In animals, circadian pacemakers respond to seasonal changes in day length by making corresponding adjustments in the durations of diurnal and nocturnal periods of circadian rhythms; these adjustments mediate effects of photoperiod on breeding and other seasonally recurring phenomena. Little is known about photoperiod responses of human circadian pacemakers. To investigate this question, we recorded and compared circadian rhythm profiles of 15 individuals after chronic exposures to short (8 h) and long (14 h) nights. As occurs in animals, durations of nocturnal periods of active melatonin secretion (11.9 +/- 1.6 vs. 10.3 +/- 1.3 h, df = 14, t = 4.583, P < 0.0005, paired t test), high prolactin secretion (12.9 +/- 2.1 vs. 9.9 +/- 2.2 h, df = 11, t = 2.917, P < 0.01), and sleep (10.6 +/- 0.8 vs. 7.6 +/- 0.4 h, df = 14, t = 17.122, P < 0.0005) were longer after exposure to long nights than after short ones. Durations of nocturnal periods of low rectal temperature (11.6 +/- 2.3 vs. 9.5 +/- 1.6 h, df = 12, t = 3.912, P < 0.001) and rising cortisol secretion (10.8 +/- 1.6 vs. 9.3 +/- 1.9 h, df = 14, t = 3.130, P < 0.005) were also longer. Some of these differences persisted during 24-h periods of enforced wakefulness in constant dim light, indicating that prior exposure to the two regimes induced abiding changes in the timing of internal processes, such as circadian pacemaker oscillations, that control the durations of nocturnal and diurnal periods of the rhythms.
In a previous paper we have described a method for the segregation of viruslike bodies from cancerous animal tissues by the use of nonpolar organic solvents that form emulsions with aqueous liquids containing highly dispersed tissues. These solvents were selected in order to have greater control over the specific gravity and viscosity of the isolating medium and, at the same time, to avoid the difficulties usually encountered in aqueous systems.We had first selected monochlorbenzene for the organic solvent since its low viscosity and specific gravity of 1.1 produced the effect that long periods of high centrifugation were avoided. In a search for solvents similar but possessing an absolute minimum of toxicity, we found the fluorocarbons to be highly suitable. Particularly, the toxicities of Freon 112 t (CCl aF-CClaF) or Genetron 226t (CFaCl-CClaF), are of the order of that of carbon dioxide. Their specific gravities of 1.6 allowed easy adjustment with an admixture of n-heptane to resemble the specific gravity of proteins. N-heptane is a saturated hydrocarbon possessing a density of Ca. 0.7, and the qualities and low toxicity of petrol ether (n-h ex an e). The latter hydrocarbon has been frequently mentioned in the literature as a useful agent to use in extracting fat without affecting the viability of viruses contained in the tissue. The surface tensions of both Freon 112 and Genetron 226 are very low, producing the effect that their emulsions with water break easily and cleanly upon standing or at low-speed centrifugation.For a suitable test virus we selected vaccinia virus grown on the chorioallantoic membranes of Ll-day-old fertile chicken eggs. The homogenization of the infected membranes and the segregation of the viruses contained in them required a total time of less than 2 hours. The viability of the obtained vaccinia viruses compared very favorably with our controls. In reality, dilutions could be made after our Freon-heptane treatment that showed a titer of 10-9 , while our control showed a titer of 10. 7 • In view of the ease of operation of this method and the freedom of the isolated viruses from nonviral proteins and lipids, we believe that the
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