The circadian clock in the suprachiasmatic nucleus of the hypothalamus (SCN) contains multiple autonomous single-cell circadian oscillators and their basic intracellular oscillatory mechanism is beginning to be identified. Less well understood is how individual SCN cells create an integrated tissue pacemaker that produces a coherent read-out to the rest of the organism. Intercellular coupling mechanisms must coordinate individual cellular periods to generate the averaged, genotype-specific circadian period of whole animals. To noninvasively dissociate this circadian oscillatory network in vivo, we (T.C. and A.D.-N.) have developed an experimental paradigm that exposes animals to exotic light-dark (LD) cycles with periods close to the limits of circadian entrainment. If individual oscillators with different periods are loosely coupled within the network, perhaps some of them would be synchronized to the external cycle while others remain unentrained. In fact, rats exposed to an artificially short 22 hr LD cycle express two stable circadian motor activity rhythms with different period lengths in individual animals. Our analysis of SCN gene expression under such conditions suggests that these two motor activity rhythms reflect the separate activities of two oscillators in the anatomically defined ventrolateral and dorsomedial SCN subdivisions. Our "forced desychronization" protocol has allowed the first stable separation of these two regional oscillators in vivo, correlating their activities to distinct behavioral outputs, and providing a powerful approach for understanding SCN tissue organization and signaling mechanisms in behaving animals.

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.

Most processes within organisms, and most interactions between organisms and their environment, have distinct time profiles. The temporal coordination of such processes is crucial across levels of biological organization, but disciplines differ widely in their approaches to study timing. Such differences are accentuated between ecologists, who are centrally concerned with a holistic view of an organism in relation to its external environment, and chronobiologists, who emphasize internal timekeeping within an organism and the mechanisms of its adjustment to the environment. We argue that ecological and chronobiological perspectives are complementary, and that studies at the intersection will enable both fields to jointly overcome obstacles that currently hinder progress. However, to achieve this integration, we first have to cross some conceptual barriers, clarifying prohibitively inaccessible terminologies. We critically assess main assumptions and concepts in either field, as well as their common interests. Both approaches intersect in their need to understand the extent and regulation of temporal plasticity, and in the concept of 'chronotype', i.e. the characteristic temporal properties of individuals which are the targets of natural and sexual selection. We then highlight promising developments, point out open questions, acknowledge difficulties and propose directions for further integration of ecological and chronobiological perspectives through Wild Clock research.

Recent studies suggest (Schwartz et al., 1979) that local cerebral glucose utilization, measured with the deoxyglucose technique (Sokoloff et al., 1977), correlates most closely with electrical activity in the neuropil in general and synaptic terminals in particular. Presumably, increased glucose utilization associated with increased impulse activity in nervous tissue is, as is oxygen consumption (Ritchie, 1967;Greengard and Ritchie, 1971: De-Weer, 1973, principally due to enhanced activity of the sodium pump. If the increased energy metabolism during impulse activity is used mainly for reconstitution of electrochemical gradients, then it is to be expected that cellular components with larger surface-to-volume ratios will have larger energy demands (Ritchie, 1967;Greengard and Ritchie, 1971;DeWeer, 1975) and, thus, greater rates of glucose utilization. It would be of value for the interpretation of studies that employ the autoradiographic deoxyglucose method to identify the cellular elements in which neural activity and energy metabolism are most closely linked. We have, therefore, studied an in vitro preparation of rat posterior pituitary, which represents a relatively enriched population of axon terminals (Nordmann, 1977) and may serve as a model for synaptic endings in the brain. Because the pituitary is a neurosecretory organ, we have also studied the influence of the secretory process in this system on energy metabolism. As an index of glucose utilization, we have measured the rate at which [ ''C]deoxyglucose is phosphorylated by hexokinase and trapped in the tissue incubated in vitro. This is the in vifro equivalent of the deoxyglucose method in which trapped [ '4C]deoxyglucose-6-phosphate is visualized and measured autoradiographically (Sokoloff et al., 1977). MATERIALS AND METHODSMale Sprague-Dawley rats (180-250 g) were decapitated, and the pituitary glands were removed rapidly and placed in balanced salt solution (BSS) consisting of 10 mM

Physiological stimulation of the hypothalamo-neurohypophysial system by salt loading of rats resulted in a dramatically increased glucose utilization in the posterior pituitary but not in the paraventricular or supraoptic nuclei. The good correlation between glucose utilization and neural activity in the posterior pituitary (that is, nerve terminals) contrasted with the lack of correlation in the paraventricular and supraoptic nuclei (that is, the sites of the cell bodies of the same neurons). This difference in the metabolic response to functional activity between the two regions of these neurons can be explained by the differences in surface-to-volume ratios of these regions.