Host resets phase of grafted suprachiasmatic nucleus: a 2-DG study of time course of entrainment

The suprachiasmatic nucleus and the circadian time-keeping system revisited

Many physiological and behavioral processes show circadian rhythms which are generated by an internal time-keeping system, the biological clock. In rodents, evidence from a variety of studies has shown the suprachiasmatic nucleus (SCN) to be the site of the master pacemaker controlling circadian rhythms. The clock of the SCN oscillates with a near 24-h period but is entrained to solar day/night rhythm by light. Much progress has been made recently in understanding the mechanisms of the circadian system of the SCN, its inputs for entrainment and its outputs for transfer of the rhythm to the rest of the brain. The present review summarizes these new developments concerning the properties of the SCN and the mechanisms of circadian time-keeping. First, we will summarize data concerning the anatomical and physiological organization of the SCN, including the roles of SCN neuropeptide/neurotransmitter systems, and our current knowledge of SCN input and output pathways. Second, we will discuss SCN transplantation studies and how they have contributed to knowledge of the intrinsic properties of the SCN, communication between the SCN and its targets, and age-related changes in the circadian system. Third, recent findings concerning the genes and molecules involved in the intrinsic pacemaker mechanisms of insect and mammalian clocks will be reviewed. Finally, we will discuss exciting new possibilities concerning the use of viral vector-mediated gene transfer as an approach to investigate mechanisms of circadian time-keeping.

Circadian rhythmicity of vasopressin levels in the cerebrospinal fluid of suprachiasmatic nucleus-lesioned and -grafted rats

Transplantation of the fetal suprachiasmatic nucleus (SCN) in arrhythmic SCN-lesioned rats can reinstate circadian drinking rhythms in 40% to 50% of the cases. In the current article, it was investigated whether the failure in the other rats could be due to the absence of a circadian rhythm in the grafted SCN, using a circadian vasopressin (VP) rhythm in the cerebrospinal fluid (CSF) as the indicator for a rhythmic SCN. CSF was sampled in continuous darkness from-intact control rats and SCN-lesioned and -grafted rats. VP could be detected in all samples, with concentrations of 15 to 30 pg/ml in the control rats and 5 to 15 pg/ml in the grafted rats. A circadian VP rhythm with a two- to threefold difference between peak and nadir values was found in all 7 control rats but in only 4 of 13 experimental rats, despite the presence of a VP-positive SCN in all grafts. A circadian VP rhythm was present in 2 drinking rhythm-recovered rats (6 of 13) and in 2 nonrecovery rats. Apparently, in these latter rats, the failure of the grafted SCN to restore a circadian drinking rhythm cannot be attributed to a lack of rhythmicity in the SCN itself. Thus, the presence of a rhythmic grafted SCN, as is deduced from a circadian CSF VP rhythm, appears not to be sufficient for restoration of a circadian drinking rhythm in SCN-lesioned arrhythmic rats.

Melatonin entrains the restored circadian activity rhythms of syrian hamsters bearing fetal suprachiasmatic nucleus grafts

A circadian pacemaker consists of at least three essential features: the ability to generate circadian oscillations, an output signal, and the ability to be entrained by external signals. In rodents, ablation of the suprachiasmatic nucleus (SCN) results in the loss of circadian rhythms in activity. Rhythmicity can be restored by transplanting fetal SCN into the brain of the lesioned animal, demonstrating the first two of the essential pacemaker features within the grafts. External signals, such as the light/dark cycle, have not, however, been shown to entrain the restored rhythms. Melatonin injections are an effective entraining stimulus in fetal and neonatal Syrian hamsters of the same developmental ages used to provide donor tissue for transplantation. Therefore, melatonin was used to test the hypothesis that SCN grafts contain an entrainable pacemaker. Daily injections of melatonin were given to SCN-lesioned hosts beginning on the day after transplantation of fetal SCN. Two groups that received melatonin at different times of day 12 hr apart each showed significantly clustered phases but with average phases that differed by 8.67 hr. Thus melatonin was able to entrain the restored circadian activity rhythms. In contrast to these initial injections, injections given 6 weeks after transplantation were unable to entrain or phase shift the rhythms. The results demonstrate that SCN grafts contain an entrainable circadian pacemaker. In addition, the results also indicate that the fetal SCN is directly sensitive to melatonin and, as with intact hamsters, sensitivity to melatonin is lost during SCN development.

Output signals of the SCN

The suprachiasmatic nucleus (SCN) of the hypothalamus controls circadian rhythmicity in mammals (for reviews, see Refs. 33 and 59). Responses modulated by the SCN are numerous and include rhythms in sleep/wake cycles, locomotor, gnawing and general activity, temperature, ingestive behavior, and rhythms of hormonal and peptide secretions. Though a great deal is known about the neuroanatomical organization of the SCN, many elements of the structure-function relationships remain to be discovered. For example, it is not known which cellular components of the SCN function as driving pacemakers or which output signal(s) of these pacemakers are important for each of its functions. While some signals from pacemaker cells reach target regions by neural efferents, there is also evidence that rhythmic responses can be controlled by diffusible signals. This article reviews output signals from the SCN. The data available suggest that neural efferents are not necessary for the control of locomotor activity rhythms. Evidence that a diffusible signal is sufficient to restore activity rhythms in SCN-lesioned animals is described. Finally, possible physiological mechanisms for diffusible signals are suggested.