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Circadian rhythms of hypothalamic catecholamines and their metabolites and of prolactin and corticosterone circulating hormones were determined over 24 h in group-housed (six per cage; control) and individually housed, 90-day-old male albino rats adapted to a 12:12 light/dark cycle. After 5 weeks of adaptation, in individually housed rats, norepinephrine (NE) turnover increased, since lower NE (p < 0.01) and higher MOPEG metabolite (p < 0.01) levels were measured. As concerns the dopaminergic system, individually housed rats increased both dopamine (DA) synthesis and turnover as revealed by higher DOPAC (p < 0.01) and HVA (p < 0.05) metabolites without modification of DA levels. Moreover, in individually housed rats, corticosterone (p < 0.01) and prolactin (p < 0.05) mean levels were higher than in the control group.
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Gambardella et al. (1994) studied this question.
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