Alan M. Rosenwasser

Circadian activity rhythms in rats with midbrain raphe lesions.

The dorsal and median mesencephalic raphe nuclei provide a robust projection to the hypothalamic suprachiasmatic nucleus, the site of a putative neuronal circadian pacemaker. Although it has been suggested that the raphe may play a role in the circadian timing system, this role has not yet been specified. In the present report, we examined the circadian activity patterns of rats with large mid-brain lesions aimed at the median and dorsal raphe nuclei under conditions of light-dark entrainment, and while free-running in constant light and constant darkness. The results indicate that midbrain raphe lesions may interfere with the expression of free-running circadian activity rhythms.

Circadian rhythmicity and behavioral depression: I. Effects of stress

Rats were exposed to repeated sessions of inescapable footshock, and behavioral depression was subsequently assessed by measuring escape performance during exposure to escapable shock in a different testing environment. Free-running circadian activity rhythms were assessed using running wheels for approximately three weeks before and after administration of inescapable shock. Several animals showed lengthening of free-running period and decreases in activity level following shock. Similar effects were also seen in rats that were removed from their running wheels, placed within the shock apparatus, and not given shock, but not in nonhandled control animals. Furthermore, period lengthening in shocked and handled rats was positively correlated with escape performance, suggesting that circadian rhythm alterations occurred in those animals that were best able to cope with shock or handling-related stressors. In contrast, individual differences in circadian period and activity level during baseline conditions were not predictive of either escape performance or circadian rhythm alterations. These results suggest that successful behavioral adaptation to stress may be associated with alterations of circadian rhythmicity.

Interactions between nocturnal feeding and wheel running patterns in the rat.

The purpose of this experiment was to investigate the relationship between daily patterns of wheel running activity and feeding behavior. Wheel running and bar pressing for food were continuously recorded from rats which had concurrent access to both wheels and food. In a second condition, wheel running and (non-reinforced) bar pressing were recorded during food deprivation. In a third condition, bar pressing for food was recorded while wheel running was prevented. During ad lib access to wheels and food, both behaviors occurred primarily during the dark phase of the light-dark cycle. However, the nocturnal distribution of these responses differed: the early dark hours were primarily devoted to running, while feeding occurred in small, frequent meals. Maximal food intake occurred during the late dark hours, when meals were larger and less frequent, and there was little running. During three days of complete food deprivation, both wheel running and meal initiation attempts (as indicated by nonreinforced bar pressing) maintained their normal daily patterns. However, during wheel deprivation, changes occurred in the temporal distribution of meals which equalized early and late dark food intake. These results show that the availability of an alternative activity, like wheel running, can be an important determinant of daily feeding patterns. However, the daily pattern of wheel running is not influenced by food availability. Different behaviors may show differential plasticity in the coordination of an overall daily behavioral program.

Circadian rhythm in metabolic activity of suprachiasmatic, supraoptic and raphe nuclei.

Previous studies using 2-deoxyglucose (2-DG) autoradiography have demonstrated that the suprachiasmatic nucleus (SCN) of the hypothalamus, a putative neural circadian pacemaker, displays circadian rhythmicity in its metabolic activity. In the present study, we show that distinct circadian variations in 2-DG uptake occur not only in the suprachiasmatic, but also in the supraoptic and median raphe nuclei of the rat brain. On the other hand, several other brain areas failed to display systematic circadian variations in 2-DG uptake. These results indicate that circadian metabolic rhythms are not unique to the SCN. Further studies are required to precisely define the extent of such phenomena.

Timing of parturition and postpartum mating in norway rats: Interaction of an interval timer and a circadian gate

Two temporal variables have been implicated in the timing of postpartum heat in Norway rats: time of day and time since parturition. The nature of the interaction between these variables is not clearly understood. We extend previous observations on this problem to freely behaving animals continuously videotaped during parturition and postpartum mating in a seminatural habitat. The timing of postpartum mating is best described by a model in which an interval timer is coupled to a circadian gating mechanism. Interactions between circadian and interval timing mechanisms may be widespread in the neuroendocrine control of behavior.

Effects of Pregnancy and Parturition on Free-Running Circadian Activity Rhythms in the Rat

Alterations in circadian rhythms have previously been associated with estrous and seasonal changes in reproductive state. In the present study we explored the effects of the reproductive events of pregnancy and parturition on free-running circadian activity rhythms in the rat. Free-running rhythms were monitored before mating, during pregnancy, and following parturition and removal of pups. Systematic and long-lasting alterations of the period of the free-running activity rhythm were seen following parturition. The effects of estrous, seasonal, and gestational reproductive states on circadian rhythms may be mediated by the endocrine events which accompany these states.