months-old) and aged (22-25 months-old) rats were investigated. The type 1 theta rhythm was measured from hippocampal EEG recorded from walking rats and the type 2 theta was measured from the EEG induced by reticular pontin oralis nucleus (PON) stimulation in urethane anesthetized rats. The peak frequency and the peak power were detected from power spectra calculated on each theta sample by fast Fourier transformation (FFT). No age-related alteration was observed on the peak frequency of type 1 theta rhythm. However, on type 2 theta rhythm, the peak frequency was decreased in the aged rats compared with the young and the mature rats. The type 2 theta rhythm is cholinergic, and therefore this result suggests that age-related deterioration can be clearly observed in the cholinergic system including the hippocampus in rats.-KEY WORDS: aging, hippocampus, rat, theta rhythm.J. Vet. Med. Sci. 61 (5): [543][544][545][546][547][548] 1999 theta rhythms and aging remains unclear.In this study, we attempted to clarify the age-related changes in the two types of hippocampal theta rhythms associated with different pharmacological systems. MATERIALS AND METHODSMale Wistar rats supplied by SLC Co., Ltd. were used in this study. For the measurement of the type 1 theta, three aged rats (22-25 months-old), four mature rats (12-13 months-old) and four young rats (3-4 months-old) were used. For the type 2 theta, four aged rats, four mature rats and three young rats were used. Aged and mature rats were purchased as retired rats (6-7 months-old) and housed in our animal facility for 6 months and 18 months respectively. The animal facility was maintained on a 12 hr light/dark cycle, at 21 ± 1°C and 50-60% humidity.The rats used for the type 1 theta experiments were deeply anesthetized by pentobarbital sodium (50 mg/kg, i.p.) and then placed into a stereotaxic frame. A pair of staggered electrodes (200 µm stainless steel wires insulated except at cut ends) were chronically implanted in the bilateral hippocampal formation. The deep electrode was directed at the molecular layer of the dentate gyrus (DG), and the surface electrode at the CA1 pyramidal cell layer. Two stainless steel screw electrodes for reference and ground were fixed in the skull above the cerebellum. After 1 week of recovery from surgery, the type 1 theta recordings were carried out. The rats were placed in an open field box (100 × 70 × 33 cm) without a roof, which was painted gray. The hippocampal EEG and the behavioral data in the freely moving rats were recorded and stored simultaneously on a video tape using a video converter system (NEC Sanei) which received the signals from a multichannel amplifier and a CCD camera. Walking was selected as a type 1 theta

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