The effect of diazepam on hippocampal EEG in relation to behavior

Male mice of the BALB/cByJ and C57BL/6 strains were implanted with electrodes in the CA1 area of the hippocampus to record rhythmic slow-wave activity (RSA) or 'theta' EEG activity. The EEG spectral characteristics and the animal's motor behavior were studied while the animals walked on a moving belt (2.2 cm/s) both before and after i.p. injections of diazepam (Valium, 2 mg/kg) or vehicle. EEG spectral analyses were carried out on-line by computer. Diazepam produced a dissociation of locomotion and RSA. (1) Uninjected and vehicle-injected mice showed typical RSA (7-8 Hz) while walking. (2) Under diazepam, 7-8 Hz RSA virtually disappeared and was replaced in the temporally averaged records by RSA with a sharp, narrow-band peak at 4-5 Hz. (3) This lower-frequency RSA was associated with immobility if, and only if, the immobility immediately followed walking. This was true whether the animal itself stopped walking or the experimenter stopped the moving belt. This theta activity predominated for about 30 s and had disappeared after 2 min. Locomotion, on the other hand, was accompanied by irregular EEG activity. (4) Scopolamine (i.p. 1 mg/kg), a cholinergic blocker, greatly reduced the diazepam-induced 4-5 Hz RSA, but also partially restored 7-8 Hz RSA. The possibility that the effects of diazepam on hippocampal EEG involve changes in septohippocampal cholinergic activity is discussed.