Behavioural Brain Research, 31 (1988) 75-84 75 Elsevier BBR00849 Hippocampal interictal spikes induced by kindling" relations to behavior and EEG Lai-Wo Stan Leung Departments of Clinical Neurological Sciences and Physiology, University of Western Ontario, London, Ont. (Canada) (Received 11 February 1988) (Revised version received 29 March 1988) (Accepted 7 April 1988) Key words: Hippocampus; Kindling; Interictal spike; Sleep; Theta rhythm; Irregular slow activity; Medial septum Hippocampal spontaneous interictal spikes (SISs) were recorded during the course of dally tetanization (kindling) of afferent fibers to the hippocampal CA1 region. SISs were detected after 3-10 tetanizations. A clear variation of SIS rate with behavior was observed. SIS rate was high during slow-wave sleep (SWS), waking immobility, face-washing and chewing and low during rapid-eye-movement sleep (REMS), walking and rearing. Scopolamine hydrochloride (2.5-5 mg/kg i.p.) increased the SIS rate during walking. Despite the negative correlation of SIS occurrence with the theta rhythm in normal rats, abolishing the theta rhythm by medial septal lesions did not affect the suppression of SISs during REMS as compared to SWS. When interictal or postictal spikes were seen together with the theta rhythm, the spikes tended to occur at a phase of about 240 ° after the positive peak of the alvear surface rhythm. INTRODUCTION Kindling offers a model of experimental epilep- sy whereby controlled, spaced and repetitive stimulation of the brain leads to a progressive development of clinical signs of seizure5. One im- portant clinical sign of epilepsy is the spontaneous interictal spike which, despite its controversial relation with the temporal and spatial onset of seizures 2'4"6'15, is widely used as a diagnostic tool in epilepsy. In an ongoing study, we have used electrical kindling of the hippocampal CA1 region as a model of the developing epileptic focus ~2,25. We believe that abnormal neural activities can be dis- tinguished only if the range of normal activities has been established. In previous research, we have systematically investigated the electro- physiological and behavioral characteristics of normal hippocampal slow waves in the rat. Active movements such as walking, rearing and postural shifts and rapid-eye-movement sleep (REMS) are always accompanied by a regular high-amplitude hippocampal theta rhythm 1,14,23. During slow- wave sleep (SWS), waking immobility, chewing and face-washing, a large irregular slow activity (ISA) dominated the hippocampal EEG. A con- tinuum of hippocampal activation has been pro- posed with large-amplitude ISA (and low-fre- quency theta rhythm) at one end and high-fre- quency theta rhythm (and low-amplitude ISA) at the other 11,~4. This activation continuum may also be inferred from hippocampal-evoked poten- tials 8'~3 or the frequency of normal hippocampal Correspondence: L.S. Leung, Department of Clinical Neurological Sciences, University of Western Ontario, London, Ont., Canada N6A 5A5. 0166-4328/88/$03.50 © 1988 Elsevier Science Publishers B.V. (Biomedical Division)