Paraquat (PQ) is a known herbicide that causes acute cell injury by undergoing redox cycling. In previous reports, it has been reported that melatonin reduces PQ-induced hepatic toxicity in vivo, but, at the moment, there is no evidence that this effect occurs in this organ in vitro. In the present study we examined the effect of melatonin on PQ-induced oxidative damage in the liver using a hepatocyte suspension as a biological model. Preincubation of hepatocytes with melatonin (0.5, 1 or 2 mM), 30 min prior to PQ (10 mM) addition, prevented in a dose-and time-dependent manner the loss of viability, the leakage of lactate dehydrogenase, depletion of intra-cellular glutathione and malondialdehyde accumulation induced by the herbicide. Melatonin at the highest dose assayed (2 mM) completely prevented cell damage caused by PQ. These effects of melatonin are similar to those described in studies carried out in vivo. These results confirm that melatonin confers protection against PQ-induced hepatic oxidative stress and show that freshly isolated hepatocyte suspension is an adequate in vitro system for evaluating the cytoprotective effects of melatonin on oxidative injury caused by xenobiotics.

The entrainment limits of the circadian rhythms of feeding activity were studied in Wistar rats exposed to gradually increasing and decreasing or to static light-dark cycles. In the former, the entrainment limits of feeding behavior were 22 h 10 min and 26 h 40 min. In the latter, the upper limit was higher, because rats under zeitgeber period ( t) length = 27 h ( t27) and t28 met the criteria of entrainment. The lower limit, on the other hand, was not modified because none of the t22 animals showed entrained rhythms and one-half of the t23 rats exhibited two components in their circadian feeding rhythms, one with a period of 23 h and the other free running. This 23-h component reflected not only the masking effect of light-dark cycles but also seemed a true light-entrained component. In well-synchronized animals, food intake seemed to depend more on the number of cycles that the animal experienced than on actual time lived; however, other feeding parameters, such as meal frequency and feeding duration, remained constant when expressed per 24 h, irrespective of the t cycle. These results concerning feeding duration, meal frequency, and food intake revealed that the homeostatic and circadian controls interacted to a degree that depended on the type of variable considered. In conclusion, the entrainment limits appeared much more imprecise than they were previously thought to be, because the circadian system can only be partially synchronized near its entrainment limits. The hypothesis that the rat’s circadian system is composed of multiple oscillators with different intrinsic frequencies and varying capacities for light synchronization would explain the partial desynchronization observed near the entrainment limits.

MADRID, J. A., P. MATAS, F. J. SJd~ICHEZ-V/~uZQUEZ AND E. M. CUENCA. A contact eatometerfor automated continuous recording of feeding behavior in rats. PHYSIOL BEHAV 57(I ) 129-134, 1995.--A new contact eatometer designed and built in our laboratory is described. The system makes possible the reliable continuous recording of feeding behavior in rats from the time of weaning, without significantly affecting food intake-and grnwth-related variables. Improved time discrimination and reliability of records -along with the small financial outlay necessary to manufacture the device-make the system appropriate for chronobiological studies which involve the simultaneous use of a large number of animals.

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