Hippocampal theta rhythm in behaving rats following ibotenic acid lesion of the septum.

The effects of ibotenic acid lesion of the septum were studied in rats implanted with chronically indwelling electrodes and septal cannula. Each rat served as its own control and the properties of the hippocampal theta rhythm were studied before and after ibotenic acid and control saline infusion into the medial septal area. Ibotenic acid preferentially killed neurons in the lateral septum, and significantly attenuated the hippocampal theta rhythm about 50% bilaterally, at both surface and deep electrodes. The coherence and the phase of the theta rhythm at the CA1 apical dendrites, with respect to a superficial electrode, also declined significantly after ibotenic acid lesion. Pilocarpine (25 mg/kg i.p.) induced a theta rhythm of 7-9 Hz during immobility in the lesioned rats that was significantly higher in frequency than that induced in intact rats (4-6 Hz). In lesioned rats, the theta rhythm during tail pinch under urethane anesthesia was largely abolished, and the theta during walking was attenuated by atropine sulfate (50 mg/kg i.p.). Phencyclidine (10 mg/kg i.p.) or parachlorophenylalanine (PCPA) alone, which was inferred to abolish an atropine-resistant theta input, did not affect the power of the walking theta rhythm in either the lesioned or the normal rat. It was concluded that the theta in the behaving rats after ibotenic acid lesion in the septum has a strong atropine-sensitive component, and that it is not predominantly atropine-resistant, as suggested previously. The lack of PCPA effect on the theta phase in intact and lesioned rats also suggested a different view of the atropine-resistant theta in hippocampal region CA1. One possible mechanism of the atropine-resistant theta at the distal dendrites of pyramidal cells may result from rhythmic inhibition by stratum lacunosum-moleculare interneurons which may be activated by either serotonergic or cholinergic inputs.