By evolutionary adaptation to the perpetual day-night changes in envi ronmental conditions, eukaryotic organisms have acquired an endogen ous programme. This mechanism exhibits the characteristics of a self sustaining oscillation the period of which approximates that of the earth's rotation. For animals such a property was first clearly demon strated by Maynard S. Johnson (1939) who recorded, in constant conditions, free-running activity rhythms of white-footed mice (Peromyscus /eucopus). Johnson concluded from his observations that "this animal has an exceptionally substantial and durable self-winding and self-regulating clock, the mechanism of which remains to be worked out". Twenty years later, the formal properties of this "circadian" clock and its use by organisms as a time-keeping device were summa rized at the Cold Spring Harbor Symposium in 1960 (Chovnick 1961). During the following two decades, investigations have turned towards an analysis of the physiological mechanisms involved in and the search for a central masterclock. These efforts led to the discovery that the pineal organ of submammalian vertebrates and the suprachiasmatic nuclei of birds and mammals are major candidates for a role as central circadian pacemakers. At the same time the neural pathways through which these structures are coupled to the light-dark cycle were identi fied. Furthermore, it was established that the pineal gland and the suprachiasmatic nuclei are closely related structures that integrate the functions of circadian timekeeping and photoperiodic time measure ment.

Inhalt
1 Introduction.- 1.1 The Search for Principles of Physiological Organization in Vertebrate Circadian Systems.- 1.2 Zeitgebers, Entrainment, and Masking: Some Unsettled Questions.- 2 Pathways of Zeitgeber Signals.- 2.1 Comparative Aspects of Retinal and Extraretinal Photosensory Input Channels Entraining Endogenous Rhythms.- 2.2 Neuroanatomical Pattern of Endocrine and Oscillatory Systems of the Brain: Retrospect and Prospect.- 2.3 Unspecific Optic Fibres and Their Terminal Fields.- 2.4 Characteristics of the Interaction Between the Central Circadian Mechanism and the Retina in Rabbits.- 3 The Role of the Nucleus Suprachiasmaticus.- 3.1 Physiological Models of the Rodent Circadian System.- 3.2 Neuroanatomical Organization and Connections of the Suprachiasmatic Nucleus.- 3.3 CNS Structures Controlling Circadian Neuroendocrine and Activity Rhythms in Rats.- 3.4 The Neurophysiology of the Mammalian Suprachiasmatic Nucleus and Its Visual Afferents.- 3.5 Neurophysiological Studies of the SCN in the Rat and in the Java Sparrow.- 3.6 Neural Mechanisms in Avian Circadian Systems: Hypothalamic Pacemaking Systems.- 3.7 Limits of Entrainment to Periodic Feeding in Rats with Suprachiasmatic Lesions.- 4 The Role of the Pineal Organ.- 4.1 Phase Responses and Characteristics of Free-Running Activity Rhythms in the Golden Hamster: Independence of the Pineal Gland.- 4.2 Circadian and Infradian Activity Rhythms in the Mammalian Pineal Body.- 4.3 Electrophysiology of the Mammalian Pineal Gland: Evidence for Rhythmical and Non-Rhythmical Elements and for Magnetic Influence on Electrical Activity.- 4.4 Circadian Rhythms of the Isolated Chicken Pineal in Vitro.- 4.5 Endogenous Oscillator and Photoreceptor for Serotonin N-Acetyltransferase Rhythm in Chicken Pineal Gland.- 5 Systemic Aspects.-5.1 Role of Hormones in the Circadian Organization of Vertebrates.- 5.2 The Neuropharmacology of Circadian Timekeeping in Mammals.- 5.3 Entraining Agents for the Circadian Adrenocortical Rhythm in the Rat.- 5.4 Splitting of the Circadian Rhythm of Activity in Hamsters.- 5.5 Phase-Response Curves and the Dual-Oscillator Model of Orcadian Pacemakers.- 5.6 Orcadian Control of Body Temperature in Primates.- 6 The Circadian Sleep-Wake Cycle.- 6.1 Circadian and Sleep-Dependent Processes in Sleep Regulation.- 6.2 Sleep Circadian Rhythms in the Rat: One or Two Clocks?.- 6.3 Sleep ECoG Rhythm in the High Mesencephalic Rat.- 6.4 Interaction Between the Sleep-Wake Cycle and the Rhythm of Rectal Temperature.- 6.5 The Phase-Shift Model of Spontaneous Internal Desynchronization in Humans.- 7 Photoperiodic Phenomena Related to the Circadian System.- 7.1 Physiology of Photoperiodic Time-Measurement.- 7.2 Pineal Influences on Circannual Cycles in European Starlings: Effects Through the Circadian System?.- 7.3 Complex Control of the Circadian Rhythm in N-Acetyltransferase Activity in the Rat Pineal Gland.- 7.4 The Critical Photoperiod in the Djungarian Hamster Phodopus sungorus.- 8 General and Functional Aspects.- 8.1 Circadian Contributions to Survival.- 8.2 Daily Temporal Organization of Metabolism in Small Mammals: Adaptation and Diversity.- 8.3 Characteristics and Variability in Entrainment of Circadian Rhythms to Light in Diurnal Rodents.- 8.4 Functional Significance of Daily Cycles in Sexual Behavior of the Male Golden Hamster.