Nocturnal synthesis of the pineal hormone melatonin (MEL) is regulated by the circadian clock in the suprachiasmatic nucleus (SCN) of the hypothalamus. We examined the hypothesis that MEL can feed back to regulate the SCN using a brain slice preparation from rat. We monitored the SCN ensemble firing rate and found that MEL advanced the time of peak firing rate by more than 3 h at restricted circadian times (CTs) near subjective dusk [CT 10-14 (10-14 h after lights on)] and dawn (CT 23-0) on days 2 and 3 after treatment. The effect of MEL at CT 10 was blocked by pertussis toxin. The protein kinase C (PKC) activator, 12-O-tetradecanoylphorbol 13-acetate, reset the SCN firing rate rhythm with a profile of temporal sensitivity congruent with that of MEL. Two specific PKC inhibitors, calphostin C and chelerythrine chloride, independently blocked MEL-induced phase advances at each sensitive period. Furthermore, MEL administration increased PKC phosphotransferase activity transiently to 200% at CT 10 and CT 23, but not at CT 6. These data demonstrate that 1) MEL can directly modulate the circadian timing of the SCN within two windows of sensitivity corresponding to dusk and dawn; and 2) MEL alters SCN cellular function via a pertussis toxin-sensitive G protein pathway that activates PKC.

Nine girls with Rett syndrome (mean age, 10.1 years) were monitored 24 hours a day over a period of 10 weeks using wrist actigraphy. Baseline sleepwake patterns were assessed for 1 week. Subsequently, patients underwent a 4-week melatonin treatment period in a double-blind, placebocontrolled, crdssover protocol that employed a 1-week washout between treatment trials. Melatonin doses ranged from 2.5 to 7.5 mg, based upon individual body weight.

No abstract

The basis of the decline in circadian rhythms with aging was addressed by comparing the patterns of three behavioral rhythms in young and old rats with the in vitro rhythm of neuronal activity in the suprachiasmatic nuclei (SCN), the primary circadian pacemaker. In some old rats, rhythms of body temperature, drinking, and activity retained significant 24-h periodicities in entraining light-dark cycles; in others, one or two of the rhythms became aperiodic. When these rats were 23-27.5 mo old they were killed, and single-unit firing rates in SCN brain slices were recorded continuously for 30 h. There was significant damping of mean peak neuronal firing rates in old rats compared with young. SCN neuronal activities were analyzed with reference to previous entrained behavioral rhythm patterns of individual rats as well. Neuronal activity from rats with prior aperiodic behavioral rhythms was erratic, as expected. Neuronal activity from rats that were still maintaining significant 24-h behavioral rhythmicity at the time they were killed was erratic in most cases but normally rhythmic in others. Thus there was no more congruence between the behavioral rhythms and the brain slice rhythms than there was among the behavioral rhythms alone. These results, the first to demonstrate aberrant SCN firing patterns and a decrease in amplitude in old rats, imply that aging could either disrupt coupling between SCN pacemaker cells or their output, or cause deterioration of the pacemaking properties of SCN cells.

The main mammalian circadian pacemaker is located in the suprachiasmatic nuclei (SCN) of the hypothalamus. Gastrin-releasing peptide (GRP) and its receptor (BB(2)) are synthesized by rodent SCN neurons, but the role of GRP in circadian rhythm processes is unknown. In this study, we examined the phase-resetting actions of GRP on the electrical activity rhythms of hamster and rat SCN neurons in vitro. In both rat and hamster SCN slices, GRP treatment during the day did not alter the time of peak SCN firing. In contrast, GRP application early in the subjective night phase-delayed, whereas similar treatment later in the subjective night phase-advanced the firing rate rhythm in rat and hamster SCN slices. These phase shifts were completely blocked by the selective BB(2) receptor antagonist, [d-Phe(6), Des-Met(14)]-bombesin 6-14 ethylamide. We also investigated the temporal changes in the expression of genes for the BB(1) and BB(2) receptors in the rat SCN using a quantitative competitive RT-PCR protocol. The expression of the genes for both receptors was easily detected, but their expression did not vary over the diurnal cycle. These data show that GRP phase-dependently phase resets the rodent SCN circadian pacemaker in vitro apparently via the BB(2) receptor. Because this pattern of phase shifting resembles that of light on rodent behavioral rhythms, these results support the contention that GRP participates in the photic entrainment of the rodent SCN circadian pacemaker.