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A Nonspecific System Provides Nonphotic Information for the Biological Clock
Marian H. Lewandowski
CONTENTS
INTRODUCTION
NONPHOTIC INFORMATION
NONSPECIFIC SYSTEMS
INTERGENICULATE LEAFLET OF THE THALAMUS
CONCLUSIONS
REFERENCES
INTRODUCTION
Rhythmicity is a key property of all living matter, starting with unicellular organisms and ending with such complex organisms as humans. All the time, numberless structures and functions of the organism undergo rhythmic changes (of seconds, minutes, and hours; circadian; seasonal). These alterations are of great adaptive significance to the organism. They permit behavioral patterns and physiological processes of the organism to be optimally synchronized with the cyclically changing environmental conditions.
In recent years, a growing interest in biological rhythms, especially in their mechanism, has been an effect of not only their endogenous origin and the discovery of genetic basis, but—above all—also of the understanding of their great significance for the proper functioning of the organism. One of the first symptoms of functional disturbances (illnesses) in a living organism is a change in the rhythmicity of its physiological processes and behavioral patterns, while interference with the rhythmic processes may be very dangerous and cause disturbances in the functioning of human organism. Therefore, the understanding of the mechanism regulating rhythmic processes, the elements of this mechanism, their reciprocal functional relations, as well as the influence of external factors on its functioning not only presents a challenge to chronobiologists, but also requires specialist knowledge on the part of physicians, pharmacologists, and psychologists, that is, all those in everyday contact with ill and healthy persons. Such information is also valuable to every one of us who listens intently to one’s own organism and wants to live in harmony with it.
From: Ophthalmology Research: Visual Transduction and Non-Visual Light Perception
Edited by: J. Tombran-Tink and C. J. Barnstable © Humana Press, Totowa, NJ
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The discovery of the suprachiasmatic nuclei (SCNs) of the anterior hypothalamus, the main neuronal element building up the mechanism of mammalian biological clock, at the beginning of the 1970s [1] was undoubtedly a giant step in chronobiological research. A direct connection between ganglion cells of the retina and the SCN via the retinohypothalamic tract (RHT) supported the notion of a significant role of light as an external synchronizer (zeitgeber, time donor) of mammalian biological clock [2, 3].
NONPHOTIC INFORMATION
However, light is not the only external stimulus that synchronizes the functioning of mammalian biological clock. Also other, nonphotic information has substantial influence on its work [4]. It is of particular significance to higher organisms, including first of all humans, for whom the cyclically changing light/darkness or ambient temperature are not the only factors affecting the normal course of rhythmic processes. Of equal importance as a stimulus synchronizing rhythmic processes is all nonphotic information, which under certain conditions may exert even a more powerful effect than light [5]. In social animals, including mammals above all, there is a strong tendency to synchronize the behavior of an individual with that of other group members. This is particularly important and clearly visible in primates, including humans first, for whom the social instructions developed by way of evolution have become dominant over physicochemical stimuli in the regulation of their behavior [6]. The contemporary lifestyle strongly emphasizes this tendency by showing how far light, present in our everyday activities (shift work, intercontinental flights, etc.), diverges from the solar cycle of day and night, often supplying the biological clock with contradictory and incorrect information about time.
A vast body of evidence indicates that synchronization of the biological clock may also take place in the absence of light. Persons kept under permanent poor lighting in specially isolated living quarters or during the polar night show a free-running rhythm, characteristic of the so-called permanent light conditions. In such circumstances, cognitive stimuli themselves and the information about the passing time are not sufficient to change this rhythm. However, the imposition of definite behavioral patterns or the maintenance of regular contacts with the remaining group members may synchronize a free-running rhythm to the circadian rhythm. A similar mechanism can be observed in blind persons, whose acquired permanent behavioral patterns inhibit generation of a free-running rhythm by synchronizing their biological clock to the circadian rhythm.
The disturbance of customary everyday behaviors or their elimination as well as deprivation of other social signals may have a serious negative effect on our well-being. A change in mood is often an effect of sudden, unexpected, mainly negative, nonphotic stimuli on the circadian mechanism of the biological clock. Due to their force and negative character, these stimuli can mask the current synchronizing effect of less-potent nonphotic information (synchronizers), thereby causing desynchronization of rhythmic processes. The disturbance of circadian rhythms that frequently accompanies depression may also be connected with behavioral dysfunction, including locomotor ones or changes in customary behaviors, which hinder the availability of social stimuli and thus their synchronizing influence. Nonphotic communication is of particular importance