Gerard Overkamp

Changes in behaviour and body weight following a single or double social defeat in rats.

In a series of experiments, the consequences of a single and double social conflict on various behaviours and body weight in rats were studied. Animals were subjected to social defeat by placing them into the territory of an aggressive male conspecific for one hour, either once, or twice at the same time on two consecutive days. To assess the consequences of social defeat, three experiments were performed with independent groups of rats. In the first experiment, an open field test was performed two days after the last conflict. Locomotor activity was strongly reduced after social defeat. There were no differences between the single and double defeat group. To assess the effects of social defeat on subsequent social behaviour, a second experiment was performed in which experimental animals were confronted with an unfamiliar non-aggressive rat two days after a single or double conflict. Social defeat resulted in a reduction of social contact with the unfamiliar conspecific. There was no difference between the single and double conflict group. In the third experiment, the effects of social conflict on food intake, body weight and saccharine preference were measured. Food intake was not affected after a single conflict, but in the double conflict group food intake was decreased for several days. Body weight gain was decreased after both single and double social defeat. The decrease was stronger in the double conflict group. Water intake and saccharine preference were not significantly affected. This study revealed that social defeat in rats causes pronounced changes in various behaviours and body weight. Different aspects of behaviour are differentially affected by defeat with respect to the magnitude and time course of the changes induced. Moreover, different behavioural parameters are differentially sensitive to repetition of the stressor.

Long-term changes in open field behaviour following a single social defeat in rats can be reversed by sleep deprivation

The long-term consequences of a single social defeat on open field behaviour in rats were studied, with special emphasis on the time course of stress-induced changes. Animals were subjected to social defeat by placing them into the territory of an aggressive male conspecific for 1 h. After the defeat session experimental animals were returned to their home cage and their own room, receiving no further cues from the resident. Other animals serving as controls were placed in a clean and empty cage for 1 h. Five-minute open field tests were performed on days 1, 2, 7, 14, and 28 after defeat, with independent groups of rats. Locomotion of the animals was recorded and analyzed with an automated video system. Social defeat resulted in a strong subsequent reduction in open field activity, which lasted till at least 7 days after the conflict. Differences in total travelled distance were no longer significant 2 weeks after the conflict. The latency for moving to the outer ring of the open field arena after the start of the test was still significantly longer 4 weeks after defeat. The stress-induced reduction in open field locomotion could be reversed by 12-h sleep deprivation during the resting phase, an intervention known to have antidepressant effects in humans. Possible relevance of the present findings with respect to human affective disorders is discussed.

Behavioural and physiological consequences of a single social defeat in Roman high- and low-avoidance rats

The behavioural and physiological consequences of a single social defeat were studied in Roman high-avoidance (RHA) and Roman low-avoidance (RLA) rats, two rat lines with a genetically determined difference in the way of responding to or coping with stressors. Animals were subjected to social defeat by placing them in the cage of an aggressive male conspecific for 1 h. In both RHA and RLA rats, social defeat induced a profound increase in body temperature during the circadian resting phase, lasting for up to 10 days after the conflict. The increase in resting temperature was paralleled by a slight decrease in spontaneous home cage activity. Food intake and growth were suppressed for a number of days, resulting in a long-lasting lower body weight compared to non-stressed control animals. An open field test 2 days after defeat showed a social stress-induced decrease in locomotion in a novel environment. Despite the well-known differentiation between RHA and RLA rats in their behavioural and neuroendocrine response pattern to acute environmental challenges, the present study did not show major differences in the long-term consequences of social defeat.

COMPENSATION IN RESTING METABOLISM FOR EXPERIMENTALLY INCREASED ACTIVITY

Abstract To study zebra finch allocation of energy to day and night at two different workloads, we assessed the daily energy turnover from: (1) metabolizable energy of the food, and (2) doubly-labeled water. In both experiments we imposed two levels of activity on captive zebra finches (Taeniopygia guttata), by applying different computer-controlled workload schedules. A low workload required 20 hops, and a high workload 40 hops to obtain 10 s access to food. In experiment 1, we further measured nocturnal energy expenditure by overnight oxygen consumption. From experiment 2 we derived an estimate of the costs of hopping activity, from inter-individual association of daily amount of hopping and daily energy expenditure. Surprisingly, the daily energy budget was, on average, reduced slightly when birds were subjected to a high workload. Since hopping activity was 50% higher during the high workload than during the low workload, the birds apparently compensated, even over-compensated, for the increased energetic demands of activity. Nocturnal energy expenditure was indeed reduced for the high workload, which was largely due to a reduction in resting metabolic rate. Economizing on energy was more than could have been accomplished by a reduction in mass alone, and we discuss the occurrence and potential mechanisms of physiological compensation. The amount of energy saved during the night did account for part of the total amount of energy saved. We surmise that the strategy of energetic compensation observed during the night was extended into the inactive hours of the day.

The Two-Oscillator Circadian System of Tree Shrews (Tupaia belangeri) and Its Response to Light and Dark Pulses

The wheel-running activity rhythm of tree shrews (tupaias; Tupaia belangeri) housed in constant darkness (DD) phase-advanced following a 3-hr light pulse at circadian time (CT) 21. Dark pulses of 3 hr presented to tupaias in bright constant light (LL) did not induce significant phase shifts of the free-running activity rhythm, irrespective of the CT. In dim LL, tupaias showed simultaneous splitting of their circadian rhythm of wheel-running activity, nest-box activity, and feeding behavior. Light pulses of 6 hr and 2300 lux were presented to 13 tupaias with split wheel-running activity rhythms. These light pulses induced immediate phase shifts in the two components of the split rhythm in opposite directions. No differences were observed between the light-pulse phase response curves of the two components. Equally large immediate phase advances were induced in both components by light pulses of 230 lux, but not by 23 lux. The final phase shifts were small at all CTs. In two tupaias, activity rhythms transiently split and re-fused. Analysis of the relative position of the components in one of these indicates asymmetry in the coupling between the components.

Costs of reproduction in the Zebra Finch Taeniopygia guttata : Manipulation of brood size in the laboratory

Brood size of Zebra Finches Taeniopygia guttata was manipulated in an attempt to identify a trade-off between current and subsequent reproduction in a laboratory situation with ad libitum food availability. The birds were able to raise a larger brood than the most frequent brood size under the same conditions. Initiation of the subsequent clutch was advanced after raising a small brood, and delayed after raising a large brood. The size of the subsequent clutch was not affected by the previous, experimental brood size. Thus, a cost of reproduction was observed in modification of the reproductive interval. Brood size also affected the prospects of the current, experimental brood. Both nestling survival and nestling weight at independence decreased with brood size. It has often been suggested that food availability limits reproduction in the field. This study shows that under unrestricted access to food, other factors restrain reproduction. Time allocation, energy expenditure and nutrient reserves of the parents are discussed as potential alternative constraints.

Activity patterns in a subterranean social rodent Spalacopus cyanus (Octodontidae).

Abstract Daily patterns of activity were studied under laboratory conditions in 12 coruros, Spalacopus cyanus, subterranean social rodents originally from Chile. When able to burrow, coruros spent 90% of the total time underground, and surface activity occurred during the 1st hours of darkness. When prevented from burrowing, locomotory activity of coruro groups peaked near the onset of darkness but also occurred during episodes of light. Individually housed coruros displayed a clear nocturnal activity pattern. Coruros exhibited endogenous circadian rhythms entrained by a light–dark cycle.

Temporal organization of feeding in Syrian hamsters with a genetically altered circadian period

The variation in spontaneous meal patterning was studied in three genotypes (tau +/+, tau +/−and tau −/−) of the Syrian hamster with an altered circadian period. Feeding activity was monitored continuously in 13 individuals from each genotype in constant dim light conditions. All three genotypes had on average six feeding episodes during the circadian cycle (about 20h in homozygous tau mutants and 22h in heterozygotes compared with 24h in wild-type hamsters). Thus, homozygous tau mutant hamsters had significantly more feeding episodes per 24h than wild types, and heterozygotes were intermediate. The average duration of feeding bouts (FBs) was indistinguishable (around 30 minutes) among the three genotypes, whereas the intermeal (IM) intervals were significantly shorter for homozygote tau mutant hamsters (99 minutes), intermediate for heterozygotes (116 minutes), and the longest for wild-type hamsters (148 minutes). Thus, the meal-to-meal duration was on average 25% shorter in homozygous tau mutants (16% in heterozygous) than in wild-type hamsters. The reduction of the circadian period has a pronounced effect on short-term feeding rhythms and meal frequency in hamsters carrying the tau mutation. (Chronobiology International, 18(4), 657–664, 2001)