Nico P.A. Bos

Organization of circadian functions: interaction with the body.

The hypothalamus integrates information from the brain and the body; this activity is essential for survival of the individual (adaptation to the environment) and the species (reproduction). As a result, countless functions are regulated by neuroendocrine and autonomic hypothalamic processes in concert with the appropriate behaviour that is mediated by neuronal influences on other brain areas. In the current chapter attention will be focussed on fundamental hypothalamic systems that control metabolism, circulation and the immune system. Herein a system is defined as a physiological and anatomical functional unit, responsible for the organisation of one of these functions. Interestingly probably because these systems are essential for survival, their function is highly dependent on each others performance and often shares same hypothalamic structures. The functioning of these systems is strongly influenced by (environmental) factors such as the time of the day, stress and sensory autonomic feedback and by circulating hormones. In order to get insight in the mechanisms of hypothalamic integration we have focussed on the influence of the biological clock; the suprachiasmatic nucleus (SCN) on processes that are organized by and in the hypothalamus. The SCN imposes its rhythm onto the body via three different routes of communication: 1.Via the secretion of hormones; 2. via the parasympathetic and 3.via the sympathetic autonomous nervous system. The SCN uses separate connections via either the sympathetic or via the parasympathetic system not only to prepare the body for the coming change in activity cycle but also to prepare the body and its organs for the hormones that are associated with such change. Up till now relatively little attention has been given to the question how peripheral information might be transmitted back to the SCN. Apart from light and melatonin little is known about other systems from the periphery that may provide information to the SCN. In this chapter attention will be paid to e.g. the role of the circumventricular organs in passing info to the SCN. Herein especially the role of the arcuate nucleus (ARC) will be highlighted. The ARC is crucial in the maintenance of energy homeostasis as an integrator of long- and short-term hunger and satiety signals. Receptors for metabolic hormones like insulin, leptin and ghrelin allow the ARC to sense information from the periphery and signal it to the central nervous system. Neuroanatomical tracing studies using injections of a retrograde and anterograde tracer into the ARC and SCN showed a reciprocal connection between the ARC and the SCN which is used to transmit feeding related signals to the SCN. The implications of multiple inputs and outputs of the SCN to the body will be discussed in relation with metabolic functions.

Circadian rhythms in spontaneous neuronal discharges of the cultured suprachiasmatic nucleus.

The suprachiasmatic nucleus (SCN) is believed to play a major role in the generation and control of circadian rhythms in mammals. In order to obtain further evidence concerning this, single and multiple neuronal discharges were continuously recorded over a period of several days in neonatal rat SCN explants. These organotypic explants, which had been cultured for several weeks in a chemically defined medium, showed alternating high and low levels of spontaneous neuronal discharges with a periodicity around 24 h. Such explants can serve as a useful model to study the neuronal mechanisms underlying the generation of mammalian circadian rhythms.

Effect of light intensity on diurnal sleep-wake distribution in young and old rats

During the aging process, the amplitude of the circadian rhythms of many physiological variables is reduced. It has been hypothesized that increasing light intensity during the light phase of the light-dark cycle might result in a reduction of age-related changes in the circadian rhythms. Indeed, in the present sleep-wake study in young and old rats it was found that (a) various parameters, such as the light-dark differences and total amounts of each behavioral state responded positively to changes in environmental light intensity (i.e., age-related trends were reversed), (b) in both age groups, the logarithm of light intensity appeared to have a linear dose-response relationship with light-dark differences of the sleep-wake states, (c) the light-dark difference of active wakefulness and quiet sleep of old rats under high light intensity were comparable to those of young rats under low light intensity. The results of the present study suggest that, under appropriate conditions, light could be of clinical use in reducing age-related circadian sleep disturbances in humans. This may, in turn, reduce the use of sedatives in elderly people.

Neurons of the rat suprachiasmatic nucleus show a circadian rhythm in membrane properties that is lost during prolonged whole-cell recording.

The suprachiasmatic nucleus is commonly considered to contain the main pacemaker of behavioral and hormonal circadian rhythms. Using whole-cell patch-clamp recordings, the membrane properties of suprachiasmatic nucleus neurons were investigated in order to get more insight in membrane physiological mechanisms underlying the circadian rhythm in firing activity. Circadian rhythmicity could not be detected either in spontaneous firing rate or in other membrane properties when whole-cell measurements were made following an initial phase shortly after membrane rupture. However, this apparent lack of rhythmicity was not due to an unhealthy slice preparation or to seal formation, as a clear day/night difference in firing rate was found in cell-attached recordings. Furthermore, in a subsequent series of whole-cell recordings, membrane properties were assessed directly after membrane rupture, and in this series we did find a significant day/night difference in spontaneous firing rate, input resistance and frequency adaptation. As concerns the participation of different subpopulations of suprachiasmatic nucleus neurons expressing circadian rhythmicity, cluster I neurons exhibited strong rhythmicity, whereas no day/night differences were found in cluster II neurons. Vasopressin-containing cells form a subpopulation of cluster I neurons and showed a more pronounced circadian rhythmicity than the total population of cluster I neurons. In addition to their strong rhythm in spontaneous firing rate they also displayed a day/night difference in membrane potential.

The effect of old age on the free-running period of circadian rhythms in rat

The free-running period is regarded to be an exclusive feature of the endogenous circadian clock. Changes during aging in the free-running period may therefore reflect age-related changes in the internal organization of this clock. However, the literature on alterations in the free-running period in aging is not unequivocal. In the present study, with various confounding factors kept to a minimum, it was found that the free-running periods for active wakefulness, body temperature, and drinking behavior were significantly shorter (by 12-17 min) in old than in young rats. In addition, it was found that the day-to-day stability of the different sleep states was reduced in old rats, whereas that of the drinking rhythm was enhanced. Transient cycles were not observed, nor were there any age-related differences in daily totals of the various sleep-wake states. The amplitudes of the circadian rhythms of active wakefulness, quiet sleep, and temperature were reduced, whereas those of paradoxical sleep and quiet wakefulness remained unchanged.

Effects of excitatory and inhibitory amino acids on neuronal discharges in the cultured suprachiasmatic nucleus

The influence of amino acids on neuronal activity was studied microiontophoretically in the cultured suprachiasmatic nucleus (SCN). Three types of SCN neurons could be characterized: silent (glutamate responsive), irregular, and regular neurons. Glutamate excited about 70% of the regular and 60% of the irregular cells. GABA inhibited both the spontaneous and the glutamate-evoked activity of more than 90% of all three types of SCN neurons. MK-801 partially blocked glutamate responses. N-acetyl-aspartyl-glutamate (NAAG), a new neurotransmitter found in the retinohypothalamic fibers, directly increased firing rate and potentiated glutamate responses in the SCN neurons that were studied. These results indicate the potential significance of the amino acids in neuronal transmission within the biological clock.

Chronic neonatal MK-801 administration leads to a long-lasting increase in seizure sensitivity during the early stages of hippocampal kindling

Persistent effects of chronic neonatal administration of the N-methyl-D-aspartate (NMDA) antagonist MK-801 were investigated by measuring susceptibility to CA1 kindling epileptogenesis in adulthood. Rat pups were chronically treated with MK-801 from postnatal day 8 through day 19. Hippocampal kindling showed an increase in electrical seizure duration in the MK-801-treated group as compared with controls along with a more severe expression of behavioral seizures during the first few kindling stimulations. These results show that neonatal interference with NMDA receptor function leads to a long-lasting increase in hippocampal excitability.

Functional deprivation of noradrenaline neurotransmission: effects of clonidine on brain development.

Publisher Summary The purpose of this chapter is to provide foundations for a working hypothesis on drug-induced functional neuroteratology. A class of centrally acting antihypertensive drugs, including clonidine, which are still being prescribed during pregnancy in humans, is used as the model. A hypothesis is put forward in this chapter which proposes that behavioral state-dependent changes in monoaminergic neuronal firing levels and patterns play a key role during early ontogeny in regulating neuron membrane potential, neurotransmitter release and neurotransmitter receptor sensitivity patterns in adulthood. Neurons show spontaneous activity that varies as a function of the behavioral state of the organism. This has been demonstrated for noradrenergic neurons of the locus coeruleus (LC), serotonergic neurons of the dorsal raphe (DR), dopaminergic neurons of the substantia nigra (SN) and cholinergic neurons of the dorsolateral part of the pons and basal forebrain. Because centrally acting antihypertensives predominantly affect the noradrenaline (NA) system in the brain, the induced modifications of central NA neurotransmission is also emphasized in this chapter.

Spectral analysis of the electroencephalogram in neonatal rats chronically treated with the NMDA antagonist MK-801

In order to study the involvement of NMDA-receptor activation in brain development, rat pups were chronically treated with the non-competitive NMDA antagonist MK-801 during the neonatal period. We recorded the cortical EEG at various vigilance states throughout the treatment period. Spectral analysis of the EEG showed reduced power in the delta (delta) frequency range (1.5-4 Hz) during quiet sleep and less power in the theta (theta) range (4-7 Hz) during REM-sleep in MK-801 animals than in controls. No significant differences were found for the total time spent in each of the different vigilance states. We conclude that chronic MK-801 treatment probably causes a developmental retardation in state-related brain activities.

NEONATAL CLONIDINE TREATMENT RESULTS IN LONG-LASTING CHANGES IN NORADRENALINE SENSITIVITY AND KINDLING EPILEPTOGENESIS

In the present experiment we tested the hypothesis that early interference with noradrenaline transmission can have permanent consequences for brain function in adulthood. Neonatal depletion of noradrenaline by daily subcutaneous injections of clonidine results in supersensitivity to noradrenaline in adult hippocampal CA1 cells as shown in our previous microiontophoretic study. These findings were confirmed and extended here with dose-response curves. Furthermore, we tested whether this form of neonatal interference with noradrenaline also permanently affects long-lasting plasticity as revealed in kindling epileptogenesis in adulthood. The initiation of the epileptic activity after the kindling stimulation was significantly delayed in the clonidine-treated group, and all measured parameters of seizure expression tended to be retarded in comparison with saline-treated control rats. This indicates that noradrenaline supersensitivity induced by neonatal clonidine treatment retards kindling development in adulthood.