Retinohypothalamic tract development: alteration by suprachiasmatic lesions in the neonatal rat

Neurobiological mechanisms for the regulation of mammalian sleep-wake behavior: reinterpretation of historical evidence and inclusion of contemporary cellular and molecular evidence

At its most basic level, the function of mammalian sleep can be described as a restorative process of the brain and body; recently, however, progressive research has revealed a host of vital functions to which sleep is essential. Although many excellent reviews on sleep behavior have been published, none have incorporated contemporary studies examining the molecular mechanisms that govern the various stages of sleep. Utilizing a holistic approach, this review is focused on the basic mechanisms involved in the transition from wakefulness, initiation of sleep and the subsequent generation of slow-wave sleep and rapid eye movement (REM) sleep. Additionally, using recent molecular studies and experimental evidence that provides a direct link to sleep as a behavior, we have developed a new model, the cellular-molecular-network model, explaining the mechanisms responsible for regulating REM sleep. By analyzing the fundamental neurobiological mechanisms responsible for the generation and maintenance of sleep-wake behavior in mammals, we intend to provide a broader understanding of our present knowledge in the field of sleep research.

BMAL1 and CLOCK, two essential components of the circadian clock, are involved in glucose homeostasis

Circadian timing is generated through a unique series of autoregulatory interactions termed the molecular clock. Behavioral rhythms subject to the molecular clock are well characterized. We demonstrate a role for Bmal1 and Clock in the regulation of glucose homeostasis. Inactivation of the known clock components Bmal1 (Mop3) and Clock suppress the diurnal variation in glucose and triglycerides. Gluconeogenesis is abolished by deletion of Bmal1 and is depressed in Clock mutants, but the counterregulatory response of corticosterone and glucagon to insulin-induced hypoglycaemia is retained. Furthermore, a high-fat diet modulates carbohydrate metabolism by amplifying circadian variation in glucose tolerance and insulin sensitivity, and mutation of Clock restores the chow-fed phenotype. Bmal1 and Clock, genes that function in the core molecular clock, exert profound control over recovery from insulin-induced hypoglycaemia. Furthermore, asynchronous dietary cues may modify glucose homeostasis via their interactions with peripheral molecular clocks.

Loss of entrainment and anatomical plasticity after lesions of the hamster retinohypothalamic tract

The suprachiasmatic nuclei receive photic input information directly through a retinohypothalamic tract (RHT) and indirectly through a projection from the intergeniculate leaflet of the lateral geniculate complex, the geniculohypothalamic tract (GHT). Prior work has established that the RHT is sufficient for entrainment, but has not shown whether it is necessary because it has not been possible to transect that pathway. The present study addresses this problem by employing knife cuts to sever the RHT in male hamsters. Three knife cut procedures were used and one of these succeeded in separating the SCN from the optic chiasm in 8 animals with limited damage to the chiasm and the SCN. The effectiveness of the RHT lesion was confirmed by cholera toxin-HRP histochemistry which demonstrated that the knife cuts eliminate the normal retinal innervation of the SCN while sparing projections to thalamic and tectal visual centers. In a light-dark cycle, the lesioned animals exhibit free-running rhythms indicating that the RHT is necessary for entrainment. A surprising observation is the presence of extensive axonal sprouting of retinal fibers in brains of animals with RHT lesions. The newly-formed axons grow extensively into the SCN, anterior hypothalamus and basal forebrain, but form anomalous axonal plexuses which have no evident function.

Persistence of circannual rhythms in ground squirrels with lesions of the suprachiasmatic nuclei

Golden-mantled ground squirrels (Spermophilus lateralis) sustained complete ablation of the suprachiasmatic nuclei (SCN) while being maintained under constant conditions of photoperiod, temperature and food availability. After SCN ablation, most squirrels observed for up to 3 years postsurgically underwent normal circannual cycles in body mass. Other squirrels with similar SCN damage evidenced various anomalies in their circannual body weight cycles. Response of squirrels to SCN ablation was not related to the phase of the annual cycle in which lesions were produced. The variable nature of the effect of SCN ablation on circannual body mass cycles suggests that the SCN are non-essential components of the system underlying the generation or expression of circannual rhythms.

The suprachiasmatic nuclei: two circadian clocks?

Unilateral lesions of the bilaterally paired suprachiasmatic nuclei of the hypothalamus were made in golden hamsters maintained in constant light (LL). Prior to surgery, the circadian rhythm of locomotor activity had dissociated or 'split' into two distinct components in 19 out of 30 hamsters, while in the remaining 11 animals (i.e. non-splitters) a normal rhythm with a single bout of activity was maintained. Following destruction of a major portion (i.e. 50-100%) of a single suprachiasmatic nucleus (SCN) in 6 'split' and 5 'non-split' hamsters, a single circadian bout of activity was observed in all animals. In 9 of these 11 animals the circadian period of the activity rhythm was altered following unilateral SCN lesions. In two split animals sustaining lesions which missed the SCN, but disrupted efferents caudal to the SCN, the split condition was also abolished. Lesions which destroyed less than 50% of a single SCN did not abolish the split condition (n = 6) and had little effect on the circadian pattern of the non-splitters (n = 5). In contrast, bilateral destruction of the suprachiasmatic nuclei eliminated the circadian pattern of activity in both 'split' (n = 5) and 'non-split' (n = 1) animals. These results demonstrate that each SCN is capable by itself of maintaining circadian rhythmicity and that at least two circadian oscillators, or oscillating systems, reside within the suprachiasmatic nuclei.

Sprouting of sympathetic fibers in the hippocampus in the absence of major target cell candidates

Noradrenergic fibers of sympathetic origin enter the hippocampal formation following removal of the cholinergic septal afferents. These sympathetic fibers apparently form synaptic contacts with cells in the hippocampus, but the identity of the postsynaptic cell is unknown. As the two major cell types of the hippocampal formation are the pyramidal cells and the dentate granule cells, we sought to determine how their absence would affect the ingrowth and distribution of the sympathetic fibers following septal lesions. Hippocampal pyramidal cells were selectively removed with kainic acid and granule cells were selectively removed with colchicine 4 days before lesions were placed in the medial septal-diagonal band nuclear complex. Animals were sacrificed 4-5 weeks after the septal lesions and the brains prepared for catecholamine fluorescence histochemistry. Sympathetic fibers innervate the hippocampal formation of brains depleted of pyramidal cells or granule cells. Thus, it appears that neither cell type is an obligatory postsynaptic target or is necessary for the ingrowth of sympathetic fibers. The anomalous sympathetic fibers may be contacting interneurons or glia.

Development of circadian rhythmicity and light responsiveness in the rat suprachiasmatic nucleus: a study using the 2-deoxy[1-14C]glucose method

The suprachiasmatic nucleus (SCN) of the hypothalamus is thought to play a critical role in circadian rhythm generation and entrainment to the light/dark cycle. In adult rats, the SCN shows a circadian rhythm in metabolic activity level as indicated by 2-deoxy[1-14C]glucose uptake. In the present study, the development of this rhythm was investigated. No diurnal difference in uptake was evident in fetal rats 1--2 days before birth. A significant diurnal difference in SCN 2-deoxyglucose uptake was preseno light at night increased the SCN metabolic levels. According to previous studies, on day 1 the SCN is poorly developed and contains few synapses. At this time the retinohypothalamic tract has not yet developed. We found progressive functional maturation of the SCN through day 21, when the rhythm and light responsiveness resembled those of adult rats.