Physiology& Behavior, Vol. 27, pp. 363-368. Pergamon Press and Brain Research Publ., 1981. Printed in the U.S.A. The Use of Sodium Pentobarbital for The Study of Immobility-Related (Type 2) Hippocampal Theta B. H. BLAND, R. S. SAINSBURY, M. SETO, B. R. SINCLAIR Department of Psychology, The University of Calgary 2500 University Drive, Calgary, Alberta, Canada, T2N IN4 AND I. Q. WHISHAW Department of Psychology, The University of Lethbridge 4401 University Drive, Lethbridge, Alberta, Canada, TIK 3M4 Received 20 October 1980 BLAND, B. H., R. S. SAINSBURY, M. SETO, B. R. SINCLAIR AND I. Q. WHISHAW. The use of sodium pentobarbi- talf i , the study of immobility-related (Type 2) hippocampal theta. PHYSIOL. BEHAV. 27(2) 363-368, 1981.--A series of experiments investigated the effects of sodium pentobarbital on the dentate electroencephalographic (EEG) activity of rats and rabbits implanted with tungsten microelectrodes. Type 1 (movement-related) theta was abolished in both species and Type 2 (immobility-related)theta was abolished in the rat but remained intact and readily elicitable by sensory stimulation in the rabbit, following anaesthetic doses of pentobarbital. It was proposed that sodium pentobarbital may be useful for the investigation of mechanisms underlying the two types of theta and that the effect of pentobarbital provides further evidence for species differences in the way the hippocampal formation produces Type 2 theta. Sodium pentobarbital Hippocamal theta Immobility THE earlier work carried out by Vanderwolf [14,15] demonstrated that there was a correlation between hip- pocampal theta activity and what he termed "voluntary" motor activities, in the rat. Complications in the generaliza- tion of these observations to other species arose when sev- eral studies reported observing theta activity coincident with immobility in rabbits and cats [7, 8, 21]. Subsequent work involving behavioral and pharmacological manipulations demonstrated that there are two types of theta activity, in both rat and rabbit [9, 16, 17, 18]. One type of theta (labelled Type 1) has a higher overall frequency range (6-12 Hz), oc- curs during voluntary movement (e.g., walking, running, rearing), is resistant to atropine, but is sensitive to anaes- thetic agents such as urethane, ethyl ether and alcohol. The second type (labelled Type 2 theta) has a lower frequency range (4-9 Hz), occurs during immobility, and is resistant to most anaesthetics but sensitive to atropine. These data, to- gether with the fact that eserine elicits Type 2 theta in both rats and rabbits suggested that Type 2 theta is mediated by cholinergic inputs to the hippocampal formation. Electrophysiological studies have shown that theta is generated in stratum oriens of the CA1 pyramidal region of the hippocampus proper and the stratum moleculare of the dentate gyrus of the rat [5,22], rabbit [2,23] and cat [3]. It has been established that both types of the theta can be recorded from the two generators and suggests that theta may be the result of cholinergic and non-cholinergic inputs to each generator [20]. While it appears that rats and rabbits have both types of theta, there are obvious species differences with respect to how readily Type 2 theta may be observed during spontaneously-occurring behavior [12]. The rabbit can produce long trains of Type 2 theta during immobility, both spontaneously and in response to various types of sensory stimulation (whistles, tones, hand claps, hand waves, strok- ing of the fur). The rat rarely (if ever) produces Type 2 theta when immobility occurs during spontaneous behavior. Short trains of Type 2 theta have been observed in the rat during conditioning studies [15,18]. Recent developmental studies also confirm that there are species differences with respect to Type 2 theta. LeBlanc and Bland [10] demon- strated that in the rat both types of theta appear at the same time, about l0 days of age. Type 2 theta was never observed during spontaneous behavior, but was elicited by injections of eserine. Creery and Bland [6] have shown that in the developing rabbit Type l theta appeared around 8 days of ~This research was supported by grants A9935 (Bland), A0361 (Sainsbury) and A8273 (Whishaw) from the Natural Sciences and Engineering Research Council of Canada. Copyright © 1981 Brain Research Publications Inc.--0031-9384/81/080363-06502.00/0