The rate of phase shift of plasma corticosterone circadian rhythm during early developmental stages in neonatally blinded rats

Effects of feeding and light cycles on activity rhythms of maternally isolated rat pups

Motor activity of infant rat pups was measured continuously between days 3 and 18 of postnatal age. Mother-reared rats on a 12:12 LD cycle exhibited significant rhythmic activity in the circadian range as early as day 5 of postnatal age. Some of the pups reared in isolation from maternal and sibling contact and kept on 12:12 LD cycles, feeding cycles, or combinations of feeding, temperature, and LD cycles also showed rhythmic activity but it was less persistent and of a lower amplitude than the rhythms of the mother-reared group. In the isolated rat pups nocturnal partitioning of activity was strengthened in the presence of both a light-dark cycle and a feeding cycle but only when the feeding resembled more natural nursing rhythms. In animals kept on constant light and a feeding cycle, activity occurred slightly more often during the 12-hr interval of decreased food intake. The addition of a temperature cycle--cooler nocturnal temperature--decreased the proportions of nocturnal motor activity. These results indicate that feeding and light-dark cycles may contribute to the synchronization of activity rhythms during the early postnatal period.

Development of the circadian rhythm of body temperature in rats

Seven male and seven female rat pups, all from separate litters of 10 pups each, had biotelemetry thermistors implanted into their peritoneal cavities when they were 10-15 days old. Body temperatures (Tb) were recorded by an Apple computer every 10 minutes until the rats were 60-75 days old so that the development of the circadian temperature rhythm (CTR) could be studied. The data were compared with eight parameters taken from adult CTRs. All rats were maintained on a 12 hr light/12 hr dark photoperiod. The results indicate that the CTR begins to develop after thermoregulatory mechanisms mature to produce an adult like daily range of TbS around day 24. From days 25 to 45, there was a steady increase in the percentage of TbS greater than the daily mean occurring in the dark. This process organized Tb into the characteristic sinusoidal waveform of the adult. Proper phasing of the trough and peak relative to the light/dark cycle took place between days 30-36. By day 42, 90% of TbS higher than the daily mean were occurring in the dark, while those lower than the mean were in the light. This is the typical adult pattern. The waveform's appearance continued to develop until day 50.

Age differences in a circadian influence on hippocampal LTP

Data from several experiments on long-term potentiation (LTP) in the rat hippocampus were examined for circadian influence. Incidence and magnitude of LTP produced in both area CA1 and area dentata were analyzed, and a reciprocal light/dark difference was found in the two areas, with pyramidal cells of area CA1 showing more LTP during the light period and granule cells of area dentata showing more LTP during the dark period. In addition, results from experiments on developing animals, suggested that the circadian influence on LTP in either area was not present before postnatal day 20. All of these experiments were from hippocampal slice preparations; therefore, it is important to note that circadian influences on hippocampal LTP are preserved in the in vitro environment where tonic extrahippocampal input has presumably been removed.