Influence of mediobasal hypothalamic lesion and catecholamine receptor antagonists on ultradian rhythm of EEG in the posterior hypothalamus of the rat

Multiple ultradian rhythms of metabolism, body temperature and activity in Djungarian hamsters

Djungarian hamsters (Phodopus sungorus) living at constant 15 °C Ta in short photoperiod (8:16 h L:D) showed pronounced ultradian rhythms (URs) of metabolic rate (MR), body temperature (Tb) and locomotor activity. The ultradian patterns differed between individuals and varied over time. The period length of URs for MR, Tb and activity was similar although not identical. Wavelet analysis showed that three different URs are existing in parallel, URs of small amplitude and short duration (URsmall), URs of medium amplitude and medium duration (URmedium) and URs of large amplitude (URlarge), superimposed on each other. URlarge were accompanied by an increase in locomotor activity, whereas URsmall and URmedium were of metabolic origin with lacking or delayed responses of activity. An energetic challenge to cold which raised total energy requirements by about 50% did not accelerate the period length of URs, but extended the amplitude of URsmall and URmedium. URlarge corresponds with the URs of activity, feeding and drinking, sleep and arousal as described in previous studies, which are related to midbrain dopaminergic signalling and hypothalamic ultradian signalling. The cause and control of URmedium and URsmall is unknown. Their periods are similar to periods of central and peripheral endocrine ultradian signalling, suggesting a link with URs of metabolism.

Silencing of ultradian rhythms and metabolic depression during spontaneous daily torpor in Djungarian hamsters

Ultradian rhythms of metabolism, body temperature and activity are attenuated or disappear completely during torpor in Djungarian hamsters, for all three ultradian periodicities (URsmall, URmedium and URlarge). URsmall and URmedium disappear during entrance into torpor, whereas URlarge disappear later or continue with a low amplitude. This suggests a tight functional link between torpor and the expression of ultradian rhythms, i.e. torpor is achieved by suppression of metabolic rate as well as silencing of ultradian rhythms. Spontaneous torpor is often initiated after an ultradian burst of activity and metabolic rate, beginning with a period of motionless rest and accompanied by a decrease of metabolic rate and body temperature. To extend previous findings on the potential role of the adrenergic system on torpor induction we analysed the influence of the ß3-adrenergic agonist Mirabegron on torpor in Djungarian hamsters, as compared to the influence of the ß-adrenergic antagonist Propranolol. Hamsters were implanted with 10 day release pellets of Mirabegron (0.06 mg day-1) or Propranolol (0.3 mg day-1). Mirabegron transiently supressed and accelerated ultradian rhythms but had no effect on torpor behaviour. Propranolol did not affect torpor behaviour nor the expression of ultradian rhythms with the dosage applied during this study.

Neurotransmitters are released in brain areas according to ultradian rhythms: Coincidence with ultradian oscillations of EEG waves

Use of the push-pull superfusing technique has shown that in the brain the release rates of endogenous catecholamines, GABA, glutamate and histamine are not constant but fluctuate temporally according to ultradian rhythms. Rhythmic fluctuations have been found in the posterior and anterior hypothalamus, the locus coeruleus, the nucleus of the solitary tract, the mammillary body and the medial amygdaloid nucleus of cats and rats. Similar fluctuations appear in the nitric oxide signal registered in the nucleus accumbens, as well as in the power of delta and theta waves of the EEG in the posterior hypothalamus. The EEG rhythmic fluctuations are generated in the arcuate nucleus because they disappear after its electrocoagulation. The frequency of the EEG fluctuations is increased, decreased or even abolished when catecholamine or histamine receptor agonists and antagonists are centrally applied showing that the EEG ultradian rhythm is controlled by catecholaminergic and histaminergic neurons. Moreover, the rhythmic fluctuations of delta and theta waves corelate negatively with those of histamine in the rat posterior hypothalamus. The possible role of these rhythmic fluctuations is discussed. Their potential importance for pharmacotherapy is still unknown.

Extracellular long-term recordings of the isolated accessory medulla, the circadian pacemaker center of the cockroach Leucophaea maderae, reveal ultradian and hint circadian rhythms

In the cockroach Leucophaea maderae transplantation studies located the circadian pacemaker center, which controls locomotor activity rhythms, to the accessory medulla (AMe), ventromedially to the medulla of the brain's optic lobes. The AMe is densely innervated via GABA- and manyfold peptide-immunoreactive neurons. They express ultradian action potential oscillations in the gamma frequency range and form phase-locked assemblies of synchronously spiking cells. Peptide application resulted in transient rises of extracellularly recorded activity. It remained unknown whether transient rises in spontaneous electrical activity as a possible indication of peptide release occur in the isolated circadian clock in a rhythmic manner. In extracellular glass electrode recordings of the isolated AMe in constant darkness, which lasted at least 12 h, the distribution of daytime-dependent changes in activity independently of the absolute action potential frequency was examined. Rapid, transient changes in activity preferentially occurred at the mid-subjective night, with a minimum at the middle of the subjective day, hinting the presence of circadian rhythms in the isolated circadian clock. Additionally, ultradian rhythms in activity change that are multiples of a fundamental 2 h period were observed. We hypothesize that circadian rhythms might originate from coupled ultradian oscillations, possibly already at the single cell level.

Rhythmic properties of the hamster suprachiasmatic nucleus in vivo

We recorded multiple unit neural activity [multiunit activity (MUA)] from inside and outside of the suprachiasmatic nucleus (SCN) in freely moving male golden hamsters housed in running-wheel cages under both light/dark cycles and constant darkness. The circadian period of MUA in the SCN matched the period of locomotor activity; it was approximately 24 hr in wild-type and 20 hr in homozygous tau mutant hamsters. The peak of MUA in the SCN always occurred in the middle of the day or, in constant darkness, the subjective day. There were circadian rhythms of MUA outside of the SCN in the ventrolateral thalamic nucleus, the caudate putamen, the accumbens nucleus, the medial septum, the lateral septum, the ventromedial hypothalamic nucleus, the medial preoptic region, and the stria medullaris. These rhythms were out-of-phase with the electrical rhythm in the SCN but in-phase with the rhythm of locomotor activity, peaking during the night or subjective night. In addition to circadian rhythms, there were significant ultradian rhythms present; one, with a period of approximately 80 min, was in antiphase between the SCN and other brain areas, and another, with a period of approximately 14 min, was in-phase between the SCN and other brain areas. The periods of these ultradian rhythms were not significantly different in wild-type and tau mutant hamsters. Of particular interest was the unique phase relationship between the MUA of the bed nucleus of the stria terminalis (BNST) and the SCN; in these two areas both circadian and ultradian components were always in-phase. This suggests that the BNST is strongly coupled to the SCN and may be one of its major output pathways. In addition to circadian and ultradian rhythms of MUA, neural activity both within and outside the SCN was acutely affected by locomotor activity. Whenever a hamster ran on its wheel, MUA in the SCN and the BNST was suppressed, and MUA in other areas was enhanced.

Influence of histamine receptor agonists and antagonists on ultradian rhythm of EEG in the posterior hypothalamus of the rat

The delta and theta frequency bands of the electroencephalogram (EEG) in the posterior hypothalamic area (PH) of rats vary according to an ultradian rhythm with a frequency of approximately 1 cycle/100 min. The influence of histamine-related drugs on the ultradian hypothalamic EEG rhythm was now studied in urethane anaesthetized rats. Injected into the lateral ventricle, metoprine (inhibitor of histamine catabolism) and alpha-fluoromethylhistidine (inhibitor of histamine synthesis) did not alter the duration of the rhythmic changes. The H1 receptor agonist 2-(2-aminoethyl)-thiazole was ineffective, while mepyramine (H1 receptor antagonist) prolonged the cycle duration of delta and theta frequency bands. Stimulation of H2 and H3 receptors by amthamine and immepip, respectively, also prolonged the cycle duration of these frequency bands, while the H2 antagonist famotidine and the H3 antagonist thioperamide exerted the opposite effects. Our results indicate that the ultradian EEG rhythm in the PH is susceptible to regulatory influences mediated by the histaminergic system of the brain.