between mother and fetus or a newborn. It prepares and optimizes its later behaviors and activities for circadian light conditions [7].

These examples permit the following conclusion: Of importance equal to photic information in man is the direct influence of strictly determined changes and behavioral activity patterns generated by nonphotic stimuli. This is a key factor synchronizing biological rhythms.

NONSPECIFIC SYSTEMS

A characteristic feature of the central nervous system is its distinct division into two autonomously separate but functionally strictly related systems. The first consists of specific pathways along which there flow centripetal sensory information and centrifugal motor input. These systems are characterized by the selectivity of the flowing information, which indicates that only information specific to a given projection (e.g., optic to the visual cortex or photic to elements of the mechanism of biological clock, including the SCN above all) flows along a definite path. The other system contains nonspecific pathways along which no specific sensory information flows from a particular reception zone or a sense organ to a specific area of the cerebral cortex. Anatomically, they form greatly “disseminated,” diffusible projections with terminals sited in many brain areas, also including the neuronal elements of the biological clock. The activity of sources of these systems causes a general excitation of the brain, the so-called arousal reaction. This arousal, with an electrophysiological picture that is represented by desynchronization of the activity of cortical neurons, makes the information flowing along specific pathways to be well “understood” by the appropriate areas of the brain, mainly the cerebral cortex. Hence, the nonspecific systems prepare the brain for the reception and adequate reaction to stimuli of diverse modality, coming in from specific systems. Only proper “cooperation” of the two systems guarantees normal brain activity—in the context not only of the cortical facilitation of sensory information transmission along nervous pathways but also of their participation in the mechanism of numerous important physiological processes, including the mechanism of mammalian biological clock.

Nonphotic stimuli such as behavioral arousal that induces hyperactivity and sleep deprivation, food shortage, as well as such social interactions as, for example, matingoriented behaviors, have no specific projections of their own in the brain. A physiological result of their influence on human organism may be and is activation of nonspecific systems and a change in human brain arousal [7], which also elevate the physical activity of man [4]. They also modulate the work of neuronal elements of the mechanism of the mammalian biological clock, in particular those receiving direct photic information from ganglion cells of the retina.

Ascending Reticular-Activating System

In 1949, Moruzzi and Magoun [8] demonstrated the indispensability of the reticular formation of the brain stem to the maintenance of cortical activity and behavioral arousal, giving that projection the name ascending reticular-activating system (ARAS). The ascending nonspecific projections of the reticular formation reach brain cortical areas by the dorsal pathway via the thalamus and by the ventral pathway via the basal

IGL

SCN

Fig. 1. The sagittal section through the rat brain illustrating the ascending reticular-activating system (ARAS). BF basal forebrain, IGL intergeniculate leaflet, LC locus coeruleus, LDT laterodorsal tegmental nuleus, PPT penduculopontine tegmental nucleus, RN raphe nuclei, SCN suprachiasmatic nuclei, TH thalamus, TMN tuberomammillary nucleus, VTA/SN ventral tegmental area/substantia nigra.

forebrain and the hypothalamus, while neurons of the descending projection of the reticular formation come up to peripheral muscles through activation of motor neurons of the medulla (Fig. 1).

The main neurotransmitter of the diffusible thalamocortical projection and some neurons of the basal forebrain is glutamate (GLU); its activity is modified by such neuromodulators of the brain stem as noradrenaline of the locus coeruleus, dopamine of the substantia nigra and ventral tegmental area, serotonin of the raphe nuclei, acetylcholine of pontomesencephalic neurons, or histamine of tuberomammillary neurons. The majority of these projections also have their terminals in the neuronal elements of the mammalian biological clock, receiving information directly or indirectly via basal nuclei of the forebrain [9]. The dominating and most significant projection for the clock activity is the serotoninergic projection from the raphe nuclei, often defined as a nonphotic projection [10].

Orexin/Hypocretin Projection

At the end of the 1990s, a group of researchers from San Diego, headed by Gregory Sutcliffe, discovered in the hypothalamic area a peptide controlling appetite and regulating body weight. In earlier studies, rats with lesioned median hypothalamus showed obesity, while those with lesion of the lateral hypothalamus displayed anorexia [11]. The above-mentioned peptide was named hypocretin (Hctr) after the site of its hypothalamic localization and the structural resemblance to the gut hormone secretin [12]. At the same time, in their search for orphan receptor ligands, Yanagisawa’s research group from Texas found two peptides binding to those receptors, orexin A and orexin B. The name orexin (OX) is derived from the Greek word for appetite; it was introduced after an increased appetite had been observed following administration of that peptide into

the lateral ventricle of rat brain. It shortly turned out that hypocretin and orexin were the same compound participating in the regulation of not only food intake, but also behaviors and behavioral states of the organism, including regulation of the sleep-and- wakefulness rhythm. Its concentration changes according to the day-and-night rhythm, reaching its maximum in the waking state and during rapid-eye movement (REM) sleep and the minimum during slow-wave sleep [13]. The blockade of orexin synthesis via degeneration of orexin neurons or genetic mutations causes narcolepsy in animals and humans [14].

To date, two orexin/hypocretin peptides, OX-A (Hctr-1) and OX-B (Hctr-2), have been recognized. They bind to two different metabotropic receptors, OX-R1 and OX-R2. OX-A has high affinity for both these receptors, while OX-B shows considerably greater affinity for the latter receptor (type 2) [15].

The peptide under description is synthesized mainly by a small group of cells of the posterior lateral hypothalamus [16]. A tiny number of small Hctr neurons were also observed in other brain regions: the lateral part of the amygdala, the anterolateral area of the bed nucleus and lateral ventricle [17], as well as in olfactory neurons [18].

Axons of hypocretin neurons, mainly those localized within the hypothalamic area, innervate numerous regions of the brain. The richest projections reach the locus coeruleus and raphe nuclei, which have descending fibers that reach motor neurons, controlling muscle tone, while the ascending fibers that innervate the forebrain are involved in sensory integration. Other sites that are strongly innervated by orexin fibers are the source of cholinergic projection of the brain stem and basal nuclei. The cholinergic projection of these areas is responsible for cortical activation, the result of which is the wakeful brain. The orexin projection is in charge of coordination of the activity of arousing cholinergic projections with the motor activity of the organism. The histaminergic cells of the posterior and other hypothalamic nuclei, which are engaged in the so-called forebrain wakefulness, are also under strict control of the orexin system [19]. The orexin projection is also in control of the dopaminergic neurons involved in reward processes and in the mechanism of wakefulness. Weaker projections innervate regions of the dorsal and ventral roots of the medulla oblongata, of motor neurons and limbic areas of the brainstem, as well as of the cerebral cortex [20] (Fig. 2).

In the majority of cases, the orexin projection is stimulatory; this also refers to γ-aminobutyric acid-ergic (GABAergic) neurons of the pars reticulata substantia nigra or the septal nucleus innervating the hippocampus. Regarding the stimulatory action of the orexin projection, of paramount importance is the fact that it has no terminals of its own on neurons of specific thalamic pathways transmitting sensory information to the cerebral cortex [21]. This is further proof helping to classify this system as a nonspecific projection of the brain.

The direct postsynaptic stimulatory action of orexin often depends on the effect of its simultaneous activation of other fibers that may have their terminals on the stimulated synapse. This happens, for example, in the case of serotonergic and noradrenergic cells that simultaneously receive direct orexin stimulation and indirect inhibition originating with GABAergic cells, also stimulated by orexin. Hence, the influence of the direct stimulatory action of orexin depends on indirect modulating effects and does not ever have to be of a stimulating nature. Such modulating action

Fig. 2. The sagittal section through the rat brain shows the key projections of hypocretin (orexin) neurons (Hctr) from the lateral hypothalamus to the main components of the brain structures. See Fig. 1 caption for definitions of abbreviations. (After [20].)

that intensifies weak signals and inhibits strong ones is also a characteristic feature of nonspecific systems.

The modulating effect of orexin projection on neuronal activity was also confirmed by the results of in vitro studies. The orexin-induced stimulation of orexin neurons in hypothalamic sections stems indirectly from the enhanced activity of glutaminergic cells adjacent to orexin ones but is not a direct effect on orexin cells [22].

The mechanism of inhibitory feedback can also be observed between the orexin system and the noradrenergic and serotoninergic ones. Orexin directly stimulates these two projections, which may attenuate its action on a feedback basis, via an inhibitory influence on the glutaminergic neurons present in the vicinity of the orexin projection. Also, the histaminergic projection originating with the protuberance of nuclei of the mammillary body exerts an inhibitory influence on orexin neurons via GABA release since histamine itself has a small effect on these neurons [22, 23].

Studies have shown that orexin release is endogenous and depends on the presence of the main generator of the mammalian biological clock: the SCNs of the hypothalamus [24]. The presence of OR-A and OR-B and OX-R1 receptors in the human retina seems particularly noteworthy [25] as this may suggest its modulatory role in the interaction among ganglionic cells of the retina, which transfer photic information to the SCN of the hypothalamus. Considering the presence of orexin OX-R1 receptors and OXcontaining fibers in the vicinity of the SCNs [25], it is proposed that this system may have a modulatory influence on the neuronal activity of the whole mechanism of mammalian biological clock. However, the unique role of orexin seems to be connected with its involvement—mediation of the transfer of nonphotic information to the intergeniculate leaflet (IGL) of the thalamus, the other (besides the SCN) extremely important neuronal element of the mechanism of the mammalian biological clock.