We investigated prolactin secretion and metabolic changes in stress response in adult male rats submitted to periodic maternal separation (MS; 180 min/day) at 2 weeks of life. Restraint and ether exposure were randomly performed when the animals were 10-12 weeks of age. Restraint exposure: the animals were placed into plastic tubes (21 cm long, 4.5 cm diameter) for 20 min. Ether exposure: the rats were exposed to ether for 10 min. Atrial cannulation for blood sampling was performed through the jugular vein 5 days before the experiments. In both protocols, blood samples were taken immediately before (0), and 5, 15 and 20 min after the beginning of stress exposure. Ours results showed attenuated endocrine and metabolic responses to ether exposure in the maternal separation (MS) group compared to the control group. The measured metabolic parameters, plasma glucose, prolactin, lactate, and insulin secretion, were 32%, 55%, 41%, 73% lower (P < 0.01), respectively, in MS than in control animals. On the other hand, the endocrine and metabolic stress responses to restraint exposure were not affected by maternal separation. There was no difference between the MS and the control groups in any of the parameters studied. Our data demonstrated that early life experiences affect the hormonal systems beyond the hypothalamic-pituitary-adrenal axis, such as the central neuronal pathways, and their activities related to hormonal and metabolic responses to stress in adulthood. More importantly, these modifications were specific, but dependent on stress situation affecting mainly the circuitry related to the stress response to ether exposure.

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In the present study, we investigated whether the daily fluctuations of internal body temperature (Tb) and spontaneous locomotor activity (SLA) interact with the thermal and neuronal adjustments induced by high-intensity aerobic exercise until fatigue. The body temperature and SLA of adult Wistar rats (n = 23) were continuously recorded by telemetry for 48 h. Then, the rats were subjected to a protocol of graded exercise until fatigue or rest on the treadmill during light and dark-phases. Tb, tail skin temperature and ambient temperature during each experimental session were recorded. At the end of the last experimental session, the animals were anaesthetized; the brains were perfused and removed for immunohistochemical analysis of c-fos neuronal activation. The daily rhythms of SLA and Tb were strongly correlated (r = 0.88 and p < 0.001), and this was followed by a daily oscillation in both the ratio and the correlation index between these variables (p < 0.001). Exercise capacity was associated with a lower resting Tb (p < 0.01) and was higher in the light-phase (p < 0.001), resulting in an increased capacity to accumulate heat during exercise (p < 0.01). Independent of time-of-day, high intensity exercise strongly activated the hypothalamic paraventricular nucleus (PVN), the supra-optic nucleus (SON) and the locus coeruleus (LC) (p < 0.001) but not the suprachiasmatic nucleus (SCN). Taken together, our results points toward a role of the circadian system in a basal activity control of the thermoregulatory system as an important component for the onset of physical activities. In fact, rather than directly limiting the adjustments induced by exercise the present study brings new evidence that the effect of time-of-day on exercise performance occurs at the threshold level for each thermoregulatory system effector activity. This assumption is based on the observed resilience of the central clock to high-intensity exercise and the similarities in exercise-induced neuronal activation in the PVN, SON, and LC.

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