William J. Schwartz

Circadian timekeeping in BALB/c and C57BL/6 inbred mouse strains

Circadian rhythms of locomotion (wheel-running activity) in 12 inbred mouse strains were recorded for interstrain differences in tau DD, the endogenous (free-running) period of the circadian pacemaker measured in constant environmental darkness. The results indicate that 1 or more genetic loci influence the value of tau DD, and a large (50 min) difference in mean tau DD between 2 of the strains, BALB/cByJ and C57BL/6J, allowed further characterization of the origins and inheritance of the polymorphic expression of this circadian pacemaker property. The interstrain difference in mean tau DD was associated with an interstrain difference in light-induced shifts of the phase of the free-running locomotor rhythm; the BALB/c strain (with the shorter mean tau DD) displayed relatively fewer advance phase shifts. Neither the history of previous light exposure, albinism, nor elevated circulating testosterone levels could account for the interstrain difference in mean tau DD. The value of tau DD based on the circadian rhythm of drinking activity (with the running wheel removed) was longer than that based on locomotion; this discrepancy was significantly greater and more variable in BALB/c than in C57BL/6 mice, though the interstrain difference in mean tau DD could not be attributed entirely to this effect. Reciprocal F1 hybrids of BALB/c x C57BL/6 matings revealed dominance of the C57BL/6 genotype, no sex linkage, and a significant (but small) maternal effect. Examination of CXB recombinant inbred strains provided no support for the hypothesis of monogenic inheritance. Further study of inherited differences in the BALB/c and C57BL/6 strains may be a useful noninvasive experimental approach for investigation of the neurobiological substrates of circadian rhythmicity.

Morning and evening circadian oscillations in the suprachiasmatic nucleus in vitro

Daily biological rhythms are governed by an innate timekeeping mechanism, or ‘circadian clock’. In mammals, a clock in the suprachiasmatic nucleus (SCN) comprises multiple autonomous single-cell oscillators, but it is unclear how SCN cells interact to form a tissue with coherent metabolic and electrical rhythms that might account for circadian animal behaviors. Here we demonstrate that the circadian rhythm of SCN electrophysiological activity, recorded as a single daytime peak in hamster hypothalamic coronal slices, shows two distinct peaks when slices are cut in the horizontal plane. Substantiating an idea initially derived from behavioral observations, the properties of these two peaks indicate functional organization of SCN tissue as a clock with two oscillating components.

In search of the pathways for light-induced pacemaker resetting in the suprachiasmatic nucleus.

Within the suprachiasmatic nucleus (SCN) of the mammalian hypothalamus is a circadian pacemaker that functions as a clock. Its endogenous period is adjusted to the external 24-h light-dark cycle, primarily by light-induced phase shifts that reset the pacemakers oscillation. Evidence using a wide variety of neurobiological and molecular genetic tools has elucidated key elements that comprise the visual input pathway for SCN photoentrainment in rodents. Important questions remain regarding the intracellular signals that reset the autoregulatory molecular loop within photoresponsive cells in the SCNs retino-recipient subdivision, as well as the intercellular coupling mechanisms that enable SCN tissue to generate phase shifts of overt behavioral and physiological circadian rhythms such as locomotion and SCN neuronal firing rate. Multiple neurotransmitters, protein kinases, and photoinducible genes add to system complexity, and we still do not fully understand how dawn and dusk light pulses ultimately produce bidirectional, advancing and delaying phase shifts for pacemaker entrainment.

Localization of immunoreactive enkephalins in GABA synthesizing neurons of the rat neostriatum

The localization of immunoreactive glutamic acid decarboxylase (GAD) and enkephalin-like immunoreactivity was examined in serial, 4-micron frozen sections of the caudate nucleus from rats pretreated with colchicine. Colocalization was found in numerous caudate neurons of medium size. Cell counts of corresponding labeled neurons in paired adjacent sections showed that GAD and enkephalin-like immunoreactivity coexist in about one half of the caudate cell populations containing each of these substances.

Tracking the seasons: the internal calendars of vertebrates

Animals have evolved many season-specific behavioural and physiological adaptations that allow them to both cope with and exploit the cyclic annual environment. Two classes of endogenous annual timekeeping mechanisms enable animals to track, anticipate and prepare for the seasons: a timer that measures an interval of several months and a clock that oscillates with a period of approximately a year. Here, we discuss the basic properties and biological substrates of these timekeeping mechanisms, as well as their reliance on, and encoding of environmental cues to accurately time seasonal events. While the separate classification of interval timers and circannual clocks has elucidated important differences in their underlying properties, comparative physiological investigations, especially those regarding seasonal prolactin secretions, hint at the possibility of common substrates.

In vivo metabolic activity of the suprachiasmatic nuclei: a comparative study.

In vivo glucose utilization was measured in the suprachiasmatic nuclei (SCN) of the rat, monkey, and cat using the 14C-labeled deoxyglucose technique. SCN metabolic activity in all species was endogenously rhythmic with high levels during the subjective daylight portion of the 24 h day. Such phase conservation across night-, day-, and randomly-active species is in agreement with formal analyses of the properties of entrainable circadian oscillators, and our data suggest that the biochemical processes which underlie the activity of this circadian clock are similar in mammals with differing patterns of expressed circadian rhythmicity.

Physiology and Pharmacology of Biological Rhythms

Abstract edited by Peter H. Redfern and Bjorn Lemmer, Springer-Verlag, 1997. £218.50 (xxiv+668 pages) ISBN 3 540 61525 3

Arginine vasopressin: a novel peptide rhythm in cerebrospinal fluid

Abstract The neuropeptide, arginine vasopressin, exhibits a prominent daily rhythm in the cerebrospinal fluid (CSF) of several mammals. The CSF rhythm is an endogenously generated, circadian rhythm and is entrained to the 24 h period by the daily light-dark cycle. The circadian regulation of vasopressin in CSF is effectively insulated from the osmotic regulation of the peptide in blood. The CSF vasopressin rhythm is produced by a neural system anatomically separate from the classical hypothalamo-neurohypophyseal system. The suprachiasmatic nuclei, the site of a hypothalamic circadian pacemaker, are an important component of this newly defined neural system and are necessary for rhythm generation. These findings support the concept that brain vasopressin is both functionally and anatomically compartmentalized for specific actions within the CNS.

Differential regulation of fos family genes in the ventrolateral and dorsomedial subdivisions of the rat suprachiasmatic nucleus.

Extensive studies have established that light regulates c-fos gene expression in the suprachiasmatic nucleus, the site of an endogenous circadian clock, but relatively little is known about the expression of genes structurally related to c-fos, including fra-1, fra-2 and fosB. We analysed the photic and temporal regulation of these genes at the messenger RNA and immunoreactive protein levels in rat suprachiasmatic nucleus, and we found different expression patterns after photic stimulation and depending on location in the ventrolateral or dorsomedial subdivisions. In the ventrolateral suprachiasmatic nucleus, c-fos, fra-2 and fosB expression was stimulated after a subjective-night (but not subjective-day) light pulse. Expression of the fra-2 gene was prolonged following photic stimulation, with elevated messenger RNA and protein levels that appeared unchanged for at least a few hours beyond the c-fos peak. Unlike c-fos and fra-2, the fosB gene appeared to be expressed constitutively in the ventrolateral suprachiasmatic nucleus throughout the circadian cycle; immunohistochemical analysis suggested that delta FosB was the protein product accounting for this constitutive expression, while FosB was induced by the subjective-night light pulse. In the dorsomedial suprachiasmatic nucleus, c-fos and fra-2 expression exhibited an endogenous circadian rhythm, with higher levels during the early subjective day, although the relative abundance was much lower than that measured after light pulses in the ventrolateral suprachiasmatic nucleus. Double-label immunohistochemistry suggested that some of the dorsomedial cells responsible for the circadian expression of c-Fos also synthesized arginine vasopressin. No evidence of suprachiasmatic nucleus fra-1 expression was found. In summary, fos family genes exhibit differences in their specific expression patterns in the suprachiasmatic nucleus, including their photic and circadian regulation in separate cell populations in the ventrolateral and dorsomedial subdivisions. The data, in combination with our previous results [Takeuchi J. et al. (1993) Neuron 11, 825-836], suggest that activator protein-1 binding sites on ventrolateral suprachiasmatic nucleus target genes are constitutively occupied by DeltaFosB/JunD complexes, and that c-Fos, Fra-2, FosB and JunB compete for binding after photic stimulation. The differential regulation of fos family genes in the ventrolateral and dorsomedial suprachiasmatic nucleus suggests that their circadian function(s) and downstream target(s) are likely to be cell specific.

A discrete lesion of ventral hypothalamus and optic chiasm that disturbed the daily temperature rhythm

SummaryA patient with a discrete metastasis in the ventral hypothalamus and optic chiasm is reported, who developed an abnormal daily rhythm of oral temperature without alteration of the 24-h mean temperature. This region, its afferents, and its efferents appear to be important in the neural regulation of human circadian rhythmicity.