53 0090-2977/08/4001-0053 © 2008 Springer Science+Business Media, Inc. Neurophysiology, Vol. 40, No. 1, 2008 REVIEWS Role of Interneuronal Systems in the Formation of Main Patterns of Field Electrical Activity in the Hippocampus О. А. Markova, 1, 2 , Т. М. Tsugorka, 1 О. V. Dovgan’, 1 А. R. Stepanyuk, 1 and V. P. Cherkas 1 Neirofiziologiya/Neurophysiology, Vol. 40, No. 1, pp. 58-68, January-February, 2008. Received November 11, 2007. Studies on the cellular and subcellular levels promote elucidation of the fundamental principles of formation of effective neuronal systems from cell units. To estimate the interrelations between electrical activity of neuronal networks and processes realized on the cellular level, we need to adequately understand the general patterns of behavior of populations of interneurons, which are components of these networks, under different physiological conditions. In this review, we describe and discuss the relations between the electrical activity of single hippocampal neurons and different components of the field electrical activity, as well as modern concepts on the mode of involvement of the system of hippocampal interneurons in the formation of physiologically important patterns of efferent activity of the above-mentioned structure (in particular in encoding of information on the neuronal level). Keywords: electrical activity of interneurons, types of interneurons, field electrical activity, principles of neuronal encoding. 1 Bogomolets Institute of Physiology, National Academy of Sciences of Ukraine, Kyiv, Ukraine. 2 Institut de Neurobiologie de la Mediterranee, Marseille, France. Correspondence should be addressed to О. А. Markova (e-mail: markova@inmed.univ-mrs.fr); Т. М. Tsugorka (e-mail:tanya@biph.kiev.ua); О. V. Dovgan’ (e-mail: rasboinik@biph.kiev.ua); А. R. Stepanyuk (e-mail: standrey@biph.kiev.ua); V. P. Cherkas (e-mail: cherkas@biph.kiev.ua). INTRODUCTION The hippocampus is the most important part of the archeocortex. Transient fixation of memory traces related to recognized life episodes (i.e., integrated multimodal representation of life events together with information on time, space, emotional state, and other aspects of the context, within which these events are realized, and integration of these memory traces in a general system of memory in the brain are the main functions of the hippocampus in mammals. The structure and physiology of the hippocampus were thoroughly examined in numerous studies on different levels. Nonetheless, a detailed integral theory of functioning of the hippocampus has not been developed. Elucidating how coordinated functioning of neurons of local hippocampal networks is related to generation of definite patterns of the field activity is one of the important tasks for successful development of such a theory. These patterns, in particular patterns related to encoding of memory traces, can be observed using electrophysiological techniques. Systems of interneurons are important components of the hippocampal local networks. The involvement of different types of interneurons in the formation of a few known patterns of field electrical activity has been discussed in several reviews. During the last three years, a considerable amount of new data relating to this problem has been stored. In addition, new studies of the mode of participation of interneuronal systems in the encoding of components of episodic memory in the hippocampus were described. Our review is an attempt to analyze these results. EFFICACY OF PYRAMIDAL- INTERNEURONAL SYNAPTIC CONNECTIONS IN THE HIPPOCAMPUS UNDER IN VIVO CONDITIONS A number of studies in vitro demonstrated the extensive variability of synaptic connections between hippocampal neurons [1-3]. We should recognize that,