Letter from the Editors

Two behavioural measures of activity at 5HT₁ receptors in mice: RU 24969-induced hyperactivity and 80H-DPAT hypothermia, were measured at 3-hr intervals throughout the 12-12 L-D cycle. Neither parameter showed significant circadian variation.     

Letter from the Editors

Transgenic Research -     

Letter to the Editors

–We have previously shown that NAD kinase and NADP phosphatase activities display circadian rhythms, in the soluble (SN) and membrane-bound (P) fractions of crude extracts of the achlorophyllous ZC mutant of the phytoflagellate Euglena gracilis (which displays circadian rhythmicity of cell division). We determined if changes in the affinity of NADP phosphatase and NAD kinase for their substrates, NADP⁺ and NAD⁺, were occurring by calculating the ratios 100(velocity found in Km conditions/velocity found in saturating conditions). The rationale was that if the affinity remained unchanged according to circadian time (CI), these values should always equal 50, independently of any changes in enzyme quantity; values greater than 50 should indicate increases in enzyme affinity, and values less than 50 decreases in affinity. Our results indicated that these values calculated for NADP phosphatase exhibited a complex pattern of rhythmicity, while those for NAD kinase displayed circadian variations strongly correlated with the rhythms in enzyme activity. The curves showed troughs at CT 00–04 both in dividing and nondividing cells and peaks at CT 18–20 or at CT 08–14 in cells sampled, respectively, from a dividing or a stationary culture. Such variations are indicative of changes in the kinetic properties of the enzyme, which may reflect modifications in its affinity either for effectors (such as Ca2⁺-calmodulin) or for its substrate, NAD⁺. This may be due to (i) the expression of different isoenzymes at different CTs; (ii) different posttranslational modifications of the enzyme; or (iii) concentrations of effectors varying in a circadian manner.     

Letter to the Editors: in Defense of Visual Inspection

Cornelissen and Haus (1) are certainly correct in their assertion that computerized data collection and analysis are useful and important tools in chronobiological research. We also agree that circadian rhythm data should be subjected to multiple complementary analyses whenever possible. In fact, both of our laboratories currently employ on-line collection of behavioural activity data, as well as several different computer-assisted statistical procedures, including analyses similar to those referred to by Cornelissen and Haus in their commentary.