In vivo metabolic activity of the suprachiasmatic nuclei: non-uniform intranuclear distribution of 14C-labeled deoxyglucose uptake

Vasculature of the Suprachiasmatic Nucleus: Pathways for Diffusible Output Signals

Transplant studies demonstrate unequivocally that the suprachiasmatic nucleus (SCN) produces diffusible signals that can sustain circadian locomotor rhythms. There is a vascular portal pathway between the SCN and the organum vasculosum of the lamina terminalis in mouse brain. Portal pathways enable low concentrations of neurosecretions to reach specialized local targets without dilution in the systemic circulation. To explore the SCN vasculature and the capillary vessels whereby SCN neurosecretions might reach portal vessels, we investigated the blood vessels (BVs) of the core and shell SCN. The arterial supply of the SCN differs among animals, and in some animals, there are differences between the 2 sides. The rostral SCN is supplied by branches from either the superior hypophyseal artery (SHpA) or the anterior cerebral artery or the anterior communicating artery. The caudal SCN is consistently supplied by the SHpA. The rostral SCN is drained by the preoptic vein, while the caudal is drained by the basal vein, with variations in laterality of draining vessels. In addition, several key features of the core and shell SCN regions differ: Median BV diameter is significantly smaller in the shell than the core based on confocal image measurements, and a similar trend occurs in iDISCO-cleared tissue. In the cleared tissue, whole BV length density and surface area density are significantly greater in the shell than the core. Finally, capillary length density is also greater in the shell than the core. The results suggest three hypotheses: First, the distinct arterial and venous systems of the rostral and caudal SCN may contribute to the in vivo variations of metabolic and neural activities observed in SCN networks. Second, the dense capillaries of the SCN shell are well positioned to transport blood-borne signals. Finally, variations in SCN vascular supply and drainage may contribute to inter-animal differences.

Disturbance and strategies for reactivation of the circadian rhythm system in aging and Alzheimer's disease

Circadian rhythm disturbances, such as sleep disorders, are frequently seen in aging and are even more pronounced in Alzheimer's disease (AD). Alterations in the biological clock, the suprachiasmatic nucleus (SCN), and the pineal gland during aging and AD are considered to be the biological basis for these circadian rhythm disturbances. Recently, our group found that pineal melatonin secretion and pineal clock gene oscillation were disrupted in AD patients, and surprisingly even in non-demented controls with the earliest signs of AD neuropathology (neuropathological Braak stages I-II), in contrast to non-demented controls without AD neuropathology. Furthermore, a functional disruption of the SCN was observed from the earliest AD stages onwards, as shown by decreased vasopressin mRNA, a clock-controlled major output of the SCN. The observed functional disconnection between the SCN and the pineal from the earliest AD stage onwards seems to account for the pineal clock gene and melatonin changes and underlies circadian rhythm disturbances in AD. This paper further discusses potential therapeutic strategies for reactivation of the circadian timing system, including melatonin and bright light therapy. As the presence of melatonin MT1 receptor in the SCN is extremely decreased in late AD patients, supplementary melatonin in the late AD stages may not lead to clear effects on circadian rhythm disorders.

Pineal clock gene oscillation is disturbed in Alzheimer's disease, due to functional disconnection from the "master clock"

The suprachiasmatic nucleus (SCN) is the "master clock" of the mammalian brain. It coordinates the peripheral clocks in the body, including the pineal clock that receives SCN input via a multisynaptic noradrenergic pathway. Rhythmic pineal melatonin production is disrupted in Alzheimer's disease (AD). Here we show that the clock genes hBmal1, hCry1, and hPer1 were rhythmically expressed in the pineal of controls (Braak 0). Moreover, hPer1 and hbeta1-adrenergic receptor (hbeta1-ADR) mRNA were positively correlated and showed a similar daily pattern. In contrast, in both preclinical (Braak I-II) and clinical AD patients (Braak V-VI), the rhythmic expression of clock genes was lost as well as the correlation between hPer1 and hbeta1-ADR mRNA. Intriguingly, hCry1 mRNA was increased in clinical AD. These changes are probably due to a disruption of the SCN control, as they were mirrored in the rat pineal deprived of SCN control. Indeed, a functional disruption of the SCN was observed from the earliest AD stages onward, as shown by decreased vasopressin mRNA, a clock-controlled major output of the SCN. Thus, a functional disconnection between the SCN and the pineal from the earliest AD stage onward could account for the pineal clock gene changes and underlie the circadian rhythm disturbances in AD.

The suprachiasmatic nucleus is a functionally heterogeneous timekeeping organ

Ever since the locus of the brain clock in the suprachiasmatic nucleus (SCN) was first described, methods available have both enabled and encumbered our understanding of its nature at the level of the cell, the tissue, and the animal. A combination of in vitro and in vivo approaches has shown that the SCN is a complex heterogeneous neuronal network. The nucleus is composed of cells that are retinorecipient and reset by photic input; those that are reset by nonphotic inputs; slave oscillators that are rhythmic only in the presence of the retinohypothalamic tract; endogenously rhythmic cells, with diverse period, phase, and amplitude responses; and cells that do not oscillate, at least on some measures. Network aspects of SCN organization are currently being revealed, but mapping these properties onto cellular characteristics of electrical responses and patterns of gene expression are in early stages. While previous mathematical models focused on properties of uniform coupled oscillators, newer models of the SCN as a brain clock now incorporate oscillator and gated, nonoscillator elements.

Cycle of period gene expression in a diurnal mammal (Spermophilus tridecemlineatus): implications for nonphotic phase shifting

Ground squirrels, Spermophilus tridecemlineatus, were kept in a 12:12 h light-dark cycle. As expected for a diurnal species, their locomotor activity occurred almost entirely in the daytime. Expression of mPer1 and mPer2 in the suprachiasmatic nucleus was studied at six time points by in situ hybridization. For both these genes, mRNA was highest in the first part of the subjective day (about zeitgeber time 5). This is close to the time when mPer1 and mPer2 expression is maximal in nocturnal rodents. These results have implications for understanding nonphotic phase response curves in diurnal species and thereby for guiding research on nonphotic phase shifting in people.

Multiple oscillators in the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) of the hypothalamus is the site of the pacemaker that controls circadian rhythms of a variety of physiological functions. Data strongly indicate the majority of the SCN neurons express self-sustaining oscillations that can be detected as rhythms in the spontaneous firing of individual neurons. The period of single SCN neurons in a dissociated cell culture is dispersed in a wide range (from 20h to 28h in rats), but that of the locomotor rhythm is close to 24h, suggesting individual oscillators are coupled to generate an averaged circadian period in the nucleus. Electrical coupling via gap junctions, glial regulation, calcium spikes, ephaptic interactions. extracellular ion flux, and diffusible substances have been discussed as possible mechanisms that mediate the interneuronal rhythm synchrony. Recently, GABA (gamma-aminobutyric acid), a major neurotransmitter in the SCN, was reported to regulate cellular communication and to synchronize rhythms through GABA(A) receptors. At present, subsequent intracellular processes that are able to reset the genetic loop of oscillations are unknown. There may be diverse mechanisms for integrating the multiple circadian oscillators in the SCN. This article reviews the knowledge about the various circadian oscillations intrinsic to the SCN, with particular focus on the intercellular signaling of coupled oscillators.

Circannual morphometric investigations of the rat suprachiasmatic nucleus after pinealectomy, ganglionectomy and thyroidectomy

We karyometrically investigated the nucleus suprachiasmaticus (SCN) which had been manipulated in several ways in order to analyze the functional importance of the pineal gland on the primary pacemaker of mammals in the course of the year. The manipulation modes were (i) pinealectomy (PX), and (ii) sympathetic denervation of the pineal by bilateral extirpation of the upper cervical ganglia (ganglionectomy, GX). Additionally, the influence of the inactivated pineal, obtained through hypothyroidism which was realized by (iii) subtotal thyroidectomy (TX), was also investigated. With respect to annual oscillations the results of our investigations were able to illustrate the following. (1) The SCN consists of at least two parts (ventrolateral and dorsomedial) each with different functions and relationships. The nuclei of the ventrolateral cells are bigger and there are many indications both in our own research and from literature that the neurons of this part are involved in the generation of rhythms. (2) The size of the cell nuclei of the ventrolateral part shows annual patterns. In the course of the year the maxima of the nuclear volume were registered in March and September (bimodal pattern, equinox, L:D = 12:12). PX, GX or TX only negligibly changed the bimodal annual pattern. However, in comparison the smallest cell nuclei were registered in the winter (short day). The much smaller cell nuclei of the dorsomedial part likewise show bimodal patterns but only in the experimental groups. The control group of this part shows an unimodal annual curve with a minimum at long-day conditions (June, L:D = 16:8). (3) All manipulations which inactivated the pineal or reduced the content of melatonin (PX, GX, and TX) were followed by an increase (activation) of cell nuclei of the SCN. In contrast to these effects, an increase of thyroxine (by exposure to cold), has an opposite effect (not documented here). In conclusion these results indicate, without a doubt, that a negative correlation exists, functionally, between the SCN and the pineal (in the same annual experiment the nuclear size of the pinealocytes was increased, under short-day conditions in December, and decreased under long-day circumstances in June). Additionally, it could be shown that the degree of negative correlation between the pineal and the SCN was seasonally dependent. The lowest effects of PX, GX and TX were registered at short-day conditions (December, L:D = 8:16).

Projections from the lateral geniculate nucleus to the hypothalamus of the Mongolian gerbil (Meriones unguiculatus): an anterograde and retrograde tracing study

The lateral geniculate nucleus of the thalamus sends efferents to the hypothalamic suprachiasmatic nucleus, which is involved in generation and entrainment of several circadian rhythms. It seems reasonable to believe that the lateral geniculate conveys visual information about the length of the photoperiod to the circadian oscillator. In order to study in more detail the topographical relationship between the lateral geniculate and the suprachiasmatic nucleus, anterograde tracing with Phaseolus vulgaris leucoagglutinin (PHA-L) and retrograde tracing with wheatgerm agglutinin coupled to horseradish peroxidase (WGA-HRP) were performed in the gerbil. After iontophoretic injections of PHA-L in the lateral geniculate, a large number of PHA-L-immunoreactive fibers and nerve terminals were observed in the ventrolateral part of the suprachiasmatic nucleus. Nerve fibers were also present in the ventromedial and dorsolateral portions, particularly in the caudal half of the nucleus. PHA-L-immunoreactive nerve fibers continued outside the borders of the suprachiasmatic nucleus to the adjacent anterior hypothalamic, the periventricular, and the subparaventricular areas. A moderate number of fibers entered the lateral hypothalamic area and the tuber cinerum via the optic tract and chiasm. Moreover, the paraventricular nucleus, the supraoptic nucleus, the medial preoptic area, the lateral preoptic area, and the supramammillary nucleus contained a few labeled fibers. In all parts of the hypothalamus receiving an input from the lateral geniculate, fine beaded immunoreactive fibers with varicosities and nerve terminals were observed, some of which were found in close apposition to hypothalamic neurons. Only after labeling of neurons in the intergeniculate leaflet of the lateral geniculate nucleus, fibers were found in the hypothalamus. This topographical organization of the geniculohypothalamic pathway was supported by retrograde tracing after injections of WGA-HRP in the suprachiasmatic area. In these experiments, retrograde labeled neurons were observed in the intergeniculate leaflet and, in agreement with the anterograde studies, most of labeling was observed in the ipsilateral side. These results confirm that the suprachiasmatic nucleus receives a substantial input from the intergeniculate leaflet of the lateral geniculate. Moreover, the present data demonstrate that the suprachiasmatic nucleus is not the only nucleus that receives a direct visual input. Thus other hypothalamic areas might be influenced by a direct rhythmic neuronal input as well.

Investigations on day-night differences of vesicle densities in synapses of the rat suprachiasmatic nucleus

The present study was conducted to test whether the well-known circadian alterations in physiological and metabolical parameters of the hypothalamic suprachiasmatic nucleus (SCN) are accompanied by day-night differences in the number of vesicles in intrinsic synapses of the nucleus. Two groups of 5 adult male rats each were killed at mid-light or mid-dark, respectively, by perfusion with Karnovsky's fluid. The SCN were removed and processed for routine electron microscopy. In medial parts of the nucleus, synapses were characterized as being of Gray type I (asymmetrical), Gray type II (symmetrical) or of intermediate form, and the vesicles per synaptic profile (VPSP) were counted over a defined area. It was found that the ratio of different types of synapses did not differ between day and night. Median VPSP numbers were slightly augmented in animals killed at mid-dark when compared to those obtained from mid-light rats, this difference, however, lacks statistical significance. The present results suggest that vesicle density as a morphological parameter does not clearly parallel the circadian changes in SCN metabolism.

Differential rates of glucose metabolism across subregions of the subfornical organ in Brattleboro rats

We applied [14C]deoxyglucose autoradiography and imaging techniques to determine rates of glucose metabolism in distinct subdivisions of the subfornical organ (SFO) of conscious Brattleboro rats. Seven anatomically-defineD SFO subregions were discerned having metabolic activities that differed from one another by as much as 29% in water-sated Brattleboro rats. The highest metabolic activity was found in the ventromedial zone of central and caudal subregions where previous studies identified the greatest densities of neurons, capillaries, putative angiotensin receptors, and angiotensin-immunoreactive fibers. Homozygous Brattleboro rats had rates of glucose metabolism that were 39-68% greater than those in corresponding SFO subregions of Long-Evans rats; these differences were accentuated by about 50% following 18 h of water deprivation. Exogenous treatment of Brattleboro rats with vasopressin uniformly normalized subregional glucose metabolism in the SFO. In Sprague-Dawley rats, water deprivation over 120 h provoked greater increases in metabolism of ventromedial than of dorsolateral SFO zones in amounts similar to the differences between Long-Evans and Brattleboro rats. The findings identify focal areas of high metabolic activity within subregions of the SFO where central responses are likely initiated to defend against homeostatic disturbances. The data represent further evidence for the probability that angiotensin II, as both hormone and neurotransmitter, is a metabolic stimulant of its target cells in the nervous system.